Asymmetric responses of biocrust respiration to precipitation manipulation under a changing semiarid climate

Abstract

Global climate change is expected to cause both increased and reduced precipitation in the next decades, especially in drylands with semi-arid climates. Biocrusts are an essential soil surface cover in global drylands, and they strongly influence most soil properties and support fundamental ecosystem functions, especially soil carbon (C) cycling. Nevertheless, the response patterns and mechanisms of biocrust respiration rate (Rs) to precipitation changes remain uncertain under changing semi-arid climates. In our study, a field experiment for biocrusts with natural precipitation (CK) and six levels of precipitation manipulation (–50 %, –30 %, –10 %, +10 %, +30 %, and +50 % of CK) were conducted from 2019 to 2021 in a semi-arid climate region of the northern Chinese Loess Plateau. We continuously measured the biocrust Rs of all the treatments in the growing seasons, and the fundamental properties of biocrusts and their temperature and water content at 5 cm depth were also measured. Our results showed that the increased precipitation inhibited biocrust Rs, while moderately reduced precipitation stimulated biocrust Rs. As compared with the CK, the precipitation manipulations of +10 %, +30 %, and +50 % decreased biocrust Rs by 8.9 %, 15.3 %, and 22.1 %, respectively, while the treatments of –10 % and –30 % increased biocrust Rs by 25.5 % and 8.0 %, respectively. However, the precipitation manipulation of –50 % suppressed biocrust Rs by 19.3 %. More importantly, the responses of biocrust Rs to precipitation changes were negative and asymmetric, which indicated a higher sensitivity of biocrust Rs to reduced precipitation in comparison to increased precipitation. Moreover, the increased precipitation (+10 % to + 50 %) raised soil water content (by 29.7 %–55.0 %), moss biomass (by 12.5 %–50.0 %), and organic matter content (by 11.3 %–12.4 %). In contrast, the reduced precipitation (–10 % to –30 %) decreased soil water content (by 5.2 %–17.6 %) and moss biomass (by 12.5 %–25.8 %). The structural equation modeling analysis showed that the responses of biocrust Rs to precipitation manipulation were mainly and directly influenced by soil temperature and biocrust properties. Although soil water content did not have a direct significant effect on biocrust Rs, it indirectly influenced biocrust Rs by affecting the fundamental properties of soil and biocrusts. Consequently, the changed soil properties and biocrust characteristics caused by precipitation shifts aggravated the negative responses of biocrust Rs to precipitation manipulation. Our findings emphasize that biocrusts have a negative and asymmetric response of Rs to precipitation changes, implying that the intensified precipitation variation in semi-arid regions caused by global climate change in future may positively affect the stability of soil C stocks contributed by biocrusts and thus reduce soil C efflux in drylands.