Vertical distribution of denitrification activity in an Andisol upland field and its relationship with dissolved organic carbon: Effect of long-term organic matter application

The denitrification enzyme activity (DEA) and dissolved organic carbon (DOC) concentration in soil from the plow layer to a depth of 5 m were measured in an Andisol upland field to which several types of organic matter had been continuously applied for 20 years. The DEA was measured using the acetylene inhibition method. The DEA values at the plow layer were 0.15-0.16 mmol N/kg per day in the chemical fertilizer plots. Higher DEA values were detected in all the plots to which organic matter had been applied: 0.19 mmol N/kg per day in the rice straw manure plot and 0.81 mmol N/kg per day in the pig dung plot. In contrast, lower DEA values were detected in the non-nitrogen and non-fertilizer plots, that is, 0.02 and 0.03 mmol N/kg per day, respectively. The DEA values correlated with total organic carbon (TOC) values, total nitrogen (T-N) values, nitrogen mineralization rate, and available P measured using the Bray No.2 test; a highly significant correlation coefficient was obtained for the DOC (pH 5.5-6.0) calibrated with pH-DOC relations. In all plots examined, the plow layers showed the highest DEA values, and these dropped abruptly between depths of 0.5 and 1.0 m. At depths lower than 1.0 m, the DEA ranged from 10E-4 to 10E-5 mmol N/kg per day in all plots despite remarkable differences in DEA in the plow layers among plots. These observations suggested that the application of organic matter enhances the organic carbon content and DEA values in the plow layer, the extent of which is largely influenced by the variety and quantity of the applied organic matter. However, the properties of the applied organic matter did not influence the organic carbon contents or the DEA values at the subsurface. Furthermore, our investigations confirmed that subsurface denitrification did not substantially decrease the nitrate concentration during downward infiltration.