REPOSITORY > RESULTS

Abstract

Nitrogen (N) is an essential nutrient for plant growth and one which has the greatest effect on the yield. Since effective indigenous sources are insufficient by far to support current levels of global agricultural outputs, N is intensively applied by farmers in the form of N-based fertilisers. However, excessive rates of fertiliser N to obtain maximum levels of crop production are associated with potential environmental, health and economic risks. The pollution of groundwater by nitrate is reported to be one of the most serious issues of environmental concern in Slovenia, considered in the context of agricultural pollution. Though it is not possible to prevent nitrate leaching, it has been demonstrated that improved management practices can reduce the potential for nitrate contamination of groundwater as a result of increased fertiliser N use efficiency.
The aim of this doctoral work was to follow the migration of nitrate in the plant – soil – groundwater system on an experimental field cultivated with white cabbage (Brassica oleracea var. capitata L.) in order to find the best N management practice for growing brassicas which would give optimal yields in terms of quantity and quality and at the same time pose the least negative environmental risk (N losses). For the first time in Slovenia, the effect of different fertilizer types and the effect of the so far poorly studied split and combined usage of synthetic (inorganic) and organic fertilizers on the isotopic composition of total N (δ15N) were investigated, and thus the suitability of the δ15N signature as a tool to distinguish between organic and conventionally grown vegetables was evaluated.
On an experimental field, located above a shallow groundwater body and cultivated with white cabbage, four different fertilization and irrigation practices were compared, namely: pre-plant broadcast fertilization and drip irrigation covering 50% of the crop’s water requirements, fertigation with drip irrigation covering 100% of the crop’s water requirements, farmer’s practice consisting of pre-plant broadcast fertilization and irrigation on the day before and after transplanting using a tank sprinkler and an unfertilized control with farmer’s practice of irrigation. Nitrate fertilizer, labelled with 15N isotope, was used as a tracer. Nitrate migration and distribution was followed by analysing nitrate and total N in plants, soil, soil solution and groundwater for content and isotopic composition of. In order to do so, different methods for isolation of nitrate N from groundwater (anion exchange method) and soil solution (Teflon trap diffusion method) were introduced, which in Slovenia have not been used previously. The studied management practices were evaluated based on N mass balance calculations (inputs – outputs) and yield quality in terms of nitrate content.
The effect of different fertilizers on δ15N levels of vegetables was studied in a greenhouse pot experiment with lettuce (Lactuca sativa L.), grown under seven different treatments comprising of single and split application of synthetic (inorganic) or organic fertilizer, as well combined usage of both fertilizers. Furthermore, in this work δ15N signatures of 14 different organic and conventionally grown vegetable varieties commercially available on the Slovenian market, as well as δ15N signatures of some synthetic (inorganic) and organic fertilizers commonly used in Slovenia were determined, which should serve as a baseline for future studies.
The results of the field experiment did not confirm our hypothesis that fertigation with drip irrigation covering 100 % of the crop’s water requirements is the best practice for growing cabbage. Under the given environmental conditions, farmer’s practice with pre-plant broadcast fertilization and irrigation on the day before and after transplanting was found to be the most appropriate practice from the economic (highest yield) as well as the environmental (lowest N losses) points of view. The results obtained in the greenhouse pot experiment provided an important contribution to our knowledge in regard to the effect on plant δ15N of split N fertilization, which could cover up the use of prohibited synthetic (inorganic) N fertilisers in organic production. The experiment revealed that based on δ15N it is possible to differentiate between organic and conventionally grown produce when fertiliser is applied in a single application, whereas in a split application it is not possible to detect low or moderate rates of synthetic fertiliser illegally applied to a lettuce crop. The analysis of different vegetable varieties of organically and conventionally grown vegetables from the Slovenian market confirmed that due to several limitations of the method applied, the δ15N signature of the produce could only be used as an additional supportive and not as a single definitive tool to control organic vegetable production.