Denitrification potential in paddy soils of Swat and Peshawar valleys[Pakistan]

A series of laboratory incubation experiments were conducted to determinethe extent of denitrification potential and its possible correlations with soilproperties in nine soil samples collected from paddy fields in Swat, RiverValley and Peshawar Valley. Soil sample were at 0-10, 10-30, 30-60 and 60-90cm depths. Part of soil samples was analyzed for important soil characteristicsincluding organic C, total N, pH, EC, and soil texture and the remaining soilsample was saved for denitrification measurement. Denitrification was measuredby, NO3-N disappearance method. For denitrification measurement soil sampleswere amended with KNO3 at 200, mg N kg-1 moist (20-30%) soil and, KH2PO4 at220 mg P kg-1 moist soil and incubated anaerobically for 0, 2 4,8, 16, and 32days at 30 plus minus 1 degree C. At each incubation period the soil sampleswere analyzed for total mineral N, NH4-N, and NO3-N. Denitrification variedgreatly with soils. The maximum mean rate of N43-N disappearance of 9.32 mugg-1 d-1 was found in Amandara soil sample (Swat R. Valley) followed by 8.20mug in Chinar Korona 2, (Peshawar Valley). However, the minimum mean rate of3.32 mug g-1 d-1 was observed in Chakdara Chowk 1 (Swat R. Valley). Theresults showed that the rate of NO3-N disappearance was rapid initially andthen declined gradually with increasing incubation periods in all soil samples.The rate of denitrification at any incubation period was significantly greaterthan that at respective successive incubation. It varied from 32.01-1.37, 20.01-1.13 16.3-1.13, 12.74-0.91, and 8.76-059 mug g-1 d-1 at 2, 4, 8, 16, and d 32,respectively, in these soils. The rate of NO3-N disappearance was more in thesurface and low in the lower depths. It varied form 32.01-2.98 at 0-10, 18.00-0.89 at 10-30, 14.20- 0.83 at 30-60, and 11.07-0.60 mug g-1 d-1 at 60-90 cm in32 days of incubation. The rate of denitrification was significantly correlatedwith soil organic C (r =0.64, p less than 0.01) and total N (r 0.63, p less than0.01). However, C: N ratio showed no significant correlation with N03-Ndisappearance. Initial NO3-N level showed no correlation with the extent ofdenitrification in the study. pH did not show any significant correlation withthe rate of denitrification. EC showed positive significant correlation (r=0.55, pless than 0.01) with denitrification. Soil texture showed weak correlations withthe rate of denitrification. Clay was positively correlated (r = 0.35, p less than0.05) while sand was negatively correlated (r = -0.32, p less than 0.05).However, silt showed no correlation.