Influence of Slurry Acidification with H2so4 on Soil N, P, S, and C Dynamics

Acidifying slurry with sulfuric acid (H2SO4) is practiced in some countries to help reduce ammonia (NH3) emissions during slurry storage and spreading to land. However, knowledge of how the application of acidified slurry affects soil health and nutrient cycling is lacking. This is particularly important since acidification with H2SO4 supplies an additional source of sulfur (S) to increasingly S-deficient agricultural soils. We hypothesized (i) that slurry acidification would increase the dissolved C, N, P and S content of the slurry, and (ii) that after application to soil it would reduce soil pH and promote nutrient availability. Using laboratory mesocosms we monitored nutrient dynamics (C, N, P and S) in an agricultural soil receiving cattle slurry (±acidification) in comparison to soil receiving no slurry, over a 2-month period. Measurements included greenhouse gases (CO2, CH4, N2O), solute concentrations (NO3-, NH4+, PO43-, SO42-, DOC, DON, EC, pH) in soil pore water, and soil extractions using distilled water and K2SO4. There were six treatments: (1) soil (control), (2) soil + slurry, (3) soil + acidified slurry (with H2SO4, pH =5.5), (4) soil + HCl, (5) soil + K2SO4, and (6) soil + K2SO4 + H2SO4. Our results showed that slurry acidification reduced soil pH and available P but increased electrical conductivity. In contrast, acidification did not affect the concentration of water extractable-SO42- despite evidence for microbial S consumption. Acidification treatments (3, 4 and 6) stimulated N mineralization and DOC production, likewise increased nitrification. Further, acidification did not affect N2O emissions but decreased net CO2 and CH4 emissions leading to an overall reduction in the soil’s total greenhouse gas footprint (expressed as CO2e). We conclude that application of acidified slurry to soil can reduce soil pH over a 2-month period with no negative impacts seen on soil and slurry C, N, P and S dynamics.