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Decomposition is a critical source of plant nutrients, and drives the largest flux of terrestrial C to the atmosphere. Decomposing soil organic matter typically contains litter from multiple plant species, yet we lack a mechanistic understanding of how species diversity influences decomposition processes. Here, we show that soil C and N cycling during decomposition are controlled by the composition and diversity of chemical compounds within plant litter mixtures, rather than by simple metrics of plant species diversity. We amended native soils with litter mixtures containing up to 4 alpine plant species, and we used 9 litter chemical traits to evaluate the chemical composition (i.e., the identity and quantity of compounds) and chemical diversity of the litter mixtures. The chemical composition of the litter mixtures was the strongest predictor of soil respiration, net N mineralization, and microbial biomass N. Soil respiration and net N mineralization rates were also significantly correlated with the chemical diversity of the litter mixtures. In contrast, soil C and N cycling rates were poorly correlated with plant species richness, and there was no relationship between species richness and the chemical diversity of the litter mixtures. These results indicate that the composition and diversity of chemical compounds in litter are potentially important functional traits affecting decomposition, and simple metrics like plant species richness may fail to capture variation in these traits. Litter chemical traits therefore provide a mechanistic link between organisms, species diversity, and key components of below-ground ecosystem function.

Atmospheric nitrogen (N) deposition has been shown to decrease plant species richness along regional deposition gradients in Europe and in experimental manipulations. However, the general response of species richness to N deposition across different vegetation types, soil conditions, and climates remains largely unknown even though responses may be contingent on these environmental factors. We assessed the effect of N deposition on herbaceous richness for 15,136 forest, woodland, shrubland, and grassland sites across the continental United States, to address how edaphic and climatic conditions altered vulnerability to this stressor. In our dataset, with N deposition ranging from 1 to 19 kg N·ha−1·y−1, we found a unimodal relationship; richness increased at low deposition levels and decreased above 8.7 and 13.4 kg N·ha−1·y−1 in open and closed-canopy vegetation, respectively. N deposition exceeded critical loads for loss of plant species richness in 24% of 15,136 sites examined nationwide. There were negative relationships between species richness and N deposition in 36% of 44 community gradients. Vulnerability to N deposition was consistently higher in more acidic soils whereas the moderating roles of temperature and precipitation varied across scales. We demonstrate here that negative relationships between N deposition and species richness are common, albeit not universal, and that fine-scale processes can moderate vegetation responses to N deposition. Our results highlight the importance of contingent factors when estimating ecosystem vulnerability to N deposition and suggest that N deposition is affecting species richness in forested and nonforested systems across much of the continental United States.

Niche complementarity, in which coexisting species use different forms of a resource, has been widely invoked to explain some of the most debated patterns in ecology, including maintenance of diversity and relationships between diversity and ecosystem function. However, classical models assume resource specialization in the form of distinct niches, which does not obviously apply to the broadly overlapping resource use in plant communities. Here we utilize an experimental framework based on competition theory to test whether plants partition resources via classical niche differentiation or via plasticity in resource use. We explore two alternatives: niche preemption, in which individuals respond to a superior competitor by switching to an alternative, less-used resource, and dominant plasticity, in which superior competitors exhibit high resource use plasticity and shift resource use depending on the competitive environment. We determined competitive ability by measuring growth responses with and without neighbors over a growing season and then used 15 N tracer techniques to measure uptake of different nitrogen (N) forms in a field setting. We show that four alpine plant species of differing competitive abilities have statistically indistinguishable uptake patterns (nitrate > ammonium > glycine) in their fundamental niche (without competitors) but differ in whether they shift these uptake patterns in their realized niche (with competitors). Competitively superior species increased their uptake of the most available N form, ammonium, when in competition with the rarer, competitively inferior species. In contrast, the competitively inferior species did not alter its N uptake pattern in competition. The existence of plasticity in resource use among the dominant species provides a mechanism that helps to explain the manner by which plant species with broadly overlapping resource use might coexist.

PurposePlants and soil microbes both influence how ecosystems respond to environmental change. Yet, we lack the ability to generalize how plants and soil microbes influence each other in the same or varying soil conditions. This limitation thwarts ecologists’ ability to understand and predict effects of environmental changes such as elevated anthropogenic nitrogen (N) deposition. Accordingly, we examined the specificity of plant species’ influence on soil microbial community composition.MethodsWe tested (1) whether congeneric grass species have unique effects on soil microbial communities, (2) how relative abundances of microbial taxa can be explained by Poa phylogeny, plant traits, and range-wide traits (annual temperature and soil pH), (3) whether N addition alters associations between Poa species and soil microbes, and (4) whether the magnitude of microbial community change in response to elevated N can be explained by plant growth responses to N. We conducted a greenhouse experiment with seven Poa species and native soils.ResultsWe found that individual Poa species were associated with different soil fungal and bacterial assemblages. Differences in microbial composition were not attributable to Poa phylogeny, the plant traits we tested, nor range-wide species traits we tested. Nitrogen addition enhanced the unique effects of Poa species on fungal and bacterial community compositions.ConclusionThese results demonstrate how ecological interactions of related plant species vary depending on resource supply, revealing important context dependency for accurately predicting microbially mediated nutrient cycling and ecosystem responses to changes in nutrient availability.

Sustained nitrogen deposition has had a detrimental effect on ecosystems in Europe and North America. Now a grassland in Slovakia is showing symptoms of extreme soil acidification not previously observed in association with nitrogen deposition. Anthropogenic nitrogen deposition over the past half century has had a detrimental impact on temperate ecosystems in Europe and North America, resulting in soil acidification and a reduction in plant biodiversity 1 , 2 . During the acidification process, soils release base cations, such as calcium and magnesium, neutralizing the increase in acidity. Once these base cations have been depleted, aluminium is released from the soils, often reaching toxic levels. Here, we present results from a nitrogen deposition experiment that suggests that a long legacy of acid deposition in the Western Tatra Mountains of Slovakia has pushed soils to a new threshold of acidification usually associated with acid mine drainage soils. We show that increases in nitrogen deposition in the region result in a depletion of both base cations and soluble aluminium, and an increase in extractable iron concentrations. In conjunction with this, we observe a nitrogen-deposition-induced reduction in the biomass of vascular plants, associated with a decrease in shoot calcium and magnesium concentrations. We suggest that this site, and potentially others in central Europe, have reached a new and potentially more toxic level of soil acidification in which aluminium release is superseded by iron release into soil water.

Biodiversity has been described as the diversity of life on earth within species, among species, and among ecosystems. The rate of biodiversity loss due to human activity in the last 50 years has been more rapid than at any other time in human history, and many of the drivers of biodiversity loss are increasing, including habitat loss, overexploitation, invasive species, climate change, and pollution, including pollution from reactive nitrogen (Nr). Of these stressors, climate change and Nr from anthropogenic activities are causing some of the most rapid changes. Climate change is causing warming trends that result in poleward and elevational range shifts of flora and fauna, and changes in phenology, particularly the earlier onset of spring events and migration, and lengthening of the growing season. Nitrogen (N) enrichment can enhance plant growth, but has been shown to favor, fast-growing, sometimes invasive, species over native species adapted to low N conditions. Although there have been only a few controlled studies on climate change and N interactions, inferences can be drawn from various field observations. For example, in arid ecosystems of southern California, elevated N deposition and changing precipitation patterns have promoted the conversion of native shrub communities to communities dominated by annual non-native grasses. Both empirical studies and modeling indicate that N and climate change can interact to drive losses in biodiversity greater than those caused by either stressor alone. Reducing inputs of anthropogenic Nr may be an effective mitigation strategy for protecting biodiversity in the face of climate change.

Questions: Plant community composition can be influenced by multiple biotic, abiotic, and stochastic factors acting on the local species pool to determine their establishment success and abundance and subsequently the diversity of the community. We asked if the influences of biotic interactions on the composition of plant species in communities, as indicated by patterns of plant species spatial associations (independent, positive or negative), vary across a productivity gradient within a single ecosystem type. Do dominant species of communities show spatial patterning suggestive of competitive interactions with interspecific neighbors? Do species that span multiple community types exhibit the same heterospecific interactions with neighbours in each community? Location: Three alpine communities in the southern Rocky Mountains. Methods: We measured the occurrence of species in a 1-cm spatial grid within 2 m × 2 m plots to determine the spatial patterns of species pairs in the three communities. A null model of independent species spatial arrangements was used to determine whether species pairs were positively, negatively or independently associated, and how these patterns differed among the communities across the gradient of resource supply and environmental stress. Results: Positive associations, indicative of facilitation between species, were most common in the most resource-poor and least productive community. However negative associations, suggestive of competitive interactions among species, were not more common in the two more resource-rich, productive communities. The dominant species of these communities did exhibit higher negative than positive associations with neighbours relative to positive patterning. Independent interspecific patterning was equally common relative to positive and negative patterns in all communities. Species that previously were shown to either facilitate other species or compete with neighbours exhibited spatial patterning consistent with the earlier experimental work. Conclusions: A large number of species exhibit a lack of net biotic interactions, and stochastic factors appear to be as important as competition and facilitation in shaping the structure of the three alpine plant communities we studied.

Inorganic nitrogen (N) availability hot spots have been documented in many ecosystems, but major uncertainties remain about their prevalence, timing, and causes. Using a novel mathematical definition of hot spots, spatially explicit measurements of KCl-extractable inorganic N, 2-week soil incubations in the field, ion-exchange resins deployed for 1 year, and a set of associated biotic and abiotic variables, we investigated inorganic N availability hot spots within a 0.89 km² alpine-subalpine ecosystem in the Colorado Front Range. Measurements of KCl-extractable NH₄⁺ and NO₃⁻ taken on multiple dates showed that hot spots of N availability were present in some but not all parts of the study site and that hot spot location varied over the course of the season. Ionexchange resins showed that over a 1-year period hot spots were important contributors to resinavailable N at the landscape level, with 14% of resin locations accounting for 58% of total resin-extractable inorganic N. The KCl-extractable and resinavailable inorganic N measurements showed that although spatial variation in the timing of hot spots (that is, hot moments) spreads the influence of short-term hot spots across the landscape to some extent, spatial variation in inorganic N availability is still important when integrated over 1 year. Resinavailable N was poorly correlated with the biotic and abiotic variables that we measured, though we did observe that hot spots of resin-available N were twice as common below tree and shrub canopies than in herbaceous areas. Beyond this relationship with canopy structure, neither KCl-extractable nor resinavailable inorganic N hot spots were closely related to plant species identity. Instead, the most effective predictor of KCl-extractable NH₄⁺ was the size of the soil organic matter (SOM) N pool, with nearly all hot spots appearing in soils that had greater than 1.4% SOM N.

The importance of interspecific competition as a cause of resource partitioning among species has been widely assumed but rarely tested. Using neighbor removals in combination with ⁱ⁵N tracer additions in the field, we examined variation among three alpine species in the uptake of ⁱ⁵N-NH₄⁺, ⁱ⁵N-NO₃⁻, and ⁱ⁵N-13C-[2]-glycine in intact neighborhoods, when paired with a specific neighbor, and when all neighbors were removed. Species varied in the capacity to take up ⁱ⁵N-labeled NH₄⁺, NO₃⁻, and glycine in intact neighborhoods and in interspecific pairs. When interspecific neighbor pairs were compared with no neighbor controls, neighbors reduced ⁱ⁵N uptake in target species by as much as 50%, indicating competition for N. Furthermore, neighbor identity influenced the capacity of species to take up different forms of N. Thus, competition within interspecific neighbor pairs often caused reduced uptake of a particular form of N, as well as shifts to uptake of an alternative form of N. Such shifts in resource use as a result of competition are an implicit assumption in studies of resource partitioning but have rarely been documented. Our study suggests that plasticity in the uptake of different forms of N may be a mechanism by which co-occurring plants reduce competition for N.

Soils contain the largest near-surface reservoir of terrestrial carbon 1 and so knowledge of the factors controlling soil carbon storage and turnover is essential for understanding the changing global carbon cycle. The influence of climate on decomposition of soil carbon has been well documented 2 , 3 , but there remains considerable uncertainty in the potential response of soil carbon dynamics to the rapid global increase in reactive nitrogen (coming largely from agricultural fertilizers and fossil fuel combustion). Here, using 14 C, 13 C and compound-specific analyses of soil carbon from long-term nitrogen fertilization plots, we show that nitrogen additions significantly accelerate decomposition of light soil carbon fractions (with decadal turnover times) while further stabilizing soil carbon compounds in heavier, mineral-associated fractions (with multidecadal to century lifetimes). Despite these changes in the dynamics of different soil pools, we observed no significant changes in bulk soil carbon, highlighting a limitation inherent to the still widely used single-pool approach to investigating soil carbon responses to changing environmental conditions. It remains to be seen if the effects observed here—caused by relatively high, short-term fertilizer additions—are similar to those arising from lower, long-term additions of nitrogen to natural ecosystems from atmospheric deposition, but our results suggest nonetheless that current models of terrestrial carbon cycling do not contain the mechanisms needed to capture the complex relationship between nitrogen availability and soil carbon storage.