A FIELD STUDY WITH NITROGEN-15 OF SOIL AND FERTILIZER NITRATE UPTAKE AND OF WATER WITHDRAWAL BY SPRING WHEAT

Abstract

In a field experiment on a Udalf soil formed from loess, we studied nitrate and water uptake of spring wheat as a function of soil depth and of time. Before the wheat was sown, 40 kilograms of nitrogen per hectare, tagged with 20 percent nitrogen-15, was applied three times. Nitrate concentration in the soil solution, soil water content, and soil water suction were determined as a function of soil depth and time by means of suction probes, a neutron meter, and tensiometers, respectively. From these data, total vertical water flow ut, capillary water flow vc, water flow through the roots vr labeled fertilizer and total nitrate content in the soil, and apparent mass flow of nitrate to the roots were computed. This was compared with N uptake of the crop at four harvests taken at the first node stage, anthesis, milk stage, and maturity. Total N uptake was 85, 134, 164, and 197 kg/ha at the respective harvests, of which 81, 71, 57, and 50 percent, respectively, were derived from the labeled fertilizer N. Uptake of labeled fertilizer N was practically complete at the second harvest at anthesis. Up to the first harvest, about 50 percent of the total uptake could be accounted for by apparent mass flow. This percentage dropped to about 30 at maturity. Seventy-six percent of the total N uptake by mass flow and 62 percent of the total water uptake came from the arable layer (0 to 30 cm). These values are comparatively high because quantity and distribution of rainfall did not require the plants to utilize the reserves of the subsoil to a large extent. The favorable water regime and relatively low nitrate concentrations were also responsible for the comparatively low contribution of mass flow to N supply.