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Theory and models of enzymatic stoichiometry have been typically used to assess microbial resource limitation, such as the ratios between the activities of carbon (C), nitrogen (N) and phosphorus (P) acquiring enzymes and vector characteristics (that is, vector length and angle). However, the validity of using these stoichiometry indicators to infer microbial resource limitation has been questioned by recent some studies. To test the validity of using these indicators in peatland ecosystems, we conducted a laboratory incubation experiment by adding available C, N and P individually as well as in combination to create various resource limitation scenarios. Results showed that the activity of C- (β-D-glucosidase, BDG) and P-acquiring enzymes (phosphatase, PHO) significantly decreased by 87% and 80% with the addition of C and P, respectively. However, the activity of N-acquiring enzymes (N-acetyl-β-glucosaminidase and leucine aminopeptidase, NAG + LAP) increased by 11.5% following N addition, introducing a bias when using the ratios of lnBDG:ln(NAG + LAP) and ln(NAG + LAP):lnPHO to assess microbial resource limitation. Although the ratios of lnBDG:lnPHO could better indicate C or P limitation, it was not valid to indicate C and P co-limitation. Overall, any individual enzymatic stoichiometric ratio only based on two enzymes (for example, BDG:NAG) could not validly indicate microbial resources co-limitation. However, vector characteristics provided an intuitive indication of microbial C and N co-limitation, as well as C and P co-limitation, but not N and P co-limitation. In summary, we recommend prioritizing the more intuitive vector characteristics as a basis for evaluating microbial resource limitation in peatland ecosystems. [ABSTRACT FROM AUTHOR]

Primary production in freshwater ecosystems is largely a function of light and nutrient availability, both of which have been changing in many lakes and reservoirs in response to anthropogenic pressures. Recent studies focusing on natural lakes have found a hump-shaped response of primary production (sometimes measured as chlorophyll-a) to dissolved organic matter (DOM, measured as dissolved organic carbon, DOC), which has both light-absorbing chromophoric properties and DOM-bound nutrients. We used the United States National Lakes Assessment dataset to integrate reservoirs into this paradigm in comparison with natural lakes and assessed the relative differences in the predicted response's model structure, regression parameter values, and drivers of the chlorophyll-a residuals. We found that chlorophyll-a in reservoirs exhibited a hump-shaped response to DOC, while natural lakes from this dataset were better fit with a linear response, differing from previous studies focused on boreal lakes. Despite this, reservoirs had a greater maximum chlorophyll-a response compared to natural lakes in this study (45.5 versus 33.8 μg L−1), which occurred at a lower DOC concentration threshold (18.3 versus 26.4 mg L−1) when compared using quadratic models. Reservoirs had lower median light:nutrient values compared to natural lakes, and greater median surface area and total phosphorus (TP), that can all influence the light environment and the peak chlorophyll-a responses. In both reservoirs and natural lakes, chlorophyll-a residuals were most strongly influenced by TP, where TP < 25–30 µg L−1 suppressed chlorophyll-a residuals and higher TP amplified them. Light:nutrient values were somewhat important predictors, and patterns with chlorophyll-a residuals supported previous work showing low light:nutrient values amplified chlorophyll-a responses and higher values suppressed them. Quantifying the shape of the response of primary production to DOM quantity and quality as well as the drivers of the residuals, namely TP for lakes and reservoirs in this dataset, will be important for understanding the effects that changes in water quality may have on primary production and freshwater ecosystem processes. [ABSTRACT FROM AUTHOR]

Warming is expected to increase the net release of carbon from peatland soils, contributing to future warming. This positive feedback may be moderated by the response of peatland vegetation to rising atmospheric [CO2] or to increased soil nutrient availability. We asked whether a gradient of whole-ecosystem warming (from + 0 °C to + 9 °C) would increase plant-available nitrogen and phosphorus in an ombrotrophic bog in northern Minnesota, USA, and whether elevated [CO2] would modify the nutrient response. We tracked changes in plant-available nutrients across space and through time and in comparison with other nutrient pools, and assessed whether nutrient warming responses were captured by a point version of the land-surface model, ELM-SPRUCE. We found that warming exponentially increased plant-available ammonium and phosphate, but that nutrient dynamics were unaffected by elevated [CO2]. The warming response increased by an order of magnitude between the first and fourth year of the experimental manipulation, perhaps because of dramatic mortality of Sphagnum mosses in the surface peat of the warmest treatments. However, neither the magnitude nor the temporal dynamics of the responses were captured by ELM-SPRUCE. Relative increases in plant-available ammonium and phosphate with warming were similar, but the response varied across raised hummocks and depressed hollows and with peat depth. Plant-available nutrient dynamics were only loosely correlated with inorganic and organic porewater nutrients, likely representing different processes. Future predictions of peatland nutrient availability under climate change scenarios must account for dynamic changes in nutrient acquisition by plants and microbes, as well as microtopography and peat depth. [ABSTRACT FROM AUTHOR]

Soil carbon losses to the atmosphere, via soil heterotrophic respiration, are expected to increase in response to global warming, resulting in a positive carbon-climate feedback. Despite the well-known suite of abiotic and biotic factors controlling soil respiration, much less is known about how the magnitude of soil respiration responses to temperature changes over soil development and across contrasting soil properties. Here we investigated the role of soil development stage and soil properties in driving the responses of soil heterotrophic respiration to temperature. We incubated soils from eight chronosequences ranging in soil age from hundreds to million years, and encompassing a wide range of vegetation types, climatic conditions and chronosequences origins, at three assay temperatures (5 °C, 15 °C and 25 °C). We found a consistent positive effect of assay temperature on soil respiration rates across the eight chronosequences evaluated. However, chronosequences parent materials (sedimentary/sand dunes or volcanic) and soil properties (pH, phosphorus content and microbial biomass) determined the magnitude of this temperature effect. Finally, we observed a positive effect of soil development stage on soil respiration across chronosequences that did not alter the magnitude of assay temperature effects. Our work reveals that key soil properties alter the magnitude of the positive effect of temperature on soil respiration found across ecosystem types and soil development stages. This information is essential to better understand the magnitude of the carbon-climate feedback and thus to establish accurate greenhouse gas emission targets. [ABSTRACT FROM AUTHOR]

Coastal wetland vegetation communities can respond to sea level rise via the encroachment of more salt- and inundation-tolerant species into existing vegetation communities. Black mangroves (Avicennia germinans L.) are encroaching on saltgrass (Distichlis spicata L.) within the Merritt Island National Wildlife Refuge in east central Florida (USA). Nine soil cores collected along three transects captured the transitions of both perceived abiotic drivers (salinity and inundation) and vegetation communities during both high- and low-water seasons to investigate patterns in soil biogeochemical cycling of carbon (C), nitrogen (N), and phosphorus (P). Results showed no change in soil carbon dioxide production along the ecotone during either season, though changes in enzyme activity and mineralization rates of N and P could indicate changes in C quality and nutrient availability affecting C degradation along the ecotone. All parameters, excluding microbial biomass carbon, showed higher rates of activity or availability during the low-water season. Long-term soil nutrient stores (total C, N, P) were greatest in the saltgrass soils and similar between the mangrove and transition zone soils, indicating a 'tipping point' in biogeochemical function where the transition zone is functionally equivalent to the encroaching mangrove zone. Indicators of current biogeochemical cycling (that is, enzyme activity, potentially mineralizable N rates, and extractable ammonium concentrations) showed alterations in activity across the ecotone, with the transition zone often functioning with lower activity than within end members. These indicators of current biogeochemical cycling change in advance of full vegetation shifts. Increases in salinity and inundation were linked to mangrove encroachment. [ABSTRACT FROM AUTHOR]

The transitions between ecosystems (ecotones) are often biodiversity hotspots, but we know little about the forces that shape them. Today, often sharp boundaries with low diversity are found between terrestrial and aquatic ecosystems. This has been attributed to environmental factors that hamper succession. However, ecosystem properties are often controlled by both bottom-up and top-down forces, but their relative importance in shaping riparian boundaries is not known. We hypothesize that (1) herbivores may enforce sharp transitions between terrestrial and aquatic ecosystems by inhibiting emergent vegetation expansion and reducing the width of the transition zone and (2) the vegetation expansion, diversity, and species turnover are related to abiotic factors in the absence of herbivores, but not in their presence. We tested these hypotheses in 50 paired grazed and ungrazed plots spread over ten wetlands, during two years. Excluding grazers increased vegetation expansion, cover, biomass, and species richness. In ungrazed plots, vegetation cover was negatively related to water depth, whereas plant species richness was negatively related to the vegetation N:P ratio. The presence of (mainly aquatic) herbivores overruled the effect of water depth on vegetation cover increase but did not interact with vegetation N:P ratio. Increased local extinction in the presence of herbivores explained the negative effect of herbivores on species richness, as local colonization rates were unaffected by grazing. We conclude that (aquatic) herbivores can strongly inhibit expansion of the riparian vegetation and reduce vegetation diversity over a range of environmental conditions. Consequently, herbivores enforce sharp boundaries between terrestrial and aquatic ecosystems. [ABSTRACT FROM AUTHOR]

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 km alpine-subalpine ecosystem in the Colorado Front Range. Measurements of KCl-extractable NH and NO 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. Ion-exchange resins showed that over a 1-year period hot spots were important contributors to resin-available N at the landscape level, with 14% of resin locations accounting for 58% of total resin-extractable inorganic N. The KCl-extractable and resin-available 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. Resin-available 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 resin-available inorganic N hot spots were closely related to plant species identity. Instead, the most effective predictor of KCl-extractable NH 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. [ABSTRACT FROM AUTHOR]

Ecosystem-level studies identify plant-soil feedbacks as important controls on soil nutrient availability, particularly for nitrogen and phosphorus. Although site- and species-specific studies of tree species-soil relationships are relatively common, comparatively fewer studies consider multiple co-existing species in old-growth forests across a range of sites that vary in underlying soil fertility. We characterized patterns in forest floor and mineral soil nutrients associated with four common tree species across eight undisturbed old-growth forests in Oregon, USA, and used two complementary conceptual models to assess tree species-soil relationships. Plant-soil feedbacks that could reinforce site-level differences in nutrient availability were assessed using the context-dependent relationships model, whereby relative species-based differences in each soil nutrient diverged or converged as nutrient status changed across sites. Tree species-soil relationships that did not reflect strong feedbacks were evaluated using a site-independent relationships model, whereby forest floor and surface mineral soil nutrient pools differed consistently by tree species across sites, without variation in deeper mineral soils. We found that the organically cycled elements carbon, nitrogen, and phosphorus exhibited context-dependent differences among species in both forest floor and mineral soil, and most often followed a divergence model, whereby species differences were greatest at high-nutrient sites. These patterns are consistent with theory emphasizing biotic control of these elements through plant-soil feedback mechanisms. Site-independent species differences were strongest for pools of the weatherable cations calcium, magnesium, potassium, as well as phosphorus, in mineral soils. Site-independent species differences in forest floor nutrients were attributable to one species that displayed significantly greater forest floor mass accumulation. Our findings confirm that site-independent and context-dependent tree species-soil relationships occur simultaneously in old-growth temperate forests, with context-dependent relationships strongest for organically cycled elements, and site-independent relationships strongest for weatherable elements with inorganic cycling phases. These models provide complementary explanations for patterns of nutrient accumulation and cycling in mixed-species old-growth temperate forests. [ABSTRACT FROM AUTHOR]

Arctic treeline positions are of fundamental importance to the function of high latitude landscapes, as regulators of surface energy exchange and carbon cycling. Most studies aimed at explaining current and predicting future treeline positions have examined growth trends in mature treeline trees, but treeline advance requires seed production, germination, seedling establishment, and recruitment of new trees beyond the treeline and these processes may not be well correlated with growth of mature trees. Tussock tundra is a widespread, microtopographically complex vegetation type, covering vast areas of northern Alaska and Siberia. We examined a site where the white spruce ( Picea glauca) treeline has recently advanced into tussock tundra and asked if white spruce seedlings (~36-cm tall) occurred in tussocks, inter-tussocks, and frost boils disproportionate to the abundance of these habitats in the treeline ecotone. We found that seedlings disproportionately occurred in tussocks and frost boils. Seedlings found in tussocks and frost boils also showed greater branch growth than those in inter-tussocks. Tussocks and frost boils had higher soil temperatures than inter-tussocks. Tussocks had higher N, P, and K availability, whereas frost boils had greater secondary and micronutrient availability. The disproportionate occurrence of seedlings in tussocks and frost boils can likely be explained by the combination of warmer soils and greater nutrient availability. It is also possible that low competition for primary nutrients contributes to the success of seedlings in frost boils. Results of the study highlight the importance of biotic and abiotic facilitation as mediators of treeline advance in a changing Arctic. [ABSTRACT FROM AUTHOR]

We investigated the role of rock-derived mineral nutrient availability on the nutrient dynamics of overlying forest communities ( Populus tremuloides and Picea engelmanni- Abies lasiocarpa v. arizonica) across three parent materials (andesite, limestone, and sandstone) in the southern Rocky Mountains of Colorado. Broad geochemical differences were observed between bedrock materials; however, bulk soil chemistries were remarkably similar between the three different sites. In contrast, soil nutrient pools were considerably different, particularly for P, Ca, and Mg concentrations. Despite variations in nutrient stocks and nutrient availability in soils, we observed relatively inflexible foliar concentrations and foliar stoichiometries for both deciduous and coniferous species. Foliar nutrient resorption (P and K) in the deciduous species followed patterns of nutrient content across substrate types, with higher resorption corresponding to lower bedrock concentrations. Work presented here indicates a complex plant response to available soil nutrients, wherein plant nutrient use compensates for variations in supply gradients and results in the maintenance of a narrow range in foliar stoichiometry. [ABSTRACT FROM AUTHOR]

Invasive species have the capacity to substantially alter soil processes, including rates of litter decomposition. Currently, the few remaining native-dominated lowland wet forests in Hawai’i are being invaded by Falcataria moluccana, a large, fast-growing, N2-fixing tree. In this study, we sought to determine the extent to which Falcataria invasion alters decomposition in these lowland wet forests, and whether changes resulted from differences in litter substrate type, lava flow age and type, forest stand type and associated soil biota, or some combination of these factors. We measured decomposition rates and nitrogen (N) and phosphorus (P) dynamics of Metrosideros polymorpha and Falcataria leaf litter in native-dominated and Falcataria-invaded stands on 48- and 300-year-old a’a lava flows and a 213-year-old pāhoehoe flow in the Puna district of eastern Hawai’i. Despite significant differences in the initial quality of Metrosideros and Falcataria litter, in nearly all cases mass remaining of the two litter types did not differ within a given forest stand, whether native-dominated or invaded. Instead, stand type accounted for large differences in the decomposition of both litter types, and litter decomposed two to 10 times faster in Falcataria-invaded stands than it did in their native-dominated counterparts on each lava flow. Dynamics of N (that is, immobilization or release) during decomposition were affected by stand, litter, and lava flow type; P dynamics were affected by stand and flow type, but not litter type. Although not definitive proof of causality, the decay rates of both species were positively correlated to previously measured inputs of N mass and P mass via litterfall as well as availability of soil N and P, characteristics that all increased substantially with Falcataria invasion. Given the degree of change to a host of ecosystem processes, including decomposition, after invasion by Falcataria, these transformed forest ecosystems may best be viewed as fundamentally new and different, in both structure and function, from the native ecosystems they have replaced. [ABSTRACT FROM AUTHOR]

Because nitrogen is considered to be the major growth-limiting element in boreal forests, the increasing nitrogen availability from deposition should lead to increasing growth. We have tested this assumption by simulating, with a simple model, carbon and nitrogen development in seven long-term fertilization experiments in three Nordic countries. The only differences between sites in the model are climate, the ambient nitrogen deposition, nitrogen fertilization regimes, and the initial conditions at the start of the experiment. The model simulates the observed stand development well as long as nitrogen remains the limiting factor. The simulated retention of deposited nitrogen is in general low (less than 50%), whereas retention of fertilizer nitrogen is higher. This seems to imply that the higher production in the fertilized stands will not be maintained once the fertilization is stopped. The model results also indicate that the major effect of climate on site productivity is through soil processes, not tree physiology. [ABSTRACT FROM AUTHOR]

Studies of long-term soil and ecosystem development on static geomorphic surfaces show that old soils become depleted in most rock-derived nutrients. As they are depleted, however, static surfaces also are dissected by fluvial erosion. This fluvial erosion leads to colluvial soil transport on the resulting slopes, which in turn can rejuvenate the supply of weathering-derived nutrients to plants. We evaluated the influence of erosion and consequent landscape evolution on nutrient availability along a slope on the Island of Kaua'i, near the oldest, most nutrient-depleted site on a substrate age gradient across the Hawaiian Islands. Noncrystalline minerals characteristic of younger Hawaiian soils increased from 3% of the soil on the static constructional surface at the top of the slope to 13% on the lower slope, and the fraction of soil phosphorus (P) that was occluded (and hence unavailable) decreased from 80% to 56% at mid-slope. Foliar nitrogen and P concentrations in Metrosideros polymorpha increased from 0.82% and 0.062% to 1.13% and 0.083% on the constructional surface and lower slope, respectively. The increase in foliar P over a horizontal difference of less than 250 m represents nearly half of the total variation in foliar P observed over 4.1 million years of soil and ecosystem development in Hawaii. The fraction of foliar strontium (Sr) derived from weathering of Hawaiian basalt was determined using 87Sr:86Sr it increased from less than 6% on the constructional surface to 13% and 31 % on lower slope and alluvial positions. Erosional processes increase both nutrient supply on this slope and the fine-scale biogeochemical diversity of this old tropical landscape; it could contribute to the relatively high level of species diversity observed on Kaua'i. [ABSTRACT FROM AUTHOR]