CARBON BUDGETS FOR A PRAIRIE AND AGROECOSYSTEMS: EFFECTS OF LAND USE AND INTERANNUAL VARIABILITY

Abstract

Midwestern grasslands have undergone dramatic changes in land use and management practices, but the effects of these changes on terrestrial carbon budgets are poorly understood. This study compared, for five years, the effects of land-use type on components of the carbon (C) budget (above- and belowground net primary production [NPP], C leaching, soil surface CO2 flux, vegetation and soil C contents, and C export from burning and grain removal) of a restored tallgrass prairie and maize agroecosystems on a silt loam soil. Interannual variation of the C budget was addressed by correlating annual fluctuations of environmental variables and soil properties with C-budget components. The C losses we estimated, in order of increasing magnitude, were C leaching, grain C removal from the maize agroecosystems, and soil surface CO2 flux. NPP was significantly greater for N-fertilized maize (10.4 Mg C·ha−1·yr−1) than unfertilized maize agroecosystems (6.2 Mg C·ha−1·yr−1), and both were significantly greater than restored prairie (2.6 Mg C·ha−1·yr−1). Tillage did not significantly affect NPP in the maize agroecosystems, but both NPP and C loss significantly increased following prescribed burning in the prairie. Soil C content in the top 1.4 m decreased significantly during the 5-yr study period for the maize agroecosystems, but not for the restored prairie. An estimate of ecosystem C balance (ECB), calculated as the difference between NPP and heterotrophic respiration with consideration of measurement errors, strongly suggested that restored prairies are not immediate C sinks. Estimates of ECB can potentially provide more insight than direct measurements into annual C changes within a system because of smaller errors. Collectively, these data indicate that land-use type and management strategy have greater and longer term effects on terrestrial ecosystems than the effects of interannual climate variations or long-term climate change.