Natural 15N abundance of paddy rice (Oryza sativa L.) grown with synthetic fertilizer, livestock manure compost, and hairy vetch

Nitrogen isotope abundance (?? ¹⁵N) of paddy rice (Oryza sativa L.) grown for 110?days after transplanting (DAT) under field conditions with ammonium sulfate (AS with ?0.4?? as a synthetic fertilizer), pig manure compost (PMC with 15.3?? as a livestock manure compost), and hairy vetch (HV with ?0.5?? as a green manure) was investigated to test the possible use of ?? ¹⁵N technique in discriminating organically grown from conventionally grown rice. At 15 DAT, the ?? ¹⁵N of whole rice decreased (P?<?0.05) in the order of 10.5?? for PMC > 5.5?? for control (without N input) > 4.0?? for HV > 1.8?? for AS. This difference seemed to reflect primarily the ?? ¹⁵N signal of N sources. Although differences in ?? ¹⁵N of rice grown with isotopically distinct N inputs (i.e. PMC vs. AS and PMC vs. HV) became smaller over time, the difference (2.8 and 3.0?? difference at harvest on 110 DAT, respectively) was still significant (P?<?0.05). However, there was no distinguishable difference between AS and HV treatment after 42 DAT. Such effect of N inputs on ?? ¹⁵N of whole rice was also observed for root, shoot, and grain at harvest. Therefore, our study suggests that it is possible to distinguish rice grown with manure composts from that grown with synthetic fertilizers. However, if green manure of preceding N₂-fixing plants is used as the N source, ?? ¹⁵N of rice may not be a good surrogate of N sources.