Soil mineral nitrogen dynamics following repeated application of dairy slurry

Repeated applications of animal slurry to soil can lead to residual nitrogen (N) effects from mineralization of organic N carried over from the previous year and from remineralization of previously immobilized N. We studied the effect of repeated slurry applications on soil mineral N (SMN : nitrate-N plus soluble, exchangeable and non-exchangeable ammonium-N) dynamics in a simplified, aerobic laboratory incubation. The experiment evaluated the effects of up to four applications (84-day intervals) of two different liquid cow slurries, ammonium sulphate and water (unfertilized control, CON) to sandy loam and clay loam soils. The slurries came from heifers (HEI) and lactating dairy cows (COW). Both soil types showed net N mineralization in HEI during each 84-day interval after application (3–6% of slurry-N), whereas decomposition of COW induced net N immobilization at 16% of slurry-N. The effect observed for COWmight have come from its larger C to organic-N ratio. After each application to the clay loam soil, 36–64% of the ammonium applied was not recoverable at day 0 because of ammonium fixation by clay minerals, and an average of 20% of fertilizer-N was measured as non-exchangeable ammonium at day 84. Recovery of N applied with both HEI and COW at day 84 increased significantly with subsequent applications to clay loam soil, but not to sandy loam soil. Residual effects in clay loam soil ranged from 2 to 11% of applied N, which probably resulted from slow mineralization of recalcitrant organic fractions in the slurry and partial stabilization of microbial by-products within the soil.