Carbon dynamics from carbonate dissolution in Australian agricultural soils

CSIRO PUBLISHING Soil Research http://dx.doi.org/10.1071/SR14060 Carbon dynamics from carbonate dissolution in Australian agricultural soils Waqar Ahmad A,D,E, Balwant Singh A, Ram C. Dalal B,C, and Feike A. Dijkstra A A Department of Environmental Sciences, Faculty of Agriculture and Environment, The University of Sydney, Eveleigh, NSW 2015, Australia. B Department of Science, Information Technology, Innovation and the Arts, 41 Boggo Road, Dutton Park, Qld 4102, Australia. C School of Agriculture and Food Sciences, University of Queensland, St Lucia, Qld 4072, Australia. D Food and Agriculture Organisation of the United Nations, NARC Premises, Park Road, Islamabad, Pakistan. E Corresponding author. Email: waqar.ahmad@sydney.edu.au Abstract. Land-use and management practices on limed acidic and carbonate-bearing soils can fundamentally alter carbon (C) dynamics, creating an important feedback to atmospheric carbon dioxide (CO2) concentrations. Transformation of carbonates in such soils and its implication for C sequestration with climate change are largely unknown and there is much speculation about inorganic C sequestration via bicarbonates. Soil carbonate equilibrium is complicated, and all reactants and reaction products need to be accounted for fully to assess whether specific processes lead to a net removal of atmospheric CO2. Data are scarce on the estimates of CaCO3 stocks and the effect of land-use management practices on these stocks, and there is a lack of understanding on the fate of CO2 released from carbonates. We estimated carbonate stocks from four major soil types in Australia (Calcarosols, Vertosols, Kandosols and Chromosols). In >200-mm rainfall zone, which is important for Australian agriculture, the CaCO3-C stocks ranged from 60.7 to 2542 Mt at 0–0.3 m depth (dissolution zone), and from 260 to 15 660 Mt at 0–1.0 m depth. The combined CaCO3-C stocks in Vertosols, Kandosols and Chromosols were about 30% of those in Calcarosols. Total average CaCO3-C stocks in the dissolution zone represented 11–23% of the stocks present at 0–1.0 m depth, across the four soil types. These estimates provide a realistic picture of the current variation of CaCO3-C stocks in Australia while offering a baseline to estimate potential CO2 emission–sequestration through land-use changes for these soil types. In addition, we provide an overview of the uncertainties in accounting for CO2 emission from soil carbonate dissolution and major inorganic C transformations in soils as affected by land-use change and management practices, including liming of acidic soils and its secondary effects on the mobility of dissolved organic C. We also consider impacts of liming on mineralisation of the native soil C, and when these transformations should be considered a net atmospheric CO2 source or sink. Additional keywords: carbonate฀dissolution,฀land฀use฀change฀and฀acidification,฀limed฀acidic฀soils,฀management฀practices,฀ soil฀carbonate฀stocks,฀soil฀types. Received฀10฀March฀2014,฀accepted฀2฀October฀2014,฀published฀online฀25฀February฀2015 Introduction Soils play an important role in the global carbon (C) cycle, and soil C is considered the largest terrestrial C pool, with nearly three times the amount of C as in living plants and twice the amount of C as in the atmosphere (Schlesinger 1990, 1995). Globally, the top 1 m of soil stores ~1500 Pg (1 Pg = 1 Gt = 1015 g) as soil organic C (SOC) and 900–1700 Pg as soil inorganic C (SIC; carbonates) (Lal 2008). Soil C is therefore of great importance in the context of climate change, because small changes in the soil C pool can have a significant impact on atmospheric CO2 concentration and subsequent global-warming potential (Ehhalt and Prather 2001). Research has largely focused on the dynamics of SOC and its management, and little attention has been given to soil carbonates in general Journal compilation  CSIRO 2015 (Monger and Martinez-Rios 2002; Mikhailova and Post 2006; Mi et al. 2008). Since SIC accounts for more than one-third of the total soil C pool, the prediction of potential responses of soil C to land-use change and management practices and future global changes cannot be based entirely on that of SOC. Recently, subsoil C losses have been advocated due to the dissolution of SIC and lack of SOC replenishment (Kalbitz et al. 2013). The baseline values of SIC stocks in different soil types could be valuable for predicting the magnitude of changes in these stocks resulting from land use and land-use change. However, data are scarce on carbonate stocks in soils and the impact of land-management practices on these stocks. Although the fate of bicarbonate ions (HCO3–) released from the dissolution of soil carbonates is considered important in the www.publish.csiro.au/journals/sr B Soil Research context of C sequestration (Nordt et al. 2000; Lal 2008; Sanderman 2012), the dynamics and fate of HCO3– ions in different soil types are not well understood. Carbonates in soil are categorised into primary or lithogenic carbonates and secondary or pedogenic carbonates. Lithogenic carbonates are derived from the weathering of calcareous parent material, whereas pedogenic carbonates are formed through the reaction of atmospheric CO2 with Ca2+ and Mg2+ brought in from outside the local ecosystem, for example in calcareous dust, irrigation water, fertilisers and manures (Lal 2008). Based on the source of Ca2+ during the precipitation of pedogenic carbonates, they are further subdivided into pedo-lithogenic and pedo-atmogenic carbonates. The source of Ca2+ for pedolithogenic carbonates is a carbonate mineral, whereas non-carbonate mineral is the precursor of Ca2+ in the pedoatmogenic carbonates (Monger and Martinez-Rios 2002). SIC stocks are dynamic and change significantly with time depending on climate, land-use change and management practices. Increased aridity may result in increased formation of stable carbonate materials; hence, there may be an increased sequestration of atmospheric CO2 under the climate change scenario of decreasing precipitation, especially in semi-arid regions. Land-use change resulting in soil acidification may increase the release of large amounts of C through carbonate dissolution from soils (Suarez 2000). Similarly, the dissolution of lime, applied for the remediation of acidic soils for agricultural production, causes the release of CO2. Considering the extent of area occupied by acidic soils and the widespread use of lime in agricultural production systems on these soils, even small changes in C dynamics could substantially contribute to atmospheric CO2. Thus, SIC stocks play an important role in the global terrestrial C cycle in the context of atmospheric CO2 sequestration through both natural and human-induced processes (Nordt et al. 2000; Drees et al. 2001; Eshel et al. 2007) and need consideration in the context of global climate change. The principal objective of this review is to evaluate the contribution of land use and management practices to CO2 emissions from the dissolution of carbonates in Australian agricultural soils. We use the term ‘soil carbonates’ for both the carbonates in calcareous soils and lime added to acidic soils. We discuss the mechanism of temperature sensitivity for carbonate dissolution and its feedback to the atmospheric CO2. Additionally, we present an estimate of the carbonate stocks from four major Australian soils used for agriculture, and highlight the secondary role of lime in the mobility of dissolved organic C (DOC) in acidic soils, which could be an important component of the terrestrial C balance. Extent, distribution and liming of acidic soils Soil acidity is considered one of the major soil constraints for crop production in the tropical and subtropical regions. In Australia, soil acidity is a natural attribute of most soils. The area of acidic soils (low pH soil) includes the low-rainfall zones of south-eastern Australia, the high-rainfall zone of northeastern Australia and the Mediterranean climatic zone of Western Australia (Carr and Ritchie 1993; Moody and Aitken 1997). Changes in land use and associated shift in management practices may directly affect C and nitrogen (N) cycles in soils and generate soil acidity. Agriculture and overgrazing have W. Ahmad et al. seriously degraded much of the Australian landscape. They have caused widespread acidification, changes in C stocks, accelerated erosion, and salinisation (Dalal and Mayer 1986a, 1986b; McKenzie et al. 2004; Dalal et al. 2005). Intensive land use for agricultural production has contributed more to acidification than less intensive and non-agricultural land use (Robinson et al. 1995). Higher acidity has been reported in soils used for cotton production than in similar soils under native vegetation (Singh et al. 2003). Annual acid addition rates of 0.50–34 kmol H+ ha–1 year–1 have been reported for a range of land uses (Table 1). Liming represents a common management practice for crop production on acidic soils. Agricultural lime (CaCO3 and (CaMg (CO3)2), as either lime sand or crushed limestone, is usually applied to ameliorate soil acidity around the globe. In Australia, ~2.5 Mt is applied annually to agricultural fields (Page et al. 2009). The loss of CO2 with other greenhouse gases such as nitrous oxide (N2O) is expected to increase significantly after the application and subsequent dissolution of carbonates from the applied lime (Page et al. 2009). We have estimated the CO2 emission due to neutralisation of acidity by the applied lime for a range of production systems in Australia with and without considering the fate of HCO3– ions evolved in the reaction (scenario 1, scenario 2, Table 1). Extent and stocks of carbonates in calcareous soils in Australia Calcareous soils cover >47% of Earth’s land area and are mainly concentrated in arid or semi-arid regions, where low precipitation and biological activity equate to relatively low acid inputs and leaching. These soils are important for agricultural production in many areas of the world, including Australia. For example, in South Australia, about 40% of the wheat (Triticum aestivum L.) crop is produced on the Eyre Peninsula, which contains >1 Mha of calcareous soils (Holloway et al. 2001). Over 0.3 Mha of calcareous Vertosols is under cotton production in New South Wales (Knowles and Singh 2003). Overall, soils that are calcareous throughout the soil profile (inland eastern Australia, mainly Vertosols) cover an area of ~2.3  106 km2, and those with calcareous subsoils (southern and inland regions of the Murray–Darling Basin) extend to an area of ~1.4  106 km2 (Fitzpatrick and Merry 2000). The dominant land uses on such calcareous subsoils are dryland cropping, irrigated grain cropping, horticulture and cotton cultivation (Fitzpatrick and Merry 2000). Accumulation and distribution of soil carbonates is greatly affected by water quality (Eshel et al. 2007; Sanderman 2012). The Lower Murray Lakes are extensively used for irrigation purposes in South Australia. From the available data (South Australia EPA 1998; Earth Systems 2008), we calculated an average HCO3– concentration of 178 mg L–1 in the Lower Murray Lakes. Irrigation with such water can potentially add 26.6 kg C ha–1 in a single irrigation event (75 mm). Addition of C through irrigated water could be a source of C sequestration or a net source of atmospheric CO2. However, whether the increase in carbonate density (accumulation) and its distribution or translocation (from upper layer to the lower soil layer) is C sequestration or just a pool transfer, and under what situations it could be considered as C sequestration, would depend on the Carbonate dissolution in agricultural soils Soil Research C Table 1. Annual acid addition rates for a range of production systems and CO2 emission from carbonates in Australian limed soils The average values of annual acid addition rates are for the two extreme values provided by the researchers as mentioned in the source column. CO2 emission has been calculated under two different scenarios: scenario 1 (IPCC Default Methodology Tier-1), scenario 2 (Page et al. 2009). The underlying hypothesis for Tier-1 methodology is that the entire C contained in carbonates is released into the atmosphere within the year of application; for scenario 2, the partial sequestration of HCO3– in water was taken into account Production system Location Annual rate (kmol H+ ha–1 year–1) Range Average Lime requirement (kg ha–1 year–1) CO2 emission (kg ha–1 year–1) Scenario 1 Scenario 2 Source Grazed clover pasture Victoria 0.8–4.4 2.6 130 52 40 Pasture cut for hay Tropical and subtropical Queensland Tropical Australia Northern Queensland and Northern Territory North-eastern Victoria North-eastern Victoria Tropical and subtropical Queensland Tropical and subtropical Queensland Tropical and subtropical Queensland Southern Queensland Victoria 1.0–6.0 3.5 175 70 53 Ridley et al. 1990; Noble et al. 1997 Moody and Aitken 1997 10.0–11.0 10.6 10.5 10.6 525 530 210 212 160 161 Moody and Aitken 1997 Noble et al. 1997 0.9–4.6 12.5 2.8–4.7 2.8 12.5 3.8 140 625 190 56 250 76 43 190 58 Slattery et al. 1998 Slattery et al. 1998 Moody and Aitken 1997 28–40 34 1700 679 518 Moody and Aitken 1997 1.3–2.5 1.9 95 38 29 Moody and Aitken 1997 0.2–5.1 0.15–4.1 2.7 2.1 135 105 54 42 41 32 0.15–0.9 0.5 25 10 8 Noble et al. 1998 Ridley et al. 1990; Slattery et al. 1998 Moody and Aitken 1997 7.9–10.4 9.1 455 182 139 5.0 5.0 250 100 76 Pasture cut for hay Stylosanthes seed production Cereals Lupins Sugarcane Banana Grapes Wheat–pasture rotation Wheat–lupin rotations Cereal–clover pasture Irrigated rice– wheat–pasture Cotton Tropical and subtropical Queensland Southern Queensland Northern New South Wales leaching environment and on the fate of HCO3–. The dynamics of HCO3– are discussed in more details in a later section. Carbonate stock estimation More than 70% of Australia is arid or semi-arid and contains large carbonate stocks. However, estimates of carbonate stocks are not available for major soil types. We provide below the estimates of carbonates stocks for 0–0.3 m depth (dissolution zone), in addition to the stock estimates for 1.0 m depth, in four major Australian soil types that dominate the arid and semi-arid region of Australia. Changes in carbonates in the topsoil depths could be particularly important for determining ecosystem response and functioning. Carbonate stocks in agricultural soils in Australia Carbonate stocks in agricultural soils (rainfall >200 mm) for 0–0.3 m and 0–1.0 m depths were estimated using published data (Stace et al. 1968; Knowles and Singh 2003; McKenzie et al. 2004) and a dataset provided by R. Dalal (dataset summary given in Dalal and Mayer 1986a). We included all soil profiles for which information on soil carbonate contents, bulk density (rb), land use and parent material was available. Additionally, for the samples where rb data were not available or were incomplete, we used a value of 1500 kg m–3 or the surface horizon rb value to calculate carbonate stocks (t ha–1) for the profiles. Noble et al. 1998; Slattery et al. 1998 Singh et al. 2003 Total average CaCO3-C stocks in the four soil types ranged from 60.7 to 2542 Mt for 0–0.3 m depth and from 260 to 15 660 Mt for 0–1.0 m depth in the agricultural regions (Table 2; Fig. 1). The total average CaCO3-C stock within the 0-0.3 m depth was in the order Calcarosols > Vertosols > Kandosols > Chromosols. The trend in the CaCO3-C stocks for 0–1.0 m depth followed a slightly different pattern, i.e. Calcarosols > Kandosols > Vertosols > Chromosols. At both depths, the pooled CaCO3-C stocks in the Vertosols, Kandosols and Chromosols constituted ~30% of the amount present in the Calcarosols. Total average CaCO3-C stocks at 0–0.3 m depth represented 11–23% of the stocks present at 0–1.0 m across the four soil types. Uncertainties in accounting for CO2 emission from carbonate dissolution Scenario 1 is being used in the Australian Carbon Accounting System to account for CO2 emission from the dissolution of carbonates in limed soils (Eqn 1). In scenario 1, it is assumed that all C contained in the applied lime is released into the atmosphere within the year of application: CO2 Cemission ¼ fðMLimestone  P  EFLimestone Þ þðMDolomite  P  EFDolomite Þg  Cg =1000 ð1Þ where CO2-C emission is annual C emissions from lime (Gg); MLimestone and MDolomite are the masses of limestone and D Soil Research Table 2. Estimates of carbonates dissolution rates from different Australian soil types under dominant land uses and CO2 emission scenarios Australian Soil Classification (Isbell 2002) Great Soil Group (Stace et al. 1968) AreaA (Mha) Description and distribution CaCO3-CB (t ha–1) Total CaCO3-C (Mt) CaCO3-C dissolution rateC (kg ha–1 year–1) Total CO2 emission (kg year–1)D Scenario 1 Scenario 2 Calcarosols (n = 16) Vertosols (n = 109) Kandosols (n = 7) Chromosols (n = 7) 0–0.3 m: range 4.8–411, 45 (42) Lack strong texture-contrast; mean 83.4, median 56.5 mostly calcareous throughout 0–1 m: range 36.3–1445, Mediterranean climatic zone mean 447, median 348 of South Australia, Western Australia and north-western Victoria Black earths; grey, 77 (75) Clay >35%, cracks, slickensides; 0-0.3 m: range 0.13–52.63, brown and red Queensland, NSW, Northern mean 7.71, median 5.39 clays Territory 0–1 m: range 3.26–196, mean 29.11, median 24.52 Red and yellow 114 (90) Lack strong texture-contrast; 0–0.3 m: range: 0.66–95.4, earths; calcareous pH >5.5 in B horizon; mean 15.94, median 2.46 red earths wheatbelt of southern NSW, 0–1 m: range 1.9–313.7, south-west of Western mean 90.7, median 21.5 Australia Non-calcic brown 22 (16) Strong texture-contrast; 0-0.3 m: range 1.6–16.2, soils; some red-brown wheatbelt of southern NSW, mean 5.88, median 2.76 earths; and a range of northern Victoria, south-west 0–1 m, range 7.4–403.4, podzolic soil of Western Australia, parts of mean 90.0, median 11.8 South Australia Solonised brown soils; grey-brown and red calcareous soils 0–0.3 m: range 216–18 495, Cereal 16.8 average 2542 0–1 m: range 1633–65 025, average 15 660 2520 1935 0–0.3 m: range 10.0–4052, average 415 0–1 m: range 251–15 092, average 1888 Sugarcane 22.8 Cotton 30 Banana 204 65 835 50 204 0–0.3 m: range 75.3–10 876, average 280 0–1 m: range 217–35 761, average 2451 Grapes 11.4 Cereals 16.8 10 716 8208 0–0.3 m: range 35.2–356, average 60.7 0–1 m: range 163–8875, average 260 Improved pasture (grazed clover pasture) 15.6 Cereal 16.8 2376 1826 A Estimated area of each soil type within the >200-mm average annual rainfall zone (Fig. 1, left panel); most of the Australian agriculture is concentrated at this rainfall zone. In parentheses are the previously reported areas of soil types within the >200-mm rainfall zone (www.nrm.gov.au; or elsewhere). The difference in the calculated area under four soil types may be because the earlier calculations used different sources of soil maps or because our generalisation of the rainfall map might have led to some variations. B Calculated for profile depths 0.3 m and 1 m for each soil type. C The dissolution from different soil types has been calculated by assuming that Calcarosols are cultivated only by cereal cropping; Vertosols are under cotton, wheat and sugarcane cropping; Kandosols are under cereals and grapes; and Chromosols are under improved pastures and cereal cropping. D Calculated by multiplying emission (kg ha–1 year–1) (scenario 1 and scenario 2) by the total cultivated area within >200-mm average rainfall zone for each soil type. Considering the skewness in data distribution (Fig. 1, right panel), the median rather than the mean value of CaCO3-C (t ha–1) stock was used for estimating the total average stocks (Mt). Further, these estimates were based on measured carbonates values for the soils, and lithogenic and pedogenic carbonates were not separated. W. Ahmad et al. Carbonate dissolution in agricultural soils Soil Research 0–1.0 m depth 0–0.3 m depth Calcarosols Soil types N W Calcarosols E E Median = 56.5 Median = 348 Mean = 83.4 Mean = 447 S Chromosols Kandosols 0 50 100 150 200 250 300 350 400 450 0 Chromosols Vertosols 0 5 Kandosols 250 500 750 1000 1250 1500 Median = 2.76 Median = 11.8 Mean = 5.88 Mean = 90.0 10 15 0 50 100 150 200 250 300 350 400 450 Median = 2.46 Median = 21.5 Mean = 15.94 Mean = 90.7 0 10 20 30 40 50 60 70 80 90 100 0 50 100 150 200 250 300 350 Vertosols 0 1 2 4 6 8 Median = 5.39 Median = 24.52 Mean = 7.71 Mean = 29.11 Decimal degrees 0 50 100 150 200 0 10 20 30 40 50 CaCO3-C (t ha–1) Fig. 1. Distribution of Calcarosols, Chromosols, Kandosols and Vertosols in the >200-mm rainfall zone (left panel), and histograms showing distribution of CaCO3-C (t ha–1) in these four selected soil types (right panel). In Australia, agriculture is largely dependent on rainfall, and three of four selected soil types are confined to the >200-mm average annual rainfall zone. Therefore, it was assumed that the area of each soil used came from the agricultural sites where average annual rainfall was >200 mm. This area for each soil type was calculated by overlaying the classified Australian rainfall map in GIS environment (ESRI 2011) with the major soil type map. Once the overlay was carried out, the area of soil types present in zone of annual cumulative rainfall >200 mm was extracted. dolomite applied to soils (t or Mg); P represents the fractional purity, 0.9 for lime stone and 0.95 for dolomite; EFLimestone and EFDolomite are IPCC (2006) default emission factors for limestone (0.12) and dolomite (0.13); and Cg equals 44/12, the factor to convert CO2-C into CO2. However, this assumption has been challenged by some researchers (West and McBride 2005; Hamilton et al. 2007; Page et al. 2009). Some of the C contained in lime may not be released into the atmosphere as CO2 because of the formation of HCO3– ions and their subsequent riverine transport into the sea after dissolution (Biasi et al. 2008; Page et al. 2009). Using scenario 2, Page et al. (2009) calculated 14–34% lower CO2 emission following lime dissolution considering 2.49 Mt of lime applied in Australia during 2002 as a baseline for comparison with scenario 1. We compared both scenarios for estimating the CO2 emission from the dissolution of carbonates in the four soil types (Table 2). The CO2 evolved from carbonate dissolution for the selected soils was calculated considering an average value of the range 0.659–0.860 Tg CO2 emitted from 2.49 Mt applied lime. About 23% reduced CO2 emission was computed from the four soils, taking into consideration the mobility of Ca2+ and accompanying HCO3–. As such, a similar decrease in the CO2 emission was predicted for a range of production systems by employing scenario 2 (Table 1). It is arguable that because of the differences in soil properties, there should not be congruent variations in the total amounts of CO2 generated from the four soil types. The acid neutralisation via soil carbonate dissolution and CO2 fluxes are variable because the magnitude and direction of CO2 fluxes are governed by site-specific conditions. The acid addition rates from different production systems could be responsible for the release of varied amounts of CO2 from the carbonates dissolution in Australia (Table 1). Additionally, factors such as Ca2+ concentration, movement of lime via erosion, and surface runoff as used by Page et al. (2009) in scenario 2 may yield different outcomes for a range of soils. Admittedly, the emission factor as used in Eqn 1 would vary depending on the soil type and its impact on lime movement, carbonate dissolution, and mobility and residence time of HCO3–. Soil-related data for the mobility of Ca2+ and associated HCO3– for the Australian soils are not available separately. The inclusion of experimental data for each mentioned parameter for various limed agricultural soils would lead to the development of more scenarios for accurate estimation of CO2 release from lime dissolution. Significant differences (although at similar rates across the soil types) are observed in the CO2 emissions from the carbonate dissolution following the two scenarios; it is argued that such uncertainties should be addressed for budgeting C emissions from the dissolution of soil carbonates. Carbonate morphology, soil attributes and role of rhizosphere Carbonates occur in different forms such as hardpans, nodular or pisolitic layers, mottled carbonate-rich layers, and calcareous F Soil Research W. Ahmad et al. fine earths (Milnes and Hutton 1983). Specific morphologies of calcite accumulation related to vegetation have been described in soils. For example, the presence of needle-shaped calcite crystals near the soil surface is attributed to the occurrence of higher biological activity. Roots are responsible for the concentration of such calcite crystals in the root channel (Jaillard et al. 1991). Further, needle-shaped crystals are due to more CO2 input into the soil via respiration, Ca2+ extraction by roots, and direct precipitation by soil organisms (Monger 2002). By contrast, the presence of soft and small-sized nodules with loose micrite crystals suggests limited development of pedogenic calcite (Khormeli et al. 2006; Owliaie 2012). Carbonate dissolution is variable and depends on the kind and amounts of acid produced during land use (Table 1). The non-biological dissolution of carbonates produces CO2 during the process of neutralising soil acidity. If the carbonates are dissolved by strong acids, 1 mole of CO2 is emitted for every mole of carbonate dissolved. However, estimates of the relative proportion of carbonate dissolved by weak or strong acids do not exist for the Australian landscape (Page et al. 2009). From several European and North American studies, it can be inferred that strong acids dissolve 12–38% of the carbonates in limed soils (West and McBride 2005; Oh and Raymond 2006; Hamilton et al. 2007). Under the prevailing intensity of acidification and current single-liming practice in Australia, the dissolution of carbonates with strong acids could be greater than the aforementioned dissolution range observed in Europe and the USA. Disturbance and displacement of soils containing carbonates under farm operations can profoundly change the soil environment. The rate of proton (H+) addition in soil is expected to be faster under intensive agriculture involving application of large amounts of nitrogenous fertilisers, irrigation and the use of deep-rooted legumes. Under such systems, carbonates continue to dissolve, resulting in increased loss of CO2, as represented by the following equation: 2Hþ þ CaCO3 !Ca2þ þ H2 O þ CO2 " ð2Þ Carbonate dissolution may further be enhanced through (i) increased surface area or smaller particle size; (ii) lower density or increased porosity; and (iii) movement of carbonates into the dissolution zone in soil from deeper horizons to near-surface horizons by erosion or physical transport upward in Vertosols (Hartwig et al. 1990; Hartwig and Loeppert 1991; Miller et al. 2007). The reactivity of carbonates is controlled by mineralogy, surface morphology, and aggregation of carbonates with other soil components (Hartwig and Loeppert 1991). Carbonates of similar size to the clay and fine-silt fractions are the most reactive form in soils (Hartwig et al. 1990). Clayey soils (e.g. Vertosols) limit the diffusion of acids produced by agricultural practices and allow the formation of soil micro-environments that shield carbonates from bulk soil conditions. The longevity of the carbonates is increased and the overall reactivity is decreased by the shielding effect, which may be predominantly observed in clayey soils. By contrast, relatively coarse-textured soils may promote rapid dissolution of carbonates. Consequently, the fraction of soil carbonate dissolution and emission as CO2 into the atmosphere may challenge the potential of soil C sequestration and augment the positive C–climate feedback (Yang et al. 2012). Apart from these determinants, the form of carbonates may be very important in determining the dissolution rate. The solubility of carbonates may vary significantly depending on the form and impurities. The presence of small amounts of iron (Fe) and aluminium (Al) in carbonate minerals can reduce their solubility. In soils, the reduction and oxidation of Fe and manganese (Mn) due to seasonal changes in soil moisture contributes to the formation of cutans, forming coatings and concretions (Zhang and Karathanasis 1997). The finer soil carbonate fraction is more rapidly altered than the coarser fraction by the soil acidity resulting from any increase in the use of nitrogenous fertilisers or introduction of perennial crops under changing land use (Rawlins et al. 2011). Finely distributed carbonate in limed acidic soil diffuses well through the soil and dissolves easily. Lime present in the form of fineearth particles could be more soluble than nodules because of its increased reactive surface area. Therefore, the role of carbonate morphology in ease of dissolution and contribution to atmospheric CO2 is important, but the order of magnitude of this process is yet to be investigated in detail. Root-induced physical and biochemical processes occur in the rhizosphere, and are responsible for the release of root exudates and H+. The degree of root-mediated pH changes in the rhizosphere varies broadly with plant species and form of applied N during plant growth. Ash alkalinity provides an estimate of H+ extrusion, and soil carbonates are solubilised through H+ release from the plant. Acidification driven by ash alkalinity may further enhance the carbonate dissolution rate and CO2 production and induce leaching of dissolved inorganic C. Thus, the rhizosphere has direct acidifying effects through the release of H+ during the excess uptake of basic cations over anions (Mubarak and Nortcliff 2010; Ahmad et al. 2013). However, the phenomenon of ash alkalinity cannot be generalised and is predominant only for legumes that are fully reliant on N2 fixation. Higher rhizospheric acidification under legumes may be due to greater excretion of H+ because of excessive cation uptake during biological N2 fixation (Tang et al. 2011). The relation between ash alkalinity and ion uptake largely depends on the form of N applied to the plants. Therefore, dissolution of soil carbonates and subsequent changes in C dynamics could be related to plant species and form of N applied to soil during the plant growing season. Direct priming effects of liming on the mineralisation of native soil carbon Microbial communities are more active in utilising recently exuded C compounds in limed soils than in unlimed soils (Rangel-Castro et al. 2005). Both limed and organic-residueamended soils are high in particulate organic C and lability. Because of such attributes, the soils are better related to microbial community structure and function than the total soil organic matter (SOM) (Briedis et al. 2012; Murphy et al. 2011). Liming may temporally reduce the stability of macro-aggregates through the decomposition of particulate organic C (Baldock et al. 1994; Chan and Heenan 1999), which could further enhance the native soil-C decomposition. Therefore, liming Carbonate dissolution in agricultural soils can potentially stimulate SOM decomposition, mainly by affecting soil pH. Enhanced CO2 release from SOM decomposition by liming of acidic soils has been reported (Bertrand et al. 2007; Dumale et al. 2011; Ahmad et al. 2014). In addition, decreased SOC contents were evidenced with lime application in cropping zones of southern New South Wales and in south-eastern Australia under a wheat– subterranean clover (Trifolium subterraneum L.) system (Coventry et al. 1992; Chan and Heenan 1999). Loss of soil carbon via leaching of the dissolved organic matter Dissolved organic matter (DOM) is an important indicator of a healthy soil system, because it is more responsive to environmental changes than total SOC (Silveira 2005). Simultaneously, DOC is the most mobile and important C source of microorganisms, and hence is arguably an important intermediate in the global C cycle (Battin and Brumaghim 2009). Lime addition has been shown to increase DOC concentration in both forest (Hildebrand and Schack-Kirchner 2000) and agricultural (Karlik 1995) soils. In fact, lime often results in enhanced biological activity (SOM transformation), which could promote the formation of leaching-susceptible, low-molecular-weight organic compounds (Andersson et al. 1994). Increased leaching of DOC from limed soils can lead to long-term changes in the migration pattern of chemical compounds and lowers organic C contents in the surface soil (Karlik 1995; Ahmad et al. 2013). For example, in a pot experiment, increased leaching of the DOC reduced organic C contents by ~9% in limed soils compared with the unlimed soil (Karlik and Zyczyfiska-Baloniak 1985). The amounts of DOM leached from 1 ha of amended and non-amended soil were 102.4 kg and 76.8 kg, respectively, during the whole experimental period of 4 years (Karlik 1995). Loss of DOC from agricultural soils has a negative impact on soil nutrient cycling and may lead to further soil degradation. Liming can influence DOC composition by increasing the fraction of hydrophobic acids, humic acids and carboxylic groups in DOC (Karlik 1995; Andersson et al. 1999). Changes in quantity and quality of DOC may affect the adsorption properties and thereby the storage and microbial availability of C and N (Andersson et al. 1999). However, migration patterns of the DOC and concomitant loss of C from the upper layers of soils could be tempered by the organo-mineral association. Global warming and soil carbonates dissolution Carbonate dissolution may be altered with an increase in temperature either directly, or indirectly through the products of SOC decomposition. Temperature effect on the solubility of carbonates is linked with the solubility of CO2 in water (devoid of soil and plant presence), which decreases when temperature increases (Duan and Sun 2003). The potential for CO2 evolution (due to more CO2 solubility) from limed agricultural soils was higher at 58 158C than 158 258C (Buysse et al. 2013). However, the contribution of soil carbonates to CO2 evolution was not isolated in this study. In contrast to the above study, we Soil Research G found a 59% increase in the cumulative release of lime-derived C when the incubation temperature was increased from 208C to 408C in an incubation study. Further, the temperature sensitivity of the native soil C was decreased in the lime-amended soils (in laboratory and growth chamber experiments; Ahmad et al. 2014; W. Ahmad, F. A. Dijkstra, R. C. Dalal, B. Singh, unpubl. data). We contend that because of organo-mineral interactions, carbonates (in limed acidic soils) may act differently from the CaCO3–H2O–CO2 equilibrium system. Additionally, if soil water content decreases with rising temperature, then more CO2 could be emitted by the soil at the same time. Higher temperatures could influence surface reactions and mass transfer, which are possibly responsible for the enhanced dissolution rates and C release from soil carbonates at increased temperature. The rate of lime dissolution in soil is perhaps controlled by the supply of H+, which may be accelerated with increased temperature due to nitrification and/or humification processes. The rhizospheric CO2 is commonly derived from the respiration of soil microbes and fauna, and plant roots, and via diffusion of CO2 from the atmosphere (Curtis et al. 1995). A warm growing season increases the CO2 concentration in the rooting zone. In addition, the dissolution of carbonates may be affected in the rhizosphere, particularly because of the decreased pH (associated with increased CO2 concentration). The influence of temperature on the dissolution of carbonates in the presence of plants has not been investigated. Knowledge of the temperature sensitivity of soil carbonates in the presence of crop plants is important for C-accounting under climate-change scenarios. Does the dissolution of soil carbonates lead to atmospheric carbon sequestration? The generation of HCO3– from limed acidic soils and from soils containing carbonates depends on the presence of weak and strong acids, and is closely related to soil pH. Dissolution by the weak carbonic acid results in sequestration of 1 mole of CO2 for each mole of CaCO3, whereas dissolution by strong acids results in the production of 1 mole of CO2 for each mole of CaCO3 (because of consumption of HCO3– into CO2). Because of the limited anion exchange capacity of soils, HCO3– ions are prone to leaching into groundwater, where their fate varies greatly. They can reappear in rivers quite quickly, where they remain in anionic form and act to sequester CO2, or react in situ with H+ to release CO2. The HCO3– ions can also be stored in the groundwater for thousands of years (Nordt et al. 2000) or reprecipitate as CaCO3 under water-deficit conditions, higher concentrations of Ca2+ or HCO3– ions, and alkaline pH. Thus, the dynamics of HCO3– ions highlights the importance of the temporal frame of reference if the mobility of HCO3– ions (containing CO2) is to be considered in the context of SIC sequestration. The quantification of HCO3– ions formed and their relative fluxes in different pathways are intricate processes. For the Australian landscape, the proportion of Ca2+ released from soil moving to waterways varies from 10% to 100%, whereas the proportion of Ca2+ leached with HCO3– ions has been estimated at 50–62% (Page et al. 2009). Of the HCO3– ions H Soil Research in waterways, 95% have been estimated to reach the ocean environment and 60% to re-precipitate to form CaCO3 (Page et al. 2009). These estimates were deduced from Australian landscape data and there is no experimental evidence to confirm these estimates. We calculated that ~30–40% of carbonates (Table 1) are converted to CO2, but more research is required to determine the fate of HCO3– originating from the soil carbonate dissolution in Australia. Perspectives Transformations in soil carbonates and the magnitude of CO2 emission are dependent on land use and management practices. We have provided estimates of inorganic C stocks in four major soil types in Australia, offering a baseline to estimate potential CO2 emission or sequestration through land-use changes. The fate of HCO3– ions produced from the carbonate dissolution is difficult to determine, which creates a large uncertainty in measuring the C sequestration potential from the dissolution of inorganic C. We contend that carbonate dissolution may be a net source of CO2. The movement of HCO3–ions is a core question and future research should be conducted to track the mobility of these anions by using 14C in conjunction with other suitable methodologies to understand the potential role of carbonates in the global terrestrial C budget. As such, liming in the long term may cause a loss of DOC from the upper soil layers that would be highly variable and depend on the nature of the soil profile. Temperature sensitivity of soil carbonate dissolution is very important under climate change. Given the importance of the rhizosphere in the dissolution of soil carbonates via root exudates and H+ in agricultural soils, carbonate dissolution is mostly plant-mediated (Mubarak and Nortcliff 2010; Ahmad et al. 2013). Based on the results from our laboratory, glasshouse and growth-chamber experiments, we conclude that the addition of lime increases the C released from the SOC. On the other hand, liming could decrease the temperature sensitivity of the SOC, which is important under the changing climate scenario. This divergent role of lime warrants detailed investigation on a variety of soils, to improve understanding of the mechanisms controlling C fluxes in such system. Moreover, such an understanding of soil carbonate dynamics, in which all reactants and products are fully accounted for, may elucidate whether or not specific processes lead to a net sequestration of C in soil. References Ahmad W, Singh B, Dijkstra FA, Dalal RC (2013) Inorganic and organic carbon dynamics in a limed acid soil are mediated by plants. Soil Biology & Biochemistry 57, 549–555. doi:10.1016/j.soilbio.2012.10.013 Ahmad W, Singh B, Dijkstra FA, Dalal RC, Geelan-Small P (2014) Temperature sensitivity and carbon release in an acidic soil amended with lime and mulch. Geoderma 214–215, 168–176. doi:10.1016/ j.geoderma.2013.09.014 Andersson S, Valeur I, Nilsson I (1994) Influence of lime on soil respiration, leaching of DOC, and C/S relationships in the mor humus of a haplic podsol. Environment International 20, 81–88. doi:10.1016/0160-4120 (94)90070-1 Andersson S, Nilsson I, Valeur I (1999) Influence of dolomitic lime on DOC and DON leaching in a forest soil. Biogeochemistry 47, 297–317. doi:10.1007/BF00992911 W. Ahmad et al. Baldock J, Aoyama M, Oades J, Susant O, Grant C (1994) Structural amelioration of a South Australian red-brown earth using calcium and organic amendments. Australian Journal of Soil Research 32, 571–594. doi:10.1071/SR9940571 Battin E, Brumaghim J (2009) Antioxidant activity of sulfur and selenium: A review of reactive oxygen species scavenging, glutathione peroxidase, and metal-binding antioxidant mechanisms. Cell Biochemistry and Biophysics 55, 1–23. doi:10.1007/s12013-009-9054-7 Bertrand I, Delfosse O, Mary B (2007) Carbon and nitrogen mineralization in acidic, limed and calcareous agricultural soils: Apparent and actual effects. Soil Biology & Biochemistry 39, 276–288. doi:10.1016/ j.soilbio.2006.07.016 Biasi C, Lind SE, Pekkarinen NM, Huttunen JT, Shurpali NJ, Hyvönen NP, Repo ME, Martikainen PJ (2008) Direct experimental evidence for the contribution of lime to CO2 release from managed peat soil. Soil Biology & Biochemistry 40, 2660–2669. doi:10.1016/j.soilbio.2008.07.011 Briedis C, de Moraes Sá JC, Caires EF, de Fátima Navarro J, Inagaki TM, Boer A, de Oliveira Ferreira A, Neto CQ, Canalli LB, Bürkner dos Santos J (2012) Changes in organic matter pools and increases in carbon sequestration in response to surface liming in an Oxisol under longterm no-till. Soil Science Society of America Journal 76, 151–160. doi:10.2136/sssaj2011.0128 Buysse P, Goffin S, Carnol M, Malchair S, Debacq A, Longdoz B, Aubinet M (2013) Short-term temperature impact on soil heterotrophic respiration in limed agricultural soil samples. Biogeochemistry 112, 441–455. doi:10.1007/s10533-012-9739-7 Carr S, Ritchie G (1993) Al toxicity of wheat grown in acidic subsoils in relation to soil solution properties and exchangeable cations. Australian Journal of Soil Research 31, 583–596. doi:10.1071/SR9930583 Chan KY, Heenan DP (1999) Lime-induced loss of soil organic carbon and effect on aggregate stability. Soil Science Society of America Journal 63, 1841–1844. doi:10.2136/sssaj1999.6361841x Coventry DR, Hirth JR, Reeves TG (1992) Interactions of tillage and lime in wheat-subterranean clover rotations on an acidic sandy clay loam in southeastern Australia. Soil & Tillage Research 25, 53–65. doi:10.1016/ 0167-1987(92)90062-G Curtis PS, O’Neill EG, Teeri JA, Zak DR, Pregitzer KS (1995) ‘Belowground responses to rising atmospheric CO2: Implications for plants, soil biota, and ecosystem processes.’ (Kluwer Academic Press: Dordrecht, The Netherlands) Dalal R, Mayer R (1986a) Long term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. I. Overall changes in soil properties and trends in winter cereal yields. Soil Research 24, 265–279. doi:10.1071/SR9860265 Dalal R, Mayer R (1986b) Long term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. II. Total organic carbon and its rate of loss from the soil profile. Australian Journal of Soil Research 24, 281–292. doi:10.1071/ SR9860281 Dalal RC, Harms B, Krull E, Wang W (2005) Total soil organic matter and its labile pools following mulga (Acacia aneura) clearing for pasture development and cropping 1. Total and labile carbon. Australian Journal of Soil Research 43, 13–20. doi:10.1071/SR04044 Drees LR, Wilding LP, Nordt LC (2001) Inorganic and organic carbon sequestration across broad geoclimatic regions. In ‘Soil carbon sequestration and the greenhouse effect’. Soil Science Society of America Special Publication No. 57. 1st edn (Ed. R Lal) pp. 155–172. (Soil Science Society of America: Madison, WI, USA) Duan Z, Sun R (2003) An improved model calculating CO2 solubility in pure water and aqueous NaCl solutions from 273 to 533 K and from 0 to 2000 bar. Chemical Geology 193, 257–271. doi:10.1016/S0009-2541(02) 00263-2 Dumale WA Jr, Miyazaki T, Hirai K, Nishimura T (2011) SOC turnover and lime-CO2 evolution during liming of an acid Andisol and Ultisol. Open Journal of Soil Science 1, 49–53. Carbonate dissolution in agricultural soils Earth Systems (2008) Management of options for acid sulfate soils in the lower Murray Lakes, South Australia. Stage 2. Preliminary Assessment of Prevention, Control and Treatment Options. Earth Systems Report for South Australian Government. Ehhalt D, Prather M (2001) ‘IPCC Third Assessment Report. The Scientific Basis. 4. Atmospheric Chemistry and Greenhouse Gases.’ pp. 241–287. (Intergovernmental Panel on Climate Change: Geneva) Eshel G, Fine P, Singer MJ (2007) Total soil carbon and water quality: An Implication for carbon sequestration. Soil Science Society of America Journal 71, 397–405. doi:10.2136/sssaj2006.0061 ESRI (2011) ‘ArcGIS Desktop: Release 10.’ (Environmental Systems Research Institute: Redlands, CA, USA) Fitzpatrick RW, Merry RH (2000) Pedogenic carbonate pools and climate change in Australia. In ‘Global climate change and pedogenic carbonates’. (Eds R Lal, JM Kimble, H Eswaran, BA Stewart) pp. 105–119. (CRC/Lewis Publishers: Boca Raton, FL, USA) Hamilton SK, Kurzman AL, Arango C, Jin L, Robertson GP (2007) Evidence for carbon sequestration by agricultural liming. Global Biogeochemical Cycles 21, GB2021. doi:10.1029/2006GB002738 Hartwig RC, Loeppert RH (1991) Pretreatment effect on dispersion of carbonates in calcareous soils. Soil Science Society of America Journal 55, 19–25. doi:10.2136/sssaj1991.03615995005500010004x Hartwig RC, Loeppert RH, Moore TJ (1990) Steady-state procedure for determining the effective particle-size distribution of soil carbonates. Soil Science Society of America Journal 54, 55–59. doi:10.2136/ sssaj1990.03615995005400010008x Hildebrand EE, Schack-Kirchner H (2000) Initial effects of lime and rock powder application on soil solution chemistry in a dystric cambisol— results of model experiments. Nutrient Cycling in Agroecosystems 56, 69–78. doi:10.1023/A:1009714927385 Holloway RE, Bertrand I, Frischke AJ, Brace DM, McLaughlin MJ, Shepperd W (2001) Improving fertiliser efficiency on calcareous and alkaline soils with fluid sources of P, N and Zn. Plant and Soil 236, 209–219. doi:10.1023/A:1012720909293 IPCC (2006) ‘IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 4. Agriculture, forestry and other land use.’ (Intergovernmental Panel on Climate Change: Geneva) Isbell RF (2002) ‘The Australian Soil Classification.’ (CSIRO Publishing: Melbourne) Jaillard B, Guyon A, Maurin AF (1991) Structure and composition of calcified roots, and their identification in calcareous soils. Geoderma 50, 197–210. doi:10.1016/0016-7061(91)90034-Q Kalbitz K, Kaiser K, Fiedler S, Kolbl A, Amelung W, Brauer T, Cao ZH, Don A, Grootes P, Jahn R, Schwark L, Vogelsang V, Wissing L, Kogel-Knabner I (2013) The carbon count of 2000 years of rice cultivation. Global Change Biology 19, 1107–1113. doi:10.1111/ gcb.12080 Karlik B (1995) Liming effect on dissolved organic matter leaching. Water, Air and Soil Pollution 85, 949–954. doi:10.1007/BF00476952 Karlik B, Zyczyfiska-Baloniak I (1985) Soil organic matter from cultivated and uncultivated soils due to liming. Intercol Bulletin 12, 103–106. Khormeli F, Abtahi A, Stoops G (2006) Micromorphology of calcitic pedofeatures in highly calcareous soils of Fars province, Southern Iran. Catena 132, 31–46. Knowles TA, Singh B (2003) Carbon storage in cotton soils of northern New South Wales. Australian Journal of Soil Research 41, 889–903. doi:10.1071/SR02023 Lal R (2008) Carbon sequestration. Philosophical Transactions of the Royal Society: Biological Sciences 363, 815–830. McKenzie N, Jacquier D, Isbell R, Brown K (2004) ‘Australian soils and landscapes.’ (CSIRO Publishing: Melbourne) Mi NA, Wang S, Liu J, Yu G, Zhang W, Jobbágy E (2008) Soil inorganic carbon storage pattern in China. Global Change Biology 14, 2380–2387. doi:10.1111/j.1365-2486.2008.01642.x Soil Research I Mikhailova EA, Post CJ (2006) Effect of land use on soil inorganic carbon stocks in the Russian Chernozem. Journal of Environmental Quality 35, 1384–1388. doi:10.2134/jeq2005.0151 Miller DL, Mora CI, Driese SG (2007) Isotopic variability in large carbonate nodules in Vertisols: Implications for climate and ecosystem assessments. Geoderma 142, 104–111. doi:10.1016/j.geoderma.2007. 08.007 Milnes AR, Hutton JT (1983) Calcretes in Australia. In ‘Soils: an Australian viewpoint’. pp. 119–162. (Division of Soils, CSIRO: Melbourne) Monger HC (2002) Pedogenic carbonate: links between biotic and abiotic CaCO3. In ‘17th World Conference of Soil Science’. 14–21 August 2002, Bangkok. (International Union of Soil Sciences) Monger HC, Martinez-Rios JJ (2002) Inorganic carbon sequestration in grazing lands. In ‘The potential of grazing lands to sequester carbon and mitigate the greenhouse gas effect’. (Eds RM Follett, JM Kimble, R Lal) pp. 87–118. (Lewis Publishers: Boca Raton, FL, USA) Moody PW, Aitken RL (1997) Soil acidification under some tropical agricultural systems. 1. Rates of acidification and contributing factors. Australian Journal of Soil Research 35, 163–173. doi:10.1071/S96069 Mubarak AR, Nortcliff S (2010) Calcium carbonate solubilization through H-proton release from some legumes grown in calcareous saline-sodic soils. Land Degradation & Development 21, 24–31. doi:10.1002/ ldr.962 Murphy DV, Cookson WR, Braimbridge M, Marschner P, Jones DL, Stockdale EA, Abbott LK (2011) Relationships between soil organic matter and the soil microbial biomass (size, functional diversity, and community structure) in crop and pasture systems in a semi-arid environment. Soil Research 49, 582–594. doi:10.1071/SR11203 Noble AD, Cannon M, Muller D (1997) Evidence of accelerated soil acidification under Stylosanthes-dominated pastures. Australian Journal of Soil Research 35, 1309–1322. doi:10.1071/S97053 Noble AD, Thompson CH, Jones RJ, Jones RM (1998) The long-term impact of two pasture production systems on soil acidification in southern Queensland. Australian Journal of Experimental Agriculture 38, 335–343. doi:10.1071/EA98039 Nordt LC, Wilding LP, Drees LR (2000) Pedogenic carbonate transformation in leaching soil systems: Implications for the global C cycle. In ‘Global climate change and pedogenic carbonate’. (Eds R Lal, JM Kimble, H Eswaran, BA Stewart) pp. 43–64. (Lewis Publishers: Boca Raton, FL, USA) Oh N-H, Raymond PA (2006) Contribution of agricultural liming to riverine bicarbonate export and CO2 sequestration in the Ohio River basin. Global Biogeochemical Cycles 20, GB3012. doi:10.1029/ 2005GB002565 Owliaie HR (2012) Micromorphology of calcitic features in calcareous soils of Kohgilouye Province, Southwestern Iran. Journal of Agriculture Science and Technology 14, 225–239. Page KL, Allen DE, Dalal RC, Slattery W (2009) Processes and magnitude of CO2, CH4, and N2O fluxes from liming of Australian acidic soils: a review. Australian Journal of Soil Research 47, 747–762. doi:10.1071/ SR09057 Rangel-Castro JI, Killham K, Ostle N, Nicol GW, Anderson IC, Scrimgeour CM, Ineson P, Meharg A, Prosser JI (2005) Stable isotope probing analysis of the influence of liming on root exudate utilization by soil microorganisms. Environmental Microbiology 7, 828–838. doi:10.1111/ j.1462-2920.2005.00756.x Rawlins BG, Henrys P, Breward N, Robinson DA, Keith AM, Garcia-Bajo M (2011) The importance of inorganic carbon in soil carbon databases and stock estimates: a case study from England. Soil Use and Management 27, 312–320. Ridley AM, Slattery WJ, Helyar KR, Cowling A (1990) The importance of the carbon cycle to acidification of a grazed annual pasture. Australian Journal of Experimental Agriculture 30, 529–537. doi:10.1071/ EA9900529 J Soil Research W. Ahmad et al. Robinson B, Malfroy H, Chartres C, Helyar K, Ayers G (1995) The sensitivity of ecosystems to acid inputs in the Hunter Valley, Australia. Water, Air, and Soil Pollution 85, 1721–1726. doi:10.1007/ BF00477228 Sanderman J (2012) Can management induced changes in the carbonate system drive soil carbon sequestration? A review with particular focus on Australia. Agriculture, Ecosystems & Environment 155, 70–77. doi:10.1016/j.agee.2012.04.015 Schlesinger WM (1990) Evidence from chronosequence studies for a low carbon-storage potential of soils. Nature 348, 232–234. doi:10.1038/ 348232a0 Schlesinger WM (1995) An overview of the C cycle. In ‘Soils and global change’. (Eds RK Lal, J Levine, E Stewart, BA Stewart) pp. 9–26. (CRC Press: Boca Raton, FL, USA) Silveira MLA (2005) Dissolved organic carbon and bioavailability of N and P as indicators of soil quality. Scientia Agricola 62, 502–508. doi:10.1590/S0103-90162005000500017 Singh B, Odeh IOA, McBratney AB (2003) Acid buffering capacity and potential acidification of cotton soils in northern New South Wales. Australian Journal of Soil Research 41, 875–888. doi:10.1071/ SR02036 Slattery WJ, Edwards DG, Bell LC, Coventry DR, Helyar KR (1998) Soil acidification and the carbon cycle in a cropping soil of northeastern Victoria. Australian Journal of Soil Research 36, 273–290. doi:10.1071/S96095 South Australia EPA (1998) Ambient water quality monitoring of Lake Alexandrina and Lake Albert. South Australian EPA Water Monitoring Report No. 1, September 1998. Stace HCT, Hubble GD, Brewer R, Northcote KH, Sleeman JR, Mulcahy MJ, Hallsworth EG (1968) ‘A handbook of Australian soils.’ (Rellim Technical Publications: Glenside, S. Aust.) Suarez DL (2000) Impact of agriculture on CO2 as affected by changes in inorganic carbon. In ‘Global climate change and pedogenic carbonates’. (Eds R Lal, JM Kimble, H Eswaran, BA Stewart) pp. 257–272. (CRC/ Lewis Publishers: Boca Raton, FL, USA) Tang C, Conyers MK, Nuruzzaman M, Poile GJ, Liu DL (2011) Biological amelioration of subsoil acidity through managing nitrate uptake by wheat crops. Plant and Soil 338, 383–397. doi:10.1007/s11104-010-0552-6 West TO, McBride AC (2005) The contribution of agricultural lime to carbon dioxide emissions in the United States: dissolution, transport, and net emissions. Agriculture, Ecosystems & Environment 108, 145–154. doi:10.1016/j.agee.2005.01.002 Yang YH, Fang JY, Ji CJ, Ma WH, Mohammat A, Wang SF, Wang SP, Datta A, Robinson D, Smith P (2012) Widespread decreases in topsoil inorganic carbon stocks across China’s grasslands during 1980s–2000s. Global Change Biology 18, 3672–3680. doi:10.1111/ gcb.12025 Zhang M, Karathanasis A (1997) Characterization of iron-manganese concretions in Kentucky Alfisols with perched water tables. Clays and Clay Minerals 45, 428–439. doi:10.1346/CCMN.1997.0450312 www.publish.csiro.au/journals/sr