Effects of long-term biochar addition on the wheat-rotated rice grain yield and the fate of N fertilizer in plant-soil system—A combined 15N study

Agricultural soil amending with biochar is an innovative strategy to modify soil property and improve crop productivity. However, the impacts and mechanisms of long-term successive biochar addition on root growth and leaf photosynthesis performance, and the fate of fertilizer-nitrogen (N) in plant-soil system, have not been well assessed. Hence, a long-term (2015–2023) in-situ field study combined with a micro-plot test (labeled with ¹⁵N fertilizer in 2023) on rice was conducted in the down reaches of Yangtze River Valley of China. Three treatments were established: no N fertilizer (PK), local recommended fertilization (NPK), NPK + crop straw derived biochar addition (NPKB). Relative to NPK, across four growing seasons of rice, NPKB increased root biomass and the ratio of root to shoot by 18.0 % and 8.1 %, respectively. Biochar addition improved photosynthetic capacity, showing higher leaf area index and SPAD value, mainly from tillering to filling, thus prolonged grain filling duration. Results showed that NPKB achieved higher rice yield, total N uptake, and fertilizer N recovery relative to NPK, however, there was no significant ‘year × treatment’ interaction in rice yield and yield components. In addition, NPKB increased N fertilizer retention after rice harvested, result in 19.1 % of the N fertilizer retained in 0–80 cm soil compared with 16.4 % with NPK. Totally, our results suggest that long-term successive biochar addition could ameliorate root growth, while promoting photosynthesis and N uptake, enabling better filling of rice and yield production, and this could be also an effective way to reduce the risk of N fertilizer leaching losses by improving N fertilizer retention and recovery.