VADOSE ZONE FERTILIZER-DERIVED NITRATE AND DELTA-N-15 EXTRACTS

Nitrate derived from commercial NH4NO3 fertilizer, deltaN-15=+1.5 parts per thousand, applied to fertility plots was extracted from two cores to depths of 46 and 52 ft. In both cores the deltaN-15 of the extracted nitrate-nitrogen (NO3-N) showed that it became heavier with depth. Although this isotope fractionation did not cause deltaN-15 values to overlap with those characteristic of animal waste sources (> + 10 parts per thousand), deltaN-15 values were as high as + 8 parts per thousand at the bottom of one profile. The gradual increase in deltaN-15 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 deltaN-15 values. Potassium chloride (KCl) extracts yielded significantly higher NO3-N concentrations (p < 0.005) and lighter deltaN-15 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 deltaN-15 of - 16.3 parts per thousand 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 KCl extractions of sediments high in smectite (> 40%) provide more realistic estimates of ground-water loading from nonpoint sources.