Effect of soil water-phosphorus coupling on the photosynthetic capacity of Robinia pseudoacacia L. seedlings in semi-arid areas of the Loess Plateau, China.

Afforestation can improve soil erosion in the ecologically fragile areas of the Loess Plateau; however, the amount of water and phosphorus fertilizer that can promote vegetation survival is unclear, which hinders the improvement of the local ecological environment and the waste of water and fertilizer. In this study, based on field surveys, water and fertilizer control tests on Robinia pseudoacacia L. seedlings in experimental fields, and fitting CO response curves to R. pseudoacacia seedlings using a Li-6400 portable photosynthesizer, we measured their leaf nutrient contents and calculated resource use efficiency. The results showed that (1) under the same moisture gradient, except for photosynthetic phosphorus utilization efficiency (PPUE), light use efficiency (LUE), water use efficiency (WUE), carbon utilization efficiency (CUE), and photosynthetic nitrogen use efficiency (PNUE) all increased with increasing phosphorus fertilizer application. Under the same phosphorus fertilizer gradient, WUE increased with decreasing water application, and LUE, CUE, PNUE, and PPUE all reached the maximum at 55-60% of field water holding capacity. (2) Net photosynthetic rate (P) of R. pseudoacacia seedlings increased with increasing intercellular carbon dioxide concentration (C), and as C continued to increase, the increase in P became slower, but no maximal electron transport rate (TPU) occurred. Under the same CO concentration, P reached a maximum at 55-60% of field water holding capacity and phosphorus fertilizer at 30 gPm·a. (3) Leaf maximum carboxylation rate (V), maximum electron transport rate (J), daily respiration (R), stomatal conductance (G), and mesophyll conductance (G) reached their maximum at 30 gPm·a of phosphorus fertilizer. V, J, and R reached their maximum at 55-60% of field water holding capacity; G and G reached their maximum at 75-80% of field water holding capacity. (4) The higher the soil phosphorus content, the lower the biochemical (l), stomatal (l), and mesophyll (l). With the increase of soil moisture, l and l are higher, and l is lower. (5) Structural equation modeling showed that water-phosphorus coupling had a less direct effect on R and a more direct impact on G and G. Relative photosynthetic limitation directly affected the photosynthetic rate, indicating that water and phosphorus affected the photosynthetic rate through relative plant limitation. It was concluded that the resource use efficiency and photosynthetic capacity reached the maximum when 55-60% of field water holding capacity was maintained, and phosphorus fertilization was at 30 gP m·a. Therefore, maintaining suitable soil moisture and phosphorus fertilizer levels in the semi-arid zone of the Loess Plateau can improve the photosynthetic capacity of R. pseudoacacia seedlings.