Temporal changes in 18O and 15N of nitrate nitrogen and H2O in shallow groundwater: Transit time and nitrate-source implications for an irrigated tract in southern Idaho

Authors: Lentz, Rodrick; Lehrsch, Gary

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2018
Publication Date: 9/6/2018
Publication URL: https://handle.nal.usda.gov/10113/6161586
Citation: Lentz, R.D., Lehrsch, G.A. 2018. Temporal changes in 18O and 15N of nitrate nitrogen and H2O in shallow groundwater: Transit time and nitrate-source implications for an irrigated tract in southern Idaho. Agricultural Water Management. 212:126-135. https://doi.org/10.1016/j.agwat.2018.08.043.
DOI: https://doi.org/10.1016/j.agwat.2018.08.043

Interpretive Summary: Intensive irrigated agriculture in semiarid southern Idaho contributes to nitrate loads in shallow groundwater. Substantial fertilizer applications are needed to attain high yields, which may cause soil nutrients to leach into ground water. Increased use of organic fertilizers such as dairy manure may also increase nutrient leaching risks. When nutrient rich groundwater seeps into surface waters, it promotes algal growth in downstream waters and depletes them of life-giving oxygen, resulting in dead zones which have dramatically reduced aquatic production. Impacts of agriculture on groundwater quality are difficult to ascertain, but this knowledge is crucial for protecting and preserving Earth’s water supply. To determine the temporal character and source of leached nitrate in shallow groundwater and understand the dominant soil nitrogen cycling process involved as nitrate transits to the groundwater, we measured stable isotope ratios of nitrate and water in 1) tunnel drain and irrigation waters during 2003-07 and 2) leachate from urea- and manure-amended surface soil. The study determined the residence time for water in the shallow groundwater and found that, on average, 1.5 times more nitrogen in groundwater sourced from fertilizer and fixed nitrogen than animal waste. The dominant N-cycling process in the system at the scale observed here is the nitrification of ammonium derived from applied fertilizer and manure, whereas denitrification has a minor influence. Increased knowledge of nitrogen cycling in both the vadose zone and shallow groundwater can improve nutrient management and cropping efficiency in the irrigation tract.

Technical Abstract: Intensive irrigated agriculture in semiarid southern Idaho contributes to nitrate loads in shallow groundwater. To determine the temporal character and source of leached nitrate and the dominant soil N cycling process involved, we measured stable isotope ratios of nitrate (15N-NO3,18O-NO3) and water (2H-H2O,18O-H2O) in 1) tunnel drain and irrigation waters during 2003-07 and 2) leachate from incubated urea- and manure-amended soil endmembers. The 18O-H2O time series revealed an 8 to 13 month lag between peak values for irrigation water (the primary source of recharge) and those of tunnel drains, indicating the likely residence time for water in the shallow groundwater. Correlations of tunnel water 18O-NO3 and 18O-H2O with previous-year mean annual and summer precipitation, and reservoir storage factors confirmed the approximate year-long residence time. Eight of ten tunnel waters (categorized hereafter as Group I) had 15N-NO3 and 18O-H2O compositions of +6.3 ± 0.6 (± Std. Dev.) and -5.9 ± 0.7, respectively. Nitrate 15N-NO3 and 18O-H2O compositions of tunnel waters plotted between those of urea-amended soil (4.6 ± 0.5 and -4.9 ± 1.4), manure-amended soil (13.4 ±1.3 and -4.4 ± 1.2), and regional groundwater endmembers. A dual-isotopic element, three-source, simple linear mixing model indicated that, on average, 1.5 times more N is sourced from fertilizer and fixed N than animal waste. The dominant N-cycling process in the system at the scale observed here is the nitrification of NH4-N derived from applied fertilizer and manure, whereas denitrification has a minor influence. Increased knowledge of N cycling in both the vadose zone and shallow groundwater can improve nutrient management and cropping efficiency in the irrigation tract.