title: Effects of management regime and plant species on the enzyme activity and genetic structure of N-fixing, denitrifying and nitrifying bacterial communities in grassland soils.
abstract: Management by combined grazing and mowing events is commonly used in grasslands, which influences the activity and composition of soil bacterial communities. Whether observed effects are mediated by management-induced disturbances, or indirectly by changes in the identity of major plant species, is still unknown. To address this issue, we quantified substrate-induced respiration (SIR), and the nitrification, denitrification and free-living N(2)-fixation enzyme activities below grass tufts of three major plant species (Holcus lanatus, Arrhenatherum elatius and Dactylis glomerata) in extensively or intensively managed grasslands. The genetic structures of eubacterial, ammonia oxidizing, nitrate reducing, and free-living N(2)-fixing communities were also characterized by ribosomal intergenic spacer analysis, and denaturing gradient gel electrophoresis (DGGE) or restriction fragment length polymorphism (RFLP) targeting group-specific genes. SIR was not influenced by management and plant species, whereas denitrification enzyme activity was influenced only by plant species, and management-plant species interactions were observed for fixation and nitrification enzyme activities. Changes in nitrification enzyme activity were likely largely explained by the observed changes in ammonium concentration, whereas N availability was not a major factor explaining changes in denitrification and fixation enzyme activities. The structures of eubacterial and free-living N(2)-fixing communities were essentially controlled by management, whereas the diversity of nitrate reducers and ammonia oxidizers depended on both management and plant species. For each functional group, changes in enzyme activity were not correlated or were weakly correlated to overall changes in genetic structure, but around 60% of activity variance was correlated to changes in five RFLP or DGGE bands. Although our conclusions should be tested for other ecosystems and seasons, these results show that predicting microbial changes induced by management in grasslands requires consideration of management-plant species interactions.
authors: UMR 5557 Ecologie Microbienne, CNRS-Université Lyon 1, USC INRA 1196, bâtiment G. Mendel, 43 boulevard du 11 novembre 1918, 69622 Villeurbanne, France.
authors: Ashok K
authors: AK
authors: Patra
authors: Luc
authors: L
authors: Abbadie
authors: Annie
authors: A
authors: Clays-Josserand
authors: Valérie
authors: V
authors: Degrange
authors: Susan J
authors: SJ
authors: Grayston
authors: Nadine
authors: N
authors: Guillaumaud
authors: Pierre
authors: P
authors: Loiseau
authors: Frédérique
authors: F
authors: Louault
authors: Shahid
authors: S
authors: Mahmood
authors: Sylvie
authors: S
authors: Nazaret
authors: Laurent
authors: L
authors: Philippot
authors: Franck
authors: F
authors: Poly
authors: James I
authors: JI
authors: Prosser
authors: Xavier
authors: X
authors: Le Roux