Publications

The future carbon balance of high-latitude ecosystems is dependent on the sensitivity of biological processes (photosynthesis and respiration) to the physical changes occurring with permafrost thaw. Predicting C exchange in these ecosystems is difficult because the thawing of permafrost is a heterogeneous process that creates a complex landscape. We measured net ecosystem exchange of C using eddy covariance (EC) in a tundra landscape visibly undergoing thaw during two 6 month campaigns in 2008 and 2009. We developed a spatially explicit quantitative metric of permafrost thaw based on variation in microtopography and incorporated it into an EC carbon flux estimate using a generalized additive model (GAM). This model allowed us to make predictions about C exchange for the landscape as a whole and for specific landscape patches throughout the continuum of permafrost thaw and ground subsidence. During June through November 2008, the GAM predicted that the landscape on average took up 337.1 g C m⁻² via photosynthesis and released 289.5 g C m⁻² via respiration, resulting in a net C gain of 47.5 g C m⁻² by the tundra ecosystem. During April through October 2009, the landscape on average took up 498.7 g C m⁻² and released 410.3 g C m⁻², resulting in a net C gain of 87.8 g C m⁻². On average, between the years, areas with the highest permafrost thaw and ground subsidence photosynthesized 17.7% more and respired 3.3% more C than the average landscape. Areas with the least thaw and subsidence photosynthesized 15% less and respired 5.1% less than the landscape on average. By incorporating spatial variation into the EC C estimate, we were able to estimate the C balance of a heterogeneous landscape and determine the collective effect of permafrost thaw on the plant and soil processes that drive ecosystem C flux. In these study years, permafrost thaw appeared to increase the amplitude of the C cycle by stimulating both C release and sequestration, while the ecosystem remained a C sink at the landscape scale.

Global environmental changes are expected to alter ecosystem carbon and nitrogen cycling, but the interactive effects of multiple simultaneous environmental changes are poorly understood. Effects of these changes on the production of nitrous oxide (N₂O), an important greenhouse gas, could accelerate climate change. We assessed the responses of soil N₂O fluxes to elevated CO₂, heat, altered precipitation, and enhanced nitrogen deposition, as well as their interactions, in an annual grassland at the Jasper Ridge Global Change Experiment (CA, USA). Measurements were conducted after 6, 7 and 8 years of treatments. Elevated precipitation increased N₂O efflux, especially in combination with added nitrogen and heat. Path analysis supported the idea that increased denitrification due to increased soil water content and higher labile carbon availability best explained increased N₂O efflux, with a smaller, indirect contribution from nitrification. In our data and across the literature, single-factor responses tended to overestimate interactive responses, except when global change was combined with disturbance by fire, in which case interactive effects were large. Thus, for chronic global environmental changes, higher order interactions dampened responses of N₂O efflux to multiple global environmental changes, but interactions were strongly positive when global change was combined with disturbance. Testing whether these responses are general should be a high priority for future research.

One difficulty in using bioremediation at a contaminated site is demonstrating that biodegradation is actually occurring <em>in situ</em>. The stable isotope composition of contaminants may help with this, since they can serve as an indicator of biological activity. To use this approach it is necessary to establish how a particular biodegradation pathway affects the isotopic composition of a contaminant. This study examined bacterial strains expressing three aerobic enzymes for their effect on the ¹³C/¹²C ratio when degrading both trichloroethene (TCE) and <em>cis</em>-1,2-dichloroethene (c-DCE): toluene 3-monoxygenase, toluene 4-monooxygenase, and toluene 2,3-dioxygenase. We found no significant differences in fractionation among the three enzymes for either compound. Aerobic degradation of c-DCE occurred with low fractionation producing δ¹³C enrichment factors of −0.9 ± 0.5 to −1.2 ± 0.5, in contrast to reported anaerobic degradation δ¹³C enrichment factors of −14.1 to −20.4‰. Aerobic degradation of TCE resulted in δ¹³C enrichment factors of −11.6 ± 4.1 to −14.7 ± 3.0‰ which overlap reported δ¹³C enrichment factors for anaerobic TCE degradation of −2.5 to −13.8‰. The data from this study suggest that stable isotopes could serve as a diagnostic for detecting aerobic biodegradation of TCE by toluene oxygenases at contaminated sites.

Carbon uptake by forests is a major sink in the global carbon cycle, helping buffer the rising concentration of CO₂ in the atmosphere, yet the potential for future carbon uptake by forests is uncertain. Climate warming and drought can reduce forest carbon uptake by reducing photosynthesis, increasing respiration, and by increasing the frequency and intensity of wildfires, leading to large releases of stored carbon. Five years of eddy covariance measurements in a ponderosa pine (<em>Pinus ponderosa</em>)-dominated ecosystem in northern Arizona showed that an intense wildfire that converted forest into sparse grassland shifted site carbon balance from sink to source for at least 15 years after burning. In contrast, recovery of carbon sink strength after thinning, a management practice used to reduce the likelihood of intense wildfires, was rapid. Comparisons between an undisturbed-control site and an experimentally thinned site showed that thinning reduced carbon sink strength only for the first two posttreatment years. In the third and fourth posttreatment years, annual carbon sink strength of the thinned site was higher than the undisturbed site because thinning reduced aridity and drought limitation to carbon uptake. As a result, annual maximum gross primary production occurred when temperature was 3 °C higher at the thinned site compared with the undisturbed site. The severe fire consistently reduced annual evapotranspiration (range of 12–30%), whereas effects of thinning were smaller and transient, and could not be detected in the fourth year after thinning. Our results show large and persistent effects of intense fire and minor and short-lived effects of thinning on southwestern ponderosa pine ecosystem carbon and water exchanges.

Six terrestrial ecosystems in the USA were exposed to elevated atmospheric CO₂ in single or multifactorial experiments for more than a decade to assess potential impacts. We retrospectively assessed soil bacterial community responses in all six-field experiments and found ecosystem-specific and common patterns of soil bacterial community response to elevated CO₂. Soil bacterial composition differed greatly across the six ecosystems. No common effect of elevated atmospheric CO₂ on bacterial biomass, richness and community composition across all of the ecosystems was identified, although significant responses were detected in individual ecosystems. The most striking common trend across the sites was a decrease of up to 3.5-fold in the relative abundance of <em>Acidobacteria</em> Group 1 bacteria in soils exposed to elevated CO₂ or other climate factors. The <em>Acidobacteria</em> Group 1 response observed in exploratory 16S rRNA gene clone library surveys was validated in one ecosystem by 100-fold deeper sequencing and semi-quantitative PCR assays. Collectively, the 16S rRNA gene sequencing approach revealed influences of elevated CO₂ on multiple ecosystems. Although few common trends across the ecosystems were detected in the small surveys, the trends may be harbingers of more substantive changes in less abundant, more sensitive taxa that can only be detected by deeper surveys.

Elevated CO₂ is expected to lower plant nutrient concentrations via carbohydrate dilution and increased nutrient use efficiency. Elevated CO₂ consistently lowers plant foliar nitrogen, but there is no consensus on CO₂ effects across the range of plant nutrients. We used meta-analysis to quantify elevated CO₂ effects on leaf, stem, root, and seed concentrations of B, Ca, Cu, Fe, K, Mg, Mn, P, S, and Zn among four plant functional groups and two levels of N fertilization. CO₂ effects on plant nutrient concentration depended on the nutrient, plant group, tissue, and N status. CO₂ reduced B, Cu, Fe, and Mg, but increased Mn concentration in the leaves of N₂ fixers. Elevated CO₂ increased Cu, Fe, and Zn, but lowered Mn concentration in grass leaves. Tree leaf responses were strongly related to N status: CO₂ significantly decreased Cu, Fe, Mg, and S at high N, but only Fe at low N. Elevated CO₂ decreased Mg and Zn in crop leaves grown with high N, and Mn at low N. Nutrient concentrations in crop roots were not affected by CO₂ enrichment, but CO₂ decreased Ca, K, Mg and P in tree roots. Crop seeds had lower S under elevated CO₂. We also tested the validity of a “dilution model.” CO₂ reduced the concentration of plant nutrients 6.6% across nutrients and plant groups, but the reduction is less than expected (18.4%) from carbohydrate accumulation alone. We found that elevated CO₂ impacts plant nutrient status differently among the nutrient elements, plant functional groups, and among plant tissues. Our synthesis suggests that differences between plant groups and plant organs, N status, and differences in nutrient chemistry in soils preclude a universal hypothesis strictly related to carbohydrate dilution regarding plant nutrient response to elevated CO₂.

The cryosphere—the portion of the Earth’s surface where water is in solid form for at least one month of the year—has been shrinking in response to climate warming. The extents of sea ice, snow, and glaciers, for example, have been decreasing. In response, the ecosystems within the cryosphere and those that depend on the cryosphere have been changing. We identify two principal aspects of ecosystem-level responses to cryosphere loss: (1) trophodynamic alterations resulting from the loss of habitat and species loss or replacement and (2) changes in the rates and mechanisms of biogeochemical storage and cycling of carbon and nutrients, caused by changes in physical forcings or ecological community functioning. These changes affect biota in positive or negative ways, depending on how they interact with the cryosphere. The important outcome, however, is the change and the response the human social system (infrastructure, food, water, recreation) will have to that change.

Deep soil profiles containing permafrost (Gelisols) were characterized for organic carbon (C) and total nitrogen (N) stocks to 3 m depths. Using the Community Climate System Model (CCSM4) we calculate cumulative distributions of active layer thickness (ALT) under current and future climates. The difference in cumulative ALT distributions over time was multiplied by C and N contents of soil horizons in Gelisol suborders to calculate newly thawed C and N. Thawing ranged from 147 PgC with 10 PgN by 2050 (representative concentration pathway RCP scenario 4.5) to 436 PgC with 29 PgN by 2100 (RCP 8.5). Organic horizons that thaw are vulnerable to combustion, and all horizon types are vulnerable to shifts in hydrology and decomposition. The rates and extent of such losses are unknown and can be further constrained by linking field and modelling approaches. These changes have the potential for strong additional loading to our atmosphere, water resources, and ecosystems.

Evidence is mounting that extinctions are altering key processes important to the productivity and sustainability of Earth’s ecosystems^{<a id="ref-link-1" title="Loreau, M., Naeem, S. &amp; Inchausti, P. Biodiversity and Ecosystem Functioning: Synthesis and perspectives (Oxford Univ. Press, 2002)" href="http://www.nature.com/nature/journal/v486/n7401/full/nature11118.html#ref1">1</a>, <a id="ref-link-2" title="Hooper, D. U. et al. Effects of biodiversity on ecosystem functioning: a consensus of current knowledge. Ecol. Monogr. 75, 3-35 (2005)" href="http://www.nature.com/nature/journal/v486/n7401/full/nature11118.html#ref2">2</a>, <a id="ref-link-3" title="Tilman, D. Ecological consequences of biodiversity: a search for general principles. Ecology 80, 1455-1474 (1999)" href="http://www.nature.com/nature/journal/v486/n7401/full/nature11118.html#ref3">3</a>, <a id="ref-link-4" title="Wardle, D. A., Bardgett, R. D., Callaway, R. M. &amp; Van der Putten, W. H. Terrestrial ecosystem responses to species gains and losses. Science 332, 1273-1277 (2011)" href="http://www.nature.com/nature/journal/v486/n7401/full/nature11118.html#ref4">4</a>}. Further species loss will accelerate change in ecosystem processes^{<a id="ref-link-5" title="Balvanera, P. et al. Quantifying the evidence for biodiversity effects on ecosystem functioning and services. Ecol. Lett. 9, 1146-1156 (2006)" href="http://www.nature.com/nature/journal/v486/n7401/full/nature11118.html#ref5">5</a>, <a id="ref-link-6" title="Cardinale, B. J. et al. The functional role of producer diversity in ecosystems. Am. J. Bot. 98, 572-592 (2011)" href="http://www.nature.com/nature/journal/v486/n7401/full/nature11118.html#ref6">6</a>, <a id="ref-link-7" title="Stachowicz, J. J., Bruno, J. F. &amp; Duffy, J. E. Understanding the effects of marine biodiversity on communities and ecosystems. Annu. Rev. Ecol. Evol. Syst. 38, 739-766 (2007)" href="http://www.nature.com/nature/journal/v486/n7401/full/nature11118.html#ref7">7</a>, <a id="ref-link-8" title="Perrings, C. et al. Ecosystem services, targets, and indicators for the conservation and sustainable use of biodiversity. Front. Ecol. Environ 9, 512-520 (2011)" href="http://www.nature.com/nature/journal/v486/n7401/full/nature11118.html#ref8">8</a>}, but it is unclear how these effects compare to the direct effects of other forms of environmental change that are both driving diversity loss and altering ecosystem function. Here we use a suite of meta-analyses of published data to show that the effects of species loss on productivity and decomposition—two processes important in all ecosystems—are of comparable magnitude to the effects of many other global environmental changes. In experiments, intermediate levels of species loss (21–40%) reduced plant production by 5–10%, comparable to previously documented effects of ultraviolet radiation and climate warming. Higher levels of extinction (41–60%) had effects rivalling those of ozone, acidification, elevated CO₂ and nutrient pollution. At intermediate levels, species loss generally had equal or greater effects on decomposition than did elevated CO₂ and nitrogen addition. The identity of species lost also had a large effect on changes in productivity and decomposition, generating a wide range of plausible outcomes for extinction. Despite the need for more studies on interactive effects of diversity loss and environmental changes, our analyses clearly show that the ecosystem consequences of local species loss are as quantitatively significant as the direct effects of several global change stressors that have mobilized major international concern and remediation efforts^{<a id="ref-link-9" title="IPCC. Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (eds Core Writing Team, Pachauri, R. K. &amp; Reisinger, A.) (IPCC, 2007)" href="http://www.nature.com/nature/journal/v486/n7401/full/nature11118.html#ref9">9</a>}.

Ecosystems regulate climate through biogeochemistry and biophysics, but current policies only recognize biogeochemical influences. A new proposal to include biophysical effects changes the climate value of ecosystems, and sets the stage to expand the suite of climate regulation services considered in global policies and carbon markets.

Recent observations suggest that permafrost thaw may create two completely different soil environments: aerobic in relatively well-drained uplands and anaerobic in poorly drained wetlands. The soil oxygen availability will dictate the rate of permafrost carbon release as carbon dioxide (<span class="fixed-roman">CO</span>₂) and as methane (<span class="fixed-roman">CH</span>₄), and the overall effects of these emitted greenhouse gases on climate. The objective of this study was to quantify <span class="fixed-roman">CO</span>₂ and <span class="fixed-roman">CH</span>₄ release over a 500-day period from permafrost soil under aerobic and anaerobic conditions in the laboratory and to compare the potential effects of these emissions on future climate by estimating their relative climate forcing. We used permafrost soils collected from Alaska and Siberia with varying organic matter characteristics and simultaneously incubated them under aerobic and anaerobic conditions to determine rates of <span class="fixed-roman">CO</span>₂ and <span class="fixed-roman">CH</span>₄ production. Over 500 days of soil incubation at 15 °C, we observed that carbon released under aerobic conditions was 3.9–10.0 times greater than anaerobic conditions. When scaled by greenhouse warming potential to account for differences between <span class="fixed-roman">CO</span>₂ and <span class="fixed-roman">CH</span>₄, relative climate forcing ranged between 1.5 and 7.1. Carbon release in organic soils was nearly 20 times greater than mineral soils on a per gram soil basis, but when compared on a per gram carbon basis, deep permafrost mineral soils showed carbon release rates similar to organic soils for some soil types. This suggests that permafrost carbon may be very labile, but that there are significant differences across soil types depending on the processes that controlled initial permafrost carbon accumulation within a particular landscape. Overall, our study showed that, independent of soil type, permafrost carbon in a relatively aerobic upland ecosystems may have a greater effect on climate when compared with a similar amount of permafrost carbon thawing in an anaerobic environment, despite the release of <span class="fixed-roman">CH</span>₄ that occurs in anaerobic conditions.

The response of northern tundra plant communities to warming temperatures is of critical concern because permafrost ecosystems play a key role in global carbon (C) storage, and climate-induced ecological shifts in the plant community will affect the transfer of carbon-dioxide between biological and atmospheric pools. This study, which focuses on the response of tundra plant growth and phenology to experimental warming, was conducted at the Carbon in Permafrost Experimental Heating Research project, located in the northern foothills of the Alaska Range. We used snow fences coupled with spring snow removal to increase deep-soil temperatures and thaw depth (winter warming), and open-top chambers to increase summer air temperatures (summer warming). Winter warming increased wintertime soil temperature (5–40 cm) by 2.3 °C, resulting in a 10% increase in growing season thaw depth. Summer warming significantly increased growing season air temperature; peak temperature differences occurred near midday when summer warming plots were approximately 1.0 °C warmer than ambient plots. Changes in the soil environment as a result of winter warming treatment resulted in a 20% increase in above-ground biomass and net primary productivity (ANPP), while there was no detected summer warming effect on ecosystem-level ANPP or biomass. Both summer and winter warming extended the growing season through earlier bud break and delayed senescence, despite equivalent snow-free days across treatments. As with ANPP, winter warming increased canopy N mass by 20%, while there was no summer warming effect on canopy N. The warming-mediated increase in N availability, coupled with phenological shifts, may have driven higher rates of ANPP in the winter warming plots, and the lack of ecosystem-level N and ANPP response to summer warming suggest continued N limitation in the summer warming plots. Synthesis: These results highlight the role of soil and permafrost dynamics in regulating plant response to climate change and provide evidence that warming may promote greater C accumulation in tundra plant biomass. While warming temperatures are expected to enhance microbial decomposition of the large pool of organic matter stored in tundra soils and permafrost, these respiratory losses may be offset, at least in part, by warming-mediated increases in plant growth.

Permafrost soils are a significant global store of carbon (C) with the potential to become a large C source to the atmosphere. Climate change is causing permafrost to thaw, which can affect primary production and decomposition, therefore affecting ecosystem C balance. To understand future responses of permafrost soils to climate change, we inventoried current soil C stocks, investigated ∆¹⁴C, C:N, δ¹³C, and δ¹⁵N depth profiles, modeled soil C accumulation rates, and calculated decadal net ecosystem production (NEP) in subarctic tundra soils undergoing minimal, moderate, and extensive permafrost thaw near Eight Mile Lake (EML) in Healy, Alaska. We modeled decadal and millennial soil C inputs, decomposition constants, and C accumulation rates by plotting cumulative C inventories against C ages based on radiocarbon dating of surface and deep soils, respectively. Soil C stocks at EML were substantial, over 50 kg C m⁻² in the top meter, and did not differ much among sites. Carbon to nitrogen ratio, δ¹³C, and δ¹⁵N depth profiles indicated most of the decomposition occurred within the organic soil horizon and practically ceased in deeper, frozen horizons. The average C accumulation rate for EML surface soils was 25.8 g C m⁻² y⁻¹ and the rate for the deep soil accumulation was 2.3 g C m⁻² y⁻¹, indicating these systems have been C sinks throughout the Holocene. Decadal net ecosystem production averaged 14.4 g C m⁻² y⁻¹. However, the shape of decadal C accumulation curves, combined with recent annual NEP measurements, indicates soil C accumulation has halted and the ecosystem may be becoming a C source. Thus, the net impact of climate warming on tundra ecosystem C balance includes not only becoming a C source but also the loss of C uptake capacity these systems have provided over the past ten thousand years.

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched ¹⁵N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem ¹⁵N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem ¹⁵N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (&lt;1 week after ¹⁵N tracer application), total ecosystem ¹⁵N recovery was negatively correlated with fine-root and soil ¹⁵N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3–18 months after ¹⁵N tracer application), total ecosystem ¹⁵N retention was negatively correlated with foliar natural-abundance ¹⁵N but was positively correlated with mineral soil C and N concentration and C : N, showing that plant and soil natural-abundance ¹⁵N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem ¹⁵N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for ¹⁵N tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem ¹⁵N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg N·ha⁻¹·yr⁻¹ above which most ecosystems showed net losses of applied ¹⁵N tracer in response to N fertilizer addition.

Arctic warming has led to permafrost degradation and ground subsidence, created as a result of ground ice melting. Frozen soil organic matter that thaws can increase carbon (C) emissions to the atmosphere, but this can be offset in part by increases in plant growth. The balance of plant and microbial processes, and how this balance changes through time, determines how permafrost ecosystems influence future climate change via the C cycle. This study addressed this question both on short (interannual) and longer (decadal) time periods by measuring C fluxes over a seven-year period at three sites representing a gradient of time since permafrost thaw. All three sites were upland tundra ecosystems located in Interior Alaska but differed in the extent of permafrost thaw and ground subsidence. Results showed an increasing growing season (May – September) trend in gross primary productivity (GPP), net ecosystem exchange (NEE), aboveground net primary productivity (ANPP), and annual NEE at all sites over the seven year study period from 2004 to 2010, but no change in annual and growing season ecosystem respiration (R_eco). These trends appeared to most closely follow increases in the depth to permafrost that occurred over the same time period. During the seven-year period, sites with more permafrost degradation had significantly greater GPP compared to where degradation was least, but also greater growing season R_eco. Adding in winter R_eco decreased, in part, the summer C sink and left only the site with the most permafrost degradation C neutral, with the other sites still C sinks. Annual C balance was strongly dependent on winter R_eco, which, compared to the growing season, was relatively data-poor due to extreme environmental conditions. As a result, we cannot yet conclude whether the increased NEE in the growing season is truly sustained on an annual basis. If it turns out that winter measurements shown here are an underestimate, we may indeed find these systems are already losing net C to the atmosphere.

Plant growth often responds rapidly to experimentally simulated climate change^{<a id="ref-link-1" title="Rustad, L. E. et al. A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming. Oecologia 126, 543-562 (2001)." href="http://www.nature.com/nclimate/journal/v2/n6/full/nclimate1486.html#ref1">1</a>, <a id="ref-link-2" title="Wu, Z., Dijkstra, P., Koch, G. W., Penuelas, J. &amp; Hungate, B. A. Responses of terrestrial ecosystems to temperature and precipitation change: A meta-analysis of experimental manipulation. Glob. Change Biol. 17, 927-942 (2011)." href="http://www.nature.com/nclimate/journal/v2/n6/full/nclimate1486.html#ref2">2</a>}. Feedbacks can modulate the initial responses^{<a id="ref-link-3" title="Harte, J. &amp; Shaw, R. Shifting dominance within a montane vegetation community: Results of a climate-warming experiment. Science 267, 876-880 (1995)." href="http://www.nature.com/nclimate/journal/v2/n6/full/nclimate1486.html#ref3">3</a>}, but these feedbacks are difficult to detect when they operate on long timescales^{<a id="ref-link-4" title="Smith, M. D., Knapp, A. K. &amp; Collins, S. L. A framework for assessing ecosystem dynamics in response to chronic resource alterations induced by global change. Ecology 90, 3279-3289 (2009)." href="http://www.nature.com/nclimate/journal/v2/n6/full/nclimate1486.html#ref4">4</a>}. We transplanted intact plant–soil mesocosms down an elevation gradient to expose them to a warmer climate and used collectors and interceptors to simulate changes in precipitation. Here, we show that warming initially increased aboveground net primary productivity in four grassland ecosystems, but the response diminished progressively over nine years. Warming altered the plant community, causing encroachment by species typical of warmer environments and loss of species from the native environment—trends associated with the declining response of plant productivity. Warming stimulated soil nitrogen turnover, which dampened but did not reverse the temporal decline in the productivity response. Warming also enhanced N losses, which may have weakened the expected biogeochemical feedback where warming stimulates N mineralization and plant growth^{<a id="ref-link-5" title="Rustad, L. E. et al. A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming. Oecologia 126, 543-562 (2001)." href="http://www.nature.com/nclimate/journal/v2/n6/full/nclimate1486.html#ref1">1</a>, <a id="ref-link-6" title="Melillo, J. M. et al. Soil warming, carbon-nitrogen interactions, and forest carbon budgets. Proc. Natl Acad. Sci. USA 108, 9508-9512 (2011)." href="http://www.nature.com/nclimate/journal/v2/n6/full/nclimate1486.html#ref5">5</a>, <a id="ref-link-7" title="Sokolov, A. P. et al. Consequences of considering carbon-nitrogen interactions on the feedbacks between climate and the terrestrial carbon cycle. J. Clim. 21, 3776-3796 (2008)." href="http://www.nature.com/nclimate/journal/v2/n6/full/nclimate1486.html#ref6">6</a>}. Our results, describing the responses of four ecosystems to nearly a decade of simulated climate change, indicate that short-term experiments are insufficient to capture the temporal variability and trend of ecosystem responses to environmental change and their modulation through biogeochemical and ecological feedbacks.