Vegetation degradation along water gradient leads to soil active organic carbon loss in Gahai wetland

Highlights

• Vegetation degradation leads to sharp reduction in SOC, LFOC, and DOC.

• Results have important implications for assessing impacts of management changes and climate change on wetland soil quality and productivity.

Abstract

Soil active organic carbon responds quickly to soil disturbances and is a sensitive indicator of early changes in soil organic carbon (SOC). In order to identify the differences in the distribution of dissolved organic carbon (DOC), light fraction organic carbon (LFOC), SOC and their changes as affected by vegetation degradation degree along water gradient in wetland, we analyzed DOC, LFOC, and SOC in the 0–100 cm soil layer under four vegetation degradation degrees: non-degradation (ND), lightly degradation (LD), moderately degradation (MD) and heavily degradation (HD). The results showed that soil DOC, LFOC and SOC in the 0–100 cm layer of ND wetland was significantly higher than the other three degradation levels. DOC, LFOC and SOC contents decreased with increasing soil depth under the four degradation degrees and the contents of soil DOC, LFOC and SOC were mainly concentrated in the soil surface (0–20 cm). The DOC, LFOC, and SOC contents in the 0–20 cm layer under all four degradation levels showed obvious seasonal changes, while the DOC, LFOC, and SOC contents in the 20–100 cm layer showed little fluctuation over the plant growing season. There was a significant positive correlation between soil DOC and SOC, and between LFOC and SOC, with correlation coefficients of 0.948 and 0.911, respectively. There was also a very significant correlation between DOC and LFOC(R² = 0.904). Soil DOC and LFOC in the 0–100 cm layer under the four degree of degradation were linearly correlated with SOC. While there was a linear correlation between DOC and LFOC in the non-degradation wetland soils, DOC and LFOC in the three degradation soils correlated exponentially correlated with SOC.

Introduction

The wetlands environment is a special ecosystem existing between land and water ecosystems. Wetlands play an important role in regulating climate, flood and drought control, controlling soil erosion, promoting silting, and reducing environmental pollution (Costanza et al., 1997). Although the area occupied by wetlands is only 2–6% of the total global land area, the carbon reserves of wetlands account for 1/3 of the world's carbon reserves, thereby playing an important role in the global carbon cycle (Kayranli et al., 2010; Wu, 2012). Soil organic carbon is an important component of soil organic matter, which can improve soil physical properties and soil quality, reduce soil nutrient loss, and increase soil effective nutrient content (Cleveland et al., 2006).

The active carbon pool accounts for only a small portion of the total SOC pool, but it has become an important focus of research in soil, environmental, and ecologyical studies. Because of its direct or indirect participation in important ecological processes, such as nutrient cycling and material transformation (Hulatt et al., 2014; Byrd et al., 2015), it can quickly and effectively reflect the transformation and change of components in the soil carbon pool (Kara and Baykara, 2014; Lacoste et al., 2015) in the short term. Soil dissolved organic carbon (DOC) and light fraction organic carbon (LFOC), are the most active carbon components in terrestrial ecosystems. They can be decomposed by soil microbes and rapidly converted into other components in the soil, thereby acting as sensitive indicators of the response of the SOC pool to climate change (Guo et al., 2011; Wang et al., 2016). Therefore, studying the characteristics of soil active carbon content should be useful in revealing the mechanism of dynamic changes in the soil carbon pool and should be helpful in evaluating future climate change impacts on wetland systems.

The northeastern edge of the Qinghai Tibet Plateau is considered one of the areas most sensitive to global climate change, and the Gahai wetland within that region was considered internationally important wetlands in 2011 (Sun et al., 2014) of great significance to the ecological safety and water conservation of the Yellow River Basin. In recent years, due to the impact of climate warming and overgrazing, the Gahai wetlands have seen serious declines in vegetation degradation (Hirota et al., 2005; Liu et al., 2013). Therefore, the objective of this study was to quantify the distribution characteristics of DOC, LFOC, and SOC in four wetlands soils differing in vegetation degradation levels in an alpine wetland to provide basic data for future studies of organic carbon reserves and carbon sequestration in alpine wetlands.