Göran I. Ågren

Environmental and stoichiometric controls on microbial carbon-use efficiency in soils

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 CO(2). 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.

Nitrogen saturation of terrestrial ecosystems

Nitrogen saturation, in the sense that nitrogen additions to an ecosystem lead to losses of the same order of magnitude, is analyzed as an interplay between a plant subsystem and a soil subsystem. The plant system is defined by its nitrogen productivity, which allows calculations of the maximum amount of nitrogen that can be held in, and the maximum nitrogen flux density that can be utilized by, the plant subsystem. The most important response of the soil subsystem is a change in the microbial nitrogen concentration, from which the nitrogen absorption capacity can be derived. It is shown that of the two subsystems the soil must always saturate first. The time to reach saturation depends strongly on site history in terms of the sources of litter forming the soil organic matter and on the ratio between the external nitrogen inflows and the litter nitrogen flow.

Soil organic matter quality interpreted thermodynamically

Abstract Soil organic matter quality in the sense of how easily carbon in the soil organic matter can be mineralised is a major determinant of soil carbon storage and rate of mineralisation of nutrients. Its origin has so far remained elusive and a number of indices, such as C-to-N-ratio, lignin concentration and other combinations of chemical constituents have been used as substitutes for quality. We suggest here that quality is the number of enzymatic steps required to release as carbon dioxide a carbon atom from an organic compound . The larger the number of steps the lower is the quality of the carbon atom. Such a measure connects quality to thermodynamics. It also explains the rapid decrease in decomposition rate with decreasing quality suggested in the q-theory of organic matter dynamics and shows that the decomposition rate of low quality substrates has a stronger temperature dependence than that of high quality substrates.

CARBON SEQUESTRATION IN ECOSYSTEMS: THE ROLE OF STOICHIOMETRY

The fate of carbon (C) in organisms, food webs, and ecosystems is to a major extent regulated by mass-balance principles and the availability of other key nutrient elements. In relative terms, nutrient limitation implies excess C, yet the fate of this C may be quite different in autotrophs and heterotrophs. For autotrophs nutrient limitation means less fixation of inorganic C or excretion of organic C, while for heterotrophs nutrient limitation means that more of ingested C will “go to waste” in the form of egestion or respiration. There is in general a mismatch between autotrophs and decomposers that have flexible but generally high C:element ratios, and consumers that have lower C:element ratios and tighter stoichiometric regulation. Thus, C-use efficiency in food webs may be governed by the element ratios in autotroph biomass and tend to increase when C:element ratios in food approach those of consumers. This tendency has a strong bearing on the sequestration of C in ecosystems, since more C will be di...

State-of-the-Art of Models of Production-Decomposition Linkages in Conifer and Grassland Ecosystems

We review the state-of-the-art of models of forests and grasslands that could be used to predict the impact of a future climate change arising from increased atmospheric carbon dioxide concentration. Four levels of resolution are recognized: physiologically based models, population models, ecosystem models, and regional or global models. At the physiological level a number of important processes can be described in great detail, but these models often treat inadequately interactions with nutrient cycles, which operate on longer time scales. Population and ecosystem models can, on the other hand, encapsulate relationships between the plants and the soil system, but at the expense of requiring more ad ho formulations of processes. At the regional and global scale we have so far only steady-state models, which cannot be used to predict transients caused by climate change. However, our conclusion is that, in spite of the gaps in knowledge, there are several models based on dominant processes that are well enough understood for the predictions of those models to be taken seriously.

RESPONSES OF N-LIMITED ECOSYSTEMS TO INCREASED CO2: A BALANCED-NUTRITION, COUPLED-ELEMENT-CYCLES MODEL

Ecosystem responses to increased CO2 are often constrained by nutrient limitation. We present a model of multiple-element limitation (MEL) and use it to analyze constraints imposed by N on the responses to an instantaneous doubling of CO2 concen- tration in a 350-yr-old eastern deciduous forest. We examine the effects of different exchange rates of inorganic N with sources and sinks external to the ecosystem (e.g., through de- position and leaching) and different initial ratios of net: gross N mineralization. Both of these factors influence the availability of N to vegetation and, therefore, have important effects on ecosystem responses to increased CO2. We conclude that reliable assessments of ecosystem responses to CO2 will require a better understanding of both these factors. The responses to increased CO2 appear on at least four characteristic time scales. (1) There is an instantaneous increase in net primary production, which results in an increase in the vegetation C:N ratio. (2) On a time scale of a few years, the vegetation responds by increasing uptake effort for available N (e.g., through increased allocation of biomass, energy, and enzymes to fine roots). (3) On a time scale of decades, there is a net movement of N from soil organic matter to vegetation, which enables vegetation biomass to accumulate. (4) On the time scale of centuries, ecosystem responses are dominated by increases in total ecosystem N, which enable organic matter to accumulate in both vegetation and soils. In general, short-term responses are markedly different from long-term responses.

The Influence of Plant Nutrition on Biomass Allocation

The influence of nutrition on the allocation of dry matter is investigated using data from previously published experiments with the forest tree species (Betula pendular Roth., Picea babies (L.) Karst., Pinus contorta Doug., and Pinus Sibbaldia L.) where the nutrient status of the plants was maintained constant over a considerable period of time and biomass increase (steady-state nutrition and growth). We demonstrated that the allocation patterns of a plant species under limiting nutrient conditions and at optimum can be derived from parameters that have been used to characterize relationships between nutrient status, nutrient uptake, and growth of the species. The properties of the plant that control biomass allocation are discussed on the basis of these findings.

Temperature sensitivity of soil respiration rates enhanced by microbial community response

Soils store about four times as much carbon as plant biomass, and soil microbial respiration releases about 60 petagrams of carbon per year to the atmosphere as carbon dioxide. Short-term experiments have shown that soil microbial respiration increases exponentially with temperature. This information has been incorporated into soil carbon and Earth-system models, which suggest that warming-induced increases in carbon dioxide release from soils represent an important positive feedback loop that could influence twenty-first-century climate change. The magnitude of this feedback remains uncertain, however, not least because the response of soil microbial communities to changing temperatures has the potential to either decrease or increase warming-induced carbon losses substantially. Here we collect soils from different ecosystems along a climate gradient from the Arctic to the Amazon and investigate how microbial community-level responses control the temperature sensitivity of soil respiration. We find that the microbial community-level response more often enhances than reduces the mid- to long-term (90 days) temperature sensitivity of respiration. Furthermore, the strongest enhancing responses were observed in soils with high carbon-to-nitrogen ratios and in soils from cold climatic regions. After 90 days, microbial community responses increased the temperature sensitivity of respiration in high-latitude soils by a factor of 1.4 compared to the instantaneous temperature response. This suggests that the substantial carbon stores in Arctic and boreal soils could be more vulnerable to climate warming than currently predicted.

Theoretical analysis of decomposition of heterogeneous substrates

A theory is proposed for the decomposition of a heterogeneous substrate, in which the heterogeneity is described by a continuously varying quality variable, q. Two microbial properties, efficiency in substrate utilization, e(q), and rate of substrate utilization, u(q), depend on the quality variable and decrease with decreasing substrate quality. General results of the theory can be displayed either in terms of time or quality. It turns out that the quality representation is both more general and more lucid. Only very weak specifications of the functions u(q) and u(q) are necessary to determine whether the decomposition process will end after a finite time and whether all substrate eventually will become mineralized. The final nitrogen-to-carbon ratio is shown to be independent of these two functions but depends on the initial nitrogen concentration and quality of the substrate. Using specific functions for u(q) and u(q) it is possible to derive a number of models used to describe decomposition and the variation in the critical nitrogen-to-carbon ratio with specific decomposition rate of the substrate. The theoretical predictions are compared to a number of decomposition experiments.

Quality : a bridge between theory and experiment in soil organic matter studies

The abstract concept quality was introduced by us ten years ago to formalise the dynamics of C, N, P, and S in soil organic matter. We present here an interpretation of decomposition studies of 19 different litter types performed at 16 different localities and including a total of 978 observations in terms of the continuous quality theory showing how quality can be estimated from conventional chemical fractionation. This provides us with a powerful tool to unify our understanding of the physical-chemical-biological complex behind the processes controlling soil organic matter dynamics.