Functional responses of biological soil crusts to simulated small precipitation pulses in the Monte desert, Argentina

Abstract

In arid areas, biological activity responds to precipitation pulses, with different activation thresholds for different organisms. Biocrusts, communities of cyanobacteria, green algae, lichens and mosses living in the soil surface, contribute to soil fertilization by fixing atmospheric nitrogen (N), carbon (C), and solubilizing phosphorus (P). In this study, we hypothesized that biocrusts respond to low simulated precipitation pulses, lower than 10 mm, increasing nutrient availability. We expected biocrust organisms to increase soil net CO2 exchange, N fixation, soil N and P availability in response to small water pulses. In order to evaluate these predictions, we artificially irrigated soils with different covers, including those dominated by bare soil, mosses, and lichens, simulating precipitation pulses of 1, 3, 5, and 10 mm. We measured variables that change at short time scales (hourly), such as net CO2 exchange, N fixation and soil nitrate and phosphate availability, and those that change at larger time scales (years), such as organic matter and total N. Organic matter, total N, and phosphate concentrations were higher in soils with moss dominated biocrusts than in bare soils, indicating their importance to soil fertility in the long term. Soil net CO2 exchange, phosphate, and nitrate availability responded to the simulated precipitation pulses in all soil covers. Net CO2 exchange increased in all soil covers, but phosphate availability decreased in response to precipitation in moss dominated biocrusts. Nitrate availability was lower in lichen dominated biocrusts, although it increased in all soil covers in response to precipitation. These results show that although all the soils analyzed respond to low simulated precipitation pulses, biocrust organisms may retain soil nutrients and decrease availability to other organisms.