The application of biochar as an organic amendment in polluted soils can facilitate their recovery by reducing the availability of contaminants. In the present work, the effect of biochar application to acid soils contaminated by heavy metal spillage is studied to assess its effect on the quantity and composition of soil organic matter (SOM), with special attention given to soil humic acids (HAs). This effect is poorly known and of great importance, as HA is one of the most active components of SOM. The field experiment was carried out in 12 field plots of fluvisols, with moderate and high contamination by trace elements (called MAS and AS, respectively), that are located in the Guadiamar Green Corridor (SW Spain), which were amended with 8 Mg·ha−1 of olive pit biochar (OB) and rice husk biochar (RB). The results indicate that 22 months after biochar application, a noticeable increase in soil water holding capacity, total organic carbon content, and soil pH were observed. The amounts of oxidisable carbon (C) and extracted HAs in the soils were not altered due to biochar addition. Thermogravimetric analyses of HAs showed an increase in the abundance of the most thermostable OM fraction of the MAS (375–650 °C), whereas the HAs of AS were enriched in the intermediate fraction (200–375 °C). Spectroscopic and chromatographic analyses indicate that the addition of biochar did not alter the composition of the organic fraction of HAs, while Cu, Fe, and as were considerably accumulated at HAs.

<p>Agriculture is facing the challenge of providing food for a growing world population in a context of climate change. The Mediterranean region is characterized for a semi-arid climate. Thus, water scarcity is coupled with the development of intensive crops that require irrigation, such as olive orchards. Recently, biochar –the solid aromatic carbonaceous product of the pyrolysis of residual biomasses– has been proposed as an amendment for reducing soil water loss [1] and increasing plant productivity [2].  The main objective of this study was to compare the effects of the application of biochar and green-compost (the organic amendment traditionally used) on soil properties and crop productivity at a super-intensive plantation of <em>arbequina</em> olive trees under deficit irrigation located at “<em>La Hampa”</em> field station (Coria del Río, Seville, Spain). Thus, soils were amended with 40 t ha<sup>-1</sup> of olive-waste biochar, green-compost or a biochar-compost mixture (50 % w/w). Un-amended plots were used as control. On a monthly basis, soil pH, water holding capacity, humidity and penetrability resistance, as well as TC and TN contents of soils were determined. Finally, the total weight of produced-olives per tree was measured.</p><p>Results showed that biochar application was the most effective amendment in increasing soil water holding capacity and moisture. All the organic amendments reduced the soil penetrability resistance. Olive production increased about 15 % at the biochar amended plots. Thus, the application of organic amendments, especially biochar, improved soil physical properties and led to a higher crop production.</p><p><em>Acknowledgements:</em> The BBVA foundation is gratefully acknowledged for funding the scholarship Leonardo to<em> “Investigadores y Creadores Culturales 2020”</em>, what made this project possible.</p><p>References:</p><p>[1] Campos et al., 2021. Agronomy 11, 1394. https://doi.org/10.3390/agronomy11071394</p><p>[2]De la Rosa et al., 2014. Science of the Total Environment 499, 175-184. http://dx.doi.org/10.1016/j.scitotenv.2014.08.025</p>

<p>Today's agriculture faces the challenge of safely feeding a growing population. This situation generates additional pressures on the environment such as increased organic waste generation, irrigated cropland and the consumption of mineral fertilizers. Moreover, in the present context of global warming, it is necessary to transform the linear economy into a circular economy, in which organic waste should be valorized and greenhouse gas emissions reduced. During the last decade the transformation of organic waste into biochar, the carbon-rich material produced during pyrolysis of biomass to be applied as soil ameliorant [1], to increase the amount of pyrogenic C at soils have been developed [2]. Here, green compost and biochar were produced from contrasting agricultural wastes and applied at greenhouse under limited irrigation conditions.</p><p>Results showed that raw material, together with the pyrolysis conditions, determined physical properties of biochars, and thus its performance as soil amendment. In all cases, an increase in the pyrogenic carbon content and a general improvement in the physical properties of agronomic interest of the technosoils were observed. However, the use of high doses of olive-pomace biochar negatively affected the germination due to its high salinity.</p><p>Biochar, although beneficial, is therefore not a universal solution and must be characterized, have the appropriate properties and be applied in a specific way to correct specific soil deficiencies.</p><p>Acknowledgements: The BBVA foundation is gratefully acknowledged for funding the scholarship Leonardo to<em> “Investigadores y Creadores Culturales 2020”</em> (<em>Proyecto realizado con la Beca Leonardo a Investigadores y Creadores Culturales 2020 de la Fundación BBVA).</em></p><p><em>References:</em></p><p>[1] Campos, P., Miller, A., Knicker, H., Costa-Pereira, M., Merino, A., De la Rosa, J.M., 2020. Waste Manag., 105, 256-267.</p><p>[2] De la Rosa, J.M., Rosado, M., Paneque, M., Miller, A.Z., Knicker, H., 2018. Sci. Tot. Environ., 613-614, 969-976.</p>

<p>It is estimated that over 37 % of degraded soils in the European Union are polluted by heavy metals [1], which are non-biodegradable and persistent pollutants in soils. The application of organic amendments to soils for their remediation has been worldwide used [2]. Several studies have shown that biochar, the carbonaceous material produced by pyrolysis of organic residues, has a high potential to stabilize trace elements in soils [3]. Biochars usually have an alkaline pH and high water holding capacity (WHC), large specific surface area and cation exchange capacity, which are appropriate characteristics to reduce the availability of heavy metals in the environment [4]. Nevertheless, recent studies exhibited that biochar recalcitrance could be much lower than assumed [5].  Beside this, the effects of the addition of biochar as a soil amendment on the composition of soil organic matter (SOM) are largely unknown. Thus, the aim of this study is to investigate the effects of the application of biochars from rice husk (RHB) and olive pit (OPB) in a Typic Xerofluvent polluted with trace-elements after 24 months at field in 12 plots installed at the surroundings of the Guadiamar Green Corridor (37° 23' 7.152"N, 6° 13' 43.175"; Southwest Spain). Specifically, for this study the effects of biochar amendment on soil physical properties (pH, water holding capacity-WHC, moisture, etc), elemental composition, total SOM, the content of oxidizable SOM as well as the content and composition of humic acids (HAs) have been assessed.</p><p>Biochar application caused an increase in soil pH (around 0.4 units), soil moisture (from 6-7% to 10-18 %) and WHC. In addition, the total organic carbon and HAs content increased slightly. Preliminary results show that biochar could become part of the humified SOM in a shorter time than initially expected. Nevertheless, the spectroscopic analyses (FT-IR and <sup>13</sup>C NMR spectroscopy) documented that the qualitative composition of soil HAs was not altered due to the biochar amendment.</p><p><strong> </strong></p><p><em>References</em>:</p><p>[1] EEA; 2007. CSI 015. Copenhagen, Denmark: European Environmental Agency.</p><p>[2] Madejón, E.; Pérez de Mora, A.; Burgos, P.; Cabrera, F.; 2006. Environ. Pollut. 139, 40-52.</p><p>[3] Campos, P., De la Rosa, J.M., 2020. Sustainability 12, 6025.Uchimiya, M.; Klasson, K.T.; Wartelle, L.H.; Lima, I.M.; 2011. Chemosphere 82, 1438-1447.</p><p>[4] Campos, P., Miller, A.Z., Knicker, H., Costa-Pereira, M.F., Merino, A., De la Rosa, J.M., 2020. Waste Manag. 105, 256-267.</p><p>[5] De la Rosa, J.M.; Rosado, M.; Paneque, M.; Miller, A.Z.; Knicker, H.; 2018. Sci. Tot Environ. 613-614, 969-976.</p><p><em>Acknowledgements</em>: The Spanish Ministry of Economy, Industry and Competitiveness (MINEICO), CSIC and AEI/FEDER are thanked for funding the project CGL2016-76498-R. P. Campos thanks the “Fundación Tatiana Pérez de Guzmán el Bueno” for funding her PhD.</p>