Effects of an eight-year nitrogen and phosphorous addition on leaf photosynthesis and chemistry of mature castanopsis sclerophylla trees in subtropical China

Abstract

How long-term nitrogen (N) deposition affects plant growth in N-rich subtropical forest ecosystems, and whether there are interactive effects between N and phosphorous (P) addition on physiological performances remain unclear. To address these questions, an eight-year nutrient addition experiment was conducted to examine the chronic effects on trees in a secondary Castanopsis sclerophylla forest in subtropical China. Our key results showed that (1) Long-term N deposition could stimulate mature C. sclerophylla growth mainly by increasing the net photosynthetic rate (Pn), soluble sugars (SS) and total non-structural carbohydrates (NSCs). Furthermore, there were significant interactive effects on growth between N and P addition. (2) Leaf chlorophyll a + b, SS and total NSCs under N, P and NP addition were all increased in two years (except the chlorophyll a + b in 2020 and total NSCs in 2019). The positive impacts of P or NP addition on leaf chlorophyll a + b, SS and total NSCs were stronger compared to N addition. (3) Compared to N addition, P addition had a positive stimulation on leaf Pn. These conclusions demonstrated that P addition either alone or together with the addition of N have positive effects on the physiological performances of mature C. sclerophylla trees.