Greenhouse gas emissions in an agroforestry system of the southeastern USA

Agroforestry systems may provide diverse ecosystem services and economic benefits that conventional agriculture cannot, e.g. potentially mitigating greenhouse gas emissions by enhancing nutrient cycling, since tree roots can capture nutrients not taken up by crops. However, greenhouse gas emission data from agroforestry systems are not available in the southeastern USA, thus limiting our ability to optimize agroforestry management strategies for the region. We hypothesized that tree-crop interactions could prevent excess N from being released to the atmosphere as nitrous oxide (N2O). We determined N2O and carbon dioxide (CO2) emissions, soil temperature, water content, and surface-soil inorganic N in an 8-year-old agroforestry site at the Center for Environmental Farming Systems in Goldsboro, North Carolina, USA. The experimental design was a factorial arrangement of soil texture (loamy sand, sandy loam, and clay loam) and canopy cover (cropped alley, margin between crops and trees, and under Pinus palustris, Pinus taeda, and Quercus pagoda) with three replications. Sampling occurred 42 times within a year using static, vented chambers exposed to the soil for 1-h periods. Soil N2O emission was lower under tree canopies than in cropped alleys, and margin areas were intermediate. Soil texture, water content, and inorganic N were key determinants of the magnitude of N2O emission. Soil CO2 emission was controlled by temperature and water content as expected, but surprisingly not by their interaction. Soil temperature was 1.8 +/- 1.3 degrees C lower and soil water content was 0.043 +/- 0.15 m(3) m(-3) lower under tree canopy than in cropped alleys, which helped to reduce CO2 emission under trees relative to that in cropped alleys. Our results provide a foundation for reducing greenhouse gas emissions in complex agricultural landscapes with varying soil texture by introducing timber production without abandoning agricultural operations.