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Spring cultivation of soil under long-term pasture to grow summer forage followed by autumn pasture renewal is a common practice on New Zealand dairy farms. Although this practice is widely used, there is limited research available on the impact that it has on nutrient losses to water. This three-year field study was conducted to quantify the effects of summer forage cropping and autumn re-grassing on nitrogen (N) and phosphorus (P) losses in mole and pipe drainage from a Pallic Soil in the Manawatu region of New Zealand. The two treatments compared were a continuation of long-term grazed pasture, and long-term pasture that was cultivated in spring, sown into turnips as a summer forage for dairy cows, followed by autumn re-grassing. Both treatments where located on artificially drained field plots. Summer forage cropping and autumn re-grassing increased total nitrogen measured in drainage water by 21 kg N/ha in total over the three seasons monitored, which was a 84% increase compared to long-term pasture. Approximately three-quarters of this increase happened during the two months following spring cultivation, which was influenced by an atypically wet late spring/early summer period that extended the drainage season. It is estimated that if the spring cultivation had been conducted in a year with a more typical drainage season, then the additional total nitrogen losses are likely to have been about 6.9 kg N/ha. Overall losses of total phosphorus in drainage were low for both treatments during the study period, with the forage cropping and autumn re-grassing increasing total phosphorus losses in drainage by 0.36 kg P/ha over the duration of the study, which was 77% higher than the long-term pasture treatment. [ABSTRACT FROM AUTHOR]

Abstract: In the design of wood-based, enhanced-denitrification bioreactors to treat nitrate in agricultural drainage, the consideration of the highly variable flow rates and nitrate concentrations inherent to many drainage systems is important. For optimized mitigation of these nitrate loads, it may be best to contain drainage water prior to treatment in order to facilitate longer, more constant retention times rather than to allow cycles of flushing and dry periods in the denitrification bioreactor. Simulated containment prior to bioreactor treatment compared to passing drainage directly through a bioreactor was investigated with the use of six pilot-scale denitrification bioreactors constructed with plywood and filled with Pinus radiata woodchips at Massey University No. 4 Dairy Farm (Palmerston North, New Zealand). Initial bromide tracer tests were followed with a series of five simulated drainage events each at successively declining inflow nitrate concentrations. During each drainage event, three pilot bioreactors received a simulated hydrograph lasting 1.5 days (Non-Containment treatment) and three pilot bioreactors received the same total drainage volume treated over 4 days at a constant flow rate (i.e. constant retention time; Containment treatment). Results showed significantly different total mass removal efficiencies of 14.0% vs. 36.9% and significantly different removal rates of 2.1gNm−3 day−1 vs. 6.7gNm−3 day−1 for the Non-Containment and Containment treatments, respectively, which indicated that treating drainage at constant retention times provided more optimized nitrate removal. While this work was done to evaluate treatment under New Zealand drainage conditions, it also provides valuable information for optimizing agricultural drainage denitrification bioreactor performance in general. [Copyright &y& Elsevier]