Abstract
Biological soil crusts (biocrusts) composed of cyanobacteria, bacteria, algae, fungi, lichens, and bryophytes stabilize the soil surface. This effect has mainly been studied in arid climates, where biocrusts constitute the main biological agent to stabilize and connect soil aggregates. Besides, biocrusts are an integral part of the soil surface under mediterranean and humid climate conditions, mainly covering open spaces in forests and on denudated lands. They often develop after vegetation disturbances, when their ability to compete with vascular plants increases, acting as pioneer communities and affecting the stability of soil aggregates. To better understand how biocrusts mediate changes in soil aggregate stability under different climate conditions, we analyzed soil aggregate samples taken under biocrust communities from four national parks in Chile along a large climatic gradient ranging from (north to south) arid (Pan de Azúcar), semi-arid (Santa Gracia), mediterranean (La Campana) to humid (Nahuelbuta). Biocrust communities showed a stabilizing effect on the soil aggregates in dry fractions for the three northern and the wet aggregates for the southernmost sites. Here, permanent vascular plants and higher contents of organic carbon and nitrogen in the soil control aggregate stability more than biocrusts, which are in intense competition to higher plant communities. Moreover, we found an increase in stability for edge aggregate size classes (< 2.0 mm and 9.5–30.0 mm). The geometric mean diameter of the soil aggregates showed a clear effect due to the climatic gradient, indicating that the aggregate stability presents a log-normal instead of a normal distribution, with a trend of low change between aggregate size fractions. Based on our results, we assume that biocrusts affect the soil structure in all climates. Their role for aggregate stability is masked under humid conditions by higher vegetation and organic matter contents in the topsoil.
Highlights
Biological soil crusts are highly variable communities of microscopic and macroscopic organisms found on the surface and in the upper centimeters of the soil (Gao et al, 2017)
Soil pH was significantly affected by the climatic gradient (Table 2), with mean values of 7.7 in PA, 6.2 in Santa Gracia Natural Reserve (SG), 5.9 in La Campana National Park (LC), and 4.4 in NA, with acidification levels of 6.2 in BSC- to 5.9 in BSC+
Bulk density (BD) showed a significant difference between the study sites (Table 2), with higher values in the two dryer sites, with 1.5 g cm-3 in PA, and 1.6 g cm-3 in SG, and a decrease in the more humid sites, with 1.2 g cm-3 in LC and 0.6 g cm-3 in NA (Table 2). 235 Total nitrogen (Nt) content was directly proportional to the climatic gradient, with values of 0.04% for PA, 0.07% for SG, 0.28% for LC and 0.51% for NA (Table 2)
Summary
Biological soil crusts (biocrusts) are highly variable communities of microscopic (cyanobacteria, algae, fungi, and bacteria) and macroscopic (lichens, bryophytes) organisms found on the surface and in the upper centimeters of the soil (Gao et al, 2017). They stabilize the soil surface (Garcia-Pichel et al, 2016), especially in arid climates, 40 where biocrusts are the main biological agents for consolidating and connecting soil aggregates (Belnap and Büdel, 2016). 50 compensate the lack of dedicated structures For this reason, biocrusts form an almost continuous layer in arid regions where water availability limits vascular plant cover (Colesie et al, 2014; Grote et al, 2010). When water demand is not restrained anymore, vascular plants have an advantage in the use of light due to their canopy development, which leads to a decrease in biocrusts (Chen et al, 2018)
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