Nutrient Availability In Ecosystems Research Articles

Looking back to look ahead: a vision for soil denitrification research.

Denitrification plays a critical role in regulating ecosystem nutrient availability and anthropogenic reactive nitrogen (N) production. Its importance has inspired an increasing number of studies, yet it remains the most poorly constrained term in terrestrial ecosystem N budgets. We censused the peer-reviewed soil denitrification literature (1975-2015) to identify opportunities for future studies to advance our understanding despite the inherent challenges in studying the process. We found that only one-third of studies reported estimates of both nitrous oxide (N2 O) and dinitrogen (N2 ) production fluxes, often the dominant end products of denitrification, while the majority of studies reported only net N2 O fluxes or denitrification potential. Of the 236 studies that measured complete denitrification to N2 , 49% used the acetylene inhibition method, 84% were conducted in the laboratory, 81% were performed on surface soils (0-20cm depth), 75% were located in North America and Europe, and 78% performed treatment manipulations, mostly of N, carbon, or water. To improve understanding of soil denitrification, we recommend broadening access to technologies for new methodologies to measure soil N2 production rates, conducting more studies in the tropics and on subsoils, performing standardized experiments on unmanipulated soils, and using more precise terminology to refer to measured process rates (e.g., net N2 O flux or denitrification potential). To overcome the greater challenges in studying soil denitrification, we envision coordinated research efforts based on standard reporting of metadata for all soil denitrification studies, standard protocols for studies contributing to a Global Denitrification Research Network, and a global consortium of denitrification researchers to facilitate sharing ideas, resources, and to provide mentorship for researchers new to the field.

High food quality increases infection of Gammarus pulex (Crustacea: Amphipoda) by the acanthocephalan parasite Pomphorhynchus laevis

Parasitism is an important process in ecosystems, but has been largely neglected in ecosystem research. However, parasites are involved in most trophic links in food webs with, in turn, a major role in community structure and ecosystem processes. Several studies have shown that higher nutrient availability in ecosystems tends to increase the prevalence of parasites. Yet, most of these studies focused on resource availability, whereas studies investigating resource quality remain scarce. In this study, we tested the impact of the quality of host food resources on infection by parasites, as well as on the consequences for the host. Three resources were used to individually feed Gammarus pulex (Crustacea: Amphipoda) experimentally infected or not infected with the acanthocephalan species Pomphorhynchus laevis: microbially conditioned leaf litter without phosphorus input (standard resource); microbially conditioned leaf litter enriched in phosphorus; and microbially conditioned leaf litter without phosphorus input but complemented with additional inputs of benthic diatoms rich in both phosphorus and eicosapentaenoic acid. During the 110 day experiment, infection rate, parasite load, host survival, and parasite-mediated behavioral traits implicated in trophic transmission were measured (refuge use, geotaxis and locomotor activity). The resources of higher quality, regardless of the infection status, reduced gammarid mortality and increased gammarid growth. In addition, higher quality resources increased the proportion of infected gammarids, and led to more cases of multi-infections. While slightly modifying the geotaxis behavior of uninfected gammarids, resource quality did not modulate the impact of parasites on host behavior. Finally, for most parameters, consumption of algal resources had a greater impact than did phosphorus-enriched leaf litter. Therefore, manipulation of resource quality significantly affected host–parasite relationships, which stressed the need for future research to investigate in natura the relationships between resource availability, resource quality and parasite prevalence.

Bushmeat biogeochemistry: hunting tropical mammals alters ecosystem phosphorus budgets.

Wild meat (or 'bushmeat') hunting is nearly ubiquitous across the tropics and is very often unsustainable-driving declines and extirpation of numerous mammal populations. Loss of these animals can alter the transport of nutrients within and between ecosystems. But whether the physical removal of vertebrate carcasses and the nutrients that they store can reduce overall nutrient availability in ecosystems has been little explored. At 32 sites on three continents, we show that annual phosphorus (P) loss via mammal exploitation was low relative to the rate of atmospheric P deposition. But at four sites in Africa and Southeast Asia, removal of P in the skeletons of hunted mammals exceeded the atmospheric input of this nutrient by 10-fold or more. Because P is the growth-limiting nutrient for many tropical terrestrial ecosystems and certain large mammals, the imbalance created by the removal of mammal biomass under very high hunting scenarios could reduce ecosystem carrying capacity if no compensatory P additions occur in the system. This biogeochemical perspective on bushmeat exploitation raises further concerns about harvest sustainability and human food security in areas where hunting rates are high and ecosystem P inputs low.

Explaining European fungal fruiting phenology with climate variability.

Here we assess the impact of geographically dependent (latitude, longitude, and altitude) changes in bioclimatic (temperature, precipitation, and primary productivity) variability on fungal fruiting phenology across Europe. Two main nutritional guilds of fungi, saprotrophic and ectomycorrhizal, were further separated into spring and autumn fruiters. We used a path analysis to investigate how biogeographic patterns in fungal fruiting phenology coincided with seasonal changes in climate and primary production. Across central to northern Europe, mean fruiting varied by approximately 25d, primarily with latitude. Altitude affected fruiting by up to 30d, with spring delays and autumnal accelerations. Fruiting was as much explained by the effects of bioclimatic variability as by their large-scale spatial patterns. Temperature drove fruiting of autumnal ectomycorrhizal and saprotrophic groups as well as spring saprotrophic groups, while primary production and precipitation were major drivers for spring-fruiting ectomycorrhizal fungi. Species-specific phenology predictors were not stable, instead deviating from the overall mean. There is significant likelihood that further climatic change, especially in temperature, will impact fungal phenology patterns at large spatial scales. The ecological implications are diverse, potentially affecting food webs (asynchrony), nutrient cycling and the timing of nutrient availability in ecosystems.

Seasonal dynamics of soil and plant nutrients at three environmentally contrasting sites along a sub-Arctic catchment sequence

Nutrient availability is one of the most important factors controlling Arctic plant productivity. It is also sensitive to climate change, with increased nitrogen (N) mineralization arising from warmer soils and deeper snow. However, warming also tends to reduce snow cover duration, leading to antagonistic effects of climate change on mineralization. Furthermore, since snow melt is also a trigger for seasonal nutrient pulses, changes to snow melt timing may alter seasonal availability to plants. To investigate the impacts of environmental conditions on ecosystem nutrient availability and seasonal dynamics, we undertook regular, high-frequency measurements of soil extractable and plant N and phosphorus (P) concentrations along with winter and summer N and P mineralization rates along a sub-Arctic catchment representing a gradient in temperature, snow melt timing and vegetation composition. Our data show that a delay in snow melt timing of 11 days did not alter the seasonal dynamics of soil or leaf N and P concentrations. Net N mineralization, however, was highest at the warmest site and at the site with the most productive vegetation, while P was strongly immobilized at all sites, both in winter and summer. N:P ratios suggest that plants were generally P limited at all sites, probably due to strong P immobilization. Our data suggest that where warming and resulting vegetation change increase net N mineralization rates, in combination with strong P immobilization this may impose greater P limitation, possibly limiting the extent to which Arctic ecosystems can increase productivity under warming.

Does infection tilt the scales? Disease effects on the mass balance of an invertebrate nutrient recycler.

While parasites are increasingly recognized as important components of ecosystems, we currently know little about how they alter ecosystem nutrient availability via host-mediated nutrient cycling. We examined whether infection alters the flow of nutrients through hosts and whether such effects depend upon host diet quality. To do so, we compared the mass specific nutrient (i.e., nitrogen and phosphorus) release rates, ingestion rates, and elemental composition of uninfected Daphnia to those infected with a bacterial parasite, P. ramosa. N and P release rates were increased by infection when Daphnia were fed P-poor diets, but we found no effect of infection on the nutrient release of individuals fed P-rich diets. Calculations based on the first law of thermodynamics indicated that infection should increase the nutrient release rates of Daphnia by decreasing nutrient accumulation rates in host tissues. Although we found reduced nutrient accumulation rates in infected Daphnia fed all diets, this reduction did not increase the nutrient release rates of Daphnia fed the P-rich diet because infected Daphnia fed this diet ingested nutrients more slowly than uninfected hosts. Our results thus indicate that parasites can significantly alter the nutrient use of animal consumers, which could affect the availability of nutrients in heavily parasitized environments.

High potential for iron reduction in upland soils.

Changes in the redox state of iron (Fe) can be coupled to the biogeochemical cycling of carbon (C), nitrogen, and phosphorus, and thus regulate soil C, ecosystem nutrient availability, and greenhouse gas production. However, its importance broadly in non-flooded upland terrestrial ecosystems is unknown. We measured Fe reduction in soil samples from an annual grassland, a drained peatland, and a humid tropical forest We incubated soil slurries in an anoxic glovebox for 5.5 days and added sodium acetate daily at rates up to 0.4 mg C x (g soil)(-1) x d(-1). Soil moisture, poorly crystalline Fe oxide concentrations, and Fe(II) concentrations differed among study sites in the following order: annual grassland < drained peatland < tropical forest (P < 0.001 for all characteristics). All of the soil samples demonstrated high Fe reduction potential with maximum rates over the course of the incubation averaging 1706 ± 66, 2016 ± 12, and 2973 ± 115 μg Fe x (g soil)(-1) x d(-1) (mean ± SE) for the tropical forest, annual grassland, and drained peatland, respectively. Our results suggest that upland soils from diverse ecosystems have the potential to exhibit high short-term rates of Fe reduction that may play an important role in driving soil biogeochemical processes during periods of anaerobiosis.

Rhizodeposition of organic carbon by plants with contrasting traits for resource acquisition: responses to different fertility regimes

Rhizodeposition plays an important role in mediating soil nutrient availability in ecosystems. However, owing to methodological difficulties (i.e., narrow zone of soil around roots, rapid assimilation by soil microbes) fertility-induced changes in rhizodeposition remain mostly unknown. We developed a novel long-term continuous 13C labelling method to address the effects of two levels of nitrogen (N) fertilization on rhizodeposited carbon (C) by species with different nutrient acquisition strategies. Fertility-induced changes in rhizodeposition were modulated by root responses to N availability rather than by changes in soil microbial biomass. Differences among species were mostly related to plant biomass: species with higher total leaf and root biomass also had higher total rhizodeposited C, whereas species with lower root biomass had higher specific rhizodeposited C (per gram root mass). Experimental controls demonstrated that most of the biases commonly associated with this type of experiment (i.e., long-term steady-state labelling) were avoided using our methodological approach. These results suggest that the amount of rhizodeposited C from plants grown under different levels of N were driven mainly by plant biomass and root morphology rather than microbial biomass. They also underline the importance of plant characteristics (i.e., biomass allocation) as opposed to traits associated with plant resource acquisition strategies in predicting total C rhizodeposition.

Changes in canopy structure and ant assemblages affect soil ecosystem variables as a foundation species declines

The decline of Tsuga canadensis (eastern hemlock)—a foundation tree species—due to infestation by Adelges tsugae (hemlock woolly adelgid) or its complete removal from a stand by salvage logging dramatically affects associated faunal assemblages. Among these assemblages, species composition (richness and abundance) of ants increases rapidly as T. canadensis is lost from the stands. Because ants live and forage at the litter‐soil interface, we hypothesized that environmental changes caused by hemlock loss (e.g., increased light and warmth at the forest floor, increased soil pH) and shifts in ant species composition would interact to alter soil ecosystem variables. In the Harvard Forest Hemlock Removal Experiment (HF‐HeRE), established in 2003, T. canadensis in large plots were killed in place or logged and removed to mimic adelgid infestation or salvage harvesting, respectively. In 2006, we built ant exclosure subplots within all of the canopy manipulation plots to examine direct and interactive effects of canopy change and ant assemblage composition on soil and litter variables. Throughout HF‐HeRE, T. canadensis was colonized by the adelgid in 2009, and the infested trees are now declining. The experimental removal of T. canadensis from the canopy was associated with an increase in the rate of cellulose decomposition by &gt;50%, and exclosure of ants from subplots directly reduced their soil nitrate availability by 56%. Partial least squares path models revealed sequential interactive effects prior to adelgid infestation: canopy change (as a proxy for associated environmental changes) altered both decomposition and ant assemblage structure; changes in ant assemblage structure and decomposition rates altered nitrogen availability. The results illustrate that biotic changes directly associated with decline of T. canadensis can have cascading effects on ecosystem nutrient availability and cycling.

Temporal-Spatial Dynamics in Orthoptera in Relation to Nutrient Availability and Plant Species Richness

Nutrient availability in ecosystems has increased dramatically over the last century. Excess reactive nitrogen deposition is known to negatively impact plant communities, e.g. by changing species composition, biomass and vegetation structure. In contrast, little is known on how such impacts propagate to higher trophic levels. To evaluate how nitrogen deposition affects plants and herbivore communities through time, we used extensive databases of spatially explicit historical records of Dutch plant species and Orthoptera (grasshoppers and crickets), a group of animals that are particularly susceptible to changes in the C:N ratio of their resources. We use robust methods that deal with the unstandardized nature of historical databases to test whether nitrogen deposition levels and plant richness changes influence the patterns of richness change of Orthoptera, taking into account Orthoptera species functional traits. Our findings show that effects indeed also propagate to higher trophic levels. Differences in functional traits affected the temporal-spatial dynamics of assemblages of Orthoptera. While nitrogen deposition affected plant diversity, contrary to our expectations, we could not find a strong significant effect of food related traits. However we found that species with low habitat specificity, limited dispersal capacity and egg deposition in the soil were more negativly affected by nitrogen deposition levels. Despite the lack of significant effect of plant richness or food related traits on Orthoptera, the negative effects of nitrogen detected within certain trait groups (e.g. groups with limited disperse ability) could be related to subtle changes in plant abundance and plant quality. Our results, however, suggest that the changes in soil conditions (where many Orthoptera species lay their eggs) or other habitat changes driven by nitrogen have a stronger influence than food related traits. To fully evaluate the negative effects of nitrogen deposition on higher trophic levels it is essential to take into account species life-history traits.

Predator-Driven Nutrient Recycling in California Stream Ecosystems

Nutrient recycling by consumers in streams can influence ecosystem nutrient availability and the assemblage and growth of photoautotrophs. Stream fishes can play a large role in nutrient recycling, but contributions by other vertebrates to overall recycling rates remain poorly studied. In tributaries of the Pacific Northwest, coastal giant salamanders (Dicamptodon tenebrosus) occur at high densities alongside steelhead trout (Oncorhynchus mykiss) and are top aquatic predators. We surveyed the density and body size distributions of D. tenebrosus and O. mykiss in a California tributary stream, combined with a field study to determine mass-specific excretion rates of ammonium (N) and total dissolved phosphorus (P) for D. tenebrosus. We estimated O. mykiss excretion rates (N, P) by bioenergetics using field-collected data on the nutrient composition of O. mykiss diets from the same system. Despite lower abundance, D. tenebrosus biomass was 2.5 times higher than O. mykiss. Mass-specific excretion summed over 170 m of stream revealed that O. mykiss recycle 1.7 times more N, and 1.2 times more P than D. tenebrosus, and had a higher N:P ratio (8.7) than that of D. tenebrosus (6.0), or the two species combined (7.5). Through simulated trade-offs in biomass, we estimate that shifts from salamander biomass toward fish biomass have the potential to ease nutrient limitation in forested tributary streams. These results suggest that natural and anthropogenic heterogeneity in the relative abundance of these vertebrates and variation in the uptake rates across river networks can affect broad-scale patterns of nutrient limitation.

Plant-herbivore-decomposer stoichiometric mismatches and nutrient cycling in ecosystems.

Plant stoichiometry is thought to have a major influence on how herbivores affect nutrient availability in ecosystems. Most conceptual models predict that plants with high nutrient contents increase nutrient excretion by herbivores, in turn raising nutrient availability. To test this hypothesis, we built a stoichiometrically explicit model that includes a simple but thorough description of the processes of herbivory and decomposition. Our results challenge traditional views of herbivore impacts on nutrient availability in many ways. They show that the relationship between plant nutrient content and the impact of herbivores predicted by conceptual models holds only at high plant nutrient contents. At low plant nutrient contents, the impact of herbivores is mediated by the mineralization/immobilization of nutrients by decomposers and by the type of resource limiting the growth of decomposers. Both parameters are functions of the mismatch between plant and decomposer stoichiometries. Our work provides new predictions about the impacts of herbivores on ecosystem fertility that depend on critical interactions between plant, herbivore and decomposer stoichiometries in ecosystems.

Using atmospheric fallout to date organic horizon layers and quantify metal dynamics during decomposition

High concentrations of metals in organic matter can inhibit decomposition and limit nutrient availability in ecosystems, but the long-term fate of metals bound to forest litter is poorly understood. Controlled experiments indicate that during the first few years of litter decay, Al, Fe, Pb, and other metals that form stable complexes with organic matter are naturally enriched by several hundred percent as carbon is oxidized. The transformation of fresh litter to humus takes decades, however, such that current datasets describing the accumulation and release of metals in decomposing organic matter are timescale limited. Here we use atmospheric 210Pb to quantify the fate of metals in canopy-derived litter during burial and decay in coniferous forests in New England and Norway where decomposition rates are slow and physical soil mixing is minimal. We measure 210Pb inventories in the O horizon and mineral soil and calculate a 60–630 year timescale for the production of mobile organo-metallic colloids from the decomposition of fresh forest detritus. This production rate is slowest at our highest elevation (∼1000 m) and highest latitude sites (>63°N) where decomposition rates are expected to be low. We calculate soil layer ages by assuming a constant supply of atmospheric 210Pb and find that they are consistent with the distribution of geochemical tracers from weapons fallout, air pollution, and a direct 207Pb application at one site. By quantifying a gradient of organic matter ages with depth in the O horizon, we describe the accumulation and loss of metals in the soil profile as organic matter transforms from fresh litter to humus. While decomposition experiments predict that Al and Fe concentrations increase during the initial few years of decay, we show here that these metals continue to accumulate in humus for decades, and that enrichment occurs at a rate higher than can be explained by quantitative retention during decomposition alone. Acid extractable Al and Fe concentrations are higher in the humus layer of the O horizon than in the mineral soil immediately beneath this layer: it is therefore unlikely that physical soil mixing introduces significant Al and Fe to humus. This continuous enrichment of Al and Fe over time may best be explained by the recent suggestion that metals are mined from deeper horizons and brought into the O horizon via mycorrhizal plants. In sharp contrast to Al and Fe, we find that Mn concentrations in decomposing litter layers decrease exponentially with age, presumably because of leaching or rapid uptake, which may explain the low levels of acid extractable Mn in the mineral soil. This study quantifies how metals are enriched and lost in decomposing organic matter over a longer timescale than previous studies have been able to characterize. We also put new limits on the rate at which metals in litter become mobile organo-metallic complexes that can migrate to deeper soil horizons or surface waters.

NUTRIENT LOSSES OVER FOUR MILLION YEARS OF TROPICAL FOREST DEVELOPMENT

Biological, atmospheric, and geochemical processes interact to shape how element cycles and nutrient losses develop within newly formed landscapes. We examined losses of nitrogen (N), phosphorus (P), and base cations across a four-million-year substrate age gradient of Hawaiian montane tropical forests, with the goal of understanding how losses depend on changes in biotic demand, weathering, and atmospheric sources. We were particularly interested in whether losses of nutrients that are not subject to traditional mechanisms of biotic availability could influence ecosystem fertility and nutrient limitation over time. Over a three-year period we sampled nutrient outputs in soil solutions below the active plant–soil system and small streams, gaseous N losses (NO, N2O, and N2), and pools and transformations of N in soils. Weathering was the major determinant of ecosystem losses of P, of base cations and Si, and of ecosystem acid–base status. Sharp reductions in weathering inputs after 20 000 years of forest development caused dramatically lower outputs of P (∼65% reduction), Ca2+ (∼99%), and Si (∼94%), to rates that matched inputs from dilute sea-salt and dust aerosols. Internal production of organic acids, in combination with low weathering, caused highly acidic soil waters (pH < 5.0) with elevated Al (up to 300 μg/L) and Ca:Al ratios (<0.3) below values considered critical thresholds for many plant species. Long-term N and P interactions were more complex than predicted by the Walker and Syers model, with important influences of N and P loss pathways that were not subject to direct biotic availability. While losses of available forms of dissolved N and P, and N gases followed (as predicted) ecosystem nutrient availability, significant dissolved organic N (DON) and dissolved organic P (DOP) losses occurred independent of ecosystem N or P status. DON losses were not sufficient (relative to external inputs) to sustain N limitation beyond 20 000 years, while dissolved P losses remained large enough to maintain P limitation in older forests. Forests developed high N:P loss ratios over time (>300 on mass weight basis) due to efficient P recycling and unexpectedly high N throughputs (4–9 kg N·ha−1·yr−1) by either N fixation and/or N deposition.

SPATIAL HETEROGENEITY OF STREAM WATER NUTRIENT CONCENTRATIONS OVER SUCCESSIONAL TIME

Nutrient availability in ecosystems is patchy both in space and in time. Whereas temporal trends have often been studied, less information exists on spatial patterns of nutrient availability, particularly in aquatic ecosystems. The goals of this study were (1) to describe and quantify patterns of nutrient concentration in surface waters of an arid land stream and (2) to compare spatial patterns of nutrient availability across nutrients and over a successional sequence. We describe changes in the spatial pattern of stream water nutrient concentrations over successional time (between floods) using quantitative measures of heterogeneity. Samples were collected from the middle of the channel every 25 m over a 10-km section of a Sonoran Desert stream during three periods: early succession (2 wk post-flood), middle succession (2 mo post-flood), and late succession (9 mo post-flood). Nutrient concentrations were extremely variable in space (coefficients of variations as high as 145%). Coefficients of variation increased over successional time and were consistently greater for nitrate-nitrogen than for soluble reactive phosphorus. Semi-variogram analysis showed that nutrient concentrations were spatially dependent on all dates, but to different degrees and over different distances. The distance over which nutrient concentrations were spatially dependent, as measured by the semi-variogram range, tended to decrease with successional time. The strength of spatial dependence, as measured by the slope of the ascending limb of the semi-variogram, increased with successional time. The limiting nutrient, nitrogen, was consistently more spatially heterogeneous than phosphorus or conductivity, both in terms of patch size (range) and strength of spatial dependence. In streams, downstream transport combined with nutrient transformation produces patches of similar nutrient concentrations that are elongated compared with nutrient patches in terrestrial soils. Variation in nutrient concentration is likely to affect the spatial distribution of organisms and rates of ecosystem processes.

Spatial Heterogeneity of Stream Water Nutrient Concentrations over Successional Time

Nutrient availability in ecosystems is patchy both in space and in time. Whereas temporal trends have often been studied, less information exists on spatial patterns of nutrient availability, particularly in aquatic ecosystems. The goals of this study were (1) to describe and quantify patterns of nutrient concentration in surface waters of an arid land stream and (2) to compare spatial patterns of nutrient availability across nutrients and over a successional sequence. We describe changes in the spatial pattern of stream water nutrient concentrations over successional time (between floods) using quantitative measures of heterogeneity. Samples were collected from the middle of the channel every 25 m over a 10-km section of a Sonoran Desert stream during three periods: early succession (2 wk post-flood), middle succession (2 mo post-flood), and late succession (9 mo post-flood). Nutrient concentrations were extremely variable in space (coefficients of variations as high as 145%). Coefficients of variation increased over successional time and were consistently greater for nitrate-nitrogen than for soluble reactive phosphorus. Semi-variogram analysis showed that nutrient concentrations were spatially dependent on all dates, but to different degrees and over different distances. The distance over which nutrient concentrations were spatially dependent, as measured by the semi-variogram range, tended to decrease with successional time. The strength of spatial dependence, as measured by the slope of the ascending limb of the semi-variogram, increased with successional time. The limiting nutrient, nitrogen, was consistently more spatially heterogeneous than phosphorus or conductivity, both in terms of patch size (range) and strength of spatial dependence. In streams, downstream transport combined with nutrient transformation produces patches of similar nutrient concentrations that are elongated compared with nutrient patches in terrestrial soils. Variation in nutrient concentration is likely to affect the spatial distribution of organisms and rates of ecosystem processes.