Mechanisms driving ecosystem carbon sequestration in a Chinese fir plantation: nitrogen versus phosphorus fertilization

Nitrogen (N) and phosphorus (P) fertilization are effective methods for increasing forest productivity and have been widely used in the management of forest plantations. However, how changes in the N:P ratio affect the sequestration of carbon in forests is unclear. We measured the net primary production (NPP) based on plot inventory, soil respiration (RS), autotrophic respiration (RA), and heterotrophic respiration (RH) using static chamber with GC method after trenching to compare the effects of low N fertilization (50 kg N ha−1 year−1), high N fertilization (100 kg N ha−1 year−1), P fertilization (50 kg P ha−1 year−1), low N plus P fertilization, high N plus P fertilization, and an unfertilized control on net ecosystem productivity (NEP). The NEP was significantly higher with 39–60% more under N and P fertilization than that of control. However, the mechanisms contributing to the difference varied among different treatments. Specifically, the RH was unchanged with P fertilization but significantly lower with N fertilization, while NPP was significantly higher with P or N fertilization. The NEP was higher with N plus P fertilization than with either N or P fertilization, although the interactive effect was not significant. Our findings suggested that P fertilization may be an effective method of improving timber yield and that adding both N and P fertilizers at a higher ratio may favor ecosystem carbon sequestration in P-limited stands.