Gross Ecosystem Productivity Research Articles

Ecosystem carbon exchange and nitrogen removal rates in two 33‐year‐old constructed salt marshes are similar to those in a nearby natural marsh

Human activities have led to 1–2% of coastal wetlands lost per year globally, with subsequent losses in ecosystem services such as nutrient filtering and carbon sequestration. Wetland construction is used to mitigate losses of marsh cover and services resulting from human impacts in coastal areas. Though marsh structure can recover relatively quickly (i.e., <10 years) after construction, there are often long‐term lags in the recovery of ecosystem functions in constructed marshes. We conducted a year‐long study comparing seasonal plant productivity, ecosystem respiration (), denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) between two 33‐year‐old constructed marshes (CON‐1, CON‐2) and a nearby natural reference marsh (NAT). We found that CON‐1 and CON‐2 were structurally similar to NAT (i.e., plant aboveground and belowground biomass did not differ). Likewise, gross ecosystem productivity (GEP), , and net ecosystem exchange (NEE) were similar across all marshes. Further, DNRA and denitrification were similar across marshes, with the exception of greater denitrification rates at CON‐2 than at the other two sites. While pore‐water ammonium concentrations were similar across all marshes, organic matter (OM) content, pore‐water phosphate, nitrate + nitrite, and hydrogen sulfide concentrations were greater in NAT than CON‐1 and CON‐2. Collectively, this work suggests that current marsh construction practices could be a suitable tool for recovering plant structure and some ecosystem functions. However, the lag in recovery of pore‐water nutrient stocks and OM content also suggests that some biogeochemical functions may take longer than a few decades to fully recover in constructed marshes.

Modes of Metabolic Performance of Pacific Reefs

AbstractIt has been hypothesized that coral reef primary production rates are characterized by a series of absolute modes associated with dominant benthic cover classes (coral, algae, and sand). However, this hypothesis has yet to be fully tested. Similarly, there has been no investigation into the influence of differential light levels on modal productivity. Findings from this study support the hypothesis of modal metabolic performance, with coral, algal, and sand communities showing differences in gross ecosystem production (GEP). This lends support to census‐based estimates of regional to global GEP. At the same time, the data reveal large differences in the responses of different communities to light. Any given day may have vastly different GEP rates depending on cloudiness, hence it is critical to minimize this variation using approaches that account for the effects of light. Importantly, future comparative studies should incorporate a number of days at each site to provide a representative metabolic rate.

Nitrogen availability and precipitation variability regulated CO2 fertilization effects on carbon fluxes in an alpine grassland

It's generally believed that elevated CO2 (eCO2) could stimulate plant growth and the ecosystem carbon (C) sink. However, great uncertainties exist in terms of the CO2 fertilization effect (CFE) magnitude, and how it is regulated by other global change factors. The lack of experimental evidence from the Alpine Region also limits our cognition on the CFE. By conducting a five-year manipulative field experiment in a semi-arid grassland of the Tibetan Plateau, we are aimed to explore the behavior of ecosystem C exchange in response to eCO2 and N availability under contrasting natural precipitation regimes. The experiment showed that eCO2 stimulated both gross ecosystem productivity (GEP) and ecosystem respiration (ER), and resulted in a neutral effect on net ecosystem productivity (NEP). The reduction of leaf N concentration under eCO2 constrained the eCO2 effects on C fluxes, especially on GEP and NEP. As N addition replenishes N availability in soil and leaf, GEP benefited more from the N addition than the ER. The eCO2 strengthened the C sink when exogenous N was added simultaneously. Furthermore, precipitation variability played an importance role in mediating the eCO2 effect among growing seasons. The eCO2 effects on C fluxes tended to decline with increased water availability. The CFE was suppressed with excessive precipitation when the water-use efficiency (WUE) response was weak and eCO2-induced water-saving disappeared. The negative impact of precipitation on the CFE may also be attributed to the short precipitation intervals and insufficient radiation caused by high-frequency precipitation. Our study demonstrates that eCO2 only stimulates net C uptake under conditions of N addition or during drier periods. Given the widespread N limitation, the efficacy of terrestrial ecosystems in mitigating climate change under rising CO2 may be weaker than projected and is closely related to the precipitation variability.

Drought Affected Ecosystem Water Use Efficiency of a Natural Oak Forest in Central China

Global climate models project more frequent drought events in Central China. However, the effect of seasonal drought on ecosystem water use efficiency (WUE) and water regulation strategy in Central China’s natural forests is poorly understood. This study investigated variations in WUE associated with drought in a natural oak (Quercus aliena) forest in Central China from 2017 to 2020 at several timescales based on continuous CO2 and water vapor flux measurements. Results showed that the 4-year mean gross ecosystem production (GEP), evapotranspiration (ET) and WUE of the natural oak forest was 1613.2 ± 116 g Cm−2, 637.8 ± 163.3 mm and 2.6 ± 0.68 g Ckg−1 H2O, with a coefficient of variation (CV) of 7.2%, 25.6% and 26.4%, respectively. The inter-annual variation in WUE was large, primarily due to the variation in ET caused by seasonal drought. Drought increased WUE distinctly in summer and decreased it slightly in autumn. During summer drought, surface conductance (gs) usually decreased with an increase in VPD, but the ratios of stomatal sensitivity (m) and reference conductance (gsref) were 0.21 and 0.3 molm−2s−1ln(kPa)−1 in the summer of 2019 and 2020. Strong drought can also affect ecosystem WUE and water regulation strategy in the next year. Decrease in precipitation in spring increased annual WUE. These results suggested that drought in different seasons had different effects on ecosystem WUE. Overall, our findings suggest that the natural oak forest did not reduce GEP by increasing WUE (i.e., reducing ET) under spring and summer drought, which could be due to its typical anisohydric characteristics, although it can also reduce stomatal opening during long-term drought.

Interannual variability in net ecosystem carbon production in a rain-fed maize ecosystem and its climatic and biotic controls during 2005-2018.

Interannual variability (IAV) in net ecosystem carbon production (NEP) plays an important role in the processes of the carbon cycle, but the long-term trends in NEP and the climatic and biotic control of IAV in NEP still remain unclear in agroecosystems. We investigated interannual variability in NEP, expressed as annual values and anomalies, and its climatic and biotic controls using an eddy-covariance dataset for 2005–2018 for rain-fed spring maize in northeastern China. Average annual NEP was 270±31 g C m−2yr −1, with no significant changes over time. The effects on interannual variability in NEP of gross ecosystem productivity (GEP) that was mainly controlled by soil water content (SWC) and leaf area index (LAI), were more than those of respiration (RE) that was controlled by temperature and LAI. Further, maximum daily NEP (NEPmax) that was dominated by summer vapor pressure deficit explained the largest fraction of annual anomalies in NEP, followed by carbon dioxide uptake period (CUP) that was defined by the beginning date (BDOY) and the end date (EDOY) of CUP. The variability in BDOY was mainly determined by spring precipitation and the effective accumulated temperature, and the variability in EDOY was determined by autumn precipitation, SWC and LAI. NEP may decrease with declining precipitation in the future due to decreasing GEP, NEPmax, or CUP, and irrigation and residues cover may be useful in efforts to maintain current NEP levels. Our results indicate that interannual variability in NEP in agroecosystems may be more sensitive to changes in water conditions (such as precipitation, SWC and VPD) induced by climate changes, while temperature may be an important indirect factor when VPD is dominated.

Soil moisture, temperature and nitrogen availability interactively regulate carbon exchange in a meadow steppe ecosystem

Primary productivity in terrestrial ecosystems can be significantly altered by the predicted increases in nitrogen (N) deposition, but it is still unclear how N deposition influences the carbon (C) exchange processes especially in dryland ecosystems. In this study, a 3-year experiment with two types of fertilizers and five N addition levels was conducted in a semiarid steppe of Erguna, Inner Mongolia to assess the effects of exogenous N input on net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (ER). Our results showed that enhanced N input significantly increased NEP and GEP when the volumetric soil water content (VWC) was greater than 15% (v/v%), but had trivial and inconsistent effects when VWC was less than that threshold. Moreover, the NEP and GEP increased significantly with increasing N addition but leveled off when the N input reached 10 g N m–2 year–1 and the VWC was above 15%. However, the ER showed a significant increase with N addition rates, regardless of soil water content. Our main findings are as follows. First, the ecosystem C exchange can be significantly enhanced by addition of N only when soil moisture exceeds certain threshold, suggesting co-limitation of water and N in the grassland ecosystem that we studied. Second, in addition to concurrent consideration of water, temperature and N as well as their interactions also need to be accounted for. Third, the frequency of days with the VWC being higher than 15% accounted only around one third of days during the three growing seasons. Taken together, our findings imply that the expected increases of carbon sequestration should be minimal under N deposition in the Inner Mongolia grassland ecosystems.

Drought years in peatland rewetting: rapid vegetation succession can maintain the net CO<sub>2</sub> sink function

Abstract. The rewetting of peatlands is regarded as an important nature-based climate solution and intended to reconcile climate protection with the restoration of self-regulating ecosystems that are resistant to climate impacts. Although the severity and frequency of droughts are predicted to increase as a consequence of climate change, it is not well understood whether such extreme events can jeopardize rewetting measures. The goal of this study was to better understand drought effects on vegetation development and the exchange of the two important greenhouse gases CO2 and CH4, especially in rewetted fens. Based on long-term reference records, we investigated anomalies in vegetation dynamics, CH4 emissions, and net CO2 exchange, including the component fluxes of ecosystem respiration (Reco) and gross ecosystem productivity (GEP), in a rewetted fen during the extreme European summer drought in 2018. Drought-induced vegetation dynamics were derived from remotely sensed data. Since flooding in 2010, the fen was characterized by a patchy mosaic of open-water surfaces and vegetated areas. After years of stagnant vegetation development, drought acted as a trigger event for pioneer species such as Tephroseris palustris and Ranunculus sceleratus to rapidly close persistent vegetation gaps. The massive spread of vegetation assimilated substantial amounts of CO2. In 2018, the annual GEP budget increased by 20 % in comparison to average years (2010–2017). Reco increased even by 40 %, but enhanced photosynthetic CO2 sequestration could compensate for half of the drought-induced increase in respiratory CO2 release. Altogether, the restored fen remained a net CO2 sink in the year of drought, though net CO2 sequestration was lower than in other years. CH4 emissions were 20 % below average on an annual basis, though stronger reduction effects occurred from August onwards, when daily fluxes were 60 % lower than in reference years. Our study reveals an important regulatory mechanism of restored fens to maintain their net CO2 sink function even in extremely dry years. It appears that, in times of more frequent climate extremes, fen restoration can create ecosystems resilient to drought. However, in order to comprehensively assess the mitigation prospects of peatland rewetting as a nature-based climate solution, further research needs to focus on the long-term effects of such extreme events beyond the actual drought period.

Deeper burning in a boreal fen peatland 1‐year post‐wildfire accelerates recovery trajectory of carbon dioxide uptake

AbstractPeatlands contain a globally significant store (30%) of soil carbon (C). Within the Canadian Western Boreal Plains, where peatlands are a dominant feature, the climate is becoming warmer and drier, coupled with an increase in forest fire incidence. The response of peatlands to forest fire is likely to be a key determinant in the future of C storage of Boreal peatlands. This study examined the impacts of fire on key environmental controls on CO2 fluxes at the plot‐scale (using static chambers) between burned and unburned understory vegetation throughout the growing season of 2017 in a treed fen impacted by the Horse River wildfire (2016) in Fort McMurray, Alberta, Canada. Both gross ecosystem productivity (GEP) and total respiration (Rtot) were less at burned plots compared with unburned. Temporal patterns varied between the plots, where both component of CO2 fluxes at the unburned plots were largest in June, whereas at the burned plots, CO2 fluxes peaked in the late growing season. GEP and net ecosystem exchange (NEE) showed a positive relationship with depth of burn, with the deepest burned areas showing significantly greater CO2 uptake, coinciding with both increased bioavailable phosphorus and greater moss recolonization. At the unburned plots, soil temperature was a dominant control on CO2 fluxes. This work demonstrates the importance of the depth of burn to post‐fire carbon fluxes and how a knowledge of burn severity and depth can inform understanding of the recovery trajectory of northern peatlands following fire disturbance.

降水量与氮添加对荒漠草原生态系统碳交换的影响

为深入了解降水格局改变和氮沉降增加对荒漠草原生态系统碳交换的影响机制，于2017年在宁夏荒漠草原设立了降水量变化（减少50%、减少30%、自然降水量、增加30%以及增加50%）和氮添加（0和5 g m^-2 a^-1）的野外试验，研究了2019年生长季（5-10月份）净生态系统碳交换（Net ecosystem carbon exchange，NEE）、生态系统呼吸（Ecosystem respiration，ER）和总生态系统生产力（Gross ecosystem productivity，GEP）的时间动态，分析了三者与植被组成以及土壤属性的关系。NEE、ER和GEP日动态和月动态均呈先增加后降低，NEE在整个生长季表现为净生态系统碳吸收。0和5 g m^-2 a^-1氮添加下，减少降水量显著降低了NEE、ER和GEP （P<0.05），增加30%降水量显著提高了三者（P<0.05）。相同降水量条件下，氮添加不同程度地提高了NEE、ER和GEP，且其效应在增加50%降水量时较为明显。净生态系统碳吸收（-NEE）、ER和GEP与群落生物量、牛枝子（Lespedeza potaninii）以及草木樨状黄芪（Astragalus melilotoides）生物量正相关。三者亦随Patrick丰富度指数和Shannon-Wiener多样性指数的增加而增加。本文结果意味着，减少降水量降低了土壤水分和养分有效性、抑制了植物生长，从而降低了生态系统碳交换。适量增加降水量则可能通过提高土壤含水量、刺激土壤酶活性、调节土壤C ： N ： P平衡特征等途径，促进了植物生长和物种多样性，从而提高了生态系统碳汇功能；氮添加亦促进了生态系统碳交换，但其与降水的交互作用尚不明显，需通过长期观测进行深入探讨。;The change in precipitation pattern and increase in atmospheric nitrogen deposition are two important aspects of global change. Both of them closely relate to soil resource availabilities, plant growth, microbial activity, etc., and further posing profound influences on the carbon dynamics in plant-soil systems. Being one of main grassland ecosystem types in northwestern China, desert steppe is limited by soil water and nitrogen availabilities and thus is sensitive to the alterations in precipitation pattern and nitrogen deposition. However, the studies on how the carbon dynamics in desert steppes respond to the two global change aspects are still lacked, especially in those located in Ningxia, northwestern China. To deeply understand the influencing mechanisms of the alterations in precipitation pattern and nitrogen deposition on the ecosystem carbon exchanges in desert steppes, a field experiment was conducted in a desert steppe of Ningxia in 2017. The experiment involved five precipitation treatments (50% reduction, 30% reduction, natural, 30% increase, and 50% increase) and two nitrogen addition treatments (0 and 5 g m^-2 a^-1). The temporal dynamics of net ecosystem carbon exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP) were monitored from May to October of 2019. Their relationships with plant community composition and soil properties were analyzed as well. The daily and monthly dynamics of NEE, ER, and GEP increased first and then decreased. NEE was shown as net ecosystem carbon absorption during the whole growing season. Under 0 and 5 g m^-2 a^-1 of nitrogen addition, the decreasing precipitation significantly reduced NEE, ER, and GEP (P<0.05), while 30% increase in precipitation significantly promoted the three indices (P<0.05). Between the same precipitation treatments, nitrogen addition also greatly increased the three indices, especially under the treatment of 50% increase in precipitation. The net ecosystem carbon absorption (represented as-NEE), ER, and GEP were positively related with plant community biomass, Lespedeza potaninii population biomass, and Astragalus melilotoides population biomass. The three indices also enhanced with the increase of Patrick richness index and Shannon-Wiener diversity index. The results above indicate that decreasing precipitation reduces soil water and nutrient availabilities, inhibits plant growth, and thus limiting ecosystem carbon exchange. An appropriate increase in precipitation can promote plant growth and species diversity through increasing soil water content, stimulating soil enzyme activities, regulating soil C:N:P stoichiometric balance, etc., consequently improving the ecosystem carbon sink function. Nitrogen addition also promotes ecosystem carbon exchange. However, its interaction with precipitation is not clear after nearly 3-year experimental treatment. Therefore, a long-term observation is needed for further deeply exploring. The results of this paper will provide data supports for the global networking experiment on ecosystem carbon cycle under global change.

Divergent forcing of water use efficiency from aridity in two meadows of the Mongolian Plateau

The future climate change predicts higher drought risks in deteriorating the functions and services of dryland ecosystems. Interpreting the sensitivity of ecosystem water use efficiency (WUE) to aridity could provide important environmental implications to water conservation and carbon budgets of drylands. The effects of temporal aridity changes on WUE in semi-arid ecosystems, particularly in meadow steppes, have been poorly investigated. In this study, we used eddy-covariance data from two representative temperate meadows on Mongolian Plateau, Changling (2007-2015, China) and Bayan (2014-2018, Mongolia), to understand the inter- and intra-annual changes of WUE by evaporative fraction (EF), as well as the climatic and biotic controls on gross ecosystem production (GEP), evapotranspiration (ET) and partitioned evaporation (T) and transpiration (E). Results showed that the response of annual WUE to EF was positive in Changling meadow with dry atmosphere but insignificant in Bayan with cold climate; the contrasting WUEs were driven by divergent GEP and ET responses to EF in two meadows. Monthly T/ET increased linearly with EF in both meadows, but logarithmically with leaf area index (LAI). Bayan and Changling meadows had a similar instantaneous WUE (WUEp, GEP/T) that were insensitive to EF change. We also found that the frequency of growing season precipitation affected WUE at both sites. We conclude that contrasting GEP responses to EF determined the meadow WUE variations. The magnitude and changes in transpiration and evaporation provide further insights on the biophysical regulations (i.e., LAI) of ecosystem water use and WUEp. This study indicated that differences in ecosystem water yield, soil water status and sensitivities of GEP and ET partitioning jointly contributed to the divergent WUE at the two meadow steppes in this semi-arid region.

Empirical evidence for resilience of tropical forest photosynthesis in a warmer world.

Tropical forests may be vulnerable to climate change1-3 if photosynthetic carbon uptake currently operates near a high temperature limit4-6. Predicting tropical forest function requires understanding the relative contributions of two mechanisms of high-temperature photosynthetic declines: stomatal limitation (H1), an indirect response due to temperature-associated changes in atmospheric vapour pressure deficit (VPD)7, and biochemical restrictions (H2), a direct temperature response8,9. Their relative control predicts different outcomes-H1 is expected to diminish with stomatal responses to future co-occurring elevated atmospheric [CO2], whereas H2 portends declining photosynthesis with increasing temperatures. Distinguishing the two mechanisms at high temperatures is therefore critical, but difficult because VPD is highly correlated with temperature in natural settings. We used a forest mesocosm to quantify the sensitivity of tropical gross ecosystem productivity (GEP) to future temperature regimes while constraining VPD by controlling humidity. We then analytically decoupled temperature and VPD effects under current climate with flux-tower-derived GEP trends in situ from four tropical forest sites. Both approaches showed consistent, negative sensitivity of GEP to VPD but little direct response to temperature. Importantly, in the mesocosm at low VPD, GEP persisted up to 38 °C, a temperature exceeding projections for tropical forests in 2100 (ref. 10). If elevated [CO2] mitigates VPD-induced stomatal limitation through enhanced water-use efficiency as hypothesized9,11, tropical forest photosynthesis may have a margin of resilience to future warming.

Drought-induced shift from a carbon sink to a carbon source in the grasslands of Inner Mongolia, China

Precipitation has an impact on both gross ecosystem productivity (GEP) and ecosystem respiration (Reco), which ultimately influences net ecosystem productivity (NEP). A positive NEP denotes an ecosystem functioning as a carbon sink; whereas, a negative NEP denotes an ecosystem functioning as a carbon source. Therefore, drought plays an important role in the carbon balance of an ecosystem. However, little is known about the point at which the ecosystem converts from a carbon sink to a carbon source in extreme droughts. Such knowledge is crucial for predicting terrestrial carbon cycling under climate change, and consequently, was the subject of this study. We imposed two types of drought treatments on desert-grassland: (1) press-drought, in which the quantity of natural precipitation was reduced by 66% from May to August; and (2) pulse-drought, in which the quantity of natural precipitation was reduced by 100% during June and July. Reco and NEP were measured and GEP was calculated and then regression analyses were employed to determine the point at which the carbon sink shifts to a carbon source. The regression equation of NEP (μmol·m−2·s−1) on Reco (μmol·m−2·s−1) took the form: NEP = 0.504 Reco – 0.086 and, consequently, when Reco equaled 0.171 μmol·m−2·s−1, there was zero change in the carbon sink and GEP also equaled 0.171 μmol·m−2·s−1. Below this value, the ecosystem functioned as a carbon source, whereas above this value, the ecosystem functioned as a carbon sink. Structural equation models (SEM) demonstrated that coverage, standing biomass, pH, soil water content and total soil carbon were the main driving factors on desert grassland ecosystem carbon fluxes.

The Impact of Seasonal and Annual Climate Variations on the Carbon Uptake Capacity of a Deciduous Forest Within the Great Lakes Region of Canada.

In eastern North America, many deciduous forest ecosystems grow at the northernmost extent of their geographical ranges, where climate change could aid or impede their growth. This region experiences frequent extreme weather conditions, allowing us to study the response of these forests to environmental conditions, reflective of future climates. Here we determined the impact of seasonal and annual climate variations and extreme weather events on the carbon (C) uptake capacity of an oak‐dominated forest in southern Ontario, Canada, from 2012 to 2016. We found that changes in meteorology during late May to mid‐July were key in determining the C sink strength of the forest, impacting the seasonal and annual variability of net ecosystem productivity (NEP). Overall, higher temperatures and dry conditions reduced ecosystem respiration (RE) much more than gross ecosystem productivity (GEP), leading to higher NEP. Variability in NEP was primarily driven by changes in RE, rather than GEP. The mean annual GEP, RE, and NEP values at our site during the study were 1,343 ± 85, 1,171 ± 139, and 206 ± 92 g C m−2 yr−1, respectively. The forest was a C sink even in years that experienced heat and water stresses. Mean annual NEP at our site was within the range of NEP (69–459 g C m−2 yr−1) observed in similar North American forests from 2012 to 2016. The growth and C sequestration capabilities of our oak‐dominated forest were not adversely impacted by changes in environmental conditions and extreme weather events experienced over the study period.

Global change alters peatland carbon cycling through plant biomass allocation

Global change is shown to significantly affect the C storage function of peatlands; however, a majority of previous research is focused on a single environmental stressor such as the increased temperature. As a result, little is known about the interactive effect of multiple environmental stressors on peatland C storage, especially in sedge-dominated fen peatlands. We performed a full factorial experiment of increased temperature and elevated atmospheric CO2 concentration on minerotrophic, sedge-dominated fen monoliths to experimentally examine the individual and interactive effects of simulated future climate conditions on peatland plant biomass, CO2 exchange, and pore water dissolved organic carbon (DOC) over one full growing season. Our study demonstrates that warming and elevated atmospheric CO2 significantly increased aboveground and belowground biomass, respectively, as well as the gross ecosystem production (GEP), while the DOC concentrations and respired CO2 from peatland soils only increased under warming Our results suggest that global change will increase both plant production and microbial decomposition, but with altered plant biomass allocation between aboveground and belowground. Our study provides experimental evidence for shifts in ecosystem-level carbon dynamics under global change for a sedge-dominated peatland, and suggests that while carbon stores may weaken, the carbon sink will be maintained in these types of northern peatlands if hydrological conditions are largely maintained.

Predominant role of soil moisture in regulating the response of ecosystem carbon fluxes to global change factors in a semi-arid grassland on the Loess Plateau

Climate warming, altered precipitation and nitrogen deposition may critically affect plant growth and ecosystem carbon fluxes. However, the underlying mechanisms are not fully understood. We conducted a 2-yr, multi-factor experiment (warming (W), altered precipitation (+30% and − 30%) and nitrogen addition (N)) in a semi-arid grassland on the Loess Plateau to study how these factors affect ecosystem carbon fluxes. Surprisingly, no interactive effects of warming, altered precipitation and nitrogen addition were detected on parameters of ecosystem carbon fluxes, including net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), gross ecosystem productivity (GEP) and soil respiration (SR). Warming marginally reduced NEE and GEP mainly due to its negative effects on them in July and August. Altered precipitation significantly affected all parameters of carbon fluxes with precipitation reduction decreasing NEE, ER and GEP, whereas precipitation addition increasing SR. In contrast, nitrogen addition had little effect on any parameters of carbon fluxes. Soil moisture was the most important driver and positively correlated with ecosystem carbon fluxes and warming impacted ecosystem carbon fluxes indirectly by decreasing soil moisture. While plant community cover did not show significant association with carbon fluxes, semi-shrubs cover was positively related to NEE, ER and GEP. Together, these results suggest that soil water availability, rather than soil temperature and nitrogen availability, may dominate the effect of the future multi-faceted global changes on semi-arid grassland carbon fluxes on the Loess Plateau.

Linking diffuse radiation and ecosystem productivity of a desert steppe ecosystem.

Radiation components have distinct effects on photosynthesis. In the desert steppe ecosystem, the influence of diffuse radiation on carbon fixation has not been thoroughly explored. We examined this diffusion and its effect on ecosystem productivity was examined during the growing season from 2014 to 2015 on the basis of eddy covariance measurements of CO2 exchange in a desert steppe ecosystem in northwest China. Our results indicated that the gross ecosystem production (GEP) and diffuse photosynthetically active radiation (PARdif) peaked when the clearness index (CI) was around 0.5. The maximum canopy photosynthesis (Pmax) under cloudy skies (CI < 0.7) was 23.7% greater than under clear skies (CI ≥ 0.7). When the skies became cloudy in the desert steppe ecosystem, PARdif had a greater effect on GEP. Additionally, lower vapor pressure deficits (VPD ≤ 1 kPa), lower air temperatures (Ta ≤ 20 °C), and non-stressed water conditions (REW ≥ 0.4) were more conducive for enhanced ecosystem photosynthesis under cloudy skies than under clear skies. This may be due to the comprehensive effects of VPD and Ta on stomatal conductance. We concluded that cloudiness can influence diffuse radiation components and that diffuse radiation can increase the ecosystem production of desert steppe ecosystems in northwest China.

Carbon fluxes response of an artificial sand-binding vegetation system to rainfall variation during the growing season in the Tengger Desert

Revegetation is considered as an effective approach for desertification control, and artificial sand-binding vegetation exerts a significant contributor to carbon cycling in arid and semiarid regions; however, this is largely determined by the rainfall regime. We measured carbon fluxes (the net ecosystem CO2 exchange (NEE), the gross ecosystem productivity (GEP) and the ecosystem respiration (Reco)) during the growing season of 2014–2016 using the eddy covariance technique and explored the effects of rainfall variables (amount, timing distribution and pulse size) and environmental factors on carbon fluxes at different time scales. The system had NEE values of −117.5 and −98.9 g C m−2 during the growing seasons of 2015 (dry year) and 2016 (wet year), respectively. When the rainfall amount did not differ significantly between spring and autumn, the cumulative GEP was greater in spring than in autumn, whereas the cumulative Reco and NEE showed the opposite pattern. Small (<5 mm) rain events failed to trigger obvious GEP and NEE pulses, whereas ≥ 5 mm or a series of small rain events led to high assimilation but with hysteresis. The magnitude of Reco increased as the rain pulses increased. The Random Forest (RF) algorithm revealed that soil water contents had a great impact on carbon fluxes at different integration periods. A correlation analysis showed that the soil water contents were positively correlated with GEP and Reco and negatively correlated with NEE over different time scales in most cases. These findings suggest that artificial vegetation not only improves habitat restoration but is a significant carbon sink during both dry and wet growing season, which is likely to supplement our knowledge gap to accurately evaluate the current carbon budget in dry land.

Carbon flux and soil organic carbon content and density of different community types in a typical steppe ecoregion of Xilin Gol in inner Mongolia, China

Carbon fluxes and contents are integral components of the global carbon cycle. The photosynthetic and respiratory rates were measured at the levels of leaves and canopies for different community types in a typical steppe ecoregion in Xilin Gol, Inner Mongolia, Northern China. Soil Organic Carbon (SOC) content and density and above-biomass were surveyed in each community type. The results show: 1) thicketized Caragana microphylla and Achnatherum splendens communities exhibit higher above-ground biomass, Maximum Net Photosynthesis Rate (Pnmax), Gross Ecosystem Productivity (GEP), Net Ecosystem Exchange (NEE), and Ecosystem Respiration (ER) (p < 0.05) than other communities; 2) degraded Artemisia frigida community contained significantly lower biomass, Pnmax, and NEE (p < 0.05), but significantly higher ER (p < 0.05) than the native Leymus chinensis community, 3) degraded Allium polyrhizum community exhibited the lowest levels of all measured indices (p < 0.05); 4) thicketized and degraded communities exhibited significantly lower SOC content and densities than the L. chinensis community (p < 0.05). The research suggests that the typical steppe communities undergoing shrub encroachment or degradation may ultimately lead to decreases in SOC content and densities by as much as 15–30% and weakened carbon sink functioning of typical steppe ecosystems.

Winter CO2 Efflux From Sagebrush Shrublands Distributed Across the Rain‐to‐Snow Transition Zone

AbstractThe projected shifts in winter weather and snowpack conditions are expected to impact carbon storage in western U.S. rangelands. Sagebrush shrublands comprise much of the western United States, yet contribution of winter CO2 efflux to the overall carbon budget of these ecosystems remains uncertain. We explored factors controlling winter CO2 efflux measured using eddy covariance at five sagebrush‐dominated sites along an elevation/climate transect extending from 1,425 to 2,111 m. Results showed that winter CO2 efflux was modest but had important impacts on annual carbon budgets, and its impact increased in high‐elevation, snow‐dominated ecosystems compared to low, rain‐dominated ones. Observed cumulative winter CO2 efflux accounted for 8–30% of annual gross ecosystem production (GEP) and roughly approximated annual net carbon uptake. Omission of winter periods would have increased net uptake by 1.5 to 2.2 times. Within‐site variability in observed 30‐min winter CO2 efflux was related to soil temperature and moisture. Between‐site variability was attributed to available carbon stocks, including soil organic carbon and the previous year's GEP. At low elevations, lack of snow cover to insulate soil from freezing, coupled with lower carbon stocks, limited CO2 efflux. Conversely, large carbon stocks and deep snowpack that prevented soil freezing at high elevation led to increased CO2 efflux. These results show how climate and biota exert strong controls on winter ecosystem respiration and extend our understanding of how state factors influence winter CO2 efflux. Collectively, our findings suggest that an upward climatic shift in the rain‐to‐snow transition elevation may alter the carbon budget of sagebrush shrublands.

中国亚热带-暖温带过渡区锐齿栎林净生态系统碳交换特征

在2017年1月1日-2017年12月31日期间，采用涡度相关法对位于亚热带-暖温带气候过渡区的河南宝天曼国家级自然保护区的65年生锐齿栎（Quercus aliena）天然次生林的碳通量进行了连续观测。结果表明：在观测期间，该森林生态系统在生长季5-10月份为碳汇，非生长季各月为碳源，净碳吸收量与释放量分别在7月和4月达到最大。净生态系统生产力为569.4 g C m^-2a^-1，生态系统呼吸为529.9 g C m^-2a^-1，总生态系统生产力为1099.3 g C m^-2a^-1。30min尺度上夜间净生态系统碳交换量与5cm深度土壤温度的关系可用指数方程表示（R²=0.21，P < 0.001），其温度敏感性系数（Temperature sensitivity coefficient，Q₁₀）为2.2。如果排除夜间通量观测的误差，处在海拔较高地区的夜间低温和非生长季的低温抑制了生态系统呼吸排放，可能导致全年生态系统呼吸量较低。在生长季5-10月份，各月的白天净生态系统碳交换量对光合有效辐射的响应符合直角双曲线模型，初始光能利用效率、平均最大光合速率和白天平均生态系统呼吸强度呈明显的季节变化，范围分别是0.06-0.12 μmol CO₂ μmol^-1 photon、0.44-1.47 mg CO₂ m^-2s^-1和0.07-0.19 mg CO₂ m^-2s^-1。夏季7、8月份，较高的饱和水汽压差对白天锐齿栎林的碳吸收有明显的抑制作用；生长季末期9月份较高的土壤含水量对白天锐齿栎林的碳吸收也产生了抑制作用，表明生长末期降水过多影响森林的碳吸收。;Oak forests comprise the largest forest area in central China and are the potential carbon sink, while we know little about the carbon dioxide flux of oak forests in the transitional zone from subtropics to warm temperate, China. Using an open-path eddy covariance system and micro-climate instruments, the CO₂ flux, photosynthetic active radiation (PAR), air temperature, soil temperature and precipitation were simultaneously observed in a natural oak (Quercus aliena) forest at Baotianman National Nature Reserve. Based on the data sets during January to December 2017, dynamic change of CO₂ flux at different temporal scales and its underlying mechanism were analyzed. The results indicated that the diurnal and seasonal variations of CO₂ fluxes showed an obvious single peak pattern. The oak forest ecosystem was a carbon sink during the growing season (May-October), while a carbon source occurred during the non-growing season. Net carbon sequestration and emissions peaked in July and April, respectively. Mean annual net ecosystem productivity (NEP), ecosystem respiration (Re) and gross ecosystem productivity (GEP) were 569.4, 529.9 and 1099.3 g C m^-2 year^-1, respectively. The relationship between net ecosystem carbon exchange (NEE) measured at half-hour interval during night and soil temperature at depth of 5 cm can be expressed by an exponential equation (R²=0.21, P < 0.001), with its temperature sensitivity coefficient (Q₁₀) of 2.2. A low temperature at night and in the non-growing season at high elevation resulted in lower Re throughout the year. The relationship between NEE and PAR at daytime could be well expressed by a rectangular hyperbolic equation during growing seasons. Monthly initial light use efficiency, maximum photosynthetic capacity and daytime Re were 0.06-0.12 μmol CO₂ μmol^-1 photon, 0.44-1.47 mg CO₂ m^-2s^-1 and 0.07-0.19 mg CO₂ m^-2s^-1, respectively. Both higher vapour pressure deficit in July and August and higher soil moisture at the end of the growing season inhibited carbon uptake of the oak forest, indicating a negative effect of increased precipitation on carbon sequestration at the end of the growing season.