In the context of global warming, increasingly widespread and frequent freezing and thawing cycles (FTCs) will have profound effects on the biogeochemical cycling of soil carbon and nitrogen. FTCs can increase soil greenhouse gas (GHG) emissions by reducing the stability of soil aggregates, promoting the release of dissolved organic carbon, decreasing the number of microorganisms, inducing cell rupture, and releasing carbon and nitrogen nutrients for use by surviving microorganisms. However, the similarity and disparity of the mechanisms potentially contributing to changes in GHGs have not been systematically evaluated. The present study consolidates the most recent findings on the dynamics of soil carbon and nitrogen, as well as GHGs, in relation to FTCs. Additionally, it analyzes the impact of FTCs on soil GHGs in a systematic manner. In this study, particular emphasis is given to the following: (i) the reaction mechanism involved; (ii) variations in soil composition in different types of land (e.g., forest, peatland, farmland, and grassland); (iii) changes in soil structure in response to cycles of freezing temperatures; (iv) alterations in microbial biomass and community structure that may provide further insight into the fluctuations in GHGs after FTCs. The challenges identified included the extension of laboratory-scale research to ecosystem scales, the performance of in-depth investigation of the coupled effects of carbon, nitrogen, and water in the freeze-thaw process, and analysis of the effects of FTCs through the use of integrated research tools. The results of this study can provide a valuable point of reference for future experimental designs and scientific investigations and can also assist in the analysis of the attributes of GHG emissions from soil and the ecological consequences of the factors that influence these emissions in the context of global permafrost warming.

Anticipated permafrost thaw in upcoming decades may exert significant impacts on forest soil nitrogen (N) dynamics. The rate of soil N mineralization (Nmin) plays a crucial role in determining soil N availability. Nevertheless, our understanding remains limited regarding how biotic and abiotic factors influence the Nmin of forest soil in response to permafrost thaw. In this study, we investigated the implications of permafrost thaw on Nmin within a hemiboreal forest based on a field investigation along the degree of permafrost thaw, having monitored permafrost conditions for eight years. The results indicate that permafrost thaw markedly decreased Nmin values. Furthermore, Nmin demonstrated positive associations with soil substrates (namely, soil organic carbon and soil total nitrogen), microbial biomass carbon and nitrogen, and soil moisture content. The decline in Nmin due to permafrost thaw was primarily attributed to the diminished quality and quantity of soil substrates rather than alterations in plant community composition. Collectively, our results underscore the pivotal role of soil substrate and microbial biomass in guiding forest soil N transformations in the face of climate-induced permafrost thaw.

The snowbed habitats represent a relevant component of the alpine tundra biome, developing in areas characterized by a long-lasting snow cover. Such areas are particularly sensitive to climate changes, because small variations in air temperature, rain, and snowfall may considerably affect the pedoclimate and plant phenology, which control the soil C and N cycling. Therefore, it is fundamental to identify the most sensitive abiotic and biotic variables affecting soil nutrient cycling. This work was performed at seven permanent snowbed sites belonging to Salicetum herbaceae vegetation community in the northwestern Italian Alps, at elevations between 2,686 and 2,840 m.a.s.l. During a four-year study, we investigated climate, pedoclimate, floristic composition, phenology, and soil C and N dynamics. We found that lower soil water content and earlier melt-out day decreased soil N-NH4 (+), N-NO3 (-), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), total dissolved nitrogen (TDN), microbial nitrogen (Nmicr), microbial carbon (Cmicr), and C:Nmicr ratio. The progression of the phenological stages of Salix herbacea reduced soil N-NH4 (+) and increased DOC. Our results showed that the snow melt-out day, soil temperature, soil water content, and plant phenological stages were the most important factors affecting soil biogeochemical cycles, and they should be taken into account when assessing the effects of climate change in alpine tundra ecosystems, in the framework of long-term ecological research.

Forest fires have significantly impacted the permafrost environment, and many research programs looking at this have been undertaken at higher latitudes. However, their impacts have not yet been systematically studied and evaluated in the northern part of northeast China at mid-latitudes. This study simultaneously measured ecological and geocryological changes at various sites in the boreal forest at different stages after forest fires (chronosequence approach) in the northern Da Xing'anling (Hinggan) Mountains, Northeast China. We obtained results through field investigations, monitoring and observations, remote sensing interpretations, and laboratory tests. The results show that forest fires have resulted in a decreased Normalized Difference Vegetation Index (NDVI) and soil moisture contents in the active layer, increased active layer thickness (ALT) and ground temperatures, and the release of a large amount of C and N from the soils in the active layer and at shallow depths of permafrost. NDVI and species biodiversity have gradually increased in the years since forest fires. However, the vegetation has not fully recovered to the climax community structures and functions of the boreal forest ecosystems. For example, ground temperatures, ALT, and soil C and N contents have been slowly recovering in the 30years after the forest fires, but they have not yet been restored to pre-fire levels. This study provides important scientific bases for assessment of the impacts of forest fires on the boreal forest ecosystems in permafrost regions, environmental restoration and management, and changes in the carbon stock of soils at shallow (<3m) depths in the Da Xingan'ling Mountains in northeast China.

To better understand the factors controlling the growth of larch trees in Arctic taiga-tundra boundary ecosystem, we conducted field measurements of photosynthesis, tree size, nitrogen (N) content, and isotopic ratios in larch needles and soil. In addition, we observed various environmental parameters, including topography and soil moisture at four sites in the Indigirka River Basin, near Chokurdakh, northeastern Siberia. Most living larch trees grow on mounds with relatively high elevations and dry soils, indicating intolerance of high soil moisture. We found that needle delta(13)c was positively correlated with needle N content and needle mass, and these parameters showed spatial patterns similar to that of tree size. These results indicate that trees with high needle N content achieved higher rates of photosynthesis, which resulted in larger amounts of C assimilation and larger C allocation to needles and led to larger tree size than trees with lower needle N content. A positive correlation was also found between needle N content and soil NK4+ pool. Thus, soil inorganic N pool may indicate N availability, which is reflected in the needle N content of the larch trees. Microtopography plays a principal role in N availability, through a change in soil moisture. Relatively dryer soil of mounds with higher elevation and larger extent causes higher rates of soil N production, leading to increased N availability for plants, in addition to larger rooting space for trees to uptake more N. (C) 2014 Elsevier B.V. and NIPR. All rights reserved.

Thaw of ice-rich permafrost soils on sloping terrain can trigger erosional disturbance events that displace large volumes of soil and sediment, kill and damage plants, and initiate secondary succession. We examined how retrogressive thaw slumps (RTS), a common form of thermo-erosional disturbance in arctic tundra, affected the local loss and re-accumulation of carbon (C) and nitrogen (N) pools in organic and surface mineral soil horizons of 18 slumps within six spatially independent sites in arctic Alaska. RTS displaced 3 kg C and 0.2 kg N per m(2) from the soil organic horizon but did not alter pools of C and N in the top 15 cm of the mineral horizon. Surface soil C pools re-accumulated rapidly (32 +/- 10 g C m(-2) yr(-1)) through the first 60 years of succession, reaching levels similar to undisturbed tundra 40-64 years after disturbance. Average N re-accumulation rates (2.2 +/- 1.1 g N m(-2) yr(-1)) were much higher than expected from atmospheric deposition and biological N fixation. Finally, plant community dominance shifted from graminoids to tall deciduous shrubs, which are likely to promote higher primary productivity, biomass accumulation, and rates of nutrient cycling.