Modern intensive agricultural practices, particularly the use of nitrogen fertilizers, have accelerated soil acidification on a global scale. The soil pH buffering capacity (pHBC) is often used to quantify the soil acidification rate. Calcareous soils have relatively higher pH and pHBC, reflecting the presence of carbonate minerals; however, the impact of long-term fertilization treatment on pH and pHBC is poorly understood for calcareous soils. Here, calcareous soil samples (0–20 cm) were collected from fields receiving six different fertilization treatments for 22 years: control (CK, unfertilized but planted); nitrogen (N); nitrogen and phosphorus (NP); nitrogen, phosphorus and potassium (NPK); combined manure and NPK (NPKM); and combined corn-stover and NPK (NPKS). Both pH and pHBC significantly decreased for all treatments relative to CK. NPKS treatment had the lowest soil pH. Compared with CK, the soil pHBC decreased 5.7 to 17.3% under different treatments. The calcium carbonate (CaCO₃) content was significantly reduced by fertilization treatments, with a maximum decrease under the NPKS treatment. Structural equation model (SEM) analysis revealed that calcium carbonate and soil organic matter (SOM) made important contributions to effective cation exchangeable capacity (ECEC). Soil pHBC was directly controlled by ECEC, while CaCO₃ and SOM indirectly contributed to the pHBC through ECEC. These results indicated that NPKS treatment induces more severe soil acidification, reflecting the higher H⁺ input and lower pHBC under this treatment.

Soil texture, coupled with environmental factors, plays an important role in soil nutrient conservation. In order to evaluate the effect of soil texture on soil fertility of fluvo-aquic soil, a long-term experiment that included four fluvo-aquic soils with different textures (sand, sandy loam, sandy clay loam and loamy clay) was established in 1990. In 2017, soil samples were collected from the 0–20 cm depth to measure soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), alkali-hydrolysable nitrogen (AN), available phosphorus (AP) and available potassium (AK) concentrations, water-stable aggregate distribution, and aggregate-associated carbon and nitrogen. After 27 years of the experiment, the sand content decreased and the silt and clay content increased in sand and sandy loam, while the sand content increased and the silt content decreased in loamy clay. The SOC, total and available N, P and K (except TK) increased more in coarse-textured soils than in finely textured soils, while the concentrations of SOC, TN, TP, AN and AK increased significantly with a decrease in the sand content. The SOC, TN, AN and AK concentrations in loamy clay were 2 to 3 times higher than those in the sand treatment. The mass percentage of each aggregate fraction, especially the content of macroaggregates, increased significantly with a decrease in the sand content. Soil aggregates play an important role in SOC and TN storage. 75–84% of SOC and 67–81% of TN were stored in macroaggregates and microaggregates. It was concluded that soil particle composition was affected by long-term cultivation. The finely textured soils had higher SOC, TN and TP storage capacity and provided more AN and AK. The formation and stability of aggregate structures were limited by the high content of sand particles, which led to a negative influence on the storage and supply of SOC and N in the fluvo-aquic soils.

Introduction: Crop residue represents the largest input of organic carbon in agricultural ecosystems and its decomposition is fundamentally mediated by soil microbial communities. However, the mechanism of N fertilization regulating decomposition of the plant residue especially the associated key microbial taxa remain unclear. Methods: To address this gap, we conducted a 100-day field decomposition experiment using the litterbag method to track temporal shifts in straw physicochemical properties and associated microbial communities under three N regimes: no nitrogen (N0), 200 kg N ha ^-1 (N200), and 300 kg N ha ^-1 (N300). Results and discussions: Results showed that nitrogen addition significantly accelerated the decomposition of wheat straw, increasing mass loss and the degradation rates of cellulose, hemicellulose, and lignin relative to N0 treatment. Enzyme activities linked to carbon acquisition, including α-glucosidase (AG), β-glucosidase (BG), cellobiohyrolase (CBH), and β-xylosidase (XYL), were consistently elevated under N-amended treatments during mid- to late-stage decomposition. Similarly, activities of N-acquiring enzymes (β-N-acetyl-glucosaminidase, NAG; leucine aminopeptidase, LAP) and oxidative enzymes (polyphenol oxidase, PPO; laccase) were significantly enhanced, particularly after Day 14. Microbial community succession was tightly coupled with decomposition progression. Random forest modeling identified key bacterial biomarkers (e.g., Terribacillus , Bacillus , Solibacillus , Oceanobacillus , and Cellulosimicrobium ) and fungal biomarkers (e.g., Neocosmospora , Actinomucor , Fusarium , Chaetomium , and Aspergillus ), all of which are known for their capacity to degrade lignocellulosic and recalcitrant substances. Variation partitioning revealed that straw properties, especially the C/N ratio, TN content, and CBH activity, collectively explained the majority of microbial community variation. These findings support a mechanistic pathway in which nitrogen fertilization reduces residue C/N, thereby reshaping microbial community composition and stimulating enzyme production, which in turn accelerates decomposition. Our study provides novel insights into how nitrogen management influences the coupling of microbial ecology and biogeochemical cycling during straw decomposition, with direct implications for optimizing N fertilization management and sustaining soil fertility in agroecosystems.

Partial substitution of synthetic nitrogen fertilizer with manure (organic substitution) is highly recommended to minimize environmental risks without compromising crop productivity in intensive agricultural systems. However, our understanding of the effect of organic substitution on soil organic carbon (OC) and total nitrogen (TN) in deep soil and its impact on crop productivity remains limited. Here we investigated OC and TN changes in soil profile down to 100 cm, crop yield, and sustainable yield index under synthetic nitrogen, phosphate, and potassium fertilizers (NPK), NPK plus straw (NPKS), and organic substitution (NPKM) treatments over two decades in four upland fields across different climate zones. Compared with the initial values, two decades of NPKM treatment significantly (p < 0.05) increased OC and TN stocks in either topsoil (by 25.6–103.8 and 15.8–89.8%) or deep soil (by 2.9–71.3 and 5.7–36.9%), respectively, across all sites. The increases in OC and TN stocks in 0–100 cm soil receiving NPKM were significantly higher than those receiving NPK at all sites and NPKS at three high-evaporation sites. Compared with NPKS and NPK treatments, crop yield and N uptake were significantly increased under NPKM treatment only at the Qiyang site. Furthermore, OC sequestration in the entire soil profile down to 100 cm and TN accumulation in topsoil exhibited significant positive correlations with crop N uptake, relative crop yield, and sustainable yield index. In conclusion, long-term partial substitution of synthetic N fertilizer with manure facilitates soil OC and TN sequestration in the entire 100 cm profile and thus maintains high crop productivity in upland areas.