Near Infrared Reflectance Spectroscopy for the Determination of Biological Activity in Agricultural Soils

Abstract

The objective of this work was to investigate the usefulness of near infrared (NIR) reflectance spectroscopy in determining biological activity in agricultural soils. A Foss-NIRSystems model 6500 spectrometer, equipped with a spinning sample cup module, was used to scan 179 soil samples obtained from experimental plots at two locations with three replicate plots under plough and no-till practices at each location with three rates of NH₄NO₃ for each plot with samples taken from five depths for a total of 180 samples (one sample lost). Biological activity as measured by four enzymes (dehydrogenase, phosphatase, arylsulfatase and urease) and nitrification potential was determined by conventional methods and NIR reflectance spectroscopy. Investigations showed NIR reflectance spectroscopy to be capable of determining biological activity as reflected by the four enzymes and nitrification potential to at least some degree. With the best R² in the range of 0.8, the results, while positive, were not as good as found previously for many other components (i.e. total C and N) in the same sample set. Efforts at simple discrimination into high, medium and low activities were not successful, and for the most part, calibrations based on subsets, such as samples from only one location, were not found to be an improvement. Correlation analysis indicated that measures of biologically-active nitrogen might be the basis for these determinations. Finally, while further research will be needed to define clearly the basis for, limitations to and usefulness of NIR reflectance spectroscopy in determining biological activity in soil samples, the results presented indicated that NIR reflectance spectroscopy might be useful for the rapid determination of such activity in cases where extreme accuracy is not required, such as spatial mapping.