Vadose Zone Fertilizer‐Derived Nitrate and δ15N Extracts

Abstract

AbstractNitrate derived from commercial NH4 NO3 fertilizer, δ15N =+1.5 %0, applied to fertility plots was extracted from two cores to depths of 46 and 52 ft. In both cores the δ15N of the extracted nitrate‐nitrogen (NO3‐N) showed that it became heavier with depth. Although this isotope fractionation did not cause δ15N values to overlap with those characteristic of animal waste sources (> + 10%0), δ15N values were as high as +8%0 at the bottom of one profile. The gradual increase in δ15N 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 NO3‐N concentrations (r =+0.9) in the core with 30% clay. Correlation of NO3‐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 NO3‐N concentrations or δ15N values.Potassium chloride (KC1) extracts yielded significantly higher NO3‐N concentrations (p < 0.005) and lighter δ15N values (p < 0.0001) than distilled deionized water (DDW) extracts. It is postulated that NO3− not extracted by DDW is trapped within the microstructure of the swelling clays. The strongly depleted δ15N 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.