Nitrate Leaching in Californian Rice Fields: A Field- and Regional-Scale Assessment

Abstract

Irrigated croplands can be a major source of nitrate-N (NO-N) in groundwater due to leaching. In California, where high NO-N levels have been found in some areas of the Central Valley aquifer, the contribution from rice systems has not been determined. Nitrate leaching from rice systems was evaluated from soil cores (0-2 m), from the fate of N fertilizer in replicated microplots, and from about 145 regional groundwater wells. Soil NO-N concentrations were ≤3.3 mg kg (usually <1 mg kg) below the root zone (below 33 cm depth). In pore-water samples, NO-N was observed only below the root zone during the first 2 wk after the onset of flooding in either the growing season or the winter fallow period and was always ≤8.4 mg L. Fertilizer N accounted for 0 to 11.8% of NO-N in pore-water samples below the root zone. One year after application, based on an analysis of soil core samples, on average 2.5% of fertilizer N was recovered as N below the root zone (33-100 cm), possibly due to leaching in permeable soils or via preferential flow through cracks in heavy clay soils. Based on a regional assessment, groundwater samples from wells that are located in proximity to rice fields all had measured median NO-N and NO-N levels below 1 mg L. These results indicate that NO-N leaching from the majority of California rice systems poses little risk to groundwater under current crop management practices.

A Bayesian approach to infer nitrogen loading rates from crop and land-use types surrounding private wells in the Central Valley, California

“…This was attributed to denitrification during the rice growing season when fields are flooded, ammonia volatilization, plant uptake, and crop management practices that contribute to the development of a hardpan layer directly below the rice root zone. The study also found very low nitrate concentrations in groundwater wells near rice fields (median value less than 1 mg L −1 NO 3 -N) (Liang et al, 2014) consistent with our estimates of Rice nitrogen loading. The GNLM mass balance estimates are outside the range of our model-predicted CIs for Rice as they reflect nitrogen losses prior to denitrification or ammonium volatilization potentially taking place in saturated clay soils of rice fields.…”

“…Credibility intervals of posterior probability densities of estimated nitrogen loading rates for selected crop groups plotted with historical field measurements of nitrogen loading from California (Devitt et al, 1976;Embleton et al, 1979;Letey et al, 1977;Pratt et al, 1972;Pratt and Adriano, 1973;Adriano et al, 1972;Allaire-Leung et al, 2001;Liang et al, 2014). Thinner lines are 95 % credibility intervals, thicker lines are 68 % credibility intervals, the solid dot is the median estimated value, and the open black circles are historical field measurements.…”

“…Thinner lines are 95 % credibility intervals, thicker lines are 68 % credibility intervals, the solid dot is the median estimated value, and the open black circles are historical field measurements. Adriano et al, 1972;Allaire-Leung et al, 2001;Liang et al, 2014). These field measurements are limited to a few crops and are not available for all crop groups analyzed in this study.…”

“…Knowledge of the highest-risk crops can aid future regulatory efforts and help in defining priority areas on a county or smaller scale. Previous fieldbased, often plot-scale research, conducted in California, has measured the amount of nitrogen leached in kg ha −1 yr −1 from several different crop types including grains, vegetables and berries, citrus, nuts, and field crops (Devitt et al, 1976;Embleton et al, 1979;Letey et al, 1977;Pratt et al, 1972;Pratt and Adriano, 1973;Adriano et al, 1972;AllaireLeung et al, 2001;Liang et al, 2014). However, these studies were largely conducted in the 1970s and 1980s, with little research conducted since then, and are subject to high variability due to various measurement methods (Viers et al, 2012).…”

Soil–aquifer phenomena affecting groundwater under vertisols: a review

“…Denitrification in the root zone and deeper in the soil profile explains the small amount of nitrate leached to the groundwater under rice fields in clayey soils in California, USA (Liang et al, 2014). Shallow groundwater (< 1.5 m) under cultivated vertisols (e.g., the Netherlands) showed large variability (spatial and temporal) in nitrate concentration, probably due to the highly variable oxygen concentrations and therefore variability in nitrogen transformations in these systems (Hendriks et al, 1999).…”