Environmental drivers of increased ecosystem respiration in a warming tundra

Arctic and alpine tundra ecosystems are large reservoirs of organic carbon1,2. Climate warming may stimulate ecosystem respiration and release carbon into the atmosphere3,4. The magnitude and persistency of this stimulation and the environmental mechanisms that drive its variation remain uncertain5-7. This hampers the accuracy of global land carbon-climate feedback projections7,8. Here we synthesize 136 datasets from 56 open-top chamber in situ warming experiments located at 28 arctic and alpine tundra sites which have been running for less than 1 year up to 25 years. We show that a mean rise of 1.4 °C [confidence interval (CI) 0.9-2.0 °C] in air and 0.4 °C [CI 0.2-0.7 °C] in soil temperature results in an increase in growing season ecosystem respiration by 30% [CI 22-38%] (n = 136). Our findings indicate that the stimulation of ecosystem respiration was due to increases in both plant-related and microbial respiration (n = 9) and continued for at least 25 years (n = 136). The magnitude of the warming effects on respiration was driven by variation in warming-induced changes in local soil conditions, that is, changes in total nitrogen concentration and pH and by context-dependent spatial variation in these conditions, in particular total nitrogen concentration and the carbon:nitrogen ratio. Tundra sites with stronger nitrogen limitations and sites in which warming had stimulated plant and microbial nutrient turnover seemed particularly sensitive in their respiration response to warming. The results highlight the importance of local soil conditions and warming-induced changes therein for future climatic impacts on respiration.

Global plant trait relationships extend to the climatic extremes of the tundra biome

The majority of variation in six traits critical to the growth, survival and reproduction of plant species is thought to be organised along just two dimensions, corresponding to strategies of plant size and resource acquisition. However, it is unknown whether global plant trait relationships extend to climatic extremes, and if these interspecific relationships are confounded by trait variation within species. We test whether trait relationships extend to the cold extremes of life on Earth using the largest database of tundra plant traits yet compiled. We show that tundra plants demonstrate remarkably similar resource economic traits, but not size traits, compared to global distributions, and exhibit the same two dimensions of trait variation. Three quarters of trait variation occurs among species, mirroring global estimates of interspecific trait variation. Plant trait relationships are thus generalizable to the edge of global trait-space, informing prediction of plant community change in a warming world.

An optode sensor array for long-term in situ oxygen measurements in soil and sediment

Long-term measurements of molecular oxygen (O) dynamics in wetlands are highly relevant for understanding the effects of water level changes on net greenhouse gas budgets in these ecosystems. However, such measurements have been limited due to a lack of suitable measuring equipment. We constructed an O optode sensor array for long-term in situ measurements in soil and sediment. The new device consists of a 1.3-m-long, cylindrical, spear-shaped rod equipped with 10 sensor spots along the shaft. Each spot contains a thermocouple fixed with a robust fiberoptic O optode made by immobilizing a layer of Pt(II) meso-tetra(pentafluorophenyl)porphine in polystyrene at the end of a 2-mm polymethyl methacrylate plastic fiber. Temperature and O optode readings are collected continuously by a data logger and a multichannel fiberoptic O meter. The construction and measuring characteristics of the sensor array system are presented along with a novel approach for temperature compensation of O optodes. During in situ application over several months in a peat bog, we used the new device to document pronounced variations in O distribution after marked shifts in water level. The measurements showed anoxic conditions below the water level but also diel variations in O concentrations in the upper layer presumably due to rhizospheric oxidation by the main vegetation The new field instrument thus enables new and more detailed insights to the in situ O dynamics in wetlands.

Controls on the distribution of productivity and organic resources in Antarctic Dry Valley soils

The Antarctic Dry Valleys are regarded as one of the harshest terrestrial habitats on Earth because of the extremely cold and dry conditions. Despite the extreme environment and scarcity of conspicuous primary producers, the soils contain organic carbon and heterotrophic micro-organisms and invertebrates. Potential sources of organic compounds to sustain soil organisms include in situ primary production by micro-organisms and mosses, spatial subsidies from lacustrine and marine-derived detritus, and temporal subsidies ('legacies') from ancient lake deposits. The contributions from these sources at different sites are likely to be influenced by local environmental conditions, especially soil moisture content, position in the landscape in relation to lake level oscillations and legacies from previous geomorphic processes. Here we review the abiotic factors that influence biological activity in Dry Valley soils and present a conceptual model that summarizes mechanisms leading to organic resources therein.