Influence of nitrogen and soil physical characteristics on belowground carbon flux dynamics of woody plants

At ecosystem level soil respiration (Rs) represents the largest carbon (C) flux after gross primary productivity, being mainly generated by root respiration (autotrophic respiration, Ra) and soil microbial respiration (heterotrophic respiration, Rh). In the case of terrestrial ecosystems, soils contain the largest C-pool, storing twice the amount of C contained in plant biomass. Soil organic matter (SOM), representing the main C storage in soil, is decomposed by soil microbial community. This process produces CO2 which is mainly released as Rh. It is thus relevant to understand how microbial activity is influenced by environmental factors like soil temperature, soil moisture and nutrient availability, since part of the CO2 produced by Rh, directly increases atmospheric CO2 concentration and therefore affects the phenomenon of climate change. Among terrestrial ecosystems, agricultural fields have traditionally been considered as sources of atmospheric CO2. In agricultural ecosystems, in particular apple orchards, I identified the role of root density, soil temperature, soil moisture and nitrogen (N) availability on Rs and on its two components, Ra and Rh. To do so I applied different techniques to separate Rs in its two components, the ”regression technique” and the “trenching technique”. I also studied the response of Ra to different levels of N availability, distributed either in a uniform or localized way, in the case of Populus tremuloides trees. The results showed that Rs is mainly driven by soil temperature, to which it is positively correlated, that high levels of soil moisture have inhibiting effects, and that N has a negligible influence on total Rs, as well as on Ra. Further I found a negative response of Rh to high N availability, suggesting that microbial decomposition processes in the soil are inhibited by the presence of N. The contribution of Ra to Rs was of 37% on average.

Purple bacteria for the biotechnological valorisation of agro-industrial streams

Waste management worldwide has received increasing attention from global policies in recent years. In particular, agro-industrial streams represent a global concern due to the huge volumes generated and a high number of residues, which produce an environmental and economic impact on the ecosystem. The use of biotechnological approaches to treat these streams could allow the production of desirable by-products to be reinjected into the production cycle through sustainable processes. Purple phototrophic bacteria (PPB) are targeted as microorganisms capable to reduce the pressure of agro-industrial streams on environmental issues, due to their metabolic versatility (autotrophic and/or heterotrophic growth under different conditions). This Ph.D. research project aims to assess the effectiveness of PPB cultivation for industrial streams valorisation in the applications of biogas desulfurization and microbial protein production. For these purposes, the first part of the present work is dedicated to the cultivation of purple sulfur bacteria (PSB) for biogas streams upgrading, cleaning biogas from sulfur compounds (H2S), and producing elemental sulfur (S0), potentially suitable as a slow-release fertilizer. The second part of the thesis, instead, sees the application of purple non-sulfur bacteria (PNSB) on streams rich in organics, such as molasses, generating biomass with high content of proteins and pigments, useful as supplements in animal feed. The assessment of the main metabolic mechanisms involved in the two processes is evaluated at a laboratory scale using flasks and a photobioreactor, to define the consumption of substrates and the accumulation of products both in the autotrophic (on biogas) and in heterotrophic grow (on molasses). In conclusion, the effectiveness of processes employing PPB for a sustainable valorisation of several agro-industrial streams has been proved promising, using actual residues, and coupling their treatments with the production of added-value by-products.