Net ecosystem carbon budget of a grassland ecosystem in central Qinghai-Tibet Plateau: integrating terrestrial and aquatic carbon fluxes at catchment scale
Graphical abstract
Introduction
Complete quantifications of all the input and output components of carbon in terrestrial ecosystems are fundamental for improving our understanding of carbon cycle and climate system evolution (Field et al., 2007; Heimann and Reichstein, 2008). Covering up to 40% of the global land surface, grassland ecosystems are globally crucial modulators on affecting the carbon cycle and climate. Model efforts and eddy covariance technique showed that most grasslands have gross primary production (GPP) exceed ecosystem respiration (Re) and thus act as carbon sinks (Keenan and Williams, 2018; Lin et al., 2017; Scurlock and Hall, 1998, Smith, 2014; Yan et al., 2015). Although eddy covariance is a powerful technique to quantify the net ecosystem exchange (NEE) between land and atmosphere carbon pools, the net ecosystem carbon budget (NECB) is a more complete framework to assess the net carbon budget for certain time and spatial scales (Chapin et al., 2006; Webb et al., 2019). The core concept of NECB is the inclusion of all physical, biological, and anthropogenic carbon sources and sinks for ecosystem carbon balance. NECB provides a simple and useful framework for quantifying the terrestrial ecosystem carbon budget (Keenan and Williams, 2018). For natural ecosystems, NECB often refers to the integrating of both terrestrial and aquatic carbon pathways.
Aquatic carbon fluxes are increasingly recognized as quantitatively important components of regional and global carbon budgets (Cole et al., 2007; Drake et al., 2018). The terrestrial and aquatic systems are intimately coupled with carbon, water, and energy fluxes. The leaching, erosion, export, and emission of various species of carbon via aquatic systems can inevitably affect the carbon balance of terrestrial ecosystems (Butman et al., 2016; Chapin et al., 2006; Ciais et al., 2008). However, the conventional ecosystem carbon budget research either considered the terrestrial carbon exclusively (Jin et al., 2015; Wang et al., 2017a) or considered only limited species of aquatic carbon (Kindler et al., 2011). The total carbon export by aquatic pathways has rarely been accounted into ecosystem scale carbon balance due to the long-standing uncertainties in aquatic carbon fluxes (Webb et al., 2019). These gaps may lead to inaccuracy of ecosystem carbon budget estimations and overestimations of terrestrial carbon sinks. To gain a better understanding of how ecosystem processes interact with carbon cycle, integrated ecosystem carbon budgets with both terrestrial and aquatic carbon flows are needed (Butman et al., 2016; Webb et al., 2019).
The study of NECB has only been emerging in the recent decade. In the northern peatland, water-borne carbon has been recognized as a critical and variable component of the ecosystem carbon cycle (Campeau et al., 2017; Nilsson et al., 2008; Olefeldt et al., 2012). In some peatland ecosystems, the export of dissolved organic carbon (DOC) can substantially decrease the NECB by 24–35% (D'Acunha et al., 2019; Dinsmore et al., 2010). CO2 emission from peatland streams was also a significant carbon flux on affecting NECB (Wallin et al., 2013). A study in European grassland showed that leaching of biogenic dissolved carbon can account for 5–98% of NEE (Kindler et al., 2011), suggesting the importance of aquatic carbon on offsetting the ecosystem carbon balance. In a recent synthesis, Webb et al. (2019) highlight that the aquatic carbon fluxes account higher proportion on offsetting NECB in ecosystems with smaller net ecosystem production (NEP) than ecosystems with larger NEP. This point that the aquatic carbon may play more important roles in NECB of grassland since their low NEP compare to other ecosystems.
The Qinghai-Tibet Plateau (QTP) is the highest plateau in the world with fragile grassland and permafrost ecosystems that sensitive to global climate change. Currently, the QTP grassland acts as a net carbon sink (Jin et al., 2015; Yan et al., 2015). The warming-induced vegetation greening (Zhang et al., 2014) may enhance the carbon assimilation by grassland ecosystems (Piao et al., 2012). In addition, the QTP grassland absorbs methane (CH4) according to direct observations (Chen et al., 2017a,b; Wei et al., 2015), which can enhance the carbon sink for grassland ecosystems. On the other hand, the river networks in QTP can export substantial flux of lateral carbon (Song et al., 2019, 2020). The supersaturation of CO2 and CH4 in lakes and rivers can lead to carbon emissions from aquatic systems to the atmosphere (Qu et al., 2017a; Yan et al. 2018). These carbon fluxes via aquatic conduits represent carbon losses to the ecosystem, which may counterbalance the grassland carbon sink. Although a few studies reported net carbon sink of QTP grassland through eddy covariance method (Kato et al., 2004, 2006; Wang et al., 2017a), these studies disregarded the loss of carbon by aquatic pathways, which lead to uncertainties on whether the QTP grassland is a carbon sink or a carbon source. As the climate warms and permafrost degrades (Yang et al., 2010), both terrestrial and aquatic carbon cycling processes may change (Schuur and Mack, 2018). Thus, there is an urgent need to account all carbon flux components and assess the NECB of QTP grassland.
In this study, we selected a typical catchment with a grassland ecosystem in the central QTP permafrost region. We used eddy covariance data to derive GPP, Re, and NEP in growing seasons. We also utilized previous in situ observation results of CH4 and non-growing season Re (Chen et al., 2017a,b; Zhang et al. 2017a) to obtain the annual terrestrial carbon budget. In addition, we calculated the annual aquatic carbon flux, including DOC, biogenic dissolved inorganic carbon (DIC), particulate carbon (PC), and CO2 emission fluxes, of the catchment. Our study sought to advance the understanding of carbon cycle in grassland ecosystems by the following aspects: (1) quantify the annual NECB with the inclusion of aquatic carbon fluxes; and (2) assess to what extent the aquatic carbon fluxes can affect the NECB.
Section snippets
Study site
This study was conducted at the Fenghuoshan (FHS) catchment in the central QTP with permafrost-underlain grassland (Fig. 1). The catchment is a tributary of the Tongtian River that represents the headwater of the Yangtze River. The total watershed drainage area is 117.0 km2. The elevation of the watershed ranges from 4720 to 5392 m above sea level. The study area has continuous permafrost distribution with thicknesses of 80 to 120 m, while active layer thicknesses range from 1.3 to 2.5 m (
Terrestrial carbon fluxes
The diurnal patterns of NEE based on EC flux measurements are shown in Fig. 2. Net CO2 uptake usually occurred between 8:00 and 18:00 of the day, with peak CO2 uptake occurred around 12:00. The diurnal NEE variation magnitude differed by months, with high diurnal CO2 uptake levels in warm months (e.g. July and August) and low or even net CO2 emission (positive NEE) levels in frozen months (e.g. May and October). The mean half-hour NEE for May, June, July, August, September, and October are
Terrestrial and aquatic carbon
The EC-derived ecosystem scale carbon accumulation depends on the difference between GPP and Re, both of which are controlled by the ecosystem processes. GPP is controlled by environmental variables including solar radiation, temperature, and water and nutrients availability (Chapin et al., 2011). The high GPP in summer is expected since the ideal environmental conditions for photosynthesis and biomass growth. The Re also peaked in summer (Fig. 3) due to the warm temperature-enhanced
Conclusions and implications
By accounting all known carbon pathways in a grassland ecosystem in central QTP, we provide an integrative characterization of the terrestrial carbon, aquatic carbon, and NECB at the catchment scale. The NEP, TAC, and NECB have strong seasonality with peak levels in summer. The catchment flipped from a carbon sink in GS to a carbon source in NGS. We found that TAC offset 14% of the net carbon assimilation for growing season and 30% of the annual net carbon assimilation by land after subtracting
Acknowledgements
This work was supported by the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (no. 2019QZKK0304), the National Natural Science Foundation of China (no. 41890821), and the China Postdoctoral Science Foundation (no. 2019M663566). We thank the reviewers for their important, thoughtful, and detailed comments that greatly improved this paper. Data used in this study is available from the corresponding author upon reasonable request.
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2022, Ecological IndicatorsCitation Excerpt :Net ecosystem CO2 (C) exchange (NEE) studies in grassland ecosystems have helped to confirm the balance of the amount of carbon sinks in soil and the volume of carbon sources of CO2 in atmosphere (Wofsy et al., 1993; Oechel et al., 1993; Kato et al., 2004). The amount of carbon sinks is determined by plants during photosynthetic activity processes (gross primary productivity, GPP) (Kuzyakov, 2006; paterson et al., 2009; Moinet et al., 2016; Sun et al., 2021b) and the volume of carbon sources is determined by plants (plants respiration, Rplant) and microorganism (heterotrophic respiration, Rh) during their respiration processes (ecosystem respiration, Reco) (Piao et al., 2009, Hao et al., 2017; Song et al., 2020). So, NEE are closely related to those processes mentioned above (Nakano et al., 2008; Du and Liu, 2013; Chen et al., 2013, Chen et al., 2016).