The largest terrestrial organic carbon pool, carbon in soils, is regulated by an intricate connection between plant carbon inputs, microbial activity, and the soil matrix. This is manifested by how microorganisms, the key players in transforming plant-derived carbon into soil organic carbon, are controlled by the physical arrangement of organic and inorganic soil particles. Here we conduct an incubation of isotopically labelled litter to study effects of soil structure on the fate of litter-derived organic matter. While microbial activity and fungal growth is enhanced in the coarser-textured soil, we show that occlusion of organic matter into aggregates and formation of organo-mineral associations occur concurrently on fresh litter surfaces regardless of soil structure. These two mechanisms—the two most prominent processes contributing to the persistence of organic matter—occur directly at plant–soil interfaces, where surfaces of litter constitute a nucleus in the build-up of soil carbon persistence. We extend the notion of plant litter, i.e., particulate organic matter, from solely an easily available and labile carbon substrate, to a functional component at which persistence of soil carbon is directly determined.
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 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 largest terrestrial organic carbon pool, carbon in soils, is regulated by the intricate connection between plant carbon inputs, microbial activity, and soil matrix. This is manifested by how microorganisms, the key players in transforming plant-derived carbon into soil organic carbon, are controlled by the physical arrangement of organic and inorganic soil particles. We studied the role of soil structure on the fate of litter-derived organic matter and we propose that the persistence of soil carbon pools is directly determined at plant–soil interfaces. We show that while microbial activity and fungal growth is controlled by soil structure, occlusion of organic matter into aggregates and formation of organo-mineral associations occur in concert on litter surfaces regardless of soil structure. These two mechanisms—the two most prominent processes contributing to the persistence of organic matter—occur directly at fresh litter that constitutes a key nucleus in the build-up of soil carbon persistence.
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.
<p>Almost half of Earth&#8217;s terrestrial surface is covered by drylands, where limitation of water restricts vascular plant growth. In these ecosystems, a substantial part of primary production instead takes place directly at the soil surface, within complex microbial communities forming biological soil crusts (biocrusts) that include intricately bound soil particles. These communities, composed mainly of cyanobacteria, algae, fungi, and bryophytes, are fundamental actors for dryland biogeochemical cycles, as they fix atmospheric CO<sub>2</sub> and N<sub>2</sub> and constitute one of the primary, if not the only, sources of soil C and N. Due to the vast spatial extent of biocrusts, accounting for up to 70% of the living land cover in drylands, their importance for C and N cycling extend to the global scale, <em>i.e. </em>accounting for 7% of global net primary production of total terrestrial vegetation. However, warming temperatures and increasing soil dryness following climate change are estimated to have critical implications on these systems; recent studies<em> </em>show <em>i.e. </em>warming-induced reduction of biocrust cover and, thus, reduced CO<sub>2</sub> uptake.</p><p>Our aim is to contribute to this relatively new research field by providing insights into biocrust-soil-microorganism interactions under elevated temperatures and drought at the process-scale. This was realized in a phytotron incubation experiment of soil-biocrust mesocosms with experimental warming and drought, during which CO<sub>2</sub> uptake and heterotrophic respiration was monitored. Dual labelling pulses (<sup>13</sup>CO<sub>2</sub> and <sup>15</sup>N<sub>2</sub>) were applied to follow the fate of recently fixed <sup>13</sup>C and <sup>15</sup>N into both particulate and mineral-associated SOM pools via physical fractionation and into microbial biomass via PLFA. Further, hyperspectral VIS-NIR images of the surface were recorded to quantify and determine crust cover and composition.</p><p>The results revealed clear drought effects&#8212;not only in a distinct reduction in CO<sub>2</sub> fixation by the biocrusts, but also in the elemental distribution of soil C underneath; the effect extended down into underlying soil layers, where biocrust-derived C contents were reduced by half due to drought. The change in the translocation of biocrust-derived C into the underlying soil was reflected in the <sup>13</sup>C-PLFA profiles, showing how mainly fungi transform recently fixed C from the biocrust into their biomass in the biocrust layer, extending down into the underlying soil via fungal hyphae expansion. While drought clearly restricted the microbial abundance, warming further induced a microbial community shift, where a greater relative fungal dominance was determined under experimental warming&#8212;a shift, however, that was not reflected under dry conditions. A further combined effect was determined in N fixation, where we confirm a decrease in biocrust-derived N under drought under warming.</p><p>Our results showcase the implications of elevated temperature and drought on C and N fixation and cycling&#8212;the two most fundamental ecosystem functions in biocrust-soil systems. The results support the growing body of evidence of major implications for biogeochemical cycles in drylands in a warming world.</p>
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