Acidified slurry reduces microbial and soil organic matter priming compared to conventional slurry

Intensive agricultural practices typically lead to a loss of soil organic carbon (SOC). Consequently, there is a need to develop more sustainable management strategies that promote greater carbon (C) storage in soil. Slurry application is a management option often used to maintain soil organic matter (SOM) stocks and recycle and replenish nutrients essential for crop growth. Slurry acidification with sulfuric acid (H₂SO₄) is a method known to enhance slurry nitrogen (N) retention (through reduced ammonia emissions), however, its broader impacts on SOC turnover are less well understood. This study assessed the priming effect of applications of conventional cattle slurry (pH 6.90 ± 0.06, n = 3) and acidified cattle slurry (pH 5.5) on three distinct soil C pools; ¹⁴C-labelled microbial biomass (apparent priming), fresh particulate organic matter (POC; ¹⁴C-labelled fresh grass crop residues; GCR) and ¹⁴C-labelled quasi-stable SOM (real priming) in a series of laboratory assays, over 6 months. All slurry treatments resulted in positive priming compared to the control (no treatment). The results indicated that slurry acidification reduced the real and apparent priming effects, relative to conventional slurry application, by 7.9 % and 11.3 %, respectively. Acidified slurry application also increased mineralisation of fresh GCR inputs promoting decomposition and nutrient release. In all pools, the effect of slurry addition was short-lived, lasting no more than six months. This suggests that applying acidified slurry may reduce C losses, potentially enhancing soil quality compared to conventional slurry, while also offering the added benefits of reduced N losses.