Direct quantification of terrestrial biosphere responses to global change is crucial for projections of future climate change in Earth system models. Here, we synthesized ecosystem carbon-cycling data from 1,119 experiments performed over the past four decades concerning changes in temperature, precipitation, CO 2 and nitrogen across major terrestrial vegetation types of the world. Most experiments manipulated single rather than multiple global change drivers in temperate ecosystems of the USA, Europe and China. The magnitudes of warming and elevated CO 2 treatments were consistent with the ranges of future projections, whereas those of precipitation changes and nitrogen inputs often exceeded the projected ranges. Increases in global change drivers consistently accelerated, but decreased precipitation slowed down carbon-cycle processes. Nonlinear (including synergistic and antagonistic) effects among global change drivers were rare. Belowground carbon allocation responded negatively to increased precipitation and nitrogen addition and positively to decreased precipitation and elevated CO 2. The sensitivities of carbon variables to multiple global change drivers depended on the background climate and ecosystem condition, suggesting that Earth system models should be evaluated using site-specific conditions for best uses of this large dataset. Together, this synthesis underscores an urgent need to explore the interactions among multiple global change drivers in underrepresented regions such as semi-arid ecosystems, forests in the tropics and subtropics, and Arctic tundra when forecasting future terrestrial carbon-climate feedback.

Summary Ecologists have tried to link plant species composition and ecosystem properties since the inception of the ecosystem concept in ecology. Many have observed that biological communities could feed back to, and not simply result from, soil properties. But which group of organisms, plants or microorganisms, drive those feedback systems? Recent research asserts that soil microorganisms preclude plant species feedback to soil nitrogen (N) transformations due to strong microbial control of soil N cycling. It has been well documented that litter properties influence soil N cycling. In this review, we stress that under many circumstances plant species exert a major influence over soil N cycling rates via unique N attainment strategies, thus influencing soil N availability and their own fitness. We offer two testable mechanisms by which plants impart active control on the N cycle and thereby allow for plant–litter–soil–plant feedback. Finally, we describe the characteristics of plants and ecosystems that are most likely to exhibit feedback.

Tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil elevation gain to avoid permanent conversion to open water. The maximal rate of SLR that these ecosystems can tolerate depends partly on mineral sediment deposition, but the accumulation of organic matter is equally important for many wetlands. Plant productivity drives organic matter dynamics and is sensitive to global change factors, such as rising atmospheric CO2 concentration. It remains unknown how global change will influence organic mechanisms that determine future tidal wetland viability. Here, we present experimental evidence that plant response to elevated atmospheric [CO2] stimulates biogenic mechanisms of elevation gain in a brackish marsh. Elevated CO2 (ambient ؉ 340 ppm) accelerated soil elevation gain by 3.9 mm yr ؊1 in this 2-year field study, an effect mediated by stimulation of below-ground plant productivity. Further, a companion greenhouse experiment revealed that the CO2 effect was enhanced under salinity and flooding conditions likely to accompany future SLR. Our results indicate that by stimulating biogenic contributions to marsh elevation, increases in the greenhouse gas, CO2, may paradoxically aid some coastal wetlands in counterbalancing rising seas.coastal wetlands ͉ nitrogen pollution ͉ tidal marsh loss ͉ root productivity ͉ salinity T he world currently loses thousands of hectares of low-lying coastal wetlands to shallow open water each year (1-3), attributable, in part, to a recent acceleration of sea-level rise (SLR) (4-6). Loss of coastal wetlands threatens critical services these ecosystems provide, such as supporting commercially important fisheries, providing a wildlife habitat, improving water quality, and buffering human populations from oceanic forces (3). Recent catastrophes, such as Hurricane Katrina and the Asian Tsunami, have underscored the importance of understanding factors that govern sustainability of coastal wetlands in the face of climate change and accelerating SLR. Marshes must build vertically through accumulation of mineral and organic matter to maintain a constant elevation relative to sea level (7). To explain the dynamics of coastal wetland elevation, researchers have traditionally focused on abiotic factors, such as reductions of mineral sediment loads from hydrologic modifications (8). However, organic matter dynamics have a clear importance in peaty soils, which are composed mostly of live and dead plant tissues (9, 10) and may also play an important role in stabilizing mineral soils (11). Organic mechanisms may be especially sensitive to other global change factors and may determine the fate of tidal wetlands.Rising atmospheric CO 2 is largely responsible for recent global warming and will continue to contribute to accelerating SLR through thermal expansion and ice melt (12). Elevated CO 2 , in addition to accelerating SLR, may have important biologically mediated effects on coastal wetland ecosystems, such as stimulating plant productivity (13). The effects ...

Terrestrial ecosystems gain carbon through photosynthesis and lose it mostly in the form of carbon dioxide (CO 2 ). The extent to which the biosphere can act as a buffer against rising atmospheric CO 2 concentration in global climate change projections remains uncertain at the present stage [1][2][3][4] . Biogeochemical theory predicts that soil nitrogen (N) scarcity may limit natural ecosystem response to elevated CO 2 concentration, diminishing the CO 2 -fertilization effect on terrestrial plant productivity in unmanaged ecosystems [3][4][5][6][7] . Recent models have incorporated such carbon-nitrogen interactions and suggest that anthropogenic N sources could help sustain the future CO 2 -fertilization effect 8,9 . However, conclusive demonstration that added N enhances plant productivity in response to CO 2 -fertilization in natural ecosystems remains elusive. Here we manipulated atmospheric CO 2 concentration and soil N availability in a herbaceous brackish wetland where plant community composition is dominated by a C 3 sedge and C 4 grasses, and is capable of responding rapidly to environmental change 10 . We found that N addition enhanced the CO 2 -stimulation of plant productivity in the first year of a multi-year experiment, indicating N-limitation of the CO 2 response. But we also found that N addition strongly promotes the encroachment of C 4 plant species that respond less strongly to elevated CO 2 concentrations. Overall, we found that the observed shift in the plant community composition ultimately suppresses the CO 2 -stimulation of plant productivity by the third and fourth years. Although extensive research has shown that global change factors such as elevated CO 2 concentrations and N pollution affect plant species differently 11-13, and that they may drive plant community changes 14-17 , we demonstrate that plant community shifts can act as a feedback effect that alters the whole ecosystem response to elevated CO 2 concentrations. Moreover, we suggest that trade-offs between the abilities of plant taxa to respond positively to different perturbations may constrain natural ecosystem response to global change.The progressive nitrogen limitation (PNL) hypothesis 7 suggests that N additions should enhance CO 2 effects on plant productivity. However, only a limited number of studies have provided direct experimental evidence that N addition actually sustains or enhances the CO 2 response of productivity 3,7 . In a pine forest, N addition amplified the CO 2 effect on woody tissue increment 5 . A CO 2 3 N experiment in a grassland reported that a positive CO 2 3 N interaction emerged after three years, indicating that N addition amplified the effect of elevated CO 2 on productivity 6 . In managed ryegrass swards, N addition yielded larger CO 2 responses, an effect that strengthened over time on a relative basis, but diminished in terms of absolute magnitude 18 .As originally articulated, the PNL hypothesis does not explicitly consider the effects that elevated CO 2 and added N can have on the ecosyst...