Characteristics of inorganic nitrogen leaching from tobacco fields in rain-fed areas

Abstract: The main purpose of this study was to analyze the characteristics of nitrogen leaching from tobacco fields in order to understand the differences of nitrogen leaching from farmlands and tobacco fields. A long-term positioning experiment was conducted in Kaiyang county, Guizhou province of China (107°06′40″E, 26°52′24″N). In this area, the soil texture is silt loam. The mean rainfall is 1 129.9-1205.9 mm. Two cropping system were included. One was rotation (tobacco-winter fallow-maize) and the other one was continuous cropping (tobacco-winter fallow-tobacco). Four treatments with rotation cropping system were set up: no fertilizer (CK), chemical fertilizer (NPK), chemical fertilizer with manure (NPK + M), chemical fertilizer with bio-organic fertilizer (NPK + BM). One treatment with continuous cropping was also set up: chemical fertilizer with continuous cropping (NPK + L). The experiments were carried out in the fields from 2015 to 2017. In 2015, the leaching barrels were buried into the soils at 90-cm depth in the fields for monitoring leaching solution from the soils. The leaching solution was collected each a month for determination of inorganic N and total N. The organic N, net N leaching rate and apparent N leaching rate were calculated. The results showed that the annual leaching amount of inorganic nitrogen was 3.62-6.08 kg/hm2, and the net leaching rate was 0.09%-3.29%. The leaching loss of inorganic nitrogen mainly occurred during the growing season of tobacco fields, especially in May to June, which accounts for 40.33%-65.86% of the total leaching amount. The nitrogen in leaching solution were mainly in organic form. During the growing season of tobacco, the inorganic nitrogen accounted for 27.82%-33.39% of the total leaching nitrogen, and organic nitrogen accounted for 66.61%-72.18% of the total leaching nitrogen. Rainfall was a main factor affecting the nitrogen leaching from tobacco fields, and the monthly leaching amount of inorganic nitrogen was nonlinearly related to the monthly rainfall. Application of chemical fertilizer resulted in a significant increase of leaching inorganic nitrogen. Organic fertilizer combined with chemical fertilizer decreased the leaching amount of soil solution. The amounts of soil leaching solution in chemical fertilizer + manure and chemical fertilizer + bio-organic fertilizer treatment were lower than that in chemical fertilizer treatment. Bio-manure combined with chemical fertilizer significantly reduced leaching of inorganic nitrogen, and its inorganic nitrogen leaching amount was decreased by 39.44% compared with chemical fertilizer treatment. Compared with the treatment of rotation, the continuous cropping treatment significantly reduced soil water leaching and reduced the net leaching rate of nitrogen fertilizer by 59.57% (P<0.05). To sum up, under the current conventional management of flue-cured tobacco, the leaching intensity of inorganic nitrogen in the tobacco field in the rainfed agricultural area is not high. The results could provide valuable information for formulating suitable strategies of fertilization management in tobacco fields.