The CO<sub>2</sub> exchange of biological soil crusts in a semiarid grass-shrubland at the northern transition zone of the Negev desert, Israel

Abstract

Abstract. Biological soil crusts (BSC) contribute significantly to the soil surface cover in many dryland ecosystems. A mixed type of BSC, which consists of cyanobacteria, mosses and cyanolichens, constitutes more than 60% of ground cover in the semiarid grass-shrub steppe at Sayeret Shaked in the northern Negev Desert, Israel. This study aimed at parameterizing the carbon sink capacity of welldeveloped BSC in undisturbed steppe systems. Mobile enclosures on permanent soil borne collars were used to investigate BSC-related CO 2 fluxes in situ and with natural moisture supply during 10 two-day field campaigns within seven months from fall 2001 to summer 2002. Highest BSC-related CO 2 deposition between -11.31 and -17.56 mmol m −2 per 15 h was found with BSC activated from rain and dew during the peak of the winter rain season. Net CO 2 deposition by BSC was calculated to compensate 120%, -26%, and less than 3% of the concurrent soil CO

Annual net primary productivity of a cyanobacteria-dominated biological soil crust in the Gulf Savannah, Queensland, Australia

“…The main difference we could find in these studies was the thickness of the biocrust plus the sub-crust (soil) layer used. While the studies revealing biocrusts to be CO 2 emitters used collars penetrating 20 to 35 cm into the soil (Bowling et al, 2011;Su et al, 2013;Darrouzet-Nardi et al, 2015), the studies attributing biocrusts to be CO 2 gainers during the course of 1 year either used pieces of biocrust of 1 to 5 cm thickness (this study; Brostoff et al, 2005;Feng et al, 2014), or a collar penetrating only 5.5 cm into the soil (Wilske et al, 2008(Wilske et al, , 2009). The metabolic activity of heterotrophic organisms as well as respiration of roots from nearby plants of deeper soil levels apparently influence the CO 2 gas exchange measurements accordingly as indicated in the investigation of Bowling et al (2011).…”

“…For example, a cyanobacteriadominated biocrust in the Mojave Desert, USA, had a C gain of 11.5 g m −2 yr −1 (Brostoff et al, 2005), 6.7 times higher than the cyanobacteria-dominated Boodjamulla biocrust. Another biocrust dominated by cyanobacteria, algae, lichens and mosses from the Negev Desert, Israel, resulted in a C gain of 0.7 to 5.1 g m −2 yr −1 (Wilske et al, 2008(Wilske et al, , 2009) and thus is pretty close to what we observed in our study, which also corresponds with the results from biocrusts composed of cyanobacteria, lichens and mosses of the Mu Us Desert in China with a C gain of 3.5 to 6.1 g m −2 yr −1 (Feng et al, 2014).…”

“…Comparing methodology and how the measurements were taken sheds some light on this phenomenon. All investigations, including our own study that showed biocrusts having a net CO 2 uptake over the year, used gas exchange devices with a separate cuvette where the samples had to be removed from the biocrust (Brostoff et al, 2005;Feng et al, 2014) except the study of Wilske et al (2008Wilske et al ( , 2009, in which a top soil chamber measured the biocrust in situ. All other studies with a negative C balance used top soil chambers where the biocrust was measured in situ (Bowling et al, 2011;Su et al, 2013;Darrouzet-Nardi et al, 2015).…”

“…There are various reasons, however; two major ones are (1) positive CO 2 uptake might only occur during a small part of the year and (2) it is difficult to separate the C balance of the biocrust from C fluxes of other organisms like microbes and roots of higher vegetation or minerals like carbonate that occur below them. Nevertheless, biocrusts are only one constituent of mature soils, and it seems plausible that measurements that include soil layers other than the biocrust itself might result in CO 2 release because of a high percentage of heterotrophic organisms (Bowling et al, 2011;Darrouzet-Nardi et al, 2015;Su et al, 2013), while those that are solely restricted to the biocrust layer might explain why they show net C uptake over the year (Brostoff et al, 2005;Wilske et al, 2008Wilske et al, , 2009Feng et al, 2014). We believe that seasonality (biocrust wet-up and dry-down) also plays an important role for several reasons.…”

“…From these results, two biocrust groups can be distinguished: one where biocrusts experienced net C uptake and the other where biocrusts experienced C loss. Examples from the net C uptake group include a biocrust from the Mojave Desert which gained 11.7 g C m −2 yr −1 (Brostoff et al, 2005), a biocrust in the northern Negev Desert, Israel, with a net C uptake of 0.7-5.1 g m −2 yr −1 (Wilske et al, 2008(Wilske et al, , 2009) and a biocrust from a desert region of north-west China showing a net C uptake of 3.5 to 6.1 g C m −2 yr −1 (Feng et al, 2014). Among the C-losing biocrusts are those of southeast Utah determined to be typical net C sources (Bowling et al, 2011), a biocrust from the Colorado Plateau, USA, also losing 62 ± 8 g C m −2 yr −1 (Darrouzet-Nardi et al, 2015) and finally biocrusts from the Gurbantunggut Desert, north-western China, showed a C release of 48.8 ± 5.4 to 50.9 ± 3.8 g m −2 yr −1 (Su et al, 2013).…”