Denitrification and greenhouse gas emissions from cultivated and wetland alluvial soils

Agricultural development in the Mississippi River Basin has contributed to an 3-fold increase in NO3 loading of the river. Increased NO3 loading is a primary cause of eutrophication in the northern Gulf of Mexico. Identification of best management practices (BMPs) to reduce NO3 loss and wetlands restoration to remove NO3 through denitrification are critically needed. The objectives of this research were to determine factors controlling denitrification potential of different landscape units in an agricultural watershed and quantify the effects of BMPs and organic C amendments on denitrification rates of cultivated lands and restored forested wetlands. N2O, CH4 and CO2 emissions from restored and natural forested wetlands were measured to determine if restoration for NO3 removal will increase greenhouse gas emissions, thereby contributing to global warming and compromising the water quality benefits of restoration. Low-elevation, wetland clay soils exhibited 6.3 and 2.5 times greater denitrification potential than the high-elevation silt-loam and low-elevation clay soils under cultivation, respectively. Denitrification potentials of vegetated ditches were 1.3 to 4.2 times greater than the unvegetated ditches and cultivated soils, respectively. Soil cores collected from forested wetlands displayed 2.0 - 6.6 times greater denitrification rates than cultivated soils when incubated at 70 to 100% water-filled pore space (WFPS). Significantly lower N2O:N2 emission ratios were observed from wetlands than from cultivated soils. Denitrification rates in the cultivated and restored forested wetland soils increased 200% and 42%, respectively, when amended with cotton gin trash (CGT). BMPs increased denitrification rates of restored wetlands. Nitrate addition to forested wetlands led to a 48% increase in N2O emissions. Forested wetlands exhibited net CH4 sink of 438-1050 g CH4 ha-1 y-1. CO2 emission decreased as WFPS increased from 40 to100% in forested wetlands. Low-elevation clay soils in agricultural watersheds are the best candidates for wetland restoration for water quality improvement. CGT amendment of cultivated and restored soils in conjunction with BMPs can help reduce on-site NO3 loss. Wetland restoration in the Lower Mississippi valley will not significantly affect the global greenhouse gas emissions budget; however, increased N2O emissions due to NO3 additions merit consideration when establishing CO2 storage credits on restored wetlands