Synergistic effects of slow-release fertilizer and biochar on crop yield and soil carbon-nitrogen dynamics under reduced nitrogen input

Reducing fertilizer input while improving crop yield and enhancing soil carbon and nitrogen storage in agricultural ecosystems remains a serious challenge. However, it remains unclear whether the addition of exogenous additives with various functions can enhance soil carbon (C) and nitrogen (N) stocks following nitrogen fertilizer reduction. Here, we report results from a wheat-maize rotation system with multiple cropping green manure designed to understand the response mechanism of soil C and N storage to slow-release fertilizer (SRF) and soil amendments (biochar and attapulgite) combination in two nitrogen level agricultural systems (no nitrogen reduction: NNR, and 30 % nitrogen reduction: NR30). We found that SRF and biochar combination significantly increased crop yield, soil nutrients (ammonium nitrogen and nitrate nitrogen), soil microbial biomass carbon (MBC) and nitrogen (MBN) in both systems, and the increase magnitude was notably greater in the NR30 system (+19.5 % for yields, +60 %～85 % for nutrients, and +18 %∼44 % for MBC and MBN) compared with the NNR system (+17.0 % for yields, +31 %∼35 % for nutrients, and +16 %∼40 % for MBC and MBN) by inducing soil negative priming effects and ensuring the continuous nitrogen supply. The combination of SRF and biochar could reduce nitrogen input by at least 30 % while still increasing soil C (1.98 %∼7.90 %) and N storage (49.83 %∼56.61 %), which attributed to the dual control mechanism that nitrogen reduction alleviated the 'nitrogen repression' of green manure and biochar enhanced the sensitivity of soil nitrogen-related extracellular enzyme activities to changes in MBN under NR30 system. This finding offers new insights for sustainable dryland agriculture.