Soil bacteria play a fundamental role in pedogenesis. However, knowledge about both the impact of climate and slope aspects on microbial communities and the consequences of these items in pedogenesis is lacking. Therefore, soil-bacterial communities from four sites and two different aspects along the climate gradient of the Chilean Coastal Cordillera were investigated. Using a combination of microbiological and physicochemical methods, soils that developed in arid, semi-arid, mediterranean, and humid climates were analyzed. Proteobacteria, Acidobacteria, Chloroflexi, Verrucomicrobia, and Planctomycetes were found to increase in abundance from arid to humid climates, while Actinobacteria and Gemmatimonadetes decreased along the transect. Bacterial-community structure varied with climate and aspect and was influenced by pH, bulk density, plant-available phosphorus, clay, and total organic-matter content. Higher bacterial specialization was found in arid and humid climates and on the south-facing slope and was likely promoted by stable microclimatic conditions. The presence of specialists was associated with ecosystem-functional traits, which shifted from pioneers that accumulated organic matter in arid climates to organic decomposers in humid climates. These findings provide new perspectives on how climate and slope aspects influence the composition and functional capabilities of bacteria, with most of these capabilities being involved in pedogenetic processes.

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 denuded 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 collected under biocrust communities from four national parks in Chile along a large climatic gradient ranging from (north to south) arid (Pan de Azúcar, PA), semi-arid (Santa Gracia, SG), Mediterranean (La Campana, LC) to humid (Nahuelbuta, NA). Biocrust communities showed a stabilizing effect on the soil aggregates in dry fractions for the three northern sites and the wet aggregates for the southernmost site. 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 with higher plant communities. Moreover, we found an increase in stability for aggregate size classes < 2.0 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 in aggregate stability is masked under humid conditions by higher vegetation and organic matter contents in the topsoil.

The O’Higgins Region, located in Central Chile, concentrates 40% of the country’s maize production, mainly under conventional tillage. This has generated soil physical degradation, modifying water movement on it, which varies even in short distances. In this study, we wanted to evaluate the spatial variability of different physical and hydraulic properties in relation to the agricultural use of Inceptisol. The study was conducted on a farm in Central Chile, in a fallow–maize rotation under conventional tillage. Penetration resistance (PR) was measured by using systematic sampling, defining areas of high and low PR, where soil samples were collected in places with frequent crossing of machinery (+M) and places without crossing (−M) and on topsoil and subsoil, establishing four treatments: +M—topsoil, +M—subsoil, −M—topsoil, and −M—subsoil. Organic matter (OM), texture, bulk density (BD), hydraulic conductivity (K), and hydrophobicity (R) were measured. The soil order was Inceptisol with a sandy-loam texture. The PR ranged from 200,000 to 2,000,000 Pa on topsoil and 600,000 to 2,400,000 Pa in subsoil, and the OM content was higher with a low PR. The K varied from 0.6 to 18 cm h⁻¹, being greater in depth, as tillage disturbs the topsoil stabilized during the season. A linear relationship was found between the K and R, explaining differences between high- and low-PR sites. There was an association between K_sat with position (subsoil/topsoil) and PR (high/low) that may allow us to use the PR as a proxy for K.

Biological soil crusts (biocrusts) composed of cyanobacteria, algae, lichens and bryophytes have a stabilizing effect on the soil surface. This effect is mostly studied in arid climates, where biocrusts are the main biological agent to steady and bind together soil aggregates. Nevertheless, biocrusts are also an integral part of the soil surface under semi-humid and humid climate conditions, mainly covering open spaces in forests and on fallow lands. As such, they often develop after vegetation disturbances, when their ability to compete with higher plants is increased. To better understand how biocrusts mediate changes in soil aggregate stability under different climatic conditions, we analyzed soil substrates taken under biocrust communities from four national parks in Chile using dry and wet sieving. These samples cover soils from a large climate gradient from arid (Pan de Azúcar), semiarid (Santa Gracia), mediterranean (La Campana) to humid (Nahuelbuta). Biocrust communities were dominated by cyanobacteria in Pan de Azúcar and Santa Gracia, bryophytes and lichens in La Campana and bryophytes in Nahuelbuta. They showed a stabilizing effect on the soil surface in three of the four investigated climates. Their presence increased the Mean Weight Diameter of the aggregates (MWD) by 102% in Pan de Azúcar, 208% in Santa Gracia and 82% in La Campana. In Nahuelbuta there was no significant increase to the condition without biocrust, because the abundance of permanent soil covering higher vegetation does not allow the effect of biocrusts to manifest. The stabilization differed between the aggregate fractions studied, being most pronounced for smaller aggregates >2 mm. The Geometric Mean Diameter (GMD) showed similar results as MWD, but with a clear effect of drying and wetting conditions, as an increase in the stability directly related to precipitation and the climatic gradient. Bulk density (BD) changed from high mean values of 1.50 g cm-3 in Pan de Azúcar and 1.63 g cm-3 in Santa Gracia (where cattle grazing was observed) to 1.16 g cm-3 in La Campana and the lowest mean of 0.62 g cm-3 in Nahuelbuta, where we observed a more developed soil structure and high organic matter content (21.58% in average). Accordingly, here we also found pronounced hydrophobicity of the soil. These preliminary findings indicate not only differences in the stability of the aggregates, but also in the state of conservation and management of the soils. Results will now be extended by further statistical analyses, which will additionally be presented at vEGU21.

Bearing capacity (BC) can explain mechanical soil quality, but its alteration generally comes accompanied by changes in other soil functions. One way to determine BC is by compression tests on undisturbed soil samples, where increasing normal loads are applied at the surface of confined soil core cylinders. Nevertheless, in natural conditions soils are semi-confined, allowing the soil particles to move upward to release the applied forces. Furthermore, in agricultural soils, the loads are primarily dynamic, such as the traffic of machinery or cattle. In this study, we hypothesized that the application of dynamic loads on unconfined soils is more detrimental than static loads, reflected as a decrease in the functionality of two agricultural soils under conventional tillage management. For this purpose, static and dynamic pre-compression tests to determine BC were evaluated in 2 soils, 2 depth levels, and 2 levels of mechanical resistance (8 treatments in total), using semi-confined cylinders, complemented with measurements of saturated hydraulic conductivity (Ks) as an indicator of pore functionality before and after loading. When comparing the dynamic with the static test, it showed no changes in BCwhich was reflected in a homogenization under the studied conditions.. Nevertheless, an increase in Ks from 66 cm h-1 in the initial condition, to 186 cm h-1 in static condition, and 295 cm h-1 in the dynamic test could be shown. The increase in the dynamic test contradicted our initial assumption and could be attributed to cracking of the soils, boosted by the partial confinement of the samples, which allowed sideways movement of the soil against the loads creating zones of preferential flow.

Biological soil crusts (biocrusts) have received increasing attention in recent years, especially in arid regions, where it is the main living cover, due to its active role in a variety of ecological services, for example regulation of nutrient and water cycles, surface stabilization and erosion protection. Nevertheless, biocrusts are present globally but understudied under climatic conditions other than arid, where they also play a role. Our study from the Chilean Coast Range shows that the mechanisms and the subsequent results of the occurrence of biocrust vary in non-arid conditions regarding sediment discharge, surface runoff, and nutrient allocation. Here we show a shift in the surface roughness from smooth to rugose as the water availability increases, resulting in shorter times to start the runoff in humid conditions but a lower total runoff volume. There were also changes in the amount and composition of the sediment discharged during erosion, with a sustained reduction of it in the presence of biocrusts but increasing carbon and nitrogen sediment contents under semiarid climates and a decrease in humid and mediterranean. This place biocrusts as a relevant community along different ecosystems, but with behavior that depends on the local conditions that must be considered.