Interactive effect of biochar and urea addition increases plant growth and rhizosphere microbial activity and decreases N2O emissions

Simultaneous addition of biochar with applied N fertilizer was proposed as useful way how to improve soil fertility and reduce negative effects of nitrogen fertilization. A hypothesis that added N decreases root growth and soil microbial activity and increases N2O emissions while addition of biochar has opposite effect was tested. We further hypothesized that increased microbial activity in biochar-amended soil stimulates growth of subse-quently cultivated plants. The supposed interactive effect of biochar and N-fertilizer was studied in pot experiment with cultivation of lettuce in soil amended with urea, biochar and their combination. In each pot, yield of plant biomass was determined and basal respiration, substrate-induced respiration and content of mineral N were determined in soil samples. Soil with roots decomposing after lettuce harvest was also analyzed and subsequently used for the cultivation of wheat plants. Emissions of N2O were measured following N application and measurement continued after harvest. Contrary to the hypothesis, N fertilization stimulated the growth of not only aboveground but also belowground plant biomass. Biochar increased growth of roots in agreement with the hypothesis. The effect of treatments on mi-crobial biomass and respiration was relatively weak in the rhizosphere of growing plants. However, when roots were decomposed, lower Nmin and higher microbial biomass were observed in biochar amended soils. Biochar amendment significantly stimulated the yield of subsequently cultivated wheat. At the beginning of the exper-iment, N2O emissions were higher in biochar-amended soil. However, at the later phase of the experiment, cumulative emissions were lower in biochar-amended soils. Results suggest that biochar supports more efficient N utilization by plants and increases soil fertility. Biochar also decreases N losses through N2O emissions, even with simultaneous N fertilization.