Effects of Fertilizer Nitrogen on Short‐Term Nitrogen Loss in Bypass Flow in a Vertisol

Bypass flow, the vertical flow of free water along the walls of macropores or preferential flow paths in the soil, can lead to movement of fertilizer nutrients beyond the reach of plants. Fertilizer type and the rate of application, as well as the amount, frequency, and intensity of rainfall, can influence the amount of fertilizer nitrogen (N) loss in leaching or bypass flow. The effect of fertilizer N form and rate of application on N recovery in bypass flow in a Kenyan Vertisol was determined. Calcium nitrate and ammonium sulfate, used to supply nitrate (NO3-)-N and ammonium (NH4+)-N, respectively, were surface-broadcast to 40-cm-long undisturbed soil columns at equivalent rates of 50, 100, and 200 kg N ha(-1). Using a rainfall simulator, two rainfall events (30mm of water applied in 1 h) were applied to the soil columns, one before and the other after fertilizer application. Total N, NO3--N, and NH4+-N concentrations in the bypass flow were determined after the second rainfall event. The application of NH4+-N, regardless of the rate, had no effect on N recovery in the bypass flow. When nitrate N was applied, the amount of fertilizer N recovered in the bypass flow significantly increased with the rate of NO3--N application. Of the total N in the bypass flow, 24 to 48% was derived from the soil, the bulk of which was organic N. It is concluded that following the application of NO3--N, bypass flow is an important avenue of loss of both fertilizer and soil N from Vertisols.