Transport of Nutrients by Overland Flow on Sites Containing Beef Cattle Manure

Abstract

In an inductive metastudy, we investigated the effects of varying rates of overland sheet flow on the transport of phosphorus (P) and nitrogen (N) from sites containing beef cattle manure. The analyzed data were collected during previously reported field rainfall simulation investigations conducted on cropland sites and a beef cattle feedlot in the US state of Nebraska. During the experiments, inflow was incrementally added to the top of the experimental plots to simulate runoff rates occurring at greater downslope distances. Runoff rates on the experimental plots ranged from 2.9 to 22.9 L min−1, and the maximum equivalent downslope distances varied from 5.3 to 42.3 m. The findings revealed that P and N transport rates increased linearly with runoff rate on sites where N was applied at rates ≤151 kg ha−1 (Condition 1). Nutrient transport rates were influenced by the quantity of nutrients released to overland flow and the amount of runoff available to transport these nutrients. Interestingly, on sites where N was added at rates >151 kg ha−1 (Condition 2) and on beef cattle feedlots (Condition 3), maximum transport rates for P were not affected by runoff rate, but N transport rates again increased linearly with runoff rate. The following hypotheses were formulated: Hypothesis 1 states that P transport rates can be directly related to runoff rates for Condition 1; Hypothesis 2 states that for Condition 2, the maximum rate at which beef cattle manure can release P to overland flow can be estimated from the P content of the applied manure; and Hypothesis 3 states that transport rates for total N can be directly related to runoff rates for Conditions 1, 2, and 3. Hypothesis testing using the student’s t-test affirmed each hypothesis. However, further assessment of these hypotheses is needed at other field sites with varying soil, cropping, and management conditions. The land application of manure to cropland areas has the dual benefit of reducing fertilizer expenses and improving soil health. However, it is essential to consider water quality concerns that may arise if nutrients in manure are transported by runoff to receiving water bodies. In this investigation, rainfall simulation data were used to identify the effects of varying runoff rates on nutrient transport from sites containing beef cattle manure. The study revealed the following findings: (1) P transport rates increase linearly with runoff rate on sites where beef cattle manure was applied at N application rates ≤151 kg ha−1, which is approximately the annual N requirement for corn; (2) P transport rates do not vary with runoff rate when N is added at rates >151 kg ha−1; (3) the maximum P transport rate is directly related to the P content of the applied manure; and (4) N transport rates increase linearly with runoff rate on sites containing beef cattle manure. These findings suggest that if nutrient transport rates can be related to runoff rates, rainfall simulation data obtained from small plots could potentially be extrapolated to greater downslope distances.