Well-constrained carbon:nitrogen:phosphorus (C:N:P) ratios in planktonic biomass, and their importance in advancing our understanding of biological processes and nutrient cycling in marine ecosystems, has motivated ecologists to search for similar patterns in terrestrial ecosystems. Recent analyses indicate the existence of “Redfield-like” ratios in plants, and such data may provide insight into the nature of nutrient limitation in terrestrial ecosystems. We searched for analogous patterns in the soil and the soil microbial biomass by conducting a review of the literature. Although soil is characterized by high biological diversity, structural complexity and spatial heterogeneity, we found remarkably consistent C:N:P ratios in both total soil pools and the soil microbial biomass. Our analysis indicates that, similar to marine phytoplankton, element concentrations of individual phylogenetic groups within the soil microbial community may vary, but on average, atomic C:N:P ratios in both the soil (186:13:1) and the soil microbial biomass (60:7:1) are well-constrained at the global scale. We did see significant variation in soil and microbial element ratios between vegetation types (i.e., forest versus grassland), but in most cases, the similarities in soil and microbial element ratios among sites and across large scales were more apparent than the differences. Consistent microbial biomass element ratios, combined with data linking specific patterns of microbial element stoichiometry with direct evidence of microbial nutrient limitation, suggest that measuring the proportions of C, N and P in the microbial biomass may represent another useful tool for assessing nutrient limitation of ecosystem processes in terrestrial ecosystems.

https://www.cfc.umt.edu/research/biogeochemistry/files/publications/BGC_85_232%20copy.pdf

C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass?

Applications of Hutchinson’s n-dimensional niche concept are often focused on the role of interspecific competition in shaping species distribution patterns. In this paper, I discuss a variety of factors, in addition to competition, that influence the observed relationship between species distribution and the availability of suitable habitat. In particular, I show that Hutchinson’s niche concept can be modified to incorporate the influences of niche width, habitat availability and dispersal, as well as interspecific competition per se. I introduce a simulation model called NICHE that embodies many of Hutchinson’s original niche concepts and use this model to predict patterns of species distribution. The model may help to clarify how dispersal, niche size and competition interact, and under what conditions species might be common in unsuitable habitat or absent from suitable habitat. A brief review of the pertinent literature suggests that species are often absent from suitable habitat and present in unsuitable habitat, in ways predicted by theory. However, most tests of niche theory are hampered by inadequate consideration of what does and does not constitute suitable habitat. More conclusive evidence for these predictions will require rigorous determination of habitat suitability under field conditions. I suggest that to do this, ecologists must measure habitat specific demography and quantify how demographic parameters vary in response to temporal and spatial variation in measurable niche dimensions.

On the relationship between niche and distribution

Global mean temperature is predicted to increase by 2–71C and precipitation to change across the globe by the end of this century. To quantify climate effects on ecosystem processes, a number of climate change experiments have been established around the world in various ecosystems. Despite these efforts, general responses of terrestrial ecosystems to changes in temperature and precipitation, and especially to their combined effects, remain unclear. We used metaanalysis to synthesize ecosystem-level responses to warming, altered precipitation, and their combination. We focused on plant growth and ecosystem carbon (C) balance, including biomass, net primary production (NPP), respiration, net ecosystem exchange (NEE), and ecosystem photosynthesis, synthesizing results from 85 studies. We found that experimental warming and increased precipitation generally stimulated plant growth and ecosystem C fluxes, whereas decreased precipitation had the opposite effects. For example, warming significantly stimulated total NPP, increased ecosystem photosynthesis, and ecosystem respiration. Experimentally reduced precipitation suppressed aboveground NPP (ANPP) and NEE, whereas supplemental precipitation enhanced ANPP and NEE. Plant productivity and ecosystem C fluxes generally showed higher sensitivities to increased precipitation than to decreased precipitation. Interactive effects of warming and altered precipitation tended to be smaller than expected from additive, single-factor effects, though low statistical power limits the strength of these conclusions. New experiments with combined temperature and precipitation manipulations are needed to conclusively determine the importance of temperature–precipitation interactions on the C balance of terrestrial ecosystems under future climate conditions.

/pdf/responses-of-terrestrial-ecosystems-to-temperature-and-y2jrrgao22.pdf

Responses of terrestrial ecosystems to temperature and precipitation change: a meta-analysis of experimental manipulation.

Global warming is predicted to be most pronounced at high latitudes, and observational evidence over the past 25 years suggests that this warming is already under way. One-third of the global soil carbon pool is stored in northern latitudes, so there is considerable interest in understanding how the carbon balance of northern ecosystems will respond to climate warming. Observations of controls over plant productivity in tundra and boreal ecosystems have been used to build a conceptual model of response to warming, where warmer soils and increased decomposition of plant litter increase nutrient availability, which, in turn, stimulates plant production and increases ecosystem carbon storage. Here we present the results of a long-term fertilization experiment in Alaskan tundra, in which increased nutrient availability caused a net ecosystem loss of almost 2,000 grams of carbon per square meter over 20 years. We found that annual aboveground plant production doubled during the experiment. Losses of carbon and nitrogen from deep soil layers, however, were substantial and more than offset the increased carbon and nitrogen storage in plant biomass and litter. Our study suggests that projected release of soil nutrients associated with high-latitude warming may further amplify carbon release from soils, causing a net loss of ecosystem carbon and a positive feedback to climate warming.

Ecosystem carbon storage in arctic tundra reduced by long-term nutrient fertilization

The elevational patterns of diversity for plants and animals have been well established over the past century. However, it is unclear whether there is a general elevational distribution pattern for microbes. Changbai Mountain is one of few well conserved natural ecosystems, where the vertical distribution of vegetation is known to mirror the vegetation horizontal zonation from temperate to frigid zones on the Eurasian continent. Here, we present a comprehensive analysis of soil bacterial community composition and diversity along six elevations representing six typical vegetation types from forest to alpine tundra using a bar-coded pyrosequencing technique. The bacterial communities differed dramatically along elevations (vegetation types), and the community composition was significantly correlated with soil pH, carbon/nitrogen ratio (C/N), moisture or total organic carbon (TOC), respectively. Phylogenetic diversity was positively correlated with soil pH ( P  = 0.024), while phylotype richness was positively correlated with soil pH ( P  = 0.004), total nitrogen (TN) ( P  = 0.030), and negatively correlated with C/N ratio ( P  = 0.021). Our results emphasize that pH is a better predictor of soil bacterial elevational distribution and also suggest that vegetation types may indirectly affect soil bacterial elevational distribution through altering soil C and N status.

Soil pH drives the spatial distribution of bacterial communities along elevation on Changbai Mountain

Previous research has shown that experimental perturbations of arctic ecosystems simulating direct and indirect effects of predicted environmental changes have led to strong responses in the plant communities, mostly associated with increased plant nutrient availability. Similarly, changes in decomposition and nutrient mineralization are likely to occur if the soil warms and the soil moisture conditions are altered. Plant and microbial responses have usually been investigated separately, and few, if any, studies have addressed simultaneous responses to environmental changes in plants and soil microorganisms, except in models.

We measured simultaneous responses in biomass, nitrogen (N), and phosphorus (P) incorporation in plants and microorganisms after five years of factorial fertilizer addition, air warming, and shading. We expected increased N and P uptake by microorganisms after fertilizer addition and also after warming, due to increases in mineralization rates in warmer soils. Plant productivity and N and P uptake were expected to increase after fertilizer addition but less after warming, because microbes were expected to absorb most of the extra released nutrients. Shading was expected to decrease plant production and also microbial biomass, due to the reduced production of labile carbon (C) in plant root exudates associated with reduced photosynthesis.

We found that the plants responded strongly to fertilizer addition by increased biomass accumulation and N and P uptake. They responded less to warming, but more than expected, showing a decline in N and P concentrations in many cases. There were few significant responses to shading. The strongest response was found in combined fertilizer addition and warming treatments. All functional vascular plant groups responded similarly. However, mosses declined under those conditions when vascular plant growth was most pronounced.

Contrary to our expectation, microbial C, N, and P did not increase after warming, but microbial N and P increased after shading. As expected, fertilizer addition led to increased microbial P content, whereas microbial N either increased or did not change. In general, microbial C did not change in any treatment. The microbes accumulated extra N and P only when soil inorganic N or P levels increased, suggesting that the soil microorganisms absorbed extra nutrients only in cases of declining N and P sink strength in plants.

Responses in microbes and plants to changed temperature, nutrient, and light regimes in the arctic

The soil microbial carbon (C), nitrogen (N) and phosphorus (P) pools were quantified in the organic horizon of soils from an arctic/alpine low-altitude heath and a high-altitude fellfield by the fumigation-extraction method before and after factorial addition of sugar, NPK fertilizer and benomyl, a fungicide. In unamended soil, microbial C, N and P made up 3.3-3.6%, 6.1-7.3% and 34.7% of the total soil C, N and P content, respectively. The inorganic extractable N pool was below 0.1% and the inorganic extractable P content slightly less than 1% of the total soil pool sizes. Benomyl addition in spring and summer did not affect microbial C or nutrient content analysed in the autumn. Sugar amendments increased microbial C by 15 and 37% in the two soils, respectively, but did not affect the microbial nutrient content, whereas inorganic N and P either declined significantly or tended to decline. The increased microbial C indicates that the microbial biomass also increased but without a proportional enhancement of N and P uptake. NPK addition did not affect the amount of microbial C but almost doubled the microbial N pool and more than doubled the P pool. A separate study has shown that CO2 evolution increased by more than 50% after sugar amendment and by about 30% after NPK and NK additions to one of the soils. Hence, the microbial biomass did not increase in response to NPK addition, but the microbes immobilized large amounts of the added nutrients and, judging by the increased CO2 evolution, their activity increased. We conclude: (1) that microbial biomass production in these soils is stimulated by labile carbon and that the microbial activity is stimulated by both labile C and by nutrients (N); (2) that the microbial biomass is a strong sink for nutrients and that the microbial community probably can withdraw substantial amounts of nutrients from the inorganic, plant-available pool, at least periodically; (3) that temporary declines in microbial populations are likely to release a flush of inorganic nutrients to the soil, particularly P of which the microbial biomass contained more than one third of the total soil pool; and (4) that the mobilization-immobilization cycles of nutrients coupled to the population dynamics of soil organisms can be a significant regulating factor for the nutrient supply to the primary producers, which are usually strongly nutrient-limited in arctic ecosystems.

Esben V. Nielsen

Papers

Responses in microbes and plants to changed temperature, nutrient, and light regimes in the arctic

Microbial biomass C, N and P in two arctic soils and responses to addition of NPK fertilizer and sugar: implications for plant nutrient uptake.