Natural 15N abundances of maize and soil amended with urea and composted pig manure

To investigate the effect of inorganic fertilizer and composted manure amendments on the N isotope composition (delta(15)N) of crop and soil, maize (Zea mays L.) was cultivated under greenhouse conditions for 30, 40, 50, 60, and 70 days. Composted pig manure (delta(15)N = + 13.9parts per thousand) and urea (-2.3parts per thousand) were applied at 0 and 0 kg N ha(-1) (C0U0), 0 and 150 kg N ha(-1) (C0U2), 150 and 0 kg N ha(-1) (C2U0), and 75 and 75 kg N ha(-1) (C1U1), respectively. The delta(15)N of total soil-N was not affected by both amendments, but delta(15)N of NH4+ and NO3- provided some information on the N isotope fractionation in soil. During the early growth stage, significant differences (P < 0.05) in delta N-15 among maize subjected to different treatments were observed. After 30 days of growth, the delta N-15 values of maize were +6.6 parts per thousand for C0U0, +1.1% for C0U2, +7.7% for C2U0, and +4.5 parts per thousand for C1U1. However, effects of urea and composted manure application on maize delta N-15 progressively decreased with increasing growth period, probably due to isotope fractionation accompanying N losses and increased uptake of soil-derived N by maize. After 70 days of growth, delta N-15 of leaves and grains of maize amended with composted pig manure were significantly (P <0.05) higher than those with urea. The temporal variations in delta(15)N of maize amended with urea and composted manure indicate that plant delta(15)N is generally not a good tracer for N sources applied to field. Our data can be used in validation of delta(15)N fractionation models in relation to N source inputs.