Seasonal and interannual variations in carbon fluxes in East Asia semi-arid grasslands

Differential responses of plant and microbial respiration to extreme precipitation and drought during spring and summer in the Eurasian meadow steppe

Increasing extreme precipitation and drought events along changes in their seasonal patterns due to climate change are expected to have profound consequences for carbon cycling. However, how these climate extremes impact ecosystem respiration (Reco) and whether these impacts differ between seasons remain unclear. Here, we reveal the responses of Reco and its components to extreme precipitation and drought in spring and summer by conducting a five-year manipulative experiment in a temperate meadow steppe. Based on a 5-year average, the seasonal mean values (±SE) of Reco, Rh, Rroot, Rabg and Rplant significantly increased (p < 0.01) under both extreme precipitation treatments: wet spring (WSP) and wet summer (WSU), and significantly decreased (p < 0.01) under both extreme drought treatments: dry spring (DSP) and dry summer (DSU), except in Rabg under DSU, which remained comparable to the control. The sensitivity of Reco, Rh, Rroot and Rplant to extreme precipitation was significantly higher in spring than in summer. On average, Rplant was the primary contributor of Reco, accounting for 37.18% and 38.31% of the total across all its components under WSP and WSU, respectively during the growing season over the five study years. Moreover, linear models revealed Rplant explained 87% of the variance in Reco. Our findings indicate that future changes in precipitation events, particularly extreme precipitation may lead to increased carbon release from ecosystems, largely driven by enhanced plant respiration rather than microbial respiration. However, due to this study focused solely on respiration and did not measure photosynthesis, the findings represent only the carbon release processes and do not account for potential carbon uptake by plants during the same conditions. These emergent identified contribution to ecosystem respiration provide valuable insights for improving model benchmarks to better predict ecosystem respiration responses to extreme climate in specified season.

Carbon fluxes and water-use efficiency in a Pinus tabuliformis plantation in Northeast China and their relationship to drought

The impact of extreme weather events on carbon fluxes and water-use efficiency (WUE) in revegetated areas under water-limited conditions is poorly understood. We analyzed changes in carbon fluxes and WUE over three years of eddy-covariance measurements in a Pinus tabuliformis plantation in Northeast China to investigate carbon fluxes and WUE responses to drought events at different time scales. Mean annual net ecosystem exchange (NEE), gross primary production (GPP), and ecosystem respiration (Re) were -368.48, 1042.42, and 673.94 g C m-2, respectively. Drought events increased NEE, as GPP was more sensitive to water stress than Re at different growing stages. Mean annual WUE was 2.46 g C kg-1 H2O, and plant phenology played a key role in WUE responses to drought. Water stress had negative and positive effects on daily WUE at the early and late growing stages, respectively, and daily WUE was generally insensitive to drought at the mid growing stage. A lagged effect existed in the carbon fluxes and WUE dynamics after drought events at various time scales. Water stress at the early growing stage was more important than that at other growing stages on annual carbon sequestration and WUE, as it dominated canopy growth in the current year. The annual mean normalized difference vegetation index controlled interannual variations in carbon fluxes and WUE in the plantation. Our findings contribute to the prediction of possible changes in carbon and water fluxes under climate warming in the afforested areas of Northeast China.

Carbon flux dynamics and influencing factors in a semi-arid dune ecosystem of northern China

The dune ecosystem plays a significant role in the global carbon cycle. The Horqin Sandy Land is a typical semi-arid fragile ecosystem in northern China. Understanding the magnitudes and dynamics of carbon dioxide fluxes within this region is essential for understanding the carbon balance. Used 6 years (2013-2018) measurements from an eddy-covariance system, we analyzed the dynamic patterns of net ecosystem carbon exchange (NEE), gross primary production (GPP), and ecosystem respiration (Reco) of the dune ecosystem in Horqin Sandy Land and examined their responses to climate factors with a focus on the precipitation. The results showed that the NEE of the dune ecosystem fluctuated from -166 to 100 gCO2·m-2·year-1 across the 6 growing seasons, with an average of -56 gCO2·m-2·year-1. The precipitation was not a key factor influencing the carbon flux variability. During the mid-growth stage, GPP was primarily affected by the effective precipitation frequency (R2 ranging from 0.65 to 0.85, P < 0.05), followed by fractional vegetation cover (R2 ranging from 0.65 to 0.68, P < 0.05). However, in the early and late growth stages, temperature predominantly drove the carbon flux (R2 = 0.75, P < 0.01). The interannual variability of carbon flux can be predominantly elucidated by phenological indicators such as CO2 uptake (CUstart), end of CO2 uptake (CUend), CO2 uptake period (CUP), and Spring lag. The results demonstrated the dune ecosystem is a weak carbon sink in semi-arid ecosystems. Furthermore, we emphasized the significance of effective precipitation frequency in regulating carbon fluxes. Our results provide a foundational understanding of the carbon balance in semi-arid ecosystems.

Assessment of Phenological Dynamics of Different Vegetation Types and Their Environmental Drivers with Near-Surface Remote Sensing: A Case Study on the Loess Plateau of China

Plant phenology is an important indicator of the impact of climate change on ecosystems. We have continuously monitored vegetation phenology using near-surface remote sensing, i.e., the PhenoCam in a gully region of the Loess Plateau of China from March 2020 to November 2022. In each image, three regions of interest (ROIs) were selected to represent different types of vegetation (scrub, arbor, and grassland), and five vegetation indexes were calculated within each ROI. The results showed that the green chromatic coordinate (GCC), excess green index (ExG), and vegetation contrast index (VCI) all well-captured seasonal changes in vegetation greenness. The PhenoCam captured seasonal trajectories of different vegetation that reflect differences in vegetation growth. Such differences may be influenced by external abiotic environmental factors. We analyzed the nonlinear response of the GCC series to environmental variables with the generalized additive model (GAM). Our results suggested that soil temperature was an important driver affecting plant phenology in the Loess gully region, especially the scrub showed a significant nonlinear response to soil temperature change. Since in situ phenology monitoring experiments of the small-scale on the Loess Plateau are still relatively rare, our work provides a reference for further understanding of vegetation phenological variations and ecosystem functions on the Loess Plateau.

Reduction in precipitation amount, precipitation events, and nitrogen addition change ecosystem carbon fluxes differently in a semi-arid grassland

The increases in extent and frequency of extreme drought events and increased nitrogen (N) deposition due to global change are expected to have profound impacts on carbon cycling in semi-arid grasslands. However, how ecosystem CO2 exchange processes respond to different drought scenarios individually and interactively with N addition remains uncertain. In this study, we experimentally explored the effects of different drought scenarios (early season extreme drought, 50 % reduction in precipitation amount, and 50 % reduction in precipitation events) and N addition on net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP) over three growing seasons (2019-2021) in a semi-arid grassland in northern China. The growing-season ecosystem carbon fluxes in response to drought and N addition were influenced by inter-annual precipitation changes, with 2019 as a normal precipitation year, and 2020 and 2021 as wet years. Early season extreme drought stimulated NEE by reducing ER. 50 % reduction in precipitation amount decreased ER and GEP consistently in three years, but only significantly suppressed NEE in 2019. 50 % reduction in precipitation events stimulated NEE. Nitrogen addition stimulated NEE, ER, and GEP, but only significantly in wet years. The structural equation models showed that changes in carbon fluxes were regulated by soil moisture, soil temperature, microbial biomass nitrogen (MBN), and the key plant functional traits. Decreased community-weighted means of specific leaf area (CWMSLA) was closely related to the reduced ER and GEP under early season extreme drought and 50 % reduction in precipitation amount. While increased community-weighted means of plant height (CWMPH) largely accounted for the stimulated ER and GEP under 50 % reduction in precipitation events. Our study stresses the distinct effects of different drought scenarios and N enrichment on carbon fluxes, and highlights the importance of soil traits and the key plant traits in determining carbon exchange in this water-limited ecosystem.

An abrupt shift in gross primary productivity over Eastern China-Mongolia and its inter-model diversity in land surface models

The terrestrial ecosystem in East Asia mainly consists of semi-arid regions that are sensitive to climate change. Therefore, gross primary productivity (GPP) in East Asia could be highly variable and vulnerable to climate change, which can significantly affect the local carbon budget. Here, we examine the spatial and temporal characteristics of GPP variability in East Asia and its relationship with climate factors over the last three decades. We detect an abrupt decrease in GPP over Eastern China-Mongolia region around the year 2000. This is attributed to an abrupt decrease in precipitation associated with the phase shift of the Pacific decadal oscillation (PDO). We also evaluate the reproducibility of offline land surface models to simulate these abrupt changes. Of the twelve models, eight were able to simulate this abrupt response, while the others failed due to the combination of an exaggerated CO2 fertilization effect and an underrated climate impact. For accurate prediction, it is necessary to improve the sensitivity of the GPP to changes in CO2 concentrations and the climate system.

Understanding elevated CO2 concentrations in East Asia relative to the global mean during boreal spring on the slow and interannual timescales

It is important to examine the physical processes that regulate current CO2 concentrations in East Asia to understand the global carbon cycle. To do this, we begin by defining the difference between East Asian and global CO2 concentrations (East Asian CO2 concentration minus global CO2 concentration), which is referred to as East Asian local CO2 concentration (i.e., EA_LCO2). Then, we examine the physical processes associated with the variability of EA_LCO2 during boreal spring (March-April-May) on the slow and interannual timescales. Our results indicate that there are two key factors leading to elevated CO2 concentrations in East Asia relative to the global mean during boreal spring; one is higher emissions in East Asia, which mostly explains the increasing in EA_LCO2 on the slow timescales. The other is a cool sea surface temperature (SST) in the eastern tropical Pacific (La-Nina-like SST), which is associated with an interannual higher CO2 concentration in East Asia than the global mean. Enhanced convective activity in the western tropical Pacific, which is associated with a La-Nina-like SST forcing, induces low-pressure circulation in the western North Pacific with northerly winds, leading to suppressed precipitation and cool surface temperature in East Asia. Subsequently, those suppress vegetation growth as well as gross primary product, resulting in relatively high CO2 concentrations in East Asia compared to the global mean.

Spatiotemporal Dynamics of the Carbon Budget and the Response to Grazing in Qinghai Grasslands

Estimating the grassland carbon budget is critically important for ensuring that grassland resources are used sustainably. However, the spatiotemporal dynamics of the carbon budget and the response to grazing have not yet been characterized in Qinghai grasslands. Here, we estimated the gross primary productivity (GPP) and net ecosystem exchange (NEE) in Qinghai grasslands using the improved Biome-BGCMuSo model to characterize the spatiotemporal dynamics of the carbon budget and the response to grazing in this region from 1979 to 2018. The GPP of Qinghai grasslands fluctuated during the study period, with an average annual value of 118.78 gC/m2. The NEE of Qinghai grasslands fluctuated from 1979 to 2018, with an average value of -5.16 gC/m2. After 2,000, GPP increased, and NEE decreased in a fluctuating manner. There were clear regional differences in GPP and NEE. GPP was low in most areas of Qinghai, and GPP was high in eastern and southern Qinghai. The southern, southeastern, and northeastern parts of Qinghai were mainly carbon sinks, and the northwestern part of Qinghai and the region between the southeastern and northeastern parts of Qinghai were mainly carbon sources. Grazing generally decreased GPP and increased NEE in Qinghai grasslands from 1979 to 2018. There was spatial heterogeneity in the effect of grazing on GPP and NEE. Under grazing, GPP and NEE were significantly decreased mainly in eastern Qinghai, and GPP and NEE were significantly increased mainly in southern and eastern Qinghai. NEE was most affected by grazing in eastern Qinghai. The results of this study aid our understanding of the mechanism driving variation in the grassland carbon budget and provide new data that could be used to support local grassland management.

Increased CO2 emissions surpass reductions of non-CO2 emissions more under higher experimental warming in an alpine meadow

It is well documented that warming can accelerate greenhouse gas (GHG) emissions, further inducing a positive feedback and reinforcing future climate warming. However, how different kinds of GHGs respond to various warming magnitudes remains largely unclear, especially in the cold regions that are more sensitive to climate warming. Here, we concurrently measured carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes and their total balance in an alpine meadow in response to three levels of warming (ambient, +1.5 °C, +3.0 °C). We found warming-induced increases in CH₄ uptake, decreases in N₂O emissions and increases in CO₂ emissions at the annual basis. Expressed as CO₂-equivalents with a global warming potential of 100 years (GWP100), the enhancement of CH₄ uptake and reduction of N₂O emissions offset only 9% of the warming-induced increase in CO₂ emissions for 1.5 °C warming, and only 7% for 3.0 °C warming. CO₂ emissions were strongly stimulated, leading to a significantly positive feedback to climate system, for 3.0 °C warming but less for 1.5 °C warming. The warming with 3.0 °C altered the total GHG balance mainly by stimulating CO₂ emissions in the non-growing season due to warmer soil temperatures, longer unfrozen period, and increased soil water content. The findings provide an empirical evidence that warming beyond global 2 °C target can trigger a positive GHG-climate feedback and highlight the contribution from non-growing season to this positive feedback loop in cold ecosystems.

Spatio-Temporal Grassland Development in Inner Mongolia after Implementation of the First Comprehensive Nation-Wide Grassland Conservation Program

Protection of the grassland’s ecological environment and improvement of people’s livelihoods are major tasks for the management of pastoral areas in Inner Mongolia. The comprehensive program for grassland conservation in China, the Subsidy and Incentive System for Grassland Conservation (SISGC), was launched in 2011. To comprehend the effects of this major step towards sustainable grassland development, this study focuses on the spatio-temporal development of grasslands in Inner Mongolia since 2011. Through the combination of MODIS (Moderate-resolution Imaging Spectroradiometer) satellite data with up to date meteorological data, we used the indicators of Fractional Vegetation Cover (FVC) and Net Primary Productivity (NPP) to analyze qualitative and quantitative grassland changes. A classification system on the pixel level, reflecting change trends and fluctuations of both FVC and NPP, was applied to monitor and analyze the grassland development from 2011 to 2019. In particular, the spatial transfer matrix of the recent two years (2018 to 2019) was analyzed to reveal the latest potential issues and random impact factors. The results show a positive overall but spatially unbalanced effect of SISGC, with a prominent positive impact in the semi-desert grassland area. The potential threats from both social and natural aspects as well as the importance of a forecast system for local stakeholders in the pastoral area are discussed.

Seasonal variation of net ecosystem CO2 exchange and its influencing factors in an apple orchard in the Loess Plateau

The Loess Plateau is the largest apple cultivation region in the world. However, the role of rain-fed apple orchards as carbon sinks or sources, including the dynamic variation and influencing factors, are still unclear. In this study, the net ecosystem CO₂ exchange (NEE) was monitored by an eddy covariance (EC) system in Loess Plateau apple orchards during 2016-2017. The results demonstrated that the annual NEE was higher in 2016 (- 698.0 g C m^-2 year^-1) than in 2017 (- 554.0 g C m^-2 year^-1). Particularly, the amount of orchard CO₂ uptake was significantly greater in 2016 (- 772.0 g C m^-2) than in 2017 (- 642.1 g C m^-2) during the carbon sink period. This difference may be attributed to the higher NEE in 2016 compared to 2017 during the fast and slow growth periods. In addition, a higher daily NEE occurred to the higher air temperature (T_a), which promoted early sprouting in 2016 (- 3.91 g C m^-2 day^-1) compared to 2017 (- 2.86 g C m^-2 day^-1) during the fast growth period. The daily NEE in 2016 (- 2.59 g C m^-2 day^-1) was remarkably higher than that in 2017 (- 1.41 g C m^-2 day^-1) during the slow growth period, owing to the greater number of cloudy and rainy days and lower temperatures in 2017. Overall, the present study demonstrated the key role played by the amount of precipitation and temperature in regulating the NEE during the growth season and provided accurate quantitative information on the carbon budget in apple orchards. Graphical abstract.

Drivers of carbon fluxes in Alpine tundra: a comparison of three empirical model approaches

In high mountains, the effects of climate change are manifesting most rapidly. This is especially critical for the high-altitude carbon cycle, for which new feedbacks could be triggered. However, mountain carbon dynamics is only partially known. In particular, models of the processes driving carbon fluxes in high-altitude grasslands and Alpine tundra need to be improved. Here, we propose a comparison of three empirical approaches using systematic statistical analysis, to identify the environmental variables controlling CO₂ fluxes. The methods were applied to a complete dataset of simultaneous in situ measurements of the net CO₂ exchange, ecosystem respiration and basic environmental variables in three sampling sites in the same catchment. Large year-to-year variations in the Gross Primary Production (GPP) and Ecosystem Respiration (ER) dependences on solar irradiance and temperature were observed. We thus implemented a multi regression model in which additional variables were introduced as perturbations of the standard exponential and rectangular hyperbolic functions for ER and GPP, respectively. A comparison of this model with other common modelling strategies showed the benefits of this approach, resulting in large explained variances (83% to 94%). The optimum ensemble of variables explaining the inter- and intra-annual flux variability included solar irradiance, soil moisture and day of the year for GPP, and air temperature, soil moisture, air pressure and day of the year for ER, in agreement with other studies. The modelling approach discussed here provides a basis for selecting drivers of carbon fluxes and understanding their role in high-altitude Alpine ecosystems, also allowing for future short-range assessments of local trends.