4.2 Impact of P addition on seed production
In our study, although the main effect of phosphorus upon seed production of S . krylovii was significant, neither low nor high P addition influenced seed production in the absence of N addition, suggesting that seed production is not limited by P availability in the temperate steppe. This finding is consistent with previous research (Li et al., 2017; Yang et al., 2014). Both low and high P addition increased seed production in the presence of N addition, and the increment was significantly higher under high N addition than under low N addition, indicating that a P limitation of seed production can be triggered by N addition. This phenomenon is supported by model simulation work (Menge & Field, 2007) as well several field experiments (Marklein & Houlton, 2012; Zheng et al., 2018). Plants capable of a high growth rate under N-rich conditions will require a greater allocation of P-rich rRNA to support macromolecular (protein, rRNA) synthesis (Niklas et al., 2005). The demand for P increases with N addition-induced growth (Li et al., 2016b). Accordingly, fertilization with P would allow for an increased allocation of P to reproductive structures inflorescences.
Phosphorus is not only a structural element of cell organelles (such as mitochondria and chloroplast) but also the primary constituent of phospholipids (ATP and NADPH) that are used for energy metabolism in light and dark reactions. Indeed, P is indispensable for plant photosynthesis and respiration, such that changes in the P concentration available for plant uptake would alter their vegetative and reproductive growth (George et al., 2016; Patel et al., 2017). An external P addition usually tends to enhance plants’ internal P concentration, accelerating their photosynthetic efficiency, and thus promoting biomass accumulation (Graciano et al., 2006; Suriyagoda et al., 2014). P enrichment can indirectly promote plant height growth and thereby augment the seed number per inflorescence (Figure 4). Higher levels of P to plants can result in more spikelets per fertile tiller (Wang et al., 2017) and an earlier plant flowering date (Petraglia et al., 2014). Both outcomes may subsequently enhance overall fecundity and prolong the seed development period, and eventually stimulate seed production.
In addition, soil P availability is highly responsive to local available N (Marklein & Houlton, 2012). Even a minor increase in available N addition can increase soil P availability by stimulating greater root surface phosphatase activity and facilitating P dissolution, which alleviates P limitation (Crowley et al., 2012;Johnson et al., 1999; Vitousek et al., 2010). Although N fertilizer can promote P cycling, the increased available P is insufficient to balance the greater plant demand for P (Li et al., 2016b); hence, P limitation will gradually predominate become predominant (Peng et al., 2017; Peñuelas et al., 2013).