Shallow groundwater inhibits soil respiration and favors carbon uptake in a wet alpine meadow ecosystem

Highlights

• Wet alpine meadow ecosystems (WAME) act as a significant carbon sink.

• Ecosystem respiration was significantly decreased when soil was nearly-saturated.

• Shallow groundwater inhibits soil respiration and favors carbon uptake of WAME.

• Warming affects carbon dynamics of WAME through changing soil hydrology.

Abstract

Wet alpine meadows generally act as a significant carbon sink, since their low rate of soil decomposition determines a much smaller ecosystem respiration (Re) than photosynthesis. However, it remains unclear whether the low soil decomposition rate is determined by low temperatures or by nearly-saturated soil moisture. We explored this issue by using five years of measurements from two eddy-covariance sites with low temperature and significantly different soil water conditions. The results showed that both sites were carbon sinks. However, despite a smaller annual gross primary productivity, the wet site with a shallow groundwater showed a much higher carbon use efficiency and larger carbon sink than the dry site (which had a deeper water table) due to its much lower Re. Our analyses showed that Re of the wet site was significantly decreased under the nearly-saturated soil condition during the unfrozen seasons. This effect of nearly-saturated soil water on Re increased with soil depths. In contrast, at the dry site the high soil water content favored Re. The corresponding soil temperature at both sites expectedly showed large and positive effects on Re. These results demonstrated that the high carbon sink of the wet alpine meadow was mainly caused by the inhibiting effects of the nearly-saturated soil condition on soil respiration rather than by the low temperatures. Therefore, we argue that a warming-induced shrinking cryosphere may affect the carbon dynamics of wet and cold ecosystems through changes in soil hydrology and its impact on soil respiration. In addition, our study highlights the different responses of soil respiration to warming across soil depths. The thawing of frozen soil may cause larger CO₂ emission in the top soil, while it may also partially contribute to slowing down soil carbon decomposition in the deep soil through decreasing metabolic activity of aerobic organisms.

Introduction

Alpine meadows are sensitive to climate change and human activities due to their extremely cold and fragile environment (Marcolla et al., 2011; Hu et al., 2016; Knowles et al., 2015; Piao et al., 2012). These ecosystems generally act as a carbon sink, since the low soil respiratory (SR) determines a much smaller ecosystem respiration (Re) than photosynthesis (Li et al., 2019; Marchin et al., 2018; Scholz et al., 2018; Wang et al., 2017). The low SR of the ecosystems is usually considered to be primarily determined by the low temperature (Saito et al., 2009). Thus, the carbon sink of the ecosystems is expected to be reduced and potentially turns into a carbon source, due to the large increases in SR carbon losses in response to future warming (Ganjurjav et al., 2018; Sun et al., 2019). According to the soil hydrologic condition, alpine meadows can be divided into wet alpine meadows and dry alpine meadows. Wet alpine meadows are saturated with water during much of the growing seasons, mostly exist a seasonally shallow water table level (WTL), and may be flooded soon after snowmelt. Dry alpine meadows may experience intermittent flooding, but are well-drained and have a deeper WTL than wet alpine meadows (Halsey, 2017; Rocchio, 2006). Except for the direct impacts of warming, the soil carbon dynamics of wet alpine meadows may be also significantly affected by the changing soil hydrologic conditions induced by climate warming and human activities (Zhang et al., 2018; Lv et al., 2020), like the situations occurred in several cold and wet ecosystems such as Arctic tundra, northern peatlands, mires, and bogs (Ise et al., 2008; McVeigh et al., 2014; Drollinger et al., 2019; Korkiakoski et al., 2019). However, the mechanisms of the soil hydrological conditions, such as the shallow WTL, on affecting carbon exchanges in wet alpine meadows are poorly understood. To maintain the carbon sink capacity of the ecosystems and predict their potential changes in response to future warming, it is critical to better understand this eco-hydrological coupling process.

A shallow WTL can largely affect carbon exchanges of cold and wet ecosystems by controlling oxygen diffusion, enzyme dynamics, and osmoregulation in the soil (Moyano et al., 2013; Yan et al., 2018). For example, in northern peatlands a shallow WTL reduced the decomposition rate of soil organic carbon (Bubier et al., 2003; Korkiakoski et al., 2019), and thus resulted in a high sensitivity of peat decomposition to climate change through WTL feedbacks (Ise et al., 2008; Luan et al., 2018). The changing soil water content from melting permafrost largely affects permafrost carbon dynamics (Walvoord & Kurylyk, 2016). For tundra ecosystem, an increase in WTL can strongly lower SR by reducing soil oxygen availability and thus decreases Re; a decreased WTL increases both gross primary productivity (GPP) and Re, while it negatively affects the ecosystem's carbon uptake because the increases in Re are greater than that in photosynthesis (Olivas et al., 2010). In addition, both measurements and model simulations demonstrated that the net ecosystem exchange (NEE) of several mire and bog ecosystems is controlled by the interactions between WTL and temperature (Yurova et al., 2007; McVeigh et al., 2014).

The Tibetan Plateau (TP) is the highest plateau in the world, with an average elevation of 4000 m above sea level and an area covering about 2.5 × 10⁶ km² (Yang et al., 2014). It is known as the “Asian Water Tower” because of its large water storage (Immerzeel et al., 2010). The alpine meadows and grassland are the dominant ecosystem over the TP, covering more than 50% of the whole TP (Liu et al., 2010; Gao et al., 2013). Wet alpine meadows often occur in upstream watersheds as well as adjacent areas, valley bottoms, lakes, and rivers on the TP (Brierley et al., 2015). The TP has been experiencing significantly warming during recent decades (Gao et al., 2015; Guo et al., 2019). The groundwater level in the wet regions on the TP is usually shallow and is significantly changing with climate warming (Wu et al., 2013). Indeed, both a warming climate and increasing anthropogenic activities have caused significant changes in carbon exchanges of the alpine meadows over the TP (Piao et al., 2012). Many studies have investigated the relationships between carbon exchanges and environmental factors of the alpine meadows (Chen et al., 2015; Hu et al., 2016; Fu et al., 2018; Ganjurjav et al., 2018; Liu et al., 2018a; Zhang et al., 2018; Lv et al., 2020; Yu et al., 2020). These studies mainly focused on the effects of temperature, precipitation, soil moisture (SM), soil temperature (ST), and grazing on NEE, GPP, and Re, but the effects of shallow WTL were rarely considered. As several previous studies demonstrated that the nearly-saturated soil water in cold regions such as Arctic tundra (Olivas et al., 2010) and boreal peatland (Korkiakoski et al., 2019) can decrease the metabolic activity of aerobic organisms and thus negatively affect SR. Based on these knowledges, we hypothesize that the high carbon use efficiency (CUE, defined as the ratio of net ecosystem productivity to GPP) and large carbon sink of the wet alpine meadows on the TP is determined by the inhibiting effects of the shallow WTL on SR (which further on Re) rather than by the low temperature. To test this hypothesis, we analyzed five years of measurements from two sites in the northeastern TP with low temperature and significantly different soil water conditions.