Impact of Nitrogen Fertilization on Soil Nitrate Concentrations in an Onion Field in the Saiss Basin

¹ Soil, Plant, and Water Laboratory, National Institute of Agronomic Research, km 10, Road Haj Kaddour, P O Box 578 VN, Meknes, Morocco.
² Department of Geology Faculty of Sciences, Moulay Ismail, University, P O Box 11202, Zitoune, Meknes, Morocco.
³ Laboratory of Geo-engineering and Environment, Department of Geology, Faculty of Sciences, Moulay Ismail University, P O Box 11202, Zitoune, Meknes, Morocco.

^* Corresponding author: nessah.chaimae@gmail.com

Abstract

High nitrogen (N) fertilizer rates can result in excessive nitrate (NO₃^-N) in soils and groundwater. The Saiss basin is an agricultural area in Morocco, where substantial increases in nonpoint source pollution from excessive N fertilization is commonly observed. The purpose of this study was to determine the short-term effect of N fertilizer application rates on soil NO₃^-N and soil water contents (SWC) in an onion (Allium cepa L.) field over two consecutive years (2021 and 2022). The field experiment was conducted in a randomized complete block design. Six N rates, namely 0, 90, 135, 180, 225, and 270 Kg N/ha were considered in drip-irrigated onion crops. Soil samples were collected from five consecutive soil layers, namely 0-20, 20-40, 40-60, 60-80, and 80-100 cm, and analyzed for NO₃^-N contents and SWC. SWC were lower before irrigation and increased significantly after irrigation for both seasons. There was no significant effect of N rates on SWC. Soil NO3-N decreased significantly with time. In the first season, lowest NO3-N levels was observed after harvest for all N rates. Visually, a marked accumulation of nitrate in soil was observed in the three highest N rates after harvest. Statistically, N rates did not show significant effect on soil NO3-N (p>0,05) in 2021 and 2022. This finding may be explained by the predominance of the denitrification process at Douyet station.