Long-term Monitoring of Belowground Black Carbon.

            Black carbon (BC, also called soot, char, biochar, and charcoal) is a solid or particulate byproduct generated from various combustion processes with largely differing chemistries. The annual production of 60- to 280-Gt BC from natural fires and industrial activities constitutes a significant component of organic- and inorganic-carbon in soils, and sediments; further, its particulate form encountered in the atmosphere and on snow and ice surfaces affects the earth’s radiative balance. Using the highly stable BC as a soil amendment affords a new venue for carbon sequestration with its concurrent benefits to soil- and water-quality. However, the residence times of BC in soil differ considerably from those of organic carbon involved in the carbon cycle because of its chemistry and physical properties. Therefore, current state-of-the-art destructive soil sampling and analysis in the laboratory is highly inadequate for tracking these various forms of carbon. We developed and constructed a new, highly specific, analytical instrument for non-destructive in situ elemental analysis, termed an INS, which constitutes a major contribution to our very limited arsenal of instruments for noninvasive studies of the belowground processes. This new “nuclear” technology, based on neutrons interacting with elements’ nuclei and gamma ray spectroscopy, is insensitive to the elements’ chemistry. In addition to interacting with carbon, neutrons may interact with many other elements in the soil’s matrix, e.g., O, Si, N, P, Al, and Na.  In addition to its high elemental specificity, employing high-energy neutrons and gamma rays non-destructively interrogates soil masses larger than 200 kg.  We tested the INS system  in the  field in stationary mode, and continuous scanning modes, demonstrating that it offers  increased sensitivity for  identifying  changes in soil carbon over shorter  times and over large landscapes than does any other method. I discuss the relevance and advantages of the INS instrument for BC monitoring.