Nitrogen deposition increases root production and turnover but slows root decomposition in Pinus elliottii plantations

Abstract

Fine roots of woody plants comprise multiple root orders, which can be functionally partitioned into two pools: absorptive fine roots (AFRs, orders 1, 2) and transport fine roots (TFRs, orders 3-5). However, the function-based fine-root dynamics and especially their responses to increased nitrogen (N) availability remain unclear. We explored dynamic responses of both AFRs and TFRs of Pinus elliottii to N addition in subtropical China based on a 4-yr minirhizotron experiment and a two-stage - early (0.5 yr) vs late (4 yr) - decomposition experiment. N addition increased the production, mortality, and turnover of AFRs but not TFRs. High rates of N persistently inhibited AFR decomposition but affected TFR decomposition differentially at the early (no effect) and late (negative effect) stages. The increased production of AFRs was driven by N-induced decrease in foliar and soil phosphorus (P) concentrations. The decreased decomposition of AFRs might be due to the increased acid-unhydrolyzable residues in decomposing roots. AFRs are the resource-acquiring module, the increased carbon allocation to AFRs may represent a P-acquiring strategy when N no longer limits growth of P. elliottii. Our results suggest that AFRs and TFRs respond differently to N deposition, both in terms of production, mortality, and turnover and in terms of decomposition.

Unaltered phenology but increased production of ectomycorrhizal roots of Pinus elliottii under 4 years of nitrogen addition

“…We found significant relationships between root number and root length for all treatments ( r 2 ranging from 0.86 to 0.96; Fig. S3), suggesting that changes in root numbers can predict root length dynamics reliably (Crocker et al ., ; Kou et al ., ).…”

“…The details of the field sampling have been described previously (Kou et al, 2018b). Briefly, we randomly selected three mature P. elliottii individuals (similar heights and diameters at breast height) in the center of each plot in 2012.…”

“…N addition increased foliar and soil N contents but decreased P content (Table S1). P plays a pivotal role in determining root production in P-deficient subtropical forests when N no longer limits growth (Kou et al, 2018b). ECM roots are vitally important in P acquisition, so the increased amount of ECM roots in the N-addition treatments could have been largely driven by decreased P availability.…”

“…We have previously reported the effects of N concentration on root traits and processes (Kou et al, 2015(Kou et al, , 2017(Kou et al, , 2018b. The input of NH 4 + can release protons via root uptake or microbial nitrification (Matson et al, 1999).…”

A Processes‐Based Dynamic Root Growth Model Integrated Into the Ecosystem Model

“…The roots of each branch order are assumed to have different longevities and turnover rates, and such large discrepancies result in uncertainties in modeling terrestrial C cycles (Guo et al, ; Högberg & Read, ). Moreover, environmental changes impact the turnover rates of roots; for example, drought conditions can increase the turnover of fine roots (Gaul et al, ; Meier & Leuschner, ), and Kou et al () reported that nitrogen addition also enhances root turnover rates. However, current terrestrial biosphere models assume constant root turnover rates, resulting in uncertainties in carbon cycle simulations (Warren et al, ).…”