Shifts in soil seed bank and plant community under nitrogen addition and mowing in an Inner Mongolian steppe

Highlights

• Mowing reversed the depletion of seed bank size under N deposition in deep soil.

• Both N deposition and mowing individually increased seed richness in deep soil.

• Mowing stimulated whereas N deposition suppressed plant species number.

Abstract

Atmospheric nitrogen (N) deposition and land management practices (e.g. mowing) are two major threats to grassland soil seed banks and plant communities. However, the interactive effects of N addition and mowing on soil seed banks and plant communities are poorly quantified. A six-year field experiment was used to examine the responses of soil seed banks and plant communities to N addition (10 g N m⁻² year⁻¹) and mowing (once a year) in a temperate steppe in northern China. Soil seed bank and plant community structure were investigated with laboratory germination and visual examination. Mowing reversed the depletion of seed bank size under N deposition at soil depths of 2–5 cm and 5–10 cm. The main effects of N addition and mowing increased 1.1 species (0.25 × 0.25 m²) independently, but no interactions of N addition and mowing on seed richness were detected at soil depths of 5–10 cm. Nitrogen addition had few effects on plant cover, but mowing suppressed it by 11.5%, mainly by decreasing grass cover. Nitrogen addition stimulated plant height by 6.25 cm by enhancing grass height whereas mowing did not influence it. Nitrogen addition decreased 2.8 plant species due to negative effects of N addition on forb. The positive effects of mowing on forb richness exceed negative effects of mowing on grass richness, leading to a net increase of 2.8 plant species with mowing. Our findings suggest that changes in soil seed bank size under N deposition can be strongly modulated by land use. Both N deposition and mowing can increase seed richness in deep soil (5–10 cm). Mowing stimulated, but N deposition suppressed, plant species richness. This is potentially important for understanding the influences of land use and environmental changes on soil seed banks, with subsequent effects on plant community dynamic.

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?