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The temperature sensitivity (Q10) of soil organic C (SOC) decomposition is an important parameter to predict C dynamics under climate change. Given that SOC is mainly protected by aggregates and minerals, differentiating the Q10 of the two C fractions helps to explain bulk soil C dynamics. In the present study, we collected agricultural soils from adjacent paddy and upland areas in mid‐temperate (Mollisols) and subtropic (Ultisols) regions of China. We employed density fractionation to separate aggregate‐protected and free mineral‐associated C fractions of soil samples and determined the Q10 of SOC and the two C fractions at 15 and 25°C incubated conditions. Results showed that the Q10 of SOC for Mollisols were lower than that for Ultisols, with an exception of aggregates in upland soils. Aggregate‐protected C had lower Q10 than free mineral‐associated C, except in the upland Mollisols. The Q10 of SOC was negatively correlated with the proportion of C protected in aggregates, whereas it was positively correlated with the proportions of mass or C of free minerals. Given that the mass and C proportion of aggregates in bulk soils of Mollisols were 271% and 80% higher than of Ultisols, respectively, the SOC of Mollisols exhibited lower Q10 than Ultisols. Therefore, stronger soil aggregation and higher proportion of aggregate‐protected C contributed to the lower temperature sensitivity of SOC in Mollisols. Consequently, agricultural practices aimed at promoting soil aggregation will alleviate SOC loss under future global warming scenarios. [ABSTRACT FROM AUTHOR]

Free‐living N2 fixation (FNF) is an important source of bioavailable nitrogen for forests. Though increasing plant species diversity (PSD) benefits soil nitrogen accumulation, how it impacts FNF rate has not been explored.Forty‐five plots covering a PSD gradient were selected in a subtropical karst forest, southwest China. FNF rates in leaf litter and soil were measured using acetylene reduction assay calibrated with a 15N2 fixation method. Diazotrophic community, litter and soil properties were determined as well.The nifH gene abundance was not significantly altered in litter but was significantly decreased in soil by higher PSD. Diazotrophic community's Shannon diversity was significantly increased by higher PSD in both litter and soil. Increasing PSD inhibited litter FNF rate via aggravating phosphorus limitation and decreasing the relative abundance of Betaproteobacteria and Deltaproteobacteria. For the mineral soil horizon, increasing PSD suppressed FNF rate via decreasing nifH gene abundance caused by elevated N availability and lowered phosphorus and iron availability or through decreasing the relative abundance of Deltaproteobacteria but increasing that of Betaproteobacteria.Our findings, for the first time, reveal the microbial and abiotic controls on FNF in litter and soil in response to PSD, and hence benefit the prediction of nitrogen input via biological N2 fixation under changes in PSD. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Bacteria, fungi, and protist are important in driving nutrient cycling. The microbiological mechanism underlying vegetation restoration during different temperature and precipitation gradients remains unclear. To determine the relationship between microbes and nutrients, we examined the changes in soil bacterial, fungal, and protist diversity and community composition under two different temperatures in a plantation forest and shrubland as well as reference cropland in a karst region. Bacterial and protist diversity in the warm and low‐altitude region of Guangxi (average temperature 20.9°C) was higher in cropland than in shrubland or plantation forest. By contrast, fungal richness was lower in cropland than in shrubland. A co‐occurrence network revealed higher numbers of correlated links among the bacterial, fungal, and protist taxa in the cool and high‐altitude region of Guizhou (average temperature 14.6°C) than in the warm and low‐altitude region of Guangxi. Stronger interactions were observed among microbial taxa under cropland than in vegetation restoration. The protist groups Cercozoa and Lobosa showed the highest numbers of links with the bacterial phyla Acidobacteria and Proteobacteria and with the fungal phylum Ascomycota. The numbers of Proteobacteria, Ascomycota, and Cercozoa were most correlated with soil nutrient levels of carbon, nitrogen, and phosphorus. Overall, microbial interactions were higher at low temperatures than at high temperatures, consequently intensifying the predation of bacteria and fungi by protists at a low‐temperature level. Furthermore, Proteobacteria, Ascomycota, and Cercozoa were the keystone taxa linked to nutrient availability; hence, effective monitoring of these dominant groups may be beneficial for increasing nutrient accumulation during vegetation restoration. [ABSTRACT FROM AUTHOR]

Soil ammonia oxidizers play a critical role in nitrogen cycling and ecological restoration. The composition and structure of soil ammonia oxidizers and their impacting factors were studied in four typical ecosystem soils, tussock (T), shrub (S), secondary forest (SF), and primary forest (PF), during vegetation restoration in the Karst region of Southwest China. The composition and structure of the ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) communities were characterized by sequencing the amoA and arch- amoA genes, respectively. The diversity of soil ammonia oxidizers (except in S) and plant Shannon diversity index gradually increased with vegetation restoration, and the ammonia oxidizer communities differed significantly ( p < 0.001). Amplicons of AOA from the Nitrososphaera cluster dominated all four ecosystem soils. AOB Nitrosospira cluster 3b only appeared in PF and SF soils, while Nitrosospira cluster 3a species were found in all soils. Changes in AOB paralleled the changes in soil ammonium content that occurred with vegetation restoration. Redundancy analysis showed that the distribution of dominant AOB species was linked to pH, soil urease activity, and soil C/N ratio, whereas the distribution of dominant AOA species was mainly influenced by litter nitrogen content and C/N ratio. These results suggested that the composition and structure of the AOB community were more sensitive to changes in vegetation and soil ammonium content, and may be an important indicator of nitrogen availability in Karst ecosystem soils. [ABSTRACT FROM AUTHOR]