The decomposition of returned straw is increasing facing the negative impacts by metal nanoparticles (NPs), however, which may be modulated by soil fauna and this modulation effect is unclear. Here, the interactive effects of ZnO NPs with soil fauna on wheat straw decomposition were investigated in a potted rice cropping system. The results showed that ZnO NP below middle concentrations did not significantly influence straw decomposition, and mass loss was mainly driven by microfauna and microbes. High concentrations of ZnO NPs significantly impeded decomposition, mainly by reducing the complexity of fungal communities. This negative effect was ascribed to the promotion of Zn solubilization by bacterial taxa such as unclassified Acidobacteria, Bacteroidetes and Gemmatimonadetes. ZnO NPs had a greater impact on soil microorganisms than on fauna, reduced microbial activity, promoted the released straw nutrients entering into the soil by damaging nutrient transferring microorganisms and dominated the effects on soil stoichiometry. However, soil fauna significantly increased the activities of C- and N-releasing enzymes, decreased the activity of P-releasing enzymes, regardless of ZnO NP concentration, and promoted straw C decomposition. ZnO NPs altered soil microbial community composition, but these changes were modulated by soil fauna. Nonetheless, nutrient transport by fungi such as Ascomycota and Zygomycota and grazing by fauna were the predominant modulators on straw stoichiometry. The results of this study revealed that rational control of soil fauna will be helpful for promoting straw decomposition and efficient recycling of straw nutrients by crops under ZnO NP contamination. High ZnO NP concentrations inhibit straw decomposition mainly by reducing diversity of fungal community.The negative effects of ZnO NPs are ascribed to Zn solubilization by bacterial taxa such as unclassified Acidobacteria, Bacteroidetes and Gemmatimonadetes.ZnO NPs have greater impact on soil microorganisms than on fauna, reduce microbial activity, promote the released nutrients into soil and dominate the effects on soil stoichiometry.Fungal transport (e.g., Ascomycota and Zygomycota) and fauna grazing are the predominant modulators on straw stoichiometry.

In Arctic soils, warming accelerates decomposition of organic matter and increases emission of greenhouse gases (GHGs), contributing to a positive feedback to climate change. Although microorganisms play a key role in the processes between decomposition of organic matter and GHGs emission, the effects of warming on temporal responses of microbial activity are still elusive. In this study, treatments of warming and precipitation were conducted from 2012 to 2018 in Cambridge Bay, Canada. Soils of organic and mineral layers were collected monthly from June to September in 2018 and analyzed for extracellular enzyme activities and bacterial community structures. The activity of hydrolases was the highest in June and decreased thereafter over summer in both organic and mineral layers. Bacterial community structures changed gradually over summer, and the responses were distinct depending on soil layers and environmental factors; water content and soil temperature affected the shift of bacterial community structures in both layers, whereas bacterial abundance, dissolved organic carbon, and inorganic nitrogen did so in the organic layer only. The activity of hydrolases and bacterial community structures did not differ significantly among treatments but among months. Our results demonstrate that temporal variations may control extracellular enzyme activities and microbial community structure rather than the small effect of warming over a long period in high Arctic soil. Although the effects of the treatments on microbial activity were minor, our study provides insight that microbial activity may increase due to an increase in carbon availability, if the growing season is prolonged in the Arctic.

Fire frequency and severity are increasing in tundra and boreal regions as climate warms, which can directly affect climate feedbacks by increasing carbon (C) emissions from combustion of the large soil C pool and indirectly via changes in vegetation, permafrost thaw, hydrology, and nutrient availability. To better understand the direct and indirect effects of changing fire regimes in northern ecosystems, we examined how differences in soil burn severity (i.e., extent of soil organic matter combustion) affect soil C, nitrogen (N), and phosphorus (P) availability and microbial processes over time. We created experimental burns of three fire severities (low, moderate, and high) in a larch forest in the northeastern Siberian Arctic and analyzed soils at 1, 8 days, and 1 year postfire. Labile dissolved C and N increased with increasing soil burn severity immediately (1 day) postfire by up to an order of magnitude, but declined significantly 1 week later; both variables were comparable or lower than unburned soils by 1 year postfire. Soil burn severity had no effect on P in the organic layer, but P increased with increasing severity in mineral soil horizons. Most extracellular enzyme activities decreased by up to 70% with increasing soil burn severity. Increasing soil burn severity reduced soil respiration 1 year postfire by 50%. However, increasing soil burn severity increased net N mineralization rates 1 year postfire, which were 10-fold higher in the highest burn severity. While fires of high severity consumed approximately five times more soil C than those of low severity, soil C pools will also be driven by indirect effects of fire on soil processes. Our data suggest that despite an initial increase in labile C and nutrients with soil burn severity, soil respiration and extracellular activities related to the turnover of organic matter were greatly reduced, which may mitigate future C losses following fire.

Geomorphic disturbances to surrounding terrain induced by thermal degradation of permafrost often lead to surface ponding or soil saturation. However, interactions between soil moisture and temperature on belowground carbon processes are not fully understood. We conducted batch incubation for three temperature treatments [constant freezing (CF), constant thawing (CT), and fluctuating temperatures (FTC)] and two soil moisture conditions (ponded and unsaturated). Extracellular enzyme activity was higher under ponded conditions than under unsaturated conditions, resulting in higher dissolved organic carbon (DOC) levels for ponded conditions. More CO2 and less CH4 were emitted under unsaturated conditions than under ponded conditions. Carbon dioxide emission was similar for CT and FTC treatments regardless of moisture conditions. However, CH4 emission was higher under ponded conditions than under unsaturated conditions for CT treatments, but was very low for FTC treatments regardless of moisture conditions. Little CO2 and CH4 were produced in CF treatments. Despite similar CO2 and CH4 emission levels for CT and FTC treatments, lower DOC levels were observed in the latter, indicating slower soil organic carbon (SOC) decomposition. Similar DOC variation patterns between CT and CF treatments indicated that SOC decomposition was considerable and further degradation to CO2 or CH4 was negligible even for CF treatments. The SOC decomposition and CO2 and CH4 emissions were considerable for FTC treatments. Our results suggest that labile-C produced during SOC decomposition in seasonally frozen soils and permafrost may provide supplemental substrates that would enhance the positive feedback to climate change with rising temperatures and wetter conditions.