1. ¹INRA, CNRS, Université de Lyon, Université Lyon 1, UMR 5557 Ecologie Microbienne, Villeurbanne, France
2. ²GSF National Research Center for Environment and Health, Institute of Soil Ecology, Neuherberg, Germany
3. ³School of Biological Sciences, University of Aberdeen, Aberdeen, UK
4. ⁴Unité d'Agronomie INRA, Clermont Ferrand, France

Correspondence: X Le Roux, INRA, CNRS, Université de Lyon, Université Lyon 1, UMR 5557 Ecologie Microbienne, USC 1193, bat G Mendel, 43 boulevard du 11 novembre 1918, Villeurbanne 69622, France. E-mail: leroux@biomserv.univ-lyon1.fr

⁵Current address: Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK.

Received 7 August 2007; Revised 29 October 2007; Accepted 29 October 2007; Published online 29 November 2007.

Abstract

The influence of switches in grassland management to or from grazing on the dynamics of nitrifier activity, as well as the abundance of ammonia-oxidizing bacteria, AOB and ammonia-oxidizing archeae, AOA, was analyzed for two years after changing management. Additionally community structure of AOB was surveyed. Four treatments were compared in mesocosms: grazing on previously grazed grassland (G-G); no grazing on ungrazed grassland (U-U); grazing on ungrazed grassland (U-G) and cessation of grazing on grazed grassland (G-U). Nitrifier activity and abundance were always higher for G-G than U-U treatments and AOB community structure differed between these treatments. AOA abundance was in the same range as AOB abundance and followed the same trend. Grazing led to a change in AOB community structure within <5 months and a subsequent (5–12 months) increase in nitrifier activity and abundance. In contrast, cessation of grazing led to a decrease in nitrifier activity and abundance within <5 months and to a later (5–12 months) change in AOB community structure. Activity in G-U and U-G was similar to that in U-U and G-G, respectively, after 12 months. Sequence analysis of 16S rRNA gene clones showed that AOB retrieved from soils fell within the Nitrosospira lineage and percentages of AOB related to known Nitrosospira groups were affected by grazing. These results demonstrate that AOB and AOA respond quickly to changes in management. The selection of nitrifiers adapted to novel environmental conditions was a prerequisite for nitrification enhancement in U-G, whereas nitrification decrease in G-U was likely due to a partial starvation and decrease in the abundance of nitrifiers initially present. The results also suggest that taxonomic affiliation does not fully infer functional traits of AOB.