Introduction

Tropical forests account for approximately one-third of global terrestrial gross primary productivity and sequester one-half of global carbon (C) stored in terrestrial vegetation (Hubau et al., 2020). Climate models predict that tropics will experience earlier climate warming compared with other climatic regions (Mora et al., 2013). Consequently, large amounts of C stored in tropical forests may be lost by the warming induced release of carbon dioxide from soil due to enhanced microbial activity (Nottingham et al., 2019, 2020). However, it remains uncertain whether tropical forests will increase C storage above-ground in plant biomass to potentially compensate for C lost from soils under climate warming (Reed et al., 2012; Wieder et al., 2015). Phosphorus (P) status could be one of the most important factors determining C balance of tropical ecosystems under warming, as P often limits net primary production in the tropics (Cleveland et al., 2011; Du et al., 2020). If plants experience greater P limitation due to warming, then plant productivity and C fixation might be significantly reduced (Sullivan et al., 2020). Currently, incomplete understanding of P dynamics in tropical forests in response to warming is a challenge for modeling the direction and magnitude of tropical C feedback in future climates (Reed et al., 2015).
The climate-demand/supply hypothesis holds that P limitation can be caused by high phosphorus demand and low phosphorus supply with climate change (Hou et al., 2021). Numerous studies have addressed plant P dynamics in response to warming (Sardans et al., 2006; Hood et al., 2018), and observed an elevated plant P demand due to increased plant production and P uptake (Rui et al., 2012; Diffenbaugh et al., 2013). To assess the potential for soil P supply under warming, most previous studies have focused on soil P availability (Sardans et al., 2006; Dijkstra et al. 2012). For instance, a decrease in plant-available P derived from the limited soil P mobility is observed with warming-induced drying soil (Dijkstra et al., 2012; Yuan & Chen, 2015; Tian et al., 2018). Based on demand/supply hypothesis, enhanced plant P demand, but decreased soil P availability with warming may increase plant and microbial P deficiency (Jonasson et al., 2004). However, in P-deficient tropical ecosystems, focusing only on available P may lead to uncertainties regarding actual soil P supply since changes in available P can be masked by the balance of rapid soil P release and plants and/or microbes P uptake. These uncertain of P supply constrain our understanding on the status of plant P demand relative to P supply.
Combining key processes of P conversion and P fractions allows the assessment of P supply. The current paradigm states that tropical P cycling is dominated by P mineralization to meet rapid P turnover and plant demand (McGroddy et al., 2008; Turner et al., 2018). In general, warming tends to increase organic P mineralization by stimulating soil phosphatase production in optimal moisture environments (Zhou et al., 2013; Gong et al., 2015; Zi et al., 2018; Zuccarini et al., 2020), which further satisfies P demand during accelerated plant growth (Sardans et al., 2006; Rui et al., 2012). However, these studies ignore a important P supply process: plant P resorption that is a crucial strategy for plants to improve P use efficiency and reduce the dependence of plant growth on soil P availability (Vitousek et al., 1982; Yuan & Chen, 2009). In resorption theory, the magnitude of plant P resorption is considered to be a trade-off with P derived from soil, especially from P mineralization (Vergutz et al., 2012; Gerdol et al., 2019). For instance, plants might retain adequate P concentrations in foliar tissues through increased plant P resorption when soil P availability is insufficient under warming (Yuan & Chen, 2015; Zong et al., 2018). Consequently, in tropical forests with high plant productivity and low P availability in weathered soils, plant P resorption may play a critical role in P supply under warming. In addition, recent research found a previously unrecognized pathway of P supply: P bound to iron (Fe) oxides and hydroxides could be dissolved and released during redox fluctuations in humid tropical soils due to the enhanced metabolism and oxygen depletion of microbes and subsequently Fe reduction (Peretyazhko & Sposito, 2005; Chacón et al., 2006; Gross et al., 2020). A reduced redox potential associated with P release from redox-sensitive dissimilatory reduction of Fe oxide-bound P was observed in a wetland ecosystem (Zhang et al., 2015). Although a large fraction of soil P may be bound to redox-sensitive Fe minerals in a humid tropical forest, significant portions of P might be released from Fe reduction due to increased microbial activity under warming (Li et al., 2016). Moreover, the loss of P also affects P supply, but most studies ignore this fact, which prevents an accurate assessment of P cycling.
Most short-term warming cannot accurately anticipate P cycling dynamics at longer term (> 5 years), although from demand/supply hypothesis, tropical plants may be prone to experience more P limitation in long-term warming as P is continuously depleted with plant uptake and leaching. Lack of long-term monitoring in tropical forests indicates our incomplete understanding of the mechanisms controlling P cycling under long-term warming. To address these knowledge gaps, we conducted a 7-year field warming experiment in a humid tropical forest using natural temperature differences across elevations. We assessed P status in plant, litter, soil, and leachate as well as soil acid phosphatase and soil Fe components in the dry and wet seasons. Our previous studies found a continuous increase in plant growth and the accelerated decomposition of soil organic matter due to warming (Fang et al., 2020; Lie et al., 2021). Hence, we hypothesized that warming would increase plant P demand with enhanced plant growth, and P supply would be affected by multiple biological and geochemical processes which are strongly dependent on temperature.