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Environmental Microbiology (2006) 8(6), 1005–1016 doi:10.1111/j.1462-2920.2006.00992.x

Effects of management regime and plant species on

the enzyme activity and genetic structure of N- xing, denitrifying and nitrifying bacterial communities in grassland soils

Ashok K. Patra,1 Luc Abbadie,2

Annie Clays-Josserand,1 Valérie Degrange,1 Susan J. Grayston,3 Nadine Guillaumaud,1 Pierre Loiseau,4 Frédérique Louault,4 Shahid Mahmood,5 Sylvie Nazaret,1

Laurent Philippot,6 Franck Poly,1 James I. Prosser5 and Xavier Le Roux1*1

UMR 5557 Ecologie Microbienne (CNRS-Université Lyon 1, USC INRA 1196), bâtiment G. Mendel, 43 boulevard du 11 novembre 1918, 69622 Villeurbanne, France.2

UMR 7625 Ecologie (CNRS-Université Paris 6-ENS), 45 rue d’Ulm, 75005 Paris, France.3

Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, Scotland.4

Unité d’Agronomie (INRA), site de Crouel, 234 avenue du Brézet, 63039 Clermont Ferrand, France.5

School of Medical Sciences, University of Aberdeen, Institute of Medical Sciences, Foresterhill, Aberdeen AB25 2ZD, Scotland.6

Laboratoire de Microbiologie des Sols (INRA), 17 rue Sully – B. P. 86510, 21065 Dijon, France.Summary

Management by combined grazing and mowingevents is commonly used in grasslands, which in u-ences the activity and composition of soil bacterialcommunities. Whether observed effects are mediatedby management-induced disturbances, or indirectlyby changes in the identity of major plant species, isstill unknown. To address this issue, we quanti edsubstrate-induced respiration (SIR), and the nitri ca-tion, denitri cation and free-living N2- xation enzymeactivities below grass tufts of three major plant spe-cies (Holcus lanatus, Arrhenatherum elatius and Dac-tylis glomerata) in extensively or intensively managedgrasslands. The genetic structures of eubacterial,

ammonia oxidizing, nitrate reducing, and free-livingN2- xing communities were also characterized byribosomal intergenic spacer analysis, and denaturinggradient gel electrophoresis (DGGE) or restrictionfragment length polymorphism (RFLP) targetinggroup-speci c genes. SIR was not in uenced by man-agement and plant species, whereas denitri cationenzyme activity was in uenced only by plant species,and management–plant species interactions wereobserved for xation and nitri cation enzyme activi-ties. Changes in nitri cation enzyme activity werelikely largely explained by the observed changes inammonium concentration, whereas N availability wasnot a major factor explaining changes in denitri ca-tion and xation enzyme activities. The structures ofeubacterial and free-living N2- xing communitieswere essentially controlled by management, whereasthe diversity of nitrate reducers and ammonia oxidiz-ers depended on both management and plant spe-cies. For each functional group, changes in enzymeactivity were not correlated or were weakly correlatedto overall changes in genetic structure, but around60% of activity variance was correlated to changes in ve RFLP or DGGE bands. Although our conclusionsshould be tested for other ecosystems and seasons,these results show that predicting microbial changesinduced by management in grasslands requires con-sideration of management–plant species interactions.Introduction

Management by combined grazing and mowing eventscan deeply affect the biodiversity and functioning of grass-land ecosystems (Collins et al., 1998). Herbivores, whichhave a more selective effect on vegetation than mowing,often enhance soil N cycling (McNaughton et al., 1997),which can result in improved N availability to plants (Ris-ser and Parton, 1982; McNaughton et al., 1997; Hamiltonand Frank, 2001). In particular, grazing can affect keymicrobially mediated processes involved in soil N cycling(i.e. nitri cation, denitri cation and N2- xation) that largelycontrol the balance between the forms of soil mineralnitrogen (NO3– versus NH4+) available to plants and Nconservation at the ecosystem level. Ecosystem-scale

Received 14 October, 2005; accepted 14 December, 2005. *Forcorrespondence. E-mail leroux@biomserv.univ-lyon1.fr; Tel.(+33) 472 431379; Fax (+33) 472 431223. Present addresses: Divi-sion of Soil Science & Agricultural Chemistry, Indian AgriculturalResearch Institute, New Delhi 110 012, India; Soil Microbial EcologyChair, Department of Forest Sciences, University of British Columbia,2424 Main Mall, Vancouver BC V6T 1Z4, Canada.

© 2006 The Authors

Journal compilation © 2006 Society for Applied Microbiology and Blackwell Publishing Ltd