Biological nitrogen fixation by common beans (Phaseolus vulgaris L.) increases with bio-char additions

This study examines the potential, magnitude, and causes of enhanced biological N-2 fixation (BNF) by common beans (Phaseolus vulgaris L.) through bio- char additions (charcoal, biomass- derived black carbon). Biochar was added at 0, 30, 60, and 90 g kg(-1) soil, and BNF was determined using the isotope dilution method after adding N-15- enriched ammonium sulfate to a Typic Haplustox cropped to a potentially nodulating bean variety (CIAT BAT 477) in comparison to its non- nodulating isoline (BAT 477NN), both inoculated with effective Rhizobium strains. The proportion of fixed N increased from 50% without biochar additions to 72% with 90 g kg(-1) bio- char added. While total N derived from the atmosphere (NdfA) significantly increased by 49 and 78% with 30 and 60 g kg(-1) bio- char added to soil, respectively, NdfA decreased to 30% above the control with 90 g kg(-1) due to low total biomass production and N uptake. The primary reason for the higher BNF with bio- char additions was the greater B and Mo availability, whereas greater K, Ca, and P availability, as well as higher pH and lower N availability and Al saturation, may have contributed to a lesser extent. Enhanced mycorrhizal infections of roots were not found to contribute to better nutrient uptake and BNF. Bean yield increased by 46% and biomass production by 39% over the control at 90 and 60 g kg(-1) bio-char, respectively. However, biomass production and total N uptake decreased when biochar applications were increased to 90 g kg(-1). Soil N uptake by N-fixing beans decreased by 14, 17, and 50% when 30, 60, and 90 g kg(-1) bio-char were added to soil, whereas the C/N ratios increased from 16 to 23.7, 28, and 35, respectively. Results demonstrate the potential of biochar applications to improve N input into agroecosystems while pointing out the needs for long-term field studies to better understand the effects of bio-char on BNF.