Compost: Production, Use and Impact on Carbon and Nitrogen Cycles

Keywords: biodegradable wastes, composting, compost maturity and quality, plant nutrients.

The importance of composting in countries of the European Union has been enhanced by recent European legislation, which has forced a switch from the landfill of organic wastes to their recycling and transformation. This should promote the concept that composting is a way of obtaining useful and valuable products from waste materials, rather than just a method to dispose of them. Different official institutions from several countries have set limits of certain non-desirable components in compost, as well as the quality parameters to define compost use, but an international standardisation is required.

At present, there is much interest in the use of composts in the agricultural and horticultural sectors. However, the great variety of biodegradable wastes which can be transformed into compost makes it necessary to adapt the well developed technology for composting of city refuse to the requirements of these other wastes. To obtain good-quality compost, the composting process should be carefully controlled, using adequate mixtures of wastes for their proper degradation and allowing enough time for maturation. In particular, the achievement of compost maturity guarantees that:

1. The organic matter (OM) is microbiologically stable, with humic characteristics.

2. Phytotoxic compounds are absent.

3. The compost is sanitised, useful as a fertiliser, providing a slow release of plant nutrients.

The transformation of wastes through composting leads to gaseous emissions into the atmosphere, mainly carbon dioxide (CO₂) and ammonia (NH₃), and to possible nutrient loss via leaching. These have an impact in the environment and, in the case of nitrogen (N), reduce the agricultural value of the compost. Strategies for reducing N-losses should be based on:

1. Technical aspects of composting installations such as leachate collection and recycling, proper aeration system and a gas filtration.

2. Aspects related to the composition of the composting mass, particularly the use of a bulking agent which provides degradable organic-C, for inducing N-immobilisation during the first steps of the process, and an adequate initial C/N ratio.

When composted wastes are added to soil, the decreased CO₂ emission from the soil, relative to their direct application without composting, can more than compensate for the CO₂ emitted from the composting system. Thus, composting improves C-conservation in the soil system, enhancing its role as a sink for CO₂ and maintaining soil fertility, at the chemical, physical and biological levels.

Dr María Pilar Bernal, Dept. Soil and Water Conservation and Organic Waste Management, Centro de Edafología y Biología Aplicada del Segura, CSIC, Campus Universitario de Espinardo, Apartado 164, 30100 Murcia, Spain.

36 pages, 7 figures, 5 tables, 72 references.