BioCycle January 2011, Vol. 52, No. 1, p. 42
Diverting and recycling close to 4 million metric tons of organic residues in 2006/07 in Australia prevented generation of methane equivalent to 4.28 million metric tons CO2-e.
Johannes Biala

IT is well documented that preventing organic residues from going to landfill avoids methane emissions, and also preserves organic carbon and nutrients for beneficial use in land management and food production. It is equally well known that ongoing use of compost improves physical, chemical and biological soil properties, and delivers a wide range of agronomic and environmental benefits. By supplying both stable and labile organic compounds, as well as plant nutrients and beneficial organisms, the agricultural and horticultural use of compost also supports climate change mitigation on two fronts:
Removal of atmospheric carbon through soil carbon sequestration, achieved directly through storage of compost carbon, and indirectly via enhanced plant growth, which in turn contributes also to increased soil carbon levels; Reduction of greenhouse gas (GHG) emissions, e.g. through reduced production and use of chemical fertilizer and pesticides, and reduced irrigation.
In 2006/07, the Australian organics recycling industry diverted at least 3.7 million metric tons of organic residues from landfill, including garden and food organics, wood and timber, biosolids and sludges. Diverting and recycling these materials prevented generation of methane equivalent to 4.28 million metric tons CO2-e. In addition, a minimum of 600,000 metric tons of manure and other agricultural residues were composted, which the International Panel on Climate Change (IPCC) recognizes as superior manure management compared to deep litter or dry storage, let alone pit or liquid storage.

Declining Soil Carbon And Productivity
Clearing of native land for agricultural use, particularly when it involves soil cultivation, results in considerable decline of soil carbon levels. It is estimated that soil carbon levels in Australia declined 30 to 60 percent after land clearing for cropping some 60 to 100 years ago. The rate of decline depends primarily on initial soil carbon levels, soil texture (clay content), rainfall and agricultural activities. However, decline of soil carbon is not a historic phenomenon – it still is widespread today as many soils have not yet reached new steady state carbon levels, and as agricultural production continues to intensify.
Soil organic matter (or carbon) levels are closely linked with soil fertility and soil productivity. Hence, declining organic matter diminishes soil fertility, resulting in less favorable soil physical characteristics, reduced water storage and availability, and reduced nutrient (N, P, K, trace elements) supply capacity.
Mineralization of organic matter provides a constant slow-release nutrient source in natural and also agricultural ecosystems. Hence, declining soil carbon also delivers significant benefits – approximately 1,200 kg of N and 100 kg of P per hectare (at soil bulk density of 1.2 t/m3) are released per one percentage point of carbon lost. Agriculture production benefited from this effect for a long time. However, increasingly unfavorable soil conditions and diminishing soil nutrient supplies due to organic matter decline resulted in increased need for mineral fertilizer and other external inputs.

Compost Is Solution
Using compost is a solution to declining soil carbon and productivity. Compost provides a two-fold benefit: it is ideal for helping rebuild soil fertility and replenishing soil carbon and nutrient stocks, while at the same time helping mitigate climate change. Compost contains macro and micronutrients, a diverse microbial population, stable organic compounds (e.g. humic compounds), and also labile organic matter, an important source of food and energy for the soil food web. Hence, compost is not “naked carbon,” but rather “humus in the making.”
Conversion of compost into humus continues after compost is added to soil, i.e., transformation of organic matter into microbial biomass, energy, CO2 and stable organic compounds. Although this mineralization process reduces the amount of compost carbon that remains in the soil, it also releases nutrients necessary for plant growth. Apart from soil type and agricultural activity, the degree to which compost carbon is converted into CO2 depends on the type and age (maturity) of compost used, as well as on environmental conditions, primarily temperature and moisture. Research has shown, for example, that 17 percent of total organic carbon added with garden and food organics compost was converted into CO2 during a one-year period. This means that 83 percent of all carbon added with compost was still in the soil after one year.

Compost Use Sequesters Carbon
Many research trials in Australia and overseas have demonstrated that ongoing compost use increases soil carbon levels. However, there are very few longer-term trials that enable the modeling of carbon sequestration associated with compost use. Fortunately, a considerable number of long-term (50-160 years) field trials in Europe and North America have demonstrated the benefits of farmyard manure in increasing soil carbon levels. In the short to medium term, a considerably higher proportion of carbon applied in compost is retained in the soil than when carbon is applied in manure. With manure, 5 to 20 percent of applied carbon is retained, while carbon retention for compost ranges between 10 percent and more than 50 percent. Therefore, it can be assumed that compost is also considerably more effective than manure in sequestering carbon in the long-term.
While it is acknowledged that increases in soil carbon levels will diminish over time and be limited by new carbon equilibria, the current assumption that this point will be reached after about 20 years will have to be revised upward. Soil organic matter levels still showed linear increases after 9 and 12 years of continuous compost use. Over a 12-year period, around one metric ton of carbon per hectare and year was sequestered for every 10 metric tons of dry matter compost (garden and food organics) applied per hectare. Sequestering one metric ton of carbon equates to 3,670 kg of CO2-e abatement.
In line with this, a simple carbon sequestration model for compost use in European conditions predicts that a new equilibrium will be reached only within a time frame of probably 200 and 300 years for annual application rates of 10 and 15 metric tons per hectare, respectively. Annual application rates between 2.5 and 5.0 metric tons/hectare prevent further decline in soil carbon levels.
Based on available research data, it is estimated that 45, 35 and 10 percent of carbon contained in mature garden organics compost is retained in the soil over 20, 50 and 100 year timeframes, respectively. Hence, use of such compost at rates of 10 t DM ha-1 will sequester carbon that is equivalent to around 5,000 kg CO2-e over 20 years, 3,500 kg CO2-e over 50 years and 1,000 kg CO2-e over 100 years.

Replacement Of Mineral Fertilizers
Production of mineral fertilizers is energy and GHG intensive, particularly for nitrogenous fertilizers. The supply of plant nutrients through compost use allows for a reduction in using mineral fertilizer, and hence also a cut in GHG emissions caused by fertilizer production.
The degree to which this is achieved depends on factors such as nutrient density in compost, nutrient replacement efficiency, compost application rate and the global warming potential allocated to fertilizer manufacturing. If, for example, garden organics compost (N: 1.1%, P: 0.2%, K: 0.55%) is applied annually at 10 metric tons dry matter per hectare, it could replace the use of approximately 44 kg of N, 20 kg of P and 55 kg of K from mineral fertilizer per year. If use of urea and single superphosphate is reduced accordingly, emissions from fertilizer production will be reduced by around 180 kg CO2-e. Savings are obviously considerably higher, if compost with higher nutrient concentrations is used, such as products that contain food organics, biosolids or manure. Also, while the above calculation assumes that plants utilize approximately 40 percent of nitrogen supplied with compost over four years, others (Favoino and Hogg, 2008) have assumed this amounts to 100 percent, resulting also in markedly higher GHG savings.
Most nitrogen (> 90%) contained in compost is organically bound, and released slowly over time. If compost is applied regularly, soil nitrogen and phosphorous reserves will build up in parallel to increasing soil carbon levels. For each metric ton of carbon stored in the soil, i.e., converted into humus, approximately 85 kg of N, 20 kg of P and 14 kg of S also are stored. These nutrients will be released slowly and become available for plant uptake at a later stage as the humus is mineralized. Over time, nutrient reserves will build up, so that probably 80 kg or more of N per hectare will be supplied annually from the soil nitrogen pool. This resembles farming conditions shortly after land clearing when soil humus levels were still high.

Reduction of Nitrous Oxide Emissions
Despite the fact that nitrous oxide (N2O) emissions represent less than 10 percent of the mitigation potential from cropland globally, they can have a significant impact, as their global warming potential is almost 300 times higher than that of CO2. Consequently, increased N2O emissions from agricultural activities can negate substantial carbon sequestration gains.
Production of N2O in soil is governed by available mineral nitrogen (both from soil and fertilization), soil aeration, moisture (water filled pore space), temperature, dissolved and readily degradable carbon, and soil pH and salinity. However, the overarching determinants for N2O emissions are nitrogen fertilization, nitrogen use efficiency and oxygen supply.
The effects of compost use on N2O emissions are not yet clearly established, probably because compost has the potential to both reduce and enhance emissions. Generally speaking, compost has low nitrogen concentration (ca. 1-3% DM), and only a small proportion is present in mineral form (ca. 0-10% of total N). From that perspective, compost poses little risk of causing N2O emissions. The soil aeration effect of using compost also helps to reduce N2O release. On the other hand, improved soil water holding capacity and the supply of dissolved/readily degradable carbon might enhance N2O emissions.

Summary And Outlook
The use of compost delivers a further range of other savings on GHG emissions, which are more difficult to measure and quantify. These savings include the following: Reduced energy use for irrigation, due to improved water storage and water use efficiency; Reduced need for biocides results in reduced GHG emissions associated with biocide production, due to improved soil and plant health; Reduced diesel use for soil cultivation, due to improved tilth; Increased carbon sequestration from higher biomass production, due to improved soil productivity; Reduced nitrogen loss that causes secondary N2O emissions, due to lower nitrogen surplus and leaching; Reduced erosion that causes loss of nutrients and organic matter, resulting in secondary N2O emissions and those associated with replacing lost nutrients.
In summary, use of compost can contribute to mitigating climate change and help open a window of opportunity (20-40 years) to find other means (technologies) of reducing/mitigating emissions. Additionally, compost utilization is one of the fastest means of improving soil carbon levels, is ideally suited as a mitigation measure in productive agricultural soils, fits easily into the Australian National Carbon Accounting System, and can attract carbon credits.
It is important to understand that using compost not only helps to sequester carbon and mitigate climate change, but also delivers many agronomic, environmental and societal benefits. Composting and compost use must be recognized as one of the best options available for mitigating climate change, while also enhancing agricultural production.

Johannes Biala is Director of The Organic Force (biala@optusnet.com.au). The above information is based on a literature review (The Benefits of Using Compost for Mitigating Climate Change) that was jointly funded by the Department of Environment, Climate Change and Water NSW and The Organic Force. BioCycle extends its sincere thanks to Mr. Biala for suggesting, orchestrating the writing of, and delivering the articles that comprise this issue’s Focus On Australia report.

References
Amlinger, F., Peyer, S., Dreher, P. (2003). Current knowledge about nitrogen leaching in agricultural systems that use compost. Final Report (in German). Austrian Federal Ministry for Agriculture and Forestry, 91 pp.
Dalal, R. C., and Chan, K. Y. (2001). “Soil organic matter in rainfed cropping systems of the Australian cereal belt.” Australian Journal of Soil Research, 39 (3): 435-464.
Favoino, E. and Hogg, D. (2008). “The potential role of compost in reducing greenhouse gases.” Waste Management & Research, 26: 61-69.
LTZ [Landwirtschaftliches Technologiezentrum Augustenberg] (2008). Sustainable compost use in agriculture. Final Report (in German), 126 pp.