The impact of long-term application of inorganic nitrogen fertilizers and manure on changes of selected properties of organic matter in sandy loam soil

The aim of the research was to assess the effect of long-term application of different doses of nitrogen fertilizers with or without manure to changes in the total organic carbon content, total nitrogen content, evaluation of dissolved organic carbon content and the value of absorbance coefficient (A4/6) in sandy loam soil. The base of research was the long-term field experiment, established in 1979 at the Wierzchucinek Experimental Station close to Bydgoszcz city - Poland. The experiment was carried out in the three-course crop rotation, potato, rye, rye in randomized split-plot design. The experimental treatments were four levels of N fertilizers in 0, 47, 93, and 140 kg*ha-1*yr-1 (N0, N1, N2, N3) as a 1st factor of experiment, and the same doses of N fertilizers with farmyard manure application (30 t*ha-1) as a 2nd factor. After 36 years of experiment the content of total organic carbon was 26% lower and the content of total nitrogen 13% higher compared to the values determined before the experiment foundation (1979). The consequence of changes in the content of organic carbon and total nitrogen, are changes in the ratio of organic carbon content and total nitrogen content. It was noticed that the use of manure and different nitrogen doses resulted in a decrease of organic carbon content and total nitrogen content value. After application of different nitrogen doses, organic carbon content ranged from 122.4 to 152.2 mg*kg-1. The same nitrogen doses applied simultaneously with farmyard manure increased the organic carbon content, which ranged from 133.5 to 166.7 mg*kg-1. The changes of the organic carbon content did not effect on percentage of this fraction in the total organic carbon. Percentage of organic carbon content in total organic carbon content was on averaged 1.6%. After the application of different nitrogen doses, humic acids of analyzed soils were characterized by lower average value of A4/6 (5.4). However, the application of nitrogen doses with manure brought a higher - 6.3 - A4/6 value. Farmyard manure application caused the formation of humic acids of lower molecular weight and a low humification degree.