Enhanced root exudation of mature broadleaf and conifer trees in a Mediterranean forest during the dry season

Abstract

Root exudates are part of the rhizodeposition process, which is the major source of soil organic carbon (C) released by plant roots. This flux of C is believed to have profound effects on C and nutrient cycling in ecosystems. The quantity of root exudates depends on the plant species, the period throughout the year, and external biotic and abiotic factors. Since root exudates of mature trees are difficult to collect in field conditions, very little is known about their flux, especially in water-limited ecosystems, such as the seasonally hot and dry Mediterranean maquis. Here, we collected exudates from DNA-identified roots in the forest from the gymnosperm Cupressus sempervirens and the evergreen angiosperm Pistacia lentiscus by 48-hour incubations on a monthly temporal resolution throughout the year. We examined relationships of the root exudate C flux to abiotic parameters of the soil (water content, water potential, temperature) and atmosphere (vapor pressure deficit, temperature). We also studied relationships toC fluxes through the leaves as indicators of tree C balance. Root exudation rates varied significantly along the year, increasing from 6 μg carbon cm−2 root day−1 in both species in the wet season, to 4-fold and 11-fold rates in Pistacia and Cupressus, respectively, in the dry season. A stepwise linear mixed-effects model showed that the three soil parameters were the most influential on exudation rates. Among biotic factors, there was a significant negative correlation of exudation rate with leaf assimilation in Cupressus; and a significant negative correlation with leaf respiration in Pistacia. Our observation of enhanced exudation flux during the dry season indicates that exudation dynamics in the field are less sensitive to the low tree carbon availability in the dry season. The two key Mediterranean forest species seem to respond to seasonal changes in the rhizosphere such as drying and warming, and therefore invest C in the rhizosphere under seasonal drought.

The Apparent Respiratory Quotient of Soils and Tree Stems and the Processes That Control It

“…We hypothesized that if re-fixation of respired CO 2 by the enzyme PEPC is the process that lowers tree-stems ARQ, organic acids that are the enzyme's products might be exported via the phloem to the roots and be secreted to the soil as root exudates (Hoffland et al, 1992;Shane et al, 2004). In agreement, a recent study in a different Mediterranean forest in Israel reported that root exudation rates were highest in the dry season and were associated with high soil temperature and low soil moisture (Jakoby et al, 2020). However, we found only weak and marginal inverse relation between ARQ ts and ARQ sa , which was expected if the organic acids are quickly decompose in the soil (Figure 7).…”

Variability of the Apparent Respiratory Quotient of a forest Soils and Tree Stems

“…We speculated that if re-fixation of respired CO 2 by the enzyme PEPC is the process that lowers tree-stems ARQ, the enzyme's products like the organic acids Confidential manuscript submitted to JGR Biogeosciences citrate and malate might be exported via the phloem to the roots and be secreted to the soil as root exudates [Hoffland et al, 2006;Shane et al, 2004]. In agreement, a recent study in a different Mediterranean forest in Israel reported that root exudation rates were highest in the dry season and were associated with high soil temperature and low soil moisture [Jakoby et al, 2020]. Indeed, the inverse relationship between ARQ ts vs. ARQ sa at cm in the deciduous Q.…”

A dynamic rhizosphere interplay between tree roots and soil bacteria under drought stress

“…Trees invest a substantial part of their photosynthesized carbon into root exudates that entice and presumably feed plant-beneficial and root-associated microbiota (Bais et al, 2006; Badri and Vivanco 2009; Karst et al, 2017; Jakoby et al, 2020). In parallel, the rhizosphere microbes can promote plant growth through various mechanisms such as increasing the availability of nutrients, secreting phytohormones, suppressing pathogens, or having positive effects on the plant metabolism (Perez-Montano et al, 2014; Zhou et al, 2015).…”

“…Recently, we have shown that roots of both conifer and broadleaf tree species in an evergreen Mediterranean forest increase their exudation flux during the long dry season (Jakoby et al, 2020). This occurred in spite of the sharp decrease of tree carbon uptake along the dry season, and more so in the coniferous Cupressus sempervirens .…”

“…Root systems are key components in struggling with this stress, as they coordinate both water and nutrient uptake, provide physical stabilization, store nutrients and carbohydrates, and provide carbon and nutrients to the soil through the process of fine-root turnover (Brunner and Godbold, 2007;Haichar et al 2008;Ryan, 2011;Harfouche 2014;Jarzyniak and Jasiński, 2014;Klein 2016). In addition to these roles, roots interact with the soil microbiome, which can confer various benefits to the trees such as suppression of bacterial and fungal diseases, promotion of plant growth and assimilation of essential nutrients that, in turn, mitigate drought effects (Berendsen et Trees invest a substantial part of their photosynthesized carbon into root exudates that entice and presumably feed plant-beneficial and root-associated microbiota (Bais et al, 2006;Badri and Vivanco 2009;Karst et al, 2017;Jakoby et al, 2020). In parallel, the rhizosphere microbes can promote plant growth through various mechanisms such as increasing the availability of nutrients, secreting phytohormones, suppressing pathogens, or having positive effects on the plant metabolism (Perez-Montano et al, 2014; Zhou et al, 2015).…”