Responses of fine root exudation, respiration and morphology in three early successional tree species to increased air humidity and different soil nitrogen sources

Abstract

Global climate change scenarios predict an increase in air temperature, precipitation, and air humidity for northern latitudes. Elevated air humidity may significantly reduce the water flux through forest canopies and affect interactions between water and nutrient uptake. However, we have limited understanding of how altered transpiration would affect root respiration and carbon (C) exudation as fine root morphology acclimates to different water flux. We investigated the effects of elevated air relative humidity (eRH) and different inorganic nitrogen sources (NO3− and NH4+) on above and belowground traits in hybrid aspen (Populus × wettsteinii Hämet-Ahti), silver birch (Betula pendula Roth.), and Scots pine (Pinus sylvestris L.) grown under controlled climate chamber conditions. The eRH significantly decreased the transpiration flux in all species, decreased root mass-specific exudation in pine, and increased root respiration in aspen. eRH also affected fine root morphology, with specific root area increasing for birch but decreasing in pine. The species comparison revealed that pine had the highest C exudation, while birch had the highest root respiration rate. Both humidity and nitrogen treatments affected the share of absorptive and pioneer roots within fine roots; however, the response was species-specific. The proportion of absorptive roots was highest in birch and aspen, the share of pioneer roots was greatest in aspen, and the share of transport roots was greatest in pine. Fine roots with lower root tissue density were associated with pioneer root tips and had a higher C exudation rate. Our findings underline the importance of considering species-specific differences in relation to air humidity and soil nitrogen availability that interactively affect the C input–output balance. We highlight the role of changes in the fine root functional distribution as an important acclimation mechanism of trees in response to environmental change.

Carbon allocation to root exudates is maintained in mature temperate tree species under drought

“…In our study, the proportions of net-C assimilation allocated to root exudation were only 0.6 AE 0.1% and 1.8 AE 0.6% in control and drought plots, respectively, which were below the 3-30% previously reported at other study sites for multiple species (Kuzyakov & Domanski, 2000;Jones et al, 2009;Finzi et al, 2015;Abramoff & Finzi, 2016;Gougherty et al, 2018). Our observed exudation rates from root branches are in line with modeled or measured root exudation rates of diverse vegetation types (Finzi et al, 2015;Dror & Klein, 2021;Rog et al, 2021;Sell et al, 2021), although they are at the lower end of reported values from comparable temperate forests (Tückmantel et al, 2017;Meier et al, 2020) and other ecosystems (summary provided by Gougherty et al (2018). Discrepancies in exudate estimates across studies may arise as a result of methodological differences such as filter size variations (0.2 vs 0.7 µm) or the use of C-free materials (Gougherty et al, 2018), bedrock characteristics (Meier et al, 2020) or potential reuptake during longer collection periods (Oburger & Jones, 2018).…”

Linking root exudation to belowground economic traits for resource acquisition

“…Plants exhibit substantial variation in root exudation and root morphology in response to resource availability among and within species (Wen et al ., 2019; Meier et al ., 2020; Williams et al ., 2021), but our understanding of how root exudation (quantity and quality) covaries with root morphology remains inconclusive (Phillips et al ., 2008; Aoki et al ., 2012; Proctor & He, 2017; Wen et al ., 2019; Sun et al ., 2021). For example, across a range of species, the release of root exudates is either negatively (Lyu et al ., 2016; Wen et al ., 2019; Honvault et al ., 2021), positively (Aoki et al ., 2012; Guyonnet et al ., 2018; Akatsuki & Makita, 2020; Meier et al ., 2020; Wang et al ., 2021b), or not (Kidd et al ., 2015; Sell et al ., 2021; Sun et al ., 2021; Williams et al ., 2021) correlated with SRL or specific root surface area (SRA). Adjustments in both root morphology and root exudation are consider as major C expenses for plant roots (Lambers et al ., 2006).…”

“…Specifically, rapid root exudation is related to low RTD and, thus, associated with a fast strategy along the root conservation gradient (i.e. RTD–root [N] axis; Sell et al ., 2021; Sun et al ., 2021; Williams et al ., 2021); however, SRL is independent of RTD and the root conservation gradient (Wang et al ., 2018; McCormack & Iversen, 2019; Bergmann et al ., 2020; Han & Zhu, 2021). Thus, different groups of species (e.g.…”

“…Remarkably, the tight correlations between exudation rate and RTD observed for general root exudates suggest RTD, rather than SRL (or SRA), is a potentially good predictor for rapid root exudation rate (Guyonnet et al ., 2018; Sell et al ., 2021; Sun et al ., 2021; Williams et al ., 2021). This significant relationship may also be applicable to P‐mobilizing exudates.…”

Effect of drought on root exudates from Quercus petraea and enzymatic activity of soil

“…Results in this study further confirm the role of small diameter young root segments in shaping root exudation. Sell et al 30 also indicated that greater mass-specific exudation flux was related to a higher proportion of pioneer roots, suggesting that an essential amount of C may be released by actively growing pioneer roots. According to Meier et al 9 , root systems with a high SRL have, on average, thinner roots, and the C costs of root construction and mycorrhizal symbiosis are reduced.…”