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UNESCO-IHE Institute for Water Education, Delft (2015)

Assessing nitrate loss from soils in Gula agricultural area, Tigray, Ethiopia via surface water, groundwater, denitrification and crop harvesting

Kahessay A.R.

Titre : Assessing nitrate loss from soils in Gula agricultural area, Tigray, Ethiopia via surface water, groundwater, denitrification and crop harvesting

Auteur : Kahessay A.R.

Etablissement de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2015

Résumé
The mountainous and steep slopes of the Gula Basin and other parts of Ethiopia contribute to the degradation of the soil fertility through erosion, but leaching, crop harvest, and microbial processes also contribute. In this study, pathways of nitrogen loss from crop soils were investigated. First, the concentration of nitrate, ammonium, and phosphate of surface water and groundwater was compared upstream and downstream of the study area in order to assess the potential for leaching loss. Second, potential denitrification was measured from different crop areas as well as in shallow groundwater behind check dams. Third, nitrogen loss from soils by crop harvest was measured. Water samples were collected for three weeks the end of the wet season to mid-dry season (November -December 2014) from surface water and ground water wells located in three adjacent tributaries from upstream to downstream of the sampling area in the Gula Basin in NW Tigray, Ethiopia. The concentration of nitrate ranged from 0.06 mg-NO3/L to 61.17 mg-NO3/L in groundwater and 1.1 mg-NO3/L to 66 mg-NO3/L in surface water. On the three tributaries the concentration of nitrate was increased when want from upstream to downstream on the same manner in groundwater and surface water. In most case the surface water has more concentration than groundwater. Most of the downstream groundwater and surface water samples had more than 50 mg/L of nitrate. The potential denitrification rate (PDR) of cropland from four types of crop was measured for three consecutive weeks. The first week, which was at the end of the rainy season, was significantly higher than the following two weeks, and this result is also supported by higher moisture content during that week. In general, the values of PDR were ranges from 0.0.096 g N2O-N m-2day-1to 12 g N2O-N m-2day-1, with the highest rates in wheat crops. These rates need to be interpreted with caution because PDR does not tell us the actual result and actually is an overestimation. The nitrogen loss through crop harvesting was measured by taking samples of the whole plant of the crop without its root in 20 cm-2quadrates and measuring its total N content. This was multiplied by the dry weight of the crop to estimate removal. These valued ranged from 11.8 g N m-2 to 88.4 g N m-2. The highest nitrogen removal was observed in teff. The nitrogen balance comparing fertilizer added with N loss through plant harvesting showed nitrogen balance ranges from -10.2 kg m-2to -30.5 kg m-2, with the most negative balances in teff crop type. Fertilizer application rates were determined by interviewing the farmers about how much they applied to their farmland, who normally used government recommendations. This balance did not consider the other sources of nitrogen such as animal manure, nitrogen fixation, but together with potential losses from leaching and denitrification are consistent with other studies that show declining soil fertility in this region. So the result becomes negative.

Sujets  : nitrate ; denitrification ; surface water ; groundwater ; soils ; Ethiopia

Présentation

Page publiée le 9 janvier 2017, mise à jour le 16 octobre 2018