Effects of plasma treatment of digestates on pH, nitrification and nitrogen turnover during storage and after soil application

Abstract

Acidification through plasma treatment technology presents a potential solution to mitigate nitrogen losses throughout the manure management chain, offering a dual benefit of atmospheric nitrogen incorporation into manures while inducing acidification. This study evaluated the use and effectiveness of plasma treatment technology as a one-time acidification strategy for digested slurry during long-term storage, its effects on slurry nitrogen (N) availability and subsequent release in the soil. A digestate from a full-scale anaerobic digester was separated using a decanter centrifuge, and the resultant liquid fraction (LF) was plasma treated to achieve three different initial pH levels (4.27, 5.03 and 5.42). After that, a storage experiment was set up for 180 days to monitor the evolution in pH and changes in NH4+, NO3- and NO2-. Concurrently, a soil incubation experiment was conducted to study the N turnover for 80 days after incorporating the organic materials (Liquid fraction, plasma-treated liquid fractions, digestate and raw slurry). Plasma treatment significantly increased total N and the proportion of inorganic N in the slurries by fixing atmospheric N as NO3- and NO2- and reduced the slurry pH depending on the treatment duration. After six months of storage, pH increased by 0.91 pH units in the plasma-treated liquid fraction with the highest initial pH (LFpH-5.42). Plasma treatment significantly increased net inorganic N release in soil by 5–14 % compared to a non-treated liquid fraction. Unexpected prolonged inhibition of NH4+ nitrification in soil for over 80 days after applying plasma-treated slurries was observed. In conclusion, plasma treatment emerges as a promising alternative to acid- and bio-acidification strategies, offering prolonged low slurry pH stabilisation, improved fertiliser value and delayed NH4+-N nitrification in soil.