Delayed plowing of forages within potato rotation reduces nitrate leaching losses

The potato industry plays an important role in the economics in Prince Edward Island (PEI), Canada. Intensive potato production has been conducted on sandy soil underlain by a semi-confined or unconfined sandstone aquifer, which provides all the drinking water and a large majority of stream flow on the island. Typically, potato is grown in rotation with grain underseeded with forages, with the latter being plowed down in the fall of the third season. High levels of nitrate leaching losses from the potato production systems have adversely affected both groundwater and associated surface water quality. Beneficial Management Practices (BMPs) were proposed for maintaining optimal crop production while mitigating the nitrate contamination in PEI. But there is limited information on the environmental performance of the BMPs in commercial fields. The potentials of delayed forage plowing from fall to spring to reduce nitrate losses were evaluated in commercial fields at two separated sites in PEI during October 2010 and March 2013. At Site 1, two tile-drained fields were paired for spring vs. fall plowing treatments (with pre-plow herbicidal treatment); weekly tile-drainage samples were collected to evidence the effects of the treatments. At Site 2, a field was split in half for spring vs. fall plowing treatments (without pre-plow herbicidal treatment). Shallow piezometers were installed at the upper and lower ends of the field for water sampling, with the upper end one indicating the effects of shallow groundwater inflow and the lower ones evidencing the combined effects of groundwater inflow and drainage from plowing treatments. Soil drainage was estimated through coupled LEACHN and MODFLOW simulations. Nitrate leaching losses were calculated as the integration of measured leached nitrate concentrations and simulated drainage. Field monitoring showed that nitrate leaching mainly occurred during the offseason. Nitrate leaching during the forage phase were estimated to be 36 and 18 kg N/ha from the fall and spring plowed fields at Site 1, respectively. The differences of leached nitrate were mainly attributed to pre-plow herbicidal treatment associated with fall plowing. Respective nitrate leaching losses were estimated to be 54 and 32 kg N/ha from the fall and spring plowed fields at Site 2. The differences of leached nitrate at Site 2 were likely due to the different timing of tillage. The results suggested that postponing plowing of forages and associated herbicidal treatments until spring reduced nitrate leaching. Nitrate leaching losses derived exclusively from the decay of plow down forages were estimated to account for 16-21% of the overall N in the plants, implying that a significant amount of N in the forages were carried over into the subsequent crop phase. This large carried-over N pool creates an opportunity for the growers to save costs on fertilizer N by crediting some of the organic N while reducing the risk of nitrate leaching to groundwater.