Carbon and Nitrogen Mineralization of Pyrolysis Chars Incorporated in Loamy Sand Soils with Low Soil Organic Carbon.

Soil amendment with pyrolysis char has been proposed for sequestering carbon and improving soil productivity, but its effects on C and N cycling need to be better understood. We conducted a series of char-amended soil incubation experiments to evaluate C and N mineralization of low temperature (400 to 500 ^oC) char from several feedstocks commonly available in the southeastern United States. Char C:N ratios ranged from 465 for pine chip (PC) char to 38 for peanut hull (PN) char. Cumulative C mineralization rates in 24-day incubations of Tifton loamy sand soil amended at 11 and 22 Mg ha^-1 equivalent rates with peanut hull (PN), pine bark (PB), hardwood (HW) and PC chars were low with total C mineralized ranging from 54 mg C/kg oven dry (OD) soil in the control to 125 mg C/kg OD soil for the PN amended soil at 22 Mg ha^-1. Incubation for 94-days with PN and PC chars at 22 and 44 Mg ha^-1 (equivalent rates) indicated there was a significant linear increase in C mineralization with char amendment. In all experiments the difference in C mineralization was due to higher CO₂ efflux in the first three days of incubation and differences between the char amended soils and the control disappeared by 24 to 38 days. There was no difference in nitrogen mineralization between the control and char amended soils. Carbon mineralization in Tifton loamy sand soils from a field experiment amended with PN and PC char 18 months before collection was not different from the control. These studies indicate that fresh char addition to a loamy sand soil with low organic carbon causes a short term increase in soil respiration compared to control soil. Relatively little char nitrogen appears to be biologically available based on mineralization rates, even in chars with low C:N ratios.