Soil structure and soil organic matter in water-stable aggregates under different application rates of biochar

The effects of biochar and biochar combined with N-fertilizer on the content of soil organic matter in water-stable aggregates were investigated. A field experiment was conducted with different biochar application rates: B0 control (0 t ha(-1)), B10 (10 t ha(-1)) and B20 (20 t ha(-1)) and 0 (no N), 1st and 2nd levels of nitrogen fertilization on silt loam Haplic Luvisol (Dolna Malanta, Slovakia), in 2014. The N doses of level 1 were calculated on required average crop production using balance method. Level 2 included additional 100% of N in year 2014 and additional 50% of N in year 2016. The effects were investigated during the growing seasons of spring barley and spring wheat in 2014 and 2016, respectively. Results indicate that the B20N2 treatment significantly increased the proportion of water-stable macroaggregates (WSA(ma)) and reduced water-stable micro-aggregates (WSA(mi)). Aggregate stability increased only in the B20N1 treatment. The B20N2 treatment showed a robust decrease by 27% in the WSA(ma) of 0.5-0.25 mm. On the other hand, an increase by 56% was observed in the content of WSA(ma) with fractions 3-2 mm compared to the B0N0 treatment. The effect of N fertilizer on WSA(ma) was confirmed only in the case of the B10N2 treatment. The proportion of WSA(ma) with fractions 3-2 mm decreased by 42%, while the size fraction of 0.5-0.25 mm increased by 30% compared to the B10N0 treatment. The content of WSA(ma) with fractions 1-0.5 mm decreased with time. On the contrary, the content of WSA(ma) with particle sizes above 5 mm increased with time in all treatments except the B10N2 and B20N2 treatments. A statistically significant trend was identified in the proportion of WSA in the B10N2 and B20N2 treatments, which indicates that biochar with higher application levels of N fertilizer stabilizes the proportion of water-stable aggregates. In all treatments, the content of soil organic carbon (SOC) and labile carbon (C-L) in WSA(mi) was lower than those in WSA(ma). A considerable decrease of SOC in the WSA(ma) >5 mm and an increase of SOC in WSA(mi) were observed when biochar was applied at the rate of 10 t ha(-1). Contents of SOC in WSA(mi) increased as a result of adding biochar combined with N fertilizer at first level. C-L in WSA significantly increased in all size fractions of WSA. (c) 2018 Vietnam Academy of Science and Technology