Potential and environmental control of carbon sequestration in major ecosystems across arid and semi-arid regions in China

Facing the challenge of NDVI dataset consistency for improved characterization of vegetation response to climate variability

Non-linear trend detection in Earth observation time series has become a standard method to characterize changes in terrestrial ecosystems. However, results are largely dependent on the quality and consistency of the input data, and only few studies have addressed the impact of data artifacts on the interpretation of detected abrupt changes. Here we study non-linear dynamics and turning points (TPs) of temperate grasslands in East Eurasia using two independent state-of-the-art satellite NDVI datasets (CGLS v3 and MODIS C6) and explore the impact of water availability on observed vegetation changes during 2001-2019. By applying the Break For Additive Season and Trend (BFAST01) method, we conducted a classification typology based on vegetation dynamics which was spatially consistent between the datasets for 40.86 % (459,669 km2) of the study area. When considering also the timing of the TPs, 27.09 % of the pixels showed consistent results between datasets, suggesting that careful interpretation was needed for most of the areas of detected vegetation dynamics when applying BFAST to a single dataset. Notably, for these areas showing identical typology we found that interrupted decreases in vegetation productivity were dominant in the transition zone between desert and steppes. Here, a strong link with changes in water availability was found for >80 % of the area, indicating that increasing drought stress had regulated vegetation productivity in recent years. This study shows the necessity of a cautious interpretation of the results when conducting advanced characterization of vegetation response to climate variability, but at the same time also the opportunities of going beyond the use of single dataset in advanced time-series approaches to better understanding dryland vegetation dynamics for improved anthropogenic interventions to combat vegetation productivity decrease.

Eddy covariance measurements reveal a decreased carbon sequestration strength 2010-2022 in an African semiarid savanna

Monitoring the changes of ecosystem functioning is pivotal for understanding the global carbon cycle. Despite its size and contribution to the global carbon cycle, Africa is largely understudied in regard to ongoing changes of its ecosystem functioning and their responses to climate change. One of the reasons is the lack of long-term in situ data. Here, we use eddy covariance to quantify the net ecosystem exchange (NEE) and its components-gross primary production (GPP) and ecosystem respiration (Reco) for years 2010-2022 for a Sahelian semiarid savanna to study trends in the fluxes. Significant negative trends were found for NEE (12.7 ± 2.8 g C m2 year-1), GPP (39.6 ± 7.9 g C m2 year-1), and Reco (32.2 ± 8.9 g C m2 year-1). We found that NEE decreased by 60% over the study period, and this decrease was mainly caused by stronger negative trends in rainy season GPP than in Reco. Additionally, we observed strong increasing trends in vapor pressure deficit, but no trends in rainfall or soil water content. Thus, a proposed explanation for the decrease in carbon sink strength is increasing atmospheric dryness. The warming climate in the Sahel, coupled with increasing evaporative demand, may thus lead to decreased GPP levels across this biome, and lowering its CO2 sequestration.

The different responses of AOA and AOB communities to irrigation systems in the semi-arid region of Northeast China

Ammonia oxidation is the rate-limiting step in nitrification and the key step in the nitrogen (N) cycle. Most soil nutrients and biological indicators are extremely sensitive to irrigation systems, from the perspective of improving soil fertility and soil ecological environment, the evaluation of different irrigation systems and suitability of selection, promote crop production and soil quality, study the influence of the soil microenvironment contribute to accurate evaluation of irrigation farmland soil health. Based on the amoA gene, the abundance and community diversity of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) and their responses to soil physicochemical factors and enzyme activities were studied in semi-arid areas of Northeast China. The study consisted of three irrigation systems: flood irrigation (FP), shallow buried drip irrigation (DI), and mulched drip irrigation (MF). The results showed that DI and MF significantly increased the contents of alkaline hydrolyzed nitrogen (AN), nitrate nitrogen (NO3--N), soil moisture, and the activities of ammonia monooxygenase (AMO) and hydroxylamine oxidase (HAO). Compared with FP, DI significantly increased the abundance of soil AOA and AOB, while MF significantly increased the abundance of soil AOB. Irrigation systems significantly affected the community composition of ammonia-oxidizing microorganisms (AOM). Also, AN and soil moisture had the greatest influence on the community composition of AOA and AOB, respectively. The AOB community had better stability and stress resistance. Moreover, the symbiotic network of AOB in the three irrigation systems was more complex than that of AOA. Compared with FP, the AOA community under treatment DI had higher complexity and stability, maintaining the versatility and sustainability of the ecosystem, while the AOB community under treatment MF had higher transfer efficiency in terms of matter and energy. In conclusion, DI and MF were more conducive to the propagation of soil AOM in the semi-arid area of Northeast China, which can provide a scientific basis for rational irrigation and N regulation from the perspective of microbiology.

Effects of aridification on soil total carbon pools in China's drylands

Drylands are important carbon pools and are highly vulnerable to climate change, particularly in the context of increasing aridity. However, there has been limited research on the effects of aridification on soil total carbon including soil organic carbon and soil inorganic carbon, which hinders comprehensive understanding and projection of soil carbon dynamics in drylands. To determine the response of soil total carbon to aridification, and to understand how aridification drives soil total carbon variation along the aridity gradient through different ecosystem attributes, we measured soil organic carbon, inorganic carbon and total carbon across a ~4000 km aridity gradient in the drylands of northern China. Distribution patterns of organic carbon, inorganic carbon, and total carbon at different sites along the aridity gradient were analyzed. Results showed that soil organic carbon and inorganic carbon had a complementary relationship, that is, an increase in soil inorganic carbon positively compensated for the decrease in organic carbon in semiarid to hyperarid regions. Soil total carbon exhibited a nonlinear change with increasing aridity, and the effect of aridity on total carbon shifted from negative to positive at an aridity level of 0.71. In less arid regions, aridification leads to a decrease in total carbon, mainly through a decrease in organic carbon, whereas in more arid regions, aridification promotes an increase in inorganic carbon and thus an increase in total carbon. Our study highlights the importance of soil inorganic carbon to total carbon and the different effects of aridity on soil carbon pools in drylands. Soil total carbon needs to be considered when developing measures to conserve the terrestrial carbon sink.

Land cover change in global drylands: A review

As a sensitive region, identifying land cover change in drylands is critical to understanding global environmental change. However, the current findings related to land cover change in drylands are not uniform due to differences in data and methods among studies. We compared and judged the spatial and temporal characteristics, driving forces, and ecological effects by identifying the main findings of land cover change in drylands at global and regional scales (especially in China) to strengthen the overall understanding of land cover change in drylands. Four main points were obtained. First, while most studies found that drylands were experiencing vegetation greening, some evidence showed decreases in vegetation and large increases in bare land due to inconsistencies in the datasets and the study phases. Second, the dominant factors affecting land cover change in drylands are precipitation, agricultural activities, and urban expansion. Third, the impact of land cover change on the water cycle, especially the impact of afforestation on water resources in drylands, is of great concern. Finally, drylands experience severe land degradation and require dataset matching (classification standards, resolution, etc.) to quantify the impact of human activities on land cover.

Response of net ecosystem CO2 exchange to precipitation events in the Badain Jaran Desert

It is of great significance to study the effects of precipitation events on carbon exchange in the ecosystem for an accurate understanding of the carbon cycle. However, the response of net ecosystem CO₂ exchange (NEE) in the desert to precipitation events is elusive. In this study, the NEE in response to precipitation events of varying intensities in the Badain Jaran Desert (BJD) in China was continuously monitored using the eddy covariance (EC) technique. The following results were obtained: (1) The BJD ecosystem was a net CO₂ sink throughout the study period, with NEE values of -113.4, -130.7, and -175.4 g C m^-2a^-1 in 2016, 2018, and 2019, respectively. The total precipitation yielded a higher carbon sequestration capacity in 2019 than in the other two years. In addition, the intensity, time, and frequency of precipitation had significant impacts on CO₂; (2) the threshold value of the NEE response to precipitation was ~1.4 mm, indicating the extreme sensitivity of the BJD to precipitation events; (3) the variations in the NEE response to precipitation events conformed to a dual exponential model. The analytical results of the model indicate that precipitation intensity was positively correlated with the carbon sequestration capacity of the desert. The model revealed that the greater the precipitation intensity, the longer it takes the NEE to reach the maximum, and the lengthier the duration of the residual effects. With an increase in the total precipitation and frequency of extreme precipitation events under warm and humidification climates, the carbon sequestration capacity of the BJD will likely be enhanced. The results of this study are of great significance for revealing the carbon cycle mechanism of the desert ecosystem.

Effect of crop cultivation on the soil carbon stock in mine dumps of the Loess Plateau, China

In the ecological restoration of mine dumps, soil carbon stock (SCS) improvement is an important issue. The type of land use and management approach taken can have a great influence on this issue. On the Loess Plateau, different crops have been cultivated on reclaimed land; however, the effect of long-term crop cultivation on SCS is poorly understood. To address this issue, a field investigation of mine dumps was performed at the Kee Open Pit Mine in Shanxi Province, China. Four sites utilizing different land management methods were analyzed: no reclamation (NR), reclamation with no crop cultivation (NC), and reclamation followed by 11 or 27 years crop cultivation (RC-11 and RC-27, respectively). SCS, associated soil properties (total nitrogen (TN), total phosphorus (TP), total potassium (TK), moisture content (MoiC), and pH), plant community (species composition, plant diversity, and traits), and microbial community operational taxonomic units (OTUs) of fungi and bacteria were determined by field investigation and laboratory analysis. Redundancy analysis was used to show the relationship between SCS and other environmental variables. Results varied by soil depth. At the depth range of 0-20 cm, the SCS of RC-11 was significantly greater compared to that in NR and NC, by 14.64- and 2.25-fold, respectively; whereas compared to RC-27, it was higher by 52.78%. At the depth of 20-40 cm, NC has the largest SCS; the SCS of RC-27 was the lowest, which was less compared to that in NC by 43.64%. Redundancy analysis showed a positive relationship between the SCS and TN, TP, MoiC, as well as average plant coverage, while the bacterial OTUs were negatively related with the SCS. This research suggests the potential of mine dumps for crop cultivation, which could improve the SCS of the mining area on the Loess Plateau.

Performance of the Remotely-Derived Products in Monitoring Gross Primary Production across Arid and Semi-Arid Ecosystems in Northwest China

As an important component to quantify the carbon budget, accurate evaluation of terrestrial gross primary production (GPP) is crucial for large-scale applications, especially in dryland ecosystems. Based on the in situ data from six flux sites in northwestern China from 2014 to 2016, this study compares seasonal and interannual dynamics of carbon fluxes between these arid and semi-arid ecosystems and the atmosphere. Meanwhile, the reliability of multiple remotely-derived GPP products in representative drylands was examined, including the Breathing Earth System Simulator (BESS), the Moderate Resolution Imaging Spectroradiometer (MODIS) and data derived from the OCO-2 solar-induced chlorophyll fluorescence (GOSIF). The results indicated that the carbon fluxes had clear seasonal patterns, with all ecosystems functioning as carbon sinks. The maize cropland had the highest GPP with 1183 g C m−2 y−1. Although the net ecosystem carbon exchange (NEE) in the Tamarix spp. ecosystem was the smallest among these flux sites, it reached 208 g C m−2 y−1. Furthermore, distinct advantages of GOSIF GPP (with R2 = 0.85–0.98, and RMSE = 0.87–2.66 g C m−2 d−1) were found with good performance. However, large underestimations in three GPP products existed during the growing seasons, except in grassland ecosystems. The main reasons can be ascribed to the uncertainties in the key model parameters, including the underestimated light use efficiency of the MODIS GPP, the same coarse land cover product for the BESS and MODIS GPP, the coarse gridded meteorological data, and distribution of C3 and C4 plants. Therefore, it still requires more work to accurately quantify the GPP across these dryland ecosystems.

Carbon dioxide fluxes in a farmland ecosystem of the southern Chinese Loess Plateau measured using a chamber-based method

Background

Farmland accounts for a relatively large fraction of the world’s vegetation cover, and the quantification of carbon fluxes over farmland is critical for understanding regional carbon budgets. The carbon cycle of farmland ecosystems has become a focus of global research in the field of carbon dynamics and cycling. The objectives of this study are to monitor the temporal variation in the net ecosystem exchange (NEE) and soil respiration in a spring maize (Zea mays L.) farmland ecosystem of the southern Loess Plateau of China.

Methods

A fully automated temperature-controlled flux chamber system was adopted in this study. The system contained nine chambers for CO₂ flux measurements, and three treatments were conducted: with and without maize plants in the chamber, as well as a bare field. Observations were conducted from June to September 2011. This time period covers the seedling, jointing, heading, grain filling, and ripening stages of spring maize. Other factors, such as air temperature (Ta), soil temperature (Ts), soil water content (SWC), photosynthetically active radiation (PAR), and precipitation (P), were simultaneously monitored.

Results

There was observed diurnal variation in the NEE of the maize ecosystem (NEE-maize). A short “noon break” occurred when the PAR intensity was at its maximum, while soil respiration rates had curves with a single peak. During the overall maize growth season, the total NEE-maize was –68.61 g C m⁻², and the soil respiration from the maize field (SR-maize) and bare field (SR-bare field) were 245.69 g C m⁻² and 114.08 g C m⁻², respectively. The temperature sensitivity of soil respiration in the maize field exceeded that in the bare field. Significant negative correlations were found between the NEE, PAR, and temperature (all p-values < 0.01), with both Ta and PAR being the primary factors that affected the CO₂ fluxes, collectively contributing 61.7%, 37.2%, and 56.8% to the NEE-maize, SR-maize, and SR-bare field, respectively. It was therefore concluded that both meteorological factors and farming practices have an important impact on the carbon balance process in corn farmland ecosystems. However, it is necessary to conduct long-term observational studies, in order to get a better understanding of the driving mechanism.

Remotely monitoring ecosystem respiration from various grasslands along a large-scale east-west transect across northern China

BACKGROUND

Grassland ecosystems play an important role in the terrestrial carbon cycles through carbon emission by ecosystem respiration (R_e) and carbon uptake by plant photosynthesis (GPP). Surprisingly, given R_e occupies a large component of annual carbon balance, rather less attention has been paid to developing the estimates of R_e compared to GPP.

RESULTS

Based on 11 flux sites over the diverse grassland ecosystems in northern China, this study examined the amounts of carbon released by R_e as well as the dominant environmental controls across temperate meadow steppe, typical steppe, desert steppe and alpine meadow, respectively. Multi-year mean R_e revealed relatively less CO₂ emitted by the desert steppe in comparison with other grassland ecosystems. Meanwhile, C emissions of all grasslands were mainly controlled by the growing period. Correlation analysis revealed that apart from air and soil temperature, soil water content exerted a strong effect on the variability in R_e, which implied the great potential to derive R_e using relevant remote sensing data. Then, these field-measured R_e data were up-scaled to large areas using time-series MODIS information and remote sensing-based piecewise regression models. These semi-empirical models appeared to work well with a small margin of error (R² and RMSE ranged from 0.45 to 0.88 and from 0.21 to 0.69 g C m^-2 d^-1, respectively).

CONCLUSIONS

Generally, the piecewise models from the growth period and dormant season performed better than model developed directly from the entire year. Moreover, the biases between annual mean R_e observations and the remotely-derived products were usually within 20%. Finally, the regional R_e emissions across northern China's grasslands was approximately 100.66 Tg C in 2010, about 1/3 of carbon fixed from the MODIS GPP product. Specially, the desert steppe exhibited the highest ratio, followed by the temperate meadow steppe, typical steppe and alpine meadow. Therefore, this work provides a novel framework to accurately predict the spatio-temporal patterns of R_e over large areas, which can greatly reduce the uncertainties in global carbon estimates and climate projections.

Evaluating the ecological benefits of plantations in arid areas from the perspective of ecosystem service supply and demand-based on emergy analysis

Plantations can significantly improve the ecological environment of arid areas. However, in the arid area of Northwestern China, plantations consume several thousand cubic meters of water per hectare per year, which also has a negative effect. By evaluating the relationship between the supply and demand of ecosystem services, the positive and negative benefits of plantations can be effectively identified for regional sustainable development. This study took typical arid regions as examples and evaluated the changes in ecosystem services before and after the conversion of a desert shrub system to a plantation system based on emergy analysis. The relationship between the supply and demand of various ecosystem services from different stakeholder perspectives was investigated and combined with the degree of ecosystem service demand obtained from the participatory survey. Results showed the following. (1) The ecosystem services provided by vegetation increased, the ecosystem services provided by water resources decreased, and the total service decreased by 6.89E+17 sej. (2) The relationship between the supply and demand of ecosystem services provided by water resources was of the "low-high" type. Dust purification was of the "high-high" type, and carbon sequestration and soil conservation were of the "high-low" type. (3) From the perspective of citizens, the matching degrees of the supply and demand of dust purification and soil conservation services were 21.75% and 9.38% higher than those of farmers, respectively. From the perspective of farmers, the matching degrees of the supply and demand of water supply, water purification, power supply and sediment transport, and carbon sequestration were 9.90%, 10.96%, 3.22%, and 18.28% higher than those of citizens, respectively. In summary, attention should be given to the efficient use and conservation of water resources and the promotion of the coordinated development of ecosystem services by comparing the differences in matching degrees of supply and demand among different stakeholders when constructing plantations. Meanwhile, the distribution of water resources should be given attention to protect the water demand of citizens in a targeted manner for effectively improving the level of regional well-being.

The effect of phenology on the carbon exchange process in grassland and maize cropland ecosystems across a semiarid area of China

Phenology plays an important role in the carbon exchange process. Seven years of continuous eddy covariance data across two different ecosystems in a semiarid area were used to investigate the variation in phenology indices, its effect on the carbon exchange process, and responses to climate change. The results showed that there was large annual variation for vegetation phenology. The GSL (growing season length) displayed an obvious increasing trend for the grassland ecosystem during the 7 years, and it was most determined by SOS (the start day of growing season). The growing season was divided into three periods, the recovery period (S1), the stable period (S2), and the senescence period (S3). Both ecosystems had a similar ratio of Re (ecosystem respiration) to GPP (gross primary production) during S1 and S2 periods but a much larger Re/GPP ratio during the last growing period. The inter-annual variation of the peak rate was most responsible for the NEP (net ecosystem production) and its components (GPP and Re) in both ecosystems. The inter-annual variation of recovery rate, GSL and SOS was found to be closely correlated to Re for the grassland ecosystem, while that could not be found for the cropland ecosystem. The temperature in June was most closely correlated with the peak rate of GPP and NEP for grassland ecosystem, with a significant correlation coefficient of -0.90 and -0.82, respectively. Meanwhile, the precipitation in July was found to be closely correlated with GPP for both ecosystems, with a similar correlation coefficient of 0.83. The precipitation and temperature roughly exhibited an inverse effect on vegetation phenology in this semiarid area. The variation of temperature in the early month and precipitation in mid growing season showed a more significant effect on main phenology indicators for the cropland ecosystem than those for grassland ecosystem.

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

Semiarid regions have substantial interannual variation in carbon exchange between terrestrial ecosystem and atmosphere but the diverse responses of carbon fluxes to climate variability are poorly known. We compared carbon exchange processes and the responses to environmental factors in a meadow steppe at Tongyu (TY) and a typical steppe at Maodeng (MD) using long-term continuous eddy covariance measurements. TY precipitation was 25% greater than MD. Both grasslands had interannual fluctuations of carbon sink and source and acted as weak carbon sinks averagely (-22.9 ± 41.0 gCm^-2 yr^-1 for TY and - 11.8 ± 45.0 gCm^-2 yr^-1 for MD). The seasonal dynamics of carbon fluxes were significantly related to water availability at MD but more strongly related to air temperature at TY. During dry years, the controlling effect of water availability on carbon fluxes increased. Summer precipitation and soil moisture played key roles in the interannual variations in carbon fluxes in both grasslands. At MD, net carbon uptake was negatively related to summer air temperature likely because warming induced water deficit decreased photosynthesis. Greenness index derived from PhenoCam images captured key phenological phases and diverse magnitude of canopy dynamics. The index was correlated with seasonal and annual variations in carbon fluxes at both grasslands, suggesting the potential of PhenoCam for monitoring the spatial and temporal variations in canopy dynamics in different semiarid grasslands.