Ecosystem Carbon Budgets under Contrasting Land Use Histories Using Eddy Covariance and Deep Core Methods

Increasing demand for agricultural production could be met through conversion of uncultivated lands and/or intensification of production on existing agricultural lands, each with distinct implications for ecosystem carbon budgets. We investigated the carbon budget of two intensively managed continuous no-till corn fields with contrasting land use histories over six years using eddy covariance (EC) and deep core methods. One treatment had been managed as Conservation Reserve Program(CRP) grassland for 22 years before conversion, thus representing conversion of uncultivated lands into agriculture, and the other treatment had been managed as conventional agriculture (AGR) in acorn-soybean rotation for several decades before conversion, thus representing agricultural intensification. We hypothesized that the AGRl and would sequester more carbon or at least remain neutral owing to the reduced soil disturbance and increased overall productivity of the land following conversion. Accordingly, we expected soil carbon to either increase or remain unchanged compared to its pre-conversion status. We also hypothesized that the CRP grassland converted to no-till would lose carbon to the atmosphere until a new equilibrium is reached owing to the decomposition of the large below- and above-ground plant biomass and soil organic matter that had accumulated during the 22 years of CRP management. Consequently, we anticipated the soil carbon to decrease compared to its pre-conversion status. The EC-derived carbon budget supported our hypothesis in that the former AGR land was on average carbon neutral (-8 g C m^-2 yr^-1) while the former CRP land emitted 301 gC m^-2 yr^-1 to the atmosphere over the six year period. In contra diction to the hypothesis, the deep core method indicated that the former AGRl and emitted 167 g C m^-2 yr-1 to the atmosphere while the former CRPl and sequestered 127 g C m^-2 yr^-1 over the same time period. This could be due to pre-conversion plant biomass and soil organic matter, and current soil erosion/deposition and leaching of dissolved soil organic carbon that could potentially modify the soil carbon stock. In general, the discrepancy in the carbon budgets likely arose due to differences in detecting the transformed or translocated carbon as uptake or release by the two methods.