Soybean yield response to residual NO3−N and applied N

Summary During 1976 through 1978, 10N treatments (combinations of N application times and rates) were used in a corn study. Those treatments created different levels of soil NO ₃ ^– –N content that were well-suited to a study of the influence of residual NO ₃ ^– –N and applied N on soybean yield. In April 1979 we applied ammonium nitrate at rates of 0, 75, or 150 kg N/ha to three subplots formed from each of the whole plots (previous N treatment plots). With N fertilization in 1979, seed yield increased where the residual NO ₃ ^– –N amount was less than 190 kg/ha but decreased where the residual amount was greater than 190 kg/ha. As the NO ₃ ^– –N content in the soil increased by 1 kg/ha, the soybean yield increase due to N fertilization in 1979 decreased by approximately 4 kg/ha.Contribution no. 82-368-J, Dep. of Agronomy, Kansas Agric. Exp. Stn., Manhattan, KS 66506, USA     

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.     

Evaluation of the availability ofAzolla-N and urea-N to rice using15N

Summary The symbiotic association of the water fernAzolla with the blue-green algaAnabaena azollae can fix 30–60 kg N ha^–1 per rice cropping season. The value of this fixed N for rice production, however, is only realized once the N is released from theAzolla biomass and taken up by the rice plants. The availability of N applied asAzolla or as urea was measured in field experiments by two¹⁵N methods. In the first,Azolla caroliniana (Willd.) was labelled with¹⁵N in nutrient solution and incorporated into the soil at a rate of 144 kg N ha^–1. The recovery ofAzolla-N in the above ground parts of rice [Oryza sativa (L) cv. Nucleoryza] was found to be 32% vs. 26% for urea applied at a rate of 100 kg N/ha; there was no significant difference in recovery. In the second, 100 kg N/ha of¹⁵N-urea was applied separately or in combination with either 250 or 330 kg N ha^–1 of unlabelledAzolla. At the higher rate, the recovery ofAzolla-N was significantly greater than that of urea. There was a significant interaction when both N sources were applied together, which resulted in a greater recovery of N from each source in comparison to that source applied separately. Increasing the combined urea andAzolla application rate from 350 kg N ha^–1 to 430 kg N ha^–1 increased the N yield but had no effect on the dry matter yield of rice plants. The additional N taken up at the higher level of N application accumulated to a greater extent in the straw compared to the panicles. Since no assumptions need to be made about the contribution of soil N in the method using¹⁵N-labelledAzolla, this method is preferable to the¹⁵N dilution technique for assessing the availability ofAzolla-N to rice. Pot trials usingAzolla stored at –20°C or following oven-drying showed that both treatments decreased the recovery of N by one third in comparison to freshAzolla.     

Quantitative effects of soil nitrate,growth potential and phenology on symbiotic nitrogen fixation of pea (Pisum sativum L.)

The influence of soil nitrate availability, crop growth rate and phenology on the activity of symbiotic nitrogen fixation (SNF) during the growth cycle of pea (Pisum sativum cv. Baccara) was investigated in the field under adequate water availability, applying various levels of fertiliser N at the time of sowing. Nitrate availability in the ploughed layer of the soil was shown to inhibit both SNF initiation and activity. Contribution of SNF to total nitrogen uptake (%Ndfa) over the growth cycle could be predicted as a linear function of mineral N content of the ploughed layer at sowing. Nitrate inhibition of SNF was absolute when mineral N at sowing was over 380 kg N ha^–1. Symbiotic nitrogen fixation was not initiated unless nitrate availability in the soil dropped below 56 kg N ha^–1. However, SNF could no longer be initiated after the beginning of seed filling (BSF). Other linear relationships were established between instantaneous %Ndfa and instantaneous nitrate availability in the ploughed layer of the soil until BSF. Instantaneous %Ndfa decreased linearly with soil nitrate availability and was nil above 48 and 34 kg N ha^–1 for the vegetative and reproductive stages, respectively, levels after which no SNF occurred. Moreover, SNF rate was shown to be closely related to the crop growth rate until BSF. The ratio of SNF rate over crop growth rate decreased linearly with thermal time. Maximum SNF rate was about 40 mg N m^–2 degree-day^–1, equivalent to 7 kg N ha^–1, regardless of the N treatment. From BSF to the end of the growth cycle, the high N requirements of the crop were supported by both SNF and nitrate root absorption but, of the two sources, nitrate root absorption seemed to be less affected by the presence of reproductive organs. However, since soil nitrate availability was low at the end of the growth cycle, SNF was the main source of nitrogen acquisition. The onset of SNF decrease at the end of the growth cycle seemed to be first due to nodule age and then associated to the slowing of the crop growth rate.     

Effect of application rates and abiotic factors on Steinernema carpocapsae for control of overwintering navel orangeworm (Lepidoptera: Pyralidae, Amyelois transitella) in pistachios

The effect of reduced application rate, soil temperature at shallow depth (2.5 cm), and soil type on the efficacy of Steinernema carpocapsae against the navel orangeworm, Amyelois transitella, was evaluated in six field trials employing 1 m² plots conducted from November 2003 through December 2004 in Madera and Kern Counties, California. Nematodes were applied at a concentration of 100,000 infective juveniles (IJs)/m² (10⁹/ha) in a volume of 187 ml water/m² (1870 L/ha) with a post-application irrigation in all trials. Mortality ranged from 7.9 to 64.9% in successful trials and percent reduction in live larvae per plot was as high as 74.6%. Percent reduction and mortality were highly correlated (r² = 0.78) and larval reduction typically was 10–11% greater than mortality for any treatment. In one trial, although nematode treatment significantly increased mortality compared to the controls, the treatment was deemed unsatisfactory because mortality was <15%. Soil temperature in this trial rose to 39 °C within 5 h after application. Nematodes failed in two other trials when soil temperature fell below freezing (minimum temperatures −3.0, −5.5 °C, respectively) several times in a 5-day period. We conclude that a commercially feasible application volume of 1870 L water/ha followed by post-application irrigation at this same rate was effective, and that soil maximum temperature at or below 32 °C during the first 24 h after application is necessary for treatment success.     

Long-term integrated soil fertility management in South-western Nigeria: Crop performance and impact on the soil fertility status

Crop response, tree biomass production and changes in soil fertility characteristics were monitored in a long-term (1986–2002) alley-cropping trial in Ibadan, Nigeria. The systems included two alley cropping systems with Leucaena leucocephala and Senna siamea on the one hand and a control (no-trees) system on the other hand, all cropped annually with a maize–cowpea rotation. All systems had a plus and minus fertilizer treatment. Over the years, the annual biomass return through tree prunings declined steadily, but more drastically for Leucaena than for Senna. In 2002, the nitrogen contribution from Leucaena residues stabilized at about 200 kg N/ha/year, while the corresponding value for Senna was about 160 kg N/ha/year. On average, the four Leucaena prunings were more equal in biomass as well as in amounts of N, P and cations, while the first Sennapruning was always contributing up to 60% of the annual biomass or nutrient return. Maize crop yields declined steadily in all treatments, but the least so in the Senna + fertilizer treatment where in 2002 still 2.2 tonnes/ha of maize were obtained. Nitrogen fertilizer use efficiency was usually higher in the Senna treatment compared to the control or the Leucaena treatment. Added benefits due to the combined use of fertilizer N and organic matter additions were observed only for the Sennatreatment and only in the last 6 years. At all other times, they remained absent or were even negative in the Leucaenatreatments for the first 3 years. Most chemical soil fertility parameters decreased in all the treatments, but less so in the alley cropping systems. The presence of trees had a positive effect on remaining carbon stocks, while they were reduced compared to the 1986 data. Trees had a positive effect on the maintenance of exchangeable cations in the top soil. Exchangeable Ca, Mg and K – and hence ECEC – were only slightly reduced after 16 years of cropping in the tree-based systems, and even increased in the Senna treatments. In the control treatments, values for all these parameters reduced to 50% or less of the original values after 16 years. All the above points to the Senna-based alley system with fertilizers as the more resilient one. This is reflected in all soil fertility parameters, in added benefits due to the combined use of fertilizer nitrogen and organic residue application and in a more stable maize yield over the years, averaging 2.8 tonnes/ha with maximal deviations from the average not exceeding 21%.     

Effect of soil compaction on root growth and uptake of phosphorus

Summary Zea mays L. andLolium rigidum Gaud. were grown for 18 and 33 days respectively in pots containing three layers of soil each weighing 1 kg. The top and bottom layers were 100 mm deep and they had a bulk density of 1200 kg m^–3, while the central layer of soil was compacted to one of 12 bulk densities between 1200 and 1750 kg m^–3. The soil was labelled with³²P and³³P so that the contribution of the different layers of soil to the phosphorus content of the plant tops could be determined. Soil water potential was maintained between –20 and –100 kPa.Total dry weight of the plant tops and total root length were slightly affected by compaction of the soil, but root distribution was greatly altered. Compaction decreased root length in the compacted soil but increased root length in the overlying soil. Where bulk density was 1550 kg m^–3, root length in the compacted soil was about 0.5 of the maximum. At that density, the penetrometer resistance of the soil was 1.25 and 5.0 MPa and air porosity was 0.05 and 0.14 at water potentials of –20 and –100 kPa respectively, and daytime oxygen concentrations in the soil atmosphere at time of harvest were about 0.1 m³m^–3. Roots failed to grow completely through the compacted layer of soil at bulk densities 1550 kg m^–3. No differences were detected in the abilities of the two species to penetrate compacted soil.Ryegrass absorbed about twice as much phosphorus from uncompacted soil per unit length of root as did maize. Uptake of phosphorus from each layer of soil was related to the length of root in that layer, but differences in uptake between layers existed. Phosphorus uptake per unit length of root was higher from compacted than from uncompacted soil, particularly in the case of ryegrass at bulk densities of 1300–1500 kg m^–3.     

Estimation of biological nitrogen fixation associated withBrachiaria andPaspalum grasses using15N labelled organic matter and fertilizer

Summary Six pasture grasses,Paspalum notatum cv batatais,P. notatum cv pensacola,Brachiaria radicans, B. ruziziensis, B. decumbens andB. humidicola, were grown in concrete cylinders (60 cm diameter) in the field for 31 months. The soil was amended with either a single addition of¹⁵N labelled organic matter or frequent small (2 kg N. ha^–1) additions of¹⁵N enriched (NH₄)₂SO₄. In the labelled fertilizer treatment soil analysis revealed that there was a very drastic change in¹⁵N enrichment in plant-available nitrogen (NO ₃ ^– +NH ₄ ⁺ ) with depth. The different grass cultivars recovered different quantities of applied labelled N, and evidence was obtained to suggest that the roots exploited the soil to different depths thus obtaining different¹⁵N enrichments in soil derived N. This invalidated the application of the isotope dilution technique to estimate the contribution of nitrogen fixation to the grass cultivars in this treatment. In the labelled organic matter treatment the¹⁵N label in the plant-available N declined at a decreasing rate during the experiment until in the last 12 months the decrease was only from 0.274 to 0.222 atom % excess. There was little change in¹⁵N enrichment of available N with depth, hence it was concluded that although the grasses recovered different quantities of labelled N, they all obtained virtually the same¹⁵N enrichment in soil derived N. Data from the final harvests of this treatment indicated thatB. humidicola andB. decumbens obtained 30 and 40% respectively of their nitrogen from N₂ fixation amounting to an input of 30 and 45 kg N.ha^–1 year^–1 respectively.     

Persistence and competitiveness of threeBradyrhizobium japonicum inoculant strains in clay loam Nile valley soil

The nitrogen contribution from the shoot and root system of symbiotically grown leucaena was evaluated in a field experiment on an Alfisol at IITA in Southern Nigeria. Maize in plots that received prunings from inoculated leucaena contained more N and grain yield was increased by 1.9 t.ha.^–1. Large quantities of nitrogen were harvested with leucaena prunings (300 kg N ha^–1 in six months) but the efficiency of utilization of this nitrogen by maize was low compared to inorganic N fertilizer (ammonium sulphate) at 80 kg N ha^–1. Maize yield data indicated that nitrogen in leucaena prunigs was 34 and 45% as efficient as 80 kg N ha^–1 of (NH₄)₂SO₄ for uninoculated and inoculated plants with Rhizobium IRc 1045, respectively. In plots where the prunings were removed, the leaf litter and decaying roots and nodules contributed N equivalent of 32 kg ha^–1. Twenty-five kg ha^–1 was the inorganic N equivalent from nitrogen fixed symbiotically by leucaena when inoculated with Rhizobium strain IRc 1045. Application of prunings from inoculated leucaena resulted in higher soil ogranic C, total N, pH and available NO₃.     

Estimation of residual N effect of faba bean and pea on two succeeding cereals using15N methodology

A field experiment was conducted using¹⁵N methodology to study the effect of cultivation of faba bean (Vicia faba L.), pea (Pisum sativum L.) and barley (Hordeum vulgare L.) on the N status of soil and their residual N effect on two succeeding cereals (sorghum (Sorghum vulgare) followed by barley). Faba bean, pea and barley took up 29.6, 34.5 and 53.0 kg N ha^–1 from the soil, but returned to soil through roots only 11.3, 10.8 and 5.7 kg N ha^–1, respectively. Hence, removal of faba bean, pea and barley straw resulted in a N-balance of about –18, –24, and –47 kg ha^–1 respectively. A soil nitrogen conserving effect was observed following the cultivation of faba bean and pea compared to barley which was of the order of 23 and 18 kg N ha^–1, respectively. Cultivation of legumes resulted in a significantly higher A_N value of the soil compared to barley. However, the A_N of the soil following fallow was significantly higher than following legumes, implying that the cultivation of the legumes had depleted the soil less than barley but had not added to the soil N compared to the fallow. The beneficial effect of legume cropping also was reflected in the N yield and dry matter production of the succeeding crops. Cultivation of legumes led to a greater exploitation of soil N by the succeeding crops. Hence, appreciable yield increases observed in the succeeding crops following legumes compared to cereal were due to a N-conserving effect, carry-over of N from the legume residue and to greater uptake of soil N by the succeeding crops when previously cropped to legumes.     

Effect of plant genotype and nitrogen fertilizer on symbiotic nitrogen fixation by soybean cultivars

Summary Isotopic as well as non-isotopic methods were used to assess symbiotic nitrogen fixation within eight soybean [Glycine max (L.) Merr.] cultivars grown at 20 and 100 kg N/ha levels of nitrogen fertilizer under field conditions.The¹⁵N methodology revealed large differences between soybean cultivars in their abilities to support nitrogen fixation. In almost all cases, the application of 100 kg N/ha resulted in lower N₂ fixed in soybean than at 20 kg N/ha in the first year of the study. However, N₂ fixed in one cultivar, Dunadja, was not significantly affected by the higher rate of N fertilizer application. These results were confirmed by measurements of acetylene reduction activity, nodule dry weight and N₂ fixed as measured by the difference method. Further proof of differences in N₂ fixed within soybean cultivars and the ability of Dunadja to fix similar amounts of N₂ at 20 and 100 kg N/ha was obtained during a second year experiment. Dunadja yield was affected by N fertilizer and produced larger yield at 100 kg N/ha than at 20 kg N/ha. This type of cultivar could be particularly useful in situations where soil N levels are high or where there is need to apply high amounts of N fertilizer.The present study reveals the great variability between legume germplasms in the ability to fix N₂ at different inorganic N levels, and also the potential that exists in breeding for nitrogen fixation associative traits. The¹⁵N methodology offers a unique tool to evaluate germplasms directly in the field for their N₂ fixation abilities at different N fertilizer levels.     

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.     

Nitrogen concentration of cauliflower organs as determined by organ size,N supply,and radiation environment

Data from field experiments carried out in three consecutive years under contrasting N supply and radiation environment altered by artificial shading were used to identify (a) the relationship between N concentration and organ size under conditions of unrestricted N supply and (b) critical levels of soil nitrate (Nmin_crit), where nitrogen concentration of cauliflower organs begin to decline because of N limitations. The decline of N concentrations in cauliflower was analysed at different levels of morphological aggregation, i.e., the whole shoot level, the organ level (leaves, stem, and curd), and within different leaf groups within the canopy. Nmin_crit values (0–60 cm soil depth) for total nitrogen concentration of cauliflower organs leaves, stem and curd were estimated at 85, 93 and 28 kg N ha^–1, respectively. Within the canopy, Nmin_crit values for total N of leaves increased from the top to the bottom from 44 to 188 kg N ha^–1. Nmin_crit values for protein N in leaves from different layers of the canopy were much lower at around 30 kg N ha^–1, without a gradient within the canopy. It is discussed that these differences in Nmin_crit values are most likely a consequence of N redistribution associated with nitrogen deficiency. The decline of average shoot nitrogen concentrations, [Nm] (%N DM), with shoot dry matter, W _sh, (t ha^–1) under conditions of optimal N supply was [Nm]= 4.84 (±0.071) W _sh ^{–0.089(± 0.011)}, r ²=0.67 (±S.E.). The reduction of radiation intensity by artificial shading (60% of control) had no significant influence on total nitrogen concentrations of leaves and only a small influence on protein nitrogen concentrations in lower layers of the canopy. The leaf nitrate nitrogen fraction of nitrogen, f _nitr (–), within the canopy decreased linearly with increased average incident irradiance in different canopy layers (I _av, W PAR m^–2) (f _Nitr. = 0.2456(±0.0188) – 0.0023(±0.0004)I _av, r ² = 0.67.     

Comparisons of primary production and nutrients absorption by aMiscanthus sinensis community in different soils

Soil properties, primary production, nitrogen and phosphorus uptake in aMiscanthus sinensis community on serpentine gangue area were compared with that on nonserpentine area. Soil water content, soil pH and nitrogen content were quite different between the serpentine gangue area and nonserpentine area; but phosphorus content of the soil was similar between the two sites. The maximum above-ground net production in the serpentine gangue and nonserpentine areas was 4.5±0.2 kg m^–2 yr^–1 and 7.8±0.2 kg m^–2 yr^–1, respectively. The total maximum standing biomass in the serpentine gangue and nonserpentine areas was 8.5±0.8 kg m^–2 and 11.9±0.4 kg m^–2, respectively. Nitrogen uptake by plants in the nonserpentine area was 2.4 times greater than that in the serpentine gangue area. Phosphorus uptake by plants were similar for the two sites. The most probable reasons for the small biomass produced by theMiscanthus sinensis community in this serpentine gangue area are the low levels of nitrogen and water availability in the soil.     

江河源区高山嵩草(Kobresia pygmaea)草甸植物和土壤碳、氮储量对覆被变化的响应

以青海省果洛州藏族自治州甘德县青珍乡高山嵩草Kobresia pygmaea草甸轻度退化草地和重度退化草地为研究对象,通过植物地上部分主要功能群(禾草类、杂类草、莎草类)、植物根系和土壤碳、氮浓度及储量动态研究,结果表明:高寒小嵩草草甸轻度退化草地地上部分主要功能群碳、氮浓度和C ∶ N比值明显高于重度退化草地的浓度.同一草地类型主要功能群比较,碳、氮浓度依次为杂类草>禾草类>莎草类;植物地上部分的碳、氮浓度明显高于地下根系的碳、氮浓度.重度退化草地植物根系碳、氮浓度高于轻度退化草地植物根系碳、氮浓度.重度退化草地土壤总有机碳浓度显著低于轻度退化草地土壤总有机碳浓度,随着土层的加深碳、氮浓度有减少的趋势.江河源区高山嵩草草甸的土壤有机碳、氮储量最大,植物根系碳、氮储量居中,植物地上部分碳、氮储量最小.重度退化草地总有机碳储量(13554.3 g/m2)较轻度退化草地储量(14669.2 g/m2)下降7.60%.其中,0～40cm土壤层碳储量下降4.10%,植物根系碳储量下降59.97%,植物地上部分碳储量下降15.39%;重度退化草地总氮储量(3780.6 g/m2)较轻度退化草地储量(3352.7 g/m2)高12.76%,其中,0～40cm土壤中总氮储量高13.07%,植物根系全氮储量下降55.09%,植物地上部分全氮下降16.00%.由于草地退化损失有机碳11149 kg/hm2,而全氮增加4278 kg/hm2.     

Field observations on nitrogen catch crops. III. Transfer of nitrogen to the succeeding main crop

In temperate climates with a precipitation surplus during autumn and winter, nitrogen (N) catch crops can help to reduce nitrogen losses from cropping systems by absorbing nitrogen from the soil and transfer it to a following main crop. In two field experiments the catch crop species winter rye (Secale cereale) and forage rape (Brassica napus ssp. oleifera (Metzg.) Sinsk) or oil radish (Raphanus sativus spp. oleiferus (DC.) Metzg.) were planted end of August and 3 weeks later with a non-limiting supply of N and zero-N controls. In the next spring catch crops were incorporated into the soil. In Expt 1, N transfer was measured as (i) the N uptake of a potato test crop, grown with zero and 12.5 g m^–2 N applied, and (ii) the increase in soil mineral N (0–30 cm) in uncropped soil covered with polythene film. In Expt 2, N transfer was measured as the increase in soil mineral N in covered cylinders placed in uncropped soil (in situ incubation). Subsidiary laboratory incubations were performed in Expt 2. In Expt 1, the apparent recovery in potato of fertilizer N (R _f) was 0.56. The recovery in potato of N mineralized from 'native' N pools other than catch crop material (R _n) ranged from 0.43 to 0.51, depending on the value assumed for the depth of N extraction by potato roots. The average recovery in potato of incorporated catch crop N (R _c) was 0.34. Expressed as `fertilizer N replacement factor' (F _r) the latter was 0.61 (i.e. 1 kg of N in catch crop material counts for 0.61 kg fertilizer N). Under the film in Expt 1 the fraction net mineralization of incorporated catch crop N (M _n) was 0.36 on August 11 and 0.43 on October 18. In Expt 2, the average value of M _n was 0.31, which was lower than in Expt 1 and probably associated with the drier soil in Expt 2. In the laboratory incubations (20°C) M _n showed values up to 0.54 after 84 days with the largest rates of change in mineralization occuring early after the start of the incubation. In conjunction with literature data it is concluded that cultivation of nitrogen catch crops shows promise as a means to reduce N input and N losses in temperate climates with wet winters.     

The effect of inoculation with Azospirillum brasilense on growth and nitrogen utilization by wheat plants

Azospirillium brasilense is a rhizosphere bacteria that has been reported to improve yield when inoculated on wheat plants. However, the mechanisms through which this effect is induced is still unclear. In the present work, we have studied the effects of inoculating a highly efficient A. brasilense strain on wheat plant grown in 5 kg pots with soil in a greenhouse, under three N regimes (0, 3 or 16 mM NO₃ ^–, 50 ml/pot once or twice-a -week), and in disinfected or non-disinfected soil. At the booting stage, the inoculated roots in both soils showed a similar colonization by Azospirillum sp. that was not affected by N addition. The plants grown in the disinfected soil showed a higher biomass, N content and N concentration than those in the non-disinfected soil, and in both soils the inoculation stimulated plant growth, N accumulation, and N and NO₃ ^– concentration in the tissues.At maturity, the inoculated plants showed a higher biomass, grain yield and N content than the uninoculated ones in both soils, and a higher grain protein concentration than the uninoculated. It is concluded that in the present experiments, A. brasilenseincreased plant growth by stimulating nitrogen uptake by the roots.     

In situ measurement of fine root water absorption in three temperate tree species—Temporal variability and control by soil and atmospheric factors

Miniature heat balance-sap flow gauges were used to measure water flows in small-diameter roots (3–4 mm) in the undisturbed soil of a mature beech–oak–spruce mixed stand. By relating sap flow to the surface area of all branch fine roots distal to the gauge, we were able to calculate real time water uptake rates per root surface area (J_s) for individual fine root systems of 0.5–1.0 m in length. Study aims were (i) to quantify root water uptake of mature trees under field conditions with respect to average rates, and diurnal and seasonal changes of J_s, and (ii) to investigate the relationship between uptake and soil moisture θ, atmospheric saturation deficit D, and radiation I. On most days, water uptake followed the diurnal course of D with a mid-day peak and low night flow. Neighbouring roots of the same species differed up to 10-fold in their daily totals of J_s (<100–2000 g m⁻² d⁻¹) indicating a large spatial heterogeneity in uptake. Beech, oak and spruce roots revealed different seasonal patterns of water uptake although they were extracting water from the same soil volume. Multiple regression analyses on the influence of D, I and θ on root water uptake showed that D was the single most influential environmental factor in beech and oak (variable selection in 77% and 79% of the investigated roots), whereas D was less important in spruce roots (50% variable selection). A comparison of root water uptake with synchronous leaf transpiration (porometer data) indicated that average water fluxes per surface area in the beech and oak trees were about 2.5 and 5.5 times smaller on the uptake side (roots) than on the loss side (leaves) given that all branch roots <2 mm were equally participating in uptake. Beech fine roots showed maximal uptake rates on mid-summer days in the range of 48–205 g m⁻² h⁻¹ (i.e. 0.7–3.2 mmol m⁻² s⁻¹), oak of 12–160 g m⁻² h⁻¹ (0.2–2.5 mmol m⁻² s⁻¹). Maximal transpiration rates ranged from 3 to 5 and from 5 to 6 mmol m⁻² s⁻¹ for sun canopy leaves of beech and oak, respectively. We conclude that instantaneous rates of root water uptake in beech, oak and spruce trees are above all controlled by atmospheric factors. The effects of different root conductivities, soil moisture, and soil hydraulic properties become increasingly important if time spans longer than a week are considered.     

Nitrogen fixation of field-inoculatedLeucaena leucocephala (Lam.) de Wit estimated by the15N and the difference methods

The amount of nitrogen fixed byLeucaena leucocephala (Lam.) de Wit was assessed on an Alfisol at the International Institute of Tropical Agriculture located in southwestern Nigeria. Estimated by the difference method, nitrogen fixation of leucaena inoculated with Rhizobium strain IRc 1045 was 133 kg ha^–1 in six months. Inoculation with Rhizobium strain IRc 1050 gave a lower nitrogen fixation of 76 kg ha^–1. Fertilization with 40 and 80 kg N ha^–1 inhibited nitrogen fixation by 43–76% and 49–71%, respectively. Estimates with the¹⁵N dilution method gave nitrogen fixation of 134 kg ha^–1 in six months when leucaena was inoculated with Rhizobium strain IRc 1045 and 98 kg ha^–1 for leucaena inoculated with Rhizobium strain IRc 1050. This nitrogen fixation represented 34–39% of the plant nitrogen. Inoculated leucaena derived 5–6% of its nitrogen from applied fertilizer and 56–54% from soil.     

Nitrogen fixation and metabolism by groundwater-dependent perennial plants in a hyperarid desert

The Central Asian Taklamakan desert is characterized by a hyperarid climate with less than 50 mm annual precipitation but a permanent shallow groundwater table. The perched groundwater (2–16 m) could present a reliable and constant source of nitrogen throughout the growing season and help overcome temporal nitrogen limitations that are common in arid environments. We investigated the importance of groundwater and nitrogen fixation in the nitrogen metabolism of desert plants by assessing the possible forms and availability of soil N and atmospheric N and the seasonal variation in concentration as well as isotopic composition of plant N. Water availability was experimentally modified in the desert foreland through simulated flooding to estimate the contribution of surface water and temporally increased soil moisture for nutrient uptake and plant–water relations. The natural vegetation of the Taklamakan desert is dominated by plants with high foliar nitrogen concentrations (2–3% DM) and leaf nitrate reductase activity (NRA) (0.2–1 mol NO₂^– g^–1 FW h^–1). There is little evidence that nitrogen is a limiting resource as all perennial plants exhibited fast rates of growth. The extremely dry soil conditions preclude all but minor contributions of soil N to total plant N so that groundwater is suggested as the dominant source of N with concentrations of 100 M NO₃^–. Flood irrigation had little beneficial effect on nitrogen metabolism and growth, further confirming the dependence on groundwater. Nitrogen fixation was determined by the ¹⁵N natural abundance method and was a significant component of the N-requirement of the legume Alhagi, the average contribution of biologically fixed nitrogen in Alhagi was 54.8%. But nitrogen fixing plants had little ecological advantage owing to the more or less constant supply of N available from groundwater. From our data we conclude that the perennial species investigated have adapted to the environmental conditions through development of root systems that access groundwater to satisfy demands for both water and nutrients. This is an ecologically favourable strategy since only groundwater is a predictable and stable resource.