Date Published: February 7, 2018
Publisher: Public Library of Science
Author(s): Xiaoyu Li, Lijuan Liu, Huijin Yang, Yan Li, Ben Bond-Lamberty.
Environmental factors and human activities play important roles in carbon fixation and emissions generated from croplands. Eddy covariance measurements in a drip-irrigated, film-mulched cotton field were used to analyze the relationships between carbon fluxes and environmental factors in Wulanwusu, northern Xinjiang, an arid region of Northwest China. Our results showed that the cumulative net carbon flux (NEE) was -304.8 g C m-2 (a strong sink) over the whole cotton growing season in 2012, which was more than that in cotton cropland without plastic film mulching and drip-irrigation. Moreover, when time is scaled up from a half-hour to a month, the correlations of gross primary production (GPP) to air temperature (Tair), net solar radiation (Rn) and soil water content (SWC) gradually become stronger due to ecosystem resistance and resilience as well as the protection of plastic film mulching. The GPP is more strongly correlated with Rn than Tair at time scales from minutes to days, while it reverses at time scales from days to weeks. This outcome is largely determined by the biochemical characteristics of photosynthesis. SWC and vapor pressure deficit (VPD) at all time scales are weakly correlated with GPP because plastic film mulching and regularly drip-irrigation allow soil to maintain sufficient water.
Carbon dioxide emissions generated from upland agro-ecosystems are a major source of atmospheric greenhouse gases . Agricultural lands occupy 37% of the Earth’s land surface and account for 20% of atmospheric carbon dioxide . Additionally, agricultural lands are a non-ignorable carbon sink. Conversely, an elevated concentration of atmosphere carbon dioxide could have a large impact on future agricultural productivity [3–5], as it can accelerate agricultural crop growth rates [6,7]. Consequently, measurements of carbon fluxes generated from agro-ecosystems in an arid region are essential to an analysis of carbon flux dynamics.
Drip irrigation under plastic mulch is widely used in Xinjiang, the largest arid region in China. This study investigated the relationships between carbon fluxes and environmental factors in a drip-irrigated, film-mulched cotton field of this water-limited region. The cotton field is a strong sink of carbon, and its cumulative NEE over the entire growing season is -304.8 g C m-2. The cumulative NEE in mulched cotton field is higher than that in non-mulched cotton field. RH, Rn, and Tair are important controlling factors of carbon balance, RH was the most limiting environmental factor. However, SWC and Ustar have slight impacts on the GPP residuals. Additionally, RH, Ustar and VPD are slightly more correlated with the simulated GPP driven by Tair than by Rn. RH, VPD and Ustar are more sensitive to Tair than Rn . High temperature decreases RH and increases VPD. Decreasing of Ustar increases the rate of wind and evaporation, which decreases leaf temperature . In general, the total daily GPP could be reliably predicted from Rn and Tair; at the same time, RH, Ustar, SWC and VPD are also important factors to constrain the response of GPP to Rn and Tair at different degrees of limitation.