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

Lentz, R.D. and 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. 6 September 2018.

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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.

Item Type:	Article
NWISRL Publication Number:	1631
Subjects:	Irrigation > Furrow irrigation > Runoff losses Manure > Runoff Manure Water > Water quality Water Water > Water quality > Nutrients
Depositing User:	Users 6 not found.
Date Deposited:	10 Sep 2018 16:51
Last Modified:	10 Sep 2018 16:51
Item ID:	1675
URI:	https://eprints.nwisrl.ars.usda.gov/id/eprint/1675