Miaojun Ma, Gansu Gannan Grassland Ecosystem National Observation and Research Station, State Key Laboratory of Herbage Improvement and Grassland Agro-ecosystems, College of Ecology, Lanzhou University, discusses his article: Multiple mechanisms associated with loss of seed bank diversity under nitrogen enrichment
Background
Anthropogenic activities have caused an increase in atmospheric nitrogen (N) deposition, which threatens the biodiversity of many ecosystems globally. The current average global N enrichment is 10 kg N hm-2 yr-1, but it may be as high as 50 kg N hm-2 yr-1 by 2050. Many studies have shown that N enrichment reduces species diversity in plant communities. Representing a plant diversity reservoir belowground, the soil seed bank can promote species coexistence through a storage effect, and can maintain species diversity in aboveground plant communities. However, although evidence concerning seed bank responses to N enrichment is accumulating, the mechanisms by which N enrichment affects seed banks have not been addressed.
To better understand the mechanisms by which N enrichment affects seed banks and the associated aboveground plant community, we investigated the direct and indirect effects of experimental N enrichment on seed banks from a long-term nitrogen addition experiment in Gansu Gannan Grassland Ecosystem National Observation and Research Station located in an alpine meadow on the eastern Tibetan Plateau. More specifically, we examined how aboveground plant communities (including live plants and their litter) and associated seed banks, seed rain, soil fungal communities and environmental conditions, and seed viability responded to N enrichment, and conducted a seed burial experiment to explore seed germination, rot, viability, and nonviability.
Hypotheses
We asked two questions:
1. What are the direct mechanisms of the effect of N enrichment on seed banks, and is the response of seeds (germination, rot, viability, and nonviability) to the mechanisms related to seed mass?
2. What are the indirect mechanisms by which N enrichment affects soil seed banks?
We predicted that N enrichment would promote seed germination or/and reduce seed viability, especially for small seeds. We further predicted that N enrichment would directly and indirectly decrease species diversity of seed banks through changes in the species diversity of the aboveground plant communities, amount of litter and seed rain, viability of seeds, and alterations in biotic and abiotic environmental factors belowground.
Key findings
We found that N enrichment decreased species diversity in the plant community and also depleted soil seed banks in the alpine meadow, although the rate of species loss from seed banks was slower than that from the plant communities. The seed burial experiment revealed some direct mechanisms by which N enrichment can deplete seed banks. N enrichment directly affected seed banks by decreasing seed viability and enhancing seed germination. Furthermore, small seeds were more prone to loss of viability and increased seed germination under N enrichment than large seeds.
Furthermore, we show that these direct effects on seed banks were coupled by multiple indirect effects that resulted from N enrichment altering species composition of the plant community and by increasing the amount of litter and fungal diversity. N enrichment also indirectly decreased the number of seeds in seed banks by decreasing the number of seeds in seed rain, with increasing negative effects due to higher levels of N enrichment.
Implications
Our study is among the first to illuminate the multiple direct and indirect interacting mechanisms by which N enrichment can deplete soil seed banks. We also demonstrate that N enrichment effects on seed banks also depend on seed traits that can feed back to aboveground plant communities. These findings suggest that reversing the negative effects on N enrichment on seed banks, the plant community, and ecosystems, may prove to be challenging, because multiple aspects that are altered by N enrichment, such as soil fungal community, litter, aboveground plant community, and N concentration in soil, will need to be reversed. Our study identifies multiple new mechanisms to explain the loss of seed bank diversity due to N enrichment, and sheds light on the mechanisms that interact to explain loss of seed bank diversity, with possible ramifications for ecosystem resilience in response to N enrichment.
