Effect of organic and mineral fertilizers on N-use by wheat under different irrigation frequencies

A field trial was established in Errachidia, southern Morocco, to investigate the interaction between wheat residue management and mineral 15N-labelled ammonium sulphate, under different irrigation treatments, applied to wheat (Triticum durum var. Karim). In treatments I1, I2, I3 and I4, plots were irrigated every 10, 15, 21 and 30 days. Each plot contained three sub-plots that received three fertilization treatments: T1 received 42 kg N ha-1 of ammonium sulphate before seedling, 42 kg N ha-1 of ammonium sulphate labelled with 9.764 at % 15N excess at tillering and 84 N kg ha-1 of ammonium sulphate at flowering; T2 received 42 kg N ha-1 of ammonium sulphate labelled with 9.764 at % 15N excess at seedling, 42 kg N ha-1 at tillering and 42 kg N ha-1 at flowering; T3 received 4800 kg ha-1 of wheat residue labelled with 1.504 at % 15N excess and 42 kg N ha-1 of ammonium sulphate before seedling and 42 kg N ha-1 of ammonium sulphate at flowering. Nitrogen fertilization with 168 kg N ha-1 did no significantly increase grain and straw yields in comparison to the 126 kg N ha-1 application. The combination of the organic input and supplementary application of mineral fertilizer N has been found as a more attractive management option. For all irrigation treatments, the % recovery of N in the whole plant was higher in plants that received 15N at tillering (63%, 49% respectively for irrigation intervals between 10 and 30 d) than in plants that received 15N just after seeding (28% for irrigation each 10- and 30-d intervals). For the irrigation treatment each 10 and 15 days, the 15N was mainly recovered by the grain for all fertilization treatments, whereas for irrigation treatment each 30 days, the grain and straw recovered nearly equal amounts of fertilizer. For grain and straw of wheat, nitrogen in the plant derived from the fertilizer was low, while most of the N was derived from the soil for all irrigation and fertilization treatments. The % nitrogen in the plant derived from the fertilizer values showed no significant difference between the different plant parts. The results suggested a dominant influence of moisture availability on the fertilizer N uptake by wheat. Under dry conditions the losses of N can be allotted to denitrification and volatilisation.     

Biomass production and nutrient cycling in Eucalyptus short rotation energy forests in New Zealand. I: Biomass and nutrient accumulation

Accumulation of biomass and nutrients (N, P, K, Ca, Mg and Mn) was measured during the first 3-year rotation of three Eucalyptus short rotation forest species (E. botryoides, E. globulus and E. ovata) irrigated with meatworks effluent compared with no irrigation. E. globulus had the highest biomass and nutrient accumulation either irrigated with effluent or without irrigation. After 3-year growth, E. globulus stands irrigated with effluent accumulated 72 oven dry t/ha of above-ground total biomass with a total of 651 kg N, 55 kg P, 393 kg K, 251 kg Ca, 35 kg Mg and 67 kg Mn. Effluent irrigation increased the accumulation of biomass, N, P, K and Mn, but tended to reduce the leaf area index and leaf biomass, and decreased the accumulation of Ca and Mg.     

Effect of irrigation frequency and nitrogen on groundnut yield and nutrient uptake

Summary Field experiments were conducted during 1979 and 1980 summer seasons on sandy loam soils of low moisture retentive capacity to study the effect of high frequency irrigation at different levels of N on groundnut yield and nutrient uptake (NPK). Four irrigation frequencies (irrigation at 2, 4, 6 and 8 cm cumulative can evaporation, corresponding to irrigation once in 3, 5, 7 and 10 days respectively) and four levels of nitrogen (0, 20, 40 and 60 kg N/ha) were tested in a factorial randomized block design with three replications. Pod yield of groundnut was maximum (3,293 kg/ha) when irrigations were scheduled at 4 cm cumulative can evaporation (once in 5 days). Addition of N did not increase the pod yield. N and P uptake by the crop was maximum (180 kg N and 18 kg P/ha) with high frequency irrigation of scheduling irrigation at 4 cm cumulative can evaporation. Highest uptake of N (183 kg/ha) and P (19 kg/ha) was with a combination of 20 kg N/ha and high frequency irrigation (4 cm CCE). K uptake was low with low irrigation frequency, while it was highest (67 kg K/ha) at 20 kg N/ha.     

Denitrification and total N losses from an irrigated sandy-clay loam under maize–wheat cropping system

Denitrification and total N losses were quantified from an irrigated field cropped to maize and wheat, each receiving urea at 100 kg N ha^-1. During the maize growing season (60 days), the denitrification loss measured directly by acetylene inhibition-soil cover method amounted 2.72 kg N ha^-1 whereas total N loss measured by ¹⁵N balance was 39 kg ha^-1. Most (87%) of the denitrification loss under maize occurred during the first two irrigation cycles. During the wheat growing season (150 days), the denitrification loss directly measured by acetylene inhibition-soil cover and acetylene inhibition-soil core methods was 1.14 and 3.39 kg N ha^-1, respectively in contrast to 33 kg N ha^-1 loss measured by ¹⁵N balance. Most (70-88%) of the denitrification loss under wheat occurred during the first three irrigation cycles. Soil moisture and NO ₃ ^- -N were the major factors limiting denitrification under both crops. Higher N losses measured by ¹⁵N balance than C₂H₂ inhibition method were perhaps due to underestimation of denitrification by C₂H₂ inhibition method and losses other than denitrification, most probably NH₃ volatilization.     

Nutrient management programs,nitrogen fertilizer practices,and groundwater quality in Nebraska's Central Platte Valley (U.S.), 1989-1998

Given the societal concern about groundwater pollution from agricultural sources, public programs have been proposed or implemented to change farmer behavior with respect to nutrient use and management. However, few of these programs designed to change farmer behavior have been evaluated due to the lack of detailed data over an appropriate time frame. The Central Platte Natural Resources District (CPNRD) in Nebraska has identified an intensively cultivated, irrigated area with average groundwater nitrate-nitrogen (N) levels about double the EPA"s safe drinking water standard. The CPNRD implemented a joint education and regulatory N management program in the mid-1980s to reduce groundwater N. This analysis reports N use and management, yield, and groundwater nitrate trends in the CPNRD for nearly 3000 continuous-corn fields from 1989 to 1998, where producers faced limits on the timing of N fertilizer application but no limits on amounts. Groundwater nitrate levels showed modest improvement over the 10 years of this analysis, falling from the 1989-1993 average of 18.9 to 18.1 mg/l during 1994-1998. The availability of N in excess of crop needs was clearly documented by the CPNRD data and was related to optimistic yield goals, irrigation water use above expected levels, and lack of adherence to commercial fertilizer application guidelines. Over the 10-year period of this analysis, producers reported harvesting an annual average of 9729 kg/ha, 1569 kg/ha (14%) below the average yield goal. During 1989-1998, producers reported annually applying an average of 162.5 kg/ha of commercial N fertilizer, 15.7 kg/ha (10%) above the guideline level. Including the N contribution from irrigation water, the potential N contribution to the environment (total N available less estimated crop use) was estimated at 71.7 kg/ha. This is an estimate of the nitrates available for denitrification, volatilization, runoff, future soil N, and leaching to groundwater. On average, between 1989-1993 and 1994-1998, producers more closely followed CPNRD N fertilizer recommendations and increased their use of postemerge N applications--an indication of improved synchrony between N availability and crop uptake.     

Long fallow periods: The key to sustenance of soil fertility?

Changes in N, P, K, Ca and Mg in soil and rice plants were investigated during a cropping season following a long fallow period in a system of traditional cultivation practised for several centuries, under a village tank irrigation system. Soil, N, P, K, Ca and Mg were not found to be deficient for rice production throughout the season. Flooding did not produce toxic levels of Fe and Na and soil pH remained at 6.4 during the season. The average grain yield (3.5 t/ha) without any addition of chemical fertilizer was almost the same as that from fields under major irrigation systems where fertilizer application (less than the recommended level) was common. The sustainability of soil fertility under the traditional system of rice cultivation appears to be dependent upon long fallow periods. The natural build-up of soil fertility during a three-year fallow was evidently adequate to support a good growth of the crop which produced a yield comparable to that obtained in chemically fertilized, more intensively cropped rice fields under major irrigation systems.     

Gaseous nitrogen losses from field plots grown with pea (Pisum sativum L.) or spring barley (Hordeum vulgare L.) estimated by 15N mass balance and acetylene inhibition techniques

In a mass balance of ¹⁵N-labelled nitrate added to soil grown with pea or barley, denitrification estimates using the acetylene-inhibition technique were compared with unaccounted for ¹⁵N. During the growth season of 1989, which was drier than average, N losses due to denitrification estimated by the acetylene-inhibition technique were negligible. A substantial amount of fertilizer N was unaccounted for by the ¹⁵N mass balance, especially in the pea plots. The loss took place during the period of grain-filling in which no leaching occurred, and was accompanied by a decrease in ¹⁵N content of the plants. Volatilization of ammonia from the aerial parts of the plants is a possible explanation of the observed loss. An estimation of denitrification relying only on the ¹⁵N mass balance would have resulted in an overestimation of denitrification.     

Nitrogen inputs and outputs for New Zealand in 2001 at national and regional scales

The Nanjing Declaration on Nitrogen Management, signed in Nanjing in October 2004, calls for national governments to optimize N management by several strategies including assessment of N cycles. Here we develop a first N budget for New Zealand (267,000 km²), at both national and regional scales. The national inputs are estimated to be 36.5 kg/ha, mainly from biological N fixation, but also increasingly from fertilizer application and atmospheric deposition. The outputs are estimated at 40.5 kg/ha. Biological N fixation from legumes in pasture was the most important input in most regions. Exceptions were Auckland, with a large urban population, and the West Coast of the South Island, with large tracts of rain forest. Outputs were distributed in the order leaching > ammonia volatilisation > erosion = produce = denitrification. These outputs are very different from global averages because of the large numbers of grazing animals on pasture. A large loss occurs between the subsoil and the oceans, and further research is needed to identify these pathways. Riverine export of N was generally well correlated with inputs.     

Nodulation,accumulation and redistribution of nitrogen in soybean (Glycine max (L.) Merrill) as influenced by seed inoculation and scheduling of irrigation

Summary A field experiment was conducted on soybean (Glycine max (L.) Merrill) with a view to find out the effect of seed inoculation and scheduling of irrigation on nodulation, accumulation and re-distribution of nitrogen in plant tops and soil. The eight treatment combinations consists of two seed inoculations,viz. uninoculated and inoculated with rhizobium culture, and four irrigation schedules,viz. irrigation water to the cumulative pan evaporation (IW/CPE) ratio of 0.5, 0.7, 0.9 and a control (rainfed). Seed inoculation by, rhizobium culture increased the number, dry-weight and N content of nodules per plant. Inoculation of seeds also increased the N accumulation rate in plant top and it was 2.48 kg/ha/day during the flower-initiation to the pod-initiation stage (30–60 days interval). At harvest, 32.2, 47.8 and 26.2 kg N/ha was re-distributed from the stems, leaves and pods-wall of inoculated plants to the grains, respectively. A total of 186.5 kg N/ha was harvested and 64.7 kg N/ha, was accumulated in soil under the inoculated condition.Scheduling of irrigation at 0.7 IW/CPE proved better, than other irrigation schedules and helped in increasing the nodulation, nitrogen accumulation and grain yield. As compared to control, 8.4, 17.8 and 18.4 kg more of N/ha was redistributed from the stems, leaves and pods-wall respectively when the irrigations were scheduled at 0.7 IW/CPE ratio. Under this irrigation schedule the total N harvest was 200.1 kg/ha while the total N increased by 55.9 kg over that present in soil at the time of sowing.     

Total denitrification and the ratio between N2O and N2 during the growth of spring barley

Summary Total denitrification (N₂O+N₂) and nitrous oxide emission were measured on intact soil cores using the acetylene inhibition technique.Total denitrification from the depth 0–8 cm during the growth period from April to August was 7 kg N/ha from plots supplied with 30 kg N/ha and 19 kg N/ha from plots supplied with 120 kg N/ha. The amounts of precipitation, plant growth, and N application were found to affect the denitrification rate. These factors also affected the ratio (N₂O+N₂)/N₂O, which varied from 1.0 to 7.2. Plant growth and precipitation increased the proportion of N₂ produced, whereas a high nitrate content increased the proportion of N₂O.     

Nitrogen relationships in intensively managed temperate grasslands

Summary Most studies of N relationships in grassland have used cut swards. These have shown that for annual inputs of 200 to 400 kg N/ha from fertilizer or fixation, 55 to 80% of the N is recovered in harvested herbage. Generally, no more than 5 to 15% is lost through leaching and denitrification with most of the remaining N incorporated into soil organic matter. The relatively high efficiency of N use by cut swards reflects rapid uptake of N and the removal of a large part of the input in herbage. Inclusion of the grazing ruminant alters the efficiency of N use; only 5–20% of the input is recovered in meat or milk, and 75 to 90% of the N ingested is excreted, mainly as urea in urine. Application of N in urine ranges from 30–100 g/m². Too much N is voided for effective recovery by the sward whilst soils usually contain insufficient C to allow appreciable immobilization. The surfeit is lost. Hydrolysis of urea is usually complete within 24 h of urine deposition. For urine-treated pasture in New Zealand (NZ) losses by NH₃ volatilization of up to 66% of applied N are found during warm dry weather, with an average of 28% for a range of seasonal conditions. In the UK, the average rate of NH₃ loss from an intensively grazed ryegrass sward was 0.75 kg N/ha/day during a 6-month season. NH ₄ ⁺ remaining in the soil may be nitrified, nitrification being complete within 3 to 6 weeks. Although some NO ₃ ^– is recovered by plants, a substantial portion is leached and/or denitrified. On average such losses were 42%, with only 30% of the added N recovered by plants in urine-treated pasture in NZ. In the UK annual leaching of 150 to 190 kg N/ha has been observed for grazed swards receiving 420 kg N/ha/yr. Low retention of N by grazing ruminants results in a breakdown of N relationships in intensively managed grasslands. The substantial losses through NH₃ volatilization, leaching and denitrification have serious agronomic, economic and environmental implications.     

Denitrification and dinitrogen fixation in two quaking fens in the Vechtplassen area,The Netherlands

This paper reports laboratory experiments on dinitrogen fixation and denitrification for two small quaking fens (discharge fen and recharge fen) using the acetylene reduction assay and the acetylene inhibition technique, respectively.Nitrogenase activity was detected in peat muck and associated with Alnus glutinosa saplings throughout the study period (May–October 1987), whereas no activity was observed with Sphagnum species. The annual amount of dinitrogen fixed was estimated at 2.1 and 12.7 kg N/ha/y for the recharge fen and the discharge fen, respectively.Denitrification at ambient nitrate levels (0.1 ppm NO₃) was absent in the discharge fen and very low in the recharge fen (0.1 g N/g/d, or 0.3 kg N/ha/y). In nitrate-amended soil samples denitrification rates were 2 to 3 orders of magnitude higher. It is argued that in situ denitrification rates in the fens studied will depend almost entirely on the nitrate supply by precipitation. Denitrification rates associated with precipitation are estimated at 1.1 kg N/ha/y for both fens.     

Nitrogen cycling in leucaena (Leucaena leucocephala) alley cropping in semi-arid tropics

In an alley cropping system, prunings from the hedgerow legume are expected to supply nitrogen (N) to the associated cereal. However, this may not be sufficient to achieve maximum crop yield. Three field experiments with alley-cropped maize were conducted in a semi-arid environment in northern Australia to determine: (1) the effect of N fertilizer on maize growth in the presence of fresh leucaena prunings; (2) the effect of incorporation of leucaena and maize residues on maize yield and the fate of plant residue¹⁵N in the alley cropping system; and (3) the¹⁵N recovery by maize from¹⁵N-labelled leucaena, maize residues and ammonium sulphate fertilizer.Leucaena residues increased maize crop yield and N uptake although they did not entirely satisfy the N requirement of the alley crop. Additional N fertilizer further increased the maize yield and N uptake in the presence of leucaena residues. Placement of leucaena residues had little effect on the availability of N to maize plants over a 2 month period. The incorporation of leucaena residues in the soil did not increase the recovery of leucaena¹⁵N by maize compared with placement of the residues on the soil surface. After 2 months, similar proportions of the residue¹⁵N were recovered by maize from mulched leucaena (6.3%), incorporated leucaena (6.1%) and incorporated maize (7.6%). By the end of one cropping season (3 months after application) about 9% of the added¹⁵N was taken up by maize from either¹⁵N-labelled leucaena as mulch or¹⁵N-labelled maize residues applied together with unlabelled fresh leucaena prunings as mulch. The recovery of the added¹⁵N was much higher (42.7%) from the¹⁵N-labelled ammonium sulphate fertilizer at 40 kg N ha^-1 in the presence of unlabelled leucaena prunings. Most of the added¹⁵N recovered in the 200 cm soil profile was distributed in the top 25 cm soil with little leached below that. About 27–41% of the leucaena¹⁵N was apparently lost, largely through denitrification from the soil and plant system, in one cropping season. This compared with 35% of the fertilizer¹⁵N lost when the N fertilizer was applied in the presence of prunings. ei]H Lambers     

Leaf-litter and changing nutrient levels in a seasonally dry tropical hardwood forest,Belize, C.A.

Summary Total above ground plant biomass in a 45 year old seasonally dry tropical hardwood forest was estimated to be approximately 56,000 kg/ha oven dry weight. Nutrients immobilized in the standing vegetation were: N, 203 kg/ha; P, 24 kg/ha; K, 234 kg/ha; Ca, 195 kg/ha; Mg, 47 kg/ha; Na, 9 kg/ha; Mn, 1 kg/ha; Cu, 0.5 kg/ha; Zn, 3 kg/ha; Fe, 4 kg/ha. Total nutrients returned each year through the litter were: N, 156 kg/ha; P, 9 kg/ha; K, 59 kg/ha; Ca, 373 kg/ha; Mg, 32 kg/ha; Na, 5 kg/ha; Mn, 1 kg/ha; Al, 21 kg/ha; Zn, 0.3 kg/ha; Fe, 9 kg/ha. Half of the nutrients immobilized in the standing vegetation were found in the leaves and are returned annually to the soil. Although litter fall is interrupted during the year, the mean nutrient content of the litter was high –5.2%.A decomposition rate of 0.48 percent per day was considered high for a seasonally dry tropical hardwood forest. Fluctuations in soil nutrient levels showed a sharp increase at the start of the rainy season. Later during the dry season nutrient levels decreased to concentrations similar to what they were just prior to the rainy season. Soil organic matter levels were very high –20% in the top 12 cm.     

In situ acetylene-ethylene assay of biological nitrogen fixation in lowland rice soils

Summary An in situ device for assaying biological nitrogen fixation in flooded rice soils, using the acetylene reduction method, was developed. Diurnal variations in acetylene reduction by an inoculated field plot and by laboratory-grown cultures of nitrogen-fixing algae showed a prominent single-peak pattern of nitrogenase activity. The peak occurred at mid-day for laboratory-grown algae and at late afternoon for the algae grown in the field plot. Some nitrogenase activity was noted during the night. Acetylene reduction studies in rice fields of Albay province, Philippines, showed an estimated fixation of 18.5 to 33.3 kg N/ha each cropping season for the fields of Puro soil and 2.3 to 5.7 kg N/ha each cropping season for the fields of Santo Domingo soil. re]19751202     

Hexadecane mineralization and denitrification in two diesel fuel-contaminated soils

The effect of nitrate, ammonium and urea on the mineralization of [(14)C]hexadecane (C(16)H(34)) and on denitrification was evaluated in two soils contaminated with diesel fuel. In soil A, addition of N fertilizers did not stimulate or inhibit background hexadecane mineralization (4.3 mg C(16)H(34) kg(-1) day(-1)). In soil B, only NaNO(3) stimulated hexadecane mineralization (0.91 mg C(16)H(34) kg(-1) day(-1)) compared to soil not supplemented with any nitrogen nutrient (0.17 mg C(16)H(34) kg(-1) day(-1)). Hexadecane mineralization was not stimulated in this soil by NH(4)NO(3) (0.13 mg C(16)H(34) kg(-1) day(-1)), but the addition of NH(4)Cl or urea suppressed hexadecane mineralization (0.015 mg C(16)H(34) kg(-1) day(-1)). Addition of 2 kPa C(2)H(2) did not inhibit the mineralization process in either soil. Denitrification occurred in both soils studied when supplemented with NaNO(3) and NH(4)NO(3), but was not detected with other N sources. Denitrification started after a longer lag in soil A (10 days) than in soil B (4 days). In soil A microcosms supplemented with NaNO(3) or NH(4)NO(3), rates of denitrification were 20.6 and 13.6 mg NO(3)(-) kg(-1) day(-1), respectively, and in soil B, they were 18.5 and 12.5 mg NO(3)(-) kg(-1) day(-1), respectively. We conclude that denitrification may lead to a substantial loss of nitrate, making it unavailable to the mineralizing bacterial population. Nitrous oxide was an important end-product accounting for 30-100% of total denitrification. These results indicate the need for preliminary treatability studies before implementing full-scale treatment processes incorporating commercial fertilizers.     

Controls Over Leaf Litter and Soil Nitrogen Fixation in Two Lowland Tropical Rain Forests

Global comparisons suggest that rates of N fixation in tropical rain forests may be among the highest on earth. However, data supporting this contention are rare, and the factors that regulate N fixation within the biome remain largely unknown. We conducted a full-factorial (N × P) fertilization experiment in two lowland tropical rain forests in Costa Rica to explore the effects of nutrient availability on rates of free-living N fixation in leaf litter and soil. P fertilization significantly increased N fixation rates in both leaf litter and soil, and the effect was dependent on sampling date. Fertilization with N did not affect rates of N fixation at any time. In addition, variation in N fixation rates measured in unfertilized plots at four sampling time points suggested seasonal variability in N fixation: leaf litter N fixation ranged from 0.36 kg/ha/yr in the dry season to 5.48 kg/ha/yr in the wet season. Soil N fixation showed similar patterns ranging from a dry season low of 0.26 kg/ha/yr to a wet season high of 2.71 kg/ha/yr. While the observed temporal variability suggests potential climatic control over free-living N fixation in these forests, data suggest that neither soil nor leaf litter moisture alone regulate N fixation rates. Instead, we hypothesize that a combination of ample C availability, low leaf litter N:P ratios, and high rainfall coincide during the latter portions of the rainy season and drive the highest free-living N fixation rates of the year.     

Utilization of poultry manure when cultivating potatoes in the southern steppe of the Republic of Bashkortostan

The goal of the research is to determine the effect of pretreated poultry manure and irrigation on the yield and quality of potatoes (Solanum tuberosum) grown in fields located on the southern steppe of the Republic of Bashkortostan. Field experiments were repeated every three years. During vegetation, potato leaves and tubers were studied at the full blossom, leaves’ decay, and potato harvest. The results showed that applying pretreated poultry manure at the rate of 120 t/ha favored higher tuber weight both on the rainfed and irrigated plots (from 0.23 to 0.82 kg/plant and from 0.24 to 1.02 kg/plant, respectively). On the rainfed and irrigated plots where poultry mature was not applied (control fields) the tubers’ weight ranged from 0.08 to 0.31 kg/plant and from 0.16 to 0.50 kg/plant, respectively. Upon application of 40 t/ha of manure, under irrigation, the highest marketable value of tubers was 78%. On the rainfed plots the same value was 72% when applying poultry manure at a dose of 120 t/ha. Soil moisture monitoring showed that the potatoes did not get adequate water during the growing season. When the soil moisture on irrigated plots was 70% less than its minimum water capacity, potato plantings were watered.     

Uptake and use patterns of nitrogen from urea,oxamide, and isobutylidene diurea by rice plants

Summary Two¹⁵N-labelled slow-release nitrogen (N) sources, oxamide and isobutylidene diurea (IBDU), each at two particle sizes, and¹⁵N-labelled urea were compared at two rates as sources of N for rice (Oryza sativa) under two watering regimes which simulated a transplant (continuous flood, CF) and a direct-seeded (A/F) system of paddy rice culture. Highest grain yields were obtained from −8+10-mesh oxamide particles applied at the rate of 2,000 mg of N/5 kg of soil, CF series; this yield was slightly higher than that obtained from −3+4-mesh oxamide, A/F series. Incubating the N fertilizers in moist (22% water) soil for 21 days immediately before flooding and transplanting rice greatly reduced N supply because of nitrification during the preflood period, followed by denitrification after flooding. This resulted in less plant uptake of N and less grain yield from urea, fine oxamide and IBDU, A/F series. For coarse oxamide, N release during the preflood period resulted in higher N uptake and grain yield in the A/F rather than in the corresponding CF series. The pattern of fertilizer N uptake by rice plants was affected by kind of fertilizer, particle size of oxamide and IBDU, and watering regime. Uptake of fertilizer N generally paralleled uptake of soil N throughout the growth period. Plant tops continued to accumulate some N during the period of grain filling, but much of the N in plant tops was translocated to the grain after heading. There was a large decrease in dry weight, N content, and¹⁵N content of tops after heading. Root weight and N content increased rapidly at first, and then at a diminishing rate until maturity. Unexplained N deficits occurred in the CF series (14–23% of the N applied, depending on N rate and source), and in the A/F series for IBDU (37–43% of the N applied).     

Lysimeter studies on recovery of15N-labeled urea in wetland rice

Summary Results of a two year study on the fate on¹⁵N-labelled urea (9.95 atoms percent excess¹⁵N) applied @ 180 kg N/ha to flooded rice in monolith lysimeters at the Punjab Agricultural University Farm, Ludhiana are reported. The soil of the experimental field was sandy clay loam in texture (Typic Ustochrept), had pH 7.9, organic carbon 0.36 percent, available N 187 kg/ha and total N 0.08 percent. The results revealed that 18.1 to 53.0 per cent of the fertilizer N was utilized by the rice plant, 25.1 to 41.1 percent was immobilized in the soil and 4.8 to 7.2 percent was lost by denitrification. The losses due to ammonia volatilization and leaching were negligible. The data on vertical distribution of labelled N in the soil profile reflected a higher concentration (38.3 to 39.5 per cent) in the surface (0–30 cm) soil. The content sharply decreased (1.8 to 2.4, percent) in lower soil layers (30–150 cm). A balance sheet of the various pathways of applied N showed that 58.8 to 72.2 and 66.2 to 83.0 percent N was recovered in 1976 and 1977, respectively and 17 to 41.2 per cent of labelled N still remained unaccounted for. Utilization of fertilizer N by rice was increased and losses decreased when N was applied in three equal splits as compared to the single N application at transplanting.Availability of fertilizer N immobilized in the soil was investigated in the succeeding crops of wheat and rice. The results showed that 2.1 tot 3.4 per cent of the N applied to the preceding rice was utilized by the second rice crop grown in succession. This may look small but cannot be neglected on a long term basis. But there is need to initiate long term studies to investigate the, turnover of residual N and to determine the fate of applied N in varying soil and cropping systems by using improved techniques.