Wednesday, November 15, 2006 - 11:00 AM
296-5
CO2, N and Diversity Increase Total Belowground Carbon Allocation in a Minnesota Grassland.
The allocation of fixed carbon (C) to belowground plant structures can be a large fraction of gross primary production and provides the primary source of detrital C to mineral soil. However, because it is difficult to measure, its controls are poorly understood. A C mass balance approach has been successfully used in forested ecosystems to estimate total belowground C allocation (TBCA) and gain insight into the factors that influence this important C flux, but it has not been widely applied to other ecosystem types. We used this C mass balance approach to estimate annual TBCA in a grassland Free Air CO2 Enrichment (FACE) experiment that investigates the influence of three major anthropogenic changes: nitrogen (N) deposition, atmospheric CO2 concentration, and plant diversity. Over six years, CO2, N, and diversity consistently increased plant allocation of C belowground (P < 0.05). On average, diversity increased TBCA 20% from 700 g C m-2 yr-1 in monocultures to 850 g C m-2 yr-1 in 9 and 16 species plots. Over all years, N additions (4 g N m-2 yr-1) increased TBCA by 8% versus ambient treatments, while elevated CO2 (560 ppm) increased TBCA by 25%. The combination of N additions and elevated CO2 increased belowground C allocation by 35% (950 g C m-2 yr-1 versus 700 g C m-2 yr-1 in ambient plots; a marginally significant CO2 x N interaction, P = 0.078). These increases in TBCA were concurrent with increases in annual soil carbon efflux, which often exceeded TBCA. This, combined with no significant change in soil C in this experiment, suggests that CO2-induced increases in plant allocation of C belowground may not necessarily lead to increased soil C sequestration.
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