Nitrogen Dynamics in Irrigated Forage Systems Fertilized with Liquid Dairy Manure

Optimal manure management that ensures adequate crop nutrition while avoiding pollution problems requires estimates of manure N availability. The present study was performed in the San Joaquin Valley (California) on three dairy forage production fields where liquid manure is applied together with flood irrigation water. The objective of this study was to determine the fate of manure N by combining field measurements with model simulations using the Root Zone Water Quality Model (RZWQM). The average annual N application to corn (Zea mays L.) and winter forage (oat [Avena sativa L.], triticale [xTriticosecale Wittmack], or Sudan grass [Sorghum bicolor (L.) Moench ssp. drummondii (Steud.) de Wet ex Davidse]) was 840 kg N ha(-1), while 490 kg N ha(-1) was removed with the harvested crops. The irrigation water input to corn ranged from 45 to 128 cm. The RZWQM described crop yield and N uptake well and accurately simulated the seasonal trends in soil moisture and mineral N content in the top 90 cm of the profile; however, the short-term changes and mineral N estimates for different soil layers were not accurate. For soil nutrient and water dynamics, sites-pecific calibration was an essential requirement. The model estimated that between 140 and 320 kg N ha(-1) was leached in a 12-mo period and up to 80 kg N ha-1 was volatilized as NH3, while losses due to denitrification were insignificant in these sandy soils. Field data and model estimates highlight the potential for a more efficient water and N use in the forage systems studied.