Relative importance of soil microbes and litter quality on decomposition and nitrogen cycling in grasslands

Effects of microplastics on litter decomposition in wetland soil

Microplastics (MPs) may interfere with the primary ecological processes of soil, which has become a growing global environmental issue. In terrestrial ecosystems, litter decomposition is the main process of nutrient cycling, particularly for carbon (C) and nitrogen (N). However, how microplastic pollution could alter wetland litter decomposition has barely been investigated. Therefore, a 100-day lab-scale litter decomposition experiment was conducted using Shengjin Lake wetland soil, which was treated with two types of MPs (polyethylene, PE and polyvinyl chloride, PVC) at three concentrations (0.1%, 0.5%, and 2.5%, w/w), to explore if and how MPs accumulation could affect wetland litter decomposition processes. According to our research, the PE and PVC greatly slowed the decomposition rate of wetland litter. Compared with control treatments, the addition of MPs decreased litter quality (high C:N), reduced litter decomposition-related soil enzyme activity, decreased the diversity of bacteria, and altered microbial community structure and potential functional gene abundance linked to litter decomposition. These findings revealed that MPs could affect the main process of C and N cycling in wetland ecosystems, providing important cues for further research on the wetland ecosystem function.

Does Faeces Excreted by Moxidectin-Treated Sheep Impact Coprophagous Insects and the Activity of Soil Microbiota in Subtropical Pastures?

Moxidectin (MOX) is used to control helminth parasites in ruminant livestock. It is released through feces and remains in the environment for a long period. This study aimed to evaluate the impact of faeces excreted by moxidectin-treated sheep on soil biodiversity (coprophagous insects, soil microbial biomass, and activity) to establish environment-related guidelines regarding the use of MOX in sheep livestock. The study consisted of two experiments. In the first one, faeces from MOX-treated (subcutaneous dose of 0.2 mg·kg-1 body weight) and nontreated rams were placed on an animal-free pasture field, protected or not against rain, for 88 days. Then, coprophagous insects were captured, identified, and counted, and faeces degradation was evaluated by measuring dry weight and carbon (C) and nitrogen (N) contents over time. Diptera, Hymenoptera, Isoptera, and Coleoptera were equally encountered in faeces from MOX-treated and nontreated animals. Faecal boluses of MOX-treated animals (with higher N content) not protected against rain degraded faster than faecal boluses of nontreated animals (with lower N content). In the second experiment, faeces from nontreated animals were amended with increasing amounts of MOX (75 to 3,000 ng·kg-1 faeces), mixed with soil samples from animal-free pasture (1.9 to 75 ng·kg-1 soil), and incubated in a greenhouse for 28 days. Increasing concentrations of MOX did not prevent the growth of cultivable bacteria, actinobacteria, or fungi in culture media. However, even the lower MOX concentration (1.9 ng·kg-1 soil) abruptly decreased soil microbial biomass, basal respiration, and N mineralization. Thus, the results indicate that faeces excreted from sheep treated with MOX under the experimental conditions of this study are not harmful to the coprophagous insects. However, adding MOX to faeces from drug-free sheep had a negative impact on soil microbial activity and biomass.

Nitrogen addition accelerates litter decomposition and arsenic release of Pteris vittata in arsenic-contaminated soil from mine

The arsenic (As) release from litter decomposition of As-hyperaccumulator (Pteris vittata L.) in mine areas poses an ecological risk for metal dispersion into the soil. However, the effect of atmospheric nitrogen (N) deposition on the litter decomposition of As-hyperaccumulator in the tailing mine area remains poorly understood. In this study, we conducted a microcosm experiment to investigate the As release during the decomposition of P. vittata litter under four gradients of N addition (0, 5, 10, and 20 mg N g-1). The N10 treatment (10 mg N g-1) enhanced As release from P. vittata litter by 1.2-2.6 folds compared to control. Furthermore, Streptomyces, Pantoea, and Curtobacterium were found to primarily affect the As release during the litter decomposition process. Additionally, N addition decreased the soil pH, subsequently increased the microbial biomass, as well as hydrolase activities (NAG) which regulated N release. Thereby, N addition increased the As release from P. vittata litter and then transferred to the soil. Moreover, this process caused a transformation of non-labile As fractions into labile forms, resulting in an increase of available As concentration by 13.02-20.16% within the soil after a 90-day incubation period. Our findings provide valuable insights into assessing the ecological risk associated with As release from the decomposition of P. vittata litter towards the soil, particularly under elevated atmospheric N deposition.

Soil microbial community structure dynamics shape the rhizosphere priming effect patterns in the paddy soil

Microbial community structure plays a crucial part in soil organic carbon (SOC) decomposition and variation of rhizosphere priming effects (RPEs) during plant growth. However, it is still uncertain how bacterial community structure regulates RPEs in soil and how RPE patterns respond to plant growth. Therefore, we conducted an experiment to examine the RPE response to plant growth and nitrogen (N) addition (0 (N0), 150 (N150), and 300 (N300) kg N ha^-1) using the ¹³C natural abundance method in a C₃ soil (paddy soil) - C₄ plant (maize, Zea mays L.) system; we then explored the underlying biotic mechanisms using 16S rRNA sequencing techniques. Networks were constructed to identify keystone taxa and to analyze the correlations between network functional modules of bacterial community and C decomposition. The results indicated that negative and positive RPEs occurred on Day 30 and Day 75 after maize planting, respectively. Bacterial community structure significantly changed and tended to shift from r-strategists toward K-strategists with changing labile C: N stoichiometry and soil pH during plant growth stages. The different network modules of bacterial community were aggregated in response to RPE pattern variation. Caulobacteraceae, Bacillus, and Chitinophagaceae were keystone taxa on Day 30, while Gemmatimonas, Candidatus Koribacter, and Xanthobacteraceae were keystone taxa on Day 75. Moreover, keystone taxa with different C utilization strategies were significantly different between the two growth stages and related closely to different RPE patterns. This study provides deeper insights into the network structure of bacterial communities corresponding to RPE patterns and emphasizes the significance of keystone taxa in RPE variation.

Changes in soil faunal density and microbial community under altered litter input in forests and grasslands

Root and foliar litter inputs are the primary sources of carbon and nutrients for soil fauna and microorganisms, yet we still lack a quantitative assessment to evaluate the effects of root and foliar litter on various groups of soil organisms across terrestrial ecosystems. Here, we compiled 978 paired observations from 68 experimental sites to assess the directions and magnitudes of adding and removing foliar and root litter on the soil faunal density and microbial biomass that was evaluated by phospholipid fatty acids (PLFAs) across forests and grasslands worldwide. We found that litter addition had only a marginal effect on soil faunal density but significantly increased the soil total microbial-, fungal- and bacterial-PLFAs by 13%, 14%, and 10%, respectively, across ecosystems, suggesting that the soil microbial community is more sensitive to carbon source addition than soil fauna, particularly in soils with low carbon to nitrogen ratios. In contrast, removing litter significantly decreased the soil faunal density by 17% but had few effects on soil microorganisms. Compared with foliar litter, root litter input had a more positive effect on the development of soil fungal taxa. The effect of both litter addition and removal on soil faunal density and microbial biomass did not differ between humid and arid regions, but a greater influence was observed in grasslands than in forests for soil microbial community. Our results highlight that the increasing litter production under a global greening scenario would stimulate microbial activity in grasslands more than in forests, and this stimulation would be greater for soil microbes than soil fauna.

The influence of increased precipitation and nitrogen deposition on the litter decomposition and soil microbial community structure in a semiarid grassland

Litter decomposition is a major method in which nutrients are recycled, especially carbon and nitrogen elements, in terrestrial ecosystems. However, how the responses of litter quality and soil microbial communities to global changes alter litter decomposition remains unclear. A 4-year field manipulative experiment based on the litterbag method was conducted in a typical temperate semiarid grassland in China to explore how increased precipitation and nitrogen deposition affect decomposition processes via litter quality and soil microbial communities. Our results showed that water and nitrogen addition treatments could accelerate litter carbon release and promote mass loss through different pathways. Water addition had a direct positive effect on litter decomposition. However, nitrogen addition could indirectly promote litter decomposition by improving litter quality and increasing the bacterial and fungal ratios. The water addition treatment increased litter mass loss by 7.37 %, and the N addition treatments increased litter mass loss by 5.83 %-16.93 %. Moreover, water and nitrogen additions had antagonistic effects on litter decomposition. These findings revealed that litter quality and the soil bacterial to fungal ratio were the factors controlling litter decomposition. The changes in precipitation and nitrogen deposition will impact ecosystem carbon and nitrogen cycling by altering litter decomposition processes in semiarid grassland ecosystems under the context of climate change.