Biochar and nitrogen fertilizers are known to increase soil carbon storage and reduce soil nitrogen loss as amendments, suggesting a promising strategy for highly effectively increasing soil productivity. However, few studies have explored the mechanisms of their effects on crop yield in terms of active carbon fraction and enzyme activity, which ultimately limits the potential for the application of biochar in combination with nitrogen fertilizers. To evaluate the effect of biochar and nitrogen fertilizer on the improvement of black soils in northeast China, a field experiment was conducted in the black soil to compare and analyze the application methods on total organic carbon (TOC), total nitrogen (TN), enzyme activities, and maize yields. Biochar rates: CK, C1, C2, and C3 (0, 9.8, 19.6, and 29.4 Mg·ha⁻¹); N fertilizer rates: N1/2 and N (30 and 60 kg·ha⁻¹). Results indicated that biochar and N fertilizer amendments significantly ameliorated soil fertility, such as TOC and TN, compared to the unamended soil. The TOC levels in the C3 treatment increased by 35.18% and the TN levels by 23.95%. The improvement in TN is more significant when biochar is blended with N fertilizer. Biochar blended with N fertilizer increased maize cellulase, urease, and invertase activities by an average of 53.12%, 58.13%, and 16.54%, respectively. Redundancy analysis showed that TOC, TN, and MBN contributed 42%, 16.2%, and 22.2%, respectively, to the maize yield indicator. Principal component analysis showed that reduced N fertilizer was more effective in improving yields, with a maximum yield increase of 50.74%. Biochar blended with N fertilizer is an effective strategy to improve the fertility and productivity of black soils in northeast China, while nitrogen fertilizer reduction is feasible and necessary for maintaining grain yield.

Managing carbon input from crop straw in cropland ecosystems could increase soil organic carbon (SOC) sequestration to achieve C neutrality and mitigate climate change. The complexity of the chemical structures of crop residue largely affects SOC sequestration. Fungi communities play an important role in the degradation of crop residues. However, the relationship between the fungal community composition and the chemical structures of crop residues remains unclear and requires further investigation. Therefore, a 120-day incubation experiment was conducted in Mollisols in Northeast China to investigate the decomposition processes and dynamics of maize straw stem (ST), leaf (LE) and sheath (SH) residues using ¹³C-NMR spectroscopy. Additionally, the microbiomes associated with these residues were analyzed through high-throughput sequencing to explore their relationship. Our results showed that the alkyl C contents in all treatments exhibited increases ranging from 15.1% to 49.1%, while the O-alkyl C contents decreased, ranging from 0.02% to 11.2%, with the incubation time. The A/OA ratios of ST, LE and SH treatments were increased by 23.7%, 43.4% and 49.3% with incubation time, respectively. During the early stages of straw decomposition, Ascomycota dominated, and in the later stage, Basidiomycota were predominant. The class of Sordariomycetes played a key role in the chemistry transformation of straw tissues during decomposition. The keystone taxa abundances, Fusarium_kyushuense, and Striatibotrys_eucylindrospora, showed strong negative correlations with di-O-alkyl C and carbonyl-C content and positive correlations with the β-glucosidase and peroxidase enzyme activity, respectively. In conclusion, our study demonstrated that the keystone taxa play a significant role in regulating the chemical structures of straw tissues, providing a better understanding of the influence of residue quality on SOC sequestration.

Aims: The cattle manure application to subsoil is an efficient and sustainable approach to increase soil fertility. However, the change of soil organic carbon (SOC) chemistry after manure amended in subsoil is remains elusive. Methods: Here, a pot experiment was used to investigated the SOC chemical structure (¹³C-NMR and FTIR), as well as microbial community composition (PLFA), with cattle manure amended in topsoil (T1M), and subsoil (T2M) and without manure in topsoil (T1) and subsoil (T2). Results: The utilization of cattle manure significantly improved SOC, total nitrogen (TN) content and PLFAs biomass, as well as the relative absorption of the band at 2920 cm^-1 and 1640 cm^-1 of SOC, while decreased the C/N ratio by 15.7-50.5%. T1M increased the proporations of O-alkyl C and Aromatic C by 3.8%-17.6% compared to T1, but decreased the proporations of Alkyl C and Carbonyl C by 9.9%-11.5% and the ratio of Alkyl C/O-Alkyl C (A/OA) by 60.0%. However, T2M showed the opposited trend compared to T2. The fungal to bacterial ratio (F/B) of T2 was lower than that of T1, while it was greater in T2M than that in T1M. Both RDA and MRT analyses demonstrated that the Cyanobacteria, Acidobacteria and Ascomycota were negatively association with O-alkyl C, and the Actinobacteria, Verrucomicrobia and Cyanobacteria were negatively association with Alkyl C. Conclusions: Our findings reveal application cattle manure in subsoil was more beneficial to decompose SOC owing the transfromation of bacterial to fungal and made the chemical structure of SOC more stabilization.