Measurements and modeling of carbon and nitrogen cycling in agroecosystemsof southern Wisconsin: Potential for SOC sequestration during the next 50 years

Land management practices such as no-tillage agriculture and tallgrass prai rie restoration have been proposed as a possible means to sequester atmosph eric carbon, helping to refurbish soil fertility and replenish organic matt er lost as a result of previous agricultural management practices. However, the relationship between land-use changes and ecosystem structure and func tioning is not yet understood. We studied soil and vegetation properties ov er a 4-year period (1995-98), and assembled measurements of microbial bioma ss, soil organic carbon (SOC) and nitrogen (N), N-mineralization, soil surf ace carbon dioxide (CO2) flux, and leached C and N in managed (maize; Zea m ays L.) and natural (prairie) ecosystems near the University of Wisconsin A gricultural Research Station at Arlington. Field data show that different m anagement practices (tillage and fertilization) and ecosystem type (prairie vs maize) have a profound influence on biogeochemistry and water budgets b etween sites. These measurements were used in conjunction with a dynamic te rrestrial ecosystem model, called IBIS (the Integrated Biosphere Simulator) , to examine the long-term effects of land-use changes on biogeochemical cy cling. Field data and modeling suggest that agricultural land management ne ar Arlington between 1860 and 1950 caused SOC to be depleted by as much as 63% (native SOC approximately 25.1 kg C m(-2)). Reductions in N-mineralizat ion and microbial biomass were also observed. Although IBIS simulations dep ict SOC recovery in no-tillage maize since the 1950s and also in the Arling ton prairie since its restoration was initiated in 1976, field data suggest otherwise for the prairie. This restoration appears to have done little to increase SOC over the past 24 years. Measurements show that this prairie c ontained between 28% and 42% less SOC (in the top 1 m) than the no-tillage maize plots and 40%-47% less than simulated potential SOC for the site in 1 999. Because IBIS simulates competition between C3 and C4 grass species, we hypothesized that current restored prairies, which include many forbs not characterized by the model, could be less capable of sequestering C than ag ricultural land planted entirely in monocultural grass in this region. Mode l output and field measurements show a potential 0.4 kg C m(-2) y(-1) diffe rence in prairie net primary production (NPP). This study indicates that hi gh-productivity C4 grasslands (NPP = 0.63 kg C m(-2) y(-1)) and high-yield maize agroecosystems (10 Mg ha(-1)) have the potential to sequester C at a rate of 74.5 g C m(-2) y(-1) and 86.3 g C m(-2) y(-1), respectively, during the next 50 years across southern Wisconsin.