Impact of biochar on functional activity dynamics of microbial communities in boreal soils

Biochar is the product of the pyrolysis of various organic materials with high carbon contents. Owing to its aromatic structure, high porosity, and cation exchange capacity, biochar has a significant impact on the physical, chemical, and biological properties of soils. The use of biochar in agriculture provides an opportunity for safe waste disposal, soil quality, and carbon sequestration. One of the few studied consequences of biochar application is the change in the structure, abundance, and ecophysiological status of soil microorganisms. In this study, we established for the first time that biochar has a multidirectional effect on the functioning of boreal soil microbial communities. The purpose of this study was to study the effects of wood biochar on basal respiration, microbial biomass carbon, and the microbiological quotients of boreal soils of various textures. The studies were conducted in the middle taiga subzone of Karelia. The sites were laid out in 2018 and 2019 on arable land plots with soils of different textures and normal moisture. The coarse soil texture was represented by the loamy sand Umbric Podzol on lake sands (UP) (Laboratory of Agrotechnology Vilga, Karelian Research Center RAS, Vidany settlement, Pryazhinsky District, Karelia, 61°52′22.8′′ N, 34°0′36′′ E). The fine soil texture was represented by the clay loam Umbric Retisol on loamy moraine (UR) (Agrobiological Station, Karelian Research Center RAS, Petrozavodsk, 61°45′3.6′′ N, 34°21′12.4′′ E). Plots of biochar and control sites with areas of 5 m2 were laid out randomly in four replicates. Birch biochar (GOST 7657-84), grade A (Russia) at a dose of 1.5 kg/m2 (15 t/ha) with a fraction size £ 1 cm was used in the study. Biochar was applied once and dug manually to a depth of 20-25 cm. No additional fertilizers were used. Potatoes were grown in the first year, and green manure was grown in the second year. Soil samples were collected monthly during the vegetation periods of 2018-2019 for plots with UP and 2019-2020 for plots with UR. Soil was collected from a depth of 5-20 cm in the arable horizon using the “envelope” method from each plot, and the mixed samples were analyzed in four replicates. The content of organic carbon (Corg.) by high-temperature catalytic combustion in a TOC-L CPN (“Shimadzu”, Japan) analyzer was determined. The rate of basal (microbial) respiration (BR) was measured in native soil incubated for 24 h at 22 °C. The content of microbial biomass carbon (Cmic.) was determined using the substrate-induced respiration method and was estimated using the rate of initial maximum respiration of microorganisms after enrichment of the soil with glucose (incubation for 1.5-2 h at 22 °C). Changes in CO2 concentration was recorded with a NDIR gas analyzer (SenseAir, Sweden). The parameters of the ecophysiological status of the microbial community were determined: the share of microbial biomass carbon in soil organic carbon Cmic./Corg., the metabolic quotient qCO2, and microbial respiration quotient QR. A study of the functional activity of microbial communities revealed the multifactorial and nonlinear effects of environmental conditions on the dynamics of the studied indicators at two-year monitoring sites in soils of different textures. Basal respiration was less dependent on weather conditions than microbial biomass carbon and related quotients, the magnitude of which was also influenced by organic residue supply. UP had lower microbial biomass, a less mature microbial community, lower stability, and lower organic matter quality than UR. The application of biochar has different effects on these soils. Despite the stable and reliable increase (р ≤ 0,05) in the total organic carbon of both soils (up to 20 %), when BC was added to UR, the BR, Cmic., and microbial quotients tended to significantly decrease (р ≤ 0,05), reflecting the lack of substrates available for microorganisms. At the same time, the qCO2 and QR indicators decreased slightly but nonsignificantly. There was an increase in BR (not statistically significant) and Cmic. (significant at р ≤ 0.05 for some dates) in UP, apparently reflecting a surge in K-strategist activity during the mineralization of hard-to-reach biochar compounds. The addition of biochar did not affect the microbial quotient of UP. At the same time, a decrease in the metabolic and microbial respiration quotients indicated that more favorable environmental conditions were being developed for microorganisms. Overall, biochar supplementation increased the adaptive potential of UP microbial communities.