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 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.