Diana H. Wall

Ecosystem carbon loss with woody plant invasion of grasslands

The invasion of woody vegetation into deserts, grasslands and savannas is generally thought to lead to an increase in the amount of carbon stored in those ecosystems. For this reason, shrub and forest expansion (for example, into grasslands) is also suggested to be a substantial, if uncertain, component of the terrestrial carbon sink. Here we investigate woody plant invasion along a precipitation gradient (200 to 1,100 mm yr-1) by comparing carbon and nitrogen budgets and soil δ13C profiles between six pairs of adjacent grasslands, in which one of each pair was invaded by woody species 30 to 100 years ago. We found a clear negative relationship between precipitation and changes in soil organic carbon and nitrogen content when grasslands were invaded by woody vegetation, with drier sites gaining, and wetter sites losing, soil organic carbon. Losses of soil organic carbon at the wetter sites were substantial enough to offset increases in plant biomass carbon, suggesting that current land-based assessments may overestimate carbon sinks. Assessments relying on carbon stored from woody plant invasions to balance emissions may therefore be incorrect.

Cross-biome metagenomic analyses of soil microbial communities and their functional attributes

For centuries ecologists have studied how the diversity and functional traits of plant and animal communities vary across biomes. In contrast, we have only just begun exploring similar questions for soil microbial communities despite soil microbes being the dominant engines of biogeochemical cycles and a major pool of living biomass in terrestrial ecosystems. We used metagenomic sequencing to compare the composition and functional attributes of 16 soil microbial communities collected from cold deserts, hot deserts, forests, grasslands, and tundra. Those communities found in plant-free cold desert soils typically had the lowest levels of functional diversity (diversity of protein-coding gene categories) and the lowest levels of phylogenetic and taxonomic diversity. Across all soils, functional beta diversity was strongly correlated with taxonomic and phylogenetic beta diversity; the desert microbial communities were clearly distinct from the nondesert communities regardless of the metric used. The desert communities had higher relative abundances of genes associated with osmoregulation and dormancy, but lower relative abundances of genes associated with nutrient cycling and the catabolism of plant-derived organic compounds. Antibiotic resistance genes were consistently threefold less abundant in the desert soils than in the nondesert soils, suggesting that abiotic conditions, not competitive interactions, are more important in shaping the desert microbial communities. As the most comprehensive survey of soil taxonomic, phylogenetic, and functional diversity to date, this study demonstrates that metagenomic approaches can be used to build a predictive understanding of how microbial diversity and function vary across terrestrial biomes.

Diversity meets decomposition

Over 100 gigatons of terrestrial plant biomass are produced globally each year. Ninety percent of this biomass escapes herbivory and enters the dead organic matter pool, thus supporting complex detritus-based food webs that determine the critical balance between carbon mineralization and sequestration. How will changes in biodiversity affect this vital component of ecosystem functioning? Based on our analysis of concepts and experiments of leaf decomposition in forest floors and streams, we suggest that changes in species diversity within and across trophic levels can significantly alter decomposition. This happens through various mechanisms that are broadly similar in forest floors and streams. Differences in diversity effects between these systems relate to divergent habitat conditions and evolutionary trajectories of aquatic and terrestrial decomposers.

Antarctic climate cooling and terrestrial ecosystem response

The average air temperature at the Earths surface has increased by 0.06 °C per decade during the 20th century, and by 0.19 °C per decade from 1979 to 1998. Climate models generally predict amplified warming in polar regions, as observed in Antarcticas peninsula region over the second half of the 20th century. Although previous reports suggest slight recent continental warming, our spatial analysis of Antarctic meteorological data demonstrates a net cooling on the Antarctic continent between 1966 and 2000, particularly during summer and autumn. The McMurdo Dry Valleys have cooled by 0.7 °C per decade between 1986 and 2000, with similar pronounced seasonal trends. Summer cooling is particularly important to Antarctic terrestrial ecosystems that are poised at the interface of ice and water. Here we present data from the dry valleys representing evidence of rapid terrestrial ecosystem response to climate cooling in Antarctica, including decreased primary productivity of lakes (6–9% per year) and declining numbers of soil invertebrates (more than 10% per year). Continental Antarctic cooling, especially the seasonality of cooling, poses challenges to models of climate and ecosystem change.

HABITAT LOSS, TROPHIC COLLAPSE, AND THE DECLINE OF ECOSYSTEM SERVICES

The provisioning of sustaining goods and services that we obtain from natural ecosystems is a strong economic justification for the conservation of biological diversity. Understanding the relationship between these goods and services and changes in the size, arrangement, and quality of natural habitats is a fundamental challenge of natural resource management. In this paper, we describe a new approach to assessing the implications of habitat loss for loss of ecosystem services by examining how the provision of different ecosystem services is dominated by species from different trophic levels. We then develop a mathematical model that illustrates how declines in habitat quality and quantity lead to sequential losses of trophic diversity. The model suggests that declines in the provisioning of services will initially be slow but will then accelerate as species from higher trophic levels are lost at faster rates. Comparison of these patterns with empirical examples of ecosystem collapse (and assembly) suggest similar patterns occur in natural systems impacted by anthropogenic change. In general, ecosystem goods and services provided by species in the upper trophic levels will be lost before those provided by species lower in the food chain. The decrease in terrestrial food chain length predicted by the model parallels that observed in the oceans following overexploitation. The large area requirements of higher trophic levels make them as susceptible to extinction as they are in marine systems where they are systematically exploited. Whereas the traditional species-area curve suggests that 50% of species are driven extinct by an order-of-magnitude decline in habitat abundance, this magnitude of loss may represent the loss of an entire trophic level and all the ecosystem services performed by the species on this trophic level.

Global decomposition experiment shows soil animal impacts on decomposition are climate-dependent

Climate and litter quality are primary drivers of terrestrial decomposition and, based on evidence from multisite experiments at regional and global scales, are universally factored into global decomposition models. In contrast, soil animals are considered key regulators of decomposition at local scales but their role at larger scales is unresolved. Soil animals are consequently excluded from global models of organic mineralization processes. Incomplete assessment of the roles of soil animals stems from the difficulties of manipulating invertebrate animals experimentally across large geographic gradients. This is compounded by deficient or inconsistent taxonomy. We report a global decomposition experiment to assess the importance of soil animals in C mineralization, in which a common grass litter substrate was exposed to natural decomposition in either control or reduced animal treatments across 30 sites distributed from 43°S to 68°N on six continents. Animals in the mesofaunal size range were recovered from the litter by Tullgren extraction and identified to common specifications, mostly at the ordinal level. The design of the trials enabled faunal contribution to be evaluated against abiotic parameters between sites. Soil animals increase decomposition rates in temperate and wet tropical climates, but have neutral effects where temperature or moisture constrain biological activity. Our findings highlight that faunal influences on decomposition are dependent on prevailing climatic conditions. We conclude that (1) inclusion of soil animals will improve the predictive capabilities of region- or biome-scale decomposition models, (2) soil animal influences on decomposition are important at the regional scale when attempting to predict global change scenarios, and (3) the statistical relationship between decomposition rates and climate, at the global scale, is robust against changes in soil faunal abundance and diversity.

Relationships at the aboveground-belowground interface: Plants, soil biota, and soil processes

Interactions at the aboveground-below ground interface provide important feedbacks that regulate ecosystem processes. Organisms within soil food webs are involved in processes of decomposition and nutrient mineralization, and their abundance and activity have been linked to plant ecophysiological traits such as species identity and the quality and quantity of plant tissue. We tested aboveground-below ground diversity relationships in a naturally developed plant community of native tallgrass prairie by taking soil samples from beneath naturally established grass tillers of chosen characteristics (e.g., homogeneous vs. heterogeneous plant combinations or C-4 vs. C-3 photosynthetic pathway) without imposing any disturbances to existing plant-soil relationships. The goal of this study was to elucidate the consequences, for soil microbiota (microflora phospholipid fatty acids, protozoa, and nematode functional groups) and for C and N mineralization, of plant community properties such as species richness, resource quality, resource heterogeneity, species identity, and presence of exotics. None of the biotic or abiotic soil variables was related to plant resource heterogeneity. Protozoa were not responsive to any of the plant community traits. Some bacterial and nematode groups were affected by plant characteristics specific to a particular plant species, but no uniform pattern emerged. Invasive and native plants generally were similar with respect to soil variables tested in this study. The lack of clear responses of soil variables to plant community traits indicates that idiosyncratic effects dominate both at the plant and soil biotic level and that generalized plant and soil diversity effects are hard to predict.

Controls on soil biodiversity: insights from extreme environments

Abstract Research in low biodiversity extreme environments allows separation of the climatic, soil and biological interactions that determine soil biodiversity and community structure. Studies focused on the response of low diversity communities in soils of the Antarctic Dry Valleys and the Chihuahuan Desert of the southwestern USA, to manipulations of soil resources and climate, offer the best opportunity to learn about the environmental controls on soil biodiversity and the role of biodiversity in soil functioning. We propose that insights based on research in these extreme environments should be applicable to understanding soil biodiversity in more complex, temperate and tropical ecosystems. The study of extreme soil ecosystems may also provide information on the response of soil biodiversity to increasing occurrences of environmental extremes that are predicted to occur from global change models. Studies from hot and cold deserts show that decomposition-based food webs can be very simple, that aridity produces similar mechanisms for survival and dispersal of organisms in temperate and polar systems, that suitable soil habitats are patchily distributed in arid environments, and the low biodiversity of extreme soil ecosystems creates little or no functional redundancy making these systems susceptible to disturbance. We suggest that species within the same functional group can have small differences in ecology that are sufficient to affect ecosystem processes. When this occurs, differential responses of species to disturbance within a functional group will not stabilise the soil ecosystem, but rather lead to dramatic changes in community composition and ecosystem process rates.

Long-term and large-scale perspectives on the relationship between biodiversity and ecosystem functioning

Abstract In a growing body of literature from a variety of ecosystems is strong evidence that various components of biodiversity have significant impacts on ecosystem functioning. However, much of this evidence comes from short-term, small-scale experiments in which communities are synthesized from relatively small species pools and conditions are highly controlled. Extrapolation of the results of such experiments to longer time scales and larger spatial scales—those of whole ecosystems—is difficult because the experiments do not incorporate natural processes such as recruitment limitation and colonization of new species. We show how long-term study of planned and accidental changes in species richness and composition suggests that the effects of biodiversity on ecosystem functioning will vary over time and space. More important, we also highlight areas of uncertainty that need to be addressed through coordinated cross-scale and cross-site research.

Climate and litter quality differently modulate the effects of soil fauna on litter decomposition across biomes

Climate and litter quality have been identified as major drivers of litter decomposition at large spatial scales. However, the role played by soil fauna remains largely unknown, despite its importance for litter fragmentation and microbial activity. We synthesised litterbag studies to quantify the effect sizes of soil fauna on litter decomposition rates at the global and biome scales, and to assess how climate, litter quality and soil fauna interact to determine such rates. Soil fauna consistently enhanced litter decomposition at both global and biome scales (average increment ~ 37%). [corrected]. However, climate and litter quality differently modulated the effects of soil fauna on decomposition rates between biomes, from climate-driven biomes to those where climate effects were mediated by changes in litter quality. Our results advocate for the inclusion of biome-specific soil fauna effects on litter decomposition as a mean to reduce the unexplained variation in large-scale decomposition models.