Net and Gross Nitrogen Turnover in Soil Amended with Acidified and Differently Dried Solids from Biogas Digestate

Core Ideas Acidification decreased the degradability of digestate solids. Acidification stabilized the gross N mineralization pattern with time. Dried acidified solids showed the highest potential N fertilizing value. Low drying temperature promoted net N mineralization from unacidified solids. The use of dried digestate solids from biogas plants as organic fertilizers may be disadvantaged by considerable N losses during thermal treatment. Acidification can decrease NH 3 emissions during drying, but little is known about the degradability and N release pattern of acidified and dried digestate solids. In a laboratory incubation, C mineralization and gross and net N mineralization from digestate solids subjected to acidification (to pH 5.5) and drying (70 or 160°C) were monitored for 160 d at 15°C. Gross N mineralization rates were determined using the 15 N isotopic pool dilution technique, and gross N transformation rates were calculated with the FLUAZ model. Irrespective of further treatment, acidified solids showed up to 50% reduced cumulative CO 2 –C respiration throughout the incubation and consequently decreased gross N mineralization rates compared with unacidified solids. Moreover, acidification of digestate solids delayed the nitrification dynamics during the first 3 wk of incubation. Throughout the experimental period, the acidified dried digestate solids had the highest inorganic N contents among all treatments. It was evident that acidification increased the stability of the organic matter and enhanced the availability of N on application to soil.