# Diagnosing Phosphorus Limitation in Subtropical Forests in China under Climate Warming
With the intensification of climate warming, it is expected that P will be a crucial nutrient factor affecting the C uptake and storage in subtropical China. However, the spatial and temporal distribution of current P consumptions and future P limitation in subtropical China are still unclear. Therefore, it is of great importance to explore the dynamic changes of P demand in subtropical China, which will be of great help for understanding the potential threats of P deficit in subtropical China and to make effective strategy to cope with climate change. Modeling and diagnosing are two major approaches to studying the P limitation in ecosystems. In terms of the modelling approach, some process-based models have been developed in the last 20 years for explicitly modeling P cycle. For example, [PERSON] et al. (2010) developed a P model to simulate the global distribution of P limitation on NPP at steady state, and they found that tropical vegetation NPP was reduced by about 20% on average by P limitation [28]. [PERSON] et al. (2012) incorporated a P cycle into a land surface model and found that the average plant uptake of P over 1970-1999 in tropical and temperate forests was in the range of 0.18-0.71 g\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\)[29]. [PERSON] et al. (2014) introduced P dynamics and C-N-P interactions into the community land model version 4 with a prognostic C and N model (CLM4-CN) to simulate the effects of elevated CO\({}_{2}\) concentration on plant P availability and got meaningful results, and they found that tropical forest response to elevated CO\({}_{2}\) can only be predicted if the interactions between C cycle and nutrient dynamics are well understood and represented in models [30]. Therefore, more scientific experiments are needed for understanding the C-P interactions, and the modeling approach for understanding P limitation still remains highly uncertain. In terms of the diagnostic approach, [PERSON] et al. (2013) used globally uniform stoichiometric ratios for vegetation and soil and calculated N and P demands for given changes in C pools during 2000-2099, and found that global P demand are ranged between \(-0.9\) and \(4.3\) Pg P for the lower projection for Representative Concentration Pathway (RCP) 2.6 and between \(-1.7\) and \(6.5\) Pg P for RCP8.5, with negative values indicating a P surplus [31]. However, the size of the global plant-accessible labile P soil pool is only 13.2 Pg P, and, hence, the limited P availability is likely to reduce future carbon storage by natural ecosystems during this century. [PERSON], et al. (2015) used the NPP outputs from the Coupled Climate Carbon Cycle Model Intercomparing Projects (CMIP5) models over 1860-2100 and estimated annual new N and P inputs to evaluate how CO\({}_{2}\) fertilization effects could be constrained by nutrient availability [32]. They found that accounting for N and P limitation would lower projected end-of-century estimates of NPP by 19%. The approaches of [PERSON] et al. (2013) [31] and [PERSON] et al. (2015) [32] were based on C stock and NPP changes, respectively. [PERSON] et al. (2017) used both C stock and NPP-based approaches in a consistent framework to investigate whether available soil P (ASP) can support the P demand with four scenarios of soil P availability, and they found that there are large differences between the NPP-based demand and C stock-based demand, and the diagnostic approach still remains uncertain [33]. Previous diagnostic approaches showed contradictory results in prediction of P limitation in subtropical China. For example, [PERSON] et al. (2017) [33] estimated the values of additional P demand under RCP8.5 in subtropical China were negative based on C stock method, but positive based on NPP method. Therefore, the estimation of P limitation in subtropical China still remains highly uncertain. The uncertainty in estimates of P limitation mainly comes from three sources: The uncertainty of the C:P ratios for plant organs, the NPP allocation to leaves, roots and wood and the estimates of the ASP.