Soil aggregate formation and the accrual of particulate and mineral-associated organic matter

Abstract

The degradation of soil aggregates appears to be a primary mechanism in the loss of organic matter caused by long-term cultivation, but little information exists on how the formation and stabilization of macroaggregates control the process of C aggradation when disturbance is reduced or eliminated. A chronosequence of restored tallgrass prairie was used to investigate the relationships between the formation of stable macroaggregates (> 212 μm dia) and the accrual of particulate and mineral-associated organic matter. Changes in the percentage of macroaggregates and in the accumulation of whole-soil organic C across the chronosequence were both described with a simple exponential model. The rate constant (k) for change in aggregation was more than 35 times the k for total organic C accumulation. Thus, the time required to reach 99% of equilibrium was 10.5 y for macroaggregates and 384 y for whole-soil organic C, providing evidence for the existence of a phased relationship between macroaggregate formation and C accrual. The input rate for whole-soil organic C to a 10-cm depth was estimated at 1.16 g kg⁻¹ y⁻¹ or 0.133 kg m⁻² y⁻¹ (assuming an average bulk density of 1150 kg m⁻³ for previously cultivated soils in the chronosequence). An increase in macroaggregate-associated C-to-N ratios with time since cultivation suggested that the accumulating organic matter was not “highly processed”, but less than 20% of the accrued C occurred in the form of particulate organic matter (density ≤ 1.85 g cm⁻³). Rather, most of the accumulated C occurred in the mineral-associated fraction of macroaggregates, suggesting that inputs of organic debris were rendered relatively rapidly into particles or colloids that are associated with mineral matter and thus are physically protected, slowing decomposition and promoting the development of stable microaggregates within macroaggregates.