SoilAggregation and Organic Matter Quality Effects on CarbonMineralization

Grandy, A.S. and G.P. Robertson

Presented at the All Scientist Meeting (2002-10-04 )

Predicting the impact that future land management will have on soil C storage requires an understanding of the mechanisms controlling organic matter stabilization.  We tested whether the interacting effects of vegetation and soil disturbance on aggregation and organic matter quality could explain soil CO2-C loss in eleven cropped and unmanaged ecosystems in Southwest Michigan.  The ecosystems represent a management intensity gradient and contain soil C concentrations ranging from 0.69 (conventionally managed corn-soybean-wheat rotation) to 2.58 kg/m2 (late successional deciduous forest).  Large aggregates, which varied from 20-50% of soil weight, responded to changes in tillage intensity as well as vegetation characteristics.  The difference in respiration rates of intact versus crushed aggregates was used to infer the effects of physical protection on C turnover.  Physical protection of organic matter reduced decomposition rates in all ecosystems and was greatest in an early successional, old-field community.  In several mid successional communities with high C concentrations, physical protection of organic matter was equal to that in annual cropping systems and lower than that in the early successional community, indicating the importance of alternative mechanisms that stabilize SOM.  We estimated organic matter quality in each ecosystem using the respiration rate of crushed aggregates corrected for the amount of carbon in the sample.  Our results show that aggregates confer a modest level of soil C protection that can be altered by tillage and plant community characteristics.

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