Impacts of Biochar and Other Amendments on Soil-Carbon and Nitrogen Stability: A Laboratory Column Study

There have been a large number of recent soil incubation studies examining the potential of biochar as a soil amendment tool for enhancing soil fertility and C sequestration. However, direct comparisons of biochar's effects to that of other nontraditional soil amendments are lacking. In addition, most soil incubation studies with biochar examined gaseous emissions in closed systems that do not allow for water flow through (i.e., leaching). Thus, three amendments-humic acid (HA), water treatment residual (WTR), and biochar produced from oak at 650 degrees C were incubated with Crosby silt-loam (fine, mixed, active, mesic Aeric Epiaqualfs) soil at a rate of 1% (w/w) in triplicated soil-columns. Effects of biochar and the other amendments on gaseous (CH4, CO2, and N2O) emissions and leaching of total organic C (TOC), NO3-, and NO2- were simultaneously monitored over a 257-d incubation period. Biochar-amended soil had reduced mean cumulative leaching of TOC by 30%, nitrate by 33%, and nitrite by 34%, compared to the control (control: 93, 75, and 2 mg kg(-1) for TOC, nitrate, and nitrite, respectively), likely due to sorption by the biochar. The WTR-amended soil exhibited similar leaching trends; however, HA-amended soil leached significantly greater (p < 0.05) TOC than either WTR or biochar. No amendments had any significant impact on gaseous emissions due to the high variability within each treatment. We hypothesize that the labile C and N compounds, which otherwise become emitted by microbial activity and produced significant differences in treatments, were probably leached out from the system as both leaching and gaseous sampling events took place at the same time due to the experimental settings employed. Thus, it is probable that the labile C and N compounds most likely to be oxidized and emitted in gaseous forms in any closed system incubations were either protected via mineral sorption or lost as leachate in the open column system in the present experiment. Considering both leaching and gaseous losses, C and N mean residence times (MRT) of 50 and 15 yr were calculated for the biochar-amended soil compared to 28 and 12 yr for the control, and 518 and 1.5 million years considering only gaseous losses, respectively. These findings show that estimates of soil-C and N stability that do not take into account aqueous losses of these elements (leaching) may tremendously overestimate C and N-MRT. Comparison with a field study using the same soil and amendment types suggests that analyses from laboratory incubation may serve as reliable proxies for trends in field soil responses to amendments.