Long-term warming increased carbon sequestration capacity in a humid subtropical forest.

Tropical and subtropical forests play a crucial role in global carbon (C) pools, and their responses to warming can significantly impact C-climate feedback and predictions of future global warming. Despite earth system models projecting reductions in land C storage with warming, the magnitude of this response varies greatly between models, particularly in tropical and subtropical regions. Here, we conducted a field ecosystem-level warming experiment in a subtropical forest in southern China, by translocating mesocosms (ecosystem composed of soils and plants) across 600 m elevation gradients with temperature gradients of 2.1°C (moderate warming), to explore the response of ecosystem C dynamics of the subtropical forest to continuous 6-year warming. Compared with the control, the ecosystem C stock decreased by 3.8% under the first year of 2.1°C warming; but increased by 13.4% by the sixth year of 2.1°C warming. The increased ecosystem C stock by the sixth year of warming was mainly attributed to a combination of sustained increased plant C stock due to the maintenance of a high plant growth rate and unchanged soil C stock. The unchanged soil C stock was driven by compensating and offsetting thermal adaptation of soil microorganisms (unresponsive soil respiration and enzyme activity, and more stable microbial community), increased plant C input, and inhibitory C loss (decreased C leaching and inhibited temperature sensitivity of soil respiration) from soil drying. These results suggest that the humid subtropical forest C pool would not necessarily diminish consistently under future long-term warming. We highlight that differential and asynchronous responses of plant and soil C processes over relatively long-term periods should be considered when predicting the effects of climate warming on ecosystem C dynamics of subtropical forests.

Significant effects of precipitation frequency on soil respiration and its components-A global synthesis.

Global warming intensifies the hydrological cycle, which results in changes in precipitation regime (frequency and amount), and will likely have significant impacts on soil respiration (Rs ). Although the responses of Rs to changes in precipitation amount have been extensively studied, there is little consensus on how Rs will be affected by changes in precipitation frequency (PF) across the globe. Here, we synthesized the field observations from 296 published papers to quantify the effects of PF on Rs and its components using meta-analysis. Our results indicated that the effects of PF on Rs decreased with an increase in background mean annual precipitation. When the data were grouped by climate conditions, increased PF showed positive effects on Rs under the arid condition but not under the semi-humid or humid conditions, whereas decreased PF suppressed Rs across all the climate conditions. The positive effects of increased PF mainly resulted from the positive response of heterotrophic respiration under the arid condition while the negative effects of decreased PF were mainly attributed to the reductions in root biomass and respiration. Overall, our global synthesis provided for the first time a comprehensive analysis of the divergent effects of PF on Rs and its components across climate regions. This study also provided a framework for understanding and modeling responses of ecosystem carbon cycling to global precipitation change.

Distinct patterns of soil bacterial and fungal community assemblages in subtropical forest ecosystems under warming.

Climate change globally affects soil microbial community assembly across ecosystems. However, little is known about the impact of warming on the structure of soil microbial communities or underlying mechanisms that shape microbial community composition in subtropical forest ecosystems. To address this gap, we utilized natural variation in temperature via an altitudinal gradient to simulate ecosystem warming. After 6 years, microbial co-occurrence network complexity increased with warming, and changes in their taxonomic composition were asynchronous, likely due to contrasting community assembly processes. We found that while stochastic processes were drivers of bacterial community composition, warming led to a shift from stochastic to deterministic drivers in dry season. Structural equation modelling highlighted that soil temperature and water content positively influenced soil microbial communities during dry season and negatively during wet season. These results facilitate our understanding of the response of soil microbial communities to climate warming and may improve predictions of ecosystem function of soil microbes in subtropical forests.

Acidity of Soil and Water Decreases in Acid-Sensitive Forests of Tropical China.

Acid deposition in China has been declining since the 2000s. While this may help mitigate acidification in forest soils and water, little is known about the recovery of soils and water from previous severe acidification in tropical China. Here, we assessed the chemistry of mineral soils, water, and acid gases (SO2 and NOx) from three successional forest types in tropical China from 2000 to 2022. Our results showed that soil pH increased synchronously from 3.9 (2000-2015) to 4.2 (2016-2022) across all three forest types, with exchangeable acid initially decreasing and thereafter stabilizing. Surface and ground water pH also gradually increased throughout the monitoring period. Soil pH recovery was stronger in the primary than in the planted forest. However, soil pH recovery lagged behind the increase in rainfall pH by approximately a decade. The recovery of soil pH was likely related to the positive effects of the dissolution of Al/Fe-hydroxysulfate mineral and subsequent sulfur desorption on soil acid-neutralizing capacity, increased soil organic matter, and climate warming, but was likely moderated by increased exchangeable aluminum and potentially proton-producing hydroxysulfate mineral dissolution that caused the lagged soil pH recovery. Surface and ground water pH recovery was attributed to increased water acid-neutralizing capacity. Our study reports the potential for the recovery of acidified soil and water following decreased acid deposition and provides new insights into the functional recovery of acid-sensitive forests.

Warming drives sustained plant phosphorus demand in a humid tropical forest.

Phosphorus (P) is often one of the most limiting nutrients in highly weathered soils of humid tropical forests and may regulate the responses of carbon (C) feedback to climate warming. However, the response of P to warming at the ecosystem level in tropical forests is not well understood because previous studies have not comprehensively assessed changes in multiple P processes associated with warming. Here, we detected changes in the ecosystem P cycle in response to a 7-year continuous warming experiment by translocating model plant-soil ecosystems across a 600-m elevation gradient, equivalent to a temperature change of 2.1°C. We found that warming increased plant P content (55.4%) and decreased foliar N:P. Increased plant P content was supplied by multiple processes, including enhanced plant P resorption (9.7%), soil P mineralization (15.5% decrease in moderately available organic P), and dissolution (6.8% decrease in iron-bound inorganic P), without changing litter P mineralization and leachate P. These findings suggest that warming sustained plant P demand by increasing the biological and geochemical controls of the plant-soil P-cycle, which has important implications for C fixation in P-deficient and highly productive tropical forests in future warmer climates.

Natural forests promote phosphorus retention in soil.

Soil phosphorus (P) availability often limits plant productivity. Classical theories suggest that total P content declines at the temporal scale of pedogenesis, and ecosystems develop toward the efficient use of scarce P during succession. However, the trajectory of ecosystem P within shorter time scales of succession remains unclear. We analyzed changes to P pools at the early (I), middle (II), and late (III) stages of growth of plantation forests (PFs) and the successional stages of natural forests (NFs) at 1969 sites in China. We found significantly lower P contents at later growth stages compared to earlier ones in the PF (p < .05), but higher contents at late successional stages than in earlier stages in the NF (p < .05). Our results indicate that increasing P demand of natural vegetation during succession, may raise, retain, and accumulate P from deeper soil layers. In contrast, ecosystem P in PF was depleted by the more rapidly increasing demand outpacing the development of a P-efficient system. We advocate for more studies to illuminate the mechanisms for determining the divergent changes, which would improve forest management and avoid the vast degradation of PF ecosystems suffering from the ongoing depletion of P.

Design of a compact off-axis freeform three-mirror system in a circular configuration.

A compact off-axis three-mirror system, especially with a wide field of view and small f-number, continues to present challenges in the optical design field. To design a compact off-axis three-mirror system based on a freeform surface, an optimization method with a circular configuration and four parameters is proposed. In the proposed method, the f-number and angles of mirrors are combined to optimize and achieve high-quality imaging, which means the modulation transfer function is close to the diffraction limit. To evaluate the performance of the proposed method, four design examples with different fields of view (4∘×4∘, 6∘×6∘) and f-numbers (2, 1.75) were created. The results showed that the compact off-axis three-mirror system based on a freeform surface can be designed with a wide field of view, small f-number, and high-quality imaging using the proposed method. Unlike other design methods, the surface parameters of mirrors are obtained with only one calculation and are close to the final optimization results, which saves both time and resources for optimization. The findings indicate that the method is accurate and effective for designing compact off-axis three-mirror systems with freeform surfaces.

Canopy and understory nitrogen addition have different effects on fine root dynamics in a temperate forest: implications for soil carbon storage.

Elucidating the effects of atmospheric nitrogen (N) deposition on fine root dynamics and the potential underlying mechanisms is required to understand the changes in belowground and aboveground carbon storage. However, research on these effects in forests has mostly involved direct understory addition of N and has ignored canopy interception and processing of N. Here, we conducted a field experiment comparing the effects of canopy addition of N (CAN) with those of understory addition of N (UAN) at three N-addition rates (0, 25 and 50 kg N ha-1 yr-1 ) on fine root dynamics in a temperate deciduous forest. Fine root production and biomass were significantly higher with CAN than with UAN. At the same N-addition rate, increases in fine root production with CAN were at least two-fold greater than with UAN. At the high N-addition rate and relative to the control, fine root biomass was significantly increased by CAN (by 23.5%) but was significantly decreased by UAN (by 12.2%). Our results indicate that traditional UAN may underestimate the responses of fine root dynamics to atmospheric N deposition in forest ecosystems. Canopy N processes should be considered for more realistic assessments of the effects of atmospheric N deposition in forests.

Warming leads to more closed nitrogen cycling in nitrogen-rich tropical forests.

Warming may have profound effects on nitrogen (N) cycling by changing plant N demand and underground N supply. However, large uncertainty exists regarding how warming affects the integrated N dynamic in tropical forests. We translocated model plant-soil ecosystems from a high-altitude site (600 m) to low-altitude sites at 300 and 30 m to simulate warming by 1.0°C and 2.1°C, respectively, in tropical China. The effects of experimental warming on N components in plant, soil, leaching, and gas were studied over 6 years. Our results showed that foliar δ15 N values and inorganic N (NH4 -N and NO3 -N) leaching were decreased under warming, with greater decreases under 2.1°C of warming than under 1.0°C of warming. The 2.1°C of warming enhanced plant growth, plant N uptake, N resorption, and fine root biomass, suggesting higher plant N demand. Soil total N concentrations, NO3 -N concentrations, microbial biomass N and arbuscular mycorrhizal fungal abundance were decreased under 2.1°C of warming, which probably restricted bioavailable N supply and arbuscular mycorrhizal contribution of N supply to plants. These changes in plants, soils and leaching indicated more closed N cycling under warming, the magnitude of which varied over time. The closed N cycling became pronounced during the first 3 years of warming where the sustained reductions in soil inorganic N could not meet plant N demand. Subsequently, the closed N cycling gradually mitigated, as observed by attenuated positive responses of plant growth and less negative responses of microbial biomass N to warming during the last 3 years. Overall, the more closed N cycling under warming could facilitate ecosystem N retention and affect production in these tropical forests, but these effects would be eventually mitigated with long-term warming probably due to the restricted plant growth and microbial acclimation.

Carbon pools in China's terrestrial ecosystems: New estimates based on an intensive field survey.

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.

The response of soil respiration to precipitation change is asymmetric and differs between grasslands and forests.

Intensification of the Earth's hydrological cycle amplifies the interannual variability of precipitation, which will significantly impact the terrestrial carbon (C) cycle. However, it is still unknown whether previously observed relationship between soil respiration (Rs ) and precipitation remains applicable under extreme precipitation change. By analyzing the observations from a much larger dataset of field experiments (248 published papers including 151 grassland studies and 97 forest studies) across a wider range of precipitation manipulation than previous studies, we found that the relationship of Rs response with precipitation change was highly nonlinear or asymmetric, and differed significantly between grasslands and forests, between moderate and extreme precipitation changes. Response of Rs to precipitation change was negatively asymmetric (concave-down) in grasslands, and double-asymmetric in forests with a positive asymmetry (concave-up) under moderate precipitation changes and a negative asymmetry (concave-down) under extreme precipitation changes. In grasslands, the negative asymmetry in Rs response was attributed to the higher sensitivities of soil moisture, microbial and root activities to decreased precipitation (DPPT) than to increased precipitation (IPPT). In forests, the positive asymmetry was predominantly driven by the significant increase in microbial respiration under moderate IPPT, while the negative asymmetry was caused by the reductions in root biomass and respiration under extreme DPPT. The different asymmetric responses of Rs between grasslands and forests will greatly improve our ability to forecast the C cycle consequences of increased precipitation variability. Specifically, the negative asymmetry of Rs response under extreme precipitation change suggests that the soil C efflux will decrease across grasslands and forests under future precipitation regime with more wet and dry extremes.

Interactions between CO2 enhancement and N addition on net primary productivity and water-use efficiency in a mesocosm with multiple subtropical tree species.

Carbon dioxide (CO2 ) enhancement (eCO2 ) and N addition (aN) have been shown to increase net primary production (NPP) and to affect water-use efficiency (WUE) for many temperate ecosystems, but few studies have been made on subtropical tree species. This study compared the responses of NPP and WUE from a mesocosm composing five subtropical tree species to eCO2 (700 ppm), aN (10 g N m(-2) yr(-1) ) and eCO2 × aN using open-top chambers. Our results showed that mean annual ecosystem NPP did not changed significantly under eCO2 , increased by 56% under aN and 64% under eCO2 × aN. Ecosystem WUE increased by 14%, 55%, and 61% under eCO2 , aN and eCO2 × aN, respectively. We found that the observed responses of ecosystem WUE were largely driven by the responses of ecosystem NPP. Statistical analysis showed that there was no significant interactions between eCO2 and aN on ecosystem NPP (P = 0.731) or WUE (P = 0.442). Our results showed that increasing N deposition was likely to have much stronger effects on ecosystem NPP and WUE than increasing CO2 concentration for the subtropical forests. However, different tree species responded quite differently. aN significantly increased annual NPP of the fast-growing species (Schima superba). Nitrogen-fixing species (Ormosia pinnata) grew significantly faster only under eCO2 × aN. eCO2 had no effects on annual NPP of those two species but significantly increased annual NPP of other two species (Castanopsis hystrix and Acmena acuminatissima). Differential responses of the NPP among different tree species to eCO2 and aN will likely have significant implications on the species composition of subtropical forests under future global change.

Responses of CO(2), N(2)O and CH(4) fluxes between atmosphere and forest soil to changes in multiple environmental conditions.

To investigate the effects of multiple environmental conditions on greenhouse gas (CO2 , N2 O, CH4 ) fluxes, we transferred three soil monoliths from Masson pine forest (PF) or coniferous and broadleaved mixed forest (MF) at Jigongshan to corresponding forest type at Dinghushan. Greenhouse gas fluxes at the in situ (Jigongshan), transported and ambient (Dinghushan) soil monoliths were measured using static chambers. When the transported soil monoliths experienced the external environmental factors (temperature, precipitation and nitrogen deposition) at Dinghushan, its annual soil CO2 emissions were 54% in PF and 60% in MF higher than those from the respective in situ treatment. Annual soil N2 O emissions were 45% in PF and 44% in MF higher than those from the respective in situ treatment. There were no significant differences in annual soil CO2 or N2 O emissions between the transported and ambient treatments. However, annual CH4 uptake by the transported soil monoliths in PF or MF was not significantly different from that at the respective in situ treatment, and was significantly lower than that at the respective ambient treatment. Therefore, external environmental factors were the major drivers of soil CO2 and N2 O emissions, while soil was the dominant controller of soil CH4 uptake. We further tested the results by developing simple empirical models using the observed fluxes of CO2 and N2 O from the in situ treatment and found that the empirical models can explain about 90% for CO2 and 40% for N2 O of the observed variations at the transported treatment. Results from this study suggest that the different responses of soil CO2 , N2 O, CH4 fluxes to changes in multiple environmental conditions need to be considered in global change study.

Substantial reorganization of China's tropical and subtropical forests: based on the permanent plots.

There is evidence that climate change induced tree mortalities in boreal and temperate forests and increased forest turnover rates (both mortality and recruitment rates) in Amazon forests. However, no study has examined China's tropical and subtropical evergreen broadleaved forests (TEBF) that cover >26% of China's terrestrial land. The sustainability of this biome is vital to the maintenance of local ecosystem services (e.g., carbon sequestration, biodiversity conservation, climatic regulation), many of which may influence patterns of atmospheric circulation and composition at regional to global scales. Here, we analyze time-series data collected from thirteen permanent plots within China's unmanaged TEBF to study whether and how this biome has changed over recent decades. We find that the numbers of individuals and species for shrub and small tree have increased since 1978, whereas the numbers of individuals and species for tree have decreased over this same time period. The shift in species composition is accompanied by a decrease in the mean diameter at breast height (DBH) for all individuals combined. China's TEBF may thereby be transitioning from cohorts of fewer and larger individuals to ones of more and smaller individuals, which shows a unique change pattern differing from the documented. Regional-scale drying is likely responsible for the biome's reorganization. This biome-wide reconstitution would deeply impact the regimes of carbon sequestration and biodiversity conservation and have implications for the sustainability of economic development in the area.

Photosynthetic and biochemical responses of four subtropical tree seedlings to reduced dry season and increased wet season precipitation and variable N deposition.

Interspecific variations in phenotypic plasticity of trees that are affected by climate change may alter the ecosystem function of forests. Seedlings of four common tree species (Castanopsis fissa, Michelia macclurei, Dalbergia odorifera and Ormosia pinnata) in subtropical plantations of southern China were grown in the field under rainout shelters and subjected to changing precipitation (48 L of water every 4 days in the dry season, 83 L of water every 1 day in the wet season; 4 g m-2 year-1 of nitrogen (N)), low N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 8 g m-2 year-1 N), high N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 10 g m-2 year-1 N) and their interactive effects. We found that the changes in seasonal precipitation reduced the light-saturated photosynthetic rate (Asat) for C. fissa due to declining area-based foliar N concentrations (Na). However, we also found that the interactive effects of changing precipitation and N deposition enhanced Asat for C. fissa by increasing foliar Na concentrations, suggesting that N deposition could alleviate N limitations associated with changing precipitation. Altered precipitation and high N deposition reduced Asat for D. odorifera by decreasing the maximum electron transport rate for RuBP regeneration (Jmax) and maximum rate of carboxylation of Rubisco (Vcmax). Ormosia pinnata under high N deposition exhibited increasing Asat due to higher stomatal conductance and Vcmax. The growth of D. odorifera might be inhibited by changes in seasonal precipitation and N deposition, while O. pinnata may benefit from increasing N deposition in future climates. Our study provides an important insight into the selection of tree species with high capacity to tolerate changing precipitation and N deposition in subtropical plantations.

Monitoring nitrogen deposition in typical forest ecosystems along a large transect in China.

The nitrogen (N) deposition fluxes were investigated in eight typical forest ecosystems along the North-South Transect of Eastern China (NSTEC; based on the ChinaFLUX network) by ion-exchange resin (IER) columns from May 2008 to April 2009. Our results demonstrated that the method of IER columns was both labor cost saving and reliable for measuring dissolved inorganic nitrogen (DIN) deposition at the remote forest stations. The deposition of DIN in the throughfall ranged from 1.3 to 29.5 kg N ha(-1) a(-1), increasing from north to south along NSTEC. The relatively high average ratio of ammonium to nitrate in deposition (1.83) indicated that the N deposition along the NSTEC in China mostly originated in farming and animal husbandry rather than in industry and vehicle activities. For seasonal variability, the DIN deposition showed a single peak in the growing season in the northern part of NSTEC, while, in the southern part, it exhibited double-peaks in the early spring and the mid-summer, respectively. On the annual scale, the DIN deposition variations of the eight sites could be mainly explained by precipitation and the distances from forest stations to provincial capital cities.

Mapping Chinese annual gross primary productivity with eddy covariance measurements and machine learning.

Annual gross primary productivity (AGPP) is the basis for grain production and terrestrial carbon sequestration. Mapping regional AGPP from site measurements provides methodological support for analysing AGPP spatiotemporal variations thereby ensures regional food security and mitigates climate change. Based on 641 site-year eddy covariance measuring AGPP from China, we built an AGPP mapping scheme based on its formation and selected the optimal mapping way, which was conducted through analysing the predicting performances of divergent mapping tools, variable combinations, and mapping approaches in predicting observed AGPP variations. The reasonability of the selected optimal scheme was confirmed by assessing the consistency between its generating AGPP and previous products in spatiotemporal variations and total amount. Random forest regression tree explained 85 % of observed AGPP variations, outperforming other machine learning algorithms and classical statistical methods. Variable combinations containing climate, soil, and biological factors showed superior performance to other variable combinations. Mapping AGPP through predicting AGPP per leaf area (PAGPP) explained 86 % of AGPP variations, which was superior to other approaches. The optimal scheme was thus using a random forest regression tree, combining climate, soil, and biological variables, and predicting PAGPP. The optimal scheme generating AGPP of Chinese terrestrial ecosystems decreased from southeast to northwest, which was highly consistent with previous products. The interannual trend and interannual variation of our generating AGPP showed a decreasing trend from east to west and from southeast to northwest, respectively, which was consistent with data-oriented products. The mean total amount of generated AGPP was 7.03 ± 0.45 PgC yr-1 falling into the range of previous works. Considering the consistency between the generated AGPP and previous products, our optimal mapping way was suitable for mapping AGPP from site measurements. Our results provided a methodological support for mapping regional AGPP and other fluxes.

Tree species richness as an important biotic factor regulates the soil phosphorus density in China's mature natural forests.

Tree species richness has been recognized as an underlying driving factor for regulating soil phosphorus (P) status in many site-specific studies. However, it remains poorly understood whether this is true at broad scales where soil P strongly rely on climate, soil type and vegetation type. Here, based on the data of 946 mature natural forest sites from a nationwide field survey in China, we analyzed the impact of tree species richness on soil P density of China's mature natural forests (deciduous coniferous forest, DCF; evergreen coniferous forest, ECF; deciduous broad-leaved forest, DBF; evergreen broad-leaved forest, EBF; and mixed coniferous and broad-leaved forest, MF). Our results showed that tree species richness had a negative effect on soil P density in China's mature natural forests. The Random Forest regression model showed that the relative importance of tree species richness to soil P density was second only to the climate factors (mean annual temperature, MAT; mean annual precipitation, MAP). In addition, the structural equation model (SEM) results showed that the goodness fit of SEM increased when the tree species richness was included into the model. These results suggested that tree species richness was an important factor in regulating the China's mature natural forests soil P density. Furthermore, the SEM results showed that the decreased soil P density was related to the increase in ANPP and the decrease in litter P concentration induced by tree species richness. This result indicates that tree species richness could facilitate plant P absorption and inhibit plant P return into the soil, and thus reducing the soil P density in China's mature natural forests. In conclusion, we found tree species richness was an important biotic factor in regulating soil P density at broad scales, which should be fully considered in Earth models that represent P cycle.

Photosynthetic capacity dominates the interannual variation of annual gross primary productivity in the Northern Hemisphere.

Annual gross primary productivity (AGPP) of terrestrial ecosystems is the largest carbon flux component in ecosystems; however, it's unclear whether photosynthetic capacity or phenology dominates interannual variation of AGPP, and a better understanding of this could contribute to estimation of carbon sinks and their interactions with climate change. In this study, observed GPP data of 494 site-years from 39 eddy covariance sites in Northern Hemisphere were used to investigate mechanisms of interannual variation of AGPP. This study first decomposed AGPP into three seasonal dynamic attribute parameters (growing season length (CUP), maximum daily GPP (GPPmax), and the ratio of mean daily GPP to GPPmax (αGPP)), and then decomposed AGPP into mean leaf area index (LAIm) and annual photosynthetic capacity per leaf area (AGPPlm). Furthermore, GPPmax was decomposed into leaf area index of DOYmax (the day when GPPmax appeared) (LAImax) and photosynthesis per leaf area of DOYmax (GPPlmax). Relative contributions of parameters to AGPP and GPPmax were then calculated. Finally, environmental variables of DOYmax were extracted to analyze factors influencing interannual variation of GPPlmax. Trends of AGPP in 39 ecosystems varied from -65.23 to 53.05 g C m-2 yr-2, with the mean value of 6.32 g C m-2 yr-2. Photosynthetic capacity (GPPmax and AGPPlm), not CUP or LAI, was the main factor dominating interannual variation of AGPP. GPPlmax determined the interannual variation of GPPmax, and temperature, water, and radiation conditions of DOYmax affected the interannual variation of GPPlmax. This study used the cascade relationship of "environmental variables-GPPlmax-GPPmax-AGPP" to explain the mechanism of interannual variation of AGPP, which can provide new ideas for the AGPP estimation based on seasonal dynamic of GPP.

[Effects of simulated acid rain on soil N2O emission from typical forest in subtropical sou-thern China.] / æ¨¡æ‹Ÿé ¸é›¨å¯¹å—äºšçƒ­å¸¦å ¸åž‹æ£®æž—åœŸå£¤N2O排放的影响.

Based on a long-term simulated acid rain experiment, soil N2O emission fluxes were measured using static chambers and the gas chromatography method in a coniferous and broadleaved mixed forest and a monsoon evergreen broadleaved forest in southern China. During the five-year observation periods (2014-2018), soil N2O emission fluxes in the two forests showed obvious seasonal variation. The soil N2O emission fluxes in wet season were significantly higher than that in dry season, with a large annual variation. Due to the decreases of precipitation, soil N2O emission fluxes of the two forests in 2017 and 2018 were generally low. Soil N2O emission flux was positively correlated with soil temperature and soil moisture. In the monsoon evergreen broadleaved forest, soil N2O emission flux in the control plot was 12.6 µg N2O·m-2·h-1. Soil N2O emission fluxes under the pH 3.5 and pH 3.0 treatments increased by 42.9% and 61.1%, respectively. Soil N2O emission was significantly increased under simulated acid rain in the monsoon evergreen broadleaved forest. Acid rain promoted soil N2O emission in the coniferous and broadleaved mixed forest, but without significant difference among the treatments. Under the scenario of increasing acid rain, soil N2O emission fluxes in typical subtropical southern China forests would increase, and the magnitude of such increase was different among forest types.