Aspects of carbon and nitrogen cycling in soils of the Bornhoved Lake district I. Microbial characteristics and emissions of carbon dioxide and nitrous oxide of arable and grassland soils

Soil microbial biomass content, organic carbon mineralization as well as arginine ammonification rates were estimated in samples from arable and grassland soils and carbon dioxide and nitrous oxide emission rates were measured in situ at four sites along a catena. Soil microbial biomass content increased in the order, maize monoculture < crop rotation < dry grassland < wet grassland. The two arable soils had similar rates of carbon mineralization in the laboratory at 22 °C (basal respiration) as well as in situ (carbon dioxide emission) at field temperature. Under crop rotation, maize monoculture and dry grassland, the arginine ammonification rate significantly correlated to the microbial biomass content. In contrast, the biomass-specific ammonification rate was low in wet grassland soil, as were in situ N₂O emission rates. Data from all sites together revealed no general relationship between microbial biomass content and C and N mineralization rates. In addition, there was no general relationship between the quantity of soil microbial biomass and the emission rates of the greenhouse gases CO₂ and N₂O. The maize monoculture induced a soil microbial community that was less efficient in using organic carbon compounds, as shown by the high metabolic quotient (respiration rate per unit of biomass). However, microbial biomass content was proportional to basal respiration rate in soils under crop rotation, dry and wet grassland. Arginine ammonification rate was related to microbial biomass content only in fertilized soils. Applications of high quantities of inorganic nitrogen and farmyard manure apparently increase in situ N₂O emission rates, particularly under crop rotation. The microbial biomass in the unfertilized wet grassland soil seems to be a sink for nitrogen.