Shifts in soil seed bank and plant community under nitrogen addition and mowing in an Inner Mongolian steppe
Introduction
Atmospheric nitrogen (N) deposition has increased and will continue increasing in the next few decades (Reay et al., 2008). Given that N availability is the major limiting nutrient for plant growth, changes in soil N content would have profound effects on plant community structure and composition (Harte and Shaw, 1995; Knapp et al., 2002; Yang et al., 2011), and subsequently influence terrestrial biodiversity (Wang et al., 2012). In addition, N deposition can also cause communities to shift from forb-dominated to highly productive grass-dominated states (You et al., 2017). Changes in plant species composition in response to N addition have been explained by species traits (Klanderud and Totland, 2005).
Responses of plant community to N deposition can be confounded by land use methods (Yang et al., 2019). For example, mowing for hay, a common land-use practice in grasslands (Valko et al., 2012; Giese et al., 2013), can affect species richness by influencing microclimate (Wan et al., 2002) and plant community characteristics (Liu et al., 2017). Furthermore, we previously showed that N deposition and mowing have interactive effects on plant growth (Liu et al., 2017), causing subsequent changes to aboveground vegetation communities. Nitrogen addition and mowing have differential effects on species richness of plant functional groups in this region (Yang et al., 2012), which can change the soil seed richness.
Soil seed banks are a key factor in determining the resistance and resilience of plant communities to ecological disturbances (Godfree et al., 2011; Zhou et al., 2019). The influence of N deposition on soil seed banks is a major concern because seed banks help to maintain plant species richness by buffering the rare species from extinction (Piessens et al., 2004). Despite extensive reports on the impacts of N deposition on plant communities, few experiments have focused on its effects on the soil seed bank. Quantifying the influence of N deposition on soil seed banks is a key missing link in the understanding of the response of terrestrial ecosystems to N addition.
Nitrogen deposition has various effects on sexual reproduction in different species (Throop, 2005). Mowing can decrease seed bank richness by increasing Ellenberg nutrient values (Klaus et al., 2018). In addition, mowing can mitigate the passive effects of nutrient addition on plant species richness by removing accumulated litter from the ecosystem (Hovd and Skogen, 2005). The interactive effects of N addition and mowing on soil seed banks are still unclear.
Nitrogen availability will likely continue to limit plant growth and species richness in terrestrial ecosystems in the future (Hungate et al., 2003; Zhou et al., 2018). Mowing is widely used in grasslands of Inner Mongolia, China (Du et al., 2018). Atmospheric N deposition and mowing may interact to change plant communities and soil seed banks, which will exacerbate the difficulty of predicting plant community composition and soil seed bank dynamics under future N deposition and mowing. We used a six-year (2012–2018) manipulative field experiment in a temperate steppe in northern China to examine: 1) How do N addition and mowing affect plant community composition and soil seed bank in the semiarid grassland? 2) Are there interactions of N addition and mowing in affecting soil seed banks in this ecosystem?
Section snippets
Study site
The study was conducted in a temperate steppe (42°02′N, 116°17′E, 1324 m a.s.l) in Inner Mongolia, China. Long-term mean annual precipitation is approximately 383 mm with 90% falling between May and October. Mean annual temperature is 2.1 °C, ranging from −17.5 °C in January to 18.9 °C in July (Gao et al., 2016; Song et al., 2016; Yang et al., 2017). The sandy soil in this area is classified as chestnut according to the Chinese classification, or haplic calcisols, with mean bulk density of
Soil seed bank size
No main effects of N addition or mowing on the total or forb seed density in soil, but a marginal interaction altered total and forb seed density from 0 to 10 cm soil depth (Table 1, Figs. 1a, and 2a). Nitrogen addition decreased total seed density by 176 m−2 without mowing but increased it by 601.6 m−2 with mowing. Mowing depressed seed density by 182.4 m−2 under ambient N, but stimulated it by 595.2 m−2 under N addition. These changes were mainly attributed to the alteration of forb seed
Effects of N addition on soil seed banks and aboveground vegetation
Total soil seed density increased marginally and total seed richness increased significantly following N addition at 5–10 cm depth, but not in other depths. This is contrary to reports that seed density and richness decreased under N enrichment (Basto et al., 2015; Klaus et al., 2018). Increased seed density and richness under N addition may be attributed to 1) a shift of aboveground plant composition and forb seed density under N addition (Table 1, Table 2; Figs. 2a, 4d, 5d), 2) changes in
Conclusions
In summary, we have revealed the weak negative effects of N addition or mowing on soil seed density, but the N addition and mowing interacted to change the seed density at 2–5 and 5–10 cm depth. Furthermore, mowing can reverse the negative effects of N deposition on soil seed density by influencing forb seed bank size in deep soil (2–5 and 5–10 cm). The main effects of N addition or mowing increased richness by 1.1 species (0.25 × 0.25 m2) at 5–10 cm depth, but no interactive effects of N
Declaration of Competing Interest
None.
Acknowledgements
This project was financially supported by the National Natural Science Foundation of China (grant numbers 31800399, 41807128, 31570429, 31600379, 31670477). We would like to thank Dr.Joshua Daskin at Yale University for his assistance with English language and grammatical editing.We also thank the anonymous reviewers for valuable comments on the manuscript.
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