Short-term CO2 mineralization after additions of biochar and switchgrass to a Typic Kandiudult

Biochar additions to soil can increase soil organic carbon (SOC) concentrations; however, minimal information is available on relationships with soil nitrogen (N) cycle. We hypothesized that biochar additions to sandy soils should be resistant to microbial mineralization in short-term studies but may prime organic carbon (OC) mineralization of fresh residue that promotes N immobilization. A laboratory pot incubation study was conducted with a Norfolk loamy sand (Fine-loamy, kaolinitic, thermic, Typic Kandiudult) mixed with pecan-shell biochar at rates of 0, 5, 10 and 20 g kg− 1 and with 0 and 10 g kg− 1 dried, ground switchgrass (Panicum virgatum L.). On days 25 and 67 of the incubation, all pots were leached with 1.2 to 1.3 pore volumes of deionized H2O and the leachate NO3–N, NH4–N, and dissolved organic (DOC) concentrations were measured. Also cumulative soil CO2 fluxes after days 25 and 67 were determined. Biochar alone and mixed with soil and switchgrass after 67 days of incubation were characterized using Fourier transformed infrared spectroscopy (FT-IR). Mixing biochar with switchgrass after 67 days caused a significant increase in SOC content while soil total nitrogen (TN) and leachate DOC concentrations showed mixed results. Biochar mineralization by itself was found to be minimal, but by days 25 and 67, soil with biochar and switchgrass exhibited higher cumulative CO2 fluxes implying stimulation of switchgrass mineralization. Significant NO3–N immobilization occurred after 25 days in treatments with biochar + switchgrass; however, by day 67 the NO3–N concentrations rebounded slightly. The FT-IR analysis revealed that switchgrass in the presence of biochar underwent some structural modifications. Biochar applications in the short-term can cause N immobilization resulting in temporary plant available NO3–N concentration reductions.