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Vadose Zone Fertilizer‐Derived Nitrate and δ 15 N Extracts
Author(s) -
Herbei Mitchell J.,
Spalding Roy F.
Publication year - 1993
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.1993.tb01838.x
Subject(s) - illite , chlorite , nitrate , chemistry , distilled water , fertilizer , clay minerals , nitrogen , potassium , denitrification , fractionation , environmental chemistry , chloride , mineralogy , zoology , geology , chromatography , paleontology , quartz , organic chemistry , biology
Nitrate derived from commercial NH 4 NO 3 fertilizer, δ 15 N =+1.5 % 0 , applied to fertility plots was extracted from two cores to depths of 46 and 52 ft. In both cores the δ 15 N of the extracted nitrate‐nitrogen (NO 3 ‐N) showed that it became heavier with depth. Although this isotope fractionation did not cause δ 15 N values to overlap with those characteristic of animal waste sources (> + 10% 0 ), δ 15 N values were as high as +8% 0 at the bottom of one profile. The gradual increase in δ 15 N with depth in the bottom 20 to 30 ft of the cores is consistent with denitrification at these depths. Clay content was much more variable with depth and was significantly associated with NO 3 ‐N concentrations (r =+0.9) in the core with 30% clay. Correlation of NO 3 ‐N concentrations to clay content was not significant in the other core with 50% clay. In both cores smectite and illite were the dominant clays; chlorite, and aluminum and iron oxyhydroxides were not detected by XRD. Anion exchange capacity measurements confirmed that it was not responsible for the observed variability in NO 3 ‐N concentrations or δ 15 N values. Potassium chloride (KC1) extracts yielded significantly higher NO 3 ‐N concentrations (p < 0.005) and lighter δ 15 N values (p < 0.0001) than distilled deionized water (DDW) extracts. It is postulated that NO 3 − not extracted by DDW is trapped within the microstructure of the swelling clays. The strongly depleted δ 15 N of–16.3 % 0 may be attributed to pH‐mediated aqueous ammonia or nitrate diffusion and/or ammonia volatilization into the smectite microstructure. Results indicated that DDW rather than KC1 extractions of sediments high in smectite (> 40%) provide more realistic estimates of ground‐water loading from nonpoint sources.