Carbon Sequestration in Ecosystems: The Role of Stoichiometry

Limnology and Oceanography, 2008

Cells and organisms, both autotrophs and heterotrophs, commonly face imbalanced access to and uptake of elements relative to their requirements. C is often in excess relative to key nutrient elements like N or P in photoautotrophs. Likewise, one of the lessons from ecological stoichiometry is that the growth of consumers, especially herbivores and detritivores, is commonly limited by N or P such that they also experience C in excess in relative terms. ''Excess'' implies wastage, yet this definition, which is consistent with purely stoichiometric arguments, is by no means straightforward. In fact, many organisms put this apparently surplus C to good use for fitness-promoting purposes like storage, structure, and defense or mutualistic goals like symbiosis. Nevertheless, genuine excesses do occur, in which case the remaining ''leftover C'' must be disposed of, either in organic or inorganic form via increased metabolic activity and respiration. These fluxes of C in various forms have major effects on the C balance of organisms, as well as governing the energy flux and C pathways at the ecosystem level. We here discuss evolutionary and ecological implications of ''excess C'' both at the organism and ecosystem level.

Verhandlungen, 2005

A key determinant o f e-flux in aquatic ecosystems is the supply o f elements relative to the demands o f producers and consumers. Uptake of e is commonly in excess relative to P or N in both autotrophs and heterotrophs, yet they may have different ways o f coping with excess e. Stoichiometric demands thus govern e-use efficiency in individual organisms, and in food webs these stoichiometric principles will affect etransfer efficiency across trophic levels. I f a high rate ofe-fixation via photosynthesis is not met by corresponding increased uptake rates ofN and P, this deficiency will yield plant biomass with low nutrient value (high e:N or e:P). Normally, plants and detritus have far higher e:N or e:P ratios than that ofthe heterotrophs (bacteria and zooplankton), which may lead to P or N-limited growth of consumers. This limitation will al so affect population dynamics o f the consumer and food web interactions. The excess e may enter the detritus pathway, it may be buried in sediments, or it may be oxidized to eoz. Thus the balance or mismatch o f elemental ratios in individual organisms and food webs will add up to a major determinant for the overall e-cycle and production:respiration ratio at the ecosystem level. This surplus of e is especially pronounced in many freshwater systems receiving high inputs of allochthonous e that will shift the balance from autotrophic to heterotrophic processes, thus reinforcing the net export of eoz from water to atrnosphere. Based on a large database o f lakes, this paper will explore an d review these stoichiometric aspects of e-metabolism and trophic transfer efficiency in lakes.

Ecology Letters, 1998

Primary producers with high nutrient contents typically exhibit high herbivory rates and fast decomposition rates. These tendencies, however, have not been generalized across ecosystems with contrasting herbivore characteristics and abiotic properties. Here we demonstrate that ecosystem types dominated by richer autotrophs (i.e. higher nutrient contents) are subject to higher rates of herbivory and decomposition in spite of differences in herbivore characteristics and environmental conditions. We further show that, as a result of these tendencies, ecosystems with richer autotrophs accumulate less carbon. These results identify autotrophic nutrient content as a main control of heterotrophic consumption and carbon accumulation in ecosystems. They also provide a basis to evaluate changes in these ecosystem properties following anthropogenic eutrophication.

Biogeosciences

The cycling of carbon (C) between the Earth surface and the atmosphere is controlled by biological and abiotic processes that regulate C storage in biogeochemical compartments and release to the atmosphere. This partitioning is quantified using various forms of C-use efficiency (CUE)-the ratio of C remaining in a system to C entering that system. Biological CUE is the fraction of C taken up allocated to biosynthesis. In soils and sediments, C storage depends also on abiotic processes, so the term C-storage efficiency (CSE) can be used. Here we first review and reconcile CUE and CSE definitions proposed for autotrophic and heterotrophic organisms and communities, food webs, whole ecosystems and watersheds, and soils and sediments using a common mathematical framework. Second, we identify general CUE patterns; for example, the actual CUE increases with improving growth conditions, and apparent CUE decreases with increasing turnover. We then synthesize > 5000 CUE estimates showing that CUE decreases with increasing biological and ecological organization-from uni-cellular to multicellular organisms and from individuals to ecosystems. We conclude that CUE is an emergent property of coupled biological-abiotic systems, and it should be regarded as a flexible and scale-dependent index of the capacity of a given system to effectively retain C.

Springer eBooks, 1995

Climate Research, 1993

Limnology and Oceanography, 2013

Based on the observation that organism-specific elemental content creates ecologically relevant mismatches such as between plant and animal tissue, it was postulated-and experimentally verified-that this would profoundly affect trophic efficiency and nutrient fluxes in ecosystems. From its beginnings as a Daphnia-centered perspective, the field of ecological stoichiometry (ES) has widened to include many organism groups, and ecosystem types, and the questions it addresses have broadened. We address some of the development of ES in aquatic sciences especially over the past 10 yr, focusing on homeostasis and mass balance in the consumer, and its effect on trophic efficiency and nutrient recycling in aquatic communities. We also discuss how ES has provided novel insights into genomic, proteomic, and cellular responses at one end of the biological scale as well as into large-scale effects related to biogeochemical couplings at the ecosystem level. The coupling of global C, N, and P cycles via their biotic interactions and their responses to climate change accentuate ES as an important toolkit for ecosystem analysis. We also point to some of the major topics and principles where ES has provided new insights. For each of these topics we also point to some novel directions where the ES concepts likely will be useful in understanding and predicting biological responses.

Soil Biology and Biochemistry, 2006

Soil food webs are mainly based on three primary carbon (C) sources: root exudates, litter, and recalcitrant soil organic matter (SOM). These C sources vary in their availability and accessibility to soil organisms, which could lead to different pathways in soil food webs. The presence of three C isotopes ( 12 C, 13 C and 14 C) offers an unique opportunity to investigate all three C sources simultaneously. In a microcosm experiment we studied the effect of food web complexity on the utilization of the three carbon sources. We choose an incomplete three factorial design with (i) living plants, (ii) litter and (iii) food web complexity. The most complex food web consisted of autochthonous microorganisms, nematodes, collembola, predatory mites, endogeic and anecic earthworms. We traced C from all three sources in soil, in CO 2 efflux and in individual organism groups by using maize grown on soil developed under C 3 vegetation and application of 14 C labelled ryegrass shoots as a litter layer. The presence of living plants had a much greater effect on C pathways than food web complexity. Litter decomposition, measured as 14 CO 2 efflux, was decreased in the presence of living plants from 71% to 33%. However, living plants increased the incorporation of litter C into microbial biomass and arrested carbon in the litter layer and in the upper soil layer. The only significant effect of food web complexity was on the litter C distribution in the soil layers. In treatments with fungivorous microarthropods (Collembola) the incorporation of litter carbon into mineral soil was reduced. Root exudates as C source were passed through rhizosphere microorganisms to the predator level (at least to the third trophic level). We conclude that living plants strongly affected C flows, directly by being a source of additional C, and indirectly by modifying the existing C flows within the food web including CO 2 efflux from the soil and litter decomposition. r

Science of The Total Environment, 2020

Interactions between the carbon (C) and nitrogen (N) cycles can impact on the sensitivity of terrestrial C storage to elevated atmospheric carbon dioxide (CO 2) concentrations (eCO 2). However, the underlying mechanisms associated with C\ \N interactions that influence terrestrial ecosystem C sequestration (C seq) remains unclear. Here, we quantitatively analyzed published C and N responses to experimentally eCO 2 using a meta-analysis approach. We determined the relative importance of three principal mechanisms (changes in the total ecosystem N amount, redistribution of N between plant and soil pools, and flexibility of the C:N ratio) that contribute to increases in ecosystem C storage in response to eCO 2. Our results showed that eCO 2 increased C and N accumulation, resulted in higher C:N ratios in plant, litter, and soil pools and induced a net shift of N from soils to vegetation. These three mechanisms largely explained the increment of ecosystem C seq under eCO 2 , although the relative contributions differed across ecosystem types, with changes in the C:N ratio contributing 50% of the increment in forests C seq , while the total N change contributed 60% of the increment in grassland C seq. In terms of temporal variation in the relative importance of each of these three mechanisms to ecosystem C seq : changes in the C:N ratio was the most important mechanism during the early years (~5 years) of eCO 2 treatment, whilst the contribution to ecosystem C seq by N redistribution remained rather small, and the contribution by total N change did not show a clear temporal pattern. This study highlights the differential contributions of the three mechanisms to C seq , which may offer important implications for future predictions of the C cycle in terrestrial ecosystems subjected to global change.

Ecol Appl, 2005

We used a simple model of carbon-nitrogen (C-N) interactions in terrestrial ecosystems to examine the responses to elevated CO 2 and to elevated CO 2 plus warming in ecosystems with the same total nitrogen loss but that differed in the ratio of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) loss. We postulate that DIN losses can be curtailed by higher N demand in response to elevated CO 2 but that DON losses cannot. We also examined simulations in which DON losses were held constant, were proportional to the amount of soil 1 organic matter, were proportional to the soil C:N ratio, or were proportional to the rate of decomposition. We found that the mode of N loss made little difference to the short-term (<60 years) rate of carbon sequestration by the ecosystem, but high DON losses resulted in much lower carbon sequestration in the long term than did low DON losses. In the short term, C sequestration was fueled by an internal redistribution of N from soils to vegetation and by increases in the C:N ratio of soils and vegetation. This sequestration was about three times larger with elevated CO 2 and warming than with elevated CO 2 alone. After year 60, C sequestration is fueled by a net accumulation of N in the ecosystem and the rate of sequestration was about the same with elevated CO 2 and warming as with elevated CO 2 alone. With high DON losses, the ecosystem either sequestered C slowly after year 60 (when DON losses were constant or proportional to soil organic matter) or lost C (when DON losses were proportional to the soil C:N ratio or to decomposition). We conclude that changes in long-term C sequestration depend not only on the magnitude of N losses but on the form of those losses as well.