Effects of alpine swamp wetland degradation on ecosystem respiration components
For a long time, the research results on the effects of glyphosate on soil microorganisms are mainly divided into two categories. On the one hand, it is believed that glyphosate has little effect on soil microbial individuals and groups, and in soil ecosystem, there is self balance between soil and microorganism. On the other hand, it is regarded that glyphosate plays an important role in soil microorganisms and affects natural ecology (Liang et al., 2021). On the effect study of glyphosate on soil respiration rate, some scholars believe that the relationship between glyphosate content and soil respiration rate does not reach a significant level (Hu et al., 2010), indicating that the effect of glyphosate concentration on soil respiration is not significant. The results show that glyphosate spraying has no significant effect on soil microbial respiration rate in alpine grassland. Based on the above results, this study showed that soil microbial respiration and soil respiration are close to each other in degraded alpine swamp ecosystem (including light and severe degradation), significantly higher than those before degradation. Grassland degradation plays an important role in soil respiration. For example, Lai et al (2019) analyzed the soil respiration of alpine meadow in the process of degradation in order to explore the effect of alpine meadow degradation on soil respiration, discovering that soil respiration increased at first and then decreased with the degradation of meadow in growing season. Wen et al (2014) measured soil respiration in alpine steppe with different degradation degree to find that the average soil respiration rate in the growing season increased first and then decreased during the degradation process, where the respiration of moderately degraded soil is significantly higher than that of non degraded and severely degraded soil without no significant difference in light degeneration. Silvola et al (1996) found that the decline of wetland water level made the peatland rapidly turn into carbon source or have no obvious effect on soil respiration. Mkiranta et al (2009) found that the decrease of wetland water level inhibited soil respiration and emission. However, some studies have shown that surface water in alpine wetlands significantly inhibited soil respiration and improved soil carbon stability (Hu et al., 2016), which is similar to our results. The reason is that the surface waterlogging environment and high water level will reduce the dissolved oxygen content and soil redox potential, and prevent O2 from entering the soil, forming an anaerobic environment (Oorschot et al., 2000), so as to reduce the decomposition rate of soil animal and plant residues and organic matter, which further reduces the amount of CO2 emitted from soil environment and inhibits soil respiration (Kraigher et al., 2006). The soil respiration rate in non degradation stage is significantly lower than that in light degradation and severe degradation stage, indicating that the soil conditions of alpine swamp wetland significantly inhibit soil respiration. The intensity and direction of the response of soil respiration to grassland degradation depend on the response of vegetation root respiration and soil microbial respiration. In this study, there is a positive correlation between root respiration and soil respiration at different degradation stages, but the degradation of alpine swamp significantly increases soil microbial respiration with no significant effect on root respiration, which is closely related to biomass allocation of degraded vegetation and soil environment. After all, in the severe degradation stage, the relationship between microbial respiration and soil temperature and moisture has a higher coefficient of determination.
Effect of alpine swamp wetland degradation on the proportion of each component of ecosystem respiration
Ecosystem respiration is divided into autotrophic respiration consumption and heterotrophic respiration consumption. There was a significant difference between the non degradation stage with higher f(v/e) and lower f(m/e) and the light and severe degradation stage with lower f(v/e) and higher f(m/e). In the three degradation stages, the proportions of non-degradation f(v/e) and f(m/e) are 81.1% and 19.0% respectively. In the latter two stages, f(v/e) is about 62% and f(m/e) is about 38%. Under different degradation stages of alpine swamp wetland, higher biomass will produce more autotrophic respiration (Cao et al., 2004), so that the f(v/e) of non-degraded alpine swamp wetland is higher. There is a significant positive correlation between vegetation respiration and ecosystem respiration. Therefore, higher f(v/e) ratio plays a leading role in ecosystem respiration at the same degradation stage. Ecosystem respiration is divided by space, and the proportion of vegetation canopy respiration and soil respiration in ecosystem respiration has no significant difference in different degradation stages with the proportion of f(s/e) more than 60%, and f (vc/e) less than 45% in three communities. In different ecosystems, the proportion of respiratory components is different. Soil respiration accounts for 70% of ecosystem respiration in forest ecosystem (Janssens et al., 2004), and in subalpine meadow, the proportion of soil respiration in ecosystem respiration is about 70% in perennial grazing meadow and seasonal grazing meadow, and the proportion of aboveground vegetation respiration is about 30% in July and August growth peak (Wang et al., 2008). In terms of absolute quantity and proportion, the aboveground vegetation respiration plays a certain role in ecosystem respiration in alpine swamp wetland and its degradation stage, but not a dominant role. Soil respiration plays a greater role in ecosystem respiration. In soil respiration, the correlation between root respiration and soil respiration is stronger than that of microbial respiration and the alpine swamp wetland and its degradation stage have high underground biomass. Therefore, it contributes a lot to soil respiration and vegetation respiration. The degradation of alpine swamp wetland leads to the decrease of biomass (Li et al., 2020), resulting in the decrease of canopy respiration, vegetation respiration and root respiration rate. In addition, the correlation between canopy respiration, soil respiration, root respiration and ecosystem respiration is different in different degradation stages. In the non degradation and severe degradation stages, canopy respiration and ecosystem respiration are significantly positively correlated, while in the light degradation stage, soil respiration, root respiration and ecosystem respiration are significantly positively correlated. The ecological process of ecosystem respiration is very complex, and the alpine swamp wetland in different degradation stages is the result of long-term climate adaptation. The reasons for the differences in the correlation between respiration components and ecosystem respiration need to be further studied and analyzed.
Response of respiratory components to soil temperature and moisture in degraded alpine swamp wetland ecosystem
The components of ecosystem respiration are affected by many factors, but most studies show that soil temperature is the most important factor affecting respiration. For example, the ecosystem respiration of alpine swamp meadow in the source region of the Yangtze River is exponentially correlated with soil temperature (Chen et al., 2020), the ecosystem respiration, soil respiration and microbial respiration of subalpine grassland in the Central Qilian Mountains are exponentially correlated with soil temperature (Xie et al., 2016), and the canopy respiration, root respiration and mineral soil respiration of Kobresia humilis meadow are significantly positively correlated with soil temperature (Wu et al., 2005). In this study, the exponential model used by most researchers is used to analyze the relationship between soil temperature and respiration rate, indicating that the exponential model can well fit the relationship between respiration and soil temperature in different degradation stages of alpine swamp wetland, and the fitting effect of soil respiration and temperature is the best.
Q10 is an important model parameter to reflect respiratory sensitivity and predict respiratory emissions, and the small change will lead to great error in respiratory estimation. The Q10 values of grassland ecosystem respiration and its components are different. Q10 values of ecosystem respiration and soil respiration of alpine grazing meadow in northern Tibet are 1.83–3.07 and 1.54–4.13, respectively (Zhou et al., 2010); The Q10 of ecosystem respiration and 5 cm soil temperature of alpine meadow and swamp meadow in Fenghuoshan of Qinghai-Tibet Plateau are 2.35 and 2.75, respectively, whose 20 cm soil temperature Q10 are 2.35 and 4.90, respectively (Zhang et al., 2018). Q10 of 5 cm soil temperature and soil respiration of non degraded alpine meadow in the Sanjiangyuan region of is 2.89 (Wei et al., 2014), respectively; The Q10 values of soil respiration, microbial respiration, root respiration and 10 cm ground temperature of alpine meadow in Aba Prefecture of Sichuan Province on the southeast edge of Qinghai-Tibet Plateau are 2.66, 2.12 and 3.46 respectively (Meng et al., 2020). The results show that ecosystem respiration and its component Q10 are different in different degradation stages, ecosystem respiration is 1.41–1.50, vegetation respiration is 1.81–1.94, canopy respiration is 4.46–6.93, soil respiration is 1.41–2.40, root respiration is 2.84–5.97, and soil microbial respiration is 1.99–3.74. Q10 has great variability in time and space (Kirschbaum, 2000), which is related to temperature, soil moisture, vegetation, climate, litter (Tong et al., 2005), measured soil temperature (Fierer et al., 2003), soil organic carbon quantity and structure (Zhang et al., 2005). Some studies have shown that even in the same study area, the Q10 value will change with the temperature, growth stage, phenology and other environmental factors (Davidson & Janssens, 2006). However, most scholars often use a constant Q10 value to simulate and predict the interaction between ecosystem and climate system, which will inevitably reduce the accuracy of simulation and prediction (Fang & Wang, 2007). Therefore, Q10 may be a value influenced by multiple factors of the audience, and can only correspond to one Q10 in one research site and one time period. It should not be regarded as a fixed value and applied to different environments and ecosystems (Xu & Zhang, 2009).
Soil moisture is also one of the main factors affecting respiration rate. Under the condition of sufficient soil moisture to guarantee the activity of roots and microorganisms, the effect of soil temperature on respiration is obvious (Wildung et al., 1975). When the soil is in extreme drought or water logging, soil moisture becomes the dominant factor affecting respiration (Davidson et al., 2000). Water change can affect root growth of grassland plants, soil metabolic activity, soil microbial community structure and activity, soil permeability and gas diffusion (Xu et al., 2004; Chen et al., 2003), so as to influence the changes in respiratory intensity. Regardless of soil temperature, soil moisture can promote the increase of respiration rate in a certain range (Zhang, 2008). The diurnal and monthly variations of grassland ecosystem respiration rate and its components are largely determined by the differences of soil temperature (Wang et al., 2007). The results show that the moisture content in different degradation stages range from 50–65%, and the respiration is positively correlated with soil moisture. However, the correlation between soil moisture and ecosystem respiration and its component respiration rate at different degradation stages of alpine swamp wetland is lower than that of soil temperature, and the water content of alpine swamp wetland is not the main limiting factor.
In fact, the water and heat conditions in the soil do not exist separately. The interaction between soil temperature and water has a significant impact on respiration. A two factor model is used to analyze the effects of soil temperature and moisture on respiration rate, indicating that the two factor model has a higher coefficient of determination than the single factor model, which is consistent with the results obtained by Hu (2016) and Yu (2014). Therefore, it is better to simulate the effect of soil temperature and moisture on respiration by using composite model.
In the future research on the respiratory components of alpine swamp wetland ecosystem, the soil microbial respiration rate can be determined by spraying herbicides. Meanwhile, our previous research results show that the carbon sink loss caused by the degradation of alpine swamp wetland is 193.44±47.01 g·m−2 at the peak of plant growth (Li et al., 2020), emphasizing the importance of classified protection and rational utilization of alpine wetlands in different degradation stages. For example, it is strictly forbidden to open ditches and drain water for non degraded or slightly degraded alpine wetlands, so as to reduce grazing pressure, and carry out zonal management for degraded alpine wetlands (Fu et al., 2017), and promote the function of alpine wetlands as an important carbon sink.