Enhancement of the net CO₂ release of a semiarid grassland in SE Spain by rain pulses

Abstract

Occasional rain events occur over the dry season in semiarid ecosystems and cause immediate, large increases in the net CO2 efflux which gradually decrease over a few days following the rain event. In a semiarid grassland located in SE Spain, these precipitation pulses represent only 7% of dry season length but provoked approximately 40% of the carbon emitted during the dry seasons over 2009–2013. We performed a manipulation experiment to decompose the net ecosystem pulse response into its biological processes in order to quantify how much of a role photosynthesis and aboveground respiration play compared to soil respiration. Experimental results showed that while soil respiration was the dominant component of the net CO2 flux (net ecosystem CO2 exchange, NEE) over the irrigation day and the day after (80% of NEE), plant photosynthesis remained inactive until 2 days after the pulse, when it appeared to become as prevalent as soil respiration (approximately 40% of NEE). Additionally, aboveground respiration was generally secondary to soil respiration over the whole experiment. However, statistical results showed that aboveground carbon exchange was not significantly affected by the rain pulse, with soil respiration being the only component significantly affected by the rain pulse.

Can land degradation drive differences in the C exchange of two similar semiarid ecosystems?

“…In this regard, the ecosystems' functioning could be divided into three different phases. The first phase corresponded to the autumn months, when the first rainfall events after the dry summer (i.e., rain pulses) activated the soil microbiota, triggering respiratory CO 2 emissions as previously measured at the same experimental sites (López-Ballesteros et al, 2016;Rey et al, 2017). During this phase, maximum net CO 2 emission was observed at the natural site, which exceeded the biological net CO 2 flux observed at the degraded site (Fig.…”

“…The functioning of both experimental sites can be divided into two main periods. On one hand, the growing season usually extends from late autumn to early spring, when the temperature starts to rise and water resources have not yet become scarce (López-Ballesteros et al, 2016;Serrano-Ortiz et al, 2014). On the other hand, a long period of hydric stress, with high temperatures and scarce precipitation, results in a prolonged dry season that usually begins in May-June and ends in SeptemberOctober, when the first autumn rainfall events occur.…”

“…In order to partition net CO 2 ecosystem exchange into gross primary production (GPP) and ecosystem respiration (R eco ), we first modeled the ventilative CO 2 efflux by adapting the approach proposed by Pérez-Priego et al (2013) using the results of previous studies at both sites (López-Ballesteros et al, 2016. Essentially, we aimed to isolate those moments when subterranean ventilation (V n ) dominates the net CO 2 fluxes (F c ) and biological fluxes are negligible.…”

“…We chose this approach given the determinant influence of hydric stress, in this case atmospheric drought (assessed via VPD), on the physiology of Machrochloa tenacissima, the dominant plant 268 A. López-Ballesteros et al: Land degradation drive differences in the C exchange of two semiarid ecosystems species of the studied semiarid ecosystems (Pugnaire et al, 1996;López-Ballesteros et al, 2016).…”

“…Regarding data processing, the half-hourly averaged fluxes were calculated from raw data collected at 10 Hz using EddyPro 5.1.1 software (Li-Cor, Inc., USA). Flux calculation, flux corrections and quality assessment were performed according to López-Ballesteros et al (2016). Additionally, flux measurements acquired under lowturbulence conditions were excluded from the analysis by using a friction velocity (u * ) threshold according to the approach proposed by Reichstein et al (2005).…”