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Abstract
Soil organic matter (SOM) is a carbon (C) and nitrogen (N) reservoir comprising a continuum of overlapping pools of different sizes, composition, turnover times and levels of significance to C and N cycling in terrestrial systems. The mineralization of SOM provides C, N and energy to microorganisms and N to plants. Soil C and N dynamics are linked to each other and N is usually the limiting substrate for C microbial turnover. Soil C and N demand increase with higher crop yields and risks of depletion of soil reserves arise when removals are larger than inputs. Assessing the C and N dynamics in sensitive SOM pools is a key to measure changes in soil quality under intensive agriculture. Short-term (18-month) C and N dynamics was studied using stable isotopes ( 15N and 13C) in four SOM pools under maize-soybean cropping systems.
Temporal changes in total soil C (TSC) and N (TSN) were related to the amount of C and N inputs and removals each cycle. The LF and MHA were the most labile fractions with greater sensitivity to C and N of residue inputs. The LF displayed the largest seasonal fluctuation in C and N gains and removals. The change in 13C and 15N enrichment revealed the sequence of C and N turnover as LF > MHA > CaHA > Humin. Biological activity dominates the C and N turnover in the LF and MHA due to their simpler chemical composition and reduced protection by minerals while the turnover of CaHA and Humin suggest that abiotic processes dominate turnover. The contribution of individual fractions to TSN follows Humin-N > CaHA-N > MHA-N > LF-N. Manipulation of C and N cycling in agricultural ecosystems depends on the rate and timing of C and N additions to soil, the removals at harvest and the changes in the physical environment. TSC gains were measured only in systems attaining high crop productivity.
