Abstract:
A loss of soil organic matter (SOM), whether through natural means or management
practices, results in soil degradation. Biochar as a soil amendment can alter soil
properties, ultimately affecting the availability of nitrogen and water to plants and thus
crop growth. The effects of biochar are not definitive, and often dependent on both the
soil type and the biochar applied. Biochar properties can change according to the
feedstock and production parameters, thus for their effective use further investigation is
required to link biochar properties to its effects in soil. A high-temperature (600˚ C)
biochar from a mixed-hardwood feedstock was investigated. The biochar increased the
soil water retention, as demonstrated by a water release curve and field trials. This
retention was predominant at higher water potentials, which was attributed to the greater
number of meso (storage) pores in the biochar. Biochar did not affect the soil’s
saturated hydraulic conductivity; this is thought to be due to the low number of macro
(transmission) pores in the biochar. Thus there was no effect on the transmission rate in
the soil. Biochar reduced gross ammonium levels in the soil via adsorption, but resulted
in increased non-exchangeable ammonium levels, possibly due to physical entrapment.
Where carbon was already abundant in the organically managed soil, the adsorbed
ammonium reduced nitrification through lower substrate availability. The range of
carbon fractions added as a result of the biochar amendment increased the total organic
carbon (TOC) content of the soil, but this supplementary carbon was released by the
microorganisms as carbon dioxide. Microorganisms in the relatively carbon poor
conventionally managed soil (with lower TOC), assimilated the additional labile carbon
increasing microbial biomass. The higher microbial biomass, combined with
improvements in pH and the higher ammonium levels (as a result of the ammoniacal
fertiliser) increased nitrification. These changes in water and nitrogen availability did
not alter crop yields as measured in the glasshouse and field trials. The effects of this
biochar in a sandy agricultural soil depended on the type and level of carbon and
nitrogen present in the soil, thus consideration of these factors should be taken when
applying.