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.