Soil bacteria play a key role in the ecological and evolutionary responses of agricultural ecosystems. Domestic herbivore grazing is known to influence soil bacterial community. However, the effects of grazing and its major driving factors on soil bacterial community remain unknown for different plant community compositions under increasing grazing intensity. Thus, to investigate soil bacterial community diversity under five plant community compositions (Grass; Leymus chinensis; Forb; L. chinensis & Forb; and Legume), we performed a four-year field experiment with different grazing intensity treatments (no grazing; light grazing, 4 sheep·ha−1; and heavy grazing, 6 sheep·ha−1) in a grassland in China. Total DNA was obtained from soil samples collected from the plots in August, and polymerase chain reaction (PCR) analysis and denaturing gradient gel electrophoresis (DGGE) fingerprinting were used to investigate soil bacterial community. The results showed that light grazing significantly increased indices of soil bacterial community diversity for the Forb and Legume groups but not the Grass and L. chinensis groups. Heavy grazing significantly reduced these soil bacterial diversity indices, except for the Pielou evenness index in the Legume group. Further analyses revealed that the soil N/P ratio, electrical conductivity (EC), total nitrogen (TN) and pH were the major environmental factors affecting the soil bacterial community. Our study suggests that the soil bacterial community diversity was influenced by grazing intensity and plant community composition in a meadow steppe. The present study provides a baseline assessment of the soil bacterial community diversity in a temperate meadow steppe.

According to the ‘novel weapons hypothesis’, invasive success depends on harmful plant biochemicals, including allelopathic antimicrobial roots exudate that directly inhibit plant growth and soil microbial activity. However, the combination of direct and soil-mediated impacts of invasive plants via allelopathy remains poorly understood. Here, we addressed the allelopathic effects of an invasive plant species (Rhus typhina) on a cultivated plant (Tagetes erecta), soil properties and microbial communities. We grew T. erecta on soil samples at increasing concentrations of R. typhina root extracts and measured both plant growth and soil physiological profile with community-level physiological profiles (CLPP) using Biolog Eco-plates incubation. We found that R. typhina root extracts inhibit both plant growth and soil microbial activity. Plant height, Root length, soil organic carbon (SOC), total nitrogen (TN) and AWCD were significantly decreased with increasing root extract concentration, and plant above-ground biomass (AGB), below-ground biomass (BGB) and total biomass (TB) were significantly decreased at 10 mg·mL-1 of root extracts. In particular, root extracts significantly reduced the carbon source utilization of carbohydrates, carboxylic acids and polymers, but enhanced phenolic acid. Redundancy analysis shows that soil pH, TN, SOC and EC were the major driving factors of soil microbial activity. Our results indicate that strong allelopathic impact of root extracts on plant growth and soil microbial activity by mimicking roots exudate, providing novel insights into the role of plant–soil microbe interactions in mediating invasion success.

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Atmospheric nitrogen deposition affects the health of forest ecosystems by altering soil microbial activity. However, the effects of nitrogen addition levels, morphology and ecosystem type on whether nitrogen addition is beneficial or detrimental to soil health is controver-sial, and most studies have focused on the negative effects on microbial structure. Based on this, this study conducted a four-year experiment of nitrogen (NaNO3) addition at two levels (10 and 20 kg N hm−2·yr−1) in the understory soil of Larix olgensis in northeastern China to study soil microbial properties, soil enzyme activities, and to analyze soil physi-cochemical properties and the correlation between them. The results showed that nitrogen addition reduced soil pH and increased soil NH4+-N and NO3−-N contents, thus promoting the activities of Urease (Ure), Acid phosphatase (ACP) and N-Acetamidoglucosidase (NAG) and inhibiting the activity of Leucine aminopeptidase (LAP) in soil, further improving the diversity and richness of soil microorganisms and increasing the dominant taxa of beneficial microorganisms. This may be due to soil acidification caused by the addition of nitrogen, which increases the effectiveness of nitrogen in the soil, improving soil properties, moving soil health in a beneficial direction, promoting beneficial microbial activity, and making the soil more suitable for the growth of the acid-loving tree species L. olgensis. In general, N addition favored the development of soil bacterial communities and the maintenance of soil nutrient status, and had a positive effect on the soil nutrient status of L. olgensis. The results of this study may provide an important scientific basis for adaptive management of forest ecosystems in the context of global nitrogen deposition.