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REVIEWS AND ANALYSES

Dissolved Organic Nitrogen: An Overlooked Pathway of Nitrogen Loss from Agricultural Systems?

^a Dep. of Plant Sciences, Univ. of California-Davis, Davis, CA 95616
^b Soil & Physical Sciences Group, Agriculture & Life Sciences Division, Lincoln Univ., PO Box 84, Lincoln 7647, New Zealand
^c Wageningen Univ., Dep. of Soil Quality, P.O. Box 47, 6700 AA, Wageningen, the Netherlands

^* Corresponding author (cvankessel{at}ucdavis.edu).

Received for publication June 18, 2008. Conventional wisdom postulates that leaching losses of N from agriculture systems are dominated by NO₃^–. Although the export of dissolved organic nitrogen (DON) into the groundwater has been recognized for more than 100 yr, it is often ignored when total N budgets are constructed. Leaching of DON into stream and drinking water reservoirs leads to eutrophication and acidification, and can pose a potential risk to human health. The main objective of this review was to determine whether DON losses from agricultural systems are significant, and to what extent they pose a risk to human health and the environment. Dissolved organic N losses across agricultural systems varied widely with minimum losses of 0.3 kg DON ha^–1yr^–1 in a pasture to a maximum loss of 127 kg DON ha^–1yr^–1 in a grassland following the application of urine. The mean and median values for DON leaching losses were found to be 12.7 and 4.0 kg N ha^–1yr^–1, respectively. On average, DON losses accounted for 26% of the total soluble N (NO₃^– plus DON) losses, with a median value of 19%. With a few exceptions, DON concentrations exceeded the criteria recommendations for drinking water quality. The extent of DON losses increased with increasing precipitation/irrigation, higher total inputs of N, and increasing sand content. It is concluded that DON leaching can be an important N loss pathway from agricultural systems. Models used to simulate and predict N losses from agricultural systems should include DON losses.

Abbreviations: DON, dissolved organic nitrogen • EU, European Union • HMW, high molecular weight • LMW, low molecular weight • SON, soluble organic nitrogen

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