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

Abstract

Waterlogged paddy and water-unsaturated upland ecosystems have contrasting soil nitrogen (N) processes that affect the natural 15N abundance (15N/14N, expressed as δ15N) in different N pools. In this study, we investigated the δ15N 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 δ15N. The higher (P < 0.001) concentration (22.1 mg N kg−1) and lower δ15N (6.9 ‰) of NH4 + in paddy than in upland soils (6.1 mg N kg−1 and 9.2 ‰, respectively) likely reflected the lower nitrification potential in paddy soils. On the other hand, a higher (P < 0.001) δ15N of NO3 − in paddy (12.7 ‰) than in upland soils (4.7 ‰) indicated higher denitrification rates in paddy soils. The positive (P < 0.01) correlation of the δ15N of soil organic N with the δ15N 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 δ15N of soil N (total N, organic N, and NH4 +) than those for upland. The δ15N 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 δ15N signatures in the soil and plant N pools between paddy and upland fields.