Measuring and modeling soil carbon respiration following repeated dairy slurry application

Farm animal manure slurries are repeatedly applied to soils. The portion of slurry organic matter not decomposed within its application year is left to mineralize over subsequent years, which results in a residual effect of high C mineralization in historically-amended vs. nonamended soils. This process was modeled in laboratory incubation by applying water, ammonium sulfate, heifer (HEI), or dairy cow (COW) slurry between 1 and 4 times (at Day 0, 84, 168, and 252) to soils of contrasting textures [sandy-loam (SL) and clayloam (CL)]. Respired CO2 was measured 6 to 10 times after each application along the trial period. Results showed daily respiration rates decreased as application frequency increased and that rates were significantly higher in slurry-amended, compared to control treatments. Furthermore, the contribution of slurry-derived CO2 respiration increased as application frequency rose (from 37 to 68% between the first and fourth applications of HEI and from 58 to 80%, respectively, of COW). The labile fraction of potentially mineralizable slurry C, estimated with a first-order kinetic decomposition model, was also increased with application frequency. The two slurries differed in potentially mineralizable C during the 84 d after the application events. However, there were no differences between the two slurries in the potential mineralizable C (1.43% of applied C) that contributed to respiration after 84 d of application. This small residual effect has never been quantified before in an incubation experiment. This small residue C is very important to farm practices in which slurries are continuously applied for decades.