Spatial and temporal effects of nitrogen addition on root morphology and growth in a boreal forest

The production and turnover of fine roots play a critical role in regulating underground carbon (C) cycling of terrestrial ecosystems, which are a biological feature in regulating the capacity of plant to capture soil nutrients. Although the importance of fine root production (FRP) and turnover (FRT) to whole-plant and ecosystem C cycling is increasingly recognized, their response to nitrogen (N) deposition remains unclear. To understand how N addition affects the FRP and FRT of fine roots, a field experiment was conducted with four N treatment levels (0, 2.5, 5.0, and 7.5 g N m(-2) yr(-1)) to quantify the effects of N deposition on fine root dynamics and vertical allocation in a boreal forest using the minirhizotron technique. Our results showed that N deposition significantly decreased total number of live and dead fine roots and total surface area of live fine roots in topsoil layers (0-20 cm soil depth), while increased in subsoil layers (20-40 cm soil depth) during 2015-2016. Average diameter of fine root was increased by N addition particularly in subsoil layers. The FRP rate was reduced by N addition in the topsoil, but increased in the subsoil layers, whereas patterns of their seasonal changes were not affected in both soil layers. The FRT rate tended to decrease under N addition compared with control in both soil layers during the observation years, potentially indicating a slower underground C cycle with N addition. Moreover, fine roots distributed more deeply in the soil due to N addition, indicating fine roots may through self-regulation and change of growth strategy to face environment stressor.