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European lacustrine systems are frequently exposed to nitrate (NO3–) pollution causing eutrophication processes. An example of these lakes is Shkodra Lake, a large, shallow lake shared by Albania and Montenegro, in the Balkans Peninsula. Shkodra Lake is a natural sink that collects NO3– from agricultural activities, widely diffused in the surrounding area. The additions of wheat straw and biochar have been suggested to increase soil NO3– retention of agricultural lands. To better understand the role of these two organic soil amendments in mitigating NO3– leaching from arable lands, a pot experiment using a representative sandy loam soil of the Skodra Lake basin was performed. More specifically, a greenhouse experiment with Lolium multiflorum L. and Zea mays L., was carried out for three months, to evaluate the concentrations of NO3–-N in leachate and the cumulative leaching losses of NO3–-N, after wheat straw (10 Mg ha–1) and biochar (10 Mg ha–1) soil addition, under the same rate of NPK fertiliser (300 kg ha–1). The effect of the two organic amendments on nitrate retention, was evaluated according to two methods: i) Soil NO3–-N leaching with distilled water; and ii) Soil NO3–-N extraction with 2M KCl. The leached NO3–-N and the Potentially Leachable NO3–-N (2M KCl extraction) were respectively determined. N uptake by plants, as well as the Nitrogen Use Efficiency were also calculated. A retention effect on nitrate was found in Lolium multiflorum L. and wheat straw treatments compared to control, by reducing leached NO3–-N almost to 35%. In SBFL (soil+biochar+fertiliser+Lolium) treatment, biochar effectively reduced the total amount of nitrate in leachate of 27% and 26% compared to SFL (soil+fertiliser+Lolium) and SSFL (soil+straw+fertiliser+Lolium) treatments, respectively. The potentially leachable NO3–-N was two to four times higher than the leached NO3–-N. The amount of potentially leachable NO3–-N per hectare ranged from 220 in SL (soil+Lolium) treatment, to 500 kg ha–1 in SFL. N plant uptake values ranged from 18.16 mg kg–1 in the non- fertilised treatment to 58.06 mg kg–1 soil in SSFM (soil+straw+fertiliser+maize) treatment. The NUE showed a similar trend (from 0 in the non-fertilised treatment to 47.9 % in SSFM). Results indicated a mitigating action of biochar on leaching of NO3–-N (leached up to 100 kg ha–1), despite the retention effect of the two different amendments applied.
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