Soil extracellular enzyme activities correspond with abiotic factors more than fungal community composition

Soil extracellular enzymes are the proximal drivers of decomposition. However, the relative influence of climate, soil nutrients and edaphic factors compared to microbial community composition on extracellular enzyme activities (EEA) is poorly resolved. Determining the relative effects of these factors on soil EEA is critical since changes in climate and microbial species composition may have large impacts on decomposition. We measured EEA from five sites during the growing season in March and 17 sites during the dry season in July throughout southern California and simultaneously collected data on climate, soil nutrients, soil edaphic factors and fungal community composition. The concentration of carbon and nitrogen in the soil and soil pH were most related to hydrolytic EEA. Conversely, oxidative EEA was mostly related to mean annual precipitation. Fungal community composition was not correlated with EEA at the species, genus, family or order levels. The hyphal length of fungi was correlated with EEA during the growing season while relative abundance of taxa within fungal phyla, in particular Chytridiomycota, was correlated with the EEA of beta-glucosidase, cellobiohydrolase, acid phosphatase and beta-xylosidase in the dry season. Overall, in the dry season, 35.3� % of the variation in all enzyme activities was accounted for by abiotic variables, while fungal composition accounted for 27.4� %. Because global change is expected to alter precipitation regimes and increase nitrogen deposition in soils, EEA may be affected, with consequences for decomposition

Evolutionary trade-offs among decomposers determine responses to nitrogen enrichment.

Evolutionary trade-offs among ecological traits are one mechanism that could determine the responses of functional groups of decomposers to global changes such as nitrogen (N) enrichment. We hypothesised that bacteria targeting recalcitrant carbon compounds require relatively high levels of N availability to support the construction costs of requisite extracellular and transport enzymes. Indeed, we found that taxa that used more recalcitrant (i.e. larger and cyclic) carbon compounds were more prevalent in ocean waters with higher nitrate concentrations. Compared to recalcitrant carbon users, labile carbon users targeted more organic N compounds, were found in relatively nitrate-poor waters, and were more common in higher latitude soils, which is consistent with the paradigm that N-limitation is stronger at higher latitudes. Altogether, evolutionary trade-offs may limit recalcitrant carbon users to habitats with higher N availability

Tropical Tree Species Effects on Soil pH and Biotic Factors and the Consequences for Macroaggregate Dynamics

Physicochemical and biotic factors influence the binding and dispersivity of soil particles, and thus control soil macroaggregate formation and stability. Although soil pH influences dispersivity, it is usually relatively constant within a site, and thus not considered a driver of aggregation dynamics. However, land-use change that results in shifts in tree-species composition can result in alteration of soil pH, owing to species-specific traits, e.g., support of nitrogen fixation and Al accumulation. In a long-term, randomized complete block experiment in which climate, soil type, and previous land-use history were similar, we evaluated effects of individual native tropical tree species on water-stable macroaggregate size distributions in an Oxisol. We conducted this study at La Selva Biological Station in Costa Rica, in six vegetation types: 25-year-old plantations of four tree species grown in monodominant stands; an unplanted Control; and an adjacent mature forest. Tree species significantly influenced aggregate proportions in smaller size classes (0.25–1.0 mm), which were correlated with fine-root growth and litterfall. Tree species altered soil pH differentially. Across all vegetation types, the proportion of smaller macroaggregates declined significantly as soil pH increased (p ≤ 0.0184). This suggests that alteration of pH influences dispersivity, and thus macroaggregate dynamics, thereby playing a role in soil C, N, and P cycling

Global diversity and distribution of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi form associations with most land plants and can control carbon, nitrogen, and phosphorus cycling between above- and belowground components of ecosystems. Current estimates of AM fungal distributions are mainly inferred from the individual distributions of plant biomes, and climatic factors. However, dispersal limitation, local environmental conditions,and interactions among AM fungal taxa may also determine local diversity and global distributions. We assessed the relative importance of these potential controls by collecting 14,961 DNA sequences from 111 published studies and testing for relationships between AM fungal community composition and geography, environment, and plant biomes. Our results indicated that the global species richness of AM fungi was up to six times higher than previously estimated, largely owing to high beta diversity among sampling sites. Geographic distance, soil temperature and moisture, and plant community type were each significantly related to AM fungal community structure, but explained only a small amount of the observed variance. AM fungal species also tended to be phylogenetically clustered within sites, further suggesting that habitat filtering or dispersal limitation is a driver of AM fungal community assembly. Therefore, predicted shifts in climate and plant species distributions under global change may alter AM fungal communities

KING OF THE HILL? HOW BIOTIC INTERACTIONS AFFECT BIOGEOGRAPHICAL PATTERN AND SPECIES RESPONSES TO CLIMATE CHANGE

As climate has warmed, many species have moved up mountains as physiological limits to their distributions have ameliorated. These distribution shifts are creating novel communities, begging the question: What happens to species at the tops of mountains as potential antagonists encroach upwards? Theory predicts that upward migrations will cause range contractions for high-elevation species because of novel interactions with encroaching antagonists. My dissertation work is one of the most comprehensive tests of this question to date, using a combination of ecological niche modeling (ENM), experiments, and demographic and trait-based modeling approaches. I created novel ENMs that suggest context-dependency of biotic interactions, where predictions of biotic interactions change from positive to negative over environmental gradients, is common over elevation gradients. Additionally, ENMs suggested the current focus on plant-plant interactions in niche modeling targets the most important biotic interaction for many species. I then constructed space-for-time experiments that transplanted alpine species into novel low elevation plant and mammal communities expected to encroach upwards, as well as into their native high elevation communities. Plant competition was manipulated by vegetation removals and mammals were excluded in a separate factorial experiment using below- and aboveground fencing. In both experiments, low elevation plant and mammal communities suppressed growth of alpine species to a greater extent than those antagonists found in their home range. However, demographic models suggested that environmental factors (e.g. temperature) other than novel plant and mammal communities are more consequential for determining population fate. The experiments validated a novel trait-based model of competitive interactions that can be broadly applied to other systems and conservation needs. My dissertation work found that alpine plants are unlikely to remain “king of the hill” under climate change, in part due to the upward encroachment of novel competitors and intensification of herbivore pressure

Global Imprint of Mycorrhizal Fungi on Whole-Plant Nutrient Economics

Mycorrhizal fungi are critical members of the plant microbiome, forming a symbiosis with the roots of most plants on Earth. Most plant species partner with either arbuscular or ectomycorrhizal fungi, and these symbioses are thought to represent plant adaptations to fast and slow soil nutrient cycling rates. This generates a second hypothesis, that arbuscular and ectomycorrhizal plant species traits complement and reinforce these fungal strategies, resulting in nutrient acquisitive vs. conservative plant trait profiles. Here we analyzed 17,764 species level trait observations from 2,940 woody plant species to show that mycorrhizal plants differ systematically in nitrogen and phosphorus economic traits. Differences were clearest in temperate latitudes, where ectomycorrhizal plant species are more nitrogen use- and phosphorus use-conservative than arbuscular mycorrhizal species. This difference is reflected in both aboveground and belowground plant traits and is robust to controlling for evolutionary history, nitrogen fixation ability, deciduousness, latitude, and species climate niche. Furthermore, mycorrhizal effects are large and frequently similar to or greater in magnitude than the influence of plant nitrogen fixation ability or deciduous vs. evergreen leaf habit. Ectomycorrhizal plants are also more nitrogen conservative than arbuscular plants in boreal and tropical ecosystems, although differences in phosphorus use are less apparent outside temperate latitudes. Our findings bolster current theories of ecosystems rooted in mycorrhizal ecology and support the hypothesis that plant mycorrhizal association is linked to the evolution of plant nutrient economic strategies

Differential Organic Carbon Mineralization Responses to Soil Moisture in Three Different Soil Orders Under Mixed Forested System

Soil microbial respiration is one of the largest sources of carbon (C) emissions to the atmosphere in terrestrial ecosystems, which is strongly dependent on multiple environmental variables including soil moisture. Soil moisture content is strongly dependent on soil texture, and the combined effects of texture and moisture on microbial respiration are complex and less explored. Therefore, this study examines the effects of soil moisture on the mineralization of soil organic C Soil organic carbon in three different soils, Ultisol, Alfisol and Vertisol, collected from mixed forests of Georgia, Missouri, and Texas, United States , respectively. A laboratory microcosm experiment was conducted for 90 days under different moisture regimes. Soil respiration was measured weekly, and destructive harvests were conducted at 1, 15, 60, and 90 days after incubation to determine extractable organic C (EOC), phospholipid fatty acid based microbial community, and C-acquiring hydrolytic extracellular enzyme activities (EEA). The highest cumulative respiration in Ultisol was observed at 50% water holding capacity (WHC), in Alfisol at 100% water holding capacity, and in Vertisol at 175% WHC. The trends in Extractable Organic Carbon were opposite to that of cumulative microbial respiration as the moisture levels showing the highest respiration showed the lowest EOC concentration in all soil types. Also, extracellular enzyme activities increased with increase in soil moisture in all soils, however, respiration and EEA showed a decoupled relationship in Ultisol and Alfisol soils. Soil moisture differences did not influence microbial community composition