R. J. Buresh

Analytical methods in15N research

Although the stable isotope15N is an indispensable tool in research to trace the fate of fertilizer nitrogen in soil/plant systems, the analytical methods used in this research are time consuming and prone to many errors. This paper outlines the methods used in an international program of nitrogen research coordinated by the International Fertilizer Development Center (IFDC). The different steps in the digestion, distillation, and isotope ratio analysis of15 N-labeled soils, plant material, and fertilizers are described. Details on the use of a series of controls to check the precision and accuracy of the methods are also given. It is hoped that this comprehensive description of procedures will encourage the expanded and proper use of15N.

Nitrogen Dynamics And Management In Rice-Legume Cropping Systems

Publisher Summary This chapter reviews nitrogen (N) dynamics in lowland rice fields with emphasis on how N dynamics are influenced by typical soil drying and wetting cycles, the influence of legumes on soil N transformations and N accretion in rice-based cropping systems, the N contribution of legumes to rice, and the integrated management of legume N and industrial fertilizer N for rice. Most research on N contributions from legumes in the tropics has been focused on short-duration legumes grown and subsequently incorporated solely as green manures immediately before the monsoon rice crop. Nitrification during nonflooded periods among rice crops and then subsequent denitrification when soil is flooded for rice may be an important avenue for N loss. The N contribution of legumes in lowland rice-legume sequences, as in upland crop-legume sequences, depend on the quantity of legume N derived from N 2 fixation, the NHI, the proportion of legume N mineralized, and the efficiency of use of this mineralized N by the succeeding crop.

Denitrification losses from puddled rice soils in the tropics

SummaryAlthough denitrification has long been considered a major loss mechanism for N fertilizer applied to lowland rice (Oryza sativa L.) soils, direct field measurements of denitrification losses from puddled rice soils in the tropics have only been made recently. This paper summarizes the results of direct measurement and indirect estimation of denitrification losses from puddled rice fields and reviews the status of research methodology for measurement of denitrification in rice fields. The direct recovery of (N2+N2O)-15N from 15N-enriched urea has recently been measured at sites in the Philippines, Thailand, and Indonesia. In all 12 studies, recoveries of (N2+N2O)-15N ranged from less than 0.1 to 2.2% of the applied N. Total gaseous N losses, estimated by the 15N-balance technique, were much greater, ranging from 10 to 56% of the applied urea-N. Denitrification was limited by the nitrate supply rather than by available C, as indicated by the values for water-soluble soil organic C, floodwater (nitrate+nitrite)-N, and evolved (N2+N2O)-15N from added nitrate. In the absence of runoff and leaching losses, the amount of (N2+N2O)-15N evolved from 15N-labeled nitrate was consistently less than the unrecovered 15N in 15N balances with labeled nitrate, which presumably represented total denitrification losses. This finding indicates that the measured recoveries of (N2+N2O)-15N had underestimated the denitrification losses from urea. Even with a probable two-or threefold underestimation, direct measurements of (N2+N2O)-15N failed to confirm the appreciable denitrification losses often estimated by the indirect difference method. This method, which determines denitrification losses by the difference between total 15N loss and determined ammonia loss, is prone to high variability. Measurements of nitrate disappearance and 15N-balance studies suggest that nitrification-denitrification occurs under alternate soil drying and wetting conditions both during the rice cropping period and between rice crops. Research is needed to determine the magnitude of denitrification losses when soils are flooded and puddled for production of rice.

Evolution and soil entrapment of nitrogen gases formed by denitrification in flooded soil

Abstract The direct field measurement of N loss by denitrification in flooded soils has been a long-standing challenge. A field experiment was conducted in puddled, flooded soil without plants to ascertain whether the directly measured evolution of (N2 + N2O)-15N from 15N-labeled nitrate applied to floodwater was an accurate measure of denitrification loss. The evolution of (N2 + N2O)-15N was determined from the 15N content of air samples collected in a chamber placed over the floodwater. The evolved (N2 + N2O)-15N increased for 2 to 3 days following addition of either 3.5, 6.9, or 10.4 kg nitrate-N-ha-1 and then decreased on succeeding days. Evolved (N2 + N2O)-15N correlated (r = 0.86) with floodwater (NO3 - + NO2 -)-N measured 48 h earlier. The total (N2 + N2O)-15N evolved over the 10 days following nitrate addition ranged from 20 to 25% of the added N. The added 15N not recovered in the soil and floodwater after 10 days ranged from 48 to 64% of the added N. This unrecovered 15N presumably represented d...

Ammonia volatilization from point-placed urea in upland, sandy soils

The upland fertilization practice in Africa of placing N fertilizer below the soil surface near the plant might be facilitated through use of urea supergranules (USG). Since little is known about N losses from point-placed urea on light-textured African soils, laboratory studies were conducted in a forced-draft system to determine (a) the influence of soil properties on ammonia loss from USG and (b) to compare N loss from USG with that from broadcast N sources. Ammonia loss from 1.1 g USG placed at a 4-cm soil depth ranged from 2.9 to 62% of the added N on six light-textured soils. Ammonia loss was correlated with soil clay content (r = −0.93**) but not with pH. A more detailed study on a soil from Niger revealed significantly less ammonia loss from either surfaced applied urea (18%) or surface-applied calcium ammonium nitrate (7%) than from USG placed at a 4-cm depth (67%). Amendment of surface-applied urea with 1.7% phenyl phosphorodiamidate (PPD), a urease inhibitor, essentially eliminated ammonia loss (1.9%). An15N balance confirmed that ammonia volatilization was the major loss mechanism for all N sources. The results suggest that point-placed urea may be prone to ammonia volatilization loss on light-textured African soils moistened by frequent light rainfall. In such cases, broadcast application of urea, CAN, or urea amended with PPD may be less prone to N loss.

Nitrogen losses in puddled soils as affected by timing of water deficit and nitrogen fertilization

Erratic rainfall in rainfed lowlands and inadequate water supply in irrigated lowlands can results in alternate soil drying and flooding during a rice (Oryza sativa L.) cropping period. Effects of alternate soil drying and flooding on N loss by nitrification-denitrification have been inconsistent in previous field research. To determine the effects of water deficit and urea timing on soil NO3 and NH4, floodwater NO3, and N loss from added 15N-labeled urea, a field experiment was conducted for 2 yr on an Andaqueptic Haplaquoll in the Philippines. Water regimes were continuously flooded, not irrigated from 15 to 35 d after transplanting (DT), or not irrigated from 41 to 63 DT. The nitrogen treatments in factorial combination with water regimes were no applied N and 80 kg urea-N ha−1, either applied half basally and half at 37 DT or half at 11 DT and half at 65 DT. Water deficit at 15 to 35 DT and 41 to 63 DT, compared with continuous soil flooding, significantly reduced extractable NH4 in the top 30-cm soil layer and resulted in significant but small (<1.0 kg N ha−1) soil NO3 accumulations. Soil NO3, which accumulated during the water deficit, rapidly disappeared after reflooding. Water deficit at 15 to 35 DT, unlike that at 41 to 63 DT, increased the gaseous loss of added urea N as determined from unrecovered 15N in 15N balances. The results indicate that application of urea to young rice in saturated or flooded soil results in large, rapid losses of N (mean = 35% of applied N), presumably by NH3 volatilization. Subsequent soil drying and flooding during the vegetative growth phase can result in additional N loss (mean = 14% of applied N), presumably by nitrification-denitrification. This additional N loss due to soil drying and flooding decreases with increasing crop age, apparently because of increased competition by rice with soil microorganisms for NH4 and NO3.

Fate of fertilizer nitrogen applied to wheat under simulated Mediterranean environmental conditions.

Triticum aestivumThe fate of fertilizer nitrogen applied to dryland wheat was studied in the greenhouse under simulated Mediterranian-type climatic conditions. Wheat, L., was grown in 76-cm-deep pots, each containing 50–70 kg of soil, and subjected to different watering regimes. Two calcareous clay soils were used in the experiments, Uvalde clay (Aridic Calciustoll) and Vernon clay (Typic Ustochrept). Fertilizer nitrogen balance studies were conducted using various15N-labeled nitrogen sources, including ammonium nitrate, urea, and urea amended with urea phosphate, phenyl phosphorodiamidate (a urease inhibitor), and dicyandiamide (a nitrification inhibitor). Wheat yields were most significantly affected by available water. With additional water during the growing period, the recovery of fertilizer nitrogen by wheat increased and the fraction of fertilizer nitrogen remaining in the soil decreased. In the driest regimes, from 40 to 65% of the fertilizer nitrogen remained in the soils. In most experiments the gaseous loss of fertilizer nitrogen, as estimated from unaccounted for15N, was not significantly affected by water regime. The15N not accounted for in the plant and the soil at harvest ranged from 12 to 25% for ammonium nitrate and from 12 to 38% for regular urea. Direct measurement of labeled ammonia loss from soil indicated that ammonia volatilization probably was the main N loss mechanism. Low unaccounted-for15N from nitrate-labeled ammonium nitrate, 4 to 10%, indicated that N losses due to denitrification, gaseous loss from plants, or shedding of anthers and pollen were small or negligible. Amendment of urea with urea phosphate to form a 36% N and 7.3% P product was ineffective in reducing N loss. Dicyandiamide did not reduce N loss from urea presumably because N was not leached from the sealed pots and denitrification was insignificant. Amendment of urea with 2% phenyl phosphorodiamidate reduced N loss significantly. However, band placement of urea at as 2-cm soil depth was more effective in reducing N loss than was amendment of broadcast urea with phenyl phosphorodiamidate.

Increasing nutrient use efficiency in rice with changing needs

The use of N fertilizer in Asia has increased from 24 to 39% of the worlds total consumption between 1973 and 1987/88. Approximately 60% of the N fertilizer consumed in Asia is used on rice (Oryza sativa L.). However, the N applied to rice, primarily as urea, is not effectively utilized by the crop. Ammonia volatilization is recognized as a major mechanism of N loss, causing ineffective N utilization. Basal incorporation of urea without standing water; deep placement of urea; and modification of urea with algicides, urea inhibitors or coatings are strategies to reduce ammonia loss. Loss of N by nitrification-denitrification may be a serious problem particularly when soil is dried between rice crops, then flooded for the subsequent rice crop. The use of organic N sources, such as green manure and organic manures, as partial substitutes of inorganic N fertilizer is receiving renewed research interest.The use of P fertilizers for rice is most necessary on Oxisols and Ultisols with high P-fixing capacity. Phosphate rock and partially acidulated phosphate rock are alternatives to soluble P sources used on these soils. Response to K is normally highest on light-textured soils. The limited available information suggests that in lowland rice-upland crop rotations, K fertilizers should be applied to the non-rice crop. Zinc deficiency can be overcome through (a) use of varieties more tolerant to zinc deficiency, (b) application of zinc sulfate, and (c) dipping seedling roots in a zinc oxide suspension.Increasing use of S-free fertilizers, intensive cropping, and use of high yielding rice varieties have led to S deficiency in many rice growing countries. Sulfur deficiency can be corrected by applying S-containing materials even with elemental S. Residual effects have also been reported even at a low rate of 20 kg S/ha. Thus, S does not need to be applied every season.To address the unresolved integrated nutrient management issues, both strategic and applied research are required on interacting soil-plant-water-nutrient-climate processes. Long-term sustainability is one of the parameters that must be considered in evaluating the desirability of alternative rice technologies.

Quantification of denitrification in flooded soils as affected by rice establishment method

Abstract The recovery of 15 N-labeled N 2 and N 2 O evolved from added 15 N-labeled fertilizers can reportedly underestimate N loss by denitrification in puddled, flooded soils. Field research was conducted for 2 yr in the Philippines to determine whether the evolution of N gases formed by denitrification is influenced by a rice ( Oryza sativa L.) establishment method. Quantification of denitrification was assessed by comparing the cumulative recovery of (N 2 + N 2 O)- 15 N evolved from added 15 N-labeled nitrate (5 kg N ha −1 ) with the total N loss by denitrification, which was estimated from the added NO 3 − - 15 N not recovered in the 15 N balance at the conclusion of the 20 day gas collection period. In both years, rice was established by either transplanting (TPR) at 20 × 20 cm spacing between hills with three seedlings per hill (75 plants m −2 ) or wet broadcast seeding of germinated seed (BSR) with a plant population of 370 m −2 in 1989 and 250 m −2 in 1990. In 1990, wet seeding of germinated seed in 20-cm-wide rows (RSR) with 250 plants m −2 was also included. Nitrate addition and gas collection commenced at 11 days after transplanting (DT) 20 or 21-days-old seedlings and 21 days after wet sowing (DS). In 1989, the recovery of added N as evolved (N 2 + N 2 O)- 15 N (54% for BSR and 47% for TPR) was comparable to total denitrification loss for BSR (53%) but not for TPR (61%). In 1990, the recovery of evolved (N 2 + N 2 O)- 15 N (32% for TPR, 61% for BSR and 40% for RSR) underestimated denitrification loss (72% for TPR and 73% for BSR and RSR), but to a lesser extent for BSR than for TPR and RSR. The greater recovery of added N as evolved (N 2 + N 2 O)- 15 N when chambers were placed over transplated rice rather than between plants (47 and 40%, respectively) suggested that rice was a conduit for transport of gas from soil. Underestimation of denitrification was attributed to entrapment of N-gases in soil. Results suggest that young BSR, because of high plant density and rapid extension of roots into the soil volume, was a more effective conduit for gas transport than TPR or RSR.

Nitrogen-15 and sulfur-35 balances for fertilizers applied to transplanted rainfed lowland rice

A field experiment was conducted on a poorly-drained Aeric Paleaquult in northeastern Thailand to determine the effect of N and S fertilizers on yield of rainfed lowland rice (Oryza sativa L.) and to determine the fate of applied15N- and35S-labeled fertilizers. Rice yield and N uptake increased with applied N but not with applied S in either sulfate or elemental S (ES) form. Rice yield was statistically greater for deep placement of urea as urea supergranules (USG) than for all other N fertilizer treatments that included prilled urea (PU), urea amended with a urease inhibitor (phenyl phosphorodiamidate), and ammonium phosphate sulfate (16% N, 8.6% P).The applied15N-labeled urea (37 kg N ha−1) not recovered in the soil/plant system at crop maturity was 85% for basal incorporation, 53% for broadcast at 12 days after transplanting (DT), 27% for broadcast at 5–7 days before panicle initiation (DBPI), and 49% for broadcast at panicle initiation (PI). The basal incorporated S (30 kg ha−1) not recovered in the soil/plant system at crop maturity was 37% for sulfate applied as single superphosphate (SSP) and 34% for ES applied as granulated triple superphosphate fortified with S (S/GTSP). Some basal incorporated15N and35S and some broadcast15N at PI was lost by runoff. Heavy rainfall at 3–4 days after basal N incorporation and at 1 day after PI resulted in water flow from rice fields at higher elevation and total inundation of the 0.15-m-high15N and35S microplot borders. Unrecovered15N was only 14% for 75 kg urea-N ha−1 deep placed as USG at transplanting. This low N loss from USG indicated that leaching was not a major N loss mechanism and that deep placement was relatively effective in preventing runoff loss.In order to assess the susceptibility of fertilizer-S to runoff loss, a subsequent field experiment was conducted to monitor35S activity in floodwater for 42 days after basal incorporation of SSP and S/GTSP. Maximum35S recoveries in the floodwater were 19% for SSP after 7 days and 7% for S/GTSP after 1 day. Recovery of35S in floodwater after 14 days was 12% for SSP and 3% for S/GTSP.This research suggests that on poorly drained soils with a low sorption capacity, a sizeable fraction of the fertilizer S and N remains in the floodwater following application. Runoff could then be an important mechanism of nutrient loss in areas with high probability for inundation following intense rainfall.