Biochar ageing in polluted soils and trace elements immobilisation in a 2-year field experiment.

Biochar application to soils has become a focus of research during the last decade due to its high potential for C sequestration. Nevertheless, there is no exhaustive information on the long-term effects of biochar application in soils contaminated with trace elements. In this work, a 2-year field experiment was conducted comprising the application of different types of biochar to acidic and moderately acidic soils with high concentrations of As, Cu, Pb, Ba and Zn. In addition, representative samples of each biochar were buried in permeable bags that allowed the flow of water and microorganisms but not their physical interaction with soil aggregates. The biochars significantly adsorbed trace elements from polluted soils. However, given the high total concentration of these persistent trace elements in the soils, the application of biochars did not succeed in reducing the concentration of available metals (CaCl2 extractable fraction). After 2 years of ageing under field conditions, some degradation of the biochars from olive pit, rice husk and wood were observed. This study provides novel information concerning the biochar alterations during ageing in polluted soils, as the decrease of aryl C signal observed by 13C nuclear magnetic resonance (NMR) spectroscopy and the presence of O-containing groups shown by Fourier Transform mid-Infrared Spectroscopy (FT-IR) in aged biochar which enhanced trace elements adsorption. Scanning electron microscopy (SEM) revealed slight changes on surface morphology of aged biochar particles.

Influence of inorganic additives on wheat straw composting: Characterization and structural composition of organic matter derived from the process.

Metallic oxides and clay minerals have gained increasing interest as additives of composting due to their influence in greenhouse gas emissions reduction and their effectivity in the stabilization of carbon both in compost and soils, leading to a cleaner compost production and potentially C sequestrant amendments. In this study, wheat straw (WS) was co-composted with iron oxide and allophanic soil and their influence on WS composting and composition of the end-products was evaluated. WS compost and their humic like-substances (HS) fraction were characterized by chemical and spectroscopic analyzes. After 126 days of process, the elemental composition showed slight differences of the N content for compost and HS, where the C/N atomic ratio tended to decrease relative to the initial material (WS; ~130). This trend was more pronounced in the HS from co-composted treatments (<30). The addition of inorganic materials increased the total acidity and phenolic-OH group contents (~15 and 14 mEq g-1 respectively, iron oxide treatment) relative to the treatment without inorganic additives. Nevertheless, the FTIR and solid-state 13CNMR spectroscopy barely support the wet chemical analysis and revealed a similar final composition between all the studied compost treatments. These results suggest that the incorporation of these materials as compost additives had no major effect on the spectroscopic features of the end-products, however, critical changes of the properties such as the extractability, functionality and composition of HS were revealed by traditional methods. In conclusion, the supply of metal oxides and clays could impact the aerobic composting of WS favorizing the stabilization of certain C pools and adsorptive properties of the end-products, that is of importance in production of amendments suitable for being used in degraded and contaminated soils. Nevertheless, under the experimental conditions of our research C stabilization apparently depends of other mechanisms that still need to be elucidate.

Comparison of the structural stability of pasture and cultivated soils.

The structural properties of two neighbouring soils from the NW of Spain were evaluated in order to elucidate the effect of management on the soil structural quality and soil organic carbon turnover. The two soils were developed on granite under a warm and humid climate, but differed in land use (pasture and cultivation). The pasture soil had more favourable structural properties than the cultivated soil, showing lower bulk density, higher porosity and water retention. Also, the pasture soil showed a higher mean aggregate diameter and aggregate stability against mechanical agitation in water, as well as lower soil loss under simulated rainfall. This increased structural stability of the pasture soil could be attributed to its higher soil organic matter (SOM) content. The effect of soil use and aggregate size on SOM mineralization was also investigated. Laboratory incubation experiments were conducted with 1-5 mm aggregates and disaggregated <1 mm soil. More C-CO(2) was released by SOM mineralization in the pasture soil than in the cultivated soil, thus indicating a higher microbial activity in the pasture soil. The respiratory quotient (C-CO(2)/Corg) was also higher in the pasture soil, which means that SOM in this soil is more accessible to microbial decomposition. Nevertheless no significant differences were observed between organic C mineralization in the disaggregated <1 mm soil and the undisturbed 5-1 mm aggregates. The overall results demonstrate the need to maintain adequate levels of OM by adding organic amendments or adopting lower impact cultivation practices such as reduced tillage.