1 INTRODUCTION
Anthropogenic-driven nutrient inputs, namely nitrogen (N) and phosphorus
(P) enrichment of terrestrial ecosystems,
have
been increasing intensively since the Industrial Revolution (Harpole et
al., 2011; Liu et al. 2021; Phoenix et al., 2012). Global cycles of N
and P have been respectively amplified by c.100% and c. 400%,
respectively, due to intensified human activities (Elser et al., 2007).
Being two crucial nutrient elements for growing plant, N and P
enrichment can profoundly influence plant growth, survival, and
reproduction, with subsequent impacts on community structure and
ecosystem functioning (Zhao et al., 2018a). Reproduction is an essential
function in the life cycles of plants that determine their fitness
(Willson, 1983). Seed production is an important index of reproduction
that strongly influences the relative ability of species to disperse and
establish as seedlings (Liu et al., 2012; Pierce et al., 2014). In
addition, seed production can affect the size and extent of soil seed
banks, and contribute to the maintenance of plant diversity and species
composition (Luzuriaga et al., 2005). Therefore, understanding the
effects of N and P enrichment upon seed production is critical for
predicting plant community structure, and consequently, ecosystem
functioning.
Nitrogen is a limiting nutrient for plant reproduction in terrestrial
ecosystems. Numerous studies have demonstrated that N enrichment tends
to augment seed production in plants (Bogdziewicz et al., 2017; Li et
al., 2016a; Ma & Herath, 2016; Shi et al., 2017). For example, in a
temperate steppe, Leymus chinensis produced more seeds via N
enrichment through enhanced spikelet and flower differentiation (Wang et
al., 2010). P plays a key role in regulating plant reproductive
processes because it can significantly affect the partitioning of
assimilation products, flowering phenology, root growth, and seed
maturation (Petraglia et al., 2014; Wang et al., 2017). Previous studies
have reported that plants under P-deficient conditions allocate little
to reproductive growth, which manifests as in a shortened flowering
period, reduced seed yield, and weakened dispersal ability (Fujita et
al., 2014; Groom & Lamont, 2009). Futhermore, the positive, negative or
neutral effects of P enrichment on seed production can all occur in
terrestrial ecosystems (Sims et al., 2012b; Singh et al., 2018; Wang et
al., 2017). For example, although P enrichment negligibly affected the
seed production of Stipa kryloii , it did increase that ofArtemisia frigida in a temperate steppe (Li et al., 2017).
The availability of N and P availability, however, may jointly affect
seed production since plant growth is predicted to be co-limited by
multiple resources (Graciano et al., 2006; Harpole et al., 2016;Harpole
& Suding, 2011; Long et al., 2016; Peñuelas et al., 2013).
Simultaneously adding N and P enhances ecosystem primary productivity
much more than adding either of them alone (Elser et al., 2007; Harpole
et al., 2011; Solis et al., 2013). By contrast, such an interactive
effect between N and P addition was not found for plant reproduction in
a temperate steppe (Li et al., 2017). Further, the growth and
reproduction of plants may respond differentially to the levels of
nutrient enrichment (Bowman et al., 2006; Tang et al., 2017). A
meta-analysis found that plant productivity in meadow steppe is
positively related to N addition under low N addition level, but it
decreases with increasing N addition under high N addition level (Tang
et al., 2017). Nonetheless, few attempts have been made to empirically
investigate how N and P addition rates and their interaction could
affect plant reproduction allocation and seed production in terrestrial
ecosystems, because most nutrient addition studies only include two
levels of nutrient treatments (i.e., control vs. nutrient enrichment).
Grassland is one of the major terrestrial ecosystems and covers 40% of
the world’s land area (Adams et al., 1990). In this respect, the
temperate steppe in northern China is representative of the typical
vegetation of the Eurasian grassland biome (Bai et al., 2010; Su et al.,
2018). A comprehensive project that used three levels of N and P
addition was begun in April 2012, aiming to examine the effects of
nutrient enrichment on community structure and ecosystem functioning in
a typical temperate steppe of Inner Mongolia, northern China. As part of
this long-term project, the present study was done to examine the
interactive effects of N and P addition rates upon seed production of
the dominant species Stipa krylovii , which is the most common
perennial grass in typical temperate steppe ecosystems in China. We
sought to address the following specific questions: (1) Do changes in N
and P availability and in N:P ratio alter the seed production of the
dominant species in temperate steppe ecosystem? (2) Which factors
determine seed production of the dominant species under the different
nutrient addition treatments?