Differential retention of carbon, nitrogen and phosphorus in grassland soil profiles with long-term manure application

Liquid hog manure (LHM) is a valuable source of nutrients for farm production. Long-term experimental plots that had received LHM applications of 0, 50, and 100 m³ ha⁻¹ annually for 20 years were analyzed for total soil C, N and P storage. Applications increased total soil N and P by 1,200 kg N ha⁻¹ and 850 kg P ha⁻¹ at 100 m⁻³ LHM year⁻¹, compared to the control treatment. However, C storage did not increase with LHM rates and was lower in the 50 m³ ha⁻¹ LHM treatment (86 Mg C ha⁻¹) than in the 0 or 100 m³ ha⁻¹ treatments (100 Mg C ha⁻¹). In addition to the limited quantities and high decomposability of the C supplied by LHM, it is hypothesized that LHM stimulated the mineralization of both native soil C and fresh root-derived material. This priming effect was particularly apparent in deeper soil horizons where the decomposability of native C may be limited by the supply of fresh C. This study indicates that while LHM can be a significant source of crop nutrients, it has limited capacity for maintaining or increasing soil C.