Grain 15N of crops applied with organic and chemical fertilizers in a four-year rotation

Variations in crop grain and soil N isotope composition (delta N-15) in relation to liquid hog manure (delta 15N of total N was +5.1 parts per thousand), solid cattle manure (+7.9 parts per thousand) and chemical fertilizer (+0.7 parts per thousand for urea and -1.9 parts per thousand for ammonium phosphate) applications, and control (no fertilizer application) were examined through a 4-year crop rotation under field conditions. Canola (Brassica napus), hull-less barley (Hordeum vulgare), wheat (Triticum aestivum), and canola were grown sequentially from 2000 (year 1) to 2003 (year 4). From year 2, hog manure or chemical fertilizers, but not cattle manure, treatments increased grain N concentrations over the control. Grain delta N-15 (+0.3 to +2.5 parts per thousand) of crops applied with chemical fertilizers was lower than those in the other treatments, reflecting the effects of the N source with a lower delta N-15, while the manure treatments tended to increase grain delta N-15. The higher grain delta N-15 of crops applied with hog manure (+5.6 to +8.4 parts per thousand) than those applied with cattle manure (+2.2 to +4.1 parts per thousand) reflected the higher N availability of liquid hog manure (up to 70% as NH4+) than solid cattle manure (99% organic N) and higher potentials for ammonia volatilization loss in hog manure rather than differences in manure delta N-15 signatures. Soil total- and extractable-N concentrations and delta N-15 tended to vary with the application of N sources with different N isotope composition and availability. Our study expanded the application of the delta N-15 technique for detecting N source (organic vs chemical) effects on N isotopic composition to field conditions and across a 4-year rotation, and revealed that N availability played a greater role than the delta N-15 signature of N sources in determining crop delta N-15 under the studied conditions.