Nitrate impacts on groundwater from irrigated-vegetable systems in a humid north-central US sand plain

Groundwater is frequently susceptible to nitrate pollution in irrigated regions possessing a humid climate, coarse soil, and shallow water table. Such pollution degrades local drinking water resources and increases watershed nitrate export. The irrigated-vegetable production agroecosystem of the Wisconsin Central Sand Plain (WCSP), north-central USA, exemplifies the problem. This study's research goal was to assess groundwater nitrate loading in this agroecosystem, with a view to manage groundwater quality and nitrate export in the WCSP and similar regions. Nitrate loading was measured beneath a 44 ha field for 4 years using a novel "water-year" method, during three crops of sweet corn (Zea mays L.) and one of potato (Solanum tuberosum L.). Nitrate-N concentrations averaged 20 mg l-1 in shallow (upper 3 m) groundwater beneath the study field. Nitrate-N loading from the sweet corn ranged from 126 to 169 kg ha-1 per year; loading from potato was 228 kg ha-1 per year. Groundwater loading amounted to 61% of total available N and 77% of fertilizer N. Measured loadings compared well with those calculated using a budget approach, supporting the validity of both methods. N budgets were calculated from average regional harvests and two fertilizer rates: a typical grower rate, and the more conservative University Extension rate. Budget-derived nitrate-N loadings from sweet corn are 151 kg ha-1 per year (typical) and 119 kg ha-1 per year (University Extension). For potato, typical and recommended fertilizer rates are equal, and the budget-derived nitrate-N loading is 203 kg ha-1 per year. Budget-derived loadings imply that limiting the basin-averaged nitrate-N concentration in groundwater to 10 mg l-1 (the US drinking water standard) would require each irrigated-vegetable hectare to be offset by 4.5-6.5 ha of land supplying nitrate-free groundwater recharge. Further, a WCSP watershed with half irrigated-vegetable cover, even with no other nitrate sources, would export 50-74 kg ha-1 per year of nitrate-N. This export is greater on a per-hectare basis than the large maize- and soybean-producing basins of Iowa, USA. Most orthodox nitrate control strategies (e.g., decreasing and splitting fertilizer, irrigation scheduling) have already been implemented in the WCSP, and have little further potential to substantially reduce loading if profitability must be maximized. Better control will require new and unconventional approaches for controlling nitrate leaching from mineralized crop residue as well as additional curbing of direct fertilizer nitrogen leaching.