Biochar, produced through pyrolysis of organic matter, is negatively charged, thus contributing to electrostatic adsorption of cations. However, due to its porous structure and contents of alkaline ashes, the determination of the cation exchange capacity (CEC) is challenging. Literature values for the CEC of biochar are surprisingly variable and are often poorly reproducible, suggesting methodological problems. Here, we modify and critically assess different steps in the existing ammonium acetate (NHOAc) method (pH 7), where ammonium (NH) is displaced by potassium chloride (KCl), following removal of excess NHOAc with isopropanol, in batch mode. We used pigeon pea biochar to develop the method and conducted a test on three additional biochars with different acid neutralizing capacity. A pretreatment step of biochar was introduced, using diluted hydrochloric acid, to decrease biochar pH to near neutral, so that 1 M NHOAc effectively buffers the biochar suspension pH at 7. This allows the CEC of all biochars to be determined at pH 7, which is crucial for biochar comparison. The dissolution of ashes may cause relatively large weight losses (e.g. for cacao shell biochar), which need to be accounted for when computing the CEC of raw biochar. The sum of NHOAC-extractable base cations provided a smaller and better estimate of the CEC than KCl-extractable NH. We hypothesize that the overestimation of the CEC based on KCl-extractable NH is due to the ineffectiveness of the relatively large isopropanol molecules to remove excess NHOAc in biochars rich in micro-pores, due to size exclusion. The amount of base cations removed in the pretreatment was about three (rice husk biochar) to ten times (pigeon pea biochar) greater than the amount of exchangeable cations. The CEC values of biochar increased from 10.8 cmol/Kg carbon to 119.6 cmol/Kg carbon. These values are smaller than reported CEC values of soil organic carbon.

Conservation farming (CF), involving basin tillage, residue retention and crop rotation, combined with biochar may help to mitigate negative impacts of conventional agriculture. In this study, the effects of CF on the amount and quality of soil organic matter (SOM) and potential nitrogen (N) mineralization were investigated in a maize-soya-maize rotation in an Acrisol in Zambia. A large field was run under CF for 7 years and in the subsequent three growing seasons (2015-2018), four management practices were introduced to study effects on soil characteristics and crop yield. We tested i) a continuation of regular CF (CF-NORM) ii) CF without residue retention (CF-NO-RES); iii) Conventional (CONV), with full tillage and removal of residues; and iv) CF with 4 ton ha-1 pigeon pea biochar inside basins and residue retention (CF-BC). The experiment involved the addition of fertilizer only to maize, while soya received none. Soya yield was significantly higher in CF systems than in CONV. Maize yields were not affected by the different management practices probably due to the ample fertilizer addition. CF-NORM had a higher stock of soil organic carbon (SOC), higher N mineralization rates, more hot-water extractable carbon (HWEC; labile SOC) and particulate organic matter (POM) inside basins compared to the surrounding soil (outside basins). Our results suggest that the input of roots inside basins are more effective increasing SOM and N mineralization, than the crop residues that are placed outside basins. CONV reduced both quality and quantity of SOM and N mineralization as compared to CF inside basins. CF-BC increased the amount of SOC as compared with CF-NORM, whereas N mineralization rate and HWEC remained unaffected. The results suggest benefits on yield of CF and none of biochar; larger impact of root biomass on the build-up of SOM than crop residues; and high stability of biochar in soil.

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Aims: To assess how biochar addition in rainfed conservation agriculture affects short-term transformation, plant uptake, retention of nitrogen (N) in soil, and nitrous oxide (N 2 O) fluxes in a tropical Arenosol planted to maize. Methods: A ten-day in situ 15 N pool dilution and N cycling experiment, using tracer amounts (0.1 g m -2 ) of 15 N labeled ammonium ( 15 NH 4 + ), nitrate ( 15 NO 3 -) or 15 N-urea, was carried out seven weeks after planting of maize (Zea mays L.) under conservation agriculture in Zambia, using planting basins without (CA) and with pigeon-pea biochar (BC) addition (4 t ha -1 ). Results: Pigeon-pea biochar increased soil NO 3 concentration, gross nitrification rate, 15 N recovery in extractable soil NO 3 -, and soil moisture. However, effects of biochar on soil N retention and plant N uptake were not significant. Likewise, biochar did not affect N 2 O fluxes. Conclusions: At low dosage, pigeon pea biochar has a positive effect on gross nitrification rate but does not affect short-term N retention in soil, N 2 O fluxes, nor does it help increasing the uptake of N by maize.