Effects of wheat straw addition on dynamics and fate of nitrogen applied to paddy soils

The effectiveness of plant residue addition to increase crop nitrogen (N) use efficiency while reducing N losses is dependent on whether residue management can improve the match between N immobilization-mineralization turnover and plant demand. To understand the effects of residue management on N cycle processes, we conducted a field study that included three treatments. These were a control without N fertilizer (CK), 15N-labelled urea (UR), and 15N-labelled urea + wheat straw (URS). Nitrogen immobilized in the top soil layer (0-20 cm) in the urea only treatment tended to decrease after the basal application and the second top dressing, but increased after the first top dressing, whereas the urea + straw treatment increased immobilization up to the second top dressing, before decreasing towards the end of the study. The addition of wheat straw led to a 24% reduction in ammonia loss after the basal application, corresponding well to lower soil extractable ammonium contents. The N uptake by the plant in the urea + straw treatment corresponded to 25% of applied N, which was higher than the 20% recorded in the urea only treatment. At harvest, 31% of the applied N was found in the 0-60 cm soil layer after the addition of wheat straw, whereas there was only 25% without straw addition. The results showed that 45% applied N was unaccounted for when wheat straw was added, compared to 55% unaccounted for without wheat straw. These indicate that the addition of wheat straw may increase plant N uptake by contributing to N immobilization early in the season, but then improving its subsequent mineralization later in the season.