Soil respiration and its environmental response varies by day/night and by growing/dormant season in a subalpine forest

Monotonic trends of soil microbiomes, metagenomic and metabolomic functioning across ecosystems along water gradients in the Altai region, northwestern China

To investigate microbial communities and their contributions to carbon and nutrient cycling along water gradients can enhance our comprehension of climate change impacts on ecosystem services. Thus, we conducted an assessment of microbial communities, metagenomic functions, and metabolomic profiles within four ecosystems, i.e., desert grassland (DG), shrub-steppe (SS), forest (FO), and marsh (MA) in the Altai region of Xinjiang, China. Our results showed that soil total carbon (TC), total nitrogen, NH4+, and NO3- increased, but pH decreased with soil water gradients. Microbial abundances and richness also increased with soil moisture except the abundances of fungi and protists being lowest in MA. A shift in microbial community composition is evident along the soil moisture gradient, with Proteobacteria, Basidiomycota, and Evosea proliferating but a decline in Actinobacteria and Cercozoa. The β-diversity of microbiomes, metagenomic, and metabolomic functioning were correlated with soil moisture gradients and have significant associations with specific soil factors of TC, NH4+, and pH. Metagenomic functions associated with carbohydrate and DNA metabolisms, as well as phages, prophages, TE, plasmids functions diminished with moisture, whereas the genes involved in nitrogen and potassium metabolism, along with certain biological interactions and environmental information processing functions, demonstrated an augmentation. Additionally, MA harbored the most abundant metabolomics dominated by lipids and lipid-like molecules and organic oxygen compounds, except certain metabolites showing decline trends along water gradients, such as N'-Hydroxymethylnorcotinine and 5-Hydroxyenterolactone. Thus, our study suggests that future ecosystem succession facilitated by changes in rainfall patterns will significantly alter soil microbial taxa, functional potential, and metabolite fractions.

Light-dark cycles may influence in situ soil bacterial networks and diurnally-sensitive taxa

Soil bacterial taxa have important functional roles in ecosystems (e.g. nutrient cycling, soil formation, plant health). Many factors influence their assembly and regulation, with land cover types (e.g. open woodlands, grasslands), land use types (e.g. nature reserves, urban green space) and plant-soil feedbacks being well-studied factors. However, changes in soil bacterial communities in situ over light-dark cycles have received little attention, despite many plants and some bacteria having endogenous circadian rhythms that could influence soil bacterial communities. We sampled surface soils in situ across 24-h light-dark cycles (at 00:00, 06:00, 12:00, 18:00) at two land cover types (remnant vegetation vs. cleared, grassy areas) and applied 16S rRNA amplicon sequencing to investigate changes in bacterial communities. We show that land cover type strongly affected soil bacterial diversity, with soils under native vegetation expressing 15.4%-16.4% lower alpha diversity but 4.9%-10.6% greater heterogeneity than soils under cleared vegetation. In addition, we report time-dependent and site-specific changes in bacterial network complexity and between 598-922 ASVs showing significant changes in relative abundance across times. Native site node degree (bacterial interactions) at the phylum level was 16.0% higher in the early morning than in the afternoon/evening. Our results demonstrate for the first time that light-dark cycles have subtle yet important effects on soil bacterial communities in situ and that land cover influences these dynamics. We provide a new view of soil microbial ecology and suggest that future studies should consider the time of day when sampling soil bacteria.

Soil greenhouse gas emissions from drained and rewetted agricultural bare peat mesocosms are linked to geochemistry

In view of climate considerations regarding the management of peatlands, there is a need to assess whether rewetting can mitigate greenhouse gas (GHG) emissions, and notably how site-specific soil-geochemistry will influence differences in emission magnitudes. However, there are inconsistent results regarding the correlation of soil properties with heterotrophic respiration (Rh) of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from bare peat. In this study, we determined 1) soil-, and site-specific geochemical components as drivers for emissions from Rh on five Danish fens and bogs, and 2) emission magnitudes under drained and rewetted conditions. For this, a mesocosm experiment was performed under equal exposure to climatic conditions and water table depths controlled to either -40 cm, or -5 cm. For the drained soils, we found that annual cumulative emissions, accounting for all three gases, were dominated by CO2, contributing with, on average, 99 % to a varying global warming potential (GWP) of 12.2-16.9 t CO2eq ha-1 yr-1. Rewetting lowered annual cumulative emissions from Rh by 3.2-5.1 t CO2eq ha-1 yr-1 for fens and bogs, respectively, despite a high variability of site-specific CH4 emissions, contributing with 0.3-3.4 t CO2 ha-1 yr-1 to the GWP. Overall, analyses using generalized additive models (GAM) showed that emission magnitudes were well explained by geochemical variables. Under drained conditions, significant soil-specific predictor variables for CO2 flux magnitudes were pH, phosphorus (P), and the soil substrate's relative water holding capacity (WHC). When rewetted, CO2 and CH4 emissions from Rh were affected by pH, WHC, as well as contents of P, total carbon and nitrogen. In conclusion, our results found the highest GHG reduction on fen peatlands, further highlighting that peat nutrient status and acidity, and the potential availability of alternative electron acceptors, might be used as proxies for prioritising peatland areas for GHG mitigation efforts by rewetting.

Temperature sensitivity of soil respiration across multiple time scales in a temperate plantation forest

Soil respiration (Rs) is the largest carbon (C) flux from terrestrial ecosystems to the atmosphere. Predictions of Rs and associated feedback to climate change remain largely uncertain, in part due to the high temporal heterogeneity of temperature sensitivity (apparent Q₁₀) of Rs under a changing climate. Therefore, it is of critical importance to provide better insight into how Q₁₀ varies across multiple temporal scales. We investigated the diurnal, seasonal, and annual variabilities in the Q₁₀ of Rs using continuous Rs measurements (at hourly intervals) over six growing seasons in a mature temperate larch plantation in North China. We found that night-time values of Q₁₀ were slightly lower than daytime values. Large seasonal and annual fluctuations of Q₁₀ were observed, as illustrated by high coefficients of variation of 15.0% and 21.8%, respectively. The higher Q₁₀ in spring and autumn were primarily regulated by fine root growth and higher soil moisture after snow melt in spring, and leaf senescence in autumn. Lower Q₁₀ in summer may have been caused by limitations in substrate availability and microbial activity resulting from drought, which also caused a decoupling of Rs from soil temperature in summer. Furthermore, a bivariate nonlinear model incorporating both soil temperature and soil moisture best explained Q₁₀ variability. Generally, lower soil temperature and higher soil moisture lead to higher values of Q₁₀, indicating that climate warming could exert a negative effect on Q₁₀, partially offsetting the warming-induced increase in soil C loss. We provide long-term field experimental evidence that it would be inappropriate to estimate Rs on a multiyear scale using a fixed Q₁₀ value or a value obtained from one season and/or one year. Thus, we emphasize the importance of incorporating the seasonal and annual heterogeneities of Q₁₀ into C cycle model simulations under future climate change scenarios.