Effect of long-term application of organic fertilizers on improving organic matter quality in acid soil

Soil organic carbon (SOC) content can greatly affect soil quality by determining and maintaining important soil functions. The study examined SOC, humic substances and molecular composition characteristics of acid soil (Dystric Retisol) with different long term (54 years) fertilization in Western Lithuania. Farmyard manure (FYM 60 t ha(-1)) and alternative organic fertilizers (wheat straw, green mass of lupine and oats, rape stubble and straw) applied on the two soil backgrounds - acid and limed. The experimental design included the following treatments: 1) unlimed soil; 2) unlimed soil + FYM; 3) unlimed soil + alternative organic fertilizers; 4) limed soil (1.0 rate every 5 years); 5) limed soil + FYM; 6) limed soil + alternative organic fertilizers. The result indicates that application of organic fertilizers exerted a significant positive effect on SOC accumulation (0.1-0.4% more carbon than the control). The combination of soil liming and FYM resulted in the accumulation of valuable humic acids. The higher amount of humic acids (0.67% of C) was maintained under FYM applied plots. The application of organic fertilizers and liming exerted the transformation of carbon compounds changes in the direction of polymerization of humic acids, which creates more favourable conditions for carbon sequestration.