Xiaobo Yuan and Yaodan Zhang, from Lanzhou University in China, discuss their article: Soil microbial networks mediate long-term effects of nitrogen fertilization on ecosystem multiservices
Nitrogen (N) fertilization caused by anthropogenic activities such as fertilizer application and fossil fuel combustion have significantly impacted the capacity of ecosystems to support key services such as nutrient cycling and soil carbon (C) sequestration. Ecosystem functions and services often exhibit inherent trade-offs and synergies because the enhancement of one function may come at the cost of another. Understanding the long-term effects of N fertilization on drylands is of critical importance for the conservation of these ecosystems and the many services they provide.
Soil microbial diversity encompasses not only species richness and abundance, but also complex biotic interactions such as predation, competition, mutualism, parasitism, and amensalism among hyper-diverse microorganisms belowground.
Yet, much less is known about the role of microbial network complexity and its ecological clustering in regulating soil multiservices under long-term atmospheric N deposition, particularly in natural ecosystems such as drylands. Moreover, systematic studies investigating the effects of N addition on soil multiservices and the extent to which regulatory mechanisms vary with soil depth remain insufficient.
Hypotheses
We expected ecosystem services to follow a hump-shaped pattern in response to N fertilization, with the highest levels observed at intermediate N addition levels. We also expected N addition effects on multiservices to be indirectly regulated by soil microbial network complexity. Since microbial resource availability decreases with increasing soil depth, this regulatory mechanism may also vary with soil depth.
Our study
We conducted a 13-year N addition experiment with six N addition levels in a semiarid grassland from China to explore how soil microbial diversity, network complexity, and ecological clusters (determined using metagenomics-based taxonomic information) affect soil multiservices composed of 17 soil functions associated with six ecosystem services including soil carbon stocks, nutrient supply, biodiversity, soil organic matter decomposition, pathogen control, and plant-soil symbiosis. These investigations were carried out in both the topsoil (0-10 cm) and subsoil (10-20 cm) layers.
Our findings
We found that the responses of dryland soil ecosystem services to fertilization strongly align with the intermediate disturbance hypothesis. Small increases in N can promote ecosystem services in nutrient-poor drylands, but excess N may collapse the capacity to maintain important ecosystem services. This concept may be applied to any limiting resource, and the strong regulatory role of soil microbial networks observed in our study deserves further attention across environmental change factors and ecosystems. We also provide new evidence that soil microbial network properties such as overall network complexity modulated the effects of N fertilization on ecosystem services, especially for topsoils.
This knowledge is critical for maintaining the sustainability of terrestrial ecosystems and suggests that the promotion of microbial diversity and preservation of topsoils will be incredibly important to mitigate the effects of N deposition, especially under more diverse and more frequent global climate change impacts in the future.
