Influence of biochar particle size on biota responses.

Despite the increasing interest for biochar as a soil amendment, a knowledge gap remains on its impacts on non-target soil and aquatic species. We hypothesised that biochar particle size and application rate can play a role in the toxicity to biota. Pine woodchip biochar was incorporated in a clean soil at three particle size classes: small (<0.5 mm), medium (1-2 mm), and large (<4 mm), and at two concentrations: 1% and 6% w/w. A laboratory screening with earthworm Eisenia andrei avoidance behaviour bioassay was carried out to test the most adequate application rates, particle sizes and soil-biochar pre-incubation period. Thereafter, a 28-day greenhouse microcosm experiment was conducted as an ecologically more representative approach. Survival, vertical distribution and weight changes of E. andrei, and bait-lamina consumption were recorded. Soil leachates from the microcosms were collected to evaluate their impact on Daphnia magna immobilisation and Vibrio fischeri (Microtox®) bioluminescence. A feeding experiment with E. andrei was also performed to address earthworm weight changes and to conduct a screening of PAH-type metabolites in their tissue. The 6% <0.5 mm treatment pre-incubated for 96 h induced significant avoidance of the earthworms. Significantly lower bait-lamina consumption was observed in microcosms for the 6% <0.5 mm treatment. Moreover, particle size was a statistically significant factor regarding the loss of weight in the feeding experiment and higher concentration of naphthalene-type metabolites detected in E. andrei tissue, when exposed to <0.5 mm biochar particles. The leachates had no adverse effects on the aquatic species. The results suggest that particles <0.5 mm of pine woodchip biochar can pose sub-lethal effects on soil biota.

Spatial distribution of pyrogenic carbon in Iberian topsoils estimated by chemometric analysis of infrared spectra.

Understanding the global carbon (C) cycle is critical to accurately model feedbacks between climate and soil. Thus, many climate change studies focused on soil organic carbon (SOC) stock changes. Pyrogenic carbon (PyC) is one of the most stable fractions of soil organic matter (SOM). Accurate maps based on measured PyC contents are required to facilitate future soil management decisions and soil-climate feedback modelling. However, consistent measurements that cover large areas are rare. Therefore, this study aimed to map the PyC content and stock of the Iberian Peninsula, which covers contrasting climatic zones and has long-term data on wildfire occurrence. A partial least square (PLS) regression using the mid-infrared spectra (1800-400 cm-1) was applied to a dataset composed of 2961 soil samples from the Iberian component of the LUCAS 2009 database. The values of PyC for LUCAS points were modelled to obtain a map of topsoil PyC by a random forest (RF) approach using 36 auxiliary variables. The results were validated through comparison with documented historical wildfire activity and anthropogenic energy production. A strong relationship was found between these sources and the distribution of PyC. Our study estimates that the accumulated PyC in Iberian Peninsula soils comprises between 3.09 and 20.39% of total organic carbon (TOC) in the topsoil. Forests have higher PyC contents than grasslands, followed by agricultural soils. The incidence of recurrent wildfires also has a notable influence on PyC contents. This study shows the potential of estimating PyC with a single, rapid, low cost, chemometric method using new or archived soil spectra, and has the ability to improve soil-climate feedback modelling. It also offers a possible tool for measuring, reporting and verifying soil C stocks, which is likely to be important moving forward if soils are used as sinks for C sequestration.

Can straw-biochar mulching mitigate erosion of wildfire-degraded soils under extreme rainfall?

High severity wildfires cause a drastic alteration of soil carbon cycling - both oxidising and thermally altering soil organic matter (SOM) - and usually are followed by strong runoff and erosion events. To restore wildfire-degraded soils, SOM needs to be rebuilt while soil erosion is prevented. Post-fire straw mulching has been shown to mitigate soil erosion by providing a protective cover against rainsplash. However, SOM takes many decades or centuries to rebuilt naturally. Biochar, co-applied with straw to the soil surface can replace the SOM of the O-horizon, while the stabilised soil - by straw mulching - may gain in SOM naturally and by downward movement of biochar. We conducted a field study to test if straw-only and straw-biochar co-application could restore soils degraded by wildfire in one high burn severity (HBS) and one moderate burn severity (MBS) study area in southern Portugal and Spain, respectively, by monitoring erosion and SOM for the most intense rainfall period of the first post-fire year. Burned sites were characterized for soil and sediment physical properties, TOC content, SOM quality by thermogravimetry (DTG) and nuclear magnetic resonance (NMR 13C) spectroscopy. Straw-biochar mulching significantly reduced soil erosion by 76% and 65% in the HBS and MBS sites, respectively, in both cases similar to the erosion reduction by straw-only mulching. DTG and NMR 13C indicated that a relatively small proportion of the biochar eroded, i.e. 0.7%, indicating that co-application of straw with biochar may help restore the SOC lost in the wildfire in the medium term. The amount of SOM eroded was lower with straw-biochar mulching than in the untreated plots for both study areas. Straw-biochar mulching mitigates erosion of wildfire-degraded soils under extreme rainfall, while a relatively small proportion of the biochar is lost by erosion. Future studies need to monitor medium term effects.

Soil monitoring in Europe: a review of existing systems and requirements for harmonisation.

Official frameworks for soil monitoring exist in most member states of the European Union. However, the uniformity of methodologies and the scope of actual monitoring are variable between national systems. This review identifies the differences between existing systems, and describes options for harmonising soil monitoring in the Member States and some neighbouring countries of the European Union. The present geographical coverage is uneven between and within countries. In general, national and regional networks are much denser in northern and eastern regions than in southern Europe. The median coverage in the 50 km x 50 km EMEP cells applied all over the European Union, is 300 km(2) for one monitoring site. Achieving such minimum density for the European Union would require 4100 new sites, mainly located in southern countries (Italy, Spain, Greece), parts of Poland, Germany, the Baltic countries, Norway, Finland and France. Options are discussed for harmonisation of site density, considering various risk area and soil quality indicator requirements.