Agronomic Management Practices for Soil, Water and Nutrients Conservation in a Humid Region

Abstract

<b>Abstract</b> <p /> The mean annual rainfall is approximately 1600 mm in the Lower Mississippi River Basin, and almost half occurred off-crop-growing season, in late fall, winter and spring before planting. Intensive rainfall often results in excessive percolation, runoff, soil erosion and nutrient losses, which contribute to degradation of surface and ground water quality. Planting cover crops (CC) off-growing season when severe rain storms often occur and application of poultry litter and biochar during the growing season were tested in fields and evaluated by the USDA-ARS Root Zone Water Quality Model (RZWQM2) in an attempt to determine their effectiveness in conservation of soil, water and nitrogen in this humid region. Compared with a no-CC scenario (i.e., bare soil), planting CC reduced drainage by 69, 53, and 51 mm in the wet, normal, and dry years, respectively. Meanwhile, the CC induced decrease in surface evaporation was 64 mm for the corn growing period and 38 mm for the soybean growth period over the past 80 years. CC can increase soil organic matter by 15%, and storage of rain water in soils by 13% during the crop growing season. Planting a winter wheat CC into a corn-soybean system improved annual denitrification by 9%, and reduced annual nitrate-N leaching by 24%. The continuous tillage system improved soil organic carbon (SOC) levels more than the no-till system in this humid region characterized by hot summers and mild winters. CC improved crop water productivity by 2.5% for corn and 1.0% for soybeans under the future RCP4.5 and RCP8.5 greenhouse gas emissions scenarios. Soil evaporation was reduced by 28%, 24% and 27% during the corn or soybean growing seasons under the baseline, RCP4.5 and RCP8.5 GHG emissions scenarios, respectively. The annual drainage below the 2-m soil depth was decreased by 8%, 12% and 15% for the CC treatment compared with the no-CC treatment under the baseline, RCP4.5 and RCP8.5 GHG emissions scenarios. The wheat CC enhanced the soil water storage, and growing CC buffered the impacts of climate extremes on water loss. The primary avenue of simulated litter N loss was leaching if applied in the fall and denitrification if applied in the spring. Spring application can reduce the loss of total mineralized litter N by 22% compared with fall application. Aggregate stability was improved by 17% after five years of poultry litter applications compared to the inorganic fertilizer treatments. Infiltration rate and saturated hydraulic connectivity were two to three times greater in the poultry litter treatments. The poultry litter addition increased field capacity of water and plant available water by 20%. Investigation on no-tilled farm fields that continuously received 6.7 Mg ha^-1 biochar for 10 years in the region found that the biochar addition increased soil total porosity, increased field capacity (26%) and permanent wilting point (43%). We found that cover crops and soil organic amendments (poultry litter and biochar) were effective agronomic management practices in reducing loss of soil, water and nutrients from fields and adverse impacts on off-site water quality. In addition, cover crops are an effective means to mitigate the impacts of climate changes and extremes in this humid environment.