Soil organic matter alteration under biochar amendment: study in the incubation experiment on the Podzol soils of the Leningrad region (Russia)

Purpose

Biochar is one of the most widely used ameliorants for soil amendment, which is known as factor which rises crop yields and levels of soil biological activity. Nowadays, it is under investigated how biochar application affects the dynamics of the humic components and whole soil organic matter (SOM) and the processes of its alteration. This investigation is aimed to evaluate the influence of biochar on the content, composition, and transformation of humic acids (HAs) as the main component of the SOM.

Materials and methods

The incubation experiment was carried out on three Podzol Antric soils, with varying amounts of initial total organic carbon. The incubation time was 90 days, using biochar gravimetric doses of 0.1 and 1.0%. The biochar was produced by fast pyrolysis of birch and aspen wood at 550 °С. Humus composition was analyzed for the organic matter fractions extracted with 0.1 M NaOH (containing HAs 1 + fulvic acids (FAs) 1) and 0.1 M Na₄P₂O₇ (containing HAs 1 + FAs 1 + HAs 2 + FAs 2). Isolated HAs were characterized for their elemental composition (C, N, H, and S) and molecular composition with the use of solid-state 13C nuclear magnetic resonance (13C-NMR) techniques.

Results and discussion

We found that 0.1% of biochar amendment does not influence SOM mineralization, but 1.0% of biochar increases the mineralization by 15–18%. This process is accompanied by changes in the composition and properties of the HS. The increased proportion of HA aromatic fragments in biochar indicates an increasing of their stability. However, in soils with high humus content and a significant amount of insoluble matter, the processes of mineralization and the growth of HAs are taking place simultaneously. The replenishment of HAs could be the outcome of both the intensification of the transformation processes (mineralization and humification) of the more sustainable insoluble matter compounds and the humification of the biochar itself.

Conclusions

The influence of biochar on humification in Podzol Antric soils was revealed on the basis of incubation experiment. Both negative and positive changes under biochar in HS system were demonstrated. The active decrease of humus total contents and also the labile HS ought to qualify as negative changes. The increase of HA chemical maturity that leads to the stability of humus in whole as well as the intensive new HA formation thought to qualify as positive changes.