Biochar application is widely recognized as an effective approach for increasing soil organic carbon (SOC) and mitigating climate change in agroecosystems. However, the effects of biochar application on net accumulations and relative contributions of different SOC sources remain unclear. Here, we explored the effects of biochar application on plant‐derived (PDC) and microbial necromass C (MNC) in a 10‐year experimental rice–wheat rotation field receiving four different intensities of biochar application (0, 2.25, 11.5, and 22.5 t ha⁻¹ for each crop season), using phospholipid fatty acids (PLFAs), lignin phenols and amino sugars as biomarkers of microbial biomass, PDC and MNC, respectively. Our results showed that biochar application increased SOC content and stock by 32.6%–203% and 26.4%–145%, respectively. Higher biochar application (11.5 and 22.5 t ha⁻¹) increased soil pH, total nitrogen (TN), total phosphorus (TP), SOC/TN, and root biomass. In addition, higher biochar application enhanced bacterial, fungal, and total microbial biomass. Plant lignin phenols and MNC contents significantly increased, whereas their contributions to SOC significantly decreased with the increase in biochar application rates due to the disproportionate increase in PDC and MNC, and SOC. Fungal necromass had a greater contribution to SOC than bacterial necromass. The fungal/bacterial necromass decreased from 2.56 to 2.26 with increasing biochar application rates, because of the higher abundances of bacteria than that of fungi as indicated by PLFAs under higher biochar application rates. Random forest analyses revealed that pH, TP, and SOC/TN were the main factors controlling plant lignin and MNC accumulation. Structural equation modeling revealed that biochar application increased lignin phenols by stimulating root biomass, whereas enhanced MNC accumulation was primarily from increased microbial biomass and lignin phenols. Overall, our findings suggest that biochar application increases the accumulation of the two SOC sources but decreases their contributions to SOC in paddy soils.

Soil acidity is a crop production problem of increasing concern in acid red soil. The potential of biochar as a soil amendment/for soil acid management in agricultural fields is a recently recognized yet underutilized technology. Related evidence is currently limited to short-term indoor experiments with one-time BC applications and no crop cultivation, yet the degree to which soil acidity may be impacted by the biochar aging process on long-time scale remains unclear. To evaluate the effects of successive seasonal applications of rice straw-derived biochar (BC) on acidity and fertility of soil, a five-year outdoor column trial was conducted using wheat-millet rotated acidic upland soils from the south of China. BC was applied to the top 0–15 cm of soil at the rates of 0 (BC0), 2.25 (BCL), and 22.5 (BCM) Mg ha⁻¹ with an identical dose of NPK fertilizers at the beginning of each crop season. Our results showed that the wheat-millet biomass yield gradually decreased over five rotation cycles in BC0 without BC application. In contrast, after five rotations, BCM led to an increase in the total wheat/millet grain yield by 138%, and the straw yield increased by 253% compared to the control. The cumulative above-ground nutrient uptake of P, K, Ca, Na, and Mg in BCM also increased by 139%, 171%, 129%, 182%, and 71%, respectively, compared to that in the control. This positive effect was attributed to the increase in soil pH (3.29 units), cation exchange capacity (5.66 cmol kg⁻¹), soil available P (241%), K (513%), Ca (245%), Mg (265%), exchange base (3.36 cmol kg⁻¹), base saturation percentage (65.7%), and decrease in the exchangeable acidity, especially exchangeable Al³⁺ content (<0.1 cmol kg⁻¹). Our results demonstrated that rice straw-derived BC application to soil at 22.5 t ha⁻¹ was found to be highly consistent in decreasing soil acidity and reducing soluble and exchangeable Al³⁺, indicating its higher ameliorating capacity in the south of China in the long run.

Soil nitrogen loss through NH₃ volatilization and N₂O emissions is a crucial issue in soil ecosystems. In this study, we explored the effects of biochar and the nitrification inhibitor DMPP (dimethyl-phenyl-piperazinium, a nitrification inhibitor) in vegetable soils under 60 and 200% WHC (water holding capacity). Five treatments were set: CK (control), urea (N), urea + biochar (N + C), urea + nitrification inhibitor (N + DMPP), and urea + nitrification inhibitor + biochar (N + C + DMPP). Results found that biochar promoted soil nitrification and ammonia volatilization under both moisture conditions, with higher NH₃ rate accumulation at 200% WHC. DMPP maintained high NH₄⁺-N concentration and increased soil ammonia volatilization, but effectively reduced N₂O emissions, especially at 200% WHC. The N + C + DMPP treatment further significantly decreased N₂O cumulative emissions compared to N + DMPP. QPCR results showed that N + C treatment significantly increased AOB (ammonia-oxidizing bacteria) copies compared to N treatment. Applying DMPP alone or with biochar reduced AOB copies by 50.0 and 45.7%, respectively. Soil ammonia-oxidizing archaea (AOA) responded oppositely to DMPP; AOA amounts in N + DMPP and N + C + DMPP treatments increased significantly during the culture. At 60% WHC, the greenhouse effect potential of N + DMPP and N + C + DMPP treatments were 39.0 and 43.2% lower than N, respectively. At 200% WHC, their GWP were decreased by 13.8 and 0.08% compared to N. Adding biochar alone increased the soil’s greenhouse potential at both water contents. In conclusion, using nitrification inhibitors alone or in combination with biochar is more effective in reducing the greenhouse effect potential of soil active nitrogen emissions.