Rehmat Ali

Nitrous oxide emissions from an irrigated sandy-clay loam cropped to maize and wheat

Abstract Nitrous oxide (N2O) emissions were measured from an irrigated sandy-clay loam cropped to maize and wheat, each receiving urea at 100 kg N ha–1. During the maize season (24 August–26 October), N2O emissions ranged between –0.94 and 1.53 g N ha–1 h–1 with peaks during different irrigation cycles (four) ranging between 0.08 and 1.53 g N ha–1 h–1. N2O sink activity during the maize season was recorded on 10 of the 29 sampling occasions and ranged between 0.18 and 0.94 g N ha–1 h–1. N2O emissions during the wheat season (22 November–20 April) varied between –0.85 and 3.27 g N ha–1 h–1, whereas peaks during different irrigation cycles (six) were in the range of 0.05–3.27 g N ha–1 h–1. N2O sink activity was recorded on 14 of the 41 samplings during the wheat season and ranged between 0.01 and 0.87 g N ha–1 h–1. Total N2O emissions were 0.16 and 0.49 kg N ha–1, whereas the total N2O sink activity was 0.04 and 0.06 kg N ha–1 during the maize and wheat seasons, respectively. N2O emissions under maize were significantly correlated with denitrification rate and soil NO3–-N but not with soil NH4+-N or soil temperature. Under wheat, however, N2O emissions showed a strong correlation with soil NH4+-N, soil NO3–-N and soil temperature but not with the denitrification rate. Under either crop, N2O emissions did not show a significant relationship with water-filled pore space or soil respiration.

Denitrification with and without maize plants (Zea mays L.) under irrigated field conditions

Abstract The study was conducted under irrigated field conditions to examine the effect of maize plants on denitrification. Both planted and unplanted field plots received 150kgNha–1 as urea. In a third treatment, which was also planted and received urea at 150kgNha–1, the soil nitrate N content was brought up to equal to that in the unplanted plots by applying additional doses of N as calcium nitrate. Soil cores were collected 24 and 72h after irrigation and the denitrification rate was measured by the acetylene inhibition method. Nitrate-N content, aerobically mineralizable C, microbial biomass carrying capacity and denitrification potential were also studied on field-moist soil. Maize plants grown under field conditions always had the potential to increase denitrification in conditions of both high and low water-filled porosity. When nitrate-N content of the planted soil decreased due to plant uptake, denitrification was reduced in the planted soils. However, when nitrate-N uptake by plants was compensated through additional doses of nitrate fertilizer, denitrification was always higher in planted than unplanted soil. The stimulatory effect of plants on denitrification was observed at both high and low soil nitrate-N concentrations, though it was more pronounced at high nitrate-N levels. The effect of plants on denitrification and related parameters was confined to the root zone.

Denitrification and total fertilizer-N losses from an irrigated cotton field

Abstract In a 2-year field study, denitrification loss was measured from an irrigated sandy-clay loam under cotton receiving urea-N at 158–173 kg ha–1. An acetylene inhibition-soil core method was employed for the direct measurement of denitrification, considering also the N2O entrapped in the soil. Taking into account the N2O evolved from soil cores and that entrapped in the soil, a total of 65.7 kg N ha–1 and 64.4 kg N ha–1 was lost due to denitrification during the 1995 and 1996 cotton-growing seasons, respectively. Most (>70%) of the denitrification loss occurred during June–August, a period characterized by high soil temperatures and heavy monsoon rains. On average, 35% of the denitrification-N2O was found entrapped in the soil and the amount of entrapped N2O was significantly correlated with head space N2O concentration and with water-filled pore space. 15N-balance during the 1996 growing season revealed a loss of 71.8 kg N ha–1. It was concluded that a substantial proportion of the fertilizer-N applied to irrigated cotton is lost under the semiarid subtropical climatic conditions prevailing in the Central Punjab region of Pakistan and that denitrification is the major N loss process under irrigated cotton in this region.

Dicyandiamide increases the fertilizer N loss from an alkaline calcareous soil treated with 15N-labelled urea under warm climate and under different crops

Using an alkaline calcareous soil, experiments were conducted to elucidate the effects of nitrification inhibitor dicyandiamide (DCD) on the fate of 15N-labelled urea applied to cotton, maize, and wheat under greenhouse conditions. Combined effects of DCD and two levels of wheat straw (applied to cotton) and of fertilizer application method (conventional broadcast vs. point injection in maize and wheat) on the recovery of the fertilizer N were also studied. High soil temperatures prevailed under cotton and maize, whereas the soil temperature was relatively moderate during the wheat growing season. The fertilizer N loss under cotton was lowest (44% of the applied) when urea was applied alone; the loss increased due to DCD (54%) or wheat straw (50–54%) and was highest (63–64%) when DCD and wheat straw were applied together. Under maize also, DCD increased the loss of the fertilizer N applied by the conventional method (51% without DCD vs. 66% with DCD) or by point injection (26% without DCD vs. 42% with DCD). With the conventional method under wheat, DCD had no effect on the fertilizer N loss (34–37% of the applied). The fertilizer N loss under wheat was least (16%) when urea solution was point-injected but increased (24–26%) due to DCD or/and when pH of the urea solution was reduced to 2. Besides, DCD significantly reduced the fertilizer N uptake and increased the fertilizer N immobilization in soil under cotton and maize. However, DCD applied in combination with a higher level of wheat straw significantly increased the cotton dry matter and N yields due to increased N availability from sources other than the fertilizer. The results suggested that the use of DCD may not be beneficial in alkaline calcareous soils and that point injection of urea solution without any amendment is more effective in conserving the fertilizer N as compared to the conventional broadcast method.

Nitrous oxide emission from an irrigated cotton field under semiarid subtropical conditions

In a 1-year study, quantification of nitrous oxide (N2O) emission was made from a flood-irrigated cotton field fertilized with urea at 100kg N ha−1 a−1. Measurements were made during the cotton-growing season (May–November) and the fallow period (December–April). Of the total 95 sampling dates, 77 showed positive N2O fluxes (range, 0.1 to 33.3g N ha−1 d−1), whereas negative fluxes (i.e., N2O sink activity) were recorded on 18 occasions (range, −0.1 to −2.2g N ha−1 d−1). Nitrous oxide sink activity was more frequently observed during the growing season (15 out of 57 sampling dates) as compared to the fallow period (3 out of 38 sampling dates). During the growing season, contribution of N2O to the denitrification gaseous N products was much less (average, 4%) as compared to that during the fallow period (average, 21%). Nitrous oxide emission integrated over the 6-month growing period amounted 324g N ha−1, whereas the corresponding figure for the 6-month fallow period was 648g N ha−1. Subtracting the N2O sink activity (30.3g N ha−1 and 3.8g N ha−1 during the growing season and fallow period, respectively), the net N2O emission amounted 938g N ha−1 a−1. Results suggested that high soil moisture and temperature prevailing under flood-irrigated cotton in the Central Punjab region of Pakistan though favor high denitrification rates, but are also conducive to N2O reduction thus leading to relatively low N2O emission.

Comparison of two versions of the acetylene inhibition/soil core method for measuring denitrification loss from an irrigated wheat field

Abstract Two versions of the acetylene inhibition (AI)/soil core method were compared for the measurement of denitrification loss from an irrigated wheat field receiving urea-N at a rate of 100 kg ha–1. With AI/soil core method A, the denitrification rate was measured by analysing the headspace N2O, followed by estimation of N2O dissolved in the solution phase using Bunsen absorption coefficients. With AI/soil core method B, N2O entrapped in the soil was measured in addition to that released from soil cores into the headspace of incubation vessels. In addition, the two methods were also compared for measurement of the soil respiration rate. Of the total N2O produced, 6–77% (average 40%) remained entrapped in the soil, whereas for CO2, the corresponding figures ranged from 12–65% (average 44%). The amount of the entrapped N2O was significantly correlated with the water-filled pore space (WFPS) and with the N2O concentration in the headspace, whereas CO2 entrapment was dependent on the headspace CO2 concentration but not on the WFPS. Due to the entrapment of N2O and CO2 in soil, the denitrification rate on several (18 of the 41) sampling dates, and soil respiration rate on almost all (27 of the 30) sampling dates were significantly higher with method B compared to method A. Averaged across sampling dates, the denitrification rate measured with method B (0.30 kg N ha–1 day–1) was twice the rate measured with method A, whereas the soil respiration rate measured with method B (34.9 kg C ha–1 day–1) was 1.6 times the rate measured with method A. Results of this study suggest that the N2O and CO2 entrapped in soil should also be measured to ensure the recovery of the gaseous products of denitrification by the soil core method.

4-Amino-1,2,4-triazole can be more effective than commercial nitrification inhibitors at high soil temperatures

Commercial nitrification inhibitors (NIs), namely nitrapyrin, 3,4-dimethylpyrazol phosphate (DMPP) and dicyandiamide (DCD), are ineffective at high temperatures. Therefore, it is imperative to explore new compounds that can be commercialised as effective NIs for warm climatic conditions. The aim of the present study was to compare the potential of 4-amino-1,2,4-triazole (ATC) with the two commercial NIs DMPP and DCD to delay nitrification of (NH4)2SO4 in an alkaline calcareous soil incubated under aerobic conditions at warm temperatures (35 and 25°C). Inhibitors were incorporated in (NH4)2SO4 granules and nitrification inhibition was calculated on the basis of net NH4+-N disappearance and net NO3–-N accumulation. At 35°C, the inhibitory effect of DCD and DMPP persisted only for 1 week, whereas ATC was effective up to 4 weeks. At 25°C, the inhibitory effect of ATC, DMPP and DCD was comparable. In another set of experiments, different concentrations of ATC (0.25–6% of N) were tested at three different temperatures (35, 25 and 18°C). At 35°C, ATC applied at 2% of N caused 63% inhibition for 2 weeks, whereas at a rate of 4–6% of N the inhibitory effect of ATC persisted up to 4 weeks (63–84% inhibition). At 25°C, ATC application at a rate of 1% of N caused 64% inhibition for 2 weeks; increasing the application rate to 2–6% of N prolonged the inhibitory effect up to 4 weeks (55–94% inhibition). At 18°C, a much lower concentration of ATC (0.25–0.5% of N) was required to achieve ≥50% inhibition for 2–4 weeks, whereas increasing the application rate to 2% of N caused 93% inhibition for 4 weeks. The results of the present study suggest that although commercially available NIs are ineffective at high summer temperatures, ATC may have the potential to be commercialised as an effective NI for warm as well as moderate climatic conditions.