Patterns of enzyme activities and nutrient availability within biocrusts under increasing aridity in Negev desert

Biocrusts are crucial for the biogeochemical cycles of semiarid ecosystems and strongly influenced by environmental factors. We examined topcrusts (0-2 mm) and the underlying subcrust (2-35 mm depth) at two sites of the Negev with contrasting rainfall. The results show that sampling sites differing in moisture regime and sampling depth had a significant effect on soil properties. The microbial biomass, the concentrations of water-soluble carbon and nitrogen fractions, and the activities of eight enzymes of the carbon and nitrogen cycles decreased with decreasing available moisture and with increasing soil depth. The biomass-specific enzyme activity of topcrusts shows an increase in enzyme activity with increasing biomass in the same proportion. Biomass-specific enzyme activity revealed a strong nutrient demand and a high relative enzyme activity in subcrusts of the drier study area. Moreover, phosphatase activity was much higher in the biocrust of the drier study site. The ratios of carbon to nitrogen to phosphorous show a strong limitation of the latter two for the subcrusts in the Negev, while the ratio of water-extractable organic nitrogen to water-extractable organic carbon underlines the high importance of leaching for subcrusts in semiarid areas. We conclude that Negev topcrusts are highly effective zones of nutrient turnover, adopting to higher amounts of moisture with an increase in biomass and enzyme activity. The subcrusts are an important section of turnover and need to be included into studies on C, N, and P storage in crust covered ecosystems to reveal the processes in deeper soil.

Automated summary

Biocrusts are crucial for the biogeochemical cycles of semiarid ecosystems and are strongly influenced by environmental factors. Differences in moisture regime and sampling depth affect soil properties, with the topcrusts being efficient zones of nutrient turnover and subcrusts also playing an important role in the cycles.