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Slow- and controlled-release (SCR) nitrogen (N) fertilizers improve crop production and N utilization efficiency. However, responses of biological soil characteristics to the long-term application of N fertilizer have received little attention. In this work, a five-year field experiment was performed to assess the influence of different N fertilizer treatments (no N fertilizer application [CK] and applications of polymer-sulfur coated urea [PSCU], conventional prilled urea [PU], and urea-formaldehyde [UF]) on soil microbial communities in yellow clayey paddy soils in southern China. The types of N fertilizers had a stronger impact on soil bacterial diversity and community structure than fungal diversity and community structure. The soil microbial communities of PU and UF treatments did not differ from those of CK regarding alpha diversity indices. However, the PSCU treatment significantly decreased the Shannon index by 2.16% for bacteria but had no significant effect on the Shannon index of fungi. This might be due to the application of PSCU significantly reducing the soil pH by 0.16 units compared to CK and leading to soil acidification. Co-occurrence network analysis determined the keystone species mainly belonging to Proteobacteria and Ascomycota. Notably, these keystone species played an important role in PSCU-treated soils. Soil Ca2+, Na+, and NH 4 + were the main determinants of microbial network stability. These results contribute to our understanding of how SCR-N fertilizers impact soil microorganisms. • The application of polymer-sulfur coated urea significantly reduced soil pH. • Nitrogen fertilizer types had more significant effects on soil bacterial diversity. • Nitrogen fertilizer types significantly affected soil bacterial community composition. • Soil nutrient availability played critical role in driving microbial community change. [ABSTRACT FROM AUTHOR]

Many Chinese croplands are suffering from soil acidification, defined as a decrease in soil acid neutralizing capacity (ANC), due to over-application of nitrogen (N) fertilizer and removal of base cations (BCs) in crop harvest. The contribution of fertilization and harvesting on soil acidification rates for different cropping systems is, however, seldom quantified under field conditions. Here, field experiments were conducted on a moderately acid silty clay soil in southwest China to quantify proton (H+) production rates under three fertilization management regimes, including control (non-N), optimized fertilization and farmers' practices (i.e. over fertilization), by measuring the inputs and outputs of major element in three cropping systems, i.e. rice-fallow, rice-wheat and maize-wheat. The maize-wheat cropping system had the highest proton production rates (2.3–10.2 kmol c H+ ha−1 yr−1), followed by rice-fallow (3.2–4.6 kmol c H+ ha−1 yr−1) and rice-wheat (2.2–4.5 kmol c H+ ha−1 yr−1) systems. Crop removal, dominated by straw removal, was the main driver of proton production in all three cropping systems, contributing over 50% of the total H+ production. Compared to the control, N fertilizer application enhanced soil acidification rates among three cropping systems, but had no significant effects on H+ production per kg grain in the rice-fallow and rice-wheat systems. The same was true for the maize-wheat system in case of optimized fertilization, but over fertilization increased both soil acidification rate and H+ production per kg grain, while grain yield was comparable to that with optimized fertilization. The change in soil ANC was dominated by accumulation of SO 4 2− (leading to potential soil acidification) that was mainly added to the soil with superphosphate containing high concentrations of both Ca2+ and SO 4 2− sometimes even causing an increase in the base cation pool, despite BC crop removal. The low contribution of N transformation processes to the total H+ production (3–47%) was due to high denitrification in this specific soil. The soil pH and base saturation did not change significantly among the three fertilization management regimes, largely due to the large BCs pools in the soil. Although the common farmers' practice does not strongly increase proton production in rice-fallow and rice-wheat systems, it still implies a waste of N fertilizer as the excess N input is either enhancing straw production or lost to air in this silty clay soil. Hence, optimized fertilization and localized BCs-rich straw return are suggested to reduce costs of N fertilizer without yield losses and mitigate soil acidification in those systems. [Display omitted] • Soil acidification rates were quantified in three cropping systems in China. • Maize-wheat rotations had higher acidification rates than rice-wheat and rice-fallow. • Optimized N fertilization reduced soil acidification rates without yield loss. • Base cation removal by crop harvesting was the main driver of soil acidification. • Low contribution of N cycle to soil acidification was due to high denitrification. [ABSTRACT FROM AUTHOR]