Effects of mixture of branch order-based roots and nitrogen addition on root decay in a subtropical pine plantation

Abstract

Non-additive effects may occur when branching order-based fine roots decompose in mixture, due to their distinct chemical composition. However, these effects and how they respond to increased nitrogen (N) availability are poorly known. We conducted a 524-day field litterbag experiment to examine the potential non-additive effects of order-based fine root decomposition and their responses to different rates of N addition in a Pinus elliottii (slash pine) plantation in China. The intact fine roots of slash pine trees were hierarchically dissected and split into two groups (lower-order roots (the first and second order roots) and higher-order roots (the third to fifth order roots)). The decomposition of these two root groups was monitored either separately or in 1:1 mixture. The non-additive effects did not occur when the two root groups decomposed in mixture. However, high rate of N addition shifted the root group interactions from additive to synergistic effects through contrasting effects on individual root group (lower-order roots). High rate of N addition retarded the decomposition of lower-order roots more in isolation than in mixture, due to the decreased binding of acid-unhydrolyzable residue to inorganic N ions in mixture. Therefore, we suggest that although stoichiometric heterogeneity holds within the branching hierarchy of fine roots, the tight coupling of N with acid-unhydrolyzable residue of root tissues may limit N use by microdecomposers, and thereby causing the additive effect. Since the mixing effect of order-based root decomposition is affected by N addition, understanding the chemical heterogeneity within fine root systems and their linkage with ambient N availability during decomposition either in isolation or in mixture may facilitate the accurate prediction of root decomposition responses to N loading.