Background and AimsPrescribed burning is a widely used management technique, often employed to restore grasslands affected by woody plants encroachment. However, its interaction with pre-existing plant species in influencing soil properties remains unclear.MethodsWe conducted a diachronic soil survey to assess the evolution of several soil properties in the mid-term (up to 18 months) after burning, including physico-chemical parameters and microbial biomass carbon on soils under vegetation patches of different plant functional types and life forms. Vegetation patches included Ericaceae and legume shrubs, ferns, and biocrusts dominated by lichens. Soil samples were taken pre-burning, immediately after burning and 9 and 18 months after.ResultsOur findings indicate that while some soil properties returned to pre-burning levels in the mid-term (i. e., soil cations and NH4+), others, such as available phosphorous (P Olsen), exhibited a significant decline that persisted even 18 months later. Furthermore, soils under legumes initially displayed higher levels of soil carbon and nitrogen compared to other vegetation patches, but this distinction diminished over time. This was likely due to legumes' susceptibility to fire damage, in contrast to the greater resilience of Ericaceae shrubs.ConclusionOur study highlights the complex vegetation patch-dependent effects of prescribed burning on soil properties. While legumes initially enhance soil carbon and nitrogen, their contribution decreases over time due to fire sensitivity. Some soil parameters recover in the mid-term, but nutrients like available phosphorus continue to decline. Fire management strategies should consider plant diversity and recovery time to mitigate soil fertility loss.

Invasive plants are often attacked by both introduced specialist and native generalist natural enemies in new ranges. Soil fertility can potentially alter the interactions of these natural enemies with native versus invasive plants in ways that have largely unexplored implications for biological invasions and biological control. A common garden experiment was conducted to compare the performance of an introduced specialist flea beetle, Agasicles hygrophila, and/or a native generalist nematode, Meloidogyne incognita, on invasive alligator weed, Alternanthera philoxeroides, and its native congener sessile joyweed, Alternanthera sessilis, under different levels of soil nitrogen (N) and phosphorus (P). At a relatively low or moderate N and P levels, the flea beetle and the nematodes were not significantly affected by each other. Under these conditions, alligator weed responded plastically by producing more branches and biomass, as well as longer stolons, in response to attack by the flea beetle and/or the nematode, compared to sessile joyweed responses to similar levels of damage. However, under a relatively high N and P levels, nematode infestations significantly reduced flea beetle damage on alligator weed, resulting in significantly greater above- and below-ground biomass and longer stolons than plants without herbivory. In contrast, beetle herbivory significantly increased the level of nematode infestations on sessile joyweed, resulting in significantly fewer fine and coarse roots, and lower above- and below-ground biomass compared to plants without herbivory. Synthesis and applications. Our findings illustrate the importance of soil fertility in mediating interactions between specialist biocontrol agents and native generalists on native versus invasive plants. High phenotypic plasticity seems to be an important attribute that contributes to the success of invasive plants like alligator weed in relatively nutrient-poor environments. However, nutrient-rich environments could potentially confer greater growth benefits on invasive plants than on native ones by changing herbivore-herbivore interactions on plants differently. There is a great need to fully investigate the direct and indirect interactions between biocontrol agents and generalists across food webs following classical biocontrol releases. Nutrient measurements of both soil and water bodies should also be incorporated into all stages of biocontrol programmes.

Many studies have reported modification in the degree of O3 damage to photosynthesis by elevated CO2 and soil N supply. However, the mechanism underlying the modification is unclear. To clarify the important determinants in the degree of O3 damage to net photosynthetic rate (A) in the leaves of Fagus crenata (Siebold's beech) under elevated CO2 and with different soil N supply, F. crenata seedlings were grown for two growing seasons under combinations of two O3 levels (low concentration at approximately 4 nmol mol-1 and two times the ambient concentration), two CO2 levels (ambient and 700 mu mol mol-1), and three levels of soil N supply (0, 50 and 100 kg N ha- 1 year -1). During the second growing season, we determined A, stomatal conductance for calculating phytotoxic O3 dose (POD), antioxidant concentrations, and antioxidative enzyme activities in the leaves for evaluating O3 detoxification capacity. We calculated the O3-induced reduction in mean A (Delta Amean) during the second growing season using the data reported in our previous study and plotted it against mean daily POD without flux threshold (POD0). There was no significant linear nor non-linear relationship, suggesting that not only POD0 but also O3 detoxification capacity are important determinants of Delta Amean under elevated CO2 and N supply. We found significant negative linear relationships of Delta Amean per unit POD0 (Delta Amean/POD0) with reduced ascorbate concentration in the low O3 treatment, and with percentage of O3-induced change in activity of monodehydroascorbate reductase (MDAR). In addition, the Delta Amean/POD0 was positively and significantly correlated with the activity ratio of ascorbate peroxidase to MDAR. These results suggest that reduced ascorbate pool and its maintenance through the action of MDAR could be important determinants in the degree of O3 damage to net photosynthesis under elevated CO2 and soil N supply.

Reduction in snow cover is a prominent aspect of global change. Freeze-thaw cycles (FTCs) of different amplitudes and durations in soil due to insufficient thermal insulation may alter microbial diversity and key ecological functions mediated by microorganisms. These changes could then further alter the cycling of material and energy in the ecosystem. Yet despite many assessments, the impact of FTCs upon microbial diversity remains poorly understood. Here, 546 observations from 61 published studies were collected for a global meta-analysis with the objective to explore how soil microbial diversity and C and N dynamics it drives respond to FTCs. The results showed that: in general, FTCs did not lead to a reduction of microbial alpha-diversity, but they did reduce levels of soil microbial biomass carbon, microbial biomass nitrogen, and phospholipid fatty acid by 7%, 12%, and 11%, respectively; they also significantly changed the microbial community structure. FTCs did not significantly affect the alpha-diversity of bacteria and fungi, but community structures of both were changed significantly, with that of the bacteria more sensitive to FTCs. FTCs were responsible for a 6% decrease in functions related to C, N cycling, which could be explained by the changes found in microbial biomass and community structure. FTCs could also indirectly impact microbial biomass via changed pH and soil water content (SWC). The response of microbial community to FTCs was related to the FTC frequency, freezing temperature and sampling time. FTCs had a large effect on the C and N pool components and fluxes in soil. It is particularly noteworthy that FTCs drove a 137% increase in N2O emission. Further, the changes in pH and SWC directly affected the C and N pool components and fluxes. The results of current meta-analysis deepen the comprehensive understanding of the effects of FTCs on the soil microbial community and C and N dynamics it mediated, and provide a reference for subsequent research in terms of experimental scheme and scientific issues requiring close attention.

Climate change is profound in the Arctic where increased snowfall during winter and warmer growing season temperatures may accelerate soil nitrogen (N) turnover and increase inorganic N availability. Nitrous oxide (N2O) is a potent greenhouse gas formed by soil microbes and in the Arctic, the production is seen as limited mainly by low inorganic N availability. Hence, it can be hypothesized that climate change in the Arctic may increase total N2O emissions, yet this topic remains understudied. We investigated the combined effects of variable snow depths and experimental warming on soil N cycling in a factorial field study established along a natural snowmelt gradient in a low Arctic heath ecosystem. The study assessed N2O surface fluxes, gross N mineralization and nitrification rates, potential denitrification activity, and the pools of soil microbial, soil organic and soil inorganic N, carbon (C) and phosphorus (P) during two growing seasons. The net fluxes of N2O averaged 1.7 mu g N2O-N m- 2 h-1 (range -3.6 to 10.5 mu g N2O-N m- 2 h-1), and generally increased from ambient (1 m) to moderate (2-3 m) snow depths. At the greatest snow depth (4 m) where snowmelt was profoundly later, N2O fluxes decreased, likely caused by combined negative effects of low summer temperatures and high soil moisture. Positive correlations between N2O and nitrate (NO3- ) and dissolved organic N (DON) suggested that the availability of N was the main controlling variable along the snowmelt gradient. The maximum N2O fluxes were observed in the second half of August associated with high NO3- concentrations. The effect of growing season experimental warming on N2O surface flux varied along the snowmelt gradient and with time. Generally, the experimental warming stimulated N2O fluxes under conditions with increased concentrations of inorganic N. In contrast, warming reduced N2O fluxes when inorganic N was low. Experimental warming had no clear effects on soil inorganic N. The study suggests that if increased winter precipitation leads to a deeper snow cover and a later snowmelt, total emissions of N2O from low Arctic heath ecosystems may be enhanced in the future and, dependent on dissolved N availability, summer warming may stimulate or reduce total emissions.

A novel laboratory method was developed to control soil freeze-thaw cycles and study the effects of freezing intensity on soil conditions and N2O emissions. The method created unidirectional freeze-thaw (top-down), similar to field conditions. Soil was placed in boxes that were insulated on the sides, heated from the bottom, and left open on the top. Snow was placed on the soil surface, and the boxes were placed in separate climate-controlled chambers to freeze (-9 degrees C) and thaw (5 degrees C). The method was used in an experiment to evaluate the links between freezing degree days (FDD), soil water content, carbon (C) and nitrogen (N) transformations, and N2O emissions. Results showed that N2O emissions were greatest from soils that experienced more freezing, with the 185 FDD treatment emitting significantly more N2O than the 50 FDD treatment (17.7 vs. 7.7 mg N2O-N M-2 d(-1)). Peaks in soil water content during thaw preceded peaks in N2O flux, but increasing water content by simulating rain (in addition to snow melt) did not increase N2O emissions compared with snow melt alone. Extractable soil C and N increased in the top 5 cm when soils froze; however, greater emissions were not linked to greater C and N concentrations at individual points in time. Higher N2O emissions at 134 and 185 FDD were associated with greater C exposure (i.e., extractable soil C concentration integrated over time) than the 50 FDD treatment.

The lack of snow cover due to winter climate change has great potential to impact winter soil nitrogen cycling in boreal forests. A snow manipulation was conducted in a Tibetan spruce forest to explore the effects of snow absence on winter soil nitrogen dynamics by shelter method. Snow absence on average reduced soil temperatures at the depths of 0 cm and 5 cm by 1.44 degrees C and 0.33 degrees C, respectively, throughout the winter. Moreover, snow absence increased soil frost and freeze-thaw cycles. Soil net nitrogen mineralization and labile nitrogen pools (ammonium, nitrate and dissolved organic nitrogen) were higher in the snow absence plots compared to control plots. Snow absence increased soil microbial biomass carbon but did not affect microbial biomass nitrogen. Nevertheless, soil enzyme activities involved in nitrogen cycles were often lowered by snow absence over the winter. The results noted in this study suggest that warming-induced absence in seasonal snowpack may stimulate winter soil nitrogen availabilities by changing soil microhabitats, which has important implications for soil biogeochemical cycles in the subalpine forest ecosystems on the eastern Tibetan Plateau.

Throughout most of the northern hemisphere, snow cover decreased in almost every winter month from 1967 to 2012. Because snow is an effective insulator, snow cover loss has likely enhanced soil freezing and the frequency of soil freeze-thaw cycles, which can disrupt soil nitrogen dynamics including the production of nitrous oxide (N2O). We used replicated automated gas flux chambers deployed in an annual cropping system in the upper Midwest US for three winters (December-March, 2011-2013) to examine the effects of snow removal and additions on N2O fluxes. Diminished snow cover resulted in increased N2O emissions each year; over the entire experiment, cumulative emissions in plots with snow removed were 69% higher than in ambient snow control plots and 95% higher than in plots that received additional snow (P < 0.001). Higher emissions coincided with a greater number of freeze-thaw cycles that broke up soil macroaggregates (250-8000 A mu m) and significantly increased soil inorganic nitrogen pools. We conclude that winters with less snow cover can be expected to accelerate N2O fluxes from agricultural soils subject to wintertime freezing.

We investigated the main parameters [e.g. mean annual air temperature , mean annual soil temperature, mean annual precipitation, soil moisture (SM), soil chemistry, and physics] influencing soil organic carbon (C-org), soil total nitrogen (N-t) as well as plant available nitrogen (N-min) at 47 sites along a 1200 km transect across the high-altitude and low-latitude permafrost region of the central-eastern Tibetan Plateau. This large-scale survey allows testing the hypothesis that beside commonly used ecological variables, diversity of pedogenesis is another major component for assessing carbon (C) and nitrogen (N) cycling. The aim of the presented research was to evaluate consequences of permafrost degradation for C and N stocks and hence nutrient supply for plants, as the transect covers all types of permafrost including heavily degraded areas and regions without permafrost. Our results show that SM is the dominant parameter explaining 64% of C-org and 60% of N variation. The extent of the effect of SM is determined by permafrost, current aeolian sedimentation occurring mostly on degraded sites, and pedogenesis. Thus, the explanatory power for C and N concentrations is significantly improved by adding CaCO3 content (P=0.012 for C-org; P=0.006 for N-t) and soil texture (P=0.077 for C-org; P=0.015 for N-t) to the model. For soil temperature, no correlations were detected indicating that in high-altitude grassland ecosystems influenced by permafrost, SM overrides soil temperature as the main driving parameter at landscape scale. It was concluded from the current study that degradation of permafrost and corresponding changes in soil hydrology combined with a shift from mature stages of pedogenesis to initial stages, have severe impact on soil C and plant available N. This may alter biodiversity patterns as well as the development and functioning of the ecosystems on the Tibetan Plateau.