Decoupling of soil microbes and plants with increasing anthropogenic nitrogen inputs in a temperate steppe Weixing Liu a, * , Lin Jiang b , Shuijin Hu c , Linghao Li a , Lingli Liu a , Shiqiang Wan d a State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Xiangshan, Beijing 100093, China b School of Biology, Georgia Institute of Technology, Atlanta, GA, USA c Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695, USA d Key State Laboratory of Cotton Biology, College of Life Sciences, Henan University, Kaifeng, Henan 475004, China article info Article history: Received 10 July 2013 Received in revised form 15 January 2014 Accepted 22 January 2014 Available online 6 February 2014 Keywords: Soil acidification Decoupling Grassland Plant Soil microbes N addition abstract Plant growth and soil microbial activity are intrinsically correlated. Numerous evidence shows that ni- trogen (N) deposition can greatly alter both processes. However, it is unknown whether such changes caused by N deposition can create new dynamics between plants and soil microbes. This study was con- ducted with an attempt to examine the plantemicrobe relationship along an N addition gradient. Eight levels of N addition (0, 1, 2, 4, 8, 16, 32, 64 g N m 2 ) were applied annually in a temperate steppe in northern China since 2003. Plant and soil samples were collected from 2005 to 2007. We found that N addition acidified soil significantly. Both plant aboveground biomass and dissolved organic carbon (DOC) increased with increasing N input. However, soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and (soil) microbial respiration showed nonlinear responses to N input. Low levels of N inputs stimulated MBC, MBN and microbial respiration, whereas high levels of N input suppressed them. Although MBC and MBN were both positively correlated with aboveground biomass at each level of N treatments, the dependence of such biomass on MBC and MBN declined with the increase in N addition, as indicated by the exponential decreases in the regression coefficients. The weakened linkage between aboveground biomass and MBC was mostly attributed to soil acidification. The decrease in soil pH caused by elevated N inputs reduced soil microbial activities, but not plant growth. Overall, our results revealed a trend of shifting plant emicrobe relationship from coupling to decoupling with the increase of N input. The divergent responses of plants and soil microbial activities under intensified N addition could have consequent impacts on ecosystem function and services. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Plants and soil microbes are critical to maintain the functions of natural terrestrial ecosystems (Reynolds et al., 2003; Wardle et al., 2004; Wagg et al., 2011). Plant productivity and soil microbial ac- tivities often tightly coupled, especially in nutrient poor ecosystems (Bardgett et al., 1999; Paterson, 2003; van der Heijden et al., 2008). Plants depend upon nutrient supply mediated by soil microbes, which mineralize nutrients from organic to inorganic forms that can be utilized by plants (De Deyn et al., 2004; Bardgett et al., 2005; van der Heijden et al., 2008). On the other hand, soil microbes rely on plants for carbon (C) substrates in the form of litter and root exu- dates (Paterson, 2003; Wardle et al., 2004; Bardgett et al., 2005). Atmospheric nitrogen (N) deposition, mainly from fossil fuel combustion and fertilizer applications, has risen dramatically since the Industrial Revolution and is projected to continually increase in the future (Erisman et al., 2011). Such global N enrichment has profoundly altered biogeochemical cycles in both aquatic and terrestrial biospheres (Gruber and Galloway, 2008). N is often the most limiting nutrient for plant growth (Vitousek and Howarth, 1991; LeBauer and Treseder, 2008). Plant growth and productivity generally increased with the increase of N deposition across various terrestrial plant species and ecosystems (Elser et al., 2007; Xia and Wan, 2008). The stimulation of plant growth and primary pro- ductivity induced by enhanced N availability will increase above- ground litter inputs to soil (Liu and Greaver, 2010), and may also result in greater belowground C input by increasing root growth and root exudates, especially in nutrient poor sites (Bradford et al., 2008). Consequently, growth and activity of soil microorganisms are expected to increase with the increase in C supply by plants. However, an increasing body of evidence has demonstrated * Corresponding author. Tel.: þ86 10 62836161; fax: þ86 10 82596134. E-mail address: lwxsx@ibcas.ac.cn (W. Liu). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soilbio.2014.01.022 Soil Biology & Biochemistry 72 (2014) 116e122