SOIL AGGREGATE FORMATION AND THE ACCRUAL OF PARTICULATE AND MINERAL-ASSOCIATED ORGANIC-MATTER

The degradation of soil aggregates appears to be a primary mechanism i n the loss of organic matter caused by long-term cultivation, but litt le information exists on how the formation and stabilization of macroa ggregates control the process of C aggradation when disturbance is red uced or eliminated. A chronosequence of restored tallgrass prairie was used to investigate the relationships between the formation of stable macroaggregates (> 212 mu m dial and the accrual of particulate and m ineral-associated organic matter. Changes in the percentage of macroag gregates and in the accumulation of whole-soil organic C across the ch ronosequence 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 relat ionship between macroaggregate formation and C accrual. The input rate for whole-soil organic C to a IO-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 sinc e cultivation suggested that the accumulating organic matter was not ' 'highly processed'', but less than 20% of the accrued C occurred in th e form of particulate organic matter (density less than or equal to 1. 85 g cm(-3)). Rather, most of the accumulated C occurred in the minera l-associated fraction of macroaggregates, suggesting that inputs of or ganic debris were rendered relatively rapidly into particles or colloi ds that are associated with mineral matter and thus are physically pro tected, slowing decomposition and promoting the development of stable microaggregates within macroaggregates. Copyright (C) 1996 Elsevier Sc ience Ltd