Biological Soil Crusts And Wetting Events: Effects On Soil N And C Cycles

1358 words - 6 pages

Discussion
As we expected, increasing length of wetting event had a noticeable effect on bare ground and biological soil crust microsites, though it was larger on the latest. Our results showed that longer wetting events produced higher mineral N and organic C production, which suggests that longer wetting events may be related to an enhancement in microbial metabolic rates such as both decomposition and mineralization rates. Our results are consistent with those of Schwinning and Sala (2004), as they suggested that there is a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses in semi-arid ecosystems. According to this idea, small watering events ...view middle of the document...

However, the presence of BSC may slow down water infiltration (Maestre et al. 2011), avoiding rapid leaching of labile solutes, which may contribute to the earlier increase in mineral and organic compound in soils. We found net changes of NH4+, NO3-, DON, phenols and carbohydrates increased for the two longest wetting events in BSC, while only net changes of NH4+ and carbohydrates showed the same trend in bare ground. Since this trend was more obvious in BSC than in bare ground, our results suggest that BSC communities may benefit these microbial activities throughout the larger amount of heterotrophic bacteria and fungi compared to bare ground (Belnap and Lange 2003), and that BSC communities plays a key role in nutrient conservation in semiarid ecosystems. Thus, the decrease of BSC cover predicted under climatic change scenarios (Maestre et al. 2013) may have consequences on the soil balance of soil labile C and N cycling, even without consideration of changes in the length of wetting events. Besides, we found that all the soil studied variables, excepting MB-N and NH4+, had a different response throughout the studied period to distinct length of wetting event when BSC were present. For all treatments, DON concentration decreased faster in BSC than in bare ground, while NO3- showed the inverse response, increasing faster in BSC than in bare ground. Increases of NH4+ were only perceptible under BSC. These findings suggest a differential response of microbes in BSC and bare ground microsites under different length of wetting events. Other researches have observed rapid physiological responses of BSC to water pulses (Wilske et al. 2008).
It is interesting to note the strong decrease of net change of MB-N found for all watering treatments and microsites. Several previous studies (Henry et al. 2005; Chung et al. 2007; Allison and Martiny 2008; Zelikova et al. 2012) found that more frequent wetting and drying cycles regarding to the control may modify microbial community composition. Zelikova et al. (2012) found a decrease in both bacterial and fungal biomass in frequently watered soils in a desert ecosystem with presence of BSC. They suggested that a direct response to watering or an indirect response to the decrease of moss cover could be potential explanations for this microbial biomass reduction. Fierer et al. (2003) found shifts in microbial processes 6 weeks after the end of several wetting and drying cycles, and these changes were related to shifts in the composition of the microbial population. Pesaro et al. (2004) found that respiration rates recovered rapidly after drought, meanwhile microbial biomass remained depressed for at least one month. Even tolerable stresses suppose C and N costs on microbes that they must overcome in order to survive and remain active, these costs may have relevant influences on ecosystem functioning (Shimel et al. 2007).
In drylands, dry soil and the quickly rewetting shock can select against gram negative...

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