The effect of tree diversity on forest productivity and resilience has been the subject of numerous research programs in the past decade. Large research projects like the BEF-China experiment, and networks like TreeDivNet and EuMIXFOR are evidence for the large investments into deciphering diversity-productivity relationships (DPR) in mixed forests around the globe (Zhang et al., 2012). For example, EuMIXFOR established a network of hundreds of research plots with a triplet design of a mixed Fagus sylvatica -Pinus sylvestris stands compared to pure stands of the two species (Ruiz-Peinado et al., 2018). A global meta-analysis showed that forest productivity increases with species richness and trait variation (Zhang et al., 2012). Mixed forests are, on average, 24% more productive than monoculture forests, with large variability among studies. Indeed, cases where mixtures are less productive than monocultures also exist (Forrester, 2014). In the BEF-China tree diversity experiment in a subtropical forest, tree growth increased with neighborhood species richness, leading to a positive DPR at the community scale (Fichtner et al., 2018). In a tropical
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.
Root exudates are thought to play an important role in plant-microbial interactions. In return for nutrition, soil bacteria can increase the bioavailability of soil nutrients. However, root exudates typically decrease in situations such as drought, calling into question the efficacy of solvation and bacteria-dependent mineral uptake in such stress. Here we tested the hypothesis of exudate-driven microbial priming on Cupressus saplings grown in forest soil in custom-made rhizotron boxes. A 1-month imposed drought and concomitant inoculations with a mix of Bacillus subtilis and Pseudomonas stutzeri, bacteria species isolated from the forest soil, were applied using factorial design. Direct bacteria counts and visualization by confocal microscopy showed that both bacteria associated with Cupressus Interestingly, root exudation rates increased 2.3-fold with bacteria under drought, as well as irrigation. Forty four metabolites in exudates were significantly different in concentration between irrigated and drought trees, including phenolic acid compounds and quinate. When adding these metabolites as carbon and nitrogen sources to bacterial cultures of both bacterial species, 8 of 9 metabolites stimulated bacterial growth. Importantly, soil phosphorous bioavailability was maintained only in inoculated trees, mitigating drought-induced decrease in leaf phosphorus and iron. Our observations of increased root exudation rate when drought and inoculation regimes were combined, support the idea of root recruitment of beneficial bacteria, especially under water stress.
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