Root-induced decomposer growth and plant N uptake are not positively associated among a set of grassland plants

It is known that plant species can induce development of different soil decomposer communities and that they differ in their influence on organic matter decomposition and N mineralization in soil. However, no study has so far assessed whether these two observations are related to each other. Based on the hypothesis that root-induced growth of soil decomposers leads to accelerated decomposition of SOM and increased plant N avail- ability in soil, we predicted that (1) among a set of grassland plants the abundance of soil decomposers in the plant rhizosphere is positively associated with plant N uptake from soil organic matter. To test this, we established grassland microcosms consisting of two plant individuals, a natural soil decomposer community and 15 N-labelled plant litter as organic N source, and compared the rhizosphere decomposer communities and litter-N uptake of a grass Holcus lanatus , an herb Plantago lanceolata and a leguminous herb Lotus corniculatus .We further predicted that (2) in terms of litter-N uptake those plant species that induce lower abundance of decomposers benefit from sharing soil with species inducing higher decom- poser abundance. To test this, we grew the three plant species in two-species combinations and compared the ability of each species to acquire litter-N when living in the monoculture and in the species combinations. We found that the three plant species induced develop- ment of different soil decomposer communities and that they acquired different amounts of litter-N. However, while L. corniculatus induced the highest abundance of decomposers, H. lanatus had the highest uptake of N from the litter, which refuted our first prediction. Since this prediction was falsified, we could not properly test the second one, but we found that litter-N uptake of H. lanatus and P. lanceolata were not significantly affected by the presence of L. corniculatus and the higher abundance of decomposers induced by L. corniculatus roots. Our results show that among the three plant species tested root-induced decomposer growth and plant N uptake from soil organic matter were not positively associated. It appears that plant traits such as competitive ability for soil mineral N were more important for plant uptake of litter-N than those that directly affected the growth of soil decomposers. (Résumé d'auteur