Forest Ecology and Management 261 (2011) 391–399 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco Early-stage impacts of sika deer on structure and function of the soil microbial food webs in a temperate forest: A large-scale experiment Shigeru Niwa a,∗ , Lucero Mariani a , Nobuhiro Kaneko a , Hiroaki Okada b , Kazunori Sakamoto c a Soil Ecology Research Group, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan b National Institute for Agro-Environmental Sciences, 3-1-3 Kan’nondai, Tsukuba 305-8604, Japan c Graduate School of Horticulture, Chiba University, 648 Matsudo, Chiba 271-8510, Japan article info Article history: Received 1 July 2010 Received in revised form 20 October 2010 Accepted 20 October 2010 Available online 18 November 2010 Keywords: Mammalian herbivore Deer browsing Dwarf bamboo Soil nematode Soil microorganism Nitrogen mineralization abstract Modification of forest vegetation caused by an overabundance of mammalian herbivores has been reported in temperate and subarctic regions all over the world. However, the indirect effects of these herbivores on the structure and functions of soil decomposer systems are not fully understood, espe- cially in temperate forests. We investigated the early effects of sika deer invasion on soil decomposer systems in a Japanese temperate forest using two large-scale experimental enclosures with low and high densities of deer (LD: 25 ha, 4 deer km -2 ; HD: 6.25 ha, 16 deer km -2 ) including control plots without deer (WD). Three years after deer introduction the understory cover of dwarf bamboo (Sasa nipponica) declined due to deer browsing in both enclosures. At the same time, measurements were made of the soil micro- bial community, soil nematode community, soil nitrogen (N) mineralization rate, and carbon (C) and N content in dwarf bamboo leaves. In LD, soil microbial biomass was lower from WD, probably due to the decrease of fresh aboveground litter from dwarf bamboo. Surprisingly, there were no cascading effects on total abundance of soil nematodes and soil total N mineralization potential which were unaffected by deer in the LD treatment, while soil NH 4 + -N content was lower and soil nematode community structure was different (abundance of 4 families was higher and that of 3 families was lower, but the functional structure was not different) from WD. Specifically, the responses to deer introduction varied between microbes and nematodes, and the change of balance in the microbial food webs may have altered N min- eralization processes. In contrast, in the HD treatment, all the variables measured were not significantly different from those of WD treatment. Intensive browsing by deer may have cancelled out the effects of the decrease in aboveground litter input on the soil decomposer systems through other pathways, such as a transitory increase in belowground litter input caused by induced changes in allocation patterns of bamboo. No changes in total N mineralization potential, leaf N, and composition of understory vegetation in both enclosures indicated that deer introduction did not facilitate nor retard N cycling regardless of deer density. This study showed that sika deer browsing can affect soil decomposer systems at an early stage of invasion even at low density, which contrasts with previous studies on the subject. Linking our findings of early-stage effects of deer on soil decomposer systems to longer-term dynamics of understory vegetation and tree regeneration will be needed to evaluate the adequacy of deer management practices with respect to the sustainability of soil nutrient supplies. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Modification of forest vegetation caused by overabundance of mammalian herbivores has been reported in temperate and sub- arctic regions all over the world (Weisberg and Bugmann, 2003). In temperate forests across Japan, Sika deer (Cervus nippon Tem- ∗ Corresponding author. Present address: Tomakomai Experimental Forest, Field Science Center for Northern Biosphere, Hokkaido University, Takaoka, Tomakomai 053-0035, Japan. Tel.: +81 144 33 2171; fax: +81 144 33 2173. E-mail address: sniwa@fsc.hokudai.ac.jp (S. Niwa). minck) populations have been expanding for the past two decades, and alterations of forest vegetation resulting from the heavy brows- ing pressure of the deer have become a serious problem (Takatsuki, 2009). Understanding the impacts of mammalian herbivores on the structure and functions of forest ecosystems is required as a basis for the development of appropriate management systems of these mammal populations. Belowground systems play vital roles in terrestrial ecosystems, such as organic matter decomposition and nutrient mineraliza- tion, which enable sustainable primary production in aboveground systems. Activities of large mammalian herbivores can alter the structure and functions of soil decomposer systems resulting from 0378-1127/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2010.10.024