The nutrients budget of wetlands: a study from Darcy’s scale to global scale

Abstract

During the last centuries, human activities have been under an extraordinary expansion as a result of the population growth; this has implied an increase in production of general goods, food, and energy, which have reflected in the contaminant load in the environment and the potential threats on the ecology and soil fertility. Here, I refer to "contaminant" as to anthropogenic substances related to agriculture such as nutrients inputs associated with fertilizers and plant protection products such as pesticides, and to industrial activities leading to nitrogen and sulfur release in the atmosphere that results in atmospheric deposition. Both forms of contaminants may reach natural ecosystems via different pathways such as direct applications, surface runoff, or atmospheric deposition. This thesis aims to understand the main processes involved in contaminants dispersion and their interactions with the nutrient cycles in wetlands soil, because their low elevations in catchments make them the final recipients. Most of the agrochemicals are used together with surfactants, enhancing the physical stability and foliar uptake. Surfactants can increase or decrease the water infiltration in soil, thus changing soil hydraulics. Agrochemicals composition may also include nutrient-rich mixtures, which can contribute to increase the nutrients inputs to wetlands. These inputs may cause yet unknown consequences to wetlands functioning, even if several different aspects of wetlands biogeochemistry have already been explored. Only a multidisciplinary approach involving hydrology, soil science, biogeochemistry, and advanced numerical methods can be devised to develop the tools to attempt a comprehensive assessment framework and unravel how the surrounding catchment and boundary conditions affect wetlands. The investigation in this doctoral thesis starts by analyzing in-depth the physical-chemical processes at the Darcy's scale. Specifically, experiments and numerical models are used to investigate the physical-chemical interactions of different organic substances (glycerol and butanol) in unsaturated water flow at the Darcy's scale and understand when to account for them in hydraulic models. Note this, the attention is then focused on the repercussions on the biogeochemical nutrient cycles in soil, that is, understanding how the micro- and macro-biota respond to anthropogenic and natural stressors and quantifying how greenhouse emissions and nutrient stocks change. An advanced biogeochemical reaction network model of the soil organic matter cycle, BAMS3, was linked to describe the nitrogen and sulfur cycles and was validated against plot scale observations of ecohydrological forcing in a wetland. Each biologically-mediated reaction is described using Michaelis-Menten-Monod (MMM) kinetics, which includes temperature, pH, substrate competition, and redox potential on microbial growth. Here, I have been studying how temperature, sulfur deposition, and plant community structure may change underlying microbial dynamics and subsequent greenhouse gas emissions. The outcomes of this study show the strict connection between microbial community and hydroclimatic conditions and the influence of sulfur deposition on CH4 emissions. A sensitivity analysis of the BAMS3 model has been conducted using the Morris screening method of Elementary Effects to highlight the main processes and test the sensitivity of some parameters. A simplified reaction network, BAMS4, has been developed based on these sensitivity analyses and was next applied on a regional scale assessment of the Australian wetlands. Here, elemental fluxes in each reaction involving carbon, nitrogen, and sulfur have been studied, and the greenhouse emissions have been quantified against hydroclimatic conditions and different nutrients inputs including direct applications of fertilizers and nitrogen and sulfur deposition, as well as indirect inputs from runoff. The same BAMS4 network has next been used for a global scale assessment of wetland greenhouse emissions and nutrient stocks for the current climate scenarios. The outcomes highlight a significant correlation between different driving forces and greenhouse gas emissions and nutrient stocks. It also points out potential new greenhouses gases hotspots consequent to projections of climate change. Finally, the achievements of this thesis justify the choice of a multidisciplinary approach to develop a new mechanistic model that can be applied to describe environmental processes and provide a tool for decision-makers to design better adaptation and restoration plans.