Acidification prior to drying of digestate solids affects nutrient uptake and fertilizer value when applied to maize

Abstract

Acidification has proven effective in minimizing NH₃ emissions during the drying of digestate bio-solids, but its impact on soil nutrient dynamics and plant growth is less understood. This study aimed to assess the nitrogen and phosphorus efficiency of acidified-dried digestate solids as starter fertilizer for maize through a pot experiment and a soil incubation study. Two types of digestates (MDS and SDS) and two acidifying agents (concentrated H₂SO₄ and alum) were used. Drying significantly lowered the nitrogen fertilizer replacement value (N-FRV) from 42% in untreated to 12% in the dried material, reducing maize biomass and N uptake by 34% and 54%, respectively. The decline mitigated by acidification, which doubled N-FRV to 28%. Drying enhanced maize P uptake by 25%, indicating dried MDS as an effective P fertilizer (P-FRV of 82%). However, alum negated the drying benefits for P uptake, aligning it with raw MDS levels. The SDS treatments showed no significant effects on maize growth or nutrient uptake, though dried SDS indicated a high N mineralization potential, N-FRV and P-FRV remained around 33% and 26%, respectively. The study concludes that H₂SO₄-acidified dried MDS could serves as a suitable starter fertilizer with balanced N and high P availability, supporting early maize development. Alum may serve to preserve N value while reducing P solubility to prevent runoff. Dried SDS is less effective as a mineral fertilizer replacement, better suited for sustaining soil organic N and P levels.