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Rhizosphere activity, grass species and N availability effects on the soil C and N cycles
Authors:E. Personeni  P. Loiseau
Affiliation:a Grassland Ecosystems Research Group, Agronomy Unit, INRA, 234 avenue du Brézet, F63039 Clermont-Ferrand Cedex 2, France
b Agroscope FAL Reckenholz, Swiss Federal Research Station for Agroecology and Agriculture, Reckenholzstrasse 191, CH-8046 Zurich, Switzerland
Abstract:We examined whether grass species and soil nitrogen (N) availability could enhance Carbon (C) and N turnover during root litter decay in grassland. Three species with increasing competitiveness (Festuca ovina, Dactylis glomerata and Lolium perenne) were grown at two N fertiliser levels in an undisturbed grassland soil, in which soil organic fractions derived for the last 9 years from Lolium root litter which was 13C-depleted. During the subsequent experimental year, the C turnover was calculated using the respective δ13C values of the old and new C in the root phytomass, in two Particulate Organic Matter (POM) fractions above 200 μm and in the lightest part of the aggregated soil fraction between 50 and 200 μm. Soil N availability was monitored during the regrowth periods with ion exchange resins (IER). The C decay rates of each particle size fraction were calculated with a simple mechanistic model of C dynamics. The N mineralisation immobilisation turnover (MIT) was characterised by dilution of 15N-labelled fertiliser in the N harvestThe C:N ratio and the residence time of C in the fractions decreased with particle size. The presence of a grass rhizosphere increased the decay rate of old C. Accumulation of new C in particle size fractions increased with species competitiveness and with N supply. Species competitiveness increased C turnover in the aggregated fraction, as a result of greater accumulation of new C and faster decay of old C. Fertiliser N increased N turnover and C mineralisation in the SOM. Species competitiveness decreased soil View the MathML source-N exchanged with the IER and increased dissolved organic C (DOC) content. The nature of the current rhizosphere is thus an important factor driving C and N transformations of the old root litter, in relation with grass species strategy. Plant competitiveness may stimulate the C and N turnover in the more evolved SOM fractions in a similar way to the mineral N supply.
Keywords:13C   15N   Rhizosphere   Species strategies   Mineralisation   Immobilisation   Root litter   Particulate organic matter   Dactylis glomerata   Festuca ovina   Lolium perenne
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