Carbon and nitrogen cycling in intensive production systems: Trade-offs between productivity and sustainability

A solid understanding of the impacts of agricultural management practices on soil biology, nitrogen and carbon dynamics and net greenhouse gas (GHG) emissions is essential to evaluating agro-ecosystem sustainability. I established a suite of experiments at the Michigan State University's (MSU) Tree Research Center (TRC) in East Lansing and at two different sites in Northern Michigan to address the following three hypotheses. Hypothesis 1: Managing Fraser-fir plantations with cover crops in a low-input system will increase soil organic carbon, total soil nitrogen, microbial biomass and functional diversity as well as tree performance compared to a high-input conventional system. Hypothesis 2: Conversion of grassland to short-rotation woody crop (SRWC) bioenergy systems will increase nitrate leaching losses and soil emissions of nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4), undermining the environmental benefits of substituting biomass energy for fossil fuels. Hypothesis 3: Adding N fertilizer to a switchgrass bioenergy system will improve the net GHG balance because enhanced CO2 fixation will exceed direct and indirect emissions of GHGs associated with fertilizer use. To test the first hypothesis, a tree-cover crop intercropping trial involving Fraser fir, two leguminous (Dutch white clover and alfalfa) and a non-leguminous (perennial ryegrass) cover crops was conducted at the TRC. For each cover crop, two competition-management practices were evaluated.;The approach used to test the second hypothesis was to set-up experimental plots cleared of existing grassland vegetation, then cultivated and planted with either willow (Salix dasyclados) or poplar ( Populus nigra). I measured soil greenhouse gas (GHG) fluxes and N leaching losses from these plots and compared these against undisturbed, reference grassland plots. I also established a switchgrass fertilization trial to test the third hypothesis. Nitrogen fertilization treatments included 0, 56 and 112 kg N ha-1 applied as urea, once early in the growing season. Direct and indirect GHG fluxes were performed and biomass yield of switchgrass was evaluated at the end of the growing season. Results indicate that cover crop treatments increased soil microbial N and available N by 35 to 80% and 1.5- and 2.2-fold, respectively, relative to the conventional treatment. Tree seedling survival and growth in cover crop treatments with strips and in the CONV plots were similar. These results demonstrate the potential for cover crops to quickly increase plantation soil nitrogen fertility and tree growth response. Converting grassland to SRWC resulted in 13.3 - 17.2-fold increases of N2O, an additional loss of 3.3 - 9.0 Mg CO2 ha -1 of CO2 emissions and an extra N leaching loss of 36 - 51 kg N ha-1, relative to the reference pasture control plots. Grassland conversion to SRWC systems incurred GHG debts of 9.4 and 14.2 Mg CO2eq ha-1 for poplar and willow plots, respectively. Overall, N fertilizer application to a switchgrass contributed little to direct GHG emissions from soil but substantially increased aboveground biomass production which sequestered an additional 2.6 - 9.4 Mg ha-1 of atmospheric CO2 relative to an unfertilized field. N fertilization of switchgrass in this region could reduce the land base needed for bioenergy production and reduce pressure on land required for food and forage crop production.