Global climate warming is characterized by diurnal and seasonal asymmetry, with greater increases at nighttime and in winter and spring.  Growing evidence has recognized that night-warming in winter and spring significantly impacts winter wheat production.  Pre-crop straw returning is the principal method for straw utilization, but the interactions between straw returning and night-warming on wheat yield and N use efficiency (NUE) remain unclear.  Here, a consecutive three-year field experiment with two straw treatments (S0, straw removal; S1, straw returning) and two warming treatments (W0, no warming control; W1, night-warming) found that both S1 and W1 improved wheat grain yield and NUE, with W1 exhibiting more pronounced improvements.  Notably, the interaction between S1 and W1 (S1W1) further enhanced yield and NUE by 13.0 and 16.5%, respectively, compared to S0W0 through increasing grain number and 1,000-grain weight (three-year average).  Additionally, root growth and topsoil inorganic N content decreased in S1 before jointing, thereby reducing plant dry matter and N accumulation.  However, W1 exhibited an opposite trend, thereby mitigating these negative effects.  Simultaneously, under S1W1, increased N translocation to grain and post-anthesis dry matter accumulation, driven by greater N distribution to leaves and higher N metabolism enzyme activity, enhanced both yield and NUE.  This improvement was supported by better root morphology and biomass, particularly in the 0–40 cm soil layer, boosting plant N absorption.  Additionally, elevated soil N-acquiring enzyme activity after jointing increased the net N mineralization rate and microbial biomass N, enhancing soil N-supply capacity.  As a result, post-jointing inorganic N content rose in the 0–20 cm layer while decreasing at 20–60 cm, thus reducing the apparent N surplus.  Collectively, straw returning, night-warming, and their interactions enhanced root distribution and N-supply capacity after jointing in the topsoil layer, thereby increasing plant N uptake and its translocation to grains, along with post-anthesis dry matter accumulation, ultimately improving grain yield and NUE.

Under accelerating climate change, elevated atmospheric CO₂ (e[CO₂]) presents a substantial challenge to nitrogen (N) cycling in agricultural systems. This study elucidated the mechanistic role of biochar in regulating soil N transformations and plant N acquisition under e[CO₂] conditions through a three-year pot experiment (2019-2022) with wheat. Using ¹⁵N isotope tracing combined with metagenomic sequencing, we examined the interactions between two CO₂ concentrations (a[CO₂] 400 μmol mol⁻¹ vs. e[CO₂] 600 μmol mol⁻¹) and 2% (w/w) biochar amendment. Our results demonstrated that under e[CO₂], biochar application reduced the incorporation of fertilizer-derived N into the recalcitrant heavy fraction organic N (HFON) by 39.4%, while enhancing the content of native soil-derived N in the light fraction N (LFON) by 34.0%. Concurrently, biochar promoted the formation of micro-aggregates (<0.25 mm) and particulate organic N (PON) by 37.3 and 13.2%, respectively. Metagenomic analysis revealed that biochar under e[CO₂] suppressed the relative abundance of key N-cycling genes (involved in assimilation, nitrification, and nitrate reduction) that were upregulated under a[CO₂] condition. These physicochemical processes, coupled with microbial modulation, resulted in a 52.6% reduction in soil NO₃^--N accumulation and a significant increase in aboveground N uptake. Structural equation modeling indicated that biochar counteracted the adverse effects of e[CO₂] on micro-aggregate stability and N-cycling gene abundance. Synergistically, biochar enhanced the uptake of fertilizer-N and native soil-N by 30.8% and 111.4%, respectively, under e[CO₂], leading to a 55.4% increase in grain N accumulation. Our findings demonstrate that biochar is an effective amendment for mitigating e[CO₂]-induced N limitation by redistributing N from recalcitrant to labile pools, enhancing N bioavailability, and ultimately supporting crop productivity in future CO₂-enriched agroecosystems.