Gross nitrogen mineralization and nitrification rates and their relationships to enzyme activities and the soil microbial biomass in soils treated with dairy shed effluent and ammonium fertilizer at different water potentials

Gross N mineralization and nitrification rates and their relationships to m icrobial biomass C and N and enzyme (protease, deaminase and urease) activi ties were determined in soils treated with dairy shed effluent (DSE) or NH4 + fertilizer (NH4Cl) at a rate equivalent to 200 kg N ha(-1) at three water potentials (0, -10 and -80 kPa) at 20 degrees C using a closed incubation technique. After 8, 16, 30, 45, 60 and 90 days of incubation, sub-samples o f soil were removed to determine gross N mineralization and nitrification r ates, enzyme activities, microbial biomass C and N, and NH4+ and NO3- conce ntrations. The addition of DSE to the soil resulted in significantly higher gross N mineralization rates (7.0-1.7 mu g N g(-1) soil day(-1)) than in t he control (3.8-1.2 mu g N g(-1) soil day(-1)), particularly during the fir st 16 days of incubation. This increase in gross mineralization rate occurr ed because of the presence of readily mineralizable organic substrates with low C:N ratios, and stimulated soil microbial and enzymatic activities by the organic C and nutrients in the DSE. The addition of NH4Cl did not incre ase the gross N mineralization rate, probably because of the lack of readil y available organic C and/or a possible adverse effect of the high NH4+ con centration on microbial activity. However, nitrification rates were highest in the NH4Cl-treated soil, followed by DSE-treated soil and then the contr ol. Soil microbial biomass, protease, deaminase and urease activities were significantly increased immediately after the addition of DSE and then decl ined gradually with time. The increased soil microbial biomass was probably due to the increased available C substrate and nutrients stimulating soil microbial growth, and this in turn resulted in higher enzyme activities. NH 4Cl had a minimal impact on the soil microbial biomass and enzyme activitie s, possibly because of the lack of readily available C substrates. The opti mum soil water potential for gross N mineralization and nitrification rates , microbial and enzyme activities was -10 kPa compared with -80 kPa and 0 k Pa. Gross N mineralization rates were positively correlated with soil micro bial biomass N and protease and urease activities in the DSE-treated soil, but no such correlations were found in the NH4Cl-treated soil. The enzyme a ctivities were also positively correlated with each other and with soil mic robial biomass C and N. The forms of N and the different water potentials h ad a significant effect on the correlation coefficients. Stepwise regressio n analysis showed that protease was the variable that most frequently accou nted for the variations of gross N mineralization rate when included in the equation, and has the potential to be used as one of the predictors for N mineralization.