Carbon Fluxes in the Soil Ecosystem

Soil is the habitat for plants, animals and microorganisms. As plants build up organic matter, soil animals feed on them and their debris, whilst microbes decompose the complex organic compounds to their mineral components and CO2.

The living soil is a central part of soil fertility, because the activity of soil organisms renders available the elements in plant residues and organic debris entering the soil. Part of this material however remains in the soil and contributes to its stabilisation by humus build up.

Mineralization of plant residues is the most important activity carried out by soil organisms, on par with the activity of plants to assimilate CO2 from the air. In the case of wheat, crop plant residues may add up to 10 t of straw per ha. At an average carbon content of 40 percent, and given the assumption that roots make up the same amount as the straw, 8000 kg of carbon enter the soil via the residue. Microbes mineralize it to CO2 and use it for biomass build up.

Microbial biomass in mg Cmic per kg soil, K2 = 100 %

Earthworms work hand in hand with fungi, bacteria and numerous other microorganisms in soil. In organically managed soils, the activity of these organisms is higher. Thus, nutrients are recycled faster and soil structure is improved.

Microbial processes in straw decomposition

A greater fraction of the straw applied to soil of the bio-dynamic system was mineralised (CO2) compared to the conventional soil. Additionally more straw-derived carbon was incorporated into the microbial biomass (Cmic). There was also finally much less untouched straw material in bio-dynamic than in conventional soils. This result shows that microorganisms in organic soils are not only mineralizing more actively, but also that they contribute to the build up of stable soil organic matter.

The role of microbial biomass in organic matter decomposition

The amount of microbial biomass and decomposition is connected: When relating light fraction organic material (which still has visible plant structures) to soil microbial biomass, the functional role of microbes and their substrates (the residues) becomes evident. At high microbial biomass levels, little light fraction material remains undecomposed and vice versa.

Enzymes are indicators of microbial functions

Microbes have activities with important functions in the soil system; soil enzymes indicate these functions. The total activity of microorganisms can be estimated by measuring the activity of a living cell-associated enzyme such as dehydrogenase. This enzyme plays a major role in the respiratory pathway. Proteases in soil, where most organic N is protein, cleave protein compounds.

Phosphatases cleave organic phosphorus compounds and thus provide a link between the plant and the stock of organic phosphorus in the soil. In soils of the organic systems soil enzyme activities were markedly higher than in the conventional soils. Dehydrogenase activity differentiated the systems in a similar manner in both 1990 and 1998.

• dehydrogenase activity in µg TPF per g soil and hour
• protease activity in µg tyrosine per g soil and hour
• alkaline phosphatase activity in µg phenol per g soil and hour

Organic crops profit from root symbioses and are better able to exploit the soil

A major part of the soil microbial biomass is composed of fungi. Important representatives of the soil fungi are the mycorrhizae that build up a symbiosis between fungus and plant. Both the plant and the fungus profit from this symbiosis: the plant gets nutrients acquired by the fungus and the fungus receives assimilates from the plant in exchange.

Mycorrhizae enlarge the plants rooting zone and can enter small pores in the soil, mobilise nutrients and carry them to the plant. Recently it was shown that mycorrhizae are able to colonize different plants at the same time and may therefore serve as a bridge between them.Moreover nitrogen bound in decaying roots may be saved from leaching. Last, but not least, mycorrhizae play a role in soil aggregate stability.

Colonization of the roots by mycorrhizae (1989–1993)

On average, mycorrhizal colonization of roots was highest in the crops of the unfertilized system, followed by the organic systems. The conventional crops had colonization levels that were 30 percent lower.

Among the crops and systems of the DOK-trial the most intense mycorrhizal root colonization was found in grass-clover, followed by the vetch rye intercrop. Roots of winter wheat were only scarcely colonized. Even when all soils were inoculated with active mycorrhizae, colonization was enhanced in organic soils (investigations of the Department of Botany, University of Basle).

This indicates that, even at an inoculum in surplus, soil nutrients at elevated levels and plant protection suppress the symbiosis. This underlines the importance of appropriate living conditions for specific organisms.