Rapid quantification of biological nitrogen fixation using optical spectroscopy.

Area of Science:

• Agricultural Science
• Plant Physiology
• Biogeochemistry

Background:

• Biological nitrogen fixation (BNF) is a critical global nitrogen input, but its accurate quantification is challenging.
• Traditional methods like isotope ratio mass spectrometry (IRMS) are time-consuming and costly.
• Leaf reflectance spectroscopy presents a faster alternative for assessing plant nitrogen status.

Purpose of the Study:

• To develop and validate a novel, rapid, and inexpensive method for quantifying BNF using optical spectroscopy.
• To compare optical spectroscopy-derived BNF measurements with traditional IRMS quantification.
• To assess the method's ability to capture BNF responses to environmental changes.

Main Methods:

• Plant tissues from controlled and field experiments were analyzed for nitrogen concentration ([N]), nitrogen isotope composition (δ15N), and N derived from atmospheric fixation (Ndfa) using IRMS.
• Optical spectroscopy was employed to predict the same parameters ([N], δ15N, Ndfa) from the same tissues.
• Spectroscopic predictions were validated against IRMS measurements.

Main Results:

• Optical spectroscopy accurately predicted Ndfa, with high R2 values (0.90 in glasshouse, 0.94 in field) compared to IRMS.
• The method demonstrated strong agreement with traditional quantification, with slopes close to 1.
• Root mean square errors for predicting legume δ15N were low (0.38 and 0.43).

Conclusions:

• Optical spectroscopy provides a rapid, scalable, and cost-effective tool for quantifying BNF.
• This technique complements existing methods and facilitates broader BNF research.
• The method effectively captures BNF dynamics under climate change scenarios like warming and drought.