Introduction
In plants, carbohydrates, which are classified as structural carbohydrates (SCs, including lignin and cellulose) and nonstructural carbohydrates (NSCs, including soluble sugar, sucrose, fructose, and starch), are of great importance to energy sources and physiological metabolism in plant life history (Ögren 2010, Dietze et al. 2014). SCs are generally used for constructing plant tissue, and NSCs mainly offer carbon (C) and energy for plant growth, respiration, and production (Würth et al. 2005, Dietze et al. 2014).
NSCs are referred to as reservoir pools, and have protective functions, including osmotically active compounds, chemical chaperones, and reactive oxygen species (ROS) scavengers (Newell et al. 2002, Ivanov et al. 2019). Generally, NSCs only account for approximately 10% of plant biomass, but their concentrations in leaves are higher than those in roots and stems under natural conditions, indicating the vital role of plant leaves in regulating the C balance between uptake and consumption (Martínez-Vilalta et al. 2016). The concentrations of NSCs can mirror the capacity of plant adaption to the various environmental conditions (Hoch et al. 2003, Richardson et al. 2015). Under exposure to various global environmental changes, such as warming, CO2 enrichment, ozone destruction, drought, and N deposition, plant survival, resistance ability, growth rate, and productivity are primarily determined by carbohydrate dynamics (Dietze et al. 2014, Martínez-Vilalta et al. 2016).
It is widely believed that N and P are two essential nutrients for plant photosynthetic C assimilation, and they also limit the net primary productivity (NPP) in terrestrial ecosystems (Vitousek and Howarth 1991, Herbert and Fownes 1995). Traditionally, N availability constrains plant productivity by limiting leaf initiation and expansion (Vos and Biemond 1992), while P availability mainly determines leaf biochemical processes such as energy exchange and nucleic acid synthesis in plant cells (Warren 2011). In the herbaceous plant yellow bluestem (Bothriochloa ischaemum ), the soluble sugar concentrations was reduced while the starch and total NSC concentrations were increased by N addition (Xiao et al. 2017). In another study, both above- and below-ground NSC (sugars and starch) concentrations in yellow bluestems were significantly increased by N addition (Ai et al. 2017). Nonetheless, both N and P addition decreased the concentrations of leaf soluble sugars and starch in two species of grass and forbs in an Inner Mongolian semi-arid grassland community (Wang et al. 2017b). Al-Hamdani and Sirna (2008) also reported that the starch and total NSC accumulation in Salvinia minimawere significantly lower under N or P addition. However, previous studies on different plant species (which mainly include herbaceous species) have found different results regarding the plant’s responses to external N and/or P addition.
In tropical forests, P is an important limiting factor for plant growth and productivity (Vitousek et al. 2010), as soil P availability generally declines with bedrock weathering and soil age (Walker and Syers 1976). Therefore, soil P availability in tropical forests may drive leaf NSC dynamics, which could reflect carbohydrate dynamics (C assimilation by photosynthesis and consumption by respiration). Although it is widely recognized that P addition could greatly increase leaf P concentrations in the tropics (Mayor et al. 2014, Schreeg et al. 2014, Wright et al. 2018), very few studies have investigated how increased leaf P concentrations affect the NSC dynamics in tropical forests.
A recent study reported that leaves tended to optimally allocate different functional P fractions (structural P, metabolic P, nucleic acid P, and residual P) to simultaneously accomplish a series of physical processes (photosynthesis) in P-limiting tropical forests (Mo et al. 2019). Given that leaf C assimilation is closely related to N and P supply (Kroth 2015), the response of NSC to long-term N and P addition is relatively fundamental for understanding the mechanisms and relationships of leaf C assimilation and P allocation in tropical forests.
Fertilization experiments involving addition of external N and P on reforestation from degraded sites have been conducted at many locations globally (Tanner et al. 1992, Ceccon et al. 2003, Mayor et al. 2014, Schreeg et al. 2014, Li et al. 2015). These experiments could be efficient ways to evaluate the effects of P limitation on key biological process (Ågren et al. 2012, Tessa et al. 2018). In tropical China, N deposition is projected to continually increase in the future (Liu et al. 2011). The increased atmospheric N input may provide higher active N for plant growth in forest ecosystems (Lu et al. 2008), which also aggravates the imbalance of soil N:P ratios in tropical forests (Du et al. 2016). According to our previous results, the plant growth in this studied secondary tropical forest has been proven to be primarily limited by soil P availability (Mo et al. 2015, Mo et al. 2019). Here, we employed a long-term manipulative field experiment to test the response of leaf NSCs to N and/or P addition in a secondary tropical forest in southern China. We tried to answer the following questions: how do leaf NSC (soluble sugars and starch) concentrations respond to continuous N deposition and P limitation? How does N and P addition regulate leaf NSC concentrations in this tropical forest? We hypothesized that: 1) the concentrations of leaf NSCs (soluble sugars and starch) would be primarily regulated by P availability rather than N availability due to the long-term P deficiency in this tropical forest, and 2) the NSC concentrations would be reduced and transformed into leaf biomass along with the increase in leaf structural P fraction under P addition (Mo et al. 2019).