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 15N abundances (δ15N) of soils and plants between fertilizer-applied upland (FU) and compost-applied upland (CU) fields was investigated to study using δ15N 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 δ15N of fertilizers and composts was −1.6±1.5‰ (n=8) and 17.4±1.2‰ (n=10), respectively. The δ15N of total soil-N was significantly (P<0.05) higher in CU fields (8.8±2.0‰) than in FU fields (5.9±0.7‰) due to long-term continuous application of 15N-enriched compost, as indicated by a positive correlation (r=0.62) between N content and δ15N of total soil-N. The NO3− pool of CU soils (11.6±4.5‰) was also significantly (P<0.05) enriched in 15N compared to FU soils (4.7±1.1‰), while the 15N contents of NH4+ pool were not different between both soils. Compost application resulted in 15N enrichment of plants; the δ15N values were 14.6±3.3‰ for CU and 4.1±1.7‰ for FU fields. These results showed that long-term application of compost resulted in a significant 15N-enrichment of soils and plants relative to fertilizer. Therefore, this study suggested that δ15N 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 δ15N guidelines for organic produce, since the δ15N 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 denitrifcation.