Divergent effects of elevated CO₂, N fertilization, and plant diversity on soil C and N dynamics in a grassland field experiment

While increased atmospheric CO₂ concentrations, increased N deposition, and changes in plant diversity have all been shown to significantly alter soil carbon (C) and nitrogen (N) dynamics, the effects of these factors have never been studied simultaneously and in combination. We studied the response of soil C and N dynamics to changes in atmospheric CO₂ (ambient, 560 ppm), N fertilization (0, 4 g N m^-2 yr^-1), plant species number (1, 4 species), and plant functional group number (1, 4 groups; all with 4 species) in a grassland field experiment in Minnesota, USA. During the fourth season of treatments, we used laboratory incubations to assess soil C pool sizes and dynamics and net N mineralization, and determined microbial C and N and total soil C and N. Elevated CO₂ increased labile C and microbial biomass, but had no effect on net N mineralization, respiration of more recalcitrant C, or total soil C and N. Nitrogen fertilization increased net N mineralization, because of faster decomposition or less immobilization by litter with higher N concentrations. In the four species plots, N fertilization also increased total soil C and N, likely because greater litter production more than offset any increases in decomposition. Increasing the species number from one to four increased C respiration that could largely be attributed to greater soil C inputs from increased biomass accumulation, but reduced net N mineralization, likely because of greater immobilization in the more productive four-species plots. An increase in functional group number did not affect any of the soil parameters measured. While elevated CO₂, N fertilization, and increased species number all increased plant biomass accumulation, they had divergent effects on soil C and N dynamics.