The Effect of Sampling Frequency on the Accuracy of Nitrogen Load Estimates from a Drained Loblolly Pine Plantation in Eastern North Carolina

Nutrient loading in drainage outflow is estimated from measured flows and nutrient concentrations in the drainage water. The loading function is ideally continuous, representing the product of continuously measured outflows and nutrient concentrations in drainage water. However, loading is often estimated as the product of continuously measured outflow and nutrient concentrations measured at less frequent time intervals (weekly, monthly etc.). In this study we investigate the effects of sampling frequency and method on estimating the loading of nitrate nitrogen (NO3-N) and total Kjeldahl nitrogen (TKN) from a drained loblolly pine plantation in Eastern North Carolina. The loading of NO3-N and TKN computed from continuous flows and daily concentrations were compared to loadings computed from weekly, bi-weekly, and monthly discrete samples. In this study, the NO3-N concentrations had a greater range than those of the TKN concentrations and had a more distinct relationship between concentrations and flow; consequently, the load estimation methods were less precise and more biased when estimating NO3-N than when estimating TKN. If NO3-N loads from small drained forest watersheds are going to be calculated from discrete samples, the sample interval should be well less than 7 days, probably in the range of 1 to 3 days. Load estimates for TKN may be acceptable from samples collected every 7 days.