Effect of soil-applied NPK fertilizers on severity of black spot disease (Alternaria brassicae) and yield of oilseed rape

Effect of soil application of eight combinations of NPK fertilizers on the severity of black spot disease (BSD), caused by Alternaria brassicae (Sacc.) Berk., and yield of short duration oilseed rape (Brassica campestris L) were investigated under both pot and field conditions in 1987–88, 1988–89 and 1990–91. The severity of BSD was significantly greater (36–48%) on plants grown in ground treated with NP (N 90 kg ha^–1+P 40 kg ha^–1) applied as urea and single superphosphate respectively than on plants from the unfertilized control (NoPoKo) (o). However, the severity of BSD was significantly smaller (25–33%) when K (40 kg ha^–1) was applied as muriate of potash than in plants from control and NP treatments. The effect of NK (N 90 kg ha^–1+K 40 kg ha^–1) in decreasing the severity of BSD was increasingly more pronounced than the effects of PK (P 40 kg ha^–1+K 40 kg ha^–1), NP and K (40 kg ha^–1) applications. The decrease in the severity of BSD due to K was due to increased production in plants of phenolics which inhibited conidial germination and decreased sporulation of A. brassicae.The decrease in the severity of BSD due to NK application gave consistently increased seed yield 68% more than those of control and other treatments. The K-fertilized plants also showed increased resistance to lodging, increased 1000-seed weight and decreased seed infection. Seeds obtained from K-fertilized plants showed good seed germinability and vigorous seeding growth.     

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.     

Competition in tree row agroforestry systems. 2. Distribution, dynamics and uptake of soil inorganic N

The effect of tree row species on the distribution of soil inorganic N and the biomass growth and N uptake of trees and crops was investigated beneath a Grevillea robustaA. Cunn. ex R. Br. (grevillea) tree row and Senna spectabilisDC. (senna) hedgerow grown with Zea mays L. (maize) and a sole maize crop, during one cropping season. The hypothesis was that a tree with a large nutrient uptake would have a greater competitive effect upon coexisting plants than a tree that takes up less and internally cycles nutrients. The field study was conducted on a kaolinitic Oxisol in the sub-humid highlands of western Kenya. Soil nitrate and ammonium were measured to 300 cm depth and 525 cm distance from the tree rows, before and after maize cropping. Ammonium concentrations were small and did not change significantly during the cropping season. There was > 8 mg nitrate kg^–1 in the upper 60 cm and at 90–180 cm depth at the start of the season, except within 300 cm of the senna hedgerow where concentrations were smaller. During the season, nitrate in the grevillea-maize system only decreased in the upper 60 cm, whereas nitrate decreased at almost every depth and distance from the senna hedgerow. Inorganic N (nitrate plus ammonium) decreased by 94 kg ha^–1 in the senna-maize system and 33 kg ha^–1 in the grevillea-maize system.The aboveground N content of the trees increased by 23 kg ha^–1 for grevillea and 39 kg ha^–1 for senna. Nitrogen uptake by maize was 85 kg ha^–1 when grown with grevillea and 65 kg ha^–1 with senna. Assuming a mineralisation input of 50 kg N ha^–1season^–1, the decrease in inorganic soil N approximately equalled plant N uptake in the grevillea-maize system, but exceeded that in the senna-maize system. Pruning and litter fall removed about 14 kg N ha^–1 a^–1 from grevillea, and > 75 kg N ha^–1 a^–1 from senna. The removal of pruned material from an agroforestry system may lead to nutrient mining and a decline in productivity.     

Effects of forest clear-cutting on the carbon and nitrogen fluxes through podzolic soil horizons

Effects of clear-cutting on the dissolved fluxes of organic C (DOC), organic N (DON), NO₃ ^– and NH₄ ⁺ through surface soil horizons were studied in a Norway spruce dominated mixed boreal forest in eastern Finland. Bulk deposition, total throughfall and soil water from below the organic (including understorey vegetation and, after clear-cutting, also logging residues), eluvial and illuvial horizons were sampled weekly from 1993 to 1999. Clear-cutting was carried out in September 1996. The removal of the tree canopy decreased the deposition of DOC and DON to the forest floor and increased that of NH₄ ⁺ and NO₃ ^– but did not affect the deposition of total N (DTN, <3 kg ha^–1 a^–1). The leaching of DOC and DON from the organic horizon increased over twofold after clear-cutting (fluxes were on an average 168 kg C and 3.3 kg N ha^–1 a^–1), but the increased outputs were effectively retained in the surface mineral soil horizons. Inorganic N deposition was mainly retained by the logging residues and organic horizon indicating microbial immobilization. Increased NO₃ ^– formation reflected as elevated concentrations in the percolate from below the mineral soil horizons were observed especially in the third year after clear-cutting. However, the changes were small and the increased leaching of DTN from below the illuvial horizon remained small (<0.4 kg ha^–1 a^–1) and mainly DON. Effects of clear-cutting on the transport of C and N to surface waters will probably be negligible.     

Exchange of N-gases at the Höglwald Forest – A summary

During 4 years continuous measurements of N-trace gas exchange were carried out at the forest floor-atmosphere interface at the Höglwald Forest that is highly affected by atmospheric N-deposition. The measurements included spruce control, spruce limed and beech sites. Based on these field measurements and on intensive laboratory measurements of N₂-emissions from the soils of the beech and spruce control sites, a total balance of N-gas emissions was calculated. NO₂-deposition was in a range of –1.6 –2.9 kg N ha^–1 yr^–1 and no huge differences between the different sites could be demonstrated. In contrast to NO₂-deposition, NO- and N₂O-emissions showed a huge variability among the different sites. NO emissions were highest at the spruce control site (6.4–9.1 kg N ha^–1 yr^–1), lowest at the beech site (2.3–3.5 kg N ha^–1 yr^–1) and intermediate at the limed spruce site (3.4–5.4 kg N ha^–1 yr^–1). With regard to N₂O-emissions, the following ranking between the sites was found: beech (1.6–6.6 kg N ha^–1 yr^–1) >> spruce limed (0.7–4.0 kg N ha^–1 yr^–1) > spruce control (0.4–3.1 kg N ha^–1 yr^–1). Average N-trace gas emissions (NO, NO₂, N₂O) for the years 1994–1997 were 6.8 kg N ha^–1 yr^–1 at the spruce control site, 3.6 kg N ha^–1 yr^–1 at the limed spruce site and 4.5 kg N ha^–1 yr^–1 at the beech site. Considering N₂-losses, which were significantly higher at the beech (12.4 kg N ha^–1 yr^–1) than at the spruce control site (7.2 kg N ha^–1 yr^–1), the magnitude of total gaseous N losses, i.e. N₂-N + NO-N + NO₂-N + N₂O-N, could be calculated for the first time for a forest ecosystem. Total gaseous N-losses were 14.0 kg N ha^–1 yr^–1 at the spruce control site and 15.5 kg N ha^–1 yr^–1 at the beech site, respectively. In view of the huge interannual variability of N-trace gas fluxes and the pronounced site differences in N-gas emissions it is concluded that more research is needed in order to fully understand patterns of microbial N-cycling and N-gas production/emission in forest ecosystems and mechanisms of reactions of forest ecosystems to the ecological stress factor of atmospheric N-input.     

The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha^–1 y^–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha^–1 y^–1 below the litter layer to around 50 kg C kg C ha^–1 y^–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO₃⁻). The dominance of NO₃^– relative to the total amount nitrate of N leaching from the soil shows that NO₃^– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.     

Differences among maize cultivars in the utilization of soil nitrate and the related losses of nitrate through leaching

In a 2-year field experiment conducted on a Gleyic Luvisol in Stuttgart-Hohenheim one experimental and nine commercial maize cultivars were compared for their ability to utilize soil nitrate and to reduce related losses of nitrate through leaching. Soil nitrate was monitored periodically in CaCl₂ extracts and in suction cup water. Nitrate concentrations in suction water were generally higher than in CaCl₂ extracts. Both methods revealed that all cultivars examined were able to extract nitrate down to a soil depth of at least 120 cm (1988 season) or 150 cm (1987 season). Significant differences among the cultivars existed in nitrate depletion particularly in the subsoil. At harvest, residual nitrate in the upper 150 cm of the profile ranged from 73–110 kg N ha^–1 in 1987 and from 59–119 kg N ha^–1 in 1988. Residual nitrate was closely correlated with nitrate losses by leaching because water infiltration at 120 cm soil depth started 4 weeks after harvest (1987) or immediately after harvest (1988) and continued until early summer of the following year. The calculated amount of nitrate lost by leaching was strongly influenced by the method of calculation. During the winter of 1987/88 nitrate leaching ranged from 57–84 kg N ha^–1 (suction cups) and 40–55 kg N ha^–1 (CaCl₂ extracts), respectively. The corresponding values for the winter of 1988/89 were 47–79 and 20–39 kg N ha^–1, respectively. ei]Section editor: B E Clothier     

Effects of planted tree fallows on soil nitrogen dynamics,above-ground and root biomass,N2-fixation and subsequent maize crop productivity in Kenya

The effects of planted fallows of Sesbania sesban (L.) Merr. and Calliandra calothyrsus (Meissner) on soil inorganic nitrogen dynamics and two subsequent maize crops were evaluated under field conditions in the highlands of eastern Kenya. Continuous unfertilised maize, maize/bean rotation and natural regrowth of vegetation (weed fallow) were used as control treatments. The proportion of symbiotic N₂-fixation was estimated by measuring both leaf ¹⁵N enrichment and whole-plant ¹⁵N enrichment by the ¹⁵N dilution technique for Sesbania and Calliandra, using Eucalyptus saligna (Sm.) and Grevillea robusta (A. Cunn) as reference species. Above- and below-ground biomass and N contents were examined in Sesbania, Calliandra, Eucalyptus and Grevillea 22 months after planting. Both the content of inorganic N in the topsoil and the quantity of N mineralised during rainy seasons were higher after the Sesbania fallows than after the other treatments. Compared to the continuous unfertilised maize treatment, both residual crop yields were significantly higher when mineral N (one application of 60 kg N ha^–1) was added. Furthermore, the second crop following the Sesbania fallow was significantly higher than the continuous maize crop. The above-ground biomass of the trees at final harvest were 31.5, 24.5, 32.5 and 43.5 Mg ha^–1 for the Sesbania, Calliandra, Grevillea and Eucalyptus, respectively. For the total below-ground biomass the values for these same tree species were 11.1, 15.5, 17.7, and 19.1 Mg ha^–1, respectively, of which coarse roots (>2 mm), including tap roots, amounted to 70–90%. About 70–90% of the N in Sesbania, and 50–70% in Calliandra, was derived from N₂-fixation. Estimates based on leaf ¹⁵N enrichment and whole-plant ¹⁵N enrichment were strongly correlated. The N added by N₂-fixation amounted to 280–360 kg N ha^–1 for Sesbania and 120–170 kg N ha^–1 for Calliandra, resulting in a positive N balance after two maize cropping seasons of 170–250 kg N ha^–1 and 90–140 kg N ha^–1, for Sesbania and Calliandra, respectively. All the other treatments gave negative N balances after two cropping seasons. We conclude that Sesbania sesban is a tree species well suited for short duration fallows due to its fast growth, high nutrient content, high litter quality and its ability to fix large amounts of N₂ from the atmosphere.     

Is a ferroxidase involved in the high-affinity iron uptake in Chlamydomonas reinhardtii?

In phosphorus deficient soils and under smallscale farming systems, the development of efficient management strategies for P fertilizers is crucial to sustain food production. A field experiment was conducted on a P-fixing Acrisol in western Kenya to study possibilities of replenishing soil P with seasonal additions of small rates of P fertilizers. Triple superphosphate was applied at 0, 10, 25, 50 and 150 kg P ha^–1 for 5 consecutive maize growing seasons followed by 4 seasons of residual crops. Maize yields and soil P fractions were determined. Although maize responded to additions of 10 kg P ha^–1 with a cumulative grain yield of 16.8 Mg ha^–1, at the end of the experiment, compared to 8.8 Mg ha^–1 in the non-P fertilized plots, soil labile P did not increase correspondingly. Seasonal additions of 150 kg P ha^–1 increased maize yields to a cumulative value of 39 Mg ha^–1 at the end of the experiment, and increased all soil inorganic P fractions. At the third season of residual phase, treatment with a cumulative addition of 750 kg P ha^–1 gave the highest yields compared to treatments in the same residual stage, but these yields were considered less than the maximum yield of the season. This indicates that the large build up of soil P was not available for crop uptake. The inorganic P fraction extracted by NaHCO₃ was the most affected by changes in management, increasing during the input phase and decreasing after interruption of P addition, for all P rates. The decrease in this pool during the residual phase could be explained by the maize uptake. This study showed that seasonal additions of 25 kg P ha^–1 can increase maize yield with gradual replenishment of soil P.     

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.     

Biogeochemistry of unpolluted forested watersheds in the Oregon Cascades: temporal patterns of precipitation and stream nitrogen fluxes

We analyzed long-term organic and inorganic nitrogen inputs and outputs in precipitation and streamwater in six watersheds at the H.J. Andrews Experimental Forest in the central Cascade Mountains of Oregon. Total bulk N deposition, averaging 1.6 to 2.0 kg N ha^–1 yr^–1, is low compared to other sites in the United States and little influenced by anthropogenic N sources. Streamwater N export is also low, averaging <1 kg ha^–1 yr^–1. DON is the predominant form of N exported from all watersheds, followed by PON, NH₄-N, and NO₃-N. Total annual stream discharge was a positive predictor of annual DON output in all six watersheds, suggesting that DON export is related to regional precipitation. In contrast, annual discharge was a positive predictor of annual NO₃-N output in one watershed, annual NH₄-N output in three watersheds, and annual PON output in three watersheds. Of the four forms of N, only DON had consistent seasonal concentration patterns in all watersheds. Peak streamwater DON concentrations occurred in November-December after the onset of fall rains but before the peak in the hydrograph, probably due to flushing of products of decomposition that had built up during the dry summer. Multiple biotic controls on the more labile nitrate and ammonium concentrations in streams may obscure temporal DIN flux patterns from the terrestrial environment. Results from this study underscore the value of using several watersheds from a single climatic zone to make inferences about controls on stream N chemistry; analysis of a single watershed may preclude identification of geographically extensive mechanisms controlling N dynamics.     

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.     

Evaluation of symbiotic properties and nitrogen contribution of mucuna to maize grown in the derived savanna of West Africa

The severity and increase of the Imperata cylindrica constraint as a weed, the decline of the traditional fallow systems as a means of soil fertility management and the lack of inorganic fertilizer appear to have created opportunities for adoption of mucuna (Mucuna pruriens) technology by smallholder farmers in some areas in the derived savanna of West Africa. What is not known, however, is the extent to which the establishment and N contribution of mucuna in these areas depend on symbiotic properties such as effective nodulation and mycorrhizal infection. Short term surveys carried out in 34 farmer's arable fields located in four different sites in the derived savanna, southern Benin, West Africa, together with results of greenhouse and field experiments showed that mycorrhizal infection rate of mucuma ranged from 2 to 31% and correlated positively with nodulation and shoot dry matter production. Nodulation occurred in 79% of the fields with numbers of nodules ranging from 0 to 135 plant^–1. Mucuna responded both to inoculation and N fertilizer in degraded soils but growth response depended on the rhizobia strains and mucuna varieties. Mucuna accumulated in 12 weeks about 313 kg N ha^–1 as either a sole crop or 166 kg N ha^–1 when mixed/intercropped with maize, respectively. Across all cropping systems it derived an average of 70% of its N from atmospheric N₂ (estimates made by the ¹⁵N isotope dilution method), representing 167 kg N ha^–1 per 12 weeks in the field. Mucuna interplanted with maize obtained a greater proportion of its nitrogen (74%) from fixation than did mucuna grown alone (66%) suggesting that competition for soil N influences the proportion of nitrogen fixed by mucuna. The total amount of N₂ fixed per hectare was, however, reduced significantly by intercropping mucuna with maize. A preceding mucuna crop provided a maize yield equivalent to 120 kg N kg ha^–1 of inorganic N fertilizer.     

Role of biological nitrogen fixation in legume based cropping systems; a case study of West Africa farming systems

Nitrogen (N) has been gradually depleted from West African soils and now poses serious threats to food production. Many ways of increasing N supply (e.g. judicious use of inorganic fertilizers and nitrogen-fixing plants) have been tried in West African farming systems. Herbaceous and woody legumes commonly contribute 40–70 kg N ha^–1 season. This represents about 30% of the total N applied as residues. Nevertheless and despite repeated demonstrations of the usefulness of green manures in enhancing soil fertility, their practices and adoption are still limited. Promiscuous soyabeans are being used to develop sustainable cropping systems in the moist savannah. Reliable estimates of N₂ fixed by soyabeans and their residual N benefits to subsequent cereal crops in the savannah zone of southern Guinea have only infrequently been made. The actual amounts measured varied between 38 and 126 kg N ha^–1 assuming that only seeds of soyabeans are removed from the plots, the net N accrual of soil nitrogen ranges between minus 8 kg N ha^–1 and plus 47 kg N ha^–1 depending on the soyabean cultivar. Residual soyabean N values of 10–24 kg N ha^–1 (14–36% of the total N in maize) were obtained in a soyabean-maize rotation. Although cereal yields following legume cultivation have been attributed to greater N accumulation, our data show that the relative increase in maize N was smaller than the relative increase in dry-matter yield. Hence, the increased yields of maize following soybeans are not entirely due to the carry-over of N from soyabean residues (as well as to conservation of soil N) but to other rotational effects as well. It is thus clear that the Nbenefit of grain legumes to non-legumes is small compared to the level of N fertilizer use in more intensive cereal production systems but is nevertheless significant in the context of the low amounts of input in subsistence farming.     

The response of field grownPhaseolus vulgaris to Rhizobium inoculation and the quantification of N2 fixation using15N

Summary A field experiment was performed to assess the effects of Rhizobium inoculation and nitrogen fertilizer (100 kg N ha^–1) on four cultivars of Phaseolus beans; Carioca, Negro Argel, Venezuela 350 and Rio Tibagi. In the inoculated treatment 2.5 kg N ha^–1 of¹⁵N labelled fertilizer was added in order to apply the isotope dilution technique to quantify the contribution of N₂ fixation to the nutrition of these cultivars.Nodulation of all cultivars in the uninoculated treatments was poor, but the cultivars Carioca and Negro Argel were well nodulated when inoculated. Even when inoculated, nodulation of the cultivars Venezuela 350 and Rio Tibagi was poor and these cultivars showed little response to inoculation in terms of nitrogen accumulation or grain yield. The estimates of the contribution of N₂ fixation estimated using the isotope dilution technique, for the Carioca and Negro Argel cultivars, amounted to 31.7 and 18.4 kg N ha^–1 respectively. These two cultivars produced 991 and 883 kg ha^–1 of grain, respectively, when inoculated and 663 and 620 kg ha^–1 with the addition of 100 kg N ha^–1 of N fertilizer. The response to nitrogen was particularly poor due to high leaching losses in the very sandy soil at the experimental site.The Venezuela 350 and Rio Tibagi cultivars only responded to N fertilizer and not to inoculation with Rhizobium which stresses the great importance of selecting plant cultivars for nitrogen fixation in the field.     

The contribution of nitrogen by promiscuous soybeans to maize based cropping the moist savanna of Nigeria

Agronomic results indicate that maize grain yields generally are higher when the crop is planted following soybean than in continuous maize cultivation in the moist savanna agroecological zones of West Africa. Many factors have been hypothesized to explain this phenomenon, including enhanced N availability and the so-called `rotational effect'. There is, however, hardly any quantitative information on the residual N benefits of promiscuous soybeans to subsequent cereal crops grown in rotation with soybean. Three IITA promiscuous soybean breeding lines and two Brazilian soybean lines were grown in 1994 and 1995 at Mokwa in the southern Guinea savanna, Nigeria, to quantify the nitrogen contribution by soybeans to a succeeding crop of maize grown in rotation with soybean for two consecutive years, 1996 and 1997 using two methods of introducing ¹⁵N into soil (fresh ¹⁵N labelling and its residual ¹⁵N) and three maize cultivars (including one cultivar with high N use efficiency) used as reference plants. The nodulating soybeans fixed between 44 and 103 kg N ha^–1 of their total N and had an estimated net N balance input from fixation following grain harvest ranging from –8 to 43 kg N ha^–1. Results in 1996 and in 1997 showed that maize growing after soybean had significantly higher grain yield (1.2 – 2.3-fold increase compared to maize control) except for maize cultivar Oba super 2 (8644-27) (a N-efficient hybrid). The ¹⁵N isotope dilution method was able to estimate N contribution by promiscuous soybeans to maize only in the first succeeding maize crop grown in 1996 but not in the second maize crop in 1997. The first crop of maize grown after soybean accumulated an average between 10 and 22 kg N ha^–1 from soybean residue, representing 17–33% of the soybean total N ha^–1. The percentage ¹⁵N derived from residue recovery in maize grown after maize was influenced by the maize cultivars. Maize crop grown after the N-efficient hybrid cultivar Oba Super 2 (844-27) had similar ¹⁵N values similar to maize grown after soybeans, confirming the ability of this cultivar to use N efficiently in low N soil due to an efficient N translocation ability. The maize crop in 1997 grown after maize had lower ¹⁵N enrichment than that grown in soybean plots, suggesting that soybean residues contributed a little to soil available N and to crop N uptake by the second maize crop. The differential mineralization and immobilization turnover of maize and soybean residues in these soils may be important and N contribution estimates in longer term rotation involving legumes and cereals may be difficult to quantify using the ¹⁵N labelling approaches. Therefore alternative methods are required to measure N release from organic residues in these cropping systems.     

Nitrogen fixation by white clover in pastures grazed by dairy cows: Temporal variation and effects of nitrogen fertilization

Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N₂ fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha^–1 and received urea at 0, 200 or 400 kg N ha^–1 yr^–1 in 8–10 split applications. A fourth farmlet received 400 kg N ha^–1 yr^–1 and had 4.4 cows ha^–1.There was large variation in annual clover production and total N₂ fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha^–1 yr^–1. Despite this variation, total pasture production in the 0 N treatment remained at 75–85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years.Fertilizer N application decreased the average proportion of clover N derived from N₂ fixation (P_N; estimated by ¹⁵N dilution) from 77% in the 0 N treatment to 43–48% in the 400 N treatments. The corresponding average total N₂ fixation decreased from 154 kg N ha^–1 yr^–1 to 39–53 kg N ha^–1 yr^–1. This includes N₂ fixation in clover tissue below grazing height estimated at 70% of N₂ fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N₂ fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N₂ fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons.In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease P_N from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N₂ fixation because lower pasture mass reduced grass competition, particularly during spring.     

Water chemistry and nutrient budgets in an undisturbed evergreen rainforest of southern Chile

In a pristine evergreen rainforest of Nothofagus betuloides, located at the Cordillera de los Andes in southern Chile (41 °S), concentrations and fluxes of nutrients in bulk precipitation, cloud water, throughfall water, stemflow water, soil infiltration and percolation water and runoff water were measured. The main objectives of this study were to investigate canopy-soil-atmosphere interactions and to calculate input-output budgets. From May 1999 till April 2000, the experimental watershed received 8121 mm water (86% incident precipitation, 14% cloud water), of which the canopy intercepted 16%. Runoff water volume amounted 9527 mm. Bulk deposition of inorganic (DIN) and organic (DON) nitrogen amounted 3.6 kg ha^–1 year^–1 and 8.2 kg ha^–1 year^–1 respectively. Occult deposition (clouds + fog) contributes for 40% to the atmospheric nitrogen input (bulk + occult deposition) of the forest. An important part of the atmospheric ammonium deposition is retained within the canopy or converted to nitrate or organic nitrogen by epiphytic bacteria or lichens. Also the export of inorganic (0.9 kg ha^–1 year^–1) and organic (5.2 kg ha^–1 year^–1) nitrogen via runoff is lower than the input to the forest floor via throughfall and stemflow water (3.2 kg DIN ha–1 year^–1 and 5.6 kg DON ha^–1 year^–1). The low concentrations of NO ₃ ^– and NH ₄ ⁺ under the rooting depth suggest an effective biological immobilization by vegetation and soil microflora. Dry deposition and foliar leaching of base cations (K⁺, Ca²⁺, Mg²⁺) was estimated using a canopy budget model. Bulk deposition accounted for about 50% of the total atmospheric input. Calculated dry and occult deposition are both of equal value (about 25%). Foliar leaching of K⁺, Ca²⁺, and Mg²⁺ accounted for 45%, 38% and 6% of throughfall deposition respectively. On an annual basis, the experimental watershed was a net source for Na⁺, Ca²⁺ and Mg²⁺.     

Nitrogen cycling in dense plantings of hybrid poplar and black alder

Summary Nitrogen cycling was studied during the third growing season in pure and mixed plantings (33×33 cm spacing) of hybrid poplar and black alder in southeastern Canada. After 3 years, hybrid poplar growth and N content of living tissues in a plot and of individual hybrid poplar plants increased with the proportion of black alder in a planting. No differences were detected among N contents of individual alder plants regardless of plot treatment. Black alder allocated a larger portion of its N to roots than hybrid poplar. Symbiotic nitrogen fixation was estimated to account for 80% of the nitrogen in aboveground alder tissues in the pure treatment using natural¹⁵N dilution. N return in leaf litter was estimated to be 70kg ha^–1 in the pure alder treatment and decreased to a minimum of 20 kg ha^–1 in the pure hybrid poplar plots. No difference was detected among treatments for throughfall N content. Nitrogen concentration in roots and leaf litterfall of black alder was higher than hybrid poplar. Significant soil N accretion occurred in mixed plantings containing two alders to one poplar and pure black alder plantings. Nitrogen availability (NO₃–N) increased with the amount of black alder in a plot. Results suggest that the early increase in nitrogen accumulation of hybrid poplar in mixed treatments can be attributed to an increase of total soil N availability resulting from the input of large amounts of N from easily mineralizable alder tissue.     

Variability in nitrogen fixation of common bean (Phaseolus vulgaris L.) intercropped with maize

Thirty one selected bean lines were evaluated in the field for ability to support N₂ fixation when intercropped with maize which received 0, 30 and 60 kg N ha^–1 as ammonium sulphate. The amount of fixed N₂ was estimated using the natural variation of ¹⁵N and wheat as the standard non-fixing crop. Nitrogen as low as 15 kg N ha^–1 at sowing suppressed nodule weight and activity (acetylene reduction activity) but not nodule number, suggesting that the main effect of mineral N was on nodule development and function. ¹⁵N data revealed a high potential of the bean genotypes to fix N₂, with the most promising ones averaging between 50–60% of seed N coming from fixation. Bean lines CNF-480, Puebla-152, Mexico-309, Negro Argel, CNF-178, Venezuela-350 and WBR22-3, WBR22-50 and WBR22-55 were ranked as good fixers.