Reflectance Spectroscopy for the Determination of Soil C: Where Are We? and What Are the Problems Which Need to Be Solved?.
James B., Reeves III, EMBUL, ANRI, ARS, USDA, BLDG 306, BARC EAST, BELTSVILLE, MD 20705, Dean Martens, SWRC, ARS, USDA, 2000 E. Allen Rd., Tucson, AZ 85719, and Gregory McCarty, HRSL, ANRI, ARS, USDA, BLDG 007, BARC WEST, Beltsville, MD 20705-2325.
Due to the Kyto accords and other efforts at reducing accumulation of C in the atmosphere, there has been increasing interest in the potential of soils to sequester C. The central problem is that to accurately access changes in soil C content will require the analysis of very large numbers of samples. Even to meet the Kyto minimum requirement for accuracy could involve 10's of millions of samples to be analyzed for C for a country such as Brazil. In addition, the form of C present in soils is an obvious factor influencing whether the C present will remain for any length of time. Present methods of soil C analysis, such as combustion, are too time consuming (~100 samples per day) and expensive ($2-3 US per sample) even for determination of total-C alone, much less for the added determination of organic versus inorganic C and for forms of C (labile, charcoal, etc.). Therefore new, rapid and accurate methods will be needed to determine soil C content and forms if C sequestration as soil C is to become practical from an economic sense. One method which has increasingly been put forward as such a method is diffuse reflectance spectroscopy using either near- (NIRS) or mid-infrared radiation (DRIFTS or Diffuse reflectance infrared Fourier transform spectroscopy). This technique consists of irradiating a ground sample in the wavelength range of interest and collecting the resulting diffusely reflected radiation. Compositional information is obtained from the radiation which penetrates particles of the sample and is reflected back to a detector assembly. Thus sample preparation as the method is presently practiced is the same as for conventional methods such as combustion, but since only one spectrum per sample is needed, multiple analyte determinations can be obtained simultaneously. In addition, the use of autosamplers with mid-infrared spectrometers allows for the analysis of at least 360 samples per day compared to perhaps 100 by combustion from which only one analyte value is obtained. While other techniques, such as Laser Induced Breakdown Spectroscopy (LIBS) can perhaps analyze samples as rapidly as NIRS or DRIFTS, LIBS can only provide measures of total C content and questions about sample preparation versus number of laser samplings needed still need to be answered. While the initial instrumental costs for combustion and LIBS are presently about the same as for NIRS and DRIFTS, the latter two methods have the potential for inexpensive instruments designed on filter technology which neither LIBS nor combustion can match. As only NIRS and DRIFTS can rapidly and simultaneously provide information on a wide range of C based soil components, they offer the best chance for being able to rapidly and inexpensively determine both soil C content and composition. While research has demonstrated that NIRS and DRIFTS can determine a wide variety of soil organic matter fractions (total-, inorganic- and total-C; various measures of biomass or bioactive C and N; organic-N, free amino acids and sugars among others) on a sample in less than 2 minutes, several areas still need to be researched. Areas presently under study include: 1. Can one calibration be used to analyze all soil samples for any given analyte, and if not, how will samples need to be divided? 2. What method, NIRS or DRIFTS, offers is the best method for soil analysis? 3. Similarly, which method, NIRs or DRIFTS, offers the best change at transferring calibrations between the same and dissimilar instruments. 4. Can filter based instrumentation be developed which will allow for the rapid, inexpensive and on-site determination of soil C content and composition, and if so, what will the limits be. With a modest increase in research aimed at answering the questions above, NIRS or DRIFTS have the best potential for providing an economical method necessary for successful determination of C sequestration in soil.