Soil amendments from recycled waste differently affect CO₂ soil emissions in restored mining soils under semiarid conditions

Organic amendments from recycled waste promote short-term carbon sequestration of restored soils in drylands

Soils are considered as a major reservoir for terrestrial carbon and it can act as a source or sink depending upon the land management activities. In semi-arid areas, the natural recovery of soils degraded by mining activities is complicated. A possible solution to recover soil quality and functionality, plant cover and carbon sequestration capacity could be the application of organic amendments. This work focuses on a restoration carried out in 2018 by applying with different composted organic amendments (stabilized sludge, gardening and greenhouse waste) in a limestone quarry under semi-arid climate (SE Spain). The objective was to evaluate the effects of different organic amendments on net CO₂ exchange in two microcosms: soil-Stipa tenacissima and soil-spontaneous vegetation. Soil physical and chemical properties, environmental and ecological variables and their interrelationship were studied in amended and unamended soils. The results obtained under soil-forming factors in the study area showed an increase in soil organic carbon and nitrogen content, improved moisture and plant growth, and plant canopy development in amended soils. Soil moisture, soil temperature and plant cover significantly influenced net CO₂ exchange. In general, microcosms with S. tenacissima showed higher carbon sequestration rates than soils with only spontaneous plant cover. Soils treated with a vegetable-only amendments showed higher plant cover and CO₂ fixation rates after significant rainfall. On the other hand, the plots treated with sludge compost presented more soil respiration than photosynthesis, especially in the wet seasons. Soils with sludge and greenhouse compost mixed had higher CO₂ fixation rates than soils restored with a mixture of sludge and garden compost. Soils with greenhouse waste compost showed CO₂ fixation in the microcosm with plants in all campaigns, being the best treatment to promote atmospheric CO₂ sequestration in soil restoration.

Responses of abundant and rare prokaryotic taxa in a controlled organic contaminated site subjected to vertical pollution-induced disturbances

Soil microbiota as the key role mediates the natural attenuation process of organic contaminated sites, and therefore illuminating the mechanisms underlying the responses of abundant and rare species is essential for understanding ecological processes, maintaining ecosystem stability, and regulating natural attenuation well. Here, we explored the distributional characteristics, ecological diversities, and co-occurrence patterns of abundant and rare prokaryotic subcommunities using 16S rRNA high-throughput sequencing in vertical soil profiles of a controlled organic contaminated site. Results showed that abundant prokaryotic taxa were widespread across all soil samples, whereas rare counterparts were unbalancedly distributed. Rare subcommunity had more taxonomic groups and higher α- and β-diversities than abundant subcommunity. Both of these two subcommunities surviving in the organic polluted site possessed the potential of degrading organic contaminants. Abundant subcommunity was little affected by abiotic factors and mainly shaped by soil depth, while rare one was sensitive to environmental disturbances and presented a non-depth-dependent structure. Co-occurrence analysis revealed that rare taxa were more situated at the center of the network and more inclined to cooperate with non-abundant species than abundant taxa, which might play crucial roles in enhancing the resilience and resistance of prokaryotic community and maintaining its structure and stability. Overall, our results suggest that abundant and rare prokaryotic subcommunities present different responses to physicochemical factors and pollution characteristics along vertical soil profiles of organic contaminated sites undergoing natural attenuation.

Benefits of applying organic amendments from recycled wastes for fungal community growth in restored soils of a limestone quarry in a semiarid environment

Applying organic amendments to recover physical, chemical, and biological qualities of soil may enable recovery of soils degraded by mining in semiarid climates. This study's aim was to investigate the development and changes in the composition of fungal communities in restored soils with five different types of organic amendments (two types of vegetable compost and sewage sludge compost, and a mixture of both) compared with unamended soils and surrounding natural soils and to examine the relationships between the fungal taxa, the new physico-chemical and biological soil properties of technosoils after 18 months of restoration, and natural soils. Restoration improved soil quality and fungal diversity, placing these soils in an intermediate position between unrestored soils (with no fungi present) and undisturbed reference soils, which were the most fungal diverse. Sewage-treated soils and their mixtures showed high nitrogen and carbohydrate content as well as high basal respiration and fatty acid content, suggesting that they provided readily biodegradable organic matter. In contrast, greenhouse compost-treated soils showed high total organic carbon and polyphenol content, whereas garden compost-treated soils showed intermediate values. The biological soil properties of both composts showed were similar to those of the reference soils, suggesting that composts contained more resilient organic matter. Organic amendments of dissimilar origin caused significantly different fungal soil communities at the genus level among the restored soils. Results indicated that soil pH, electrical conductivity, total nitrogen content, soil basal respiration, fungi/bacteria-PLFA ratio, and dehydrogenase and β-glucosidase activities, together with Pearson's correlations, revealed that these properties and nutrient content (total organic carbon, C/N ratio, carbohydrates, and polyphenols) influenced 40 soil fungal taxa. Therefore, the organic amendments led to changes in soil properties that favoured plant cover by promoting the soil fungal community growth beneficial to the carbon cycle and symbiotic with plants.

Functional and Taxonomic Effects of Organic Amendments on the Restoration of Semiarid Quarry Soils

The application of organic amendments to mining soils has been shown to be a successful method of restoration, improving key physicochemical soil properties. However, there is a lack of a clear understanding of the soil bacterial community taxonomic and functional changes that are brought about by these treatments. We present further metagenomic sequencing (MGS) profiling of the effects of different restoration treatments applied to degraded, arid quarry soils in southern Spain which had previously been profiled only with 16S rRNA gene (16S) and physicochemical analyses. Both taxonomic and functional MGS profiles showed clear separation of organic treatment amendments from control samples, and although taxonomic differences were quite clear, functional redundancy was higher than expected and the majority of the latter signal came from the aggregation of minor (<0.1%) community differences. Significant taxonomic differences were seen with the presumably less-biased MGS-for example, the phylum Actinobacteria and the two genera Chloracidobacterium (Acidobacteria) and Paenibacillus (Firmicutes) were determined to be major players by the MGS and this was consistent with their potential functional roles. The former phylum was much less present, and the latter two genera were either minor components or not detected in the 16S data. Mapping of reads to MetaCyc/BioCyc categories showed overall slightly higher biosynthesis and degradation capabilities in all treatments versus control soils, with sewage amendments showing highest values and vegetable-based amendments being at intermediate levels, matching higher nutrient levels, respiration rates, enzyme activities, and bacterial biomass previously observed in the treated soils. IMPORTANCE The restoration of soils impacted by human activities poses specific challenges regarding the reestablishment of functional microbial communities which will further support the reintroduction of plant species. Organic fertilizers, originating from either treated sewage or vegetable wastes, have shown promise in restoration experiments; however, we still do not have a clear understanding of the functional and taxonomic changes that occur during these treatments. We used metagenomics to profile restoration treatments applied to degraded, arid quarry soils in southern Spain. We found that the assortments of individual functions and taxa within each soil could clearly identify treatments, while at the same time they demonstrated high functional redundancy. Functions grouped into higher pathways tended to match physicochemical measurements made on the same soils. In contrast, significant taxonomic differences were seen when the treatments were previously studied with a single marker gene, highlighting the advantage of metagenomic analysis for complex soil communities.