Varying soil respiration under long-term warming and clipping due to shifting carbon allocation toward below-ground

https://doi.org/10.1016/j.agrformet.2021.108408Get rights and content

Highlights

  • Rh and Ra are highly sensitive to warming and clipping.

  • Increased allocation of C toward below-ground cause the increasing in Ra and Rs.

  • Due to the changes in ANPP/BNPP and ST, warming effect vary largely with years.

  • Plant C allocation is an emerging property in determining Rs under warming.

Abstract

Global warming and land-use change have profound impacts on soil respiration (Rs), with consequent changes in carbon exchange between terrestrial ecosystem and atmosphere and its feedback to climate change. However, it is not clear how soil respiration and its components (heterotrophic respiration, Rh; and autotrophic respiration, Ra) respond to long-term warming and clipping in alpine meadow, and what are the controlling factors for the responses of soil respiration. To answer these questions, we conducted a field experiment of warming and clipping in an alpine meadow on the Qinghai-Tibet Plateau from 2014 to 2019. Infrared heaters were used to simulate climate warming and plots were clipped once a year to mimic hay or biofuel harvest. The results showed that warming decreased Rh by 5.06% but stimulated Ra by 10.08% across years, while clipping enhanced Rs, Rh and Ra by 12.31%, 18.65% and 1.29%, respectively, and marginally interacted with warming in impacting Ra across the years. Furthermore, the warming effect on soil respiration and its components varied largely over years, which was mostly attributable to the changes in the ratio of above-ground to below-ground net primary productivity (ANPP/BNPP, A/B) and soil temperature. Increased allocation of photosynthate toward below-ground caused the gradual increase in Ra and Rs under long-term warming. While soil temperature changes explained the variation of warming effects on Rh across years. The results demonstrated that soil respiration and its components in the alpine meadow are highly sensitive to climate warming and hay harvest but with large interannual variations in their responses. We highlight that plant carbon allocation is an emerging property in determining soil respiration under climate warming.

Introduction

Soil respiration (Rs) is the second largest carbon (C) flux in terrestrial ecosystem (Ben and Allison, 2010; Raich and Potter, 1995; Unwin, 1997), releasing nearly one twelfth of carbon dioxide (CO2) stock from the soil surface to the atmosphere through microbe and root respiration (Kristiina et al., 2014). As soil organic C content (1500 – 2400 Pg C) is over three times larger than atmosphere (589 – 750 Pg C) or plant biomass (450 – 650 Pg C) (Carvalhais et al., 2014; Lal, 2004; Pries et al., 2017), even a small change in Rs has considerable impacts on atmosphere CO2 concentration and terrestrial C-climate feedbacks (Jenkinson et al., 1991; Schimel et al., 1994). Although many filed experiments (Bronson et al., 2008; Carey et al., 2016; Luo et al., 2009; Luo et al., 2001) and modeling analyses (Friedlingstein et al., 2006; Jones et al., 2005; Raich et al., 2002) showed that Rs changes substantially with warming, the mechanisms underlying its responses are not well understood (Jones et al., 2003; Trumbore, 2010), especially for the responses of Rs components: autotrophic respiration (Ra, respiration of root, mycorrhizal and rhizosphere) and heterotrophic respiration (Rh, microbial decomposition of plant and soil organic matter) (Wang et al., 2019a).

Although partitioning Rs into Ra and Rh has received considerable attention, it is still a challenge to quantify these two processes under global climate change (Baggs, 2006). It has been reported that Rh is more sensitive to warming than Ra (Lin et al., 2001), while other study showed opposite result (Zhou et al., 2007). Furthermore, previous studies also reported that the response of Rs to temperature is controlled more by Ra than Rh (Lavigne et al., 2003) or Ra and Rh respond equally to temperature increase (Noh et al., 2017). As Ra and Rh have shown different responses to warming, understanding the differential controls of Ra and Rh would help to reveal the mechanisms underlying Rs in response and feedback to global warming (Zhou et al., 2010).

Soil respiration in response to warming varies largely across years. Warming initially stimulates Rs by enhancing decomposition (Lloyd and Taylor, 1994). However, the depletion of labile C in soil under higher microbial activity (Bradford et al., 2008; Eliasson et al., 2005) dampens the stimulation effect (Contosta et al., 2015), which means short-term (<3 years) responses of ecosystems and its processes to warming always differ from long-term (>3 years) responses (Bradford et al., 2008; Wang et al., 2019b). For example, long-term warming increases Rs (Pries et al., 2015), whereas Rs shows a neutral (Li et al., 2013), negative (Reynolds et al., 2015) or positive (Zhou et al., 2016) response to short-term warming in different ecosystems. Therefore, studies of long-term experiments are necessary to understand the responses of soil respiration and its components to global warming.

In grassland ecosystems, the responses of soil respiration to warming were always confounded with human disturbance, such as mowing or clipping for hay (Luo et al., 2009; Niu et al., 2013). Clipping substantially affects ecosystem C fluxes of grassland (Bahn et al., 2006) through influencing soil microclimate (Luo et al., 2010), photosynthetic activity (Anten and Ackerly, 2001) and nutrient cycling (Ross et al., 1999). Since soil temperature is usually higher and soil moisture is lower under warming and clipping than that under warming or clipping treatment alone, soil respiration is expected to change more under warming plus clipping treatment (Xu et al., 2014). However, a previous study has reported that clipping significantly enhances ecosystem C fluxes, but it does not significantly interact with warming in a tallgrass prairie ecosystem (Niu et al., 2013), suggesting that the effect of warming and clipping on ecosystem C fluxes are statistically independent. In addition, no interaction between warming and clipping was found in a 3-year experiment conducted in Tibetan plateau (Lin et al., 2011). On contrary, a field experiment conducted in Great Plain Apiaries suggested that warming and clipping interacted to affect soil microbial community interactively (Zhou et al., 2006). These contradictory results are probably due to that the responses of soil CO2 fluxes to warming and clipping depend on hydrological variations (Peng et al., 2014), precipitation distribution and summer severe drought (Zhou et al., 2007). Therefore, how warming interacts with clipping in changing soil respiration and its components remains highly uncertain.

Warming induced changes in net primary productivity (NPP), either positive (Rustad et al., 2001) or negative (Wu et al., 2011), is considered an important cause for soil respiration responses. For example, warming induced decrease in NPP reduces Ra and Rh through a decrease in the supply of photosynthate (BhupinderpalSingh et al., 2003). Below-ground net primary productivity (BNPP), which is more sensitive to warming than above-ground net primary productivity (ANPP) (Xu et al., 2012), represents more than half of primary productions in grasslands and plays an important role in providing organic matter to soil (Luo et al., 2009). Moreover, the ratio of ANPP to BNPP (A/B) is a central issue in plant ecology and evolution, which shows the strategy of plants or ecosystems to partition photosynthate in above-ground versus below-ground tissues (Vogt et al., 1998) under global climate change (McCarthy and Enquist, 2007). Understanding A/B dynamics is pivotal to improving our knowledge of C allocation and the driving factors in Rs. To date, however, little work has been devoted to exploring how the response of soil respiration and its components to warming relates to the C allocation, especially from long-term field experiments.

Qinghai-Tibet Plateau warms more rapidly than the global average and also is one of the most sensitive ecosystems to global climate change (Crowther et al., 2016; Ma et al., 2018). Grazing or clipping for hay harvest is the most prevalent land-use pattern in this region. However, the response of soil respiration and its components under both global warming and clipping has received less attention due to the tough environmental conditions in this ecosystem. In this study, we conducted a six-year warming and clipping field experiment in an alpine meadow ecosystem on the Qinghai-Tibet Plateau. The specific questions we addressed are (1) how climate warming interacts with clipping to change soil respiration and its components? (2) how the responses of soil respiration and its components change with years? and (3) how C allocation changes and relates to the responses of soil respiration and its components under long-term warming? Accordingly, we hypothesized that (1) clipping stimulates warming impact on soil respiration and its components, (2) long-term warming stimulates soil respiration and its components but the responses vary largely over years, and (3) changes in C allocation contribute to the responses of soil respiration.

Section snippets

Study site

The study was conducted in an alpine meadow on the eastern edge of Qinghai-Tibetan Plateau (32°48′N, 102°33′E), which is located in Hongyuan County, Sichuan province of China, at a mean altitude around 3500 m. The average annual precipitation is approximately 753 mm (1961-2013), of which more than 80% occurs during May to September, and the mean annual evaporation is 684.2 mm. The site belong to continental monsoon climate, with a mean annual temperature of 1.5°C, while winters are severe and

Soil microclimate

Warming significantly increased soil temperature (ST; P < 0.001), whereas the main effects of clipping showed no significant effect on ST (Table 1). The warming treatments W1.5 and W2.5 increased ST by an average of 1.56°C and 2.34°C, respectively, in the no clipping plots and by 1.69°C and 2.39°C, respectively, in the clipping plots at the depth of 10 cm in comparing with the control (P < 0.001; Fig. 1). Soil moisture (SM) was reduced by an average of 2.49% and 4.93% with W1.5 and W2.5

Discussion

Based on a 6-year experiment investigating the response of soil respiration and its components to warming and clipping, this study indicated that Rh and Ra had different sensitivities to warming, with Rh decreasing whereas Ra increasing with warming, leading to no significant impacts on Rs. Clipping significantly stimulated Rs, Rh and Ra as well as their responses to warming. Interestingly, warming effect varied largely with years, which was due to the changes in the ratio of ANPP to BNPP and

Declaration of Competing Interest

The authors declare no competing financial interests.

Acknowledgements

The authors thank the staff of Institute of Qinghai-Tibetan Plateau in Southwest University for Nationalities. This study was financially supported by the National Natural Science Foundation of China (31988102, 31625006), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA23080302).

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