Organic Matter in Calcimorphic Biological Soil Crusts in Semiarid Ecosystems: When Humic-Type Substances are Formed From Lignin- Lacking Terrestrial Biomass

摘要

Biological Soil Crusts (BSCs) are formed from close association between soil particles and cyanobacteria, algae, lichens, bryophytes and microfungi, and are the dominant cover in Mediterranean arid and semiarid zones (up to 70% of its surface). In general BSCs are beneficial for ecosystems [1] as regards improving resistance to erosion, moisture retention [2–4] and availability of nutrients [5, 6]. In fact, BSCs produce polysaccharides contributing to soil aggregation [7]. Despite BSCs improve quality and resilience of soils, they are especially fragile against anthropogenic disturbances [8] (i.e., land use change) [9–13] the extent of which could be studied from biogeochemical proxies, such as the composition of the humic acids (HAs). However, humification in most semiarid BSCs have some characteristics in common with the formation of aquatic humic matter, where autochthonous biomass lacks lignin, and biodegradation-resistant humic-type substances could be formed from abiotic condensations of lipid and diagenetic alterations of carbohydrate and protein [14, 15]. Consequently, and based on spectroscopic indices showing the extent to which the HA-like fractions in BSCs are similar to ‘matured’ HAs in mineral soils, this study aims to evaluate possible changes in humus quality in two BSCs subjected to different anthropogenic pressures.