Mineralization and immobilization of C and N from dairy farm effluent (DFE) and glucose plus ammonium chloride solution in three grassland topsoils

Net carbon and nitrogen mineralization and soil microbial biomass dynamics were determined during a 6-month period following surface application of dairy farm effluent (DFE) at the allowable annual loading rate onto three grassland topsoils from two major dairying regions in New Zealand. Turnover of the complex substrate DFE was compared to turnover of similar amounts of carbon and nitrogen amended in glucose and ammonium chloride, which is more accessible to microorganisms and served as a simple model system. Intact soil cores from the three topsoils were incubated in the laboratory at 25°C and −10 kPa. Net carbon (CO2 evolution) and nitrogen mineralization (change in inorganic N) were calculated from the difference of the treatments to a water-amended control. Soil texture had no clear effect on C mineralization, but N mineralization was faster in the coarser soils than in the finer soil. Soil structure influenced glucose mineralization in one soil (Te Kowhai) through preferential flow. Net mineralization of DFE-C was finished by day 64 (Horotiu), 112 (Te Kowhai) and 141 (Templeton) and amounted to 27.9±11.1% (Horotiu), 29.5±6.1% (Te Kowhai), and 48.9±12.4% (Templeton) of the applied C. Net mineralization of glucose-C was higher and, apart from Te Kowhai, finished earlier. It amounted to 67.4±5.7% (Horotiu, day 24), 76.3±8.5% (Templeton, day 85), and 95.7±3.9% (Te Kowhai, day 112). The Te Kowhai silty clay was still slightly in the N immobilization phase at day 183, whereas net N mineralization was observed in the two sandy loams, equivalent to 38% of the organic DFE-N in the Templeton and 44% in the more porous Horotiu soil. Microbial biomass estimates were particularly variable after glucose amendment and there were only a few significant differences between amended soils and control soils. We therefore suggest that the suitability of the CFE method soon after the application of easily available substrates be further investigated. Depending on the objective of a study, the value of using inherently more variable intact cores, which better mimic field conditions, must be compared against the more uniform results achievable by sieved and mixed soil samples.