Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma Diameter-related variations in root decomposition of three common subalpine tree species in southwestern China Liyan Zhuang, Wanqin Yang, Fuzhong Wu, Bo Tan, Li Zhang, Kaijun Yang, Ruoyang He, Zhijie Li, Zhenfeng Xu ⁎ Key Laboratory of Ecological Forestry Engineering, Institute of Ecology & Forest, Sichuan Agricultural University, Chengdu 611130, China ARTICLE INFO Handling Editor: I. Kögel-Knabner Keywords: Root decomposition Root diameter Carbon Nitrogen Phosphorus Stoichiometry ABSTRACT Despite the importance of root quality for its decomposition, the effects of forest conversion and diameter size on root decomposition still remains poorly understood. A two-year field experiment was conducted to examine the mass loss and nutrient release of three root diameter classes (0–2 mm, 2–5 mm and 5–10 mm) in three subalpine tree species (Abies faxoniana, Picea asperata and Betula albosinensis) using a litter-bag method on the eastern Tibetan Plateau of China. The roots of B. albosinensis decayed faster compared to those of two conifer trees (A. faxoniana and P. asperata) in the 2–5 mm and 5–10 mm roots. Root diameter and decay rate exhibited a sig- nificantly negative correlation for two conifer trees. Regardless of tree species and diameter classes, all root litters experienced significant net nitrogen (N) and phosphorus (P) immobilization during the first winter. Both 2–5 mm and 5–10 mm roots tended to release more N than fine roots after 2-year incubation. Almost no obvious P release was observed for all root litters over the experimental period. Irrespective of tree species, both C:N and C:P ratios followed a trend of 0–2 mm roots < 2–5 mm roots < 5–10 mm roots during the experimental period. The root decomposition and N release were strongly associated with initial root quality (e.g., N and C:N). Our results suggest that diameter-associated variations in substrate quality could be an important driver of root decomposition and nutrient dynamics. Moreover, the diameter effects are dependent on tree species and de- composition period. 1. Introduction The decomposition of root litter is crucial to soil carbon (C) and nutrient cycling in terrestrial ecosystems (Lal, 2004). Recent evidence indicates that plant roots could be more important to accumulate stable soil organic matter as compared to aboveground leaves (Mambelli et al., 2011). Given the importance of fine root in C and nutrient cy- cling, it arose a great interest in unraveling the decomposition patterns of fine root. Traditionally, coarse roots (> 2 mm) plays a key role in the transport and storage of C and nutrients (Fogel, 1983; Palviainen and Finér, 2015). Coarse roots comprise over 80% of total root biomass (Butnor et al., 2014), which has important implications for long-term ecosystem productivity and C dynamic (Sierra et al., 2007). However, the decomposition dynamic of coarse roots and its controls are not as well understood as those on fine roots, especially in boreal forests at high-latitude and high-altitudes sites. At the global scale, substrate quality is one of the most important factors controlling decomposition process (Lin et al., 2011). For a given site, the rates of root decomposition could be primarily controlled by substrate properties (Silver and Miya, 2001). In general, root quality largely depends on tree species and root size in forest ecosystems (e.g., Makita et al., 2015; Sariyildiz, 2015). Forest land-use change often result in complete shift of tree species, which could, in turn, exert significant effects on ecosystem C cycling by altering root quality. For example, conifer tree roots generally have more recalcitrant C compo- nents and lower N and P contents as compared to broadleaf tree roots (Yang et al., 2004; Sun et al., 2013). Root diameter is a key factor that regulates root decay because it integrates physico-chemical traits as- sociated with root development (Fahey and Arthur, 1994). Different root diameters generally show marked differences in tissue substrate chemistry, with higher nutrients (e.g., N and P contents) in small roots (Birouste et al., 2012; Goebel et al., 2011). Thus, diameter-associated differences in root quality have the potential to affect the decomposi- tion patterns of the root system (Silver and Miya, 2001; Sun et al., 2013). Available studies have mainly focused on diameter-related variations within a fine root system (e.g., < 0.5 mm vs. 0.5–1 mm or 0.5–2 mm) (e.g., Yang et al., 2004; Sun et al., 2013; Wang et al., 2014). However, the decomposition heterogeneity between fine root and http://dx.doi.org/10.1016/j.geoderma.2017.09.041 Received 27 June 2017; Received in revised form 28 September 2017; Accepted 29 September 2017 ⁎ Corresponding author at: Institute of Ecology and Forestry, Sichuan Agricultural University, 211 Huimin Road, Wenjiang District, Chengdu 611130, Sichuan, China. E-mail address: xuzf@sicau.edu.cn (Z. Xu). Geoderma 311 (2018) 1–8 0016-7061/ © 2017 Elsevier B.V. All rights reserved. MARK