Fulvic and humic acids have a large variability in binding to metal (hydr) oxide surfaces and interact differently with oxyanions, as examined here experimentally. Pyrogenic humic acid has been included in our study since it will be released to the environment in the case of large-scale application of biochar, potentially creating Darks Earths or Terra Preta soils. A surface complexation approach has been developed that aims to describe the competitive behavior of natural organic matter (NOM) in soil as well as model systems. Modeling points unexpectedly to a strong change of the molecular conformation of humic acid (HA) with a predominant adsorption in the Stern layer domain at low NOM loading. In soil, mineral oxide surfaces remain efficiently loaded by mineral-protected organic carbon (OC), equivalent with a layer thickness of ≥ ~0.5 nm that represents at least 0.1-1.0% OC, while surface-associated OC may be even three times higher. In natural systems, surface complexation modeling should account for this pervasive NOM coverage. With our charge distribution model for NOM (NOM-CD), the pH-dependent oxyanion competition of the organo-mineral oxide fraction can be described. For pyrogenic HA, a more than 10-fold increase in dissolved phosphate is predicted at long-term applications of biochar or black carbon.

Biochar, a form of pyrogenic carbon, can contribute to agricultural and environmental sustainability by increasing soil reactivity. In soils, biochar could change its role over time through alterations in its surface chemistry. However, a mechanistic understanding of the aging process and its role in ionic nutrient adsorption and supply remain unclear. Here, we aged a wood biochar (550 °C) by chemical oxidation with 5-15% HO and investigated the changes in surface chemistry and the adsorption behavior of ammonium and phosphate. Oxidation changed the functionality of biochar with the introduction of carboxylic and phenolic groups, a reduction of oxonium groups and the transformation of pyridine to pyridone. After oxidation, the adsorption of ammonium increased while phosphate adsorption decreased. Ammonium adsorption capacity was nonlinearly related to the biochar's surface charge density (r = 0.94) while electrostatic repulsion and loss of positive charge due to destruction of oxonium and pyridine, possibly caused the reduced phosphate adsorption. However, the oxidized biochar substantially adsorbed both ammonium and phosphate when biochar derived organic matter (BDOM) was included. Our results suggest that aging of biochar could reverse its capacity for the adsorption of cationic and anionic species but the inclusion of BDOM could increase ionic nutrient and contaminant retention.

Increased biological nitrogen fixation (BNF) by legumes has been reported following biochar application to soils, but the mechanisms behind this phenomenon remain poorly elucidated.We investigated the effects of different biochar application rates on BNF in red clover (Trifolium pratense L.) Red clover was grown in mono or mixed cultures with red fescue grass (Festuca rubra L.) and plantain (Plantago lanceolata L.) at a range of different biochar application rates (0, 10, 50 and 120 t ha -1 ). In a separate experiment, nutrient effects of biochar on BNF were investigated using nitrogen, phosphorous and potassium (N, P, K) and micronutrient fertilisation using the same plant species.Biochar addition increased BNF and biochar applied at a rate of 10 t ha -1 led to the highest rate of BNF. Total biomass also showed the greatest increase at this application rate. An application rate of 120 t ha -1 significantly decreased biomass production in both single and mixed cultures when compared to the control, with the greatest reduction occurring in red clover. Furthermore, BNF was significantly higher in pots in which red clover was grown in mixed cultures compared to monocultures. In the absence of biochar, K fertilization caused a significant increase in BNF. For N, P, and micronutrient fertilization, BNF did not significantly differ between treatments with and without biochar addition.We conclude that different biochar applications rates lead to different effects in terms of BNF and biomass production. However, due to the high variety of biochar properties, different application rates should be investigated on a case specific basis to determine the optimum biochar application strategies.4

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Biochar is a pyrolysed biomass and largely consists of pyrogenic carbon (C), which takes much longer to decompose compared to the biomass it is made from. When applied to soil, it could increase agricultural productivity through nutrient retention and changing soil properties. The biochar-mediated nutrient retention capacity depends on the biochar properties, which change with time, and on soil properties. Here, we examined the effects of a wood biochar (20 t ha À1 ), that has aged (21 months) in a grassland field, on gross nitrogen (N) mineralization (GNM) and 15 N recovery using a 15 N tracer. A field experiment was conducted in two soil types, that is a Tenosol and a Dermosol, and also included a phosphorus (P) addition treatment (1 kg ha À1). Compared to the control, biochar with P addition significantly increased GNM in the Tenosol. Possibly, biochar and P addition enhanced nutrient availability in this nutrient-limited soil, thereby stimulating microbial activity. In contrast, biochar addition reduced GNM in the Dermosol, possibly by protecting soil organic matter (SOM) from decomposition through sorption onto biochar surfaces and enhanced formation of organo-mineral complexes in this soil that had a higher clay content (29% vs. 8% in the Tenosol). Compared to the control, biochar significantly increased total 15 N recovery in the Tenosol (on average by 12%) and reduced leaching to subsurface soil layers (on average by 52%). Overall, 15N recovery was greater in the Dermosol (83%) than the Tenosol (63%), but was not affected by biochar or P. The increased N recovery with biochar addition in the sandy Tenosol may be due to NH þ 4 -N retention at exchange sites on aged biochar, while such beneficial effects may not be visible in soils with higher clay content. Our results suggest that aged biochar may increase N use efficiency through reduced leaching or gaseous losses in sandy soils.