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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]

Dissolved organic matter (DOM) plays an important role in soil biological activity and transport of pollutants and nutrients in soils, but very little information is available with regard to the long-term impact of agricultural management practices on the dynamics and fate of DOM in acidic soils. The seasonal and inter-annual variation of dissolved organic carbon (DOC) and nitrogen (DON) contents and leaching were investigated in an acidic clay soil (Ferric Acrisol) by a long-term field lysimeter experiment in subtropical China. The experiment was conducted from 2002 to 2010 with 4 fertilization treatments under maize monoculture: no manure (CK), low-rate manure with 150 kg N ha −1 y −1 (LM), high-rate manure with 600 kg N ha −1 y −1 (HM), and high-rate manure with 600 kg N ha −1 y −1 and lime at 3000 kg Ca(OH) 2 ha −1 3 y −1 (HML). Manure application resulted in a seasonal variation of soil DOC and DON, and significant effects were observed by manure DOC, microbial biomass and soil water content. Soil DOC, which was mainly determined by soil organic matter and soil water content, increased yearly until the seventh year when it was stabilized. Manure application on acidic clay soil did not alter DOC leaching, whereas DON leaching clearly increased after three years of high manure application of 600 kg N ha –1 y –1 . The average annual DON leaching losses under long-term manure application had a range of 3.8–5.4 kg ha –1 , accounting for 6–11% of total nitrogen leached. The addition of lime, combined with manure application, produced no impact on soil dynamics and leaching of DOC and DON, with the exception of increasing emission of CO 2 . [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]

• Manuring affects positively soil structure and porosity. • Climate plays an important role in visual indicators' score. • LOC content correlates negatively with silt content and positively with clay. • SOM correlates positively with clay for soils of the control group. • Strong positive correlation between LOC and SOM for soils of the control group. A study was carried out to assess if the visual soil assessment method (VSA) would allow recognizing differences between soils receiving organic matter (OM) amendments and similar control soils, by the observation of visual soil quality indicators' score. 36 practices were identified across 8 pedoclimatic zones. These fields/plots were paired with nearby control fields/plots, without OM amendments, sharing similar farming features. A survey, comprising a VSA of the soil structure status, surface ponding, signs of erosion, earthworm counts and soil stability (slake test), complemented by measurements of soil organic matter (SOM) and permanganate oxidizable organic carbon (LOC) content, soil pH, penetration resistance and texture, on soils of both management system groups (OM addition and Control), was performed in 2016. Correlations of the visual soil quality indicators' score with SOM, LOC, other soil properties and climate variables and indices were calculated within each group; the correlations between soil properties, and between soil properties (SOM and LOC) and climate variables were also calculated. A statistically significantly higher proportion of soils of the OM group had a good score for "soil structure and consistency" and "soil porosity". These differences are not directly explained by non-inherent soil properties. No statistically significant Spearman's correlation coefficients were observed between "soil structure and consistency" and either soil properties or climate variables; concerning "soil porosity", distinct statistically significant correlations were observed between the two groups with different climate variables and indices. Correlations between the scores of the visual soil quality indicators and climate variables were found to follow the same directions of correlations of LOC content with the same climate variables, although the latter correlations were weak. Mean SOM and LOC content, were slightly higher in the OM group, although differences were not statistically significant. A high linear correlation between LOC (mg/g) and SOM (%) (r = 0.65, n = 26) exists within the Control group, but not within the OM group (r = 0.20 and n = 26). When the relationship of SOM and LOC content with visual soil quality indicators' score was studied, statistically significant correlations were only observed between SOM and "earthworm count" within the Control group (r s = 0.44), and between LOC and "soil colour" within the OM group (r s =-0.52). Both LOC content and LOC status (ranked as a function of LOC content and soil texture), had only negative statistically significant correlations with visual soil quality indicators' score , questioning their worth as stand-alone soil quality indicators. [ABSTRACT FROM AUTHOR]