Runoff nutrient loads as affected by residue cover, manure application rate, and flow rate

Manure is applied to cropland areas with varying surface cover to meet single-year or multiple-year crop nutrient requirements. The objectives of this field study were to: (1) examine runoff water quality characteristics following land application of manure to sites with and without wheat residue, (2) compare the water quality impacts of land application of manure to meet 0-, 1-, 2-, 4-, and 8-year P-based requirements for corn, and (3) evaluate the effects of varying runoff rates on runoff nutrient loads. Three 30-min simulated rainfall events, separated by 24 h intervals, were applied at an intensity of 70 mm h-1 to 0.75 m wide by 2.0 m long plots on which manure has been previously applied and incorporated. Runoff loads of dissolved phosphorus (DP), total phosphorus (TP), NO3-N, NH4-N, and total nitrogen (TN) were significantly greater on the plots with residue cover. Manure application rate significantly influenced runoff loads of DP, TP, NO3-N, NH4-N, and TN. No significant differences in runoff loads of DP and TP were found between sites where manure was applied to meet a 1-year or 2-year P requirement for corn. However, runoff loads of DP and TP were significantly greater on the sites where manure was applied to meet a 4-year rather than a 2-year P requirement. Each of the measured water quality parameters except electrical conductivity (EC) was significantly influenced by runoff rate. The application of manure to meet multiple-year crop nutrient requirements may increase nutrient loads in runoff.