Study of the effect of organic amendments on soil functionality, and CO2 emission and fixation patterns in restored technosols in a limestone quarry in a semi-arid climate

Résumé

The soil is the largest terrestrial carbon reservoir, but its formation is a slow and limited process, which makes it susceptible to degradation by human activity. Soil degradation alters biochemical cycles and biodiversity, and can make ecosystems more vulnerable to global change pressure. Degraded soils lose their quality and functionality, reducing their ability to produce environmental goods and ecosystem services (e.g., carbon sequestration and water cycling). These threatens can be reversed and soil can become a source of CO2 and other greenhouse gases. Soil restoration and conservation is especially important in the arid and semi-arid areas of the Mediterranean region, where processes leading to soil degradation due to human activities, such as open-pit mining for aggregate extraction, can cause a loss of soil quality, reducing its ability to perform its functions. Recovery of soil functions can be accelerated through the application of appropriate techniques. One key strategy for restoring degraded soils due to open-pit mining in semi-arid regions is the application of organic amendments. However, the selection of appropriate organic amendments is essential, in order to improve, in the short term, soil quality and functionality, without contributing to increase CO₂ emissions into the atmosphere. The objective of this thesis is a better understanding of (i) the biogeochemical processes in the stabilization of exogenous organic matter applied through different types of organic amendments, (ii) their evolution in the development of technosols, and (iii) the impacts on soil functionality and quality in a current context of climate change. This research is part of a project devoted to the restoration of degraded soils by open-pit mining activities, namely the restoration of a quarry for aggregates extraction in the Sierra de Gádor (Almería), where several restoration treatments based on different organic amendments were applied. These treatments consisted of stabilized sewage sludge, vegetable compost from garden waste, vegetable compost from greenhouse crop residues, and two mixtures of sewage sludge with the different vegetable composts. Native plants were also used in the restored plots. In addition, control plots without organic amendments were established, and natural soils from the closer areas were selected as reference level. To carry out the research, field and laboratory experiments were conducted by an integrated and functional perspective, in order to effectively provide answers to the proposed specific research questions. The accomplisment of the partial objectives also lead to the development of scientific articles, which have been included as chapters of this thesis. In Chapter 1, the short-term effects of organic amendments on different physical, chemical, and biological indicators of soil quality and functionality were examined, with a particular emphasis on organic matter mineralization and CO2 release due to the priming effect. For this purpose, a set of traditional physical and chemical indicators of soil quality were determined, different enzymatic activity involved in the C, N and P cycles were quantified, together with the study of soil basal respiration and fatty acid content of bacteria and fungi. Additionally, an IRMS analyzer was used to determine the δ13C isotopic signal, which allowed for the determination of the priming effect and short-term C mineralization. Chapter 2 addressed the partial objective of improving the understanding of the short- and medium-term evolution of exogenous soil organic matter applied in technosols, through the study of their structure and molecular composition, as well as their implications for CO2 emissions. To this goal, the content of humic and fulvic acids was analyzed, and thermogravimetry (TG), differential scanning calorimetry (DSC) and analytical pyrolysis (Py-GC/MS) analyses were carried out. Chapters 3 and 4 aimed to evaluate the impact of different types of organic amendments on CO2 emissions as well as to improve our understanding of processes associated to CO2 exchange in soil and plants (e.g., respiration, photosynthesis). To this goal, field measurement campaigns were carried out to monitor CO₂ fluxes, using two IRGA teams (PP-Systems EGM4 with dark chamber and Licor LI-840a with transparent chamber), covering a wide range of environmental conditions. The overall results of the study have shown that, among the tested organic amendments, vegetable compost from greenhouses and, by a lower extent, vegetable compost from gardening were more appropiate than stabilized sewage sludge and its mixtures for the restoration of a limestone quarry in a semi-arid climate, in terms of soil properties, CO2 emission and fixation, priming effect, organic matter quality, and plant growth. Then, the first two treatments were the most favorable in terms of functional recovery in the short to medium term. In addition, the results of this thesis highlighted the importance of broading our understanding on how the restoration measures can be used as a climate change mitigation strategy in degraded semiarid areas, where extreme environmental conditions can lead soils to lose their carbon retention capacity. These findings can be useful in developing more effective and sustainable restoration treatments to promote soil recovery, establish a stable vegetation cover, increase CO2 sequestration capacity, and improve ecosystem resilience in a changing climate scenario.