Placement of Acidified Biowastes: Unveiling Abiotic and Biotic Effects on Soil P Dynamics Using 18O as a Tracer

Abstract

Placement and acidification can improve phosphorus (P) availability from biowastes. However, little is known about how the placement of acidified biowastes affects biotic and abiotic processes in the soil. Thus, we selected two biowastes: digestate solid fraction (DSF) and meat and bone meal (MBM). Both were applied in their untreated and acidified forms. We hypothesised that the acidification would affect biotic and abiotic processes and, consequently, the P dynamics in the soil. All fertilisers were incubated for 12 days to evaluate abiotic and biotic processes in the placement zone and in the adjacent soil. Assessments included resin-extractable P (resin P) and microbial P contents and δ¹⁸O values at different distances from the placement zone. Microbial respiration was also measured. Acidification significantly increased P release for DSF and MBM. The soil resin P content of acidified biowastes was larger even at greater distances (10–12 mm). For untreated MBM, soil resin P was significantly larger than the negative control up to 4 mm from the placement zone (50–60 mg kg⁻¹). For this treatment, microbial P was relatively increased even at greater distances (150 mg kg⁻¹ at 6–8 mm). Acidification suppressed microbial activity and resulted in lower respiration rates for both MBM and DSF. In addition to that, our results showed a significant correlation between ¹⁸O incorporation into microbial P and microbial respiration. Thus, the greater the microbial activity, the more P is biologically cycled in the microbial biomass. However, no correlation was found between respiration and ¹⁸O incorporation into resin P. These results may indicate an insufficient incubation time for microbes to release P into the soil and/or the co-occurrence of abiotic processes which are not exchanging oxygen between water and phosphate (e.g., desorption). We conclude that for untreated MBM, biotic processes may be the main driver of P movement in the soil. In the case of acidified biowaste, diffusion is the main process moving the P in the soil. This research shows that acidifying biowastes like DSF and MBM boosts P availability through abiotic processes. These findings suggest that acidification can enhance nutrient use efficiency and improve soil fertility. However, further studies are needed to assess the long-term effects on microbial communities and soil health.