Particulate organic matter as a source of variation in denitrification in clods of soil

Existing mechanistic models of denitrification in clods of soil describe th e anaerobic activity in the centres of the clods, but they neglect the role of particulate organic materials. We therefore studied the effect of parti culate organic matter on denitrification in soil both under pasture and in arable cultivation. Clods were separated into two compartments: (i) particu late organic matter and adhering soil exceeding 200 mu m (coated POM), and (ii) the matrix (the rest of the soil). Potential denitrification and produ ction of CO2 were then estimated on coated POM, matrix and unfractionated s oil. The quantity and the quality of coated POM were assessed individually in 100 clods from the pasture and 100 from the arable land. The rate of pot ential denitrification was similar in unfractionated soil from these treatm ents. However, it was 70 times greater in the coated POM than in the matrix from the arable soil. Production of CO2 was nine times greater in the coat ed POM than in the matrix from the pasture soil and 33 times greater in the arable soils. These observations were the basis for a mechanistic model of denitrification, taking into account contributions from coated POM and the matrix. Denitrification rates in a computerized representation of clods fr om both pasture and arable soil had approximately the same distribution as experimental data both in the matrix and coated POM contributions in the cr opped soil and in the matrix alone in the pasture soil. Coated particulate organic matter can explain more than half of the denitrification and most o f the variation in denitrification when it increases microbial activity suf ficiently and the soil structure limits the supply of O-2.