Soils constitute the largest terrestrial organic carbon (C) pool, which is three times the amount of CO2 currently in the atmosphere and several times the current annual fossil fuel emissions. Thus, increasing net soil C storage can represent a substantial C sink potential with the concomitant effect in the reduction of greenhouse gas (GHG) emissions under climate change scenario. Soils in central areas of Chile showed very low organic C levels as consequence of an intensive activity of different industries such as agriculture and mining. In this sense, the high incorporation of agrochemicals in conventional and intensive agriculture and the inadequate disposal of wastes in mining operations can negatively impact the different natural and agro-ecosystems, with detrimental effects on the soil ranging from erosion, loss of organic matter (OM) and C, and a large accumulation of trace elements, which ultimately results in a significant decrease in the quality of this resource. A significant increase in the accumulation of a wide range of heavy metals in the soil, including copper, lead, cadmium and zinc among others have been widely described and associated to mining and agricultural industries.
On the other hand, under the context of sustainable agriculture, the use of agricultural wastes and thus the recycling of nutrients through biotechnological strategies is receiving increasing attention. Globally, the annual production of agricultural residues is approximately 3.7 Pg of dry matter. Straw, roots, stems and other tissues of corn, wheat and rice, are the main annual crop residues, which represent approximately 40.6%, 24.2% and 15.7%, respectively, of world production. Moreover, according to the literature, approximately 3.8 billion tons of livestock (pig and cow) and poultry manure is produced annually. For the sustainable management of these residues, aerobic composting has served as technological approach to transform this organic material into soil amendments used to increase C stock and improve different physical, chemical and biological properties of soil. However, some type of composts is degraded relatively fast under field conditions producing important nutrient losses and GHG emissions among others.
The utilization of clay minerals and recalcitrant C sources as pyrogenic C (biochar) produced by pyrolysis, have gained increasing interest as additives of composting operations, due to their influence in the reduction of GHG emissions and their effectivity in the stabilization of C both in compost and soils, leading to a cleaner compost production (reduced in emissions and leaching) and potentially C sequestrant amendments. In our previous research we reported an increase in the stabilization of C during the composting of beef manure and wheat straw mixtures and the potential sequestration of C once end-products were applied to grassland soil by the effect of halloysite (nanoclay) and oat-based biochar applied as additives. Therefore, and taking into account the positive effect related to the utilization of these additives in the composting mixture, to know how the composting process of pig manure-slurry (which is highly in heavy metal contents and GHG and odors emissions) and corn stubble can be affected by the utilization of these additives (e.g., biochar and halloysite nanoclay) in the stability of the final product has not been studied yet and result as an interesting topic of research. Moreover, the influence of additives in GHG emissions, odors, C stabilization, metal availability and the main physico chemical and biological properties during the process of composting of these organic material mixtures is also not known. Furthermore, to understand how the incorporation of this additives can also contribute in the accumulation of C and potential C sequestration once end-products are applied to metal polluted soils, and how the produced amendments affects the metal availability in soil also needs to be elucidate. Hence, this proposal suggests as main objectives 1.- To study the influence of oat based-biochar and halloysite nanoclay as additives in the composting of pig manure-slurry/corn stubble and
2.- To evaluate the effects of the end-products from composting process in the restoration of metal polluted soils from mining and agricultural industries. In a first stage, a composting study using agricultural waste co-composted with biochar and nanoclays will be conducted wherein C stabilization and gas emissions will be analyzed. Then, in order to determine the decomposition and the environmental impacts of compost in soils such as nutrient leaching and gas emissions, the produced compost will be mixed with soil and incubated. Finally, with the aim to analyze the impacts of compost in metal availability in heavy metal polluted soil, a mesocosm experiment under greenhouse conditions, and field trials will be conducted. For a better understanding of C stabilization, GHG emission, heavy metal availability and the potential mechanisms associated to the process, analytical techniques such as stable C isotope ratios (δ13C), CPMAS13CNMR spectroscopy and ICP-OES spectroscopy will be used in collaboration with the Instituto de Recursos Naturales y Agrobiología de Sevilla, Spain (IRNAS-CSIC) and ICA-CSIC Madrid. It expects that the use of described additives will improve the composting of pig manure-slurry mixture producing amendments able to be efficiently used in the remediation of contaminated soils by increasing the preservation of C and reducing the availability of heavy metals minimizing its environmental impacts and losses