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.     

Impact of residue quality on the C and N mineralization of leaf and root residues of three agroforestry species

A laboratory incubation experiment with ¹⁵N labeled root and leaf residues of 3 agroforestry species (Leucaena leucocephala, Dactyladenia barteri and Flemingia macrophylla) was conducted under controlled conditions (25 C) for 56 days to quantify residue C and N mineralization and its relationship with residue quality.No uniform relation was found between the chemical composition of the above and below residues. The leucaena and dactyladenia roots contained more lignin (8 and 26% respectively) and less N (2.0 and 1.0% respectively) than the respective leaves (2 and 13% lignin and 2.9 and 1.4% N, respectively), whereas the differences between the lignin and N contents of the flemingia leaves and roots were not significant (4.6 and 3.0% lignin and 2.63 and 2.68% N, respectively). The leucaena leaves contained more polyphenols than the roots (6.4 and 3.6%), while the polyphenol content of the leaves and roots of the other residues was similar (5.0 and 5.1% for dactyladenia and 4.0 and 3.5% for flemingia).Three patterns of N mineralization could be distinguished. A first pattern, followed by residues producing the highest amounts of CO₂, showed an initial immobilization of soil derived N, followed by a net release of both soil and residue derived N after 7 days of incubation. A second pattern, followed by the flemingia leaf residues which produced intermediate amounts of CO₂ and had an intermediate quality, showed no significant immobilization of soil derived N, and significant mineralization of residue N. A third pattern, followed by both low quality dactyladenia residues, showed a low release of residue derived N and a continued inmobilization of soil derived N.Residue C mineralization was significantly (p<0.05) correlated with the residue lignin content, C-to-N ratio, and polyphenol-to-N ratio. The proportion of residue N mineralized (immobilized) after 56 days of incubation was significantly correlated with the residue N content (p<0.01) and the C-to-N ratio (p<0.05). The relations were quadratic, rather than linear. The ratio of the proportion of residue N mineralized (immobilized) over the proportion of residue C mineralized after 56 days was highly significantly correlated with the lignin content (p<0.01) and C-to-N (p<0.001), lignin-to-N (p<0.01), polyphenol-to-N (p<0.01) and (lignin+polyphenol)-to-N ratios (p<0.01) in a linear way. This indicates that due to the low availability of the residue C, relatively less N is immobilized for the very low quality residues ((lignin+polyphenol)-to-N ratio: 29.7) than for the residues with a relatively higher quality ((lignin+polyphenol)-to-N ratios between 3.3 and 12.5).     

Management of biological N2 fixation in alley cropping systems: Estimation and contribution to N balance

Alley cropping is being widely tested in the tropics for its potential to sustain adequate food production with low agricultural inputs, while conserving the resource base. Fast growth and N yield of most trees used as hedgerows in alley cropping is due greatly to their ability to fix N₂ symbiotically with Rhizobium. Measurements of biological N₂ fixation (BNF) in alley cropping systems show that some tree species such as Leucaena leucocephala, Gliricidia sepium and Acacia mangium can derive between 100 and 300 kg N ha^-1 yr^–1 from atmospheric N₂, while species such as Faidherbia albida and Acacia senegal might fix less than 20 kg N ha^-1 yr^-1. Other tree species such as Senna siamea and S. spectabilis are also used in alley cropping, although they do not nodulate and therefore do not fix N₂. The long-term evaluation of the potential or actual amounts of N₂ fixed in trees however, poses problems that are associated with their perennial nature and massive size, the great difficulty in obtaining representative samples and applying reliable methodologies for measuring N₂ fixed. Strategies for obtaining representative samples (as against the whole tree or destructive plant sampling), the application of ¹⁵N procedures and the selection criteria for appropriate reference plants have been discussed.Little is known about the effect of environmental factors and management practices such as tree cutting or pruning and residue management on BNF and eventually their N contribution in alley cropping. Data using the ¹⁵N labelling techniques have indicated that up to 50% or more of the tree's N may be below ground after pruning. In this case, quantification of N₂ fixed that disregards roots, nodules and crowns would result in serious errors and the amount of N₂ fixed may be largely underestimated. Large quantities of N are harvested with hedgerow prunings (>300 kg N ha^-1 yr^-1) but N contribution to crops is commonly in the range of 40–70 kg N ha^-1 season. This represents about 30% of N applied as prunings; however, N recoveries as low as 5–10% have been reported. The low N recovery in maize (Zea mays) is partly caused by lack of synchronization between the hedgerow trees N release and the associated food crop N demand. The N not taken up by the associated crop can be immobilized in soil organic matter or assimilated by the hedgerow trees and thus remain in the system. This N can also be lost from the system through denitrification, volatilization or is leached beyond the rooting zone. Below ground contribution (from root turnover and nodule decay) to an associated food crop in alley cropping is estimated at about 25–102 kg N ha^-1 season^-1. Timing and severity of pruning may allow for some management of underground transfer of fixed N₂ to associated crops. However many aspects of root dynamics in alley cropping systems are poorly understood. Current research projects based on ¹⁵N labelling techniques or ¹⁵N natural abundance measurements are outlined. These would lead to estimates of N₂ fixation and N saving resulting from the management of N₂ fixation in alley cropping systems.     

Root distributions partially explain 15N uptake patterns in Gliricidia and Peltophorum hedgerow intercropping systems

The relative distributions of tree and crop roots in agroforestry associations may affect the degree of complementarity which can be achieved in their capture of below ground resources. Trees which root more deeply than crops may intercept leaching nitrogen and thus improve nitrogen use efficiency. This hypothesis was tested by injection of small doses of (¹⁵NH₄)₂SO₄ at 21.8 atom% ¹⁵N at different soil depths within established hedgerow intercropping systems on an Ultisol in Lampung, Indonesia. In the top 10 cm of soil in intercrops of maize and trees, root length density (L_rv) of maize was greater than that of Gliricidia sepium trees, which had greater L_rv in this topsoil layer than Peltophorum dasyrrachis trees. Peltophorum trees had a greater proportion of their roots in deeper soil layers than Gliricidia or maize. These vertical root distributions were related to the pattern of recovery of ¹⁵N placed at different soil depths; more ¹⁵N was recovered by maize and Gliricidia from placements at 5 cm depth than from placements at 45 or 65 cm depth. Peltophorum recovered similar amounts of ¹⁵N from placements at each of these depths, and hence had a deeper N uptake distribution than Gliricidiaor maize. Differences in tree L_rv across the cropping alley were comparatively small, and there was no significant difference (P<0.05) in the uptake of ¹⁵N placed in topsoil at different distances from hedgerows. A greater proportion of the ¹⁵N recovered by maize was found in grain following ¹⁵N placement at 45 cm or 65 cm depth than following placement at 5 cm depth, reflecting the later arrival of maize roots in these deeper soil layers. Thus trees have an important role in preventing N leaching from subsoil during early crop establishment, although they themselves showed a lag phase in ¹⁵N uptake after pruning. Residual ¹⁵N enrichment in soil was strongly related to application depth even 406 days after ¹⁵N placement, demonstrating the validity of this approach to mapping root activity distributions.     

Spatial and temporal variation in islands of fertility in the Sonoran Desert

In many arid and semi-arid ecosystems, canopy trees and shrubs have a strong positive influence on soil moisture and nutrient availability, creating islands of fertility where organic matter and nutrients are high relative to areas outside the canopy. Previous studies of canopy effects on soil processes have rarely considered how landscape context may modulate these effects. We measured the effects of velvet mesquite trees (Prosopis velutina) on soil moisture and the biogeochemistry of nitrogen at different positions along a topographic gradient from upland desert to riparian zone in the Sonoran Desert of central Arizona. We also examined how landscape position and patterns of precipitation interact to determine the influence of P. velutina on soil moisture, N availability assessed using ion exchange resins, net N mineralization and net nitrification, and microbial biomass C and N. P. velutina clearly created islands of fertility with higher soil organic matter, net N mineralization and net nitrification rates, and microbial biomass under mesquite canopies. These effects were consistent across the landscape and showed little temporal variability. Magnitude and direction of effect of mesquite on soil moisture changed with landscape position, from positive in the upland to negative in the terrace, but only when soil moisture was >4%. Resin N showed responses to mesquite that depended on precipitation and topographic position, with highest values during wet seasons and under mesquite on terraces. We suggest changes in proximity of P. velutina to groundwater lead to shifts in biogeochemical processes and species interactions with change in landscape position along a topographic gradient.     

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.     

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

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

Soil nitrogen heterogeneity in a Dehesa ecosystem

The C mineralization and N transformations during the decomposition of sunflower stalks (Helianthus annuus L.) and wheat straw (Triticum aestivum L.) with and without addition of (NH₄)₂SO₄ (27.53 atom% ¹⁵N) were studied in a Vertisol. Soil samples were incubated under aerobic conditions for 224 days at 22 °C. The plant residues were added at a rate of 5.2 g kg^-1 soil. Nitrogen was applied at a rate of 50.7 mg N kg^-1 soil. Carbon dioxide emission and inorganic N content in soil were periodically determined. Gross N immobilization and remineralization were calculated on the basis of the isotopic dilution technique. At the end of the incubation period a ¹⁵N balance was established. Respectively, 68 and 45% of the applied residue-C mineralized from the sunflower stalks and wheat straw after 224 days. Both crop residues caused losses of up to 25% of added ¹⁵N after 224 days of incubation. These ¹⁵N losses were about three times larger than in the control soil, and were probably due to denitrification. The net immobilization of soil derived N following residue incorporation was largest in the case of wheat straw, depleting all soil inorganic N. In the wheat straw treatment with added (NH₄)₂SO₄ soil inorganic N remained available, resulting in an enhanced initial C mineralization and N immobilization compared to the treatment without added N. In the case of the sunflower stalks, the high inorganic N content of the stalks suppressed the effects of N addition on C mineralization and N immobilization/mineralization. Gross N immobilization amounted to 31.9 and 28.2 mg N g^-1 added C after 14 days for wheat straw and sunflower stalks, respectively. At the end of the incubation, about 35% of the newly immobilized N was remineralized in both plant residue treatments. Gross N immobilization plotted against decomposed C suggests that fairly uniform C-N relationships exist during the decomposition of divers C substrates. The results demonstrate that low fertilizer N use efficiencies may be expected in a wheat-sunflower cropping system with incorporation of crop residues, as the fertilizer N applied becomes largely immobilized in the soil organic fraction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrogen fixation efficiency,natural15N abundance,and morphology of mesquite (Prosopis glandulosa) root nodules

Summary The relative nitrogen fixation efficiencies (RE 1-[H₂ evolved÷C₂H₂ reduced]·100) of four mesquite (Prosopis glandulosa var.torreyana) rhizobia (Strains WR 1001, WR 1002, L5, L9) and a cowpea rhizobia (Strain 176A32) on mesquite were evaluated in a glasshouse experiment. Plant yield, shoot N accumulation, and the natural¹⁵N abundance (¹⁵N) of nodule tissue were determined. Strain WR 1002 failed to nodulate mesquite and strain L5 produced ineffective nodules. Among the three effective strains (WR 1001, L9, 176A32) the cowpea strain (176A32) and strain L9 had significantly higher RE than strain WR 1001. Differences in RE, however, were not accompanied by significantly higher plant yield and shoot N accumulation. The difference in¹⁵N abundance between foliar tissue and nodules (nodules minus leaves) was 0.47 ¹⁵N for the ineffective L5 nodules, while for the effective WR 1001, L9, and 176A32 nodules, respectively, this difference was 8.35, 7.81, and 8.35 ¹⁵N. This indicates a similar relationship between N₂-fixing effectiveness and natural¹⁵N enrichment of nodules that was previously observed in soybeans (Glycine max, L. Merr.). Strains WR 1001 and L9 produced elongate, indeterminate nodules typical for mesquite. The ineffective L5 nodules had few infected cells and an abundance of cortical amyloplasts. Mesquite nodules produced by the cowpea strain were spherical and were somewhat more similar in internal morphology to determinate nodules typical of cowpea than indeterminate nodules normally associated with mesquite.     

Influence of pruning management on P and N distribution and use efficiency by N2 fixing and non-N2 fixing trees used in alley cropping systems

Pruning of hedgerow trees is an important management practice for the successful establishment of an alley cropping system. Although pruning affects biomass production, only meager evidence of this management on distribution of nutrients among the different plant organs after tree regrowth is available. This study examined the effect of pruning on the distribution and use efficiency of N and P in a N₂ fixing leguminous tree species, Gliricidia sepium, and two non-N₂ fixing leguminous tree species, Senna siamea and S. spectabilis, grown in a field on an Alfisol (low in P) at Fashola (Guinea Savanna Zone), Southwestern Nigeria. Four P rates, 0, 20, 40 and 80 kg P ha^–1 as single superphosphate were used and management treatments included pruned versus unpruned plants. The ¹⁵N isotope dilution technique was used to measure N₂ fixation in G. sepium. Partitioning of total P among different plant organs was influenced by plant species and pruning management, but was not affected by P application rates. The distribution of total P in the various plant organs followed that of dry matter yield while N partitioning had a different pattern. Pruned plants distributed about 118% more total P to branches and had a higher physiological P use efficiency (PPUE) than unpruned plants. Leaves were the biggest sink for total N and N allocation in the other plant organs was influenced by plant species and pruning management, G. sepium had relatively more of its total N and P partitioned into roots (about double that of the non-N₂ fixing trees) but had a lower PPUE. Unpruned and pruned G. sepium derived 35 and 54% respectively of their total N from atmospheric N2, with about 54% of the fixed N2 being allocated to leaves and roots. Results showed that N and P pools turned over in the branches during plant regrowth after pruning but the causative factors associated with this phenomenon were not clear.     

Intracanopy variation in nitrogen cycling through leaves is influenced by irradiance and proximity to developing fruit in mature walnut trees

Intracanopy variation in net leaf nitrogen (N) resorption and N cycling through leaves in mature walnut (Juglans regia L. cv Hartley) trees were monitored in 3 different years. Differential irradiance among the spurs sampled was inferred from differences among leaves in dry weight per unit area (LW/LA) which varied from 4.0 mg · cm^–2 to 7.0 mg · cm^–2 in shaded (S) and exposed (E) canopy positions, respectively. Our results, using ¹⁵N-depleted (NH₄)₂SO₄ validated the concept that N influx and efflux through fully expanded leaves occurred concurrently during the period of embryo growth. Additionally, it also suggested that N influx into leaves was substantially greater in exposed as compared with shaded canopy positions. Because of its well documented phloem immobility, leaf Ca accumulation was used to better estimate the relative influx of N into exposed and shaded leaves. N cycling varied locally within the tree canopy, i. e. Ca (and presumably N) influx was 100% greater in exposed than shaded tree canopy positions, but influx was not influenced significantly by the proximity of developing fruit. In contrast, both the amount and percentage N efflux was significantly greater during embryo growth in fruit-bearing than defruited spurs. Net leaf N resorption averaged 2–4 times greater (25–30%) in fruit-bearing spurs than the 5–10% decrease in the leaf N content in defruited spurs. Since about 90% of leaf N content reportedly occurs as protein, fruit N demand apparently influenced protein turnover and catalysis in associated spur leaves. The amount of leaf N resorption was greater in exposed than shaded positions in the tree canopy in 2 of the 3 years of data collection. Our data show that like leaf N content, N influx, N efflux and net leaf N resorption vary throughout mature walnut tree canopies under the combined local influences of fruiting and irradiance.     

Decomposition of leaf and root tissue of three perennial grass species grown at two levels of atmospheric CO2 and N supply

Leaf and root tissue of Lolium perenne L., Agrostis capillaris L. and Festuca ovina L. grown under ambient (350 μl l^-1 CO₂) and elevated (700 μl l^-1) CO₂ in a continuously ¹⁴C-labelled atmosphere and at two soil N levels, were incubated at 14°C for 222 days. Decomposition of leaf and root tissue grown in the low N treatment was not affected by elevated [CO₂], whereas decomposition in the high N treatment was significantly reduced by 7% after 222 days. Despite the increased C/N ratio (g g^-1) of tissue cultivated at elevated [CO₂] when compared with the corresponding ambient tissue, there was no significant correlation between initial C/N ratio and ¹⁴C respired. This finding suggests that the CO₂-induced changes in decomposition rates do not occur via CO₂-induced changes in C/N ratios of plant materials. We combined the decomposition data with data on ¹⁴C uptake and allocation for the same plants, and give evidence that elevated [CO₂] has the potential to increase soil C stores in grassland via increasing C uptake and shifting C allocation towards the roots, with an inherent slower decomposition rate than the leaves. An overall increase of 15% in ¹⁴C remaining after 222 days was estimated for the combined tissues, i.e., the whole plants; the leaves made a much smaller contribution to the C remaining (+6%) than the roots (+26%). This shows the importance of clarifying the contribution of roots and leaves with respect to the question whether grassland soils act as a sink or source for atmospheric CO₂. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogen gas (N2+N2O) flux from urea applied to lowland rice as affected by green manure

A field study was conducted on a clay soil (Andaqueptic Haplaquoll) in the Philippines to directly measure the evolution of (N₂+N₂O)−¹⁵N from 98 atom %¹⁵N-labeled urea broadcast at 29 kg N ha⁻¹ into 0.05-m-deep floodwater at 15 days after transplanting (DT) rice. The flux of (N₂+N₂O)−¹⁵N during the 19 days following urea application never exceeded 28 g N ha⁻¹ day⁻¹. The total recovery of (N₂+N₂O)−¹⁵N evolved from the field was only 0.51% of the applied N, whereas total gaseous¹⁵N loss estimated from unrecovered¹⁵N in the¹⁵N balance was 41% of the applied N. Floodwater (nitrate+nitrite)−N in the 5 days following urea application never exceeded 0.14 g N m⁻³ or 0.3% of the applied N. Prior cropping of cowpea [Vigna unguiculata (L.) Walp.] to flowering with subsequent incorporation of the green manure (dry matter=2.5 Mg ha⁻¹, C/N=15) at 15 days before rice transplanting had no effect on fate of urea applied to rice at 15 DT. The recovery of (N₂+N₂O)−¹⁵N and total¹⁵N loss during the 19 days following urea application were 0.46 and 40%, respectively. Direct recovery of evolved (N₂+N₂O)−¹⁵N and total¹⁵N loss from 27 kg applied nitrate-N ha⁻¹ were 20% and 53% during the same 19-day period. The failure of directly-recovered (N₂+N₂O)−¹⁵N to match total¹⁵N loss from added nitrate-¹⁵N might be due to entrapment of denitrification end products in soil or transport of gaseous end products to the atmosphere through rice plants. The rapid conversion of added nitrate-N to (N₂+N₂O)−N, the apparently sufficient water soluble soil organic C for denitrification (101 μg C g⁻¹ in the top 0.15-m soil layer), and the low floodwater nitrate following urea application suggested that denitrification loss from urea was controlled by supply of nitrate rather than by availability of organic C.     

Carbon, nitrogen and phosphorus in volcanic soils following afforestation with native birch (Betula pubescens) and introduced larch (Larix sibirica) in Iceland

Afforestation has become an important tool for soil protection and land reclamation in Iceland. Nevertheless, the harsh climate and degraded soils are growth-limiting for trees, and little is know about changes in soil nutrients in maturing forests planted on the volcanic soils. In the present chronosequence study, changes in C, N and total P in soil (0–10 and 10–20 cm depth) and C and N in foliar tissue were investigated in stands of native Downy birch (Betula pubescens Enrh.) and the in Iceland introduced Siberian larch (Larix sibirica Ledeb.). The forest stands were between 14 and 97 years old and were established on heath land that had been treeless for centuries. Soils were Andosols derived from basaltic material and rhyolitic volcanic ash. A significant effect of tree species was only found for the N content in foliar tissue. Foliar N concentrations were significantly higher and foliar C/N ratios significantly lower in larch needles than in birch leaves. There was no effect of stand age. Changes in soil C and the soil nutrient status with time after afforestation were little significant. Soil C concentrations in 0–10 cm depth in forest stands older than 30 years were significantly higher than in heath land and forest stands younger than 30 years. This was attributed to a slow accumulation of organic matter. Soil N concentrations and soil P_tot were not affected by stand age. Nutrient pools in the two soil layers were calculated for an average weight of soil material (400 Mg soil ha⁻¹ in 0–10 cm depth and 600 Mg soil ha⁻¹ in 10–20 cm depth, respectively). Soil nutrient pools did not change significantly with time. Soil C pools were in average 23.6 Mg ha⁻¹ in the upper soil layer and 16.9 Mg ha⁻¹ in the lower soil layer. The highest annual increase in soil C under forest compared to heath land was 0.23 Mg C ha⁻¹ year⁻¹ in 0–10 cm depth calculated for the 53-year-old larch stand. Soil N pools were in average 1.0 Mg N ha⁻¹ in both soil layers and did not decrease with time despite a low N deposition and the uptake and accumulation of N in biomass of the growing trees. Soil P_tot pools were in average 220 and 320 kg P ha⁻¹ in the upper and lower soil layer, respectively. It was assumed that mycorrhizal fungi present in the stands had an influence on the availability of N and P to the trees. Responsible Editor: Hans Lambers.     

Precipitation pulse use by an invasive woody legume: the role of soil texture and pulse size

Plant metabolic activity in arid and semi-arid environments is largely tied to episodic precipitation events or “pulses”. The ability of plants to take up and utilize rain pulses during the growing season in these water-limited ecosystems is determined in part by pulse timing, intensity and amount, and by hydrological properties of the soil that translate precipitation into plant-available soil moisture. We assessed the sensitivity of an invasive woody plant, velvet mesquite (Prosopis velutina Woot.), to large (35 mm) and small (10 mm) isotopically labeled irrigation pulses on two contrasting soil textures (sandy-loam vs. loamy-clay) in semi-desert grassland in southeastern Arizona, USA. Predawn leaf water potential (Ψ_pd), the isotopic abundance of deuterium in stem water (δD), the abundance of ¹³C in soluble leaf sugar (δ¹³C), and percent volumetric soil water content (θ_v) were measured prior to irrigation and repeatedly for 2 weeks following irrigation. Plant water potential and the percent of pulse water present in the stem xylem indicated that although mesquite trees on both coarse- and fine-textured soils quickly responded to the large irrigation pulse, the magnitude and duration of this response substantially differed between soil textures. After reaching a maximum 4 days after the irrigation, the fraction of pulse water in stem xylem decreased more rapidly on the loamy-clay soil than the sandy-loam soil. Similarly, on both soil textures mesquite significantly responded to the 10-mm pulse. However, the magnitude of this response was substantially greater for mesquite on the sandy-loam soil compared to loamy-clay soil. The relationship between Ψ_pd and δ¹³C of leaf-soluble carbohydrates over the pulse period did not differ between plants at the two sites, indicating that differences in photosynthetic response of mesquite trees to the moisture pulses was a function of soil water availability within the rooting zone rather than differences in plant biochemical or physiological constraints. Patterns of resource acquisition by mesquite during the dynamic wetting–drying cycle following rainfall pulses is controlled by a complex interaction between pulse size and soil hydraulic properties. A better understanding of how this interaction affects plant water availability and photosynthetic response is needed to predict how grassland structure and function will respond to climate change.     

Alder and lupine enhance nitrogen cycling in a degraded forest soil in Northern Sweden

Positive effects of legumes and actinorhizal plants on N-poor soils have been observed in many studies but few have been done at high latitudes, which was the location of our study. We measured N₂ fixation and several indices of soil N at a site near the Arctic Circle in northern Sweden. More than 20 years ago lupine (Lupinus nootkatensis Donn) and gray alder (Alnus incana L. Moench) were planted on this degraded forest site. We measured total soil N, net N mineralization and nitrification with a buried bag technique, and fluxes of NH⁺ ₄ and NO^– ₃ as collected on ion exchange membranes. We also estimated N₂ fixation activity of the N₂-fixing plants by the natural abundance of ¹⁵N of leaves with Betula pendula Roth. as reference species. Foliar nitrogen in the N₂-fixing plants was almost totally derived from N₂ fixation. Plots containing N₂-fixing species generally had significantly higher soil N and N availability than a control plot without N₂-fixing plants. Taken together, all measurements indicated that N₂-fixing plants can be used to effectively improve soil fertility at high latitudes in northern Sweden.     

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%.     

Minimizing the effect of mineral nitrogen on biological nitrogen fixation in common bean by increasing nutrient levels

Although common bean (Phaseolus vulgaris L.) has good potential for N₂ fixation, some additional N provided through fertilizer usually is required for a maximum yield. In this study the suppressive effect of N on nodulation and N₂ fixation was evaluated in an unfertile soil under greenhouse conditions with different levels of soil fertility (low=no P, K and S additions; medium = 50, 63 and 10 mg kg^–1 soil and high = 200, 256 and 40 mg kg^–1 soil, respectively) and combined with 5, 15, 60 and 120 mg N kg^–1 soil of ¹⁵N-labelled urea. The overall average nodule number and weight increased under high fertility levels. At low N applications, nitrogen had a synergistic effect on N₂ fixation, by stimulating nodule formation, nitrogenase activity and plant growth. At high fertility and at the highest N rate (120 mg kg^–1 soil), the stimulatory effect of N fertilizer on N₂ fixation was still observed, increasing the amounts of N₂ fixed from 88 up to 375 mg N plant^–1. These results indicate that a suitable balance of soil nutrients is essential to obtain high N₂ fixation rates and yield in common beans.     

Effects of N-fertilizers,straw, and dry fallow on the nitrogen balance of a flooded soil planted with rice

Summary Nitrogen balance studies were made on rice (Oryza sativa) grown in flooded soil in pots. A low rate of fertilizer (5.64 mg N. kg⁻¹ soil) did not depress the N gain, but a high rate (99.72 mg N. kg⁻¹ soil) elminated the N gain. Soil N loss was negligible since¹⁵N applied as ammonium sulfate and thoroughly mixed with the soil was recovered from the soil-plant system after 3 crops. The observed N gain, therefore, was caused by N₂-fixation, not by a reduction of soil N loss. Straw enhanced N gain at the rate of 2–4 mg per g straw. However, this gain was not observed when soil N availability was high. Dry fallow between rice crops decreased the N gain.     

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.