Effect of decline in soil depth and water resource on the photosynthesis of two grasses under mixed plantation in Karst regions

Uneven soil depth and karst drought are key ecological factors affecting plant survival, growth, and reproduction in karst regions. The change in global precipitation patterns will exacerbate the frequency and intensity of karst drought. The amount of water and nutrients absorbed and transported to the aboveground parts by roots systems can affect plant photosynthesis. Shallow-rooted plants mainly use water and nutrients from the top layers of the soil, whereas deep-rooted plants can use water and nutrients from the deeper soil layers. Therefore, when deep-rooted plants and shallow-rooted plants coexist, they might alleviate the inhibitory effect of resource limitation on plant photosynthesis because of their complementary root niche. We grew Festuca arundinacea (deep-rooted) and Lolium perenne (shallow-rooted) under normal versus shallow soil depth and well watered versus drought treatments in a block design, and studied the effect of the decline in soil depth, water availability, and both these resources (soil depth plus water availability) on the photosynthesis of two grasses under mixed plantation by controlled experiment. The results showed that: 1) at high resource levels, no significant difference occurred in net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and water use efficiency (WUE) of F. arundinacea between mixed plantation and in monoculture (P0.05); Pn, Gs, and Tr of L. perenne was lower in mixed plantations than in monoculture, whereas WUE was significantly higher in mixed plantations than in monoculture (P0.05). 2) Overall, Pn, Gs, and Tr of F. arundinacea and L. perenne decreased and WUE of L. perenne increased with the decline of resources (soil depth, water, and soil depth plus water) either in mixture or in monoculture. Moreover, the range of decrease or increase was lower in mixed plantations than in monoculture; WUE of F. arundinacea increased in monoculture but decreased in mixed plantations. 3) At low resource levels, Pn, Gs, and Tr of F. arundinacea was higher in mixed plantations than in monoculture, whereas WUE was lower in mixed plantations than in monoculture. No significant difference was found in Pn, Gs, and Tr of L. perenne between mixed plantations and monoculture, whereas WUE was higher in mixed plantations than in monoculture. Our results showed that owing to the complementary niche and hydraulic lift of deep-rooted plants, the inhibitory effect of resource limitation on photosynthesis of the deep-rooted and shallow-rooted plants are alleviated when they are planted together, but at the expense of WUE. Therefore, in karst regions, mixed planting with plant of different root depth systems may be an effective method of resolving the increasing drought and poor soil conditions.