 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1436-8730/06/0408-481 J. Plant Nutr. Soil Sci. 2006, 169, 481–487 DOI: 10.1002/jpln.200521922 481 Clonal and seasonal shifts in communities of saprotrophic microfungi and soil enzyme activities in the mycorrhizosphere of Salix spp. Christel Baum 1 * and Katarzyna Hrynkiewicz 2 1 Institute for Land Use, University of Rostock, Justus-von-Liebig-Weg 6, D-18059 Rostock, Germany 2 Dept. of Microbiology, N. Copernicus University, ul. Gagarina 9, PL-89–100 Torun, Poland Accepted February 22, 2006 PNSS P192/2B Summary The species-specific microbial root and rhizosphere coloniza- tion contributes essentially to the plant nutrient supply. The species number and colonization densities of cultivable saprotrophic microfungi and the activities of nutrient-releas- ing soil enzymes (protease, acid and alkaline phosphatase, arylsulfatase) were investigated in the rhizosphere of one low mycorrhizal (Salix viminalis) and one higher mycorrhizal (S. × dasyclados) willow clone at a Eutric Cambisol in N Germany. After soil washing, in total 32 and 28 saprotrophic microfungal species were isolated and identified microscopically from the rhizosphere of S. viminalis and S. × dasyclados, respectively . The fungal species composition changed within the growing season but the species number was always lower under S. × dasyclados than under S. viminalis. Under both willow clones, the fungal colonization density was largest in spring, and the species number was largest in autumn. Acid-phosphatase activity (p < 0.001) and protease activity (p < 0.003) were significantly affected by the Salix clone, whereas arylsulfa- tase and alkaline-phosphatase activities did not show clone- specific differences. All enzyme activities reached their max- ima in the summer sampling. Rhizosphere colonization with Acremonium butyri, Cladosporium herbarum, and Penicillium janthinellum contributed significantly to explain the activities of acid phosphatase. Rhizosphere colonization with Cylindro- carpon destructans, Penicillium spinulosum, Plectosphaerella cucumerina, and Trichoderma polysporum contributed signifi- cantly to explain the arylsulfatase activities. Effects of the saprotrophic fungal colonization densities on the protease activities in the rhizosphere were low. Acid- and alkaline- phosphatase and arylsulfatase activities in the rhizosphere soil were stronger affected by the composition of the sapro- trophic fungal communities than by the Salix clone itself. In conclusion, the colonization density of some saprotrophic microfungi in the rhizosphere contributed to explain shifts in soil-enzyme activities of the P and S cycles under different willow clones. Key words: saprotrophic microfungi / fungus-plant interactions / willows / short-rotation forestry / Euric Cambisol 1 Introduction Microbial interactions in the mycorrhizosphere are important in maintaining soil fertility especially in low-input cropping systems (Johansson et al., 2004). Mycorrhizal fungi can improve significantly the plant nutrition of their host plants but also influence the diversity and colonization density of sapro- trophic microfungi and bacteria in the rhizosphere (Bansal and Mukerji, 1994; McAllister et al., 1997; Marschner et al., 2001). Saprotrophic microfungi and bacteria are main decom- posers of litter and contribute significantly to the nutrient mobilization. Among these two groups, fungi normally are quantitatively more important in the microbial litter decompo- sition in soils than bacteria (Anderson and Domsch, 1978). The enzyme activities of saprotrophic microorganisms are significantly correlated with the level of nutrient concentra- tions in their habitat (Kayang, 2001; Waldrop et al., 2003). Therefore, enzyme assays are useful to ascertain variations in the soil’s potential to mobilize nutrients (Naseby et al., 1998). Salicaceae (Salix and Populus spp.) are used for biomass production in low-input cropping systems (Jug et al., 1999). They are dual mycorrhizal plants (Khan, 1993) and can signif- icantly increase their biomass production by mycorrhization (Backhaus et al., 1986). This effect lasted for inoculated Populus spp. over an investigated period of 7 y (Hofmann and Baum, 2005). Salix viminalis and S. × dasyclados clones have high capacities for biomass production, but differ in their mycorrhizal colonization and the decomposition rates of their litter (Püttsepp, 2004). Mycorrhizal colonization of Salix vimi- nalis is lower (Baum, 2002; Püttsepp et al., 2004; Kahle et al., 2005), and litter-decomposition rate higher than those of Salix × dasyclados caused by qualitative differences in the litter composition (Püttsepp et al., 2004). Different litter quality can cause shifts in the microbial communities and activities (Conn and Dighton, 2000). To the best of our knowledge, published information is avail- able neither about the clone-specific rhizosphere colonization with saprotrophic microfungi of Salix spp. nor about clonal effects on soil-enzyme activities under these plant taxa. Therefore, the objectives of the present study were (1) to document the colonization of saprotrophic microfungi and soil-enzyme activities in the mycorrhizosphere (rhizo- sphere and bulk soil from the same samples) of these two Salix clones with different mycorrhizal colonization, and (2) to derive relationships between the abundance of single fungal species and the enzyme activities in the rhizosphere. We hypothesize that the diversity and colonization densities of saprotrophic microfungi in the rhizosphere contribute to explain clone-specific differences in the nutrient mobilization in the rhizosphere soil during the growing season. * Correspondence: PD Dr. C. Baum; e-mail: christel.baum@uni-rostock.de