Nitrate leaching from potato production in Eastern Canada

Abstract

Nitrate leaching from potato production systems is of economic importance, and also of concern for both drinking water quality and the health of aquatic ecosystems. We examine the processes, timing and magnitude of nitrate leaching, and examine practices developed to reduce nitrate leaching from potato production systems, with a particular focus on Prince Edward Island. Results from tile-drain experiments indicate that nitrate leaching occurs primarily during late autumn winter and early spring when crop uptake diminishes, and elevated nitrate concentrations coexist with water movement from the root zone, with the timing of nitrate leaching generally corresponding with major recharge events. Based on stable isotopic signatures, nitrate leached during the growing season is primarily from mineral fertilizers and mineralization of soil organic N whereas nitrate leached outside of the growing season originates primarily from mineralized N (including microbially-modified fertilizer N). Nitrate concentrations in tile-drain water from potato plots were commonly above the 10 mg NO3-N L-1 drinking water quality guideline, and were higher from potatoes than from red clover or cereal plots. Simulations using LEACHN predicted that >80% of nitrate leaching occurred outside of the growing season, a finding consistent with water balance calculations in intensively cultivated watersheds. Nitrate leaching under conventional potato-barley-red clover rotations were predicted to be 27-91 kg N ha-1 (average 56 kg N ha-1), corresponding to concentrations of nitrate in drainage of 5.6-18 mg N L-1 (average 10.7 mg N L-1). Nitrate leaching was predicted to increase with fertilizer N rate for potato, and decreased with rotation length. Nitrate leaching can be reduced through implementation of practices which reduce the accumulation of nitrate in the root zone, particularly at potato harvest. These practices may include improved crediting of soil N mineralization from soil organic matter and crop residues, use of in-season measures of crop N status to guide in-season N management, and introducing potato cultivars with lower fertilizer N requirements or higher N use efficiency. The requirement for high N inputs to obtain economic tuber yields, in combination with the high risk of nitrate loss from the root zone, present significant challenges to growers with respect to N management. Reducing the frequency of potato in the crop rotation, and use of practices to reduce residual nitrate in potato production, will be most effective in reducing nitrate leaching losses.