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

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

“…Despite the functional redundancy “dampening” the major signals in the MGS data, various modest conclusions can still be drawn about the overall community functioning of the control and treatment restoration plots. The MetaCyc results showing slightly higher total biosynthesis and degradation mapping in all amended soils than in controls accords with higher nutrient levels (total organic C, total N, and assimilable P), higher respiration rates, increased enzyme activities, and 10- to 40-fold increases in bacterial biomass (via fatty acid methyl ester [FAME] analysis) observed in the treated soils as part of the companion studies on the quarry restoration project ( 16 , 26 , 30 ). Similarly, the overall MetaCyc mapping order of SS > (COVG/COHort) > control matches the pattern of increased organic matter remineralization in all treatments over control soils, with the highest in sewage, followed by COVG and COHort at intermediate levels ( 16 ).…”

“…The similarity to natural soils was therefore quite surprising given that every single measure of soil function (chemistry, CO 2 flux/respiration, enzyme activities, lipid/carbohydrate contents, etc.) indicated the control soils were of poor biological quality and only the compost treatment plots showed improvement for supporting introduced vegetation ( 16 , 26 , 30 ). It could therefore be that the only two things separating the control mining soils from assuming more “natural soil function” are the topsoil nutrients removed during mining (reintroduced in the compost plots) and the rates of growth plus expression of the same bacteria that are present in both, which could be elucidated by metatranscriptomics, as discussed in context below.…”

“…Changes in the taxonomic composition and functions, especially related to the carbon cycle, were examined in detail between soils restored with the different treatments compared to degraded unrestored soils in the quarry (Control) and surrounding natural soils undisturbed by mining activity (Natural). Given the results of the previous studies on these restoration experimental plots, our working hypotheses were (i) that experimental and control plots within the degraded soils would show stark differences from nearby natural soils; (ii) that the same major players detected in the 16S analyses would also be recovered in the metagenomic taxonomic analysis; (iii) that the functional data unique to metagenomics would discriminate among soil types, while still displaying a minor amount of functional redundancy between them; and (iv) that the SS treatment would show the highest functional diversity due to its previously observed highest biological activity of all the treatments ( 26 , 30 ).…”

“…The application of organic amendments to mining soils has been shown to be a successful method of restoration, improving physical (e.g., moisture and aggregate stability) and chemical (e.g., pH, organic matter, total N, available P, etc.) soil properties and key indicator enzymatic activities (involved in the C, N, and P cycles) ( 19 , 24 , 26 – 29 ) and influencing CO 2 fluxes ( 30 ). After the application of organic amendments in mining soils, the changes in the abovementioned physicochemical soil properties (which are considered key factors in directing the structure of soil microbial communities) ( 31 – 36 ) indirectly influence soil bacterial community composition ( 16 , 24 ).…”

“…As metagenomic analysis forgoes using selective primers and PCR, it is widely considered a much less biased approach to analyze communities—although challenges still remain in having adequate database coverage of novel organisms, the preponderance of unknown functions, and the fact that gene presence does not necessarily equate to expressed function ( 52 , 53 ). That being said, given the greater resolution of this technique, the main goal of the work presented here is the further functional profiling of the effects of different soil restoration treatments (of differing organic amendments from recycled materials) applied to degraded, arid quarry soils in southern Spain which had previously only been molecularly profiled with 16S analyses ( 16 ) but which have been extensively studied from a physicochemical/biological activity perspective as part of a multiannual restoration experiment ( 26 , 30 ). The organic amendments applied were (i) 100% vegetable compost manufactured from garden waste (COVG), (ii) 100% vegetable compost manufactured from horticultural greenhouse crop waste (COHort); (iii) treated sewage sludge waste (SS), (iv) an amendment made from a 50:50 mixture of COVG + SS, and (v) an amendment made from a 50:50 mixture of COHort + SS.…”