Nitrogen is essential for plant growth and makes up 78% of the atmosphere. However, most plants cannot use atmospheric nitrogen directly. Only certain plants can fix atmospheric nitrogen through a symbiotic relationship with nitrogen-fixing microbes.
The Haber-Bosch process revolutionized agriculture by allowing the industrial production of ammonia from hydrogen and atmospheric nitrogen under high pressure. This method, developed by chemists Fritz Haber and Carl Bosch, enabled the large-scale production of nitrogen fertilizer. While this process has greatly increased agricultural productivity, it has also led to environmental issues such as eutrophication, water pollution, and biodiversity loss due to excess nitrogen.
Nitrogen in soil exists in various forms, such as ammonium, nitrate, and organic nitrogen, which plants can use. Stable isotopes of nitrogen, specifically 14N and 15N, help researchers study nitrogen transformation and usage in plants. Different isotopes of nitrogen have distinct physical properties, allowing them to be used as tracers in these studies. Researchers often use δ15N to represent the ratio of these isotopes in plant tissues and soil nitrogen forms.
New Comprehensive Global 15N Datasets
Previous efforts have collected δ15N data for plants and soils separately on a global scale. A recent study by Hu et al. compiled the most comprehensive global δ15N dataset for both plants (including leaves, stems, and roots) and soils (including ammonium, nitrate, organic nitrogen, and total extractable nitrogen). This study revealed significant differences in δ15N values among different plant life forms (herbs, shrubs, trees) and mycorrhizal associations. It also found that within individual plants, δ15N varies among leaves, stems, and roots, with leaves having a higher δ15N. This intra-plant δ15N variation is unique and could guide future isotope analysis.
Hu et al. proposed a new term, δ15NPUN (Plant-Used Nitrogen), to describe the δ15N of plant nitrogen use. This term accounts for plant life forms and mycorrhizal types and could potentially replace the commonly used leaf δ15N as a quantitative tool for assessing soil nitrogen contributions to plants.
Nitrogen Use Patterns and Influencing Factors
Hu et al. used δ15NPUN and δ15N data from various soil components to identify plant nitrogen sources and the factors controlling nitrogen use patterns. They found that plant nitrogen use is primarily influenced by mean annual temperature. δ15NPUN increased linearly with rising temperatures globally. Higher temperatures stimulate soil nitrogen cycles, release more 14N, and increase nitrogen uptake by plants.
The study also found that atmospheric nitrogen deposition does not significantly affect plant nitrogen utilization. This is contrary to the common perception that atmospheric nitrogen deposition widely influences plant nitrogen use. Factors contributing to this observation include the relatively low levels of atmospheric nitrogen deposition in many regions and the small amount of nitrogen deposited compared to the total available nitrogen in soil. This finding challenges the belief that nitrogen availability is declining globally, as some reports suggest.
Additionally, annual precipitation did not significantly impact plant nitrogen utilization, which contrasts with regional studies linking foliar δ15N to precipitation. This discrepancy highlights the effectiveness of the δ15NPUN term in capturing plant-nitrogen interactions.
Organic Nitrogen Use and Preference
Hu et al. found that organic nitrogen accounts for nearly one-third of global plant nitrogen uptake. This high level of organic nitrogen use is surprising and calls for further investigation. The study also highlights plant nitrogen uptake preferences. While ammonium assimilation is less energy-intensive than nitrate, excess ammonium can be toxic, leading some plants to prefer ammonium or nitrate based on availability. The study identified a global threshold where ammonium contributes up to 46% of plant nitrogen.
Limitations and Future Directions
Hu et al.’s study is based on extensive observations but faces limitations due to the lack of simultaneous δ15N measurements for plant stems, roots, and soil components. Variability in sample sizes may affect the accuracy of source contribution estimates. More field studies with simultaneous δ15N measurements are needed to refine this framework.
The study also excluded urban and agricultural landscapes, where nitrogen dynamics can differ from natural systems. For example, nitrogen uptake patterns in crops may vary from those in wild plants. Future research should include data from urban and agricultural systems to provide a more comprehensive understanding of nitrogen use.