Biochar (BC) addition to soil is a proposed strategy to enhance soil fertility and crop productivity. However, there is limited knowledge regarding responses of soil respiration and C-cycle enzyme activities to BC and nitrogen (N) additions in a calcareous soil. A 56-day incubation experiment was conducted to investigate the combined effects of BC addition rates (0, 0.5, 1.0, 2.5 and 5.0% by mass) and urea (U) application on soil nutrients, soil respiration and C-cycle enzyme activities in a calcareous soil in the North China Plain. Our results showed soil pH values in both U-only and U plus BC treatments significantly decreased within the first 14 days and then stabilized, and CO₂emission rate in all U plus BC soils decreased exponentially, while there was no significant difference in the contents of soil total organic carbon (TOC), dissolved organic carbon (DOC), total nitrogen (TN), and C/N ratio in each treatment over time. At each incubation time, soil pH, electrical conductivity (EC), TOC, TN, C/N ratio, DOC and cumulative CO₂ emission significantly increased with increasing BC addition rate, while soil potential activities of the four hydrolytic enzymes increased first and then decreased with increasing BC addition rate, with the largest values in the U + 1.0%BC treatment. However, phenol oxidase activity in all U plus BC soils showed a decreasing trend with the increase of BC addition rate. Our results suggest that U plus BC application at a rate of 1% promotes increases in hydrolytic enzymes, does not highly increase C/N and C mineralization, and can improve in soil fertility.

The use of biochar as a soil amendment has substantial potential to enhance soil quality and carbon sequestration. However, the responses to the addition of biochar based on soil microbial residues are not well understood, particularly at the aggregate level. Herein, a two-year field experiment investigated the characteristics of distribution of microbial residues in calcareous fluvo-aquic soil aggregates (SA) in Henan Province, China. Four treatments were established as follows: no fertilizer (CK), chemical fertilizer (NPK), biochar (BC), and biochar combined with chemical fertilizer (NPK + BC). The results showed that the effects of particle size substantially impacted the microbial residues with 2–0.25 mm SA having the largest contents of amino sugars and microbial residual carbon (MRC), followed by >2 mm SA. Compared with the CK, the NPK treatment markedly enhanced the levels of glucosamine (GluN), galactosamine (GalN), muramic acid (MurA), total amino sugar (TAS), and MRC in the 2–0.25 mm SA by 26.69%, 24.0%, 23.62%, 25.11%, and 24.82%, respectively. The NPK + BC treatment significantly increased the contents of GluN, GalN, TAS, and MRC in the bulk soil and 0.25–0.053 mm SA compared with the NPK treatment. Bacterial biomass and the activity of N-acetyl-glucosaminidase in the bulk soil and SA markedly and positively affected the content of carbon in the amino sugars and microbial residues. Overall, the 2–0.25 mm SA were microenvironments with the largest accumulation of soil microbial residues, and the combined application of NPK + BC was more effective at increasing the accumulation of microbial residues in the SA, which provides an ideal fertilization strategy to improve the soil microenvironment and enhance soil quality.

Vicia villosa, a high-quality green manure crop, helps to increase the content of soil phosphorus (P) by returning to the field. Soil P deprivation is a severe constraint on crop productivity that triggers the low P stress response in plants, which is controlled by various transcriptional regulatory network pathways. Identifying key genes from these transcriptional regulatory networks can help in developing low P-tolerant crops. In this study, we performed physiological observations and transcriptome sequencing of seedlings from the two Vicia villosa varieties, Xu Shao 3 and Soviet Vicia villosa, under P starvation conditions. The results showed that the main root length, plant height, shoot dry weight, root acid phosphatase activity, and inorganic P content of Xu Shao 3 were significantly higher than those of Soviet Vicia villosa under low P conditions. Based on transcriptome data analysis, 183 (shoot) + 144 (root) differential genes (DEGs) between the two varieties were identified; 144 (shoot) + 79 (root) were upregulated, and 69 (shoot) + 65 (root) were downregulated. KEGG analysis found that DEGs in shoots were significantly enriched in photosynthesis pathways, such as vitamin B6 and riboflavin metabolism. Meanwhile, DEGs in roots were enriched in plant signal transduction, fatty acid degradation, citric acid cycle, pentose, glucuronic acid conversion, etc. GO enrichment analysis suggested that DEGs in shoots were significantly enriched in biological processes, including cell response to P stress, intracellular ion homeostasis, etc., and molecular functions, including phosphate ester hydrolase, phosphatase, acid phosphatase activity, etc. Furthermore, DEGs associated with low P tolerance included three acid phosphatases, a phosphoesterase, a sulfoquinovosyl diacylglycerol synthase, a phosphoenolpyruvate carboxylase, six phosphate transporters and glycerol-3-phosphate transporters, eight SPX, and two PHL genes. In conclusion, Xu Shao 3 exhibited stronger inorganic P accumulation ability and a lesser effect on growth than Soviet Vicia villosa under low P conditions, which might result from photosynthesis, sugar, and P metabolism differences between the two varieties. Acid phosphatase, phosphoesterase, phosphoenolpyruvate carboxylase, sulfoquinovosyl diacylglycerol synthase, phosphate transporter, glycerol-3-phosphate transporter, and SPX were key DEGs leading to the difference in low P stress tolerance between the two varieties.

Soil labile organic carbon (C) fractions play a key role in agricultural soil fertility. However, the effects of long-term organic substitution regimes on soil organic carbon (SOC), its labile fractions, stability, and vegetable yields as well as the relationships among these factors in the open-field are less well-studied. Hence, the objective of this study was to analyze the effects of long-term organic substitution regimes on SOC sequestration, labile C fractions [particulate organic C (POC), microbial biomass carbon (MBC), dissolved organic C (DOC), and readily oxidizable C (ROC)], SOC stability, the C pool management index (CMI), and vegetable yields in a long-term (13 years) open-field experiment. Five treatments were examined: 100% chemical nitrogen fertilizer (CN), substituting 25% of the chemical N with manure (MN) or straw (SN), and substituting 50% of the chemical N with manure (2MN) or manure plus straw (MSN). Compared to the CN, organic substitution treatments increased the average yields of vegetable, the SOC, the labile C fractions’ contents, and the C pool management index (CMI) to varying degrees, but only MSN reached significant levels for these factors. However, the MSN treatment had a significantly lower C stability index (SI) than the CN. ¹³C-NMR analyses also confirmed that organic substitution treatments increased the proportion of O-alkyl C and the OA/A, but reduced SOC stability. Pearson correlation analysis and the partial least squares path model indicated that labile C fractions were the mainly direct contributors to yield and SOC stability. Overall, substituting 50% of the chemical N with manure plus straw is a relatively ideal fertilization practice to improve vegetable yields and enhance C activity in an open field.