Legacy Nutrient Transport by Overland Sheet Flow

Abstract

The transport of phosphorus (P) and nitrogen (N) from sites containing varying quantities of legacy nutrients was evaluated in this investigation. The data that were examined were collected during four previously reported field hydrologic studies performed in eastern Nebraska. Additional flow was introduced to the top of the rainfall simulation plots to replicate runoff conditions occurring along a hillslope. It was observed that both P and N delivery were influenced by runoff rate on sites where beef cattle manure or inorganic fertilizer had been applied. Legacy nutrient delivery appeared to have been influenced by the quantity of nutrients released at a particular runoff rate and the amount of overland sheet flow available to transport the nutrients. Two hypotheses were formulated based on experimental observations. Hypothesis 1: P transport rates can be estimated from measurements of soil P content. Hypothesis 2: Linear regression equations can be used to relate total N delivery to runoff rate. Each hypothesis was affirmed using the student’s t-test. Additional assessment of the proposed nutrient transport equations is needed at other locations maintained under different management conditions. Data from four rainfall simulation studies were analyzed to identify the effects of varying discharge rates on the delivery of nutrients from cropland sites containing legacy nutrients. It was observed that nutrient delivery increased in a linear fashion with runoff rate on sites where beef cattle manure or inorganic fertilizer had been applied. Legacy nutrient delivery appeared to have been influenced by the quantity of nutrients released at a particular runoff rate and the amount of overland sheet flow present to transport the nutrients. Measurements of soil phosphorus (P) content were used to determine the slope factor contained in the linear P transport equations. Transport rates for total nitrogen were found to be directly related to runoff rates. The nutrient transport concepts introduced in this study could be incorporated into existing computer models that route overland flow along hill slopes.