Softwood biochar as a soil amendment material for boreal agriculture

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Biochar is a porous carbonaceous solid material produced by pyrolysis. Application of biochar is considered as an efficient way of carbon (C) sequestration since the C in biochar is relatively resistant to microbial degradation. Furthermore, previous research in (sub-) tropical conditions suggests that it may enhance soil fertility and the yields of agricultural crops. To target the lack of knowledge about the effects of biochar in the boreal zone, softwood biochar was added to two boreal soils (a fertile Stagnosol and a nutrient deficient Umbrisol) in laboratory and field experiments in southern Finland in 2010-2012. The study focused on the effects of biochar on 1) the mineralisation of nitrogen (N) of organic fertilisers, 2) the physicochemical properties of soil, 3) earthworm abundance and behaviour, and 4) the yield formation of wheat, turnip rape and faba bean.

Biochar application to soils caused an initial reduction in N availability, probably by N immobilisation due to increased microbial biomass. The effect was greater when the biochar application was combined with an organic fertiliser with a high C:N ratio than when one with a low C:N ratio was used. In the field experiments, however, the N immobilisation was moderate, as the N uptake of crops was not affected. Furthermore, signs of turnover of microbial biomass in the second year were seen in the Umbrisol field.

Biochar application increased the contents of C and exchangeable potassium (K) in the soil, but had no significant effects on other soil chemical properties within the first two to three years of the experiments. Biochar effects on soil physical properties varied. In the Stagnosol with a sandy clay loam texture, the application slightly increased topsoil moisture content, but did not affect soil water retention or porosity. In the Umbrisol with a loamy sand texture, biochar increased the plant-available water content of the topsoil in the first year and soil porosity in the second year but did not affect the moisture content of the soil.

In the laboratory, biochar did not affect the habitat choice of earthworms when the test lasted for 2 days, but after 2 weeks, biochar-treated soil was avoided. The avoidance effect was associated with a slight decline in soil water potential. This avoidance effect was not observed under field conditions, where there was even an indication of increased abundance and biomass of earthworms in biochar-amended soil.

The effects of biochar application on the plant growth dynamics and N uptake of turnip rape and wheat were not significant, but the enhanced accumulation of biomass and N uptake of faba bean during the initial N immobilisation phase may be related to possibly enhanced biological N fixation. In dry years, biochar addition affected the yield formation of crops, as it was associated with decreased plant density and increased number of reproductive units (pods, siliques or ears) per plant. The latter was attributed to two additive mechanisms, the compensation for decreased plant density and relieved moderate water deficit. Biochar did not however affect the crop yield significantly, irrespective of the fertiliser treatments or the soil types studied.

It can be concluded that the application of biochar in combination with inorganic fertilisers or with meat bone meal to boreal soils with near neutral pH and relatively high original SOM content may reduce deficits in both K and water, but should not be expected to significantly affect yields of faba bean, turnip rape and wheat during the first few years. As added biochar had no negative effects on crop yields or earthworms, it can be suggested that softwood biochar application is an agriculturally safe way of sequestering C. Considering the longevity of biochar in soils, future studies are needed for monitoring the long-term effects of biochar under field conditions.
Original languageEnglish
Place of PublicationHelsinki
Print ISBNs978-952-10-8895-7
Electronic ISBNs978-952-10-8896-4
Publication statusPublished - 2014
MoE publication typeG5 Doctoral dissertation (article)

Note regarding dissertation

The public examination was held in lecture room Walter, Agnes Sjöberg street 2, Viikki on May 9th 2014, at 12 o’clock noon.

Fields of Science

  • 4111 Agronomy
  • Biochar
  • 415 Other agricultural sciences
  • SOIL

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