Movement of nitrogen in a sandy loam soil under a continuous maize-wheat cropping system

<p class=MsoNormal align=left style='text-align:left;mso-layout-grid-align: none;text-autospace:none'>Nitrogen (N) movement in the soil resulting from the long-term application of fertilizer N is an environmental concern when it reaches the groundwater. The distribution of N in the profile of an alkaline sandy loam soil (Typic Haplustept) and its relationship with N uptake by plants was studied after 22 years of continuous cultivation in an annual crop rotation involving maize (<i>Zea mays</i> L.) and wheat (<i>Triticum aestivum</i> L.). Soil samples were collected to a depth of 1.2 m from the 0-0.15, 0.15-0.30, 0.30-0.45, 0.45-0.60, 0.60-0.90 and 0.90-1.20 m layers and analysed for alkaline KMnO_4­-oxidisable N (available N) and mineral N (NH₄-N and NO₃-N). The continuous addition of increasing levels of N resulted in an increase in N content, whereas the combined application of N, P and K caused a decline in its availability. Mineral N (2 M KCl-extractable NH₄-N and NO₃-N) was the lowest in the N₁₂₀P₃₅K_33.2 treatment plot. The available N and NH₄-N decreased with increasing soil depth. However, variations in NO₃-N concentration due to differential rates of fertilizer application were observed only to a depth of 0.45 m. This effect was more pronounced in the N₁₈₀P_17.5K_33.2 plot. Regression equations were used to predict N uptake by wheat using the N status in different soil layers as independent variables. Multiple regression analysis indicated that the predictability of the relationship between N uptake and available N improved considerably when its status to a soil depth of 0.45 m was included. In the case of NH₄-N, a noticeable increase in the coefficient of determination (R²) occurred to a depth of 0.90 m. The R²value of NO₃-N with the N uptake by wheat was quite low in the top layers (to a depth of 0.30 m). However, an increase in the R²value was observed when lower depths (beyond 0.30 m) were included in the regression analysis, suggesting that the inclusion of subsoil N status is important to achieve better and profitable N supply systems in crop production.</p> <p class=MsoNormal> </p>