Transient biochar effects on decomposer microbial growth rates: evidence from two agricultural case‐studies

Abstract

Summary We investigated the impact of biochar application on fungal (acetate incorporation into ergosterol) and bacterial (leucine incorporation) growth rates in two case studies: a temperate UK pasture soil and a Mediterranean Australian agricultural soil. We added biochar at similar rates per unit of soil organic carbon (SOC) and monitored both the immediate (after 1 week equilibration) and longer‐term (1–3 years) effects. The immediate effect of the biochar applied to the UK soil was a decreased fungal‐to‐bacterial growth ratio, driven by greater bacterial growth. The immediate effect of biochar application to the Australian soils was subtle, only slightly increasing the fungal‐to‐bacterial growth ratio. In both case studies, the biochar effects were transient, and no long‐term effects (1–3 years) on microbial growth rates could be detected in either soil. The bacterial growth increase in the UK soil was probably related to a release of large amounts of labile C from the biochar, or as C released from the resident SOM caused by the biochar‐induced pH increase. The increase in fungal‐to‐bacterial growth ratio could be related to a release of poor quality C in the Australian soils. There were immediate effects of biochar application on microbial growth in agricultural soils, but they were disparate between cases, making any generalization of mechanisms difficult. However, the microbial responses were consistently transient. Taken together, biochar application to agricultural soil appears to have an impact upon the decomposer community, suggesting limited resistance. However, the microbial functioning appeared resilient to these effects, stabilizing microbial communities to their initial state within 1–3 years of application.

Soil microbial responses over 2 years following biochar addition to a north temperate forest

“…Our null result is likely linked to the fact that there was no overall effect of adding biochar at 5 t ha −1 on soil pH, as discussed previously. Changes in microbial community composition after biochar addition are often correlated with pH (Rousk et al 2013;Anders et al 2013), and in the present study, soil pH was the main control on the fungi/bacteria ratio in the organic layer and pH was also correlated with bacterial community composition in 2012. There was also a significant decrease in the fungi/bacteria ratio in the organic layer of the maple biochar plots, which may have been linked to pH changes.…”

“…Soil nutrient availability was not measured in 2013, but Sackett et al (2014 found that most nutrient effects observed within 2 to 6 weeks after biochar addition were no longer present by the end of the first year (other than increased Ca 2+ levels), suggesting that many nutrient effects at this site were transient. Similarly, the resiliency of the microbial community should likewise result in only transient effects of biochar additions (Quilliam et al 2012;Rousk et al 2013). In this study, we found effects on the mineral soil microbial community composition, but not in the organic soil layer, in the second year after biochar addition.…”

“…Even though a recent meta-analysis found that MB-C typically increases after biochar addition (Biederman and Harpole 2013), other medium-term field studies of biochar addition have similarly found a negligible change in microbial biomass several years after the biochar was added, suggesting that these effects are generally short-lived Rousk et al 2013). Observed increases in microbial biomass are commonly attributed to greater C and nutrient availability (Lehmann et al 2011;Biederman and Harpole 2013), either supplied directly or via increased plant root activity, so these results may indicate that by 12 months after biochar addition, there was no labile C remaining for use by the microbial community (Anders et al 2013).…”

“…After addition to soil, these compounds are rapidly mineralized, whereas mineralization of the aromatic structures is minimal, meaning that there is generally an initial pulse in C and mineral nutrient release after biochar addition, but over time, the differences between control and biochar-amended soils decline Quilliam et al 2012;Ameloot et al 2013;Rousk et al 2013). Soil nutrient availability was not measured in 2013, but Sackett et al (2014 found that most nutrient effects observed within 2 to 6 weeks after biochar addition were no longer present by the end of the first year (other than increased Ca 2+ levels), suggesting that many nutrient effects at this site were transient.…”

“…While a few long-term (>12 months) field studies examining the effect of biochar on microbial biomass and growth rates, activity, and composition Quilliam et al 2012;Rousk et al 2013;Domene et al 2014) are beginning to emerge, none of them have been conducted in forested ecosystems. This study focuses on the effects of hardwoodand softwood-derived biochar and P addition on soil microbial community composition and functioning 1 and 2 years after addition in a temperate hardwood forest in Central Ontario.…”