Impacts of snow-farming on alpine soil and vegetation: A case study from the Swiss Alps.

Snow-farming is one of the adaptive strategies used to face the snow deficit in ski resorts. We studied the impact of a shifting snow-farming technique on a pasture slope in Adelboden, Switzerland. Specifically, we compared plots covered by a compressed snow pile for 1.5, 2.5 or 3.5 years, which then recovered from the snow cover for three, two or one vegetation seasons, respectively, with control plots situated around the snow pile. In plots with >1.5 years of compressed snow pile, plant mortality was high, recovery of vegetation was very slow, and few plant species recolonized the bare surface. Soil biological activity decreased persistently under prolonged snow cover, as indicated by reduced soil respiration. The prolonged absence of fresh plant litter and root exudates led to carbon (C) limitation for soil microbial respiration, which resulted in a significant decrease in the ratio of total organic carbon to total nitrogen (TOC/TN) under the snow pile. Microbial C, nitrogen (N) and phosphorus (P) immobilization decreased, while dissolved N concentration increased with compressed snow cover. Longer snow cover and a subsequent shorter recovery period led to higher microbial C/P and N/P but lower microbial C/N. Nitrate and ammonium were released massively once the biological activity resumed after snow clearance and soil aeration. The soil microbial community composition persistently shifted towards oxygen-limited microbes with prolonged compressed snow cover. This shift reflected declines in the abundance of sensitive microorganisms, such as plant-associated symbionts, due to plant mortality or root die-off. In parallel, resistant taxa that benefit from environmental changes increased, including facultative anaerobic bacteria (Bacteroidota, Chloroflexota), obligate anaerobes (Euryarchaeota), and saprophytic plant degraders. We recommend keeping snow piles in the same spot year after year to minimize the area of the impacted soil surface and plan from the beginning soil and ecosystem restoration measures.

Nitrogen translocation by Highland cattle grazing in Alnus viridis-encroached pastures.

During the last decades, Alnus viridis has expanded over former montane pastures and meadows, due to land use and abandonment. This nitrogen-fixing woody species has triggered negative agro-environmental impacts, such as nitrogen (N) leaching, soil acidification and a reduced biodiversity. The aim of this study was to estimate the N translocation from A. viridis-encroached areas to adjacent open pastures by Highland cattle. In 2019 and 2020, Highland cattle herds equipped with GPS collars were placed in four A. viridis-encroached paddocks across Italy and Switzerland. The N content was measured in A. viridis leaves, herbaceous vegetation, and cattle dung pats, which were collected throughout the grazing season. Using GPS locations and collar activity sensors, livestock activity phases were discriminated. The N ingested by cattle was estimated through the N content of herbaceous vegetation and A. viridis leaves of vegetation patches visited by cattle during 24 h before dung sampling (N24H). The N content of herbaceous vegetation significantly increased with increasing A. viridis cover. The average N content in dung pats (31.2 ± 3.4 g.kg-1 DM) was higher than average values from literature on grazing cattle. Moreover, it was positively related to the N24H. Most of this N (29.5 ± 10.3 kg ha-1 yr-1) was translocated towards resting areas, which generally occurred on flat open pastures. Our results highlight the potential of Highland cattle to effectively translocate part of the ingested N from A. viridis-encroached towards targeted open areas, thus bringing new perspective for forage yield and quality improvement in the long-term. Supplementary Information: The online version contains supplementary material available at 10.1007/s10705-023-10282-0.

Regional and local determinants of drought resilience in tropical forests.

The increase in severity of droughts associated with greater mortality and reduced vegetation growth is one of the main threats to tropical forests. Drought resilience of tropical forests is affected by multiple biotic and abiotic factors varying at different scales. Identifying those factors can help understanding the resilience to ongoing and future climate change. Altitude leads to high climate variation and to different forest formations, principally moist or dry tropical forests with contrasted vegetation structure. Each tropical forest can show distinct responses to droughts. Locally, topography is also a key factor controlling biotic and abiotic factors related to drought resilience in each forest type. Here, we show that topography has key roles controlling biotic and abiotic factors in each forest type. The most important abiotic factors are soil nutrients, water availability, and microclimate. The most important biotic factors are leaf economic and hydraulic plant traits, and vegetation structure. Both dry tropical forests and ridges (steeper and drier habitats) are more sensitive to droughts than moist tropical forest and valleys (flatter and wetter habitats). The higher mortality in ridges suggests that conservative traits are not sufficient to protect plants from drought in drier steeper habitats. Our synthesis highlights that altitude and topography gradients are essential to understand mechanisms of tropical forest's resilience to future drought events. We described important factors related to drought resilience, however, many important knowledge gaps remain. Filling those gaps will help improve future practices and studies about mitigation capacity, conservation, and restoration of tropical ecosystems.

Lowland plant arrival in alpine ecosystems facilitates a decrease in soil carbon content under experimental climate warming.

Climate warming is releasing carbon from soils around the world, constituting a positive climate feedback. Warming is also causing species to expand their ranges into new ecosystems. Yet, in most ecosystems, whether range expanding species will amplify or buffer expected soil carbon loss is unknown. Here, we used two whole-community transplant experiments and a follow-up glasshouse experiment to determine whether the establishment of herbaceous lowland plants in alpine ecosystems influences soil carbon content under warming. We found that warming (transplantation to low elevation) led to a negligible decrease in alpine soil carbon content, but its effects became significant and 52% ± 31% (mean ± 95% confidence intervals) larger after lowland plants were introduced at low density into the ecosystem. We present evidence that decreases in soil carbon content likely occurred via lowland plants increasing rates of root exudation, soil microbial respiration, and CO2 release under warming. Our findings suggest that warming-induced range expansions of herbaceous plants have the potential to alter climate feedbacks from this system, and that plant range expansions among herbaceous communities may be an overlooked mediator of warming effects on carbon dynamics.

In a terrestrial ecosystem, the carbon cycle primarily represents the balance between plants consuming carbon dioxide from the atmosphere and soil microbes releasing carbon stored in the soil into the atmosphere (mostly as carbon dioxide). Given that carbon dioxide traps heat in the atmosphere, the balance of carbon inputs and outputs from an ecosystem can have important consequences for climate change. Rising temperatures caused by climate warming have led plants from lowland ecosystems to migrate uphill and start growing in alpine ecosystems, where temperatures are lower and most carbon is stored in the soil. Soil microbes use carbon stored in the soil and exuded from plants to grow, and they release this carbon ­ in the form of carbon dioxide ­ into the atmosphere through respiration. Walker et al. wanted to know how the arrival of lowland plants in alpine ecosystems under climate warming would affect carbon stores in the soil. To answer this question, Walker et al. simulated warmer temperatures by moving turfs (plants and soil) from alpine ecosystems to a warmer downhill site and planting lowland plants into the turfs. They compared the concentration of soil carbon in these turfs to that of soil in alpine turfs that had not been moved downhill and had no lowland plants. Their results showed that the warmed turfs containing lowland plants had a lower concentration of soil carbon. This suggests that climate warming will lead to more soil carbon being released into the atmosphere if lowland plants also migrate into alpine ecosystems. Walker et al. also wanted to know the mechanism through which lowland plants were decreasing soil carbon concentration under warming. They find that lowland plants probably release more small molecules into the soil than alpine plants. Soil microbes use the carbon and nutrients in these molecules to break down more complex molecules in the soil, thereby releasing nutrients and carbon that can then be used in respiration. This finding suggests that soil microbes breakdown and respire native soil carbon faster in the presence of lowland plants, releasing more carbon dioxide into the atmosphere and reducing carbon stores in the soil. Walker et al.'s results reveal a new mechanism through which uphill migration of lowland plants could increase the effects of climate change, in a feedback loop. Further research as to whether this mechanism occurs in different regions and ecosystems could help to quantify the magnitude of this feedback and allow scientists to make more accurate predictions about climate change.

Behavior and interactions of the plant growth-promoting bacteria Azospirillum oryzae NBT506 and Bacillus velezensis UTB96 in a co-culture system.

The objective of the present study was to evaluate possible interactions between two potential plant growth-promoting bacteria (PGPB): Azospirillum oryzae strain NBT506 and Bacillus velezensis strain UTB96. To do this, the growth kinetic, biofilm formation, motility, surfactin production, indole-3-acetic acid (IAA) production, phosphate solubilization and enzyme activities of the strains were measured in monoculture and co-culture. The maximum biomass production for the strains in monoculture and co-culture was about 1011 CFU/ml, confirming that these two strains have the potential to grow in co-culture without reduction of biomass efficiency. The co-culture system showed more stable biofilm formation until the end of day 3. Azospirillum showed the maximum IAA production (41.5 mg/l) in a monoculture compared to other treatments. Surfactin promoted both swimming and swarming motility in all treatments. The Bacillus strain in the monoculture and co-culture showed high phosphate solubilizing capability, which increased continuously in the co-culture system after 6 days. The strains showed protease, amylase and cellulase activities in both monoculture and co-culture forms. Chitinase and lipase activities were observed in both the monoculture of the Bacillus strain and the co-culture. Overall, our findings highlight the promotion of biological and beneficial effects of these bacteria when growing together in co-culture.

Plant traits and soil fertility mediate productivity losses under extreme drought in C3 grasslands.

Extreme drought decreases aboveground net primary production (ANPP) in most grasslands, but the magnitude of ANPP reductions varies especially in C3 -dominated grasslands. Because the mechanisms underlying such differential ecosystem responses to drought are not well resolved, we experimentally imposed an extreme 4-yr drought (2015-2018) in two C3 grasslands that differed in aridity. These sites had similar annual precipitation and dominant grass species (Leymus chinensis) but different annual temperatures and thus water availability. Drought treatments differentially affected these two semiarid grasslands, with ANPP of the drier site reduced more than at the wetter site. Structural equation modeling revealed that community-weighted means for some traits modified relationships between soil moisture and ANPP, often due to intraspecific variation. Specifically, drought reduced community mean plant height at both sites, resulting in a reduction in ANPP beyond that attributable to reduced soil moisture alone. Higher community mean leaf carbon content enhanced the negative effects of drought on ANPP at the drier site, and ANPP-soil-moisture relationships were influenced by soil C:N ratio at the wetter site. Importantly, neither species richness nor functional dispersion were significantly correlated with ANPP at either site. Overall, as expected, soil moisture was a dominant, direct driver of ANPP response to drought, but differential sensitivity to drought in these two grasslands was also related to soil fertility and plant traits.

Topography and vegetation structure mediate drought impacts on the understory of the South American Atlantic Forest.

Droughts have increased in frequency, duration, and severity across most of the tropics but their effect on forest communities remain not fully understood. Here we assessed the effects of a severe El Niño-induced drought event on dominant and low abundance understory plant species and the consequent impacts on ecosystem functions in the South American Atlantic Forest. We established 20 permanent plots with contrasting vegetation structure and topography. In each plot, we measured the stem diameter at breast height (DBH) of every understory woody plant (i.e. 1 to 10 cm stem diameter) before and after a severe 4-year drought event to calculate relative growth and mortality rates after drought. Litter biomass, litter nutrient content and soil nutrients, as well as tree canopy cover, were also quantified. High stem density reduced survival to drought for both dominant and low abundance understory woody species. The growth rate of dominant and low abundance species was lower on steeper slopes during the drought. Dominant species were the main contributor of litter biomass production whereas low abundance species were important drivers of litter quality. Overall, our findings suggest that habitats with low tree density and larger trees on flat areas, such as in valleys, can act as refuges for understory plant species during drought periods. These habitats are resource-rich, providing nutrients and water during unfavorable drought periods and might improve forest resilience to climate change in the long term.

Suppression of arbuscular mycorrhizal fungi decreases the temporal stability of community productivity under elevated temperature and nitrogen addition in a temperate meadow.

Global change alters how terrestrial ecosystems function and makes them less stable over time. Global change can also suppress the development and effectiveness of arbuscular mycorrhizal fungi (AMF). This is concerning, as AMF have been shown to alleviate the negative influence of global changes on plant growth and maintain species coexistence. However, how AMF and global change interact and influence community temporal stability remains poorly understood. Here, we conducted a 4-year field experiment and used structural equation modeling (SEM) to explore the influence of elevated temperature, nitrogen (N) addition and AMF suppression on community temporal stability (quantified as the ratio of the mean community productivity to its standard deviation) in a temperate meadow in northern China. We found that elevated temperature and AMF suppression independently decreased the community temporal stability but that N addition had no impact. Community temporal stability was mainly driven by elevated temperature, N addition and AMF suppression that modulated the dominant species stability; to a lesser extent by the elevated temperature and AMF suppression that modulated AMF richness associated with community asynchrony; and finally by the N addition and AMF suppression that modulated mycorrhizal colonization. In addition, although N addition, AMF suppression and elevated temperature plus AMF suppression reduced plant species richness, there was no evidence that changes in community temporal stability were linked to changes in plant richness. SEM further showed that elevated temperature, N addition and AMF suppression regulated community temporal stability by influencing both the temporal mean and variation in community productivity. Our results suggest that global environmental changes may have appreciable consequences for the stability of temperate meadows while also highlighting the role of belowground AMF status in the responses of plant community temporal stability to global change.

Seasonal variability of groundwater level effects on the growth of Carex cinerascens in lake wetlands.

Groundwater level is crucial for wetland plant growth and reproduction, but the extent of its effect on plant growth can vary along with changed precipitation and temperature at different seasons. In this context, we investigated the effect of two groundwater levels (10 cm vs. 20 cm depth) on growth and reproductive parameters of Carex cinerascens, a dominant plant species in the Poyang Lake wetland, during three seasons (spring, summer, and autumn) and during two consecutive years (2015 and 2016). Carex cinerascens showed low stem number, height, and individual and population biomass in summer compared to spring and autumn. 10 cm groundwater level was overall more suitable for plant growth resulting in higher stem height and biomass. However, the interactive effect between groundwater level and season clearly demonstrated that the effect of groundwater level on plant growth occurred mainly in autumn. After the withering of the plant population in summer, we observed that C. cinerascens growth recovered in autumn to similar values observed in spring only with 10 cm groundwater level. Consequently, we could deduce that lowering groundwater level in the studied Poyang Lake wetland will negatively impact C. cinerascens regeneration and growth particularly during the second growth cycle occurring in autumn. Additionally, our results showed that, independently of the season and groundwater level, population biomass of C. cinerascens was lower during drier year. Altogether, our findings suggest that water limitation due to both reduction in precipitation and decreased groundwater level during the year can strongly impact plant communities.

Responses of plant leaf economic and hydraulic traits mediate the effects of early- and late-season drought on grassland productivity.

Drought can occur at different times during the grassland growing season, likely having contrasting effects on forage production when happening early or later in the season. However, knowledge about the interacting effects of the timing of drought and the development stage of the vegetation during the growing season is still scarce, thus limiting our ability to accurately predict forage quantity losses. To investigate plant community responses to drought seasonality (early- vs. late-season), we established a drought experiment in two permanent grasslands of the Swiss Jura Mountains that are used for forage production. We measured three plant functional traits, including two leaf traits related to plant economics (specific leaf area, SLA; leaf dry matter content, LDMC) and one hydraulic trait related to physiological function (predicted percentage loss of hydraulic conductance, PLCp), of the most abundant species, and plant above-ground biomass production. Plant species composition was also determined to calculate community-weighted mean (CWM) traits. First, we observed that CWM trait values strongly varied during the growing season. Second, we found that late-season drought had stronger effects on CWM trait values than early-season drought and that the plant hydraulic trait was the most variable functional trait. Using a structural equation model, we also showed that reduction in soil moisture had no direct impacts on above-ground biomass production. Instead, we observed that the drought-induced decrease in above-ground biomass production was mediated by a higher CWM PLCp (i.e. higher risk of hydraulic failure) and lower CWM SLA under drought. Change in CWM SLA in response to drought was the best predictor of community above-ground biomass production. Our findings reveal the importance of drought timing together with the plant trait responses to assess drought impacts on grassland biomass production and suggest that incorporating these factors into mechanistic models could considerably improve predictions of climate change impacts.

Linking soil's volatilome to microbes and plant roots highlights the importance of microbes as emitters of belowground volatile signals.

Plants and microbes release a plethora of volatiles that act as signals in plant-microbe interactions. Characterizing soil's volatilome and microbiome might shed light on the nature of relevant volatile signals and on their emitters. This hypothesis was tested by characterizing plant cover, soil's volatilome, nutrient content and microbiomes in three grasslands of the Swiss Jura Mountains. The fingerprints of soil's volatiles were generated by solid-phase micro-extraction gas chromatography/mass spectrometry, whereas high-throughput sequencing was used to create a snapshot of soil's microbial communities. A high similarity was observed in plant communities of two out of three sites, which was mirrored by the soil's volatilome. Multiple factor analysis evidenced a strong association among soil's volatilome, plant and microbial communities. The proportion of volatiles correlated to single bacterial and fungal taxa was higher than for plants. This suggests that those organisms might be major contributors to the volatilome of grassland soils. These findings illustrate that key volatiles in grassland soils might be emitted by a handful of organisms that include specific plants and microbes. Further work will be needed to unravel the structure of belowground volatiles and understand their implications for plant health and development.

Long-term regeneration of a tropical plant community after sand mining.

Sandy coastal plant communities in tropical regions have been historically under strong anthropic pressure. In Brazil, these systems shelter communities with highly plastic plant species. However, the potential of these systems to regenerate without human assistance after disturbances has hardly been examined. We determined the natural regeneration of a coastal sandy plain vegetation (restinga) in Brazil, 16 years after the end of sand removal. We inventoried 38 plots: 20 within a sand-mined site and 18 in an adjacent undisturbed site. We expected lower diversity values in the sand-mined site compared to the undisturbed site, but similar species composition between the two sites due to the spatial proximity of the two sites and the high plasticity of restinga species. Species were ranked using abundance and importance value index in both sites, and comparisons were performed using Rényi entropy profiles, rarefaction curves, principal component analysis, and redundancy analysis. Species composition and dominant species differed markedly between the two sites. Bromeliads and Clusia hilariana, well-known nurse plants, dominated the undisturbed site but were almost absent in the regenerating site. Species richness did not differ between both sites, but diversity was higher in the undisturbed site. Within-site composition differences in the mined area were associated with field characteristics. Interestingly, species classified as subordinate or rare in the undisturbed site became dominants in the regenerating site. These newer dominants in the sand-mined site are not those known as nurse plants in other restingas, thus yielding strong implications for restoration.

Mineral-Associated Soil Carbon is Resistant to Drought but Sensitive to Legumes and Microbial Biomass in an Australian Grassland.

Drought is predicted to increase in many areas of the world with consequences for soil carbon (C) dynamics. Plant litter, root exudates and microbial biomass can be used as C substrates to form organo-mineral complexes. Drought effects on plants and microbes could potentially compromise these relative stable soil C pools, by reducing plant C inputs and/or microbial activity. We conducted a 2-year drought experiment using rainout shelters in a semi-natural grassland. We measured aboveground biomass and C and nitrogen (N) in particulate organic matter (Pom), the organo-mineral fraction (Omin), and microbial biomass within the first 15 cm of soil. Aboveground plant biomass was reduced by 50% under drought in both years, but only the dominant C4 grasses were significantly affected. Soil C pools were not affected by drought, but were significantly higher in the relatively wet second year compared to the first year. Omin-C was positively related to microbial C during the first year, and positively related to clay and silt content in the second year. Increases in Omin-C in the second year were explained by increases in legume biomass and its effect on Pom-N and microbial biomass N (MBN) through structural equation modeling. In conclusion, soil C pools were unaffected by the drought treatment. Drought resistant legumes enhanced formation of organo-mineral complexes through increasing Pom-N and MBN. Our findings also indicate the importance of microbes for the formation of Omin-C as long as soil minerals have not reached their maximum capacity to bind with C (that is, saturation).

Tipping point in plant-fungal interactions under severe drought causes abrupt rise in peatland ecosystem respiration.

Ecosystems are increasingly prone to climate extremes, such as drought, with long-lasting effects on both plant and soil communities and, subsequently, on carbon (C) cycling. However, recent studies underlined the strong variability in ecosystem's response to droughts, raising the issue of nonlinear responses in plant and soil communities. The conundrum is what causes ecosystems to shift in response to drought. Here, we investigated the response of plant and soil fungi to drought of different intensities using a water table gradient in peatlands-a major C sink ecosystem. Using moving window structural equation models, we show that substantial changes in ecosystem respiration, plant and soil fungal communities occurred when the water level fell below a tipping point of -24 cm. As a corollary, ecosystem respiration was the greatest when graminoids and saprotrophic fungi became prevalent as a response to the extreme drought. Graminoids indirectly influenced fungal functional composition and soil enzyme activities through their direct effect on dissolved organic matter quality, while saprotrophic fungi directly influenced soil enzyme activities. In turn, increasing enzyme activities promoted ecosystem respiration. We show that functional transitions in ecosystem respiration critically depend on the degree of response of graminoids and saprotrophic fungi to drought. Our results represent a major advance in understanding the nonlinear nature of ecosystem properties to drought and pave the way towards a truly mechanistic understanding of the effects of drought on ecosystem processes.

Plant-Soil Feedback: Bridging Natural and Agricultural Sciences.

In agricultural and natural systems researchers have demonstrated large effects of plant-soil feedback (PSF) on plant growth. However, the concepts and approaches used in these two types of systems have developed, for the most part, independently. Here, we present a conceptual framework that integrates knowledge and approaches from these two contrasting systems. We use this integrated framework to demonstrate (i) how knowledge from complex natural systems can be used to increase agricultural resource-use efficiency and productivity and (ii) how research in agricultural systems can be used to test hypotheses and approaches developed in natural systems. Using this framework, we discuss avenues for new research toward an ecologically sustainable and climate-smart future.

Arbuscular mycorrhizal fungi reduce the differences in competitiveness between dominant and subordinate plant species.

In grassland communities, plants can be classified as dominants or subordinates according to their relative abundances, but the factors controlling such distributions remain unclear. Here, we test whether the presence of the arbuscular mycorrhizal (AM) fungus Glomus intraradices affects the competitiveness of two dominant (Taraxacum officinale and Agrostis capillaris) and two subordinate species (Prunella vulgaris and Achillea millefolium). Plants were grown in pots in the presence or absence of the fungus, in monoculture and in mixtures of both species groups with two and four species. In the absence of G. intraradices, dominants were clearly more competitive than subordinates. In inoculated pots, the fungus acted towards the parasitic end of the mutualism-parasitism continuum and had an overall negative effect on the growth of the plant species. However, the negative effects of the AM fungus were more pronounced on dominant species reducing the differences in competitiveness between dominant and subordinate species. The effects of G. intraradices varied with species composition highlighting the importance of plant community to mediate the effects of AM fungi. Dominant species were negatively affected from the AM fungus in mixtures, while subordinates grew identically with and without the fungus. Therefore, our findings predict that the plant dominance hierarchy may flatten out when dominant species are more reduced than subordinate species in an unfavourable AM fungal relationship (parasitism).

Nationwide investigation of extended-spectrum beta-lactamases, metallo-beta-lactamases, and extended-spectrum oxacillinases produced by ceftazidime-resistant Pseudomonas aeruginosa strains in France.

A nationwide study aimed to identify the extended-spectrum beta-lactamases (ESBLs), metallo-beta-lactamases (MBLs), and extended-spectrum oxacillinases (ES-OXAs) in a French collection of 140 clinical Pseudomonas aeruginosa isolates highly resistant to ceftazidime. Six ESBLs (PER-1, n=3; SHV-2a, n=2; VEB-1a, n=1), four MBLs (VIM-2, n=3; IMP-18, n=1), and five ES-OXAs (OXA-19, n=4; OXA-28, n=1) were identified in 13 isolates (9.3% of the collection). The prevalence of these enzymes is still low in French clinical P. aeruginosa isolates but deserves to be closely monitored.