C and N mineralization of composted and anaerobically stored ruminant manure in differently textured soils

Abstract

Three animal manures cross-labelled with ¹⁵N in either the urine, faeces or straw fractions were prepared. After a storage period of 86 days when the manures were exposed to either composting or to anaerobic storage, portions of the manures were incubated in six differently textured soils with clay contents ranging from 11 to 45%. Evolved CO₂-C was determined during a 266 day incubation and inorganic N and ¹⁵N in soil were measured at the termination of the incubation. The mineralization of C was analysed using first-order kinetics, and two C pools with fast (P₁) and slow (P₂) turnover rates were estimated. The total conversion of added C (P_s) was estimated as P_s=P₁+P₂.

The cumulated CO₂ production was considerably higher from soils incubated with anaerobically stored manure compared with soils amended with composted manure. CO₂ production levelled off after c. 60 days in the three sandier soils whereas CO₂ continued to be produced throughout the incubation from the three soils with the highest clay content. More C was assigned to the easily decomposable P₁ pool in the sandiest soils whereas the more recalcitrant P₂ pool was larger in the soils with higher clay content. Because of the different relationships between soil texture and C pools, P_s ended up being similar for five of the six soils. When taking C losses during the preceding storage into account, the accumulated C losses during storage and after incubation in soil accounted for 60 and 54% of C initially present in the composted and anaerobically stored manure, respectively.

Net N mineralization which averaged 16% of applied organic N took place in all soils amended with composted manure. Soils with anaerobically stored manure showed net immobilization after the 266 days of incubation. The amount of N immobilized accounted for up to 30% of the inorganic N applied with the manure. As anaerobically stored manure generally loses less inorganic N during storage, it may contain more inorganic N than composted manure at the time of field application. Because of the immobilization that takes place after application of anaerobically stored manure to soil, the immediate levels of plant available N in soil may not be as different from soil supplied with composted manure as could be expected from the inorganic N content in the two types of manure. However, when considering the manure as a N resource, anaerobic storage is superior to composting.