Soil and plant nitrogen pools in paddy and upland ecosystems have contrasting δ15N

Waterlogged paddy and water-unsaturated upland ecosystems have contrasting soil nitrogen (N) processes that affect the natural N-15 abundance (N-15/N-14, expressed as delta N-15) in different N pools. In this study, we investigated the delta N-15 patterns in soil and plant N pools of paddy and upland ecosystems. Samples were collected from 20 each of paddy and upland fields at the active growing season and analyzed for N concentration and delta N-15. The higher (P < 0.001) concentration (22.1 mg N kg(-1)) and lower delta N-15 (6.9 aEuro degrees) of NH4 (+) in paddy than in upland soils (6.1 mg N kg(-1) and 9.2 aEuro degrees, respectively) likely reflected the lower nitrification potential in paddy soils. On the other hand, a higher (P < 0.001) delta N-15 of NO3 (-) in paddy (12.7 aEuro degrees) than in upland soils (4.7 aEuro degrees) indicated higher denitrification rates in paddy soils. The positive (P < 0.01) correlation of the delta N-15 of soil organic N with the delta N-15 of NH4 (+) rather than with that of NO3 (-) suggested a tight linkage between organic N and NH4 (+) as a result of immobilization-mineralization turnover of NH4 (+). Therefore, in waterlogged paddy soils where nitrification was restricted, a high rate of microbial immobilization-mineralization turnover might lead to a lower delta N-15 of soil N (total N, organic N, and NH4 (+)) than those for upland. The delta N-15 in plant N pools also reflected the dominant mineral N species in paddy (NH4 (+)) and upland soils (NO3 (-)). We conclude that contrasting soil N processes (particularly nitrification) leave distinct delta N-15 signatures in the soil and plant N pools between paddy and upland fields.