The effects of biochar on soil organic matter pools are not influenced by climate change.

The sustainability of Mediterranean croplands is threatened by climate warming and rainfall reduction. The use of biochar as an amendment represents a tool to store organic carbon (C) in soil. The vulnerability of soil organic C (SOC) to the joint effects of climate change and biochar application needs to be better understood by investigating its main pools. Here, we evaluated the effects of partial rain exclusion (∼30%) and temperature increase (∼2 °C), combined with biochar amendment, on the distribution of soil organic matter (SOM) into particulate organic matter (POM) and the mineral-associated organic matter (MAOM). A set of indices suggested an increase in thermal stability in response to biochar addition in both POM and MAOM fractions. The MAOM fraction, compared to the POM, was particularly enriched in labile substances. Data from micro-Raman spectroscopy suggested that the POM fraction contained biochar particles with a more ordered structure, whereas the structural order decreased in the MAOM fraction, especially after climate manipulation. Crystalline Fe oxides (hematite) and a mix of ferrihydrite and hematite were detected in the POM and in the MAOM fraction, respectively, of the unamended plots under climate manipulation, but not under ambient conditions. Conversely, in the amended soil, climate manipulation did not induce changes in Fe speciation. Our work underlines the importance of discretely taking into account responses of both MAOM and POM to better understand the mechanistic drivers of SOC storage and dynamics.

Hydrothermal treatment as a complementary tool to control the invasive Pampas grass (Cortaderia selloana).

The rapid spread of invasive Pampas grass (PG) is having not only ecosystems impact, but also significant economic and social effects. The tonnes of bulky waste from the plant disposal require proper treatment to avoid seed dispersal, greenhouse gas emissions and landscape damage. In the pursuit of zero-waste management, hydrothermal treatment (HT) appears as a challenging alternative. The possibility of mobile HT systems offers an alternative to accomplish on-site both the PG waste management and the application of the resulting by-products within a circular framework. As a first step, this research shows that, without a prior drying step, the hydrothermal treatment at 100-230 °C under autogenous water vapor pressure for only 30 min allows safe seeds inertization, while a stable carbon-enriched solid and an aqueous stream are generated. Prolonging the process for 2 h has no profitable effects. As the reaction temperature increases, the PG residue is converted into a material with 49-58 wt% of carbon, 41-32 wt% of oxygen and 3-4 wt% of ash. The pH (~6.3), low electrical conductivity (1.21-0.86 dS/m), high carbon content, open porosity (5-8 m2/g) and improved performance in seed germination and in the early growth test suggest the potential of HT-solids derived at 100-120 °C as amendment to sequester carbon in the soil and improve its physico-biological properties. The phytotoxicity detected in the peat/lignite-like solids obtained at 200-230 °C limits its application in soil, but calorific values of 22-24 MJ/kg indicate their suitability as CO2-neutral fuel. The agrochemical analysis of the liquid by-products indicates poor value on their own, but their use supplemented with compost may be an option.

Hydrothermal carbonization as a sustainable strategy for integral valorisation of apple waste.

Hydrothermal carbonization makes feasible the integral and profitable recovery of industrial apple waste within a zero-residue bio-economy. 82-96% of the energy and 80-93% of the C in the apple bagasse are retained in the solids generated by hydrothermal treatment at 180 and 230 °C for 2 and 4 h. Such processes stabilize the apple waste and lead to CO2 neutral solid fuels with calorific value close to 30 MJ/kg. The agrochemical properties of the solid by-products suggest their potential to improve soil quality. Aqueous streams containing valuable phenolic compounds and saturated fatty acids are generated simultaneously, which provide additional cost-effectiveness. The by-products characteristics can be suited to the final application by selecting the reaction temperature, whereas the process duration has less impact. Optical microscopy and reflectance measurements are presented, for the first time, as powerful tools for assessing the biomass transformation when subjected to hydrothermal treatment under different conditions.