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Both elevated atmospheric carbon dioxide (CO) and nitrogen (N) deposition may induce changes in C:N ratios in plant tissues and mineral soil. However, the potential mechanisms driving the stoichiometric shifts remain elusive. In this study, we examined the responses of C:N ratios in both plant tissues and mineral soil to elevated CO and N deposition using data extracted from 140 peer-reviewed publications. Our results indicated that C:N ratios in both plant tissues and mineral soil exhibited consistent increases under elevated CO regimes whereas decreases in C:N ratios were observed in response to experimental N addition. Moreover, soil C:N ratio was less sensitive than plant C:N ratio to both global change scenarios. Our results also showed that the responses of stoichiometric ratios were highly variable among different studies. The changes in C:N ratio did not exhibit strong correlations with C dynamics but were negatively associated with corresponding changes in N content. These results suggest that N dynamics drive stoichiometric shifts in both plant tissues and mineral soil under both elevated CO and N deposition scenarios. [ABSTRACT FROM AUTHOR]

• Anthropogenic nitrogen (N) addition may substantially alter the terrestrial N cycle. However, a comprehensive understanding of how the ecosystem N cycle responds to external N input remains elusive. • Here, we evaluated the central tendencies of the responses of 15 variables associated with the ecosystem N cycle to N addition, using data extracted from 206 peer-reviewed papers. • Our results showed that the largest changes in the ecosystem N cycle caused by N addition were increases in soil inorganic N leaching (461%), soil NO3- concentration (429%), nitrification (154%), nitrous oxide emission (134%), and denitrification (84%). N addition also substantially increased soil NH4+ concentration (47%), and the N content in belowground (53%) and aboveground (44%) plant pools, leaves (24%), litter (24%) and dissolved organic N (21%). Total N content in the organic horizon (6.1%) and mineral soil (6.2%) slightly increased in response to N addition. However, N addition induced a decrease in microbial biomass N by 5.8%. • The increases in N effluxes caused by N addition were much greater than those in plant and soil pools except soil NO3-, suggesting a leaky terrestrial N system. [ABSTRACT FROM AUTHOR]

Knowledge of biomass partitioning is essential for estimating spatial patterns and temporal dynamics of root biomass in terrestrial ecosystems. The isometric hypothesis predicts that aboveground biomass scales isometrically with belowground biomass across both individual plants and community types (i.e. the slope of the log-log relationship between above- and belowground biomass is not significantly different from 1.0), and that the isometric pattern is independent of variation in environmental conditions. However, current evidence primarily comes from observations over space. It is still unknown whether biomass partitioning patterns occurring over time are similar to those observed over space. In this study, we explored biomass partitioning patterns in forest ecosystems over space and time by synthesizing biomass measurements made in 112 stands extracted from 16 age sequences around the world. We characterized biomass partitioning patterns in forest ecosystems using both root: shoot ratio and the relationship between above- and belowground biomass. Data across various individual studies were organized to reflect biomass partitioning patterns over space, while data within each individual study were used to illustrate biomass allocation patterns over time. Our results showed that root:shoot ratio did not exhibit any significant trend with stand age over space. Similarly, root:shoot ratio remained relatively constant over time in 10 out of 16 age sequences. Reduced major axis analysis indicated that the slope of the log-log relationship between above- and belowground biomass did not reveal a significant difference from 1.0 over space, supporting the isometric hypothesis. Likewise, the slope was not statistically different from 1.0 in 10 out of 16 age sequences, nor did it show any significant change with climatic factors and stand age. Thus, the isometric hypothesis also most probably holds true during forest stand development. Synthesis. Our results demonstrate that biomass partitioning patterns occurring over time are consistent with those observed over space, suggesting that belowground biomass dynamics in forest ecosystems may be reliably estimated from aboveground biomass using biomass partitioning patterns generated over space. [ABSTRACT FROM AUTHOR]

Aim To investigate large-scale patterns of above-ground and below-ground biomass partitioning in grassland ecosystems and to test the isometric theory at the community level. Location Northern China, in diverse grassland types spanning temperate grasslands in arid and semi-arid regions to alpine grasslands on the Tibetan Plateau. Methods We investigated above-ground and below-ground biomass in China's grasslands by conducting five consecutive sampling campaigns across the northern part of the country during 2001–05. We then documented the root : shoot ratio (R/S) and its relationship with climatic factors for China's grasslands. We further explored relationships between above-ground and below-ground biomass across different grassland types. Results Our results indicated that the overall R/S of China's grasslands was larger than the global average (6.3 vs. 3.7). The R/S for China's grasslands did not show any significant trend with either mean annual temperature or mean annual precipitation. Above-ground biomass was nearly proportional to below-ground biomass with a scaling exponent (the slope of log–log linear relationship between above-ground and below-ground biomass) of 1.02 across various grassland types. The slope did not differ significantly between temperate and alpine grasslands or between steppe and meadow. Main conclusions Our findings support the isometric theory of above-ground and below-ground biomass partitioning, and suggest that above-ground biomass scales isometrically with below-ground biomass at the community level. [ABSTRACT FROM AUTHOR]

The soils of the Qinghai-Tibetan Plateau store a large amount of organic carbon, but the magnitude, spatial patterns and environmental controls of the storage are little investigated. In this study, using data of soil organic carbon (SOC) in 405 profiles collected from 135 sites across the plateau and a satellite-based dataset of enhanced vegetation index (EVI) during 2001–2004, we estimated storage and spatial patterns of SOC in the alpine grasslands. We also explored the relationships between SOC density (soil carbon storage per area) and climatic variables and soil texture. Our results indicated that SOC storage in the top 1 m in the alpine grasslands was estimated at 7.4 Pg C (1 Pg=1015 g), with an average density of 6.5 kg m−2. The density of SOC decreased from the southeastern to the northwestern areas, corresponding to the precipitation gradient. The SOC density increased significantly with soil moisture, clay and silt content, but weakly with mean annual temperature. These variables could together explain about 72% of total variation in SOC density, of which 54% was attributed to soil moisture, suggesting a key role of soil moisture in shaping spatial patterns of SOC density in the alpine grasslands. [ABSTRACT FROM AUTHOR]

Climate warming leads to widespread permafrost thaw with a fraction of the thawed permafrost carbon (C) being released as carbon dioxide (CO2), thus triggering a positive permafrost C-climate feedback. However, large uncertainty exists in the size of this model-projected feedback, partly owing to the limited understanding of permafrost CO2 release through the priming effect (i.e., the stimulation of soil organic matter decomposition by external C inputs) upon thaw. By combining permafrost sampling from 24 sites on the Tibetan Plateau and laboratory incubation, we detected an overall positive priming effect (an increase in soil C decomposition by up to 31%) upon permafrost thaw, which increased with permafrost C density (C storage per area). We then assessed the magnitude of thawed permafrost C under future climate scenarios by coupling increases in active layer thickness over half a century with spatial and vertical distributions of soil C density. The thawed C stocks in the top 3 m of soils from the present (2000-2015) to the future period (2061-2080) were estimated at 1.0 (95% confidence interval (CI): 0.8-1.2) and 1.3 (95% CI: 1.0-1.7) Pg (1 Pg = 1015 g) C under moderate and high Representative Concentration Pathway (RCP) scenarios 4.5 and 8.5, respectively. We further predicted permafrost priming effect potential (priming intensity under optimal conditions) based on the thawed C and the empirical relationship between the priming effect and permafrost C density. By the period 2061-2080, the regional priming potentials could be 8.8 (95% CI: 7.4-10.2) and 10.0 (95% CI: 8.3-11.6) Tg (1 Tg = 1012 g) C year-1 under the RCP 4.5 and RCP 8.5 scenarios, respectively. This large CO2 emission potential induced by the priming effect highlights the complex permafrost C dynamics upon thaw, potentially reinforcing permafrost C-climate feedback. [ABSTRACT FROM AUTHOR]

Aim Root production and turnover play a key role in regulating carbon (C) flow in terrestrial ecosystems. However, a general pattern reflecting the responses of roots to increasing nitrogen (N) input has yet to be described. Location Global terrestrial ecosystems. Methods We conducted a meta-analysis to assess the central tendencies of root production, turnover rate and standing crop with respect to the experimental addition of N. We evaluated the effect of the form of N, root diameter and climatic (mean annual temperature, MAT; mean annual precipitation, MAP), biotic (ecosystem type, plant type and forest stand age) and forcing factors (experimental duration, N addition rate and cumulative amount of N) on the variations in root response. Results Globally, the addition of N significantly decreased root production and turnover rate but had only a minor impact on root standing crop. In different ecosystems, the three root variables exhibited heterogeneous responses to N enrichment. Additionally, root production and turnover rate responded distinctly to diverse forms of N. The responses of root production and turnover rate to the addition of N were generally positively correlated with MAT and MAP but negatively related to forest stand age and experimental duration. The response pattern of root standing crop was negatively affected by MAT, MAP and forest stand age. However, none of the three root parameters had any obvious correlations with N addition rate or cumulative amount of N. Main conclusions Our results demonstrate that, on aggregate, the addition of N decreased root production and turnover rate at the global scale. These root response patterns and the regulatory factors can be incorporated into earth system models to improve the prediction of belowground C dynamics. [ABSTRACT FROM AUTHOR]

Significant attention has been given to the way in which the soil nitrogen (N) cycle responds to permafrost thaw in recent years, yet little is known about anaerobic N transformations in thermokarst lakes, which account for more than one-third of thermokarst landforms across permafrost regions. Based on the N isotope dilution and tracing technique, combined with qPCR and high-throughput sequencing, we presented large-scale measurements of anaerobic N transformations of sediments across 30 thermokarst lakes over the Tibetan alpine permafrost region. Our results showed that gross N mineralization, ammonium immobilization, and dissimilatory nitrate reduction rates in thermokarst lakes were higher in the eastern part of our study area than in the west. Denitrification dominated in the dissimilatory nitrate reduction processes, being two and one orders of magnitude higher than anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA), respectively. The abundances of the dissimilatory nitrate reduction genes (nirK, nirS, hzsB, and nrfA) exhibited patterns consistent with sediment N transformation rates, while a diversity did not. The inter-lake variability in gross N mineralization and ammonium immobilization was dominantly driven by microbial biomass, while the variability in anammox and DNRA was driven by substrate supply and organic carbon content, respectively. Denitrification was jointly affected by nirS abundance and organic carbon content. Overall, the patterns and drivers of anaerobic N transformation rates detected in this study provide a new perspective on potential N release, retention, and removal upon the formation and development of thermokarst lakes. [ABSTRACT FROM AUTHOR]

We conducted a field study in Danjiangkou Reservoir region of central China to evaluate soil C and N dynamics following afforestation by comparing soil organic C and N (SOC and SON), soil net N mineralization and nitrification, and inorganic N concentrations in the plant rhizosphere and open areas in the forest, shrubland and adjacent cropland. Afforestation increased SOC but did not significantly affect SON in the plant rhizosphere. Due to large quantity of low-quality litter (with high C:N ratios) inputs, afforestation enhanced soil C recalcitrant indexes (RIC) but decreased soil N recalcitrance indexes (RIN) in the plant rhizosphere. Both SON and RIN significantly decreased following afforestation in the open areas. Afforestation decreased inorganic N concentrations and net N mineralization. Soil net N mineralization were negatively correlated with soil C:N ratios across land use types. These results suggest that afforestation could increase SOC stocks resulting from large low-quality litter input, but over the long-term, this increase was likely limited due to decreased soil N availability. [ABSTRACT FROM AUTHOR]

Aim Our objective was to document the general relationship between plant species richness (SR) and above-ground net primary productivity (ANPP) at different spatial scales and the environmental influence on this relationship. Location Temperate and alpine grasslands of China. Methods We investigated SR and ANPP at 321 field sites (1355 plots) across the widely distributed temperate and alpine grasslands of China. Ordinary least squares (OLS) regressions were used to test SR–ANPP relationships among site means. Plot-level data of SR and ANPP were analysed with general linear models (GLMs) and the correlation between SR and ANPP was decomposed into covariance components to test the influence of climatic variables, region, vegetation type and remaining variation among sites on SR, ANPP and their relationship. Results We found positive linear relationships between SR and ANPP among sites in both the alpine and temperate grassland regions and in different grassland vegetation types of these biomes. Environmental gradients such as growing-season precipitation affected both SR and ANPP in parallel. However, after removing the among-site environmental variation, residual SR and ANPP were no longer correlated at the pooled within-site level. Main conclusions The positive SR–ANPP relationship across large-scale environmental gradients among sites was most likely the result of climatic variables influencing SR and ANPP in parallel. Our results suggest that in China's natural grasslands there is no direct relationship between SR and ANPP, presumably because the pool of available species for local community assembly is large, in contrast to experiments where species pools are artificially reduced. [ABSTRACT FROM AUTHOR]

The article presents a ecology research report on temperature sensitivity of permafrost carbon release mediated by mineral and microbial properties. Topics include temperature sensitivity (Q10) of permafrost carbon (C) release upon thaw is a vital parameter for projecting permafrost C dynamics under climate warming; and mineral protection and microbial properties are two types of crucial predictors of permafrost C dynamics upon thaw.

China's terrestrial ecosystems have functioned as important carbon sinks. However, previous estimates of carbon budgets have included large uncertainties owing to the limitations of sample size, multiple data sources, and inconsistent methodologies. In this study, we conducted an intensive field campaign involving 14,371 field plots to investigate all sectors of carbon stocks in China's forests, shrublands, grasslands, and croplands to better estimate the regional and national carbon pools and to explore the biogeographical patterns and potential drivers of these pools. The total carbon pool in these four ecosystems was 79.24 ± 2.42 Pg C, of which 82.9% was stored in soil (to a depth of 1 m), 16.5% in biomass, and 0.60% in litter. Forests, shrublands, grasslands, and croplands contained 30.83 ± 1.57 Pg C, 6.69 ± 0.32 Pg C, 25.40 ± 1.49 Pg C, and 16.32 ± 0.41 Pg C, respectively. When all terrestrial ecosystems are taken into account, the country's total carbon pool is 89.27 ± 1.05 Pg C. The carbon density of the forests, shrublands, and grasslands exhibited a strong correlation with climate: it decreased with increasing temperature but increased with increasing precipitation. Our analysis also suggests a significant sequestration potential of 1.9-3.4 Pg C in forest biomass in the next 10-20 years assuming no removals, mainly because of forest growth. Our results update the estimates of carbon pools in China's terrestrial ecosystems based on direct field measurements, and these estimates are essential to the validation and parameterization of carbon models in China and globally. [ABSTRACT FROM AUTHOR]

Permafrost thaw alters the physical and environmental conditions of soil and may thus cause a positive feedback to climate warming through increased methane emissions. However, the current knowledge of methane emissions following thermokarst development is primarily based on expanding lakes and wetlands, with upland thermokarst being studied less often. In this study, we monitored the methane emissions during the peak growing seasons of two consecutive years along a thaw sequence within a thermo-erosion gully in a Tibetan swamp meadow. Both years had consistent results, with the early and midthaw stages (3 to 12 years since thaw) exhibiting low methane emissions that were similar to those in the undisturbed meadow, while the emissions from the late thaw stage (20 years since thaw) were 3.5 times higher. Our results also showed that the soil water-filled pore space, rather than the soil moisture per se, in combination with the sand content, were the main factors that caused increased methane emissions. These findings differ from the traditional view that upland thermokarst could reduce methane emissions owing to the improvement of drainage conditions, suggesting that upland thermokarst development does not always result in a decrease in methane emissions. [ABSTRACT FROM AUTHOR]

Forests in the middle and high latitudes of the northern hemisphere function as a significant sink for atmospheric carbon dioxide (CO2). This carbon (C) sink has been attributed to two processes: age-related growth after land use change and growth enhancement due to environmental changes, such as elevated CO2, nitrogen deposition, and climate change. However, attribution between these two processes is largely controversial. Here, using a unique time series of an age-class dataset from six national forest inventories in Japan and a new approach developed in this study (i.e., examining changes in biomass density at each age class over the inventory periods), we quantify the growth enhancement due to environmental changes and its contribution to biomass C sink in Japan's forests. We show that the growth enhancement for four major plantations was 4.0∼7.7 Mg C·ha-1 from 1980 to 2005, being 8.4-21.6% of biomass C sequestration per hectare and 4.1-35.5% of the country's total net biomass increase of each forest type. The growth enhancement differs among forest types, age classes, and regions. Our results provide, to our knowledge, the first ground-based evidence that global environmental changes can increase C sequestration in forests on a broad geographic scale and imply that both the traits and age of trees regulate the responses of forest growth to environmental changes. These findings should be incorporated into the prediction of forest C cycling under a changing climate. [ABSTRACT FROM AUTHOR]