Non-rainfall water inputs: a key water source for biocrust carbon fixation

Links between water and carbon (C) cycles in drylands are strongly regulated by biocrusts. These widespread communities in the intershrub spaces of drylands are able to use non-rainfall water inputs (NRWI) (fog, dewfall and water vapour) to become active and fix carbon dioxide (CO2), converting biocrusts into the main soil C contributors during periods in which vegetation remains inactive. In this study, we first evaluated the influence of biocrust type on NRWI uptake using automated microlysimeters, and second, we performed an outdoor experiment to examine how NRWI affected C exchange (photosynthesis and respiration) in biocrusts. NRWI uptake increased from incipient cyanobacteria to well-developed cyanobacteria and lichen biocrusts. NRWI triggered biocrust activity but with contrasting effects on CO2 fluxes depending on the main NRWI source. Fog mainly stimulated respiration of biocrust-covered soils, reaching net CO2 emissions of 0.68 μmol m-2 s-1, while dew had a greater effect stimulating biocrust photosynthesis and resulted in net CO2 uptake of 0.66 μmol m-2 s-1. These findings demonstrate the key role that NRWI play in biocrust activity and the soil C balance in drylands.This work has been supported by the following projects: REBIOARID (RTI2018-101921-B-I00), funded by the FEDER/Science and Innovation Ministry-National Research Agency through the Spanish National Plan for Research and the European Union including European Funds for Regional Development, the RH2OARID (P18-RT-5130) funded by Junta de Andalucía and the European Union including European Funds for Regional Development and the BIOCOST project funded by the Biodiversity Foundation of the Ministry for the Ecological Transition. S.C. and E.R.C. were supported by a UAL-Hipatia postdoctoral fellowship funded by Plan Propio of the University of Almería