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.

Large herbivores grazing is a major disturbance that can cause significant changes of soil environment in grassland ecosystems. However, it remains unclear how soil microbial metabolic activity responses to different grazing intensities. We analyzed the relationships between soil microbial carbon source utilization and grazing intensity, and further assessed the main factors determining soil microbial metabolic activity in a meadow steppe. Soil samples were analyzed along different grazing intensities (no grazing, light grazing and heavy grazing) with community‐level physiological profiles (CLPP) using Biolog Eco‐plates incubation to estimate soil microbial carbon source utilization patterns. Redundancy analysis (RDA) was performed to explore the major factors influencing soil microbial metabolic activity in a five‐year grazing grassland in northeast of China. Grazing significantly improved soil microbial community carbon utilization and increased utilization of carbohydrates, amino acids, phenolic acids and amines, while significantly decreased utilization of carboxylic acids. Among edaphic properties and plant traits, plant density, total biomass (TB), soil water content (SW) and C:N ratio (C/N) were main driving forces contributing to the carbon source utilization structure of the soil microbial communities. Soil microbial metabolic activity was promoted by grazing through altering plant traits and abiotic soil properties, and soil‐related factors were primary and direct driving force for soil microbial metabolic activity in grasslands of northeast China. The present study demonstrated differential soil microbial responses along grazing intensities and has important applications for better management practices in the grassland ecosystem.

Cadmium (Cd) and lead (Pb) generally occur simultaneously with low concentration in soil. However, anthropogenic activities have significantly raised these non-biodegradable heavy metals and caused long-term deleterious effects on ecosystem health. To study single or combined effects of Cd and Pb on seed germination, early seedling growth and physiological response in Rhus typhina, a seed germination and sand culture experiment was established completely randomized with 0, 100, 300, and 500 mg•L -1 Pb(NO 3 ) 2 or 0, 25, 75, 125 mg•L -1 CdCl 2 individually or in combination. The present results showed seed germination and seedling growth of Rhus typhina decreased with increasing Cd and Pb, and the joint effect was more serious than single heavy metal stress. The lowest of seed germination rate (GR), germination index (GI), root length (RL) and shoot length (SL) in Rhus typhina decreased 65.85%, 73.46%, 84.33% and 61.95% compared to control in soil supplemented with combined Cd and Pb, respectively. The activity of superoxide dismutase (SOD), peroxidase (POD), malondialdehyde (MDA) and soluble protein (SP) changed significantly with increasing concentration of Cd and Pb, and MDA and POD played important roles in resisting Cd and Pb stress because of their significant correlation with seed germination and early seedling growth.

The ascorbate (AsA)–glutathione (GSH) metabolism pathway is an important antioxidant system in cadmium (Cd) detoxification; the AsA–GSHpathway is generally regulated by a specific set of functional genes. However, transcription factors involved in AsA–GSH pathway have yet to be identified. Herein, we transformed a heat shock transcription factor SpHsfA4c from Sedum plumbizincicola into Populus. × canescens. Under 100 μM CdCl2 stress for 30 d, the leaf chlorosis of wild-type poplars (WT) is more serious than that in transgenic poplars. The root biomass, shoot biomass and tolerance index (TIs) of transgenic poplars were higher than those in WT. In addition, transgenic poplars have higher Cd2+ uptake and Cd content. Compared with WT, the contents of hydrogen peroxide (H2O2) and superoxide anion (O2•−) in transgenic poplars were significantly reduced in leaves under Cd treatment. The expression levels of five enzymes (ascorbate peroxidase (APX), catalases (CAT), superoxide dismutase (SOD), peroxidase (POD) and glutathione S-transferase (GST)) were higher in transgenic poplars than those in WT. Transgenic poplars contained higher concentrations of intermediate metabolites, including GSH, AsA and phytochelatins (PCs), and a higher GSH/GSSG ratio in the AsA–GSH metabolism pathway. In Fourier transform infrared (FTIR) spectra, the characteristic peaks indicated that the contents of cysteine, GSH and AsA in transgenic poplars were exceeded compared to those in WT. These results suggested that SpHsfA4c can activate the AsA–GSH metabolism pathway to reduce Cd-associated oxidative stress. Therefore, overexpressing SpHsfA4c in P. × canescens can give rise to a superior Cd tolerance. Our results provide a theoretical significance for breeding potential new germplasm resources with high biomass and high Cd tolerance for remediation of soil heavy metal pollution.