ABSTRACT This review presents the basic research and some applications of the gamma-ray attenuation, neutron gauges, and 137Cs fallout techniques for studying soil physical processes and properties. The selected studies aimed to give the readers a general idea of the use of these nuclear techniques carried out by Brazilian researchers in the past decades. It is expected to assist future researchers by identifying knowledge gaps and opportunities for applying the methods presented here. Around 100 studies were selected for this review. The papers dealing with gamma-ray attenuation are mostly related to the analysis of soil radiation interactions, the measurement of basic soil physical properties, the evaluation of hydraulic conductivity, water retention curve, and soil mechanical analysis. Neutron gauge applications are related to monitoring the water distribution and balance at the field scale, procedures for calibrating the existing gauge for the Brazilian soils, and analyzing the spatial and temporal variability of the soil water content. The 137Cs methodology involves studies about the erosion and sediment deposition in small watersheds and riparian zones, the spatial variability of 137Cs inventories at reference sites, and the measurement of sediment spatial distributions. Future studies with the gamma-ray attenuation methodology should focus on a better comprehension of the photon interaction with the soil and a correct selection of photon energies to investigate contrasting soils. This is mandatory for adopting it as a reliable tool for soil characterization. This review also revealed that the challenges for the future use of the 137Cs fallout technique involve the continuous decrease of the 137Cs activity worldwide, including in Brazil. Advances in detection systems (gamma spectrometers) will be required to overcome this issue. Future studies should focus on the use of correction factors related to the enrichment of fine particles during the transport of sediments to improve the estimates obtained through the conversion models. The use of neutron gauges to detect soil water content at the field scale depends on the adoption of reliable calibration curves. Then, comprehending how soil properties alter this curve and how it affects the water balance is a study of great interest. Motivated by strict regulations on the use of radioactive materials, the adoption of gauges with less activity is becoming a new goal. Thus, the development of more effective systems of neutron detection is crucial.

É apresentada, aqui, uma metodologia de campo para a determinação da condutividade hidráulica de um solo em declive, minimizando o corte de terra para posterior inundação e redistribuição da água. A garantia da ocorrência de fluxo de água apenas na vertical, é feita através de isolinhas de potencial da água. A condutividade hidráulica foi determinada pelo método do perfil instantâneo e os resultados para os nove tubos de acesso de sonda de nêutrons indicaram que uma única relação K(teta) é suficiente para representar a parcela.

A field methodology is presented for the measurement of the soil hydraulic conductivity in a sloping field, minimizing the leveling soil movement before water pounding and redistribution. The assurance of vertical flow only is performed through soil water potential isolines. The hydraulic conductivity was determined by the instantaneous profile method. Results for the nine neutron probe access tubes indicate that one single K(theta) relation is sufficient to represent the experimental site.

São apresentados resultados de um experimento sobre matéria orgânica em cultura de cana-de-açúcar, relativos ao primeiro ano (cana planta). A cultura foi instalada em outubro de 1997 e marcada com um pulso de fertilizante 15N, para estudar o destino da matéria orgânica no sistema solo-planta. É apresentado um balanço de nitrogênio, subdividindo o sistema em componentes de planta (colmo, ponteiro e palha), componentes de solo (cinco frações de matéria orgânica do solo) e estimando perdas por lixiviação. O 15N permitiu a determinação das quantidades de nitrogênio provenientes do fertilizante nos compartimentos acima mencionados, no final do ciclo da cultura.

Results of an organic matter management experiment of a sugar cane crop are reported for the first cropping year. Sugar cane was planted in October 1997, and labeled with a 15N fertilizer pulse to study the fate of organic matter in the soil-plant system. A nitrogen balance is presented, partitioning the system in plant components (stalk, tip and straw), soil components (five soil organic matter fractions) and evaluating leaching losses. The 15N label permitted to determine, at the end of the growing season, amounts of nitrogen derived from the fertilizer, present in the above mentioned compartments.