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

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, whereas 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.

Eutrophication reduced the release of dissolved organic carbon from tropical seagrass roots through exudation and decomposition.

Seagrass bed ecosystem is one of the most effective carbon capture and storage systems on earth. Seagrass roots are the key link of carbon flow between leaf-root-sediment, and the release of dissolved organic carbon (DOC) from seagrass roots through exudation and decomposition are vital sources to the sediment organic carbon (SOC) in the seagrass beds. Unfortunately, human-induced eutrophication may change the release process of DOC from seagrass roots, thereby affecting the sediment carbon storage capacity. However, little is known about the effect of nutrient enrichment on the release of DOC from seagrass roots, hindering the development of seagrass underground ecology. Therefore, we selected Thalassia hemprichii, the tropical dominant seagrass species, as the research object, and made a comparison of the release of DOC from roots through exudation and decomposition under different nitrate treatments. We found that under control, 10 µmol L-1, 20 µmol L-1 and 40 µmol L-1 nitrate treatments, soluble sugar of T. hemprichii roots were 71.37 ± 3.43 mg g-1, 67.03 ± 5.33 mg g-1, 49.14 ± 3.48 mg g-1, and 18.51 ± 2.09 mg g-1, respectively, while the corresponding root DOC exudation rates were 7.00 ± 0.97 mg g DW root-1 h-1, 5.11 ± 0.42 mg g DW root-1 h-1, 4.08 ± 0.23 mg g DW root-1 h-1, and 3.78 ± 0.74 mg g DW root-1 h-1, respectively. There was a significant positive correlation between root soluble sugar and DOC exudation rate. DOC concentration of sediment porewater and SOC content also decreased under nitrate enrichment (though not significantly), which were both significantly positively correlated with the rate of root exuded DOC. Meanwhile, nitrate enrichment also reduced the release rate of DOC from seagrass roots during initial decomposition, and the release flux of DOC from decomposition. Therefore, nutrient enrichment could decrease nonstructural carbohydrates of seagrass roots, reducing the rate of root exuded DOC, thereby lowered SOC, as well as the DOC release from seagrass root decomposition. In order to increase the release of DOC from seagrass roots and improve the carbon sequestration capacity of seagrass beds, effective measures should be taken to control the coastal nutrients input into seagrass beds.

Interspecific differences in root exudation for three tropical seagrasses and sediment pore-water dissolved organic carbon beneath them.

Seagrass beds act as blue carbon sinks globally; however, little attention has been given to carbon dynamics in the seagrass rhizosphere. Hence, in this study, the quantity and characteristics of dissolved organic carbon (DOC) from root exudation of the three dominant tropical seagrasses (Thalassia hemprichii, Enhalus acoroides, and Cymodocea rotundata) and sediment pore water beneath them were compared, to examine their interspecific differences, and to establish a connection between seagrass root exudation and sediment carbon. The rate of root-exuded DOC from T. hemprichii (2.15 ± 1.06 mg g DW root-1 h-1) was significantly higher (p < 0.05) than that from E. acoroides (0.72 ± 0.39 mg g DW root-1 h-1) and C. rotundata (0.46 ± 0.25 mg g DW root-1 h-1). Root exudation rates were more affected by root hair density and root hair length than by root carbon, nitrogen, and soluble sugar content. Simultaneously, DOC concentrations of the sediment pore water beneath T. hemprichii, E. acoroides and C. rotundata were 22.05 ± 11.61 mg l-1, 15.55 ± 2. 66 mg l-1, and 14.32 ± 1.82 mg l-1, respectively. The corresponding absorption coefficients at 254 nm (a254) were 30.53 ± 18.00, 17.31 ± 2.24, and 14.07 ± 2.03, respectively, while the relevant specific ultraviolet absorbances at 254 nm (SUVA254) were 1.38 ± 0.29, 1.19 ± 0.26 and 1.03 ± 0.28, respectively. Therefore, the roots of T. hemprichii exuded DOC at a higher rate, leading to a higher pore-water DOC pool in the sediment. This suggests that T. hemprichii played a greater role in the sediment carbon pool through root exudation. Thus, it can be considered as the priority species for transplantation to promote the carbon sink function of seagrass beds.