Low-dose acidification of cattle slurry with sulfuric acid enhances oilseed-rape yield but risks elevated sulfide formation during prolonged storage.

Slurry acidification using sulfuric acid (HSO) is an effective strategy for reducing ammonia and methane emissions during storage while providing a valuable source of sulfur (S) for crops. However, the optimal HSO dose for balancing emission mitigation, S availability, and cost-effectiveness remains unclear, particularly concerning the formation of undesirable sulfides during storage. This study investigated the effects of HSO dose on S transformations during storage of cattle slurry (CS), anaerobically digested slurry (DS), the liquid fraction of separated digested slurry (LFDS), and the subsequent S fertilizer replacement value (SFRV) for oilseed rape (OSR). A 5-month storage experiment at 10 °C was conducted, followed by a pot experiment. Each slurry type received three HSO doses targeting pH values of 6.5, 5.5, and 4.5. Slurry pH, redox potential, dissolved sulfate, and sulfide were monitored during storage. S offtake in OSR biomass was measured at harvest, and the SFRV was assessed. Low-dose acidification of CS to pH 6.5 triggered rapid sulfate reduction to sulfide within 4 weeks of storage, while in the DS and LFDS treatments, sulfide formation was delayed at the low acid dose. The highest dose of acidification to pH ≤ 4.5 maintained lower pH and sulfide levels in all stored slurry types. The SFRV of non-acidified slurry ranged from 19 to 24 % of total slurry S, increasing with acidification and higher sulfate+sulfide content. These results suggest that low-dose acidification with HSO can satisfy crop S demands but may lead to undesired sulfide formation during storage, although this effect is delayed in digested slurries.