Biol Fertil Soils (1991) 12:147-153 Biology and Fert~i'ty ofSoil s 9 Springer-Verlag 1991 Effects of organic manure on nitrification in arable soils* H.J. Laanbroek and S. Gerards Department of Soil Biology, Institute for EcologicalResearch, P.O. Box 40, 6666 ZG Heteren, The Netherlands Received February 4, 1991 Summary. The production of nitrate by the process of nitrification is highly dependent on other N-transforming processes in the soil. Hence, changes in the type of N compound applied to enrich agricultural soils may affect the production of nitrate. The size and activity of the chemolithotrophic bacterial community were studied in an integrated farming system, with increased inputs of or- ganic manure and reduced inputs of mineral nitrogenous fertilizer, versus conventional farming. The integrated farming had a positive effect on potential nitrifying activ- ity, but not on the numbers of chemolithotrophic nitrify- ing bacteria as determined by a most probable number technique or by fluorescence antibody microscopy. Cells of the recently described nitrite-oxidizing species Nitro- bacter hamburgensis and Nitrobacter vulgaris were just as common as the cells of the well known species Nitro- bacter winogradskyi. It was concluded that nitrification is stimulated by integrated farming, presumably by an in- creased mineralization of ammonium which is not imme- diately consumed by the crop or immobilized in the heterotrophic microflora of the soil. Since nitrifying bac- teria are involved in the production of NO and N20, in- tegrated farming with the application of manure may fa- vour the production of noxious N-oxides. Key words: Nitrification - Integrated farming - Con- ventional farming - Ammonification - Fluorescence antibody microscopy - Nitrobacter spp. In the Netherlands as well as in other western countries, intensive arable farming faces several problems related to both soil fertility and environment (Brussaard et al. 1988). Among these urgent problems are the contamina- tion of ground and surface water and the atmosphere, due to intensive use of fertilizers and biocides. Recently, * Communication no. 40 of the Dutch Programme on Soil Ecologyof Arable Farming Systems Offprint requests to: H.J. Laanbroek integrated arable farming has been proposed as a way of reducing the input of chemicals into the agroecosystem. One of the changes in farming practices proposed in- volves a partial replacement of mineral fertilizers by ma- nure. In natural ecosystems, there is usually a good balance between the production of ammonium from organic-N compounds and its immobilization by microorganisms and plants (Woldendorp 1981). A disturbance in this equilibrium, for example by deposition of ammonium from the atmosphere, may lead to the production of ni- trate that is prone to leaching (Van Breemen et al. 1982). In agroecosystems, the N cycle is manipulated by the in- troduction of inorganic nitrogenous fertilizers. The addi- tion of mineral fertilizers is intended to match the needs of the crop. However, ammonium might be produced from manure at times when there is no need for mineral N by the plants or by heterotrophic microflora growing on plant residues. At these times, nitrifying bacteria may be activated by an excess of ammonium. The occurrence of nitrification may lead to undesired effects on the envi- ronment, i.e., leaching of nitrate into ground water or the production of N-oxides by denitrifying bacteria. Recent- ly, chemolithotrophic nitrifiers themselves have been rec- ognized as important producers of N-oxides in arable soils (Tortoso and Hutchinson 1990). Hence, nitrification should not be stimulated. This paper studies the effect of integrated farming, in particular the application of organic manure, on the pro- cess of nitrification. Since none of the methods normally used to describe the quality of a nitrifying bacterial com- munity in the soil is sufficiently accurate (Woldendorp and Laanbroek 1989), different methods were used to de- termine bacterial activity and numbers of bacteria. Materials and methods Description of the experimental farming fields The fields studied are part of the experimentalfarm Dr. H.J. Lovink- hoevenear the villageof Marknesse in the Noordoostpolder, The Neth-