The Relationship between Carbon Input, Aggregation, and Soil Organic Carbon Stabilization in Sustainable Cropping Systems

the underlying processes, capacity, and longevity of C pools in agricultural lands. One of our current challenges is to quantify the mechanisms, capacStudies have shown that increases in SOC levels are ity, and longevity of C stabilization in agricultural lands. The objectives directly linked to the return of fresh organic material of this study were to evaluate the long-term (10 yr) role of C input in soil organic carbon (SOC) sequestration and to identify underlying to soil (Rasmussen et al., 1980; Cole et al., 1993). Agromechanisms of C stabilization in soils. Carbon input and SOC sequesnomic practices that influence yield and, therefore, aftration, as governed by crop management strategies, were assessed across fect the proportion of crop residues returned to the 10 Mediterranean cropping systems. Empirically derived relationships soil, are likely to influence C levels in agricultural soils. between yield and aboveground plus belowground crop biomass as Therefore, the inclusion of legumes and cover crops well as estimates of C contributions from crop residues and manure (Kuo et al., 1997), the addition of manure and fertilizer amendments were used to quantify cumulative C inputs into each crop(Hartwig and Ammon, 2002), and the reduction in falping system. Soil samples were separated into four aggregate size classes low frequency (Rasmussen et al., 1980) linearly increase ( 2000, 250–2000, 53–250, and 53 m) and into three soil organic SOC levels. Nevertheless, Campbell et al. (1991) found matter (SOM) fractions within the large ( 2000 m) and small (250– no effect of varying C inputs on SOC levels for a high 2000 m) macroaggregates. Aggregate stability increased linearly with both C input (r 2 0.75, p 0.001) and SOC (r 2 0.63, p 0.006). organic matter soil at Melfort, Saskatchewan (Canada), Across the 10 cropping systems, annual soil C sequestration rates thereby suggesting that these soils may be C saturated ranged from 0.35 to 0.56 Mg C ha 1 yr 1. We found a strong linear (Six et al., 2002). Carbon saturation implies that once relationship (r 2 0.70, p 0.003) between SOC sequestration and the capacity for a soil to stabilize C is reached, additional cumulative C input, with a residue-C conversion to SOC rate of 7.6%. C inputs will not be stabilized as SOC (Paustian et al., This linear relationship suggests that these soils have not reached an 1997b; Six et al., 2002). Therefore, determining the C upper limit of C sequestration (i.e., not C saturated). In addition, C status of a soil relative to C saturation is important to shifted from the 53m fraction in low C input systems to the large gauging the potential for C sequestration. and small macroaggregates in high C input systems. A majority of the Several studies have illuminated that, besides C input, accumulation of SOC due to additional C inputs was preferentially sesoil aggregate dynamics also strongly influence C sequesquestered in the microaggregates-within-small-macroaggregates (mM). tration and cycling (Tisdall and Oades, 1982; Jastrow, Hence, the mM fraction is an ideal indicator for C sequestration potential in sustainable agroecosystems. 1996; Six et al., 1998). Tisdall and Oades (1982) presented a hierarchical model, which suggested that three different classes of organic matter, persistent, transient, and temporary, are associated with three different physical C crop-based agriculture, excluding soil fractions, that is, 250m macroaggregates, 53to pastureland, occupies 1.7 billion hectares globally 250m microaggregates, and 53m silt-and-clay, re(Paustian et al., 1997a). It is estimated that 111 to 170 Pg spectively. Recently, several studies have shown the imC or approximately 10% of the earth’s total soil C (1500 portance of microaggregates (Jastrow, 1996; Six et al., Pg) (Post et al., 1990; Eswaran et al., 1993) is stored within 1998; Gale et al., 2000; Puget et al., 2000) and especially agricultural soils (Schlesinger, 1984; Paustian et al., 1997a). microaggregates-within-macroaggregates (Six et al., 2000; Revived interest in SOC is due partly to its role as an Denef et al., 2004) in the protection and stabilization of important indicator of soil quality (Gregorich and CarC. Denef et al. (2004) showed that microaggregatester, 1997; Lal, 1997), and its potential function as a C within-macroaggregates could explain almost the entire sink (Bruce et al., 1998; Paustian et al., 1997a). Farming difference in SOC between no-tillage and conventional systems that utilize best management practices hold tillage systems. promise for sequestering soil C, which has the potential The objectives of this study were to (i) quantify the to enhance agricultural sustainability, reduce negative relationship between C input and SOC sequestration in environmental impacts, and attenuate anthropogenic whole soil and SOM fractions and (ii) identify mechacarbon dioxide emissions. If the potential benefits of nisms of long-term soil C stabilization in cropping sysSOC sequestration are to be validated, then there exists a substantial need to elucidate and accurately quantify tems that represented a gradient of C input levels. Abbreviations: CMT, conventional–maize–tomato; cPOM, coarse parDep. of Plant Sciences, Univ. of California, Davis, CA 95616. Received ticulate organic matter; CWT, conventional–wheat–tomato; IWC, irri29 June 2004. *Corresponding author ([email protected]). gated–wheat–control; IWF, irrigated–wheat–fallow; IWL, irrigated– wheat–legume; LMT, legume–maize–tomato; mM, microaggregatesPublished in Soil Sci. Soc. Am. J. 69:1078–1085 (2005). Soil Biology & Biochemistry within-macroaggregates; MWD, mean weight diameter; OMT, organic– maize–tomato; RWC, rainfed–wheat–control; RWF, rainfed–wheat– doi:10.2136/sssaj2004.0215 © Soil Science Society of America fallow; RWL, rainfed–wheat–legume; sc-M, silt-and-clay-within-macroaggregates; SOC, soil organic carbon; SOM, soil organic matter. 677 S. Segoe Rd., Madison, WI 53711 USA 1078 Published online June 2, 2005