Studies were conducted on denitrification losses from irrigated maize and wheat fields. The study site at the Nuclear Institute for Agriculture and Biology, Faisalabad comprises a well-drained sandy-clay-loam soil that has been under maize-wheat cropping system for the past ten years. Direct measurements of denitrification loss were made for one year by acetylene inhibition technique. Effect of different fertilizer treatments on denitrification and related parameters was also studied for one year. Effect of plants on denitrification was investigated in a field experiment with maize.
From maize field fertilized at recommended rate (100 kg urea-N ha-1), denitrification loss measured by acetylene inhibition/soil cover method amounted 2.7 kg N ha-1 during the growing season (24 August-26 October). Denitrification peaks during initial two irrigation cycles to maize ranged between 23-27 g N ha-1 h-1 and were 7-14 times higher than those recorded during subsequent irrigations. Consequently, most (87%) of the total denitrification loss during maize season occurred during first two irrigation cycles. From the wheat field fertilized at recommended rate (100 kg urea-N ha-1), denitrification loss was measured by acetylene inhibition/soil cover and acetylene inhibition/soil core methods. On 26 of the 45 sampling occasions, denitrification rate was 2-28 times higher with soil core than soil cover method. Total denitrification loss during wheat growing period (22 November-20 April) amounted 1.14 and 3.39 kg N ha-1 by soil cover and soil core methods, respectively. Peaks of denitrification during different irrigation cycles to wheat ranged from 1.3 to 5.6 g K ha-1 h-1 by soil cover method whereas with soil core method, these peaks ranged between 0.7 and 23.9 g N ha-1 h-1. Under wheat. higher rates were recorded during initial three irrigation cycles which collectively contributed to 70 and 88% of the total denitrification loss measured by soil cover and soil core methods, respectively. In a 96-hour irrigation cycle, N2O surface flux was compared with the N2O concentrations in different soil profile layers. Results revealed that all the N2O produced under a C2H2-treated site was not collected by the soil cover; its downward and perhaps lateral movement also took place.
Effect of different fertilizer treatments on denitrification under wheat-maize cropping system was studied by acetylene inhibition/soil core method using intact soil cores from arable (0-15 cm) soil layer. Treatments included: N-100 (urea at 100 kg N ha-1 a-1 N-200 (urea at 200 kg N ha-1 a-1), FYM-16 (farmyard manure at 16 t ha-1 a-1), FYM-32 (farmyard manure at 32 t ha-1 a-1) and control (unfertilized). To mineral treatments (N-100 and N-200), half of the stated dose of urea was applied to each crop whereas in farmyard manure treatments (FYM-16 and FYM-32), all the farmyard manure was applied at the time of sowing wheat. These treatments have been in effect for the past ten years with wheat (November-April) followed by maize fodder (August-October). During wheat growing season, average denitrification rate was highest in N-200 (83 g N ha-1 d-1) followed by FYM-32 (60 g N ha-1 d-1), N-100 (51 g N ha-1 d-1), FYM-16 (47 g N ha-1 d-1) and the control (33 g N ha-1 d-1). Similar trend prevailed for the total denitrification loss during wheat season which amounted 3.94, 3.15, 2.44, 2.15 and 1.51 kg N ha-1 for N-200, FYM-32, N-100, FYM-16 and the control respectively. Averaged across sampling dates during maize growing season, farmyard manure-treated plots de nitrified at higher rates (FYM-32, 525 g N ha-1 d-1; FYM-16, 408 g N ha-1 d-1) than urea-treated (N-200, 372 g N ha-1 d-1; N-100, 262 g N ha-1 d-1) and the control (203 g N ha-1 d-1). Similarly, total N de nitrified during maize season was maximum for FYM-32 (10.8 kg N ha-1) followed by FYM-16 (8.5 kg N ha-1), N-200 (7.9 kg N ha-1), N-l00 (5.5 kg N ha-1) and the control (4.3 kg N ha-1). Although. the growing period of maize fodder was much shorter than that of wheat (60 versus 150 days), the amount of N de nitrified in different fertilizer treatments was 2-4 fold higher under maize. During active period of denitrification (initial three irrigation cycles), soil NO3-N and temperature remained higher under maize than wheat thus supporting higher denitrification rates during maize season.
Simple linear regression analyses were performed to assess the influence of different environmental factors on denitrification. Combining the data from both crops (n=130) revealed highly significant correlation of denitrification with soil NO3-N (r=0.519;p< 0.001), water-filled pore space (r=0.495;P < 0.001) and soil temperature (r=0.261;P< 0.001). Under wheat (n = 75), soil NO3-N still being the most important factor governing denitrification, was followed by water-filled pore space. However, under maize (n = 55); water-filled pore space was the most important factor regulating denitrification and was followed by soil NO3-N.
Combined effect of different edaphic factors on denitrification was evaluated by multiple regression analyses. The best multiple-factor model obtained for combined crop data (n = 130) accounted for 56% of the variability in denitrification (p < 0.01) with water-filled pore space, NO3-N and respiration as the independent variables. For wheat data (n = 75), water-filled pore space combined with NO3-N explained 52% of the variation in denitrification (p < 0.01). The best multiple-factor model obtained for maize crop data (n =55) included water-filled pore space and respiration as the independent variables and accounted for 70% of the variability in denitrification (p < 0.01).
Total fertilizer nitrogen losses as measured by 15N-balance method, were 33.1 and 39.2 kg N ha-l from wheat and maize fields, respectively. Maximum denitrification loss nom similarly treated (100 kg N ha-1 crop-1) wheat and maize fields as measured by C2H2-inhibition technique, was 3.9 and 7.9 kg N ha-1, respectively. Subtracting the N de nitrified from unfertilized fields of wheat (1.5 kg N ha-1) and maize (4.3 kg N ha-1), only 2 to 4% of the applied fertilizer N was lost due to denitrification. Therefore, denitrification contributed to only 7 and 9% of the total fertilizer N loss from wheat-and maize fields, respectively. Assuming that denitrification was not underestimated by C2H2-inhibition method, the higher N losses measured by 15N-balance method may be attributed to losses other than denitrification, most probably NH3-volatilization.
Effect of plants on denitrification was studied by growing maize under irrigated field conditions. Treatments included: planted (Tl), unplanted (T2) and planted + nitrate (T3). To all treatments, urea was applied at 150 kg N ha-1 whereas T3 occasionally received additional N inputs as calcium nitrate to equalize its soil NO3-N with that of the unplanted. At almost all stages of crop growth, carbon availability to soil heterotrophs as indicated by soil respiration rate, aerobically mineralizable C and microbial biomass carrying capacity, was higher in planted than unplanted soils. Similarly, under denitrifying conditions, planted soils generally showed higher carbon availability as indicated by higher denitrification potential of the planted than unplanted soils. Comparing planted (TI) and unplanted soils, plants stimulated denitrification only at early growth stage when NO3-N concentration was high. However, when plant uptake decreased the soil NO3-N content, the planted soil (Tl) always denitrified at lower rates than the unplanted. The positive effect of plants on denitrification could be demonstrated at all growth stages when unplanted soil is compared with the planted soil receiving additional inputs of NO3-N (T3). These results indicate that plants have potential to increase denitrification through decreased O2 in soil and by supplying C substrate to denitrifiers. However, such stimulatory effect of plants could only be observed when sufficient NO3-N is present in soil to support denitrifying populations; the situation which may exist particularly at early stages of growth when N requirement of crop is low. During later growth phase, the rapid uptake by crop plants leaves a little NO3-N in the soil and thus denitrification may be substantially reduced in the presence of plants. Nitrous oxide emissions from irrigated maize and wheat fields were also measured by soil cover method. These measurements were concurrent to the denitrification measurements on similar but C2H2-treated site. During maize growing period (24 August-26 October), N2O flux ranged between -0.9 and 1.5 g N ha-1 h-1 with peaks during different irrigation cycles varying from 0.1-1.5g N ha-1 h-1. Nitrous oxide emissions during wheat growing period (22 November-20 April) varied between -0.9 and 3.3 g N ha-1 h-1 while peaks ranged between 0.05 and 3.3 g N ha-1 h-1. Integrated over the whole crop periods, net N2O emissions amounted 0.16 and 0.50 kg N ha-1 during maize and wheat seasons, respectively. Nitrous oxide sink activity was also recorded on several sampling occasions both under maize and wheat. Total N2O sink activity during maize and wheat growing period was 0.04 and 0.06 kg N ha'l, respectively. Comparing with the concurrently measured denitrification loss, N2O constituted 6 and 44% of the total gaseous N evolved during maize and wheat seasons, respectively. Although it was not possible to differentiate between the N2O evolved from denitrification and that from nitrification, the latter appeared to be the main source during 2nd irrigation cycle to wheat. This was evidenced from the N2O mole fraction in denitrification gaseous N products which remained much above unity during the 2nd irrigation cycle to wheat. The N2O from nitrification was responsible for the relatively high proportion of N2O in denitrification gaseous N products during wheat growing season.
Results of the present study imply that denitrification is not an important N loss mechanism in this well-drained sandy-clay-loam under irrigated maize-wheat cropping system receiving fertilizer inputs in the range of 100-200 kg N ha-1 a-1. The annual denitrification loss in terms of the applied N fertilizer ranged between 2-3 and 4-5% for urea and farmyard manure-treated fields, respectively. The major reason for the observed low figures of denitrification is that conditions of high soil moisture combined with adequate supply of NO3-N to favour high denitrification rates, were restricted to only few events during the whole year.