Soil properties across a hydrological gradient in saladas from northeast Spain: what are the implications for soil carbon stocks, CO2 efflux and microbial communities in a warming world?

Abstract

Numerous permanent and temporary wetlands occur throughout the world’s drylands. Although characterised by diverse water inundation frequencies, durations and depths, these wetlands in drylands are typically hotspots of biological activity and productivity. The healthy functioning and even existence of many wetlands in drylands, however, is threatened by desiccation resulting from a combination of climate change and human disturbance. Near Alcañiz in arid northeast Spain, three adjacent saladas (playas) with contrasting hydroperiods provide an opportunity to investigate how moisture availability affects their soil carbon (C) stocks, CO2 efflux, and microbial communities. Predicting the impacts of changes in moisture availability on the C cycle in wetlands in drylands is challenging but important because many contain large C stocks and may be significant sources and sinks of greenhouse gases. Frequent inundation and/or near-permanent soil water saturation supports the generation of organic C from a range of different sources. Soil inorganic C was greatest on the driest salada (3.8%) compared to the wetter saladas (3.0% and 2.1%) owing to evaporative concentration and the reaction of CO2 with available Ca2+, Mg2+ and Na+ ions. CO2 efflux was greatest at intermediate moisture levels (142 mg CO2 m−2 h−1), but the spatial and temporal variability in CO2 efflux on salada surfaces is large, demonstrating the need for intensive sampling regimes to provide realistic estimates of their contribution to atmospheric CO2 exchanges. Different microbial community structures also characterise each salada. The saladas near Alcañiz, and many other similar features in northeast Spain, are renowned for their rare and threatened flora and fauna, yet their soil C cycle characteristics and soil microbial communities provide additional reasons to monitor the impacts of climate change and protect these vulnerable environments from further anthropogenic disturbances.