Methodology for studying fluxes of soil mineralN in situ

Summary-Improved methods are needed to measure the mineralization and uptake of soil N under field conditions. Because both soil disturbance (e.g. sieving, drying and rewetting) and environmental conditions markedly affect rates of soil N mineralization, rates measured in disturbed soils in the laboratory are unlikely to be quantitatively similar to, and may not be a reliable index of those in the field. It is generally impossible to assess the usefulness of most estimates of N mineraliition as predictors of field rates because of the unknown effects of assay conditions (especially soil mixing). We show that depending on soil type, sieving can either increase net N mineralization, cause immobilization of N, alter the proportion of nitrate nitrogen produced uring incubation, or induce or obscure the effects of previous fertilization on rates of N mineralization. A methodology for studying fluxes of mineral-N in soils, based on sequential soil coring and in situ exposure of largely undisturbed soil columns confined within metal or PVC tubes is described aad evaluated. The sequential measurement of changes in the quantity of soil mineral-N ia confined (no N uptake by roots) and unconfined soil allows rates of net N mineralization (or immobilization). plant uptake of N, and maximum N leaching to be calculated. It is argued that the method causes less disturbance to natural processes than others, and that it provides reliable quantitative stimates of fluxes of mineral-N in field soils. The severing of roots appeared to have no effect on the pattern of accumulation of mineral-N in several undisturbed forest soils exposed in situ for up to 130 days