The properties of soil organic carbon (SOC) required for carbon sequestration and nutrient availability are contradictory, and the changes in SOC caused by agricultural land use changes remain elusive. Data on the total soil organic carbon (TOC) and labile organic carbon, including easily oxidizable organic carbon (EOC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC), of the soil profile were analysed for four typical agricultural land use scenarios in the Chengdu Plain, China. The impacts of agricultural land use changes on sequestration and nutrient availability of SOC were assessed in this urban agricultural area using the space‐for‐time substitution method. Conversion of land use from a traditional agricultural rotation (rice‐wheat/rapeseed rotation) to afforestation increased the MBC content and decreased the contents of EOC, DOC, and TOC due to the lower input of organic matter, improved aeration of the soil profile, and growth of aboveground biomass. Conversion of a traditional rotation to a rice–garlic rotation resulted in a significant increase in topsoil TOC, slight but insignificant decreases in subsoil TOC, and clear increases in labile organic carbon because of rice planting, rice straw mulch, and reasonable application of chemical fertilizers. In contrast, the conversion of a traditional rotation to a rice–leafy vegetable rotation decreased MBC due to the excessive use of chemical fertilizers that consequently increased EOC, DOC, and TOC. We conclude that afforestation on paddy soil has negative consequences for soil carbon sequestration and a rice–leafy vegetable rotation contributes to carbon sequestration but is detrimental to soil fertility. In addition, the MBC ratio in soil could be the optimal indicator for assessing SOC stability and soil fertility, and more attention should be paid to subsoil carbon changes.

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Background: Agricultural intensification induces prolonged impacts on soil acidification (SA), whereas linking the mechanisms therein to structural and functional evolution of soil microbiota remain unclear, especially over longer time scales. Methods: To investigate the short- and long-term effects of agricultural intensification on soil microbiome and its association with SA severity, we performed metagenomic sequencing of soil samples from typical rice-vegetable rotations for 0 (control, V0), 10 (V10), 20 (V20) years. The objectives were to clarify biomarkers that indicative of SA and its putative functions of acid production. Results: Rice-vegetable rotations for 0, 10, 20 years yielded three well-defined soil clusters with differential acid saturation (V0: 4.5%; V10: 11.8%; V20: 40.7%). Soil pH declined significantly (p < 0.05) from the 6.38 (V0) to 4.82 (V20). Acid cations (H + , Al 3+ ) dominating V20’s exchangeable cation pool, suggesting that soils in V20 were highly acidic and acid-sensitive, which recruited distinct microbiomes. The increased acid cations and NO 3 - -N concentrations are driving forces that lead to a higher abundance of Rhodanobacter , Gemmatirosa , Sphingomonas and Streptomyces in V20 samples, which acting as aciduric biomarkers that significantly and positively correlated with soil acidity. Furthermore, functional modules of microbial genes revealed V20 samples contained more genes associated with“oxidative phosphorylation” and “two-component system”, particularly enriched multiple pathways of cytoplasmic pH homeostasis. More specifically, we found increased genetic abundances of active efflux of protons and cytoplasmic proton consumption in V20 samples over than in V0, which are both involved in the microbial dehydrogenation and acid production, and favoured the growth of Rhodanbacter , Gemmatirosa , Sphingomonas , Streptomyces . Conclusions: Collectively, the identified aciduric biomarkers was closely associated with SA severity in intensive agroecosystems. An overall increase with the ongoing of intensive cropping history in the abundance of indicative genes associated with dehydrogenation and acid production pathways. The differential community compositions provide new clues to elucidate the interaction between the soil microbiome and SA severity, and varying dehydrogenation and acid production pathways provide novel insights into the exacerbation of SA. Keywords: Agricultural intensification, Soil acidification, Metagenomics, Soil microbiome, pH homeostasis