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Field application of biochar is intended to increase soil carbon (C) storage. The assessment of C storage potential of biochars lacks methods and standard materials. The reactivity of biochars and hydrochars may be one possible means of evaluating their environmental stability. The aim of this study was to evaluate the reactivity of biochar produced by gasification (GS) and hydrochar produced by hydrothermal carbonisation (HTC). The approach included analysis of the two different char types produced from the same three feedstocks. Moreover, we analysed the reactivity of Holocene charcoal (150 and 2000 yr old) to evaluate whether or not their use as standard materials to represent stable biochar is meaningful. We assessed carbon loss following oxidation with acid dichromate as well as hydrolysis with HCl. Our results showed that chemical reactivity is not a straightforward approach for characterising the stability of biochar and hydrochar. Acid hydrolysis showed little difference between HTCs and GSs, despite the contrasting elemental composition. Using acid dichromate oxidation, we determined that GSs contained ca. 70% of oxidation resistant C while the proportion for HTCs was 2000 yr old charcoal > HTCs > feedstock and was related to elemental composition. This shows that acid dichromate oxidation may allow differentiation of the reactivity of modern biochars but that there is not necessarily a relationship between reactivity and age of Holocene charcoals. As the chemical reactivity of biochars may change with exposure time in soil, it is poorly suited for assessing their environmental residence time.

Small portable rainfall simulators are considered a useful tool to analyze soil erosion processes in cultivated lands. European research groups in Spain (Valencia, Málaga, Lleida, Madrid and La Rioja), France (Reims) and Germany (Trier) have used different rainfall simulators (varying in drop size distribution and fall velocities, kinetic energy, plot forms and sizes, and field of application) to study soil loss, surface flow, runoff and infiltration coefficients in different experimental plots (Valencia, Montes de Málaga, Penedès, Campo Real and La Rioja in Spain, Champagne in France and Mosel-Ruwer valley in Germany). The measurements and experiments developed by these research teams give an overview of the variety of methodologies used in rainfall simulations to study the problem of soil erosion and describe the erosion features in different climatic environments, management practices and soil types. The aims of this study are: (i) to investigate where, how and why researchers from different wine-growing regions applied rainfall simulations with successful results as a tool to measure soil erosion processes; (ii) to make a qualitative comparison about the general soil erosion processes in European terroirs; (iii) to demonstrate the importance of the development of standard method for measurement of soil erosion processes in vineyards, using rainfall simulators; and (iv) and to analyze the key factors that should be taken into account to carry out rainfall simulations. The rainfall simulations in all cases allowed infiltration capacity, susceptibility of the soil to detachment and generation of sediment loads to runoff to be determined. Despite using small plots, the experiments were useful to analyze the influence of soil cover to reduce soil erosion, to make comparisons between different locations, and to evaluate the influence of different soil characteristics. The comparative analysis of the studies performed in different study areas points out the need to define an operational methodology to carry out rainfall simulations, which allows us to obtain representative and comparable results and to avoid errors in the interpretation in order to achieve comparable information about runoff and soil loss., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)., We acknowledge all the co-authors for the fast and friendly response to take part in this paper and their useful suggestions and corrections. Furthermore, we also thank the Caixa-Bank and DAAD (Deutscher Akademischer Austauschdienst) for the Scholarship grant awarded to J. Rodrigo-Comino.

[b]Purpose[/b] The impact of biochar on subsoil organic carbon mineralisation has never been assessed despite its susceptibility to downward transport after soil amendment. In this study, we analysed the potential mineralisation of biochar and plant material as well as their effect on native soil organic matter (SOM) decomposition in subsoil horizons.[br/][b]Materials and methods[/b] We used C-13-labelled biochar and plant material to allow disentangling substrate mineralisation and priming effects. The substrates were added to two mountain subsoils under different land use and incubated for 1 year under optimum conditions. We analysed for physical parameters and C mineralisation in the two soils. Moreover, microbial communities were assessed by phospholipid fatty acid (PLFA) analyses.[br/][b]Results and discussion[/b] Our results indicated contrasting potential C mineralisation of subsoils under different land use probably related to sampling depth, contribution of stabilised organic matter compounds, carbon content as well as quality. In general, very low proportions of biochar were mineralised in the two soils as compared to plant material. The mineralisation of each of the added substrates (biochar and plant material) was slightly, but significantly different in the two soils. Native C mineralisation was much higher after plant material addition than after biochar addition. Subsoil type influenced the kind and magnitude of priming effects for both added substrates.[br/][b]Conclusion[/b] Biochar mineralisation and its priming effects in subsoil are small as compared to uncharred plant litter. We suggest that substrate mineralization and priming effects induced on subsoil organic matter are dependent on the composition of the added substrate, as well as soil parameters rather than microbial community characteristics.

International audience; This study was conducted in Champagne vineyards in France, and the objectives were to compare the main cultivation practices in Champagne vineyards and to specify the conditions required for the optimum effect of inter-row grass cover on runoff and erosion in experimental plots of 0.25 m(2) under simulated rainfall. Three types of ground cover were studied. In the bark-and-vine-prunings plots, the runoff coefficient (RC) ranged from 1.3 to 4.0% and soil losses were

International audience; Physical weathering can modify the stability of biochar after field exposure. The aim of our study was to determine the potential carbon sequestration of the two chars at different timescales. We investigated the modification in composition and stability resulting from physical weathering of two different chars produced (i) at low temperature (250 degrees C) by hydrothermal carbonization (HTC); and (ii) at high temperature (1200 degrees C) by gasification (GS) using contrasting feedstocks. Physical weathering of HTC and GS placed on a water permeable canvas was performed through successive wetting/drying and freezing/thawing cycles. Carbon loss was assessed by mass balance. Chemical stability of the remaining material was evaluated as resistance to acid dichromate oxidation, and biological stability was assessed during laboratory incubation. Moreover, we assessed modification in potential priming effects due to physical weathering. Physical weathering induced a carbon loss ranging between 10 and 40% of the total C mass depending on the feedstock. This C loss is most probably related to leaching of small particulate and dissolved compounds. GS produced from maize silage showed the highest C loss. The chemical stability of HTC and GS was unaffected by physical weathering. In contrast, physical weathering strongly increased the biological stability of HTC and GS char produced from maize silage. After physical weathering, the half-life (t(1/2)) of GS was doubled but only slight increase was noted for those of HTC. During the first weeks of incubation, HTC addition to soil stimulated native soil organic matter (SOM) mineralization (positive priming effect), while the GS addition led to protection of the native SOM against biologic degradation (negative priming effect). Physical weathering led to reduction in these priming effects. Model extrapolations based on our data showed that decadal C sequestration potential of GS and HTC is globally equivalent when all losses including those due to priming and physical weathering were taken into account. However, at century scale only GS may have the potential to increase soil C storage.

International audience; Managed temperate grassland has the potential to sequester carbon if management practices are improved. In this study, CO2 flux was measured by the eddy covariance technique in two identical temperate sown grasslands under different managements, viz. mowing and grazing, to estimate and compare net carbon storage under both the management systems. Results: In both mowing and grazing systems, the averaged annual gross plant productivity, ecosystem respiration and net ecosystem exchange were –1720 and –1741, 1244 and 1510, and –476 and –231 g C m–2 year–1, respectively. Although the management practices did not significantly influence gross plant productivity (p > 0.05), grazing system increased Reco significantly by 21% (p < 0.05) but reduced net ecosystem exchange by 52% (p < 0.05) compared to mowing system. However, averaged annual net carbon storage were 23 and 141 g C m–2 year–1 under mowing and grazing, respectively. Conclusion: The results indicate that temperate sown grassland has the potential to sequester carbon under grazing.

International audience; Hydrothermal carbonization (HTC) of biomass may be a suitable technique to increase its carbon sequestration potential when applied to soils. However, the properties of end products of HTC (hydrochars) could be significantly influenced by feedstock source and temperature during the carbonization process. This study focused on chemical modification of wheat straw, poplar wood and olive residues through HTC at different temperatures (180 degrees C, 210 degrees C and 230 degrees C). Besides general properties such as pH, electrical conductivity (EC), ash content, elemental composition and yield, we evaluated bulk chemical composition (C-13 NMR) and contribution of specific compounds (lignin and black carbon). Moreover, the possible environmental risk of using hydrochars was assessed by determining their polycyclic aromatic hydrocarbon (PAH) and their dioxin contents. Our results showed that hydrochars were generally acidic with a pH value below 5. The highest EC (1710 mu S/cm) and ash content (10.9%) were found in wheat straw derived hydrochars. Hydrochar yields and C recovery decreased with increasing temperature to about 50% and 75%, respectively for all feedstocks at 230 degrees C. N recovery increased with increasing temperature but N content of feedstock is more important. H/C and O/C ratios showed a linear decrease with increasing production temperature for all feedstocks. O-alkyl C decreased while alkyl C and aromatic C increased with increasing temperature and no significant feedstock dependence could be observed. Carboxyl C was not influenced by feedstock and temperature. Lignin content decreased with increasing temperature, while its oxidation degree and the content of black carbon and PAH contents increased. We conclude that transformation of biomass was most advanced at 230 degrees C only. Feedstock did not significantly influence the chemical composition of the hydrochars apart from N content and recovery. Instead, HTC temperature is the main driver determining the chemical composition of hydrochars. Environmental risk of investigated hydrochars is low with respect to PAH and dioxin contents. Despite the advanced biomass transformation during the HTC process at 230 degrees C, chemical properties indicated that the end product might have a less stable structure than pyrochar. Considering the higher hydrochar yields and C and N recoveries, its C and N sequestration potential in soil could have some advantages over hydrochars but this still remains to be evaluated. (C) 2012 Elsevier Ltd. All rights reserved.

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