Biochar: Effects and Benefits

Recalcitrance

Obviously charcoal lasts for a very long time, but not forever.

Enhanced nutrient retention capacity

Reduced nitrous oxide emissions

Rondon et al. (2005) found substantially reduced nitrous oxide emissions at bio-char additions of 20 g kg−1 soil to potted grass (80% reduction) and soybeans (50% reduction). Soil scientist Lucas Van Zweiten observed a 5 to 10 fold reduction in nitrous oxide emmissions with biochar in an agricultural setting. Generally, soil with elevated soil nitrate levels in the presence of sufficient moisture and robust soil organic matter will have higher nitrous oxide production, and thus will be more likely to benefit at the levels observed by Van Zweiten. However,

USDA_ARS research has correlated the effect with improved bulk density.

Increased soil pH

Raising soil pH is arguably biochar's most important contribution to influencing soil quality. (Steiner, 2006) The ash component (as opposed to the black carbon component) of biochar tends to be alkaline. The degree of alkalinity depends o the temperature of pyrolysis and the biomass source of the charcoal. The ash content increases as pyrolysis charcoal yield decreases. Measuring the acid-neutralizing capacity of charcoal is relatively easy, and recommended for any aggressive application rate.

Soil pH influences the relative availability of nutrients. At low pH, aluminum toxicity is particularly harmful to plant growth. Aluminum toxicity is an extensive and severe soil problem and biochar is the most available and obvious solution that we have to combat it. Soil phosphorus availability is highly dependent on soil pH range, and thus biochar can be used to substantially increase phosphorus availability in acidic soils.

Suppressed methane emissions

Rondon et al. (2005) found a virtually complete suppression of methane emissions at bio-char additions of 20 g kg−1 soil to potted grass and soybeans.

Mycorrhizal effects

The structure of the charcoal provide a refuge for small beneficial soil organisms, such as symbiotic mycorrhyzal fungi.

Bio-char is able to serve as a habitat for extraradical fungal hyphae that sporulate in their micropores due to lower competition from saprophytes (Saito and Marumoto, 2002 as reported in Lehmann, 2006)

Nishio (1996) states “the idea that the application of charcoal stimulates indigenous arbuscular mycorrhiza fungi in soil and thus promotes plant growth is relatively well-known in Japan, although the actual application of charcoal is limited due to its high cost”. The relationship between mycorrhizal fungi and charcoal may be important in realising the potential of charcoal to improve fertility. Nishio (1996) also reports that charcoal was found to be ineffective at stimulating alfalfa growth when added to sterilised soil, but that alfalfa growth was increased by a factor of 1.7-1.8 when unsterilised soil containing native mycorrizal fungi was also added. Warnock (2007) suggests four possible mechanisms by which biochar might influence mycorrhizal fungi abundance. These are (in decreasing order of currently available evidence supporting them): “alteration of soil physico-chemical properties; indirect effects on mycorrhizae through effects on other soil microbes; plant–fungus signalling interference and detoxification of allelochemicals on biochar; and provision of refugia from fungal grazers. (Woolf, 2006)

Metabolic effects

Increased CO2 emissions with biochar additions are apparently due to enhanced mineralization of biogenic soil organic matter. Increased respiration also implies enhanced microbial activity and nutrient cycling.