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 μm. The density-based fractionation scheme identified heavy-fraction C pools (SOM > 1.6 g cm−3 plus SOM < 53 μm), particularly those in macroaggregates (>250 μm), 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.
Keywordsaggregates agriculture C-sequestration forest C organic tillage succession
Support for this work was provided by NSF (LTER, DDIG, and REU programs), USDA-CSREES (Sustainable Agriculture and CASMGS programs) and the Michigan Agricultural Experiment Station. We thank L. Faber, Brian Rensch, C. Szekely, and S. Warners for assistance in the field and lab and A.T. Corbin for assistance with the C/N analysis. S. K. Hamilton, M. J. Klug, J. C. Neff, and A. J. M. Smucker provided insightful comments on an earlier draft. We are particularly grateful to S. Bohm for his many suggestions regarding the long-term mineralization assays and modeling and to two anonymous reviewers that provided exceptionally thorough and helpful reviews of this manuscript.
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