Effect of Soil Air and Water on Greenhouse Gases Emissions in a Corn-Soybean Rotation

Knowledge of the impact of soil and crop management practices on soil processes is important in the study of greenhouse gases emissions from agricultural fields. We assessed the effect of soil air (pore space indices) and water (content, θ; and potential, Ψ) on greenhouse gases emissions in corn/soybean field. The study was conducted in 2011 and 2012 on a silt loam soil at Freeman farm of Lincoln University. Soil samples were collected at four depths: 0-10, 10-20, 20-40 and 40-60cm and they were oven dried at 105oC for 72h for the calculation of air filled porosity (AFP), total pore space (TPS) and other soil physical properties. Pore space indices were computed using diffusivity models based on AFP and TPS. Soil samples were later saturated then brought into a pressure plate for measurements of moisture content (θ) at five different water potentials (Ψ). Soil air samples for the measurements of greenhouse gases emissions were collected using static and vented chambers of 30cm height and 20cm diameter. The concentrations of CO2, CH4 and N2O in soil air samples were determined using a Gas Chromatograph GC-14. Results showed that pore space indices significantly correlated with greenhouse gases fluxes (p<0.05) with correlation coefficient (r) ranged from 0.27 to 0.53. More correlations were found in 2012 than 2011. Similarly, significant correlations were found between greenhouse gases and θ at Ψ = 0 and Ψ = -0.05. Moisture content (θ) held at Ψ = 0 positively correlated with CO2 (r = 0.49), N2O (r = 0.64) and negatively correlated with CH4 (r = - 0.43) at p<0.05. Soil pore space indices and soil water (content and potential) seem to control greenhouse gases emissions in this soil. Inclusion of these controlling factors in models will certainly improve our understanding of the dynamics of greenhouse gases fluxes from soil.