Soil carbon dynamics in saline and sodic soils: A review

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Soil carbon dynamics in saline and sodic soils: A review Article in Soil Use and Management · December 2009 DOI: 10.1111/j.1475-2743.2009.00251.x

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SoilUse and Management doi: 10.1111/j.1475-2743.2009.00251.x

Soil Use and Management, March 2010, 26, 2–11

REVIEW ARTICLE

Soil carbon dynamics in saline and sodic soils: a review V. N. L. Wong1,2,3, R. S. B. Greene1,2, R. C. Dalal3,4 & B. W. Murphy5 1

Fenner School of Environment and Society, The Australian National University, Canberra ACT 0200, Australia, Co-operative Research Centre for Landscape Environments and Mineral Exploration, Canberra ACT 0200, Australia, 3 Co-operative Research Centre for Greenhouse Accounting, Canberra ACT 0200, Australia, 4Queensland Department of Natural Resources and Water, 80 Meiers Rd, Indooroopilly, Qld 4068, Australia, and 5New South Wales Department of Environment and Climate Change, PO Box 445, Cowra NSW 2794, Australia 2

Abstract Soil salinity (high levels of water-soluble salt) and sodicity (high levels of exchangeable sodium), called collectively salt-affected soils, affect approximately 932 million ha of land globally. Saline and sodic landscapes are subjected to modified hydrologic processes which can impact upon soil chemistry, carbon and nutrient cycling, and organic matter decomposition. The soil organic carbon (SOC) pool is the largest terrestrial carbon pool, with the level of SOC an important measure of a soil’s health. Because the SOC pool is dependent on inputs from vegetation, the effects of salinity and sodicity on plant health adversely impacts upon SOC stocks in salt-affected areas, generally leading to less SOC. Saline and sodic soils are subjected to a number of opposing processes which affect the soil microbial biomass and microbial activity, changing CO2 fluxes and the nature and delivery of nutrients to vegetation. Sodic soils compound SOC loss by increasing dispersion of aggregates, which increases SOC mineralisation, and increasing bulk density which restricts access to substrate for mineralisation. Saline conditions can increase the decomposability of soil organic matter but also restrict access to substrates due to flocculation of aggregates as a result of high concentrations of soluble salts. Saline and sodic soils usually contain carbonates, which complicates the carbon (C) dynamics. This paper reviews soil processes that commonly occur in saline and sodic soils, and their effect on C stocks and fluxes to identify the key issues involved in the decomposition of soil organic matter and soil aggregation processes which need to be addressed to fully understand C dynamics in salt-affected soils.

Keywords: Salinity, sodicity, decomposition, dispersion, soil organic matter

Introduction Worldwide, approximately 950 million ha of land are estimated to be salt affected, with salinity affecting 23% of arable land and saline-sodic soils affecting a further 10% (Szabolcs, 1994). In Australia, it is estimated that salinity affects an estimated 17 million ha while sodicity affects approximately 340 million ha of land (NLWRA, 2001). Salinity alters the osmotic and matric potential of the soil solution to adversely affect soil biota and vegetation. Sodicity directly alters soil physical properties, causing a decline in soil structure because of increased swelling, dispersion and slaking upon wetting and increased crusting and hardsetting on drying, with a concomitant decline in permeability, infiltration and hydraulic conductivity. Many Correspondence: V. N. L. Wong. E-mail: [email protected] Received April 2009; accepted after revision October 2009

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sodic areas also exhibit severe erosion, particularly sheet and gully erosion, in addition to extended periods of waterlogging and altered hydrologic processes. A number of these processes impact on soil carbon (C) dynamics, but our understanding about C stocks and fluxes in saline and sodic soils is limited. There have been a number of previous studies focusing separately on soil salinity, sodicity or soil C dynamics, particularly with regards to soil structure and plant health (e.g. Chartres, 1993; Allen et al., 1994; Akilan et al., 1997; Guo & Gifford, 2002; Rengasamy, 2002; Gardner, 2004). However, none have drawn these three topics together. Soil C stocks and fluxes are directly related to aboveground biomass production. The issue of C dynamics as influenced by salt-related degradation will increase in significance in the future, as the extent of salinisation and sodification is projected to increase by up to 40% in some dryland areas (NLWRA, 2001) which will directly affect plant health and

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Soil carbon dynamics in saline and sodic soils 3

Osmotic effects

Ion excess

EC

therefore, soil C stocks and fluxes. In these areas, C stocks and fluxes are directly related to decreased plant inputs due to low biomass production and hence, low soil organic matter (SOM) accumulation which will, therefore, affect soil C and nutrient cycling. However, the issue of C turnover as affected by salinity and sodicity is complicated by processes associated with salt-affected soils such as waterlogging and the presence of inorganic C, usually in the form of CaCO3 and NaHCO3. This review presents an overview of research into salinity, sodicity, soil organic C (SOC), and the relationship between the three issues to identify where knowledge gaps exist.

SALINESODIC Sodium induced ion toxicity and deficiency

Exacerbated with increasing pH

Ion deficiency

SODIC Soil structure decline

Saline and sodic soils

SAR

The issue of salinity and its subsequent impacts on plant health has received much attention in the past (e.g. Clemens et al., 1983; Craig et al., 1990; Allen et al., 1994; Akilan et al., 1997) as a result of anthropogenic-related changes in landscape hydrology and subsequent redistribution of salts. These activities are largely related to the widespread removal of deep-rooted perennial native vegetation and its replacement with shallow-rooted annual crops and pastures, causing an increase in the amount of water infiltrating through the soil profile, which mobilises and transports soluble salts (Burch, 1986). Sodic soils are defined by high levels of exchangeable sodium; in Australia a soil is considered sodic when the exchangeable sodium percentage (ESP) ‡6 (Isbell, 1996). The sodium adsorption ratio (SAR) is also frequently used to describe the sodicity level of the irrigation water or soil solution, with a soil considered to be sodic when the SAR1:5 > 3 (Rengasamy & Olsson, 1991). Effects of salinity are largely directly related to changed soil chemical properties and osmotic potential affecting plant growth, but effects related to sodicity are largely indirect due to influences on soil physical properties adversely affecting nutrient and water supply (Figure 1). Soil structural decline with increasing sodicity reduces nutrient mobility, which leads to nutrient deficiencies. Conversely, in saline-sodic soils, an excess of ions occurs as a result of higher salt concentrations (Naidu & Rengasamy, 1993). For example, toxicity can occur where salts contain high concentrations of B(OH)4) and ⁄ or CO32) (Rengasamy, 2002).

Soil carbon stocks Soil is the largest terrestrial C sink, and contains two thirds of the world’s terrestrial C (Schimel et al., 1994) with approximately 1500 Gt of organic C in the top metre (Eswaran et al., 1993). The SOC pool contains twice as much C as the atmospheric pool, three times as much as the terrestrial biotic pool (Lal et al., 1995) and is therefore an important C store, with the potential to be a large C source under different environmental conditions. The rate of net

Figure 1 Nutrient constraints on plant growth in sodic and salinesodic soils (Naidu & Rengasamy, 1993).

organic C accumulation or loss is a function of inputs and outputs. Inputs from decomposition of photosynthetic products are dependent on productivity and the quality of the substrate being decomposed, while outputs are due to heterotrophic respiration by the microbial biomass, leaching, erosion and burning. The amount of C in the soil at any particular time is dominated by inputs from vegetation and as a result, global C gradients largely follow that of plant biomass production, with SOC stocks increasing with increasing precipitation (Burke et al., 1989) and decreasing temperature (Post et al., 1982) due to increasing biomass production and decreasing decomposition rates. Salt-affected soils usually exhibit low organic matter contents primarily due to poor plant growth leading to low inputs of organic materials into the soil. These soils are also subjected to increased losses due to dispersion, erosion and leaching. As a result, SOC contents are frequently smaller in salt-affected soils than in the adjacent non-degraded soils (Table 1). Moreover, in Australia, this can be compounded by the generally lower C concentrations in soils compared to other soils globally (Hubble et al., 1983).

Soil carbon pools Soil organic carbon can be partitioned into discrete pools according to its age or the amount of time it takes to turn over (Jenkinson & Raynor, 1977; Parton et al., 1987). Mean residence times of these pools are dependent on resistance to decay and the extent of protection against decomposition. The three main SOC pools are: (i) the active pool, with a turnover time in the order of weeks; (ii) the slow pool with a turnover time in the order of decades; and (iii) the passive pool with a turnover time in the order of millennia. The active pool is made up of readily oxidisable materials including, the microbial biomass and its metabolites, and is

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largely controlled by climate and residue inputs, which provide a nutrient source for plants (Schnurer et al., 1985). The slow and ⁄ or very slow pools contain moderately decomposable material within macro- and microaggregates and particulate organic carbon (POC; Parton et al., 1987). The passive or recalcitrant pool includes stable C formed from the turnover of microbial and slow SOC that are chemically resistant to, or protected from further microbial degradation (Schimel et al., 1994). A large proportion of this pool is charcoal, which is found in all Australian soils (Skjemstad et al., 1996, 2002; Clough & Skjemstad, 2000; Lehmann et al., 2008) (Figure 2). The active soil C pool is frequently used as an early indicator to SOM dynamics due to its faster turnover (e.g. Alvarez et al., 1998), so that changes caused by management or environmental stresses can be detected earlier in this pool than in the SOM pool as a whole. This can be particularly important in cases where environmental conditions change over a relatively short time. Consequently the effects on microbial activity due to declining vegetation health caused by increasing salinity and sodicity should precede changes in the total C stock.

The role of inorganic carbon Sodic soils often coincide with alkaline conditions and dense subsoils. Large amounts of insoluble inorganic C (SIC) exist

Table 1 SOC concentrations in some salt-affected soils ECe (dS ⁄ m)

ESP

SOC (%)

Depth (m)

0.5 0.5 1.5 2.5 6.4 nd

2.0 3.8 17.7 65.1 88.8 15.7

0.78 0.40 0.42 0.26 0.32 0.63

0–0.15 0–0.15 0–0.15 0–0.15 0–0.15 0–0.25

13 25 11 10 6 3 20.3 7.9 5.2 3.6 61.2 3.3 1.2

0.7 1.9 2.3 0.8 1.4 1.5 0.3 2.3 1.5 2.7 0.2 2.0 2.4

0–0.10 0–0.10 0–0.10 0–0.10 0–0.10 0–0.10 0–0.05 0–0.05 0–0.05 0–0.05 0–0.05 0–0.05 0–0.05

6.5 4.0 1.0 0.3 0.1 0.1 0.2 0.2 0.2 0.2 0.1 0.1 0.1

(EC1:5) (EC1:5) (EC1:5) (EC1:5) (EC1:5) (EC1:5) (EC1:5) (EC1:5) (EC1:5) (EC1:5) (EC1:5) (EC1:5) (EC1:5)

Reference Pathak & Rao (1998)

Tejada et al. (2006) Pankhurst et al. (2001)

Wong et al. (2008b)

SOC, soil organic carbon; ESP, exchangeable sodium percentage. nd indicates that no data is available.

Plant residue input

Decomposable plant material

Resistant plant material

Litter pools

CO2

Active soil Carbon

Soil carbon pools CO2

Slow soil Carbon Passive soil Carbon

Figure 2 Conceptual model of soil C pools and turnover (Jenkinson & Raynor, 1977).

in the subsoil of these soils; with the potential to play an important role in C cycling. Globally, the SIC pool has been estimated to contain approximately 940 Pg of C to 1 m depth (Eswaran et al., 2000). Although the SOC pool dominates in soils of humid regions, SIC is the most common form of C in arid and semiarid regions, where precipitation is usually 250 lm) transiently stable aggregates Polysaccharides decrease in importance with increasing organic matter contents Decomposed rapidly by microorganisms Associated with the growth of root systems and fungal hyphae Most likely associated with young macroaggregates Dominate in microaggregates Particles of clay sorbed on to organic matter core, rather than organic matter sorbed on to clay surfaces Most likely includes complexes of clay-polyvalent metal-organic matter Degraded aromatic humic material associated with amorphous iron, aluminium and aluminosilicates to form the organomineral fraction of soil

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increased solubility, decomposability and accessibility of SOM. Similarly, Wong et al. (2009) found that following addition of organic material to a highly saline-sodic soil, SMB and respiration initially increased. It is also possible that additional substrate can become available for decomposition when SOC is released from clays with increasing salinity. Under such conditions, SOM sorbed on clays is released due to a solvation effect of the salt as increasing ionic strength can alter the composition of exchange sites (Wiklander, 1975; Kaiser et al., 1996). Mineralisation of ground plant C has been found to increase with sodicity and decrease with salinity, corresponding with the initial rapid phase of decomposition. Nelson et al. (1996) attributed this effect to the increased solubility of organic matter in the presence of Na. Because Na is more soluble than Ca, mineralisation may be stimulated, causing increased C loss from dissolved organic matter (Nelson & Oades, 1998) and transport off-site in leachate and runoff. However, following the initial flush of available substrate, the SMB is then placed under increasing stress as substrate availability and decomposability decline. Over time, the microbial population can become adapted to a high salt environment (Polonenko et al., 1981; Zahran, 1997) and continue to mineralise the remaining native SOM. In general, the effects of salinity on microbial activity have been attributed to similar deleterious effects on plant health, dominated by osmotic effects with increasing salt concentration, and specific ion toxicities causing nutritional imbalance for microbial growth and enzyme synthesis (Batra & Manna, 1997). Therefore, in the longer term, such as the time periods over which salinisation and sodication occur, decadally cycling (slow) pools continue to lose C at rates that are significant in terms of ecosystem C storage. Soil respiration is frequently used as a measure of microbial activity, and to determine whether a microbial population is under stress. Soils showed low microbial activity in an arid saline soil in southeast Spain, with the lowest values found at the most degraded site where soil respiration was inhibited at high EC levels (Garcia et al., 1994). The combined effects of salinity and low pH lead to the conclusion that salinisation has a depressive effect on the microbial biomass. This is most likely due to a shift in community structure, from one dominated by fungi to one dominated by prokaryotic microorganisms consisting mainly of bacteria which may be less active, competitive and diverse (Pankhurst et al., 2001; Sadinha et al., 2003). Chander et al. (1994) found the rate of mineralisation of organic matter increased while the SMB decreased with increasing sodicity. It was suggested that the smaller microbial population was most likely the result of decreased plant C input due to stresses placed on plants with increasing sodicity, while direct toxic effects and environmental stress played a smaller role. In contrast, Nelson et al. (1997) found a slightly negative effect of sodicity on mineralisation, which may have been due

to differences in the amount and quality of substrate added. In naturally saline soils, the SMB is generally negatively correlated with the concentration of soluble salts and positively correlated with SOC contents. Pathak & Rao (1998) found C mineralisation decreased with increasing salinity due to decreasing microbial activity. Increasing levels of salinity also decreases soil enzyme activities (Batra & Manna, 1997), with inhibition of enzymatic and microbial activity greater with NaCl, than with CaCl2 or Na2SO4 (Frankenberger & Bingham, 1982). Laura (1973) has shown losses in total C increase with increasing concentrations of Na2CO3 during decomposition of organic material. With increasing Na2CO3 concentrations, exchangeable sodium increased, resulting in higher ESP, while pH increased as CaCO3 and MgCO3 precipitated, resulting in declines in SOC. However, biochemical mineralisation by soil enzymes can still occur in highly saline and alkaline conditions (Pathak & Rao, 1998). It is suggested that with increasing sodicity, C substrates that are amenable to dissolution are likely to increase in solubility, while those that are less readily soluble and decomposable become even less soluble. Increasing erosion, common in saline and sodic landscapes as a result of scalding of the soil surface, has the potential to cause substantial SOC losses. Erosional processes can deplete the SOC content of the surface layer due to its lower density and higher erodibility. Because the labile particulate fraction is relatively unconsolidated, of relatively low density and concentrated close to the soil surface, it is most prone to removal (Lal, 2001). Furthermore, as soil aggregates break down during the process of erosion, increasing exposure to microbial processes will increase mineralisation. Eroded materials, which usually consist of humus and clay fractions, can contain 3.5 times more C than the original soil (Lal, 2001). In scalded soils, the A horizon has frequently been eroded, with the less fertile B horizon remaining as the soil surface. Because SOC generally decreases with depth (Murphy et al., 1998), erosion and increased mineralisation of the SOM in the B horizon results in a substantial loss of soil C, causing lower SOC levels in eroded compared to uneroded soils. Similarly, Wong et al. (2008b) found that 0–0.3 m depth of an eroded profile contained half the SOC stock (7.7 t ⁄ ha) compared to the same depth of an uneroded unscalded profile (19.8 t ⁄ ha). This indicates that a substantial amount of SOC can be lost in scalded soils as a result of erosion, particularly where the topsoil is lost in the process. The downslope deposition of eroded material may be protected from decomposition due to its deep burial, or translocation into lakes, reservoirs and other aquatic systems, which may result in sequestration (van Noordwijk et al., 1997; Izaurralde et al., 2001; McCarty & Ritchie, 2002). As waterlogging of lower-lying areas is common in salt-affected soils, material high in SOC eroded from upslope areas may be deposited in marshy areas and protected from

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Soil carbon dynamics in saline and sodic soils 7

decomposition. However, in general, most displaced SOC is mineralised, and this effect is compounded by the decreased biomass capacity of eroded soils (Jacinthe & Lal, 2001). Although saline and sodic soils are subjected to prolonged waterlogging, a condition which normally favours restriction in the rate of decomposition of SOM, in these soils, the adverse effects on plant growth generally result in an overall loss in SOM.

Carbon dynamics during rehabilitation of saline and sodic areas with ameliorants Previous studies have shown mixed results on soil physical and chemical properties following organic material additions to sodic soils. Gypsum (CaSO4 2H2O) and other Ca compounds are the most commonly used ameliorants in sodic soils (Gupta & Abrol, 1990; Oster et al., 1996). Addition of Ca as gypsum or lime is usually critical for plant growth in saline and sodic sites, which are frequently Ca-limited, resulting in Ca deficiencies in plants (Reid & Smith, 2000). The accumulation of organic matter in sodic soils is difficult as Na-organic linkages are highly soluble, with organic matter dissolving in runoff and percolating water in the form of soluble Na-humates, enhancing clay dispersion, mobilisation and losses from leaching (Sumner et al., 1998). Highly alkaline soils containing Na2CO3 are unlikely to retain products of decomposition because organomineral interactions depend primarily on cation bridges involving mainly Ca2+ rather than Na+ (Naidu & Rengasamy, 1993). In addition, those soils with a high base status typically have higher clay contents, and are generally more fertile with greater vegetation inputs (Baldock & Nelson, 2000), subsequently producing more SOM. In sodic soils however, Na+ must first be replaced by polyvalent cations, such as Ca2+ which enable the formation of stable linkages between Ca2+ saturated particles and organic matter (Rengasamy & Olsson, 1991). Additions of organic matter to calcareous and noncalcareous soils have been shown to cause increases in clay dispersion at constant pH and high SAR values (Gupta et al., 1984). This was attributed to the effects of increasing soil pH following addition of manure, which increased the cation exchange capacity (CEC) and changed the surface properties of the clays, thus promoting dispersion. Similarly Sumner (1993), found that the addition of humic materials to a soil with an ESP of between 10 and 30 also increased clay dispersion due to the greater preference of the organic matter than the clay for Ca2+ rather than Na+. This subsequently caused enrichment of Na+ in the inorganic clay fraction and the contribution of low molecular ligands from the added organic matter also promoted dispersion. Conversely, Barzegar et al. (1997) found spontaneously dispersible clay decreased following the addition of pea straw, irrespective of SAR, indicating that the dominant binding

mechanisms were not ionic. They suggested that the addition of organic materials to sodic soils could be expected to improve structural stability without initial remediation of sodicity. This occurred irrespective of clay type or sodicity, with the effect greatest at high organic matter contents and low ESP. Similarly, Chorom & Rengasamy (1997) found the application of green manure reduced soil pH in an alkaline sodic soil as a result of the decomposition and microbial respiration of the manure. Decomposition of the added manure caused an increase in the partial pressure of CO2 which increased the solubility of CaCO3. Where green manure was added in conjunction with gypsum, decomposition was enhanced, accelerating changes in soil solution composition. The addition of organic matter in conjunction with gypsum has also been successful in reducing adverse soil properties associated with sodic soils and in decreasing the ESP. Vance et al. (1998) found that addition of organic matter and gypsum to a surface soil decreased spontaneous dispersion and EC more than gypsum alone.

Carbon dynamics during phytoremediation of saline and sodic areas There is a large potential for building up SOC stocks in saltscalded areas because of the depleted SOC stocks. Wong et al. (2008b) showed that the low SOC stocks found in the scalded and eroded profiles, which were about a third of those in vegetated profiles, were most likely caused by the very small plant biomass C input in the scalded areas. Similarly, Pankhurst et al. (2001) found lower SOC levels in saline soils compared to non-saline soils (Table 1) and attributed this to reduced inputs of organic matter due to sparser plant cover and the reduced presence of salt sensitive pasture. The successful revegetation of these scalded landscapes can result in rapid SOC accumulation. For example, revegetation with introduced pasture can result in an increase in SOC stocks to levels similar to those found under native pasture after only 10 years (Wong et al., 2008b). Soil inorganic carbon in the form of carbonates is frequently present in sodic and saline-sodic soils. Revegetation of these areas with trees or crops, which can enhance the dissolution of SIC, has also facilitated soil reclamation where the vegetation could tolerate adverse soil conditions (Qadir et al., 2007), with the potential to accumulate SOC. The use of leguminous trees in India has been shown to reduce the ESP at depth as well as in the surface layers (Garg, 1999), to decrease pH and increase the SMB (Bhojvaid & Timmer, 1998; Mishra & Sharma, 2003). The restoration process by trees is primarily driven by two parallel mechanisms, the fertility building processes associated with organic matter addition, N accretion and nutrient cycling, and alleviation processes driven by improved leaching which reduces soil dispersion and Na toxicity (Bhojvaid &

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Root respiration increase CO2 in soil atmosphere

CO2 + H2O

H2CO3

H+ + CaCO3(soil)

H+ + HCO3–

Ca2+ + HCO3–

Increase Ca2+ in solution: Na+—Ca2+ exchange

unique to Australia and are characteristic of saline, sodic, and saline-sodic soils globally. Currently, in terms of C accounting, data on how these salt-affected areas relate to C stocks are almost non-existent while data related to C dynamics is contradictory. As areas affected by salinity and sodicity continue to increase, and focus on C sequestration processes also increase, a number of key issues will need to be addressed: 1. Estimation of C stocks in salt-affected areas to ascertain the amount of both SOC and SIC that has been lost or gained as these areas become degraded. 2. The role of SIC dynamics and the consideration of salinity and sodicity drivers when assessing soil C turnover; and 3. The assessment of how rehabilitation processes affect C cycling and C stocks, and how to maximise the accumulation of C stocks in areas where SOC stocks are very small. These issues will need to be addressed, if C cycling in these degraded areas is to be better understood.

Na+ leached

Acknowledgements Figure 3 Processes involved in the removal of Na by vegetation in sodic soils (source: Qadir et al., 2003).

Timmer, 1998). Carbon dioxide from plant root respiration dissolves in the soil solution (Figure 3), while increasing the partial pressure of CO2 due to the microbial decomposition processes also causes a decrease in pH through the production of H2CO3. Concurrently, H+ is also released from plant roots. These processes facilitate the dissolution of carbonate minerals, including CaCO3, which increases the concentration of Ca2+ and displaces exchangeable Na+ (Mishra & Sharma, 2003; Qadir et al., 2005). In northern Egypt, Ghaly (2002) found that, after the second year, both ponding and gypsum were less effective in reducing salt content than the native grass species. This was attributed to increased salt uptake, as evidenced by increased Na in the grass shoots, and greater C inputs resulting in reclamation within two years.

Conclusion The extent of saline and sodic soils is predicted to increase in Australia and globally which will result in significant SOC losses. The conflicting effects of adding organic matter to these soils are probably the result of variation in the balance between the opposing saline and sodic processes and the effects of organic matter. Soils in salt-affected landscapes produce less biomass than non saline soils resulting in less SOC and in turn more erosion which further accentuates SOC losses due to the dominance of plant inputs in the accumulation of organic matter. These processes are not

The authors would like to acknowledge the Co-operative Research Centre for Greenhouse Accounting and the Cooperative Research Centre for Landscape Environments and Mineral Exploration for funding, the valuable comments of Drs Warren Hicks, Keith Scott and two anonymous reviewers in reviewing this manuscript.

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