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low levels of humus and active mineralization of plant residues in the soils along the Danube (Kirilov et al., 2015). The climatic factor is basic in relation to soil organic matter (Zhiyanski et al., 2012). The ratio of C / N in soils, which is an indicator of the rate at which mineralization of organic matter in the soil occurs, is low to moderate and confirmed by other authors (Kirilov et al., 2015). Most quickly is carried out the mineralization of the organic substance in the soils of the control.
Table 4 shows the composition of organic matter in soils, which is highly sensitive through management applied and widely is used as an indicator of soil quality (Thomazini et al., 2015). We separate two types of organic acids - humic and fulvic acids. The humic acids are stable carboxylic acids which dissolve in NaOH but do not dissolve in HCl. Their higher content is associated with higher soil organic matter stability and a stronger bonding of organic carbon to the mineral soil and better sequestration in the soil. On the contrary, fulvic acids are taken as the more mobile part of soil organic matter. They are with lower molecular weight and dissolve in NaOH and HCl. The results show that in control there is the lowest percentage of humic acids, which is accompanied by the highest percentage of fulvic acids and especially high percentage of the most reactive part of organic matter so called “aggressive fulvic acids”. These data support the view that the organic substance in the control is the most unproductive and with lower carbon sequestration ability in soil. There are the other studies that found that trees provides continuous input of liable organic matter by litterfall and there were observed higher content of liable and soluble carbon in agroforestry systems and native forest in comparison with agricultural systems (Thomazini et al., 2015). Obviously, in our study agricultural crops play an role of enhancing the recalcitrant form of carbon in the system. The practice of agroforestry is a good tool to improve the content and composition of the soil organic matter in the system of poplar plantations. For all soils studied, organic acids are 100% linked to sesquioxides and do not bounded with Ca. The data confirm those of Mihaylov (1988) who claim that the carbonate horizon of these soils is down from 190 cm. Summarizing the results leads us to conclusion that agroforestry enhances soil fertility. This is in comply with other studies of the other authors (Neupane and Thara, 2001; Tsonkova et al., 2012; Chen et al., 2019). Our study confirms the statement that agroforestry systems supplies sustainable nutrient security and long term soil productivity (Schwab et al., 2015)
Agroforestry is a multifunctional, environmentally-friendly and modern system of land use. We achieved good results in the establishment of agroforestry systems in growing Populus sp. along the Danube concerning enhancing their productivity and improving soil quality. Planting agricultural crops among tree saplings is a good method to increase their dendrometrical indicators as average diameter and average height. As a modern form of land use, agroforestry is also a tool to enhance soil organic matter content. Agroforestry practices is especially good to improve soil organic matter composition – the amount of carbon which is bounded in recalcitrant part is increase and