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 da iry farm effluent (DFE) at the allowable annual loading rate onto three gra ssland topsoils from two major dairying regions in New Zealand. Turnover of the complex substrate DFE was compared to turnover of similar amounts of c arbon and nitrogen amended in glucose and ammonium chloride, which is more accessible to microorganisms and served as a simple model system. Intact so il cores from the three topsoils were incubated in the laboratory at 25 deg reesC and -10 kPa. Net carbon (CO2 evolution) and nitrogen mineralization ( change in inorganic N) were calculated from the difference of the treatment s to a water-amended control. Soil texture had no clear effect on C mineral ization, but N mineralization was faster in the coarser soils than in the f iner soil. Soil structure influenced glucose mineralization in one soil (Te Kowhai) through preferential Row. Net mineralization of DFE-C was finished by day 64 (Horotiu), 112 (Te Kowhai) and 141 (Templeton) and amounted to 2 7.9 +/- 11.1% (Horotiu), 29.5 +/- 6.1% (Te Kowhai), and 48.9 +/- 12.4% (Tem pleton) of the applied C. Net mineralization of glucose-C was higher and, a part from Te Kowhai, finished earlier. It amounted to 67.4 +/- 5.7% (Horoti u, 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 immobiliza tion 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 4 4% in the more porous Horotiu soil. Microbial biomass estimates were partic ularly variable after glucose amendment and there were only a few significa nt differences between amended soils and control soils. We therefore sugges t that the suitability of the CFE method soon after the application of easi ly available substrates be further investigated. Depending on the objective of a study, the value of using inherently more variable intact cores, whic h better mimic field conditions, must be compared against the more uniform results achievable by sieved and mixed soil samples. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.