Net Ecosystem Carbon Budget of Dairy Feed Crop Systems
Dairy production emits significant amount of greenhouse gases (GHG) to the atmosphere. Cultivation of hay and corn for dairy feed have the potential to partially mitigate GHG emission from dairy production by sequestering plant assimilated carbon in soil as stabilized soil organic matter. The research presented here investigated the net ecosystem carbon budget (NECB) and greenhouse gas balance (GHGB) of hay and corn over three years, and compared methods for measuring the gas exchange needed for estimating budget components. Maintaining the sampling height (z) for net ecosystem exchange (NEE) measurement above the roughness sublayer (RSL) is challenging when measuring over fast-growing agricultural crops in a multi-plot study, which is often fetch-limited. Net ecosystem exchange measured over hay and corn using the flux-gradient technique (FG) at various z were compared to measurements made using the eddy covariance (EC) technique. Net ecosystem exchange measured using FG were comparable to EC but disagreed when z was less than 1.4 times the mean canopy height. Soil GHG flux measured using chambers complements measurement made using micrometeorological methods but chambers lack in spatial and temporal resolution. Comparison of nitrous oxide (N2O) and methane (CH4) fluxes measured using chambers and micrometeorological methods over corn revealed an overestimation of cumulative trace gas flux when linear interpolation was used to estimate daily mean fluxes. Average NECB over three years showed corn was a higher carbon emitter than hay. Components of the NECB were similar on average between hay and corn, except for the amount of carbon removed from harvest. Emissions of N2O were higher in corn than in hay throughout the three years. Higher NECB and N2O emission ultimately resulted in corn having higher GHGB than hay. The lower carbon emitted from hay however came at a cost of lower feed production compared to corn. This research provides new information on dairy cropping systems that can be used to estimate the carbon sequestration potential of annual and perennial crops.