The aim of the study was to assess the effect of land use on the forms of carbon and nitrogen in humic horizons of Stagnic Luvisols located in the area of Sławieńska Plain (northern Poland). Soil samples were collected form six stands with different land uses located in the same soil complex: more than 100-year-old beech-oak forest (BOF), meadow (M), arable field (AF), fallow (F), secondary afforestation with 15- year-old birch trees (SAB) and secondary afforestation with 30-year-old alder trees (SAA). In every stand, soils were sampled in five replications and analyzed with standard methods used in soil science. The content of different forms of carbon and nitrogen was analyzed after sequential extraction in 0.25 mol KCl·dm-3, 0.25 mol H2SO4·dm-3 and 2.5 mol H2SO4·dm-3 (Becher, Kalembasa 2011). Different land uses were reflected in the properties of humic horizon of the investigated soils. Particularly large differences were observed between the forest soils never used for agriculture, and arable or post-arable soils. Tillage caused the increase of the A horizon depth from 9.2 to 26.4-35.6 cm, and the increase of its volumetric density. The highest content of soil organic matter (SOM), total organic carbon (TOC) and total nitrogen (TN) as well as the highest TOC:TN ratio were observed in BOF stand. Statistically significant differences were observed between the stands in the content of carbon forms. Nonhydrolyzable carbon was dominat in the soils (55.98-68.11% of TOC), and dissolved organic carbon (DOC) had the smallest contribution (1.43-3.70% in TOC). In general, higher contribution of DOC in TOC was observed in arable and post-arable soils than in soils under forest. The content of mineral nitrogen (NO3-N + NH4-N) in the studied soils ranged from 0.028 to 0.053 g·kg-1, and NH4-N dominated in the pool. The lowest concentration of mineral nitrogen was observed in arable soils, and significantly higher in the soils under forest and fallow. Easily hydrolyzable nitrogen (EHN), weakly hydrolyzable nitrogen (WHN) and nonhydrolyzable nitrogen (NHN) were the main forms of the element in the studied soils and their content was closely related to the content of SOM. The highest concentration of the forms was noticed in BOF stand. There were differences between the stands in DOC:DON, EHC:EHN, WHC:WHN and NHC:NHN ratios, which indicate varied biological activity of the soils under different uses.

19.4.2014