The Response of Carbon Dioxide Flux to Rain Pulses at a Saline Desert

Soil carbon dioxide (CO₂) flux, often referred to as soil respiration, is a substantial component of the carbon budget for terrestrial ecosystems. A slight modification in soil CO₂ flux could significantly slow or accelerate the increase in atmospheric CO₂, and result in a significant change to the global C cycle. Thus, it is important to understand the mechanisms that regulate soil CO₂ flux, that is, which factors control soil CO₂ flux and how these factors affect CO₂ release from the soil surface.

Temperature sensitivity of soil CO₂ flux has been considered a typically reliable predictor, but other factors can also affect soil CO₂ flux, such as soil moisture content, soil organic matter quantity and quality. Soil rewetting following rainfall events also has been shown to be an important controlling factor in regulation of soil CO₂ flux, especially during drought.

In arid regions, precipitation is often regarded as infrequent pulse events, distributed unevenly throughout the year and may become the crucial limiting factor for soil processes via effects on soil water availability. However, few discussions were conducted on soil CO₂ flux in arid regions because of a lack of available data.

To examine how rain pulses alter the patterns of soil CO₂ flux and investigate the effect of rain pulses on C release in arid regions, Ma Jie et al. from XIEG conducted the ecological experiment at the Fukang Station of Desert Ecology, CAS, located at the north of the highest peak of the eastern Tianshan Mountains and the southern periphery of the Gurbantunggut Desert. Soil CO₂ flux was measured continuously during the whole growing season of 2009.

The result showed that there were remarkable changes in amplitude or diurnal patterns of soil CO₂ flux induced by rainfall events. Further analysis indicated that there was a significant linear relationship (P<0.001) between soil CO₂ flux and soil temperature (T_soil). However, a hysteresis between the waveform of diurnal course of CO₂ flux and T_soil was observed: with soil CO₂ flux always peaked earlier than T_soil.

Furthermore, a double exponential decay function was fitted to the soil CO₂ flux after rainfall, and total C releases were estimated by numerical integration for rainfall events. The relative enhancement and total C release, in association with the rain pulses, was linearly related to the amount of precipitation. According to the size and frequency of rainfall events, the total amount of C release induced by rain pulses was computed as much as 7.88 g C · m^–2 in 2009, equivalent to 10.25% of gross primary production.

These results indicated that rain pulses played a significant role in the carbon budget of this saline desert ecosystem, and the size of them was a good indicator of rain-induced flux enhancement. The result was published in Hydrological Processes on December 2012, 26: 4029-4037.