Land-use intensity effects on soil organic carbon accumulation rates and mechanisms

Restoring soil C pools by reducing land use intensity is a potentially high impact, rapidly deployable strategy for partially offsetting atmospheric CO2 increases. However, rates of C accumulation and underlying mechanisms have rarely been determined for a range of managed and successional ecosystems on the same soil type. We determined soil organic matter (SOM) fractions with the highest potential for sequestering C in ten ecosystems on the same soil series using both density- and incubation-based fractionation methods. Ecosystems included four annual row-crop systems (conventional, low input, organic and no-till), two perennial cropping systems (alfalfa and poplar), and four native ecosystems (early successional, midsuccessional historically tilled, midsuccessional never-tilled, and late successional forest). Enhanced C storage to 5 cm relative to conventional agriculture ranged from 8.9 g C m(-2) y(-1) in low input row crops to 31.6 g C m(-2) y(-1) in the early successional ecosystem. Carbon sequestration across all ecosystems occurred in aggregate-associated pools larger than 53 mu m. The density-based fractionation scheme identified heavy-fraction C pools (SOM > 1.6 g cm(-3) plus SOM < 53 mu m), particularly those in macroaggregates (> 250 mu m), as having the highest potential C accumulation rates, ranging from 8.79 g C m(-2) y(-1) in low input row crops to 29.22 g C m(-2) y(-1) in the alfalfa ecosystem. Intra-aggregate light fraction pools accumulated C at slower rates, but generally faster than in inter-aggregate LF pools. Incubation-based methods that fractionated soil into active, slow and passive pools showed that C accumulated primarily in slow and resistant pools. However, crushing aggregates in a manner that simulates tillage resulted in a substantial transfer of C from slow pools with field mean residence times of decades to active pools with mean residence times of only weeks. Our results demonstrate that soil C accumulates almost entirely in soil aggregates, mostly in macroaggregates, following reductions in land use intensity. The potentially rapid destruction of macroaggregates following tillage, however, raises concerns about the long-term persistence of these C pools.