Biochar influences soil health but not yield in 3-year processing tomato field trials

Biochar can deliver many agronomic benefits, though effects may be greatest in suboptimal agricultural soils. The objective of this study was to investigate biochar in prime agricultural soils in a Mediterranean climate. In 3-year irrigated processing tomato (Solanum lycopersicum) field trials, in which three biochars were amended to a sandy and silt loam, biochar did not influence yield. Given the global policy focus on biochar for climate change mitigation and adaptation, however, it is essential to measure more than just agronomic parameters. To this end, a soil health assessment was conducted 2.5 years after biochar amendment. In the fertile silt loam and more nutrient-limited sandy loam, biochars had a variable but positive effect on pH, electrical conductivity, soil potassium, water stable aggregation, and permanganate oxidizable carbon. There was no effect on soil moisture nor potentially mineralizable nitrogen or carbon. In the silt loam, there was no change in microbial biomass or phospholipid fatty acid (PLFA) composition. In the sandy loam, almond shell biochars were associated with a slight increase in total biomass, an increase in the ratio of fungal to bacteria PLFAs, and reductions in multiple PLFA ratios were typically interpreted as indicators of environmental stress. Results of this study demonstrate that biochar may deliver soil health benefits in agricultural soils in a Mediterranean climate. However, increased soil health does not necessarily engender increased agricultural productivity within a 3-year period. Furthermore, increases in soil health indicators were inconsistent across soil and biochar types, underscoring the need for site- and material-specific investigation. Climate policies may bolster biochar application to working lands, irrespective of soil texture or fertility. Data on potential agronomic and soil health outcomes are needed, particularly for prime agricultural soils. In 3-year field trials, biochar did not positively or negatively impact processing tomato yield. Soil health indicators increased more in sandy loam than fertile silt loam, stressing need for site-specific data. Incentives should cover biochar costs if no agronomic benefits are expected, and negative impacts are unlikely.