Precipitation events reduce soil respiration in a coastal wetland based on four-year continuous field measurements

Abstract

Coastal wetlands are considered as a significant sink for global carbon because their organic-rich soils. Given exposed to shallow water tables, water from groundwater is transported upward to the root zone through capillary rise, thus soil moisture in coastal wetlands is relatively high even when there is no precipitation. We expected that as precipitation occurred, the soils in coastal wetlands might become quickly saturated and lead to the development of anoxic conditions. We further hypothesized that such anoxic conditions might decrease soil respiration by limiting oxygen availability and biological activities of roots and microorganisms. Based on continuous automated soil respiration data collected in a coastal wetland in the Yellow River Delta over 4 years (2012)(2013)(2014)(2015), the results showed that on the annual scale, cumulative soil respiration was 317, 321, 231, and 274 g C m −2 yr -1 for 2012, 2013, 2014, and 2015, respectively, with an average of 286 g C m −2 yr -1 . The rate of soil respiration increased exponentially with soil temperature during each year and its two seasons (growing season and non-growing season). In addition, soil respiration was significantly related to soil moisture during the growing season, but was not affected by soil moisture during the non-growing season. After each precipitation event, soil respiration was significantly negatively correlated with soil moisture under different initial soil water contents. There was a significant positive correlation between changes in soil respiration and changes in soil moisture following precipitation events. Moreover, the increase of soil moisture following precipitation events changed the temperature response of soil respiration. Our study indicated that precipitation events could decrease soil respiration by increasing soil moisture and inducing anoxic conditions in the coastal wetland. Therefore, we speculate that the continuation of decreasing precipitation and increasing temperature trends in the Yellow River Delta may increase soil carbon losses in the coastal wetland due to the increase in soil respiration.

Plant biomass and soil organic carbon are main factors influencing dry-season ecosystem carbon rates in the coastal zone of the Yellow River Delta

“…Previous study has demonstrated that the average annual precipitation has decreased with a rate of 4.5 mm yr -1 and the annual air temperature has increased by 1.7°C over the past 55 years in the YRD [28]. This shift of precipitation and air temperature might induce drier and hotter seasons in the YRD in the future, which has a great possibility to increase ER, thus greater C loss from this coastal wetland.…”

“…Microbiological processes and the roles of microorganisms could be another reason for the variation in ecosystem CO 2 and CH 4 rates. Their activities are controlled by several biological, chemical, and physical factors in soil [19,20,22,28,29,44]. Therefore, soil properties including soil organic carbon (SOC), total nitrogen (TN), inorganic nitrogen, total phosphorus (TP), available phosphorus (AP), salinity, and pH are closely related to CO 2 and CH 4 fluxes [45].…”

“…The average annual precipitation is 530–630 mm and the rainfall mostly precipitates from July to September (rainy season). The dry season (April to June) accounts for about 30% of the annual precipitation [28]. The abundant vegetation of this research area includes Phragmites australis , Suaeda heteroptera Kitag , and Tamarix chinensis .…”

“…The Yellow River Delta (YRD) wetland, which is the largest wetland ecosystem in the warm temperate zone of China, has an obvious dry season (April to June) and rainy season (July to September) [26–28]. Previous studies have reported seasonal variations in ecosystem C fluxes (CO 2 and CH 4 ) based on continuous flux measurements [28–30] and found that dry-season had the second largest contribution to the CO 2 and CH 4 emissions in the YRD [29]. Therefore, in this study, we emphasized the relationship between dry-season ecosystem C rates (GEP, ER, NEE, and CH 4 ) with vegetation and soil properties and disentangled their contributions to the C rates.…”

Soil Respiration from Different Halophytic Plants in Coastal Saline-Alkali Soils

“…Coastal wetlands are considered a significant sink for global carbon [30], and Xie et al [31] reported that alkaline/saline soils can absorb CO 2 at a rate of 0.3-3.0 μmol m -2 s -1 with an inorganic, non-biological process. However, unlike these earlier studies, our research has shown that the mean soil respiration of growing seasons in salinized bare land was 0.59 μmol m -2 s -1 , which is similar to the observations of Lai et al [32], who found negative soil respiration only when the temperature was below 5ºC in natural ecosystems of saline/alkaline soils.…”