Nitrogen fertilizer replacement indexes of legume cover crops in the derived savanna of West Africa

Legume cover crops are a potential means for overcoming N depletion in the derived savanna of West Africa. A 3-year trial was, therefore, conducted near Ibadan, southwestern Nigeria to measure the N contribution of 13 legume cover crops as compared to urea –N, using a N fertilizer replacement index for a maize test crop. Two series of trials involved the following legume cover crop species: Aeschynomene histrix, Centrosema brasilianum, Centrosema pascuorum, Chamaecrista rotundifolia, Cajanus cajan, Crotalaria verrucosa, Crotalaria ochroleuca, Lablab purpureus, Mucuna pruriens, Psophocarpus palustris, Pseudovigna argentea, Pueraria phaseoloides and Stylosanthes hamata. Trials were undertaken using a complete block design. Cover crops were planted in 1994 (Series 1) and 1995 (Series 2) in separate sites and each series was subsequently slashed and planted for one season with maize (Zea mays) in 1995 and 1996. At the 50% flowering stage, N concentration of above-ground vegetation of cover crops ranged from 21 to 38 g N kg^–1. Nitrogen accumulated by 4.5-month old cover crops ranged from 14 to 240 kg N ha^–1, depending on species and year. Cover crops increased grain yield of the subsequent maize crop by 25–136% over the control without N application. Nitrogen uptake by the maize crop was higher following cover crops than after maize or natural grass. The N fertilizer replacement index of cover crops for maize ranged from 11 (A. histrix) to 96 kg N ha^–1 (C. cajan) in Series 2. Perennial (C. brasilianum, S. hamata, C. cajan, P. phaseoloides and C. verrucosa) and annual (C. rotundifolia, M. pruriens, C. ochroleuca and L. purpureus) species could potentially save 50 to 100 kg N ha^–1 for maize crops. The cover crops accumulated more N in the wetter than in the drier year. However, the N fertilizer replacement index was higher for subsequent maize grown in the drier year. The cover crop-N recovery in maize was also higher than the urea-N uptake in the drier year. The N fertilizer replacement indexes can be predicted using the above-ground biomass amount of cover crops at 20 weeks after planting (drier year) or the N concentration at that stage (wetter year).     

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 management in `adequate' input maize-based agriculture in the derived savanna benchmark zone of Benin Republic

Although the West-African moist savanna zone has a high potential for crop production, yields on farmers' fields are, on average, far below this potential, mainly due to the low use of external sources of nutrients. Since the mid-1990s, it has become clear that in order to upgrade crop production to levels needed to sustain the growing population without further degrading the soil resource base, inorganic fertilizers are required. Due to the physico-chemical nature of these soils and the relatively high cost of inorganic fertilizers, a general consensus exists in the research and development community that these inorganic inputs need to be complemented with organic matter. Here, we explore options to produce organic matter in-situ and evaluate the impact of combining inorganic and organic sources of N on maize yields, focusing on the densely populated derived savanna (DS) benchmark of Benin Republic. Although most of the farmers (93%) in this benchmark use inorganic fertilizer, applications rates are low (on average, 27 kg N ha^–1). A significant response to N was observed for 96% of the studied farmers' fields.Grain and herbaceous legumes were observed to produce between 383 and 8700 kg dry matter ha^–1 in the benchmark area. Inoculation with Rhizobia and inorganic P additions were shown to significantly improve biomass production on sites with low contents of Rhizobia and P. Although maize grain yield was observed to increase significantly following a legume compared with following a maize crop or natural fallow, these increases were insufficient in the case of a cowpea crop or were obtained at the cost of leaving the field `idle' for a whole year in the case of a herbaceous Mucuna fallow. Topping up a cowpea haulms equivalent of 45 kg N ha^–1 with 45 kg urea–N ha^–1 was shown to give maize yields similar to the yields obtained after applying 90 kg urea–N ha^–1 on the poorest fields. Moreover, on these fields, a positive interaction between cowpea–N and urea–N sources of 200 kg grain ha^–1 was observed. On the richest fields, the effects of applied organic matter and fertilizer were additive.Agroforestry systems are alternative cropping systems that produce organic matter in-situ. As tree roots go down below the rooting depth of food crops, sub-soil fertility was observed to influence tree biomass production. Yield increases in tree-crop intercrop systems – such as alley cropping – in the absence of inorganic inputs are often reduced by the occurrence of tree-crop competition. In cut-and-carry systems, where tree prunings are harvested from a field adjacent to the crop land, increases in maize grain yield caused by addition of those prunings were observed to be on the low side. Mixing these residues with urea, however, was shown to lead to added benefits of about 500 kg grains ha^–1, relative to the treatments with sole inputs of organic matter or urea. Although residue quality was shown to affect maize N uptake in a pot trial, its impact under field conditions was minimal for the range of considered residue qualities. In an alley cropping trial, maize yield was shown to be sustained on a non-degraded site and enhanced on a degraded site, when a minimal amount of mineral fertilizer was added with the prunings, whereas fertilizer application alone failed to do so in both cases.     

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.     

Physiological variation among native and exotic winter annual plants associated with microbiotic crusts in the Mojave Desert

An experiment was established in 1992 in eastern Ontario, Canada to determine the effects of crop rotation (continuous maize, soybean-maize and alfalfa-maize) and nitrogen (N) amendment [0, 100 and 200 kg N ha^–1 of fertilizer (NH₄NO₃), and 50 and 100 Mg ha^–1 (wet wt.) each of stockpiled and rotted dairy manure] on maize production and soil properties. From 1997 to 1999, an additional study was added to the experiment to investigate treatment effects on the susceptibility of maize hybrids to gibberella ear rot. A moderately resistant and a susceptible hybrid were planted in each plot and inoculated with a macroconidial suspension of Fusarium graminearum by both the silk channel injection and the kernel-wound techniques. At harvest, ears were rated for the severity of disease symptoms and harvested kernels were analyzed for the mycotoxin deoxynivalenol (DON). The greatest number of significant N effects were found in the continuous maize treatments and with the susceptible hybrid. Most N amendments decreased both disease severity and DON accumulation in the susceptible hybrid. The most consistent effect was a decrease in disease severity with 100 kg N ha^–1 fertilizer and an increase in disease severity with the higher rate of 200 kg N ha^–1. This study is the first to report on the effects of soil N amendments on gibberella ear rot susceptibility.     

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.     

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.     

In Situ Nitrogen Mineralization,Nitrification, and Ammonia Volatilization in Maize Field Fertilized with Urea in Huanghuaihai Region of Northern China

Nitrogen (N) fertilization potentially affects soil N mineralization and leaching, and can enhance NH₃ volatilization, thus impacting crop production. A fertilizer experiment with five levels of N addition (0, 79, 147, 215 and 375 kg N ha^-1) was performed in 2009 and 2010 in a maize field in Huanghuaihai region, China, where > 300 kg N ha^-1 has been routinely applied to soil during maize growth period of 120 days. Responses of net N mineralization, inorganic N flux (0–10cm), NH₃ volatilization, and maize yield to N fertilization were measured. During the growth period, net N mineralization and nitrification varied seasonally, with higher rates occurring in August and coinciding with the R1 stage of maize growth. Soil NO₃ ⁻-N contributed to more than 60% of inorganic N flux during maize growth. Cumulative NH₃ volatilization increased with N additions, with total NH₃ volatilization during maize growth accounting for about 4% of added N. Relative to the control, mean maize yield in the fertilizer treatments increased by 17% and 20% in 2009 and 2010, respectively. However, grain yield, aboveground biomass, and plant N accumulation did not increase with added N at levels > 215 kg N ha^-1. These results suggest that the current N rate of 300 kg N ha^-1 is not only excessive, but also reduces fertilizer efficacy and may contribute to environmental problems such as global warming and eutrophication of ground water and streams.     

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.     

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.     

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.     

Fertilizer nitrogen budgets of two doses of Na15NO3 dressings split-applied to winter wheat in microplots on a loam soil

In a field experiment performed in microplots, winter wheat was fertilized at two different total N dressings (135 and 180 kg ha^–1) split-applied as Na¹⁵NO₃ in three equal applications at tillering, stem elongation, and flag leaf.No significant differences were found in the percentage recovery values for the entire plant at the three split applications between the two N dressings. The total percentage recovery of fertilizer N by the plant was high and practically equal at both fertilization levels (76.65% and 75.84% for 135 and 180 kg N ha^–1, respectively); crop yields were also similar. In contrast, gaseous losses calculated after drawing up the balance sheet were, in absolute values, higher for the tillering and stem elongation split applications when using the 180 kg N ha^–1 dressing (7.67 and 4.84 kg N ha^–1, respectively) than for the 135 kg N ha^–1 dressing (3.45 and 1.26 kg N ha^–1, respectively). They were found to be zero at flag leaf at both fertilization levels. The amount of applied fertilizer N did not influence the amount of N taken up from the soil which was about 143 kg ha^–1.     

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.     

Response of field-grown chickpea (Cicer arietinum L.) to phosphorus fertilization for yield and nitrogen fixation

Application of phosphorus at 40, 60, 80 and 100 kg P₂O₅ ha^–1 in the presence of a uniform dressing of nitrogen (N) and potash (K₂O) each applied at 20 and 24 kg ha^–1 to chickpea (CM-88) grown in sandy loam soil in a replicated field experiment improved the nodulation response of the crop, increased its grain yield (ka ha^–1) by 18, 59, 40 and 14 percent, biomass yield (ka ha^–1) by 32, 32, 54 and 14 percent, biomass N (kg ha^–1) by 31, 48, 49, 19 percent, and biomass P (kg ha^–1) by 26, 40, 41 and 11 percent, respectively. The effect of phosphorus on the nitrogenase activity of the excised roots of chickpea was, however, inconsistent.     

Transformation of15N-labelled urea in rice-wheat cropping system

Summary In a udic chromusterts the transformation of an initial application of¹⁵N-urea @ 80 kg N ha^–1 to rice (Oryza sativa L.) in rice-wheat (R-W) and to wheat (Triticum aestivum L.) in wheat-rice (W-R) rotations was followed in 6 successive crops in each rotation. All rice crops were grown in irrigated wetland and wheat in irrigated upland conditions.The first wheat crop in W-R rotation utilized 22 kg fertilizer N ha^–1 as compared to 19 kg by the corresponding rice crop in R-W rotation. But the latter absorbed more soil N than the former. About 69% of the total N uptake in rice was derived from mineralization of soil organic N as compared to 61% in wheat.The succeeding wheat crop in R-W rotation utilized 6.7% of the residual fertilizer N in the soil but the corresponding rice crop in W-R rotation only 2.2%. The higher utilization appeared to be related to a greater incorporation of labelled fertilizer N in mineral and hexosamine fractions of the soil N. After the second crop in each rotation, the average residual fertilizer N utilization in the next 4 crops ranged between 3 and 4%.The total recovery of¹⁵N-urea in all crops amounted to 21.7 and 24.3 kg N ha^–1 in R-W and W-R rotation, respectively. At the end of the experiment, about 9 to 10 kg ha^–1 of the applied labelled N was found in soil upto 60 cm depth. Most of the labelled soil N (69–76%) was located in the upper 0–20 cm soil layer indicating little movement to lower depths despite intensive cropping for 4 years.     

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.     

N2(C2H2−C2H4) fixation in two species of Ceanothus seedlings in second year postfire chaparral

Nitrogen fixation in excised root nodules of 2-year-old, postfireCeanothus tomentosus andC. leucodermis seedlings was measured over an 8-month period using the acetylene reduction method. High levels of NO₃–N and NH₄–N present in postfire soils were limited to the upper 10 cm and did not inhibit nodulation in these deeper-rooting seedlings. Decreases in acetylene reduction activity occurred with decreased soil moisture and increased soil temperature. Nitrogen gains from these two Ceanothus shrub seedlings totalled 1.6 kg N ha^–1 yr^–1.     

Effects of Rhizobium inoculation on growth and nodulation ofAlbizia falcataria (L.) Fosh. andAcacia mangium Willd. in the nursery

Three separate experiments were conducted in the nursery using grassland soil as a growing medium. The first experiment was conducted to assess the nodulation of the two legume trees grown in unamended soil, the second was done to determine the effects of N-fertilizer application on the interaction of the five Rhizobium isolates withA. falcataria and the third experiment was conducted to determine the effects of liming on theRhizobium × A. mangium interaction. Three local Rhizobium isolates, A₁₆, A₁₈, and A₁₄ were effective forA. falcataria with A₁₆ as the most promising strain under the conditions described. In general, application of combined nitrogen suppressed nodulation ofA. falcataria. Nodulation in the absence of combined N exceeded those fertilized with 30 kg N ha⁻¹ by 114.0%, 60 kg N ha⁻¹ by 209.6% and 100 kg N ha⁻¹ by 237.1%. Two local isolates, Am₁₀ and Am₂, and an introduced strain, NA 1533 from Australia were promising forA. mangium. Unlike inA. falcataria, the application of combined nitrogen at the rate of 100 kg N ha⁻¹ did not suppress nodulation inA. mangium. Liming the soil to pH 6.5 regardless of nitrogen fertilizer application improved the performance of the Rhizobium—A. mangium symbiosis.     

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.     

Alley cropping with Senna siamea in South-western Nigeria: II. Dry matter,total N,and urea-derived N dynamics of the Senna and maize roots

Crop and tree roots are crucial in the nutrient recycling hypotheses related to alley cropping systems. At the same time, they are the least understood components of these systems. The biomass, total N content and urea-derived N content of the Senna and maize roots in a Senna-maize alley cropping system were followed for a period of 1.5 years (1 maize-cowpea rotation followed by 1 maize season) to a depth of 90 cm, after the application of ¹⁵N labeled urea. The highest maize root biomass was found in the 0–10 cm layer and this biomass peaked at 38 and 67 days after planting the 1994 maize (DAP) between the maize rows (112 kg ha^–1, on average) and at 38, 67 and 107 DAP under the maize plants (4101 kg ha^–1, on average). Almost no maize roots were found below 60 cm at any sampling date. Senna root biomass decreased with time in all soil layers (from 512 to 68 kg ha^–1 for the 0–10 cm layer between 0 and 480 DAP). Below 10 cm, at least 62% of the total root biomass consisted of Senna roots and this value increased to 87% between 60 and 90 cm. Although these observations support the existence of a Senna root `safety net' between the alleys which could reduce nutrient leaching losses, the depth of such a net may be limited as the root biomass of the Senna trees in the 60–90 cm layer was below 100 kg ha^–1, equivalent to a root length density of only < 0.05 cm cm^–3. The proportion of maize root N derived from the applied urea (%Ndfu) decreased significantly with time (from 21% at 21 DAP to 8% at 107 DAP), while %Ndfu of the maize roots at the second harvest (480 DAP) was only 0.6%. The %Ndfu of the Senna roots never exceeded 4% at any depth or sampling time, but decreased less rapidly compared to the %Ndfu of the maize roots. The higher %Ndfu of the maize roots indicates that maize is more efficient in retrieving urea-derived N. The differences in dynamics of the %Ndfu also indicate that the turnover of N through the maize roots is much faster than the turnover of N through the Senna roots. The recovery of applied urea-N by the maize roots was highest in the top 0–10 cm of soil and never exceeded 0.4% (at 38 DAP) between the rows and 7.1% (at 67 DAP) under the rows. Total urea N recovery by the maize roots increased from 1.8 to 3.2% during the 1994 maize season, while the Senna roots never recovered more than 0.8% of the applied urea-N at any time during the experimental period. These values are low and signify that the roots of both plants will only marginally affect the total recovery of the applied urea-N. Measurement of the dynamics of the biomass and N content of the maize and Senna roots helps to explain the observed recovery of applied urea-N in the aboveground compartments of the alley cropping system.