By altering the physical properties of soil through root activity, plants can act as important agents in affecting soil hydrothermal properties. However, we still know little about how plant roots regulate these properties in certain ecosystems, such as alpine meadows. Thus, we studied the influence of roots on soil hydrothermal properties in the Qinghai-Tibet Plateau (QTP). Root biomass as well as soil physicochemical and hydrothermal properties were examined at a depth of 0-30 cm at three study sites in the QTP. The relationship between root biomass and saturated soil hydraulic conductivity (K-s) was examined, as was the applicability of common soil hydrothermal properties models to the alpine meadow system. Results revealed that approximately 91.10%, 72.52%, and 76.84% of root biomass was located in the top 0-10 cm of soil at Maqu, Arou, and Naqu, respectively. Compared with the bulk soil, the water-holding capacity of rhizosphere soil was enhanced by 20%-50%, while K-s was decreased by at least 2- to 3-fold. The thermal conductivity (lambda) of rhizosphere soils was lower than that of the bulk soil by 0.23-0.82 W m(-1).K-1 on average. Lastly, soil hydrothermal properties models that do not explicitly consider root effects overestimated the Ks and lambda in the rhizosphere soil of these systems. Overall, our results revealed distinctive differences in soil hydrothermal properties between the rhizosphere soil and the bulk soil in the QTP. This research has important implications for future modeling of soil hydrothermal processes of alpine meadow soils.

1. Climate warming is faster in the Arctic than the global average. Nutrient availability in the tundra soil is expected to increase by climate warming through (i) accelerated nutrient mobilization in the surface soil layers, and (ii) increased thawing depths during the growing season which increases accessibility of nutrients in the deeper soil layers. Both processes may initiate shifts in tundra vegetation composition. It is important to understand the effects of these two processes on tundra plant functional types. 2. We manipulated soil thawing depth and nutrient availability at a Northeast-Siberian tundra site to investigate their effects on above- and below-ground responses of four plant functional types (grasses, sedges, deciduous shrubs and evergreen shrubs). Seasonal thawing was accelerated with heating cables at c. 15 cm depth without warming the surface soil, whereas nutrient availability was increased in the surface soil by adding slow-release NPK fertilizer at c. 5 cm depth. A combination of these two treatments was also included. This is the first field experiment specifically investigating the effects of accelerated thawing in tundra ecosystems. 3. Deep soil heating increased the above-ground biomass of sedges, the deepest rooted plant functional type in our study, but did not affect biomass of the other plant functional types. In contrast, fertilization increased above-ground biomass of the two dwarf shrub functional types, both of which had very shallow root systems. Grasses showed the strongest response to fertilization, both above-and below-ground. Grasses were deep-rooted, and they showed the highest plasticity in terms of vertical root distribution, as grass root distribution shifted to deep and surface soil in response to deep soil heating and surface soil fertilization respectively. 4. Synthesis. Our results indicate that increased thawing depth can only benefit deep-rooted sedges, while the shallow-rooted dwarf shrubs, as well as flexible-rooted grasses, take advantage of increased nutrient availability in the upper soil layers. Our results suggest that grasses have the highest root plasticity, which enables them to be more competitive in rapidly changing environments. We conclude that root vertical distribution strategies are important for vegetation responses to climate-induced increases in soil nutrient availability in Arctic tundra, and that future shifts in vegetation composition will depend on the balance between changes in thawing depth and nutrient availability in the surface soil.