Biochar and soil carbon sinks encourage 'soil-first' farming

23:02, Oct 10 2013

Where is the best place to store carbon? Is it in the air, oceans, trees or soils? Where is the most effective place to store water for crops? Is it in man-made irrigation dams or where it falls, in soils?

Despite environmental restrictions on urban fire emissions from domestic fires, many orchardists regularly use low-cost burnoff methods, which can reduce regional air quality and increase public health risks. These burning-off practices are an echo of historic practices that cleared and burnt large tracts of bush for human settlement in Aotearoa and elsewhere.

In contrast, green wastes can offer new environmental solutions and economic opportunities to make renewable "syngas" fuels, while also producing biochar as a byproduct to improve soil and air quality, improve crop productivity, increase the effectiveness of fertiliser and water use, and reduce input costs. The "source - sink" concept describes processes that release or store resources transferred between different parts of natural or industrialised ecosystems.

The atmosphere is a sink for greenhouse gases methane and carbon dioxide. Plant leaves use photosynthesis to transform sunlight and carbon dioxide into energy in the form of plant sugars, starch and wood. Plant roots provide energy to symbiotic soil microbes in exchange for nutrients and water.

Decayed plant and animal wastes eventually may become soil carbon sinks, including humus or fossil fuels. Soil is the second largest reservoir of carbon compounds on the planet, exceeded only by oceans and lakes.

Biochar is a name for charcoal used to adapt soils for particular purposes. Charcoal is produced in kilns by heating plant or animal waste in the absence of oxygen. Compared with other organic matter, biochar resists decay and lasts for several thousand years and is thus a very effective carbon sink.


Biochar binds on to nutrients in a similar way that Velcro binds to fluff. Biochar retains soil nutrients in topsoil where they are available for crops. Biochar could thus help communities to protect riparian or surface water quality and avoid nitrate contamination of drinking water or groundwater.

Carbon-rich compost, soil organic matter and biochar increase the capacity of soil to store water where it lands and is needed by plants and soil biology. The micro pore structures contained in biochar provide environmentally stable micro-habitats for beneficial soil fungi and bacteria, which enhance plant roots access to soil nutrients and water.

As organic matter decomposes, it releases mineral nutrients, feeds soil biology and improves soil and crop health. Regardless of whether we choose to add, remove or prevent organic matter supplies being returned to soil food webs, soil microbes decompose organic matter and produce carbon dioxide or methane.

The rate that soil carbon is lost to the atmosphere or physically removed by wind or water erosion is accelerated where soil surfaces are laid bare by herbicides, the removal or burning of crop residues, intensive ploughing or rotary hoeing.

Research continues to seek improved methods to avoid losses of soil carbon stores and soil fertility, which underpin valuable primary industries. New Zealand farmers, regional authorities and researchers have decades of experience planting millions of trees in an attempt to slow soil erosion rates and protect waterways from excessive siltation effects.

Planting ground covers or green manure plants during or between crop rotations can protect bare soil surfaces under vines or orchard trees. Mulching soil surfaces, "no-dig" gardening, "no-tillage" or direct seed drilling methods minimise effects of soil disturbance and vegetation clearance during crop establishment.

Pre-contact Maori and Amazonian Indians added biochar to enrich physical and biological qualities in garden soils. Pre-plough gardening methods and a cultural heritage of biochar use may provide lessons from the past and inspiration to guide future generations of gardeners and farmers to adapt to the land use challenges of climate change.

"Soil-first" farming methods offer new ways of looking at old traditions of storing water, nutrients and carbon in soils.