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Multiscale evaluation of biochar for the delivery of agronomic and soil health benefits in California


Scientists, policymakers, and growers are increasingly interested in the use of biochar, or pyrolyzed biomass, as an agricultural soil amendment. The number of published biochar studies has increased at a near exponential rate, from one publication per annum in the early 2000s, to over 4,100 in 2020. Policymakers have also taken notice, resulting in biochar included as a leading natural climate solution in the 2018 Intergovernmental Panel on Climate Change (IPCC) Special Report, and in California, the creation of the Biochar Research Advisory Group by the Governor’s Office of Planning and Research. As scientific and policy interest in biochar grows, so too does the size of the biochar market. Since 2009, 920 patent applications mentioning biochar have been submitted to the United States Patent and Trademark Office.

While interest in biochar is evident, many questions remain about the efficacy of biochar as a soil amendment. Due to its high surface area, low bulk density, reactive surface functional groups, and recalcitrant carbon, the material is purported to deliver many agronomic and environmental benefits when added to the soil. These benefits include increased water holding capacity, nutrient retention, crop yield, and soil carbon stocks, as well as enhanced microbial activity and the promotion of soil health. Despite the proliferation of biochar studies, research shows inconsistent results on the ability of biochar to deliver these benefits, due to differences in biochar feedstock, production methods, soil properties, climate, and cropping systems. It is especially difficult to interpret results for the fertile agricultural soils of California’s Mediterranean climate, as biochar has been shown to have the greatest impact in more acidic, nutrient-limited soils. Furthermore, results from the scientific literature have limited relevance to production agriculture, as biochar studies are dominated by short-term laboratory experiments that are difficult to extrapolate to field-scale. To inform the use and regulation of biochar in California, it is essential that farmers and policymakers have access to reliable, location-based data that evaluates biochar across scales.

This project fills a gap in the literature by providing mechanistic laboratory studies that are linked to pot trials and long-term, field-scale data about the agronomic and soil health potential of biochar as a soil amendment in California. To carry out this work, seven biochars were obtained from commercial companies at multiple production temperatures from various feedstocks. The potential for these biochars to impact the soil physical, chemical, and microbial environment was investigated across scales. In the laboratory, biochars were tested for their ability to physically or chemically retain nitrate and ammonium. Nearly every biochar tested exhibited a strong chemical affinity for ammonium, likely due to the attraction between their negatively charged surfaces and the positively charged ammonium ion. This was evident in that ammonium retention was strongest in biochars with high cation exchange capacity (CEC), oxygen-containing functional groups, and high oxygen to carbon ratios. Biochars exhibited little to no chemical affinity for nitrate, though one biochar reduced nitrate leaching, to a small extent, in soil column studies. This result was linked to the biochar’s high surface area and low CEC. The ability of biochar to alter the soil physical environment was most evident in its effect on saturated hydraulic conductivity (Ksat). Broadly, biochars increased Ksat in a silt loam but had a mixed effect in a sandy loam. An additional small-scale study was carried out in a growth chamber, in which biochar-amended soil was observed to substantially increase the yield of romaine lettuce (Lactuca sativa) when compared to the unamended control.

Collectively, the short-term laboratory experiments in this study demonstrated that these biochars could improve ammonium retention, water conductivity, and crop yield when added to the soil. However, in three-year field trials with the same biochars in the same soils, similar benefits were not observed. Seven biochars were amended to soils at two rates, combined with two synthetic nitrogen (N) fertilizer rates, in two California locations. Processing tomatoes (Solanum lycopersicum) were grown for three years, and data was collected on the influence of biochar on plant and soil properties. Biochar had minor effects on soil pH, EC, and N content, though the effects varied by biochar, location, and year, and were not substantial enough to impact plant yield or quality parameters. Under no combination of experimental conditions was biochar observed to increase processing tomato yield, plant nitrogen uptake, or soil moisture. Field trial results are consistent with those from other studies, which indicate biochar may confer limited benefits in soils which do not require conditioning for the successful growth of crops. Furthermore, the discrepancy between results from experiments at different scales demonstrate that short-term laboratory trials are not sufficient to make conclusions about field-scale agriculture.

While biochar did not deliver tangible agricultural benefits in field trials, further investigation was made into its influence on parameters that constitute current notions of soil health. Soils were sampled 2.5 years after amendment with almond shell biochars produced at 500 or 800 °C, or a softwood biochar produced at 500 °C, for a comprehensive soil health assessment. To varying effects, biochars were observed to increase labile carbon, water stable aggregates, pH, and EC in both the silt loam and sandy loam. In the finer textured silt loam, which had higher fertility and organic matter concentration, results were not substantial enough to influence the microbial community. Phospholipid fatty acid (PLFA) analysis from the silt loam revealed that biochar had no effect on community composition or on the PLFA ratios typically interpreted to denote microbial stress. In the coarser, more nutrient-limited sandy loam, however, a canonical correspondence analysis (CCA) revealed microbial communities responded to the increase in water stable aggregates, pH, and potassium conferred by the addition of biochar. This resulted in a distinct community composition, as well as reduced indicators of microbial stress. Results were greatest in plots amended with almond shell biochars, likely due to the high potassium content of almond shell, and to the high pH and ash concentration of these biochars. The soil health assessment indicates that, while biochar may not deliver agronomic improvements in fertile agricultural soils, it may confer other ecological or environmental benefits, or have the potential to deliver agronomic benefits across a longer time horizon.

Importantly, results from multiple scales indicate biochar may be added to agricultural soils with few negative consequences for cropping systems. However, there is a growing body of research which suggests some biochars may contain potentially toxic properties that pose a threat to human health when airborne biochar is inhaled. In order to optimize the ecological benefits of adding biochar to soils, care should be taken to select biochars which conform to quality standards established by the International Biochar Initiative or the European Biochar Certificate. Care should also be taken to amend biochars to soils under conditions which minimize dust emissions, and to equip farmworkers with respirators and appropriate protective attire. As there may be limited financial incentive for California growers to add biochar to their soils, cost-share and incentive programs should be considered. Together, data from this project can assist policymakers and land managers in California in making decisions about amending biochar to working lands. Realistic expectations should be established for the agronomic benefits of adding biochar to California cropping systems. Meanwhile, carbon sequestration or soil health projects may be pursued with minimal consequence, given the safe and appropriate selection of biochars.

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