Qiong Chen and Zuoqiang Yuan, Northwestern Polytechnical University in China, discuss their article: Global mycorrhizal status drives leaf δ15N patterns
Nitrogen: A Vital Nutrient for Plants
Nitrogen is a crucial nutrient for plants. It plays a vital role in their growth, development, and overall productivity. The availability of nitrogen is a key factor in understanding nitrogen dynamics at the ecosystem level. Nitrogen availability refers to how much nitrogen is available to terrestrial plants and microorganisms relative to their needs. One effective way to measure this availability is by examining a specific stable isotope of nitrogen. This isotope is known as the δ15N value. It is a valuable tool for understanding nitrogen dynamics across different ecosystems. That’s because it reflects the processes involved in nitrogen cycling, such as mineralisation and denitrification.
Mycorrhizae: Symbiotic Fungi with Plants
Mycorrhizae are fungi that form symbiotic relationships with plants. Mycorrhizae help plants absorb nutrients, especially nitrogen, from the soil, and over 85% of terrestrial vascular plants form partnerships with mycorrhizae. Most of these mycorrhizae can be classified as arbuscular mycorrhizae, ectomycorrhizae, orchid mycorrhizae, or ericoid mycorrhizae. Each type of mycorrhizae has a different capacity to assist plants in acquiring and transferring nitrogen. This can significantly influence plant growth and ecosystem functioning. Understanding the impact of these fungi on nitrogen cycles could provide key insights, helping us know how ecosystems function and how plants interact with their environment. However, despite their importance, few studies have considered how different types of mycorrhizal fungi affect the variation in nitrogen present in the tissues of plants, such as levels of foliar δ15N, in relation to climate and soil conditions. This leaves a critical gap in our understanding of nitrogen dynamics.
Our Research: Filling the Knowledge Gap
To address this knowledge gap, we used a machine learning approach and produced a global map of foliar δ15N. This was based on climatic and edaphic factors, as well as vegetation and dominant mycorrhizal characteristics. Our study revealed that the predicted global average foliar δ15N value was approximately 0.48‰. We also discovered that mean annual temperature was the strongest factor driving these global patterns. In detail, plants in tropical areas exhibited much higher foliar δ15N values compared to those in subtropical, temperate, and boreal regions. Warmer regions, like tropical and subtropical areas, tended to experience less nitrogen limitation compared to cooler regions, such as temperate and boreal zones. Furthermore, we observed significant differences in foliar δ15N values between plants with different types of mycorrhizal associations. Plants that did not associate with any mycorrhizae had higher foliar δ15N values compared to those that were associated with mycorrhizal species. Among plants that were associated with mycorrhizal fungi, foliar δ15N values were highest for those associated with arbuscular mycorrhizae, followed by orchid mycorrhizae, ectomycorrhiza, and ericoid mycorrhizae. These findings suggest that mycorrhizal types play a significant role in determining how plants access and utilise nitrogen.
Collectively, our findings reveal the pivotal role of climatic conditions and mycorrhizal associations in regulating N cycling and foliar δ15N values.
The Significance of Our Research
Our research highlights the importance of considering mycorrhizal data when assessing nitrogen availability and cycling in ecosystems. While much of the focus in nitrogen research has traditionally been on soil conditions and climate, mycorrhizal fungi also deserve attention. This is especially true because of their role in nutrient acquisition. By factoring in these associations, we can create more accurate models of nitrogen dynamics. These models will reflect the complexity of natural systems.
