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If we want to develop farming on soil effectively and ecologically, we have to know the soil characteristics, the reasons for the potential low fertility and the ways how to eliminate them. Only this approach allows the rational utilization of the soil fund and achievement of the high effectiveness of the costs needed for the stabilization and increase of fertility and land capability. Recently, many scientific teams have focused their attention on the biochar, a lot of recommendations have been published which are dealing with its application into soil. However, the principal attention has been drawn to the impact of biochar on the particular soils and under the particular conditions. Far less information has been presented about the mutual interactions between the further significant agronomical factors in the combination with biochar. In this primary study, we analyze two new experiments established in the southwest part of Slovakia at the 1 Dolná Streda (sandy soil) and 2 Veľké Uľany (loamy soil) Localities. We discussed (1) the impact of the individual factors on the changes of soil characteristics, and (2) the impact of the individual interactions, such as: soil class – fertilization – biochar on the changes of the soil characteristics. The results indicated that the most significant factor, which influences the monitored soil parameters, is the soil class. The fertilization proved to be a factor which has a negative impact on the humus parameters; on the other hand, it improved the soil sorption. Biochar increased the content of the organic substances in soil and also its environmental effect of retention and immobilization of harmful elements and its positive effect on the soil structure was indicated. The highest frequency of the interactions between the monitored parameters related to the changes of soil characteristics was recorded in the combination fertilization x biochar, and also the soil class x fertilization x biochar.

Since biochar is considered to be a significant source of carbon, in this work we have evaluated the changes in soil organic matter (SOM) and soil structure due to application of biochar and biochar with N fertilization, and have considered the interrelationships between the SOM parameters and the soil structure. The soil samples were collected from Haplic Luvisol at the locality of Dolná Malanta (Slovakia) during 2017. The field experiment included three rates of biochar application (B0 - no biochar, B10 - biochar at the rate of 10 t ha-1, B20 - biochar at the rate of 20 t ha-1) and three levels of N fertilization (N0 - no nitrogen, N160 - nitrogen at the rate of 160 kg ha-1, N240 - nitrogen at the rate of 240 kg ha-1). The rate of biochar at 20 t ha-1 caused an increase in the organic carbon (Corg) content. The combination of both rates of biochar with 160 and 240 kg N ha-1 also caused an increase in Corg. In the case of B20 the extractability of humic substances carbon (CHS) was 17.79% lower than at B0. A significant drop was also observed in the values of the extraction of humic acids carbon (CHA) and fulvic acids carbon (CFA) after the addition of biochar at a dose of 20 t ha-1 with 160 kg N ha-1. However, both rates of biochar had a significant effect at 240 kg N ha-1. After application of 20 t ha-1 of biochar the content of water-stable macro-aggregates (WSAma) significantly increased compared to control. This rate of biochar also increased the mean weight diameter (MWDW) and the index of water-stable aggregates (Sw) and decreased the coefficient of vulnerability (Kv). The biochar at a rate of 20 t ha-1 with 240 kg N ha-1 the value of MWDW increased and value of Kv decreased significantly. The contents of Corg and CL correlated positively with WSAma, MWDW and Sw and negatively with WSAmi and Kv. The extraction of CHA and CFA was in negative relationship with MWDW. We conclude that the application of biochar and biochar combined with N fertilizer had a positive influence on SOM and soil structure.