Soil microbial communities responded to biochar application in temperate soils and slowly metabolized 13C‐labelled biochar as revealed by 13C PLFA analyses: results from a short‐term incubation and pot experiment

Abstract

SummaryThis study investigates (i) the effect of biochar amendments on soil microbial communities in temperate agricultural soils, (ii) the involvement of microorganisms (MOs) in degradation of biochar and (iii) techniques to quantify degradation of biochar in short‐term experiments. The study involved an incubation experiment and a pot experiment with two arable soils (a sandy acidic Planosol and a calcareous loamy Chernozem) amended with 13C‐depleted biochar from wheat husk and willow plants. Phospholipid fatty acids (PLFAs), 13C‐PLFA, CO2, 13C‐CO2, soil organic carbon (Corg) and 13C‐Corg were monitored for 100 days. Effects of biochar application on the soil microorganisms (MOs) were generally minor. In the incubation experiment, microbial biomass was elevated by wheat husk biochar, especially in the Planosol. The increase in PLFAs was attributed to Gram‐negative bacteria and actinomycetes. Fungi and Gram‐positive bacteria were less affected. In the pot experiment, MOs did not respond to the addition of willow biochar. The effects of biochar were mainly attributed to an increase in the pH of the Planosol. Additionally, MOs were probably less responsive to inorganic fertilizer in biochar‐amended soil. In the incubation, only the actinomycetal PLFA 10Me18:0 incorporated biochar C, while in the pot experiment, Gram‐negative bacterial PLFAs (16:1ω7c, 16:1ω5c, 18:1ω7c) and Me16:0 & i17:1ω8 and i17:0 indicated degradation of biochar after 5 weeks. Uptake of around 20% biochar C in these PLFAs was monitored, which accounts for 2% biochar C in the total microbial biomass. From the PLFA data the mean residence time of biochar carbon was estimated in time‐scales of centuries to millennia. The CO2 concentration decreased after biochar addition until its production was masked by root respiration. The use of 13C‐CO2 labelling to estimate degradation was complicated by the interference with an initial negative priming effect and the dissolution/precipitation of carbonate. In conclusion, soil MOs were not particularly affected by addition of biochar, and the effects recorded were mainly attributed to changing environmental conditions after biochar addition. Nonetheless, uptake of 13C label into microbial PLFAs was successfully used to estimate microbial degradation of biochar in short‐term experiments.