Patterns of natural 15N in soils and plants from chemically and organically fertilized uplands

Diagnostic tests for organic production of crops would be useful. In this study, the difference in natural N-15 abundances (delta(15)N) of soils and plants between fertilizer-applied upland (FU) and compost-applied upland (CU) fields was investigated to study using delta(15)N as a marker of organic produce. Twenty samples each of soils and plants were collected from each field in early summer after applying fertilizer or compost. The delta(15)N of fertilizers and composts was - 1.6 +/- 1.5parts per thousand (n = 8) and 17.4 +/- 1.2parts per thousand (n = 10), respectively. The delta(15)N of total soil-N was significantly (P < 0.05) higher in CU fields (8.8 +/-2.0parts per thousand) than in FU fields (5.9 4 +/- 0.7parts per thousand) due to long-term continuous application of N-15-enriched compost, as indicated by a positive correlation (r = 0.62) between N content and delta(15)N of total soil-N. The NO3- Pool Of CU Soils (11.6 +/- 4.5parts per thousand) was also significantly (P < 0.05) enriched in N-15 compared to FU soils (4.7 +/- 1.1parts per thousand), while the N-15 contents of NH4+ Pool were not different between both soils. Compost application resulted in N-15 enrichment of plants; the delta(15)N values were 14.6 +/- 3.3parts per thousand for CU and 4.1 +/- 1.7parts per thousand for FU fields. These results showed that long-term application of compost resulted in a significant N-15-enrichment of soils and plants relative to fertilizer. Therefore, this study suggested that delta(15)N could serve as promising indicators of organic fertilizers application when used with other independent evidence. However, further studies under many conditions should be conducted to prepare reliable delta(15)N guidelines for organic produce, since the delta(15)N of inorganic soil-N and plant-N are influenced by various factors such as soil type, plant species, the rate of N application, and processes such as mineralization, nitrification, and dnitrification. (C) 2003 Elsevier Ltd. All rights reserved.