Fate Of Soil-Applied Black Carbon: Downward Migration, Leaching And Soil Respiration

Abstract

Black carbon (BC) is an important pool of the global C cycle, because it cycles much more slowly than others and may even be managed for C sequestration. Using stable isotope techniques, we investigated the fate of BC applied to a savanna Oxisol in Colombia at rates of 0, 11.6, 23.2 and 116.1￢ﾀﾃt￢ﾀﾃBC￢ﾀﾃha￢ﾈﾒ1, as well as its effect on non￢ﾀﾐBC soil organic C. During the rainy seasons of 2005 and 2006, soil respiration was measured using soda lime traps, particulate and dissolved organic C (POC and DOC) moving by saturated flow was sampled continuously at 0.15 and 0.3￢ﾀﾃm, and soil was sampled to 2.0￢ﾀﾃm. Black C was found below the application depth of 0￢ﾀﾓ0.1￢ﾀﾃm in the 0.15￢ﾀﾓ0.3￢ﾀﾃm depth interval, with migration rates of 52.4ￂﾱ14.5, 51.8ￂﾱ18.5 and 378.7ￂﾱ196.9￢ﾀﾃkg￢ﾀﾃC￢ﾀﾃha￢ﾈﾒ1￢ﾀﾃyr￢ﾈﾒ1 (ￂﾱSE) where 11.6, 23.2 and 116.1￢ﾀﾃt￢ﾀﾃBC￢ﾀﾃha￢ﾈﾒ1, respectively, had been applied. Over 2 years after application, 2.2% of BC applied at 23.2￢ﾀﾃt￢ﾀﾃBC￢ﾀﾃha￢ﾈﾒ1 was lost by respiration, and an even smaller fraction of 1% was mobilized by percolating water. Carbon from BC moved to a greater extent as DOC than POC. The largest flux of BC from the field (20￢ﾀﾓ53% of applied BC) was not accounted for by our measurements and is assumed to have occurred by surface runoff during intense rain events. Black C caused a 189% increase in aboveground biomass production measured 5 months after application (2.4￢ﾀﾓ4.5￢ﾀﾃt additional dry biomass￢ﾀﾃha￢ﾈﾒ1 where BC was applied), and this resulted in greater amounts of non￢ﾀﾐBC being respired, leached and found in soil for the duration of the experiment. These increases can be quantitatively explained by estimates of greater belowground net primary productivity with BC addition.