Aim of the study:
The aim of the study was to determine the effect of deforestation of different genesis on the spatial differentiation of NO3– concentration in the Polish Tatras.

Material and methods:
In 2018, 728 water samples were collected in the Tatras by hydrological and chemical survey. In the Hydrological and Chemical Laboratory of IGiGP UJ, ion chromatography determined: Ca2+, Mg2+, Na+, K+, HCO3–, SO42–, Cl–, NO3–, NO2–, NH4+, PO43–, Li+, F– and Br–. Nitrates from 1719 water samples were analyzed in order to compare their current concentration to the period of 2007–09 (n = 991 water samples). The Tatras were divided into 13 areas.

Results and conclusions:
The chemical composition of waters draining both forested and deforested Tatra slopes showed that in the decade from 2007–09 to 2018 the concentrations of NO3 increased rapidly. In 2018, an average NO3– concentration expressed by median was higher by as much as 60.2%, rising from 1.76 to 2.82 mg · dm–3. The fluctuations in maximum concentrations were more pronounced, because their multiple increase occurred in five valleys, with the largest in: The Lejowa (by 348.2%) from 4.75 to 21.30 mg · dm–3; over double: Mała Łąka, Za Bramką and in Suchy Żleb; Kościeliska with Staników Żleb; Strążyska, Ku Dziurze and Spadowiec and Olczyska. Only in two areas the concentrations were lower. Comparison of the average NO3– concentration in Tatra waters has definitely proven its higher levels than in other regions of Poland in forested catchments or agricultural catchments, with the exception of catchments with intensive agricultural activities. Diverse genesis of deforestation result in strong mosaic–like spatial differentiation of the NO3– concentration. In waters draining the slopes deforested due to windfall, there is a concentration of NO3– > 10 mg · dm–3, which can potentially cause an unfavorable phenomenon of eutrophication of waters, however, it has a natural cause.