Nitrification is the key process determining N use efficiency in paddy soils

Nitrogen (N) fertilizer use efficiency (NUE) in flooded paddy fields is relatively low. Many N fertilizer management options have been proposed to enhance NUE and minimize environmental damage. However, few investigations are focusing on the role of the characteristics of soil N transformations in regulating NUE and N losses in paddy fields. In this study, we test the role of soil N transformations on NUE and N losses under rice growth conditions in two paddy soils collected from Jiangxi (JX) and Sichuan (SC) in China. The N recoveries of applied ¹⁵N either as nitrate or ammonium in plant and soil, and N losses estimated by ¹⁵N balance were investigated in rice pot experiments using a ¹⁵N tracing technique. The results showed that gross nitrification rates in soil collected from JX were much lower than those in soils collected from SC either at 60% water holding capacity (WHC) or rice growth (flooding) conditions, which could be due to the difference in soil pH. The ‐N concentration in soil solution was maintained at a relatively high level for a long time period after N fertilizer application in the JX soil (41 d) compared to the SC soil (26 d), caused by different nitrification rates owing to different soil pH. The ¹⁵N uptake by rice in the JX soil (29–78%) was always significantly higher than that in the SC soil (22–54%), while N losses from the plant–soil system in the JX soil (17–21%) were always significantly lower than those from the SC soil (20–34%) at the same rice growth stage in the labeled ¹⁵N ammonium treatment. However, there were no significant differences in ¹⁵N uptake by rice and N losses in applied treatment between the two studied soils. These results indicate that nitrification, not denitrification, was the key process determining NUE and N losses in paddy soils. The results of the N application gradient experiment also indicated that higher amounts of N fertilizer should be applied for the same amount of N uptake, however, this caused higher N losses, in soils characterized by high nitrification rate (e.g ., the alkaline soil). Results highlighted that soil N transformations in particular nitrification rate provided a very good guideline for an optimized N management.