Competition for 15N-labeled fertilizer in a pecan (Carya illinoensis K. Koch)-cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

The degree of tree-crop competition for nitrogen (N) and its effect on fertilizer-use efficiency and N movement were examined in a pecan (Carya illinoensis K. Koch)-cotton (Gossypium hirsutum L.) alley cropping system. Assessment of competition was accomplished via the installation of a belowground polyethylene root barrier in half the number of plots in order to provide two treatments–barrier and non-barrier. The percentage of N derived from fertilizer (NDF) and fertilizer-use efficiency (UFN) were determined using ¹⁵N-enriched ammonium sulfate (5% atom enrichment) applied at 89.6 kg N ha^–1. In cotton, the barrier treatment resulted in higher leaf (38%), stem (66%), seed cotton (55%) and total (58%) biomass compared to the non-barrier treatment. Total N content in leaf, stem and seed cotton was 67% higher in barrier compared to non-barrier treatment. Percentage of NDF in cotton leaf and stem was significantly lower in barrier (15.8% and 17.3%, respectively) compared to non-barrier treatment (20.4% and 21.2%, respectively). For UFN, this trend was reversed, with plants in barrier treatment having a higher percentage of UFN. Root trenching did not affect pecan foliar N concentration, canopy N content, NDF or UFN. In soil, N recovery at 90–120 cm depth was lower in non-barrier treatment, indicating tree root uptake of fertilizer N. Although tree roots in non-barrier treatment had access to fertilizer N, competition was mainly for N already in the soil, since fertilizer was applied after major seasonal nutrient demands of the trees had been met. Overall, the alley cropping system in this study exhibits potential for efficient N cycling, given the apparent ability of pecan trees to intercept and uptake N fertilizer from deeper soil layers and return to surface soil via litterfall.     

Losses of fertilizer-derived N from transplanted rice after heading

Microplot field experiments with ¹⁵N-labeled (NH₄)₂SO₄ were conducted to investigate the changes in the percentage of fertilizer ¹⁵N recovery (%FNR) of transplanted rice (Oryza sativa L.) with basal (4 g m^–2) and topdress (3 g m^–2) applications for two years. In 1999, %FNR from the basal ¹⁵N application decreased by 6% (0.23 g N m^–2)from heading + 2 d to maturity, and %FNR from the topdress ¹⁵N application decreased by 11% (0.32 g N m^–2)from heading – 4 d to maturity. In 2000, %FNR from the topdress ¹⁵N application decreased by 9% (0.26 g N m^–2) from heading – 1 d to maturity, whereas there was no significant decline in %FNR from the basal ¹⁵N application after heading. A decrease in %FNR occurred even when total N of rice plants showed no decrease after heading. Nitrogen harvest index of fertilizer ¹⁵N with the topdress ¹⁵N application was not significantly different from that with the basal ¹⁵N application in 1999, whereas N harvest index of fertilizer ¹⁵N with the topdress ¹⁵N application was higher than that with the basal ¹⁵N application in 2000. N harvest index of fertilizer ¹⁵N, that indicated the proportion of the ¹⁵N remobilized to panicles after heading, was substantially responsible for changes in %FNR from the basal and topdress ¹⁵N applications after heading.     

Recovery of fertilizer-derived inorganic-15N in a vegetable field soil as affected by application of an organic amendment

To examine the effect of organic amendment application on the fate of inorganic-N accumulated in a vegetable field soil during conversion from inorganic to organic input, a pot experiment using ¹⁵N-labeled soil was conducted. The soil was labeled with ¹⁵N through addition of urea-¹⁵N (98 atom % ¹⁵N) and was then incubated for 1 year resulting in inorganic soil-N concentration and ¹⁵N abundance of 211 mg kg^–1 soil and 4.950 atom %, respectively. Chinese cabbage [Brassica campestris (L.) Samjin] plants were grown in the labeled soil for 30 and 60 days after application of organic amendment at the rates of 0 (control), 200, 400, and 600 mg N kg^–1 soil. Although organic amendment application did not show any significant effect on the uptake efficiency of inorganic-N by Chinese cabbage during the first 30 days, it significantly (P<0.05) increased inorganic-N uptake efficiency as well as total-N uptake and dry matter yield at the end of the 60-day growth period. Application of the organic amendment also increased microbial immobilization of inorganic-N in both growth periods. Between 30 and 60 days of growth, however, the amount of immobilized N from the inorganic-¹⁵N pool decreased, indicating re-mineralization of previously immobilized N. Although the amount of inorganic-¹⁵N lost was virtually the same among treatments at day 30, increased immobilization of inorganic-¹⁵N caused by organic amendment application led to the higher retention of inorganic-N in the soil and less loss of N at day 60 as compared to the control. These results indicate that increased immobilization by organic amendment application in the early growth season and the subsequent gradual re-mineralization may play an important role in increasing plant uptake of inorganic-¹⁵N, while minimizing N loss.     

Turnover of residual 15N-labelled fertilizer N in soil following harvest of oilseed rape (t Brassica napus L.)

We studied the fate of ¹⁵N-labelled fertilizer nitrogen in a sandy loam soil after harvest of winter oilseed rape (Brassica napus L. cv. Ceres) given 100 or 200 kg N ha^-1 in spring, with or without irrigation. Our main objective was to quantify the temporal variations of the soil mineral N, the extractable soil organic N and soil microbial biomass N, and fertilizer derived N in these pools during autumn and winter. Nitrogen use efficiency of the oilseed rape crop varied from 47% of applied N in the 100N, irrigated treatment to 34% in the 200N, non-irrigated treatment. However, only in the latter treatment did we find significantly higher fertilizer derived soil mineral N than in the three other treatments which all had low soil mineral N contents at the first sampling after harvest (8 days after stubble tillage). Between 31% and 42% of the applied N could not be accounted for in the harvested plants or 0-15 cm soil layer at this first sampling. Over the following autumn and winter none of the remaining fertilizer derived soil N was lost from the 0–5 cm depth, but from the 5–15 cm depth a marked proportion of N derived from fertilizer was lost, probably by leaching. Negligible amounts of fertilizer derived extractable soil organic and mineral N (<1 kg N ha^-1, 0-15 cm) were found in all treatments after the first sampling.Soil microbial biomass N was not significantly affected by treatments and showed only small temporal variability (±11% of the mean 76 kg N ha^-1, 0- 15 cm depth). Surprisingly, the average amount of soil microbial biomass N derived from fertilizer was significantly affected by the treatments, with the extremes being 5.5 and 3.1 kg N ha^-1 in the 200N, non-irrigated and 100N, irrigated treatments, respectively. Also, the estimated exponential decay rate of microbial biomass N derived from fertilizer, differed greatly (2 fold) between these two treatments, indicating highly different microbial turnover rates in spite of the similar total microbial biomass N values. In studies utilising ¹⁵N labelling to estimate turnover rates of different soil organic matter pools this finding is of great importance, because it may question the assumption that turnover rates are not affected by the insertion of the label.     

Crop performance, nitrogen and water use in flooded and aerobic rice

Irrigated aerobic rice is a new system being developed for lowland areas with water shortage and for favorable upland areas with access to supplementary irrigation. It entails the cultivation of nutrient-responsive cultivars in nonsaturated soil with sufficient external inputs to reach yields of 70–80% of high-input flooded rice. To obtain insights into crop performance, water use, and N use of aerobic rice, a field experiment was conducted in the dry seasons of 2002 and 2003 in the Philippines. Cultivar Apo was grown under flooded and aerobic conditions at 0 and at 150 kg fertilizer N ha^–1. The aerobic fields were flush irrigated when the soil water potential at 15-cm depth reached –30 kPa. A ¹⁵N isotope study was carried out in microplots within the 150-N plots to determine the fate of applied N. The yield under aerobic conditions with 150 kg N ha^–1 was 6.3 t ha^–1 in 2002 and 4.2 t ha^–1 in 2003, and the irrigation water input was 778 mm in 2002 and 826 mm in 2003. Compared with flooded conditions, the yield was 15 and 39% lower, and the irrigation water use 36 and 41% lower in aerobic plots in 2002 and 2003, respectively. N content at 150 kg N ha^–1 in leaves and total plant was nearly the same for aerobic and flooded conditions, indicating that crop growth under aerobic conditions was limited by water deficit and not by N deficit. Under aerobic conditions, average fertilizer N recovery was 22% in both the main field and the microplot, whereas under flooded conditions, it was 49% in the main field and 36% in the microplot. Under both flooded and aerobic conditions, the fraction of ¹⁵N that was determined in the soil after the growing season was 23%. Since nitrate contents in leachate water were negligible, we hypothesized that the N unaccounted for were gaseous losses. The N unaccounted for was higher under aerobic conditions than under flooded conditions. For aerobic rice, trials are suggested for optimizing dose and timing of N fertilizer. Also further improvements in water regime should be made to reduce crop water stress.     

减量施氮对玉米-大豆套作系统下作物氮素吸收和利用效率的影响

为探索玉米-大豆套作系统中作物对N素吸收的差异特性,揭示减量施N对玉米-大豆套作系统的N高效利用机理。利用15N同位素示踪技术,结合小区套微区多年定位试验,研究了玉米单作(MM)、大豆单作(SS)、玉米-大豆套作(IMS)及不施N(NN)、减量施N(RN:180 kg N/hm2)、常量施N(CN:240 kg N/hm2)下玉米、大豆的生物量、吸N量、N肥利用率及土壤N素含量变化。结果表明,与MM(SS)相比,IMS下玉米茎叶及籽粒的生物量、吸N量降低,15N%丰度及15N吸收量增加,大豆籽粒及植株的生物量、吸N量及15N吸收量显著提高;IMS下玉米、大豆植株的N肥利用率、土壤N贡献率、土壤15N%丰度降低,15N回收率显著增加。施N与不施N相比,显著提高了单、套作下玉米、大豆植株的生物量、吸N量、15N丰度及15N吸收量;RN与CN相比,IMS下,RN的玉米、大豆植株总吸N量提高13.4%和12.4%,N肥利用率提高213.0%和117.5%,土壤总N含量提高12.2%和11.6%,土壤N贡献率降低12.0%和11.2%,玉米植株15N吸收量与15N回收率提高14.4%和52.5%,大豆的则降低57.1%和42.8%,单作与套作的变化规律一致。玉米-大豆套作系统中作物对N素吸收存在数量及形态差异,减量施N有利于玉米-大豆套作系统对N肥的高效吸收与利用,实现作物持续增产与土壤培肥。     

Crop uptake and leaching of 15N applied in ruminant slurry with selectively labelled faeces and urine fractions

Two animal slurries either labelled with ¹⁵N in the urine or in the faeces fraction, were produced by feeding a sheep with unlabelled and ¹⁵N-labelled hay and collecting faeces and urine separately. The slurries were applied (12 g total N ^-2) to a coarse sand and a sandy loam soil confined in lysimeters and growing spring barley (Hordeum vulgare L). Reference lysimeters without slurry were supplied with¹⁵ NH₄ ¹⁵NO³ corresponding to the inorganic N applied with the slurries (6 g N m^-2). In the second year, all lysimeters received unlabelled mineral fertilizer (6 g N m^-2) and grew spring barley. N harvested in the two crops (grain + straw) and the loss of nitrate by leaching were determined. ¹⁵N in the urine fraction was less available for crop uptake than mineral fertilizer ¹⁵N. The first barley crop on the sandy loam removed 49% of the ¹⁵N applied in mineral fertilizer and 36% of that applied with urine. The availability of fertilizer ¹⁵N (36%) and urine¹⁵ N (32%) differed less on the coarse sand. Of the¹⁵ N added with the faeces fraction, 12–14% was taken up by the barley crop on the two soils. N mineralized from faeces compensated for the reduced availability of urine N providing a similar or higher crop N uptake in manured lysimeters compared with mineral fertilized ones.About half of the total N uptake in the first crop originated from the N applied either as slurry or mineral fertilizer. The remaining N was derived from the soil N pool. Substantially smaller but similar proportions of¹⁵ N from faeces, urine and fertilizer were found in the second crop. The similar recoveries indicated a slow mineralization rate of the residual faeces N since more faeces was left in the soil after the first crop.More N was lost by leaching from manured lysimeters but as a percentage of N applied, losses were similar to those from mineral fertilizer. During the first and second winter, 3–5% and 1–3%, respectively, of the ¹⁵N in slurry and mineral fertilizer was leached as nitrate. Thus slurry N applied in spring just before sowing did not appear to be more prone to loss by nitrate leaching than N given in mineral fertilizer. Slurry N accounted for a higher proportion of the N leached, however, because more N was added in this treatment.     

15N abundance of surface soils,roots and mycorrhizas in profiles of European forest soils

¹⁵N natural abundances of soil total N, roots and mycorrhizas were studied in surface soil profiles in coniferous and broadleaved forests along a transect from central to northern Europe. Under conditions of N limitation in Sweden, there was an increase in ¹⁵N of soil total N of up to 9% from the uppermost horizon of the organic mor layer down to the upper 0–5 cm of the mineral soil. The ¹⁵N of roots was only slightly lower than that of soil total N in the upper organic horizon, but further down roots were up to 5% depleted under such conditions. In experimentally N-enriched forest in Sweden, i.e. in plots which have received an average of c. 100 kg N ha^–1 year^–1 for 20 years and which retain less than 50% of this added N in the stand and the soil down to 20 cm depth, and in some forests in central Europe, the increase in ¹⁵N with depth in soil total N was smaller. An increase in ¹⁵N of the surface soil was even observed on experimentally N-enriched plots, although other data suggest that the N fertilizer added was depleted in¹⁵N. In such cases roots could be enriched in¹⁵N relative to soil total N, suggesting that labelling of the surface soil is via the pathway: — available pools of N-plant N-litter N. Under N-limiting conditions roots of different species sampled from the same soil horizon showed similar ¹⁵N. By contrast, in experimentally N-enriched forest ¹⁵N of roots increased in the sequence: ericaceous dwarf shrubs15N enriched compounds in fungal material, which could contribute to explain the observed ¹⁵N profiles if fungal material is enriched, because it is a precursor of stable organic matter and recalcitrant N. This could act in addition to the previous explanation of the isotopically lighter soil surface in forests: plant uptake of ¹⁵N-depleted N and its redeposition onto the soil surface by litter-fall.     

Land-use change effects on soil C and N transformations in soils of high N status: comparisons under indigenous forest, pasture and pine plantation

Globally, land-use change is occurring rapidly, and impacts on biogeochemical cycling may be influenced by previous land uses. We examined differences in soil C and N cycling during long-term laboratory incubations for the following land-use sequence: indigenous forest (soil age = 1800 yr); 70-year-old pasture planted after forest clearance; 22-year-old pine (Pinus radiata) planted into pasture. No N fertilizer had been applied but the pasture contained N-fixing legumes. The sites were adjacent and received 3–6 kg ha^–1 yr^–1volcanic N in rain; NO₃ ^–-N leaching losses to streamwater were 5–21 kg ha^–1 yr^–1, and followed the order forest < pasture = pine. Soil C concentration in 0–10 cm mineral soil followed the order: pasture > pine = forest, and total N: pasture > pine > forest. Nitrogen mineralization followed the order: pasture > pine > forest for mineral soil, and was weakly related to C mineralization. Based on radiocarbon data, the indigenous forest 0–10 cm soil contained more pre-bomb C than the other soils, partly as a result of microbial processing of recent C in the surface litter layer. Heterotrophic activity appeared to be somewhat N limited in the indigenous forest soil, and gross nitrification was delayed. In contrast, the pasture soil was rich in labile N arising from N fixation by clover, and net nitrification occurred readily. Gross N cycling rates in the pine mineral soil (per unit N) were similar to those under pasture, reflecting the legacy of N inputs by the previous pasture. Change in land use from indigenous forest to pasture and pine resulted in increased gross nitrification, net nitrification and thence leaching of NO₃ ^–-N.     

Rate of accumulation and emission of N2, N2O and CH4 from a flooded rice soil

In a field experiment using microplots, a flooded Crowley silt loam (Typic Albaqualfs) rice soil was fertilized with ¹⁵N labelled (60–74 atom %) urea and KNO₃. Emission of N₂, N₂O and CH₄ and accumulation in soil were measured for 21 d after fertilizer application.Emission of ¹⁵N₂-N measured from the urea and KNO₃ treated plots ranged from <15 to 570 and from 330 to 3,420 g ha^–1 d^–1, respectively. Entrapped ¹⁵N₂-N in the urea treated microplots was significantly lower (<15 g to 2.1 kg ha^–1) on all sampling dates compared to the ¹⁵N₂-N gas accumulation in the KNO₃ treated plots (6.4 to 31.5 kg ha^–1). Emissions of N₂O-N were low and did not exceed 4 g ha^–1 d^–1. Fluxes of CH₄ from the fertilizer and control plots were low and never exceeded 33 g ha^–1 d^–1. Maximum accumulation of CH₄ in the flooded soil measured 460 and 195 g ha^–1 for the urea and KNO₃ treatments, respectively.     

Effects of fertigation scheme on N uptake and N use efficiency in cotton

While fertigation can increase fertilizer use efficiency, there is an uncertainly as to whether the fertilizer should be introduced at the beginning of the irrigation or at the end, or introduced during irrigation. Our objective was to determine the effect of different fertigation schemes on nitrogen (N) uptake and N use efficiency (NUE) in cotton plants. A pot experiment was conducted under greenhouse conditions in year 2004 and 2005. According to the application timing of nitrogen (N) fertilizer solution and water (W) involved in an irrigation cycle, four nitrogen fertigation schemes [nitrogen applied at the beginning of the irrigation cycle (N–W), nitrogen applied at the end of the irrigation cycle (W–N), nitrogen applied in the middle of the irrigation cycle (W–N–W) and nitrogen applied throughout the irrigation cycle (N&W)] were employed in a completely randomized design with four replications. Cotton was grown in plastic containers with a volume of 84 l, which were filled with a clay loam soil and fertilized with 6.4 g of N per pot as unlabeled and ¹⁵N-labeled urea for 2004 and 2005, respectively. Plant total dry matter (DM) and N content in N–W was significantly higher than in N&W in both seasons, but these were not consistent for W–N and W–N–W treatments. In year 2005, a significantly higher nitrogen derived from fertilizer (NDFF) for the whole plant was found in W–N and N–W than that in W–N–W and N&W. Fertigation scheme had a consistent effect on total NUE: N–W had the highest NUE for the whole plant, but this was not significantly different from W–N. Treatments W–N and W–N–W had similar total NUE, and N&W had the lowest total NUE. After harvesting, the total residual fertilizer N in the soil was highest in W–N, lowest in N–W, but this was not significantly different from N&W and W–N–W treatments. Total residual NO₃–N in the soil in N&W and W–N treatments was 20.7 and 21.2% higher than that in N–W, respectively. The total ¹⁵N recovery was not statistically significant between the four fertigation schemes. In this study, the fertigation scheme N–W (nitrogen applied at the beginning of an irrigation cycle) increased DM accumulation, N uptake and NUE of cotton. This study indicates that Nitrogen application at the beginning of an irrigation cycle has an advantage on N uptake and NUE of cotton. Therefore, NUE could be enhanced by optimizing fertilization schemes with drip irrigation.     

Estimation of symbiotically fixed nitrogen in field grown soybeans: An application of natural15N/14N abundance and a low level15N-tracer technique

Summary Plants from agricultural and natural upland ecosystem were investigated for¹⁵N content to evaluate the role of symbiotic N₂-fixation in the nitrogen nutrition of soybean. Increased yields and lower δ¹⁵N values of nodulating soybeansvs, non-nodulating isolines gave semi-quantitative estimates of N₂ fixation. A fairly large discrepancy was found between estimations by δ¹⁵N and by N yield at 0 kg N/ha of fertilizer. More precise estimates were made by following changes in plant δ¹⁵N when fertilizer δ¹⁵N was varied near¹⁵N natural abundance level. Clearcut linear relationships between δ¹⁵N values of whole plants and of fertilizer were obtained at 30 kg N/ha of fertilizer for three kinds of soils. In experimental field plots, nodulating soybeans obtained 13±1% of their nitrogen from fertilizer, 66±8% from N₂ fixation and 21±10% from soil nitrogen in Andosol brown soil; 30%, 16% and 54% in Andosol black soil; 7%, 77% and 16% in Alluvial soil, respectively. These values for N₂ fixation coincided with each corresponding estimation by N yield method. Other results include: 1)¹⁵N content in upland soils and plants was variable, and may reflect differences in the mode of mineralization of soil organics, and 2) nitrogen isotopic discrimination during fertilizer uptake (δ¹⁵N of plant minus fertilizer) ranged from −2.2 to +4.9‰ at 0–30 kg N/ha of fertilizer, depending on soil type and plant species. The proposed method can accurately and relatively simply establish the importance of symbiotic nitrogen fixation for soybeans growing in agricultural settings.     

The effect of cropping and fertiliser nitrogen rates on nitrogen balance in soil

Summary Fertilizer/soil N balance of cropped and fallow soil has been studied in a pot experiment carried out with grey forest soil (southern part of Moscow region) at increasing rates of¹⁵N labelled ammonium sulfate (0; 8; 16; 32 mg N/100 g of soil). The fertilizer¹⁵N balance has been shown to depend upon its application rate and the presence of growing plants. Fertilizer N uptake efficiency was maximum (72.5%) and gaseous losses-minimum (12.5%) at the application rate of 16 mg N/100 g of soil. Fertilizer N losses from the fallow soil were 130–220% versus those from the cropped soil. At the application of fertilizer N the plant uptake of soil N was 170–240% and the amount of soil N as N–NH₄ exchangeable + N–NO₃ in fallow was 350–440% as compared to the control treatment without nitrogen (PK).After cropping without or with N fertilizer application at the rates of 8 and 32 mg N/100 g of soil, a positive nitrogen balance has been found which is likely due to nonsymbiotic (associative) N-fixation. It has been shown that biologically fixed nitrogen contributes to plant nutrition.     

Natural 15N abundances of maize and soil amended with urea and composted pig manure

To investigate the effect of inorganic fertilizer and composted manure amendments on the N isotope composition (delta ¹⁵N) of crop and soil, maize (Zea mays L.) was cultivated under greenhouse conditions for 30, 40, 50, 60, and 70 days. Composted pig manure (delta ¹⁵N= +13.9) and urea (-2.3) were applied at 0 and 0 kg N ha^–1 (C0U0), 0 and 150 kg N ha^–1 (C0U2), 150 and 0 kg N ha^–1 (C2U0), and 75 and 75 kg N ha^–1 (C1U1), respectively. The delta ¹⁵N of total soil-N was not affected by both amendments, but delta ¹⁵N of NH⁺ ₄ and NO^– ₃ provided some information on the N isotope fractionation in soil. During the early growth stage, significant differences (P < 0.05) in delta ¹⁵N among maize subjected to different treatments were observed. After 30 days of growth, the delta ¹⁵N values of maize were +6.6 for C0U0, +1.1 for C0U2, +7.7 for C2U0, and +4.5 for C1U1. However, effects of urea and composted manure application on maize delta ¹⁵N progressively decreased with increasing growth period, probably due to isotope fractionation accompanying N losses and increased uptake of soil-derived N by maize. After 70 days of growth, delta ¹⁵N of leaves and grains of maize amended with composted pig manure were significantly (P < 0.05) higher than those with urea. The temporal variations in delta ¹⁵N of maize amended with urea and composted manure indicate that plant delta ¹⁵N is generally not a good tracer for N sources applied to field. Our data can be used in validation of delta ¹⁵N fractionation models in relation to N source inputs.     

Fertilizer and soil nitrogen accumulation by irrigated barley grown on a red-brown earth in south-eastern Australia

¹⁵N labelled (NH₄)₂SO₄ was applied to barley at 5 g N m⁻² (50 kg N ha⁻¹) in microplots at sowing to study the timing of the N losses and the contribution of soil and fertilizer N to the plant. Water treatments included rainfed and irrigation at 45–50 mm deficit beginning in the spring. Recovery of¹⁵N in the plant increased to a maximum of about 20% within 91 days after sowing (DAS 91) and then remained constant. Approximately 16% (0.8 g N m⁻²) of the fertilizer was in the stem and leaves at DAS 91 and this N was subsequently redistributed to the head. At maturity, approximately 75% of the¹⁵N assimilated by the tops was recovered in the grain. Soil N contributed 3.6 g N m⁻² to the head; 2.2 g N m⁻² was remobilized from the stem and leaves, and the balance, approximately 1.4 g N m⁻², was taken up from the soil between DAS 69 to 91. Effects of irrigation treatments on N accumulation were not significant. Residual¹⁵N fertilizer in the soil decreased with time from sowing, and at maturity 40% of the applied N was recovered in the surface 0.15 m.¹⁵N movement to depth was limited and less than 5% of the fertilizer was recovered below 0.15 m. Irrigation had no effect on the¹⁵N recovery at depth. Total recovery of the¹⁵N varied between 60 and 67% and implies that 33–40% was lost from the soil-plant system. The total recovery in the soil and plant was not affected by time or irrigation in the interval DAS 39 to 134. Losses occurred before DAS 39 when crop uptake of N was small and soil mineral N content was high. There was an apparent loss of 1.9 g fertilizer N m⁻² (i.e. 38% of that applied) between DAS 1 and 15. This loss occurred before crop emergence when rainfall provided conditions suitable for denitrification.     

Influence of urea on biological N2 fixation and N transfer from Azolla intercropped with rice

The N₂ fixed by Azolla before and after urea application during the rice cycle, the mineralisation of Azolla-N as well as its availability to rice was studied in two greenhouse experiments conducted in 1996 and 1997 and in June 1998 in Goettingen (Germany). Dry matter production of the various rice parts of experiment 1 showed a clear positive synergism between treatment with Azolla and urea with a resulting apparent N recovery by rice increasing from 40% (without Azolla) to 57% in the presence of Azolla. Part of this increase may be due to N fixed biologically by Azolla and transferred to the rice. The second experiment shed some light on the role of BNF. Using an iterative method of estimation, the daily rate of N fixation was estimated at 0.6 – 0.7 kg N ha^–1. The rate was not so much affected by the age of the Azolla crop. At this rate, the BNF would amount to up to 100 kg N ha^–1 over a 130-day season. Assuming that BNF may be inhibited for a period of 5 – 10 days following urea application due to high levels of N in the floodwater, this might reduce the BNF by between 6 and 14 kg N ha over the season. Using the mean-pool-abundance concept, it was estimated that around 75 – 80% of the Azolla-N mineralized during the growth period was actually absorbed by the rice plants. Of the N taken up by rice around 28% was derived from the biologically fixed Azolla N, the remainder was urea N cycled through the Azolla. Azolla also seems to help sustain the soil N supply by returning N to the soil in quantities roughly equal to those extracted from the soil by the rice plant.     

The role of Azolla cover in improving the nitrogen use efficiency of lowland rice

The development of management techniques to improve the poor N use efficiency by lowland rice (Oryza sativa L.) and reduce the high N losses has been an important focus of agronomic research. The potential of an Azolla cover in combination with urea was assessed under field conditions in Laguna, Philippines. Two on-station field experiments were established in the 1998–1999 dry season and eight on-farm experiments per season were carried out in the 2000–2001 wet and dry seasons. Treatment combinations consisting of N levels applied alone or combined with Azolla were evaluated with respect to floodwater chemistry, ¹⁵N recovery, crop growth, and grain yield. A full Azolla cover on the floodwater surface at the time of urea application prevented the rapid and large increase in floodwater pH and floodwater temperature. As a consequence, the partial pressure of ammonia (NH₃), which is an indicator of potential NH₃ volatilization, was significantly depressed. ¹⁵N recovery was higher in plots covered with Azolla where the total ¹⁵N recovery ranged between 77 and 99%, and the aboveground (grain and straw) recovery by rice ranged between 32 and 61%. The tiller count in Azolla-covered plots was significantly increased by 50% more than the uncovered plots at all urea levels. Consequently, the grain yield was likewise improved. Grain yields from the 16 on-farm trials increased by as much as 40% at lower N rates (40 and 50 kg N ha^–1) and by as much as 29% at higher N rates (80 and 100 kg N ha^–1). In addition, response of rice to treatments with lower N rates with an Azolla cover was comparable to that obtained with the higher N rates without a cover. Thus, using Azolla as a surface cover in combination with urea can be an alternative management practice worth considering as a means to reduce NH₃ volatilization losses and improve N use efficiency.     

Survival and colonization of inoculated bacteria in kallar grass rhizosphere and quantification of N2-fixation

Two experiments have been conducted, one in semi-solid Hoagland nutrient medium and the other in shallow pots containing saline soil. N₂-fixing bacteria belonging toAzospirillum, Azotobacter, Klebsiella andEnterobacter were inoculated separately on kallar grass grown in semi-solid nutrient medium. It was shown that inoculation affects root proliferation and also results in¹⁵N isotopic dilution. The % Ndfa ranged from 47–70 whereas no significant effect on the total nitrogen uptake was observed. The bacterial colonization of the root surface and the presence of enteric bacteria inside the root hair cells is reported. In a soil pot experiment, non-N₂-fixingPolypogon monspeliensis was used as a reference plant (control). A treatment receiving a high rate of nitrogen was also used as a non-N₂-fixing control.¹⁵N-labelled ammonium sulphate at 20 kg N ha^–1 and 90 kg N ha^–1 was used. The % Ndfa in the aerial parts of kallar grass was 12–15 whenP. monspeliensis was used as reference plant whereas 37–39% Ndfa was estimated when the treatment receiving high nitrogen fertilizer was used as a non-N₂-fixing control. These investigations revealed some problems of methodology which are discussed.     

Transformation of15N-labelled urea and its modified forms in tropical wetland rice culture

Summary The fate of 100 kg N ha^–1 applied as¹⁵N-urea and its modified forms was followed in 4 successive field-grown wetland rice crops in a vertisol. The first wet season crop recovered about 27 to 36.6% of the applied N depending upon the N source. In subsequent seasons the average uptake was very small and it gradually decreased from 1.4 to 0.5 kg N ha^–1 although about 18 to 20, 12 to 17 and 14 to 18 kg ha^–1 residual fertilizer N was available in the root zone after harvest of first, second and third crops, respectively. The average uptake of the residual fertilizer N was only 7.6% in the second crop and it decreased to 4.5% in the third and to 3.2% in the fourth crop although all these crops were adequately fertilized with unlabelled urea. The basal application of neem coated urea was more effective in controlling the leaching loss of labelled NH₄+NO₃–N than split application of uncoated urea. In the first 3 seasons in which¹⁵N was detectable, the loss of fertilizer N through leaching as NH₄+NO₃–N amounted to 0.5 kg ha^–1 from neem-coated urea, 1.5 kg from split urea and 4.1 kg from coal tar-coated urea. At the end of 4 crops, most of the labelled fertilizer N (about 69% on average) was located in the upper 0–20 cm soil layer showing very little movement beyond this depth. In the profile sampled upto 60 cm depth, totally about 13.8 kg labelled fertilizer N ha^–1 from neem-coated urea, 12.7 kg from coal-tar coated urea, and 11.8 kg from split urea were recovered. The average recovery of labelled urea-N in crops and soil during the entire experimental period ranged between 42 and 51%. After correcting for leaching losses, the remaining 47 to 56% appeared to have been lost through ammonia volatilization and denitrification.     

Response to inoculation and N fertilization for increased yield and biological nitrogen fixation of common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is able to fix 20–60 kg N ha^–1 under tropical environments in Brazil, but these amounts are inadequate to meet the N requirement for economically attractive seed yields. When the plant is supplemented with N fertilizer, N₂ fixation by Rhizobium can be suppressed even at low rates of N. Using the ¹⁵N enriched method, two field experiments were conducted to compare the effect of foliar and soil applications of N-urea on N₂ fixation traits and seed yield. All treatments received a similar fertilization including 10 kg N ha^–1 at sowing. Increasing rates of N (10, 30 and 50 kg N ha^–1) were applied for both methods. Foliar application significantly enhanced nodulation, N₂ fixation (acetylene reduction activity) and yield at low N level (10 kg N ha^–1). Foliar nitrogen was less suppressive to nodulation, even at higher N levels, than soil N treatments. In the site where established Rhizobium was in low numbers, inoculation contributed substantially to increased N₂ fixation traits and yield. Both foliar and soil methods inhibited nodulation at high N rates and did not significantly increase bean yield, when comparing low (10 kg N ha^–1) and high (50 kg N ha^–1) rates applied after emergence. In both experiments, up to 30 kg N ha^–1 of biologically fixed N₂ were obtained when low rates of N were applied onto the leaves.