Carbon (C) metabolism is at the core of ecosystem function. Decomposers play a critical role in this metabolism as they drive soil C cycle by mineralizing organic matter to CO2. Their growth depends ...on the carbon-use efficiency (CUE), defined as the ratio of growth over C uptake. By definition, high CUE promotes growth and possibly C stabilization in soils, while low CUE favors respiration. Despite the importance of this variable, flexibility in CUE for terrestrial decomposers is still poorly characterized and is not represented in most biogeochemical models. Here, we synthesize the theoretical and empirical basis of changes in CUE across aquatic and terrestrial ecosystems, highlighting common patterns and hypothesizing changes in CUE under future climates. Both theoretical considerations and empirical evidence from aquatic organisms indicate that CUE decreases as temperature increases and nutrient availability decreases. More limited evidence shows a similar sensitivity of CUE to temperature and nutrient availability in terrestrial decomposers. Increasing CUE with improved nutrient availability might explain observed declines in respiration from fertilized stands, while decreased CUE with increasing temperature and plant C : N ratios might decrease soil C storage. Current biogeochemical models could be improved by accounting for these CUE responses along environmental and stoichiometric gradients.
All plant species require at least 16 elements for their growth and survival but the relative requirements and the variability at different organizational scales is not well understood.
We use a ...fertiliser experiment with six willow (Salix spp.) genotypes to evaluate a methodology based on Euclidian distances for stoichiometric analysis of the variability in leaf nutrient relations of twelve of those (C, N, P, K, Ca, Mg, Mn, S, Fe, Zn, B, Cu) plus Na and Al.
Differences in availability of the elements in the environment was the major driver of variation. Variability between leaves within a plant or between individuals of the same genotype growing in close proximity was as large as variability between genotypes.
Elements could be grouped by influence on growth: N, P, S and Mn concentrations follow each other and increase with growth rate; K, Ca and Mg uptake follow the increase in biomass; but uptake of Fe, B, Zn and Al seems to be limited. The position of Cu lies between the first two groups. Only for Na is there a difference in element concentrations between genotypes. The three groups of elements can be associated with different biochemical functions.
• Growth of plants in terrestrial ecosystems is often limited by the availability of nitrogen (N) or phosphorous (P) Liebig’s law of the minimum states that the nutrient in least supply relative to ...the plant’s requirement will limit the plant’s growth. An alternative to the law of the minimum is the multiple limitation hypothesis (MLH) which states that plants adjust their growth patterns such that they are limited by several resources simultaneously. • We use a simple model of plant growth and nutrient uptake to explore the consequences for the plant’s relative growth rate of letting plants invest differentially in N and P uptake. • We find a smooth transition between limiting elements, in contrast to the strict transition in Liebig’s law of the minimum. At N : P supply ratios where the two elements simultaneously limit growth, an increase in either of the nutrients will increase the growth rate because more resources can be allocated towards the limiting element, as suggested by the multiple limitation hypothesis. However, the further the supply ratio deviates from these supply rates, the more the plants will follow the law of the minimum. • Liebig’s law of the minimum will in many cases be a useful first‐order approximation.
Nutrient elements are important for plant growth. Element stoichiometry considers the balance between different nutrients and how this balance is affected by the environment. So far, focus of plant ...stoichiometry has mainly been on the three elements carbon (C), nitrogen (N), and phosphorus (P), but many additional elements are essential for proper plant growth. Our overall aim is to test the scaling relations of various additional elements (K, Ca, Mg, S, Cu, Zn, Fe, Mn), by using ten data sets from a range of plant functional types and environmental conditions. To simultaneously handle more than one element, we define a stoichiometric niche volume as the volume of an abstract multidimensional shape in
dimensions, with the
sides of this shape defined by the plant properties in question, here their element concentrations. Thus, a stoichiometric niche volume is here defined as the product of element concentrations. The volumes of N and P (
) are used as the basis, and we investigate how the volume of other elements (
) scales with respect to
with the intention to explore if the concentrations of other elements increase faster (scaling exponent > 1) or slower (<1) than the concentrations of N and P. For example, scaling exponents >1 suggest that favorable conditions for plant growth, i.e., environments rich in N and P, may require proportionally higher uptake of other essential elements than poor conditions. We show that the scaling exponent is rather insensitive to environmental conditions or plant species, and ranges from 0.900 to 2.479 (average 1.58) in nine out of ten data sets. For single elements, Mg has the smallest scaling exponent (0.031) and Mn the largest (2.147). Comparison between laboratory determined stoichiometric relations and field observations suggest that element uptake in field conditions often exceeds the minimal physiological requirements. The results provide evidence for the view that the scaling relations previously reported for N and P can be extended to other elements; and that N and P are the driving elements in plant stoichiometric relations. The stoichiometric niche volumes defined here could be used to predict plant performances in different environments.
Summary
Extramatrical mycelia (EMM) of ectomycorrhizal fungi are important in carbon (C) and nitrogen (N) cycling in forests, but poor knowledge about EMM biomass and necromass turnovers makes the ...quantification of their role problematic.
We studied the impacts of elevated CO2 and N fertilization on EMM production and turnover in a Pinus taeda forest. EMM C was determined by the analysis of ergosterol (biomass), chitin (total bio‐ and necromass) and total organic C (TOC) of sand‐filled mycelium in‐growth bags. The production and turnover of EMM bio‐ and necromass and total C were estimated by modelling.
N fertilization reduced the standing EMM biomass C to 57% and its production to 51% of the control (from 238 to 122 kg C ha−1 yr−1), whereas elevated CO2 had no detectable effects. Biomass turnover was high (˜13 yr−1) and unchanged by the treatments. Necromass turnover was slow and was reduced from 1.5 yr−1 in the control to 0.65 yr−1 in the N‐fertilized treatment. However, TOC data did not support an N effect on necromass turnover.
An estimated EMM production ranging from 2.5 to 6% of net primary production stresses the importance of its inclusion in C models. A slow EMM necromass turnover indicates an importance in building up forest humus.
Stoichiometric relations in plants, with emphasis on C:N:P, are reviewed. Both theoretically and empirically it is found for whole plants as well as for different tissues that nitrogen concentrations ...increase slower than phosphorus concentrations. A lack of data prevents the establishment of relations between nitrogen and other elements. Optimal element ratios where elements are simultaneously limiting growth can be established. There is a considerable variability around these optimal ratios in observed values. Conclusions about the ecological significance of stoichiometric relations based on these observations may therefore be biased. The significance of this variability remains to be established.
Tree growth in boreal forests is limited by nitrogen (N) availability. Most boreal forest trees form symbiotic associations with ectomycorrhizal (ECM) fungi, which improve the uptake of inorganic N ...and also have the capacity to decompose soil organic matter (SOM) and to mobilize organic N (‘ECM decomposition’).
To study the effects of ‘ECM decomposition’ on ecosystem carbon (C) and N balances, we performed a sensitivity analysis on a model of C and N flows between plants, SOM, saprotrophs, ECM fungi, and inorganic N stores.
The analysis indicates that C and N balances were sensitive to model parameters regulating ECM biomass and decomposition. Under low N availability, the optimal C allocation to ECM fungi, above which the symbiosis switches from mutualism to parasitism, increases with increasing relative involvement of ECM fungi in SOM decomposition. Under low N conditions, increased ECM organic N mining promotes tree growth but decreases soil C storage, leading to a negative correlation between C stores above- and below-ground.
The interplay between plant production and soil C storage is sensitive to the partitioning of decomposition between ECM fungi and saprotrophs. Better understanding of interactions between functional guilds of soil fungi may significantly improve predictions of ecosystem responses to environmental change.
Soil carbon diversity can be an important property for the stability of soil carbon. A problem is the lack of techniques for measuring this diversity. I suggest here the use of a combination of a ...general statistical principle, MAXimum ENTropy (MaxEnt), and a mechanistic model of organic matter decomposition, the Q model. The Q model provides the temporal development of the average carbon quality of litter and amount of soil organic C, which can be applied in a MaxEnt calculation to obtain a distribution of soil C over qualities. This distribution may not be the actual distribution but it is the most probable one. This distribution can be used to calculate aggregate properties for the total of soil C. I will use this distribution to calculate the temporal development of the variance in C quality as an expression of C diversity. The general tendency is that the variance declines with time of decomposition. Six long-term bare fallow (LTBF) from different climatic and management conditions were used to investigate which system properties are most important for the temporal development of the variance. The initial quality of the litter forming soil C is the dominant property. Chemical shifts in NMR spectra were tested as a possible way of measuring the variance in C quality.
Many ecology textbooks present the interaction between mycorrhizal fungi and their host plants as the archetype of symbiosis or mutualism. However, mycorrhiza drains carbon directly from the plant ...and also competes with the plant for soil inorganic nitrogen. We developed hypotheses based on a simple model to qualitatively investigate when, in a nitrogen-limited system, the fungal partner returns sufficient extra nitrogen to compensate for the amount of carbon allocated to it by the plant. We showed when the mycorrhizal association can be beneficial to the plant, but also when mycorrhizal immobilization of soil inorganic nitrogen can be a limitation. The amount of carbon and nitrogen that the mycorrhizal fungus can obtain from soil organic matter, by producing extracellular enzymes, is also important. Saprotrophic capability decreases the value of the fungus to the plant, as fungal uptake of soil carbon augments the use of the plant-supplied carbon and increases the fungal requirement for N. The stoichiometric mismatch between low-N soil organic matter and high-N fungal biochemistry turned out to be a bottleneck in making the fungus a net provider of additional N to the plant. The most important properties determining the usefulness to a plant of a mycorrhizal symbiont are plant nitrogen use efficiency and the amount of inorganic N taken up per unit extra fungal growth. The fraction of carbon the fungus allocates to its own growth, relative to its investment in exocellular enzymes, is also a critical property. Our results show that plants could benefit from the association with the fungus, which could explain the ubiquitous nature of this association between fungi and plants.
Soil organic carbon (SOC) is a substantial source of atmospheric CO2, but also a large cause of uncertainties in Earth-system models. A principal control on soil CO2 release is the carbon-use ...efficiency (CUE) of microbial communities, which partitions the carbon (C) allocation between biosynthetic stabilization and CO2 respiration during SOC decomposition. In Earth-system models, CUE is commonly considered as a constant, although it should be susceptible to environmental factors such as temperature. We explored CUE across a set of land-uses and temperatures, and we show the hitherto neglected phenomenon that land-use can alter the temperature response of CUE. In arable soils, CUE was constant over a temperature range between 5 and 20 °C, but it decreased with temperature in ley farming, grassland, and forest soils at temperatures above 12.5 °C. The decrease in CUE was strongest for forest soils. Implementing our findings into a soil-C model revealed substantial differences in projected SOC losses: Assuming an increase of mean annual temperature of 2 or 4 °C, soils were projected to lose up to 6 or 15% of their current SOC, respectively, until they reach a new steady-state. These projections varied among land-uses. Our findings confront the current representation of CUE in global C models and challenges C sequestration strategies based on land-use changes, because land-uses such as e.g. forest ecosystems with current high C storage may lose substantially more C than agricultural soils due to strong declines of CUE.
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•Temperature responses of carbon-use efficiency (CUE) differed across land-uses.•CUE decreased strongly in ley farming, grassland, and forest soils beyond 12.5 °C.•Land-use specific CUE temperature responses have profound impact on SOC projection.•Novel consumption-based thermodynamic approach ensures similar microbial workloads.•Microbial community composition was not a major driver of differences in CUE.
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