Assessment of fertilizer nitrogen accumulation in Pinus sylvestris trees and retention in soil by 15N recovery technique

The distribution and quantitative recovery of fertilizer N were determined in three 29‐ to 43‐year‐old stands of Scots pine, located in western Uppland, Central Sweden. The experimental technique involved was based on the use of ¹⁵N‐labelled fertilizer materials and non‐trenched microplots of special design. The standard dose of nitrogen applied was 160 kg N ha^‐1. The primary topics examined were (1) fertilizer nitrogen accumulation in trees and in the soil system as influenced by nitrogen source, nitrogen application rate, time of application during the growing season, granule size of the nitrogen material, and method of placement, (2) distribution of labelled N within the tree (different organs of the tree), and (3) redistribution of accumulated N in the tree over an observation period of two to three growing seasons. When quantified at the end of the second growing season, the labelled N accumulation in trees, concerning plots treated with calcium nitrate or ammonium nitrate, averaged 36% (SD=7). The accumulation resulting from split doses did not differ significantly from that resulting from a single‐dose application, nor did granule size of the nitrogen materials have any significant effect on accumulation. For urea source of nitrogen, the recovery figure was markedly lower, averaging 28% (SD=6). When expressed as a percentage, the accumulation in trees was not significantly different for 40 and 160 kg nitrogen application rates. In one of the experiments application timing during the growing season proved to be an important factor determining the extent of fertilizer nitrogen accumulation in trees. The figures for total recovery of labelled nitrogen in above‐ground parts of trees and in soil, when restricting measurements to the forest floor and the 0–32 cm mineral soil, ranged from 46 to 84%, with an average of 60%. The recovery was lowest for a treatment with calcium nitrate, when applied relatively late in the growing season, and highest for a split application of urea. Treatments with urea source of nitrogen were characterized by a high accumulation of immobilized N in the organic LFH layer of the soils. Recovery figures exceeded 82 % for a 40‐kg N dose of nitrogen, regardless of the nitrogen source.     

Long-term effects of repeated urea fertilization in Douglas-fir stands on forest floor nitrogen pools and nitrogen mineralization

In six Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stands in the Puget Sound Region in Western Washington/USA, forest floor C and N pools were quantified on control plots and on plots that had been fertilized repeatedly with urea 8–30 years ago (total amount of applied N 0.9–1.1 Mg ha⁻¹). Additionally, net N mineralization and nitrification rates were assessed in field and laboratory incubation experiments. Forest floor C/N ratios were decreased on the fertilized plots of all sites compared to the respective control plots. The decreases were particularly strong at sites with initial C/N ratios larger than 30. On sites with low productivity (site index at age 50: <33 m), N fertilization resulted in considerable increases in forest floor N pools. Net N mineralization and nitrification during 12-week field incubation was negligible for the unfertilized and fertilized plots of all except one site (Pack Forest), where the stand had been clear-cut 2 years ago. The increases in N mineralization rates during 12-week laboratory incubation induced by repeated N fertilization showed an inverse relationship to the time elapsed since the last fertilizer application, and were generally larger at sites with initial forest floor C/N ratios >30. For the investigated sites, fertilization effects on net N mineralization sustained for at least 11 years after the last fertilizer application. Nitrification correlated strongly with the forest floor pH; significant formation of NO₃⁻ was observed only for O layers with a pH (H₂O) higher than 4.5.     

Mixed-species plantations of Acacia mangium and Eucalyptus grandis in Brazil: 2: Nitrogen accumulation in the stands and biological N2 fixation

The sustainability of plantation forests is closely dependent on soil nitrogen availability in short-rotation forests established on low-fertility soils. Planting an understorey of nitrogen-fixing trees might be an attractive option for maintaining the N fertility of soils. The development of mono-specific stands of Acacia mangium (100A:0E) and Eucalyptus grandis (0A:100E) was compared with mixed-species plantations, where A. mangium was planted in a mixture at a density of 50% of that of E. grandis (50A:100E). N₂ fixation by A. mangium was quantified in 100A:0E and 50A:100E at age 18 and 30 months by the ¹⁵N natural abundance method and in 50A:100E at age 30 months by the ¹⁵N dilution method. The consistency of results obtained by isotopic methods was checked against observations of nodulation, Specific Acetylene Reduction Activity (SARA), as well as the dynamics of N accumulation within both species. The different tree components (leaves, branches, stems, stumps, coarse roots, medium-sized roots and fine roots) were sampled on 5–10 trees per species for each age. Litter fall was assessed up to 30 months after planting and used to estimate fine root mortality. Higher N concentrations in A. mangium tree components than in E. grandis might be a result of N₂ fixation. However, no evidence of N transfer from A. mangium to E. grandis was found. SARA values were not significantly different in 100A:0E and 50A:100E but the biomass of nodules was 20–30 times higher in 100A:0E than in 50A:100E. At age 18 months, higher δ¹⁵N values found in A. mangium tree components than in E. grandis components prevented reliable estimations of the percentage of N derived from atmospheric fixation (%Ndfa). At age 30 months, %Ndfa estimated by natural abundance and by ¹⁵N dilution amounted to 10–20 and 60%, respectively. The amount of N derived from N₂ fixation in the standing biomass was estimated at 62 kg N ha⁻¹ in 100A:0E and 3 kg N ha⁻¹ in 50A:100E by the ¹⁵N natural abundance method, and 16 kg N ha⁻¹ in 50A:100E by the ¹⁵N dilution method. The total amount of atmospheric N₂ fixed since planting (including fine root mortality and litter fall) was estimated at 66 kg N ha⁻¹ in 100A:0E and 7 kg N ha⁻¹ in 50A:100E by the ¹⁵N natural abundance method, and 31 kg N ha⁻¹ in 50A:100E by the ¹⁵N dilution method. The most reliable estimation of N₂ fixation was likely to be achieved using the ¹⁵N dilution method and sampling the whole plant.     

The effects of Douglas-fir on tree-specific arthropod communities in mixed species stands with European beech and Norway spruce

The ecological effects of planting exotic Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] in Central Europe are still poorly understood. The aim of this study was to answer the question of whether Douglas-fir affects tree specific arthropod communities in different mature forest types (Douglas-fir, spruce and beech dominated) in Southern Germany. Therefore, arthropod communities of stem and tree crown strata of Douglas-fir and spruce (Picea abies L.) were sampled in the years 1999–2001 using arboreal photo-eclectors and flight interception traps. Statistical analysis was conducted for all species and focused on conifer specialists at three levels: (1) species diversity, (2) guild structure and (3) community structure. Within the stem stratum, species diversity was significantly higher on spruce than on Douglas-fir independent of year and stand composition. This could not be explained by a single feeding guild, rather by species changing strata during the vegetation period. In contrast, species diversity in tree crowns was approximately the same for both conifer species. However, communities in Douglas-fir crowns were conspicuously different from those in spruce crowns, especially in the Douglas-fir dominated stand type. While zoophagous insects exhibited higher activity on Douglas-fir in 2000, xylophagous beetles were more abundant on spruce in 2001. In European beech stands with widely spaced Douglas-fir trees, the site specific and broad-leaved tree related fauna might be maintained. In addition, Douglas-fir with its resource of Adelges cooleyi and crowns that overtop the broad-leaved tree canopy, offer additional resources for several aphidophagous and thermophile species.     

Tracing the fate of mineral N compounds under high ambient N deposition in a Norway spruce forest at Solling/Germany

The fate of high and equally distributed ammonium and nitrate deposition was followed in a 72-year-old roofed Norway spruce forest at Solling in central Germany by separately adding ¹⁵NH₄⁺ and ¹⁵NO₃⁻ to throughfall water since November 2001. The objective was to quantify the retention of atmospheric ammonium and nitrate in different ecosystem compartments as well as the leaching loss from the forest ecosystem. δ¹⁵N excess in tree tissues (needles, twigs, branches and bole woods) decreased with increased tissue age. Clear ¹⁵N signals in old tree tissues indicated that the added ¹⁵N was not only assimilated to newly produced tree tissues but also retranslocated to old ones. During a period of over 3-year ¹⁵N addition, 30% of ¹⁵NH₄⁺ and 36% of ¹⁵NO₃⁻ were found in tree compartments. For both ¹⁵N tracers, 15% of added ¹⁵N was found in needles, followed by woody tissues (twigs, branches and boles, 7–13%) and live fine roots (7%). The recovery of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ in the live fine roots differed with soil depth. The recovery of ¹⁵NH₄⁺ tended to be higher in the live fine roots in the organic layer than in the upper mineral soil. In the live fine roots in deeper soil, the recovery of ¹⁵NO₃⁻ tended to be higher than that of ¹⁵NH₄⁺. Soil retained the largest proportion of ¹⁵N, accounting for 71% of ¹⁵NH₄⁺ and 42% of ¹⁵NO₃⁻. Most of ¹⁵NH₄⁺ was recovered in the organic layer (65%) and the recovery decreased with soil depth. Conversely, only 8% of ¹⁵NO₃⁻ was found in the organic layer and 34% of ¹⁵NO₃⁻ was evenly distributed throughout the mineral soil layers. Nitrate leaching accounted for 3% of ¹⁵NH₄⁺ and 19% of ¹⁵NO₃⁻. Only less than 1% of the both added ¹⁵N was leached as DON. These results suggested that trees had a high contribution to the retention of atmospheric N and soil retention capacity determined the loss of atmospheric N by nitrate leaching.     

Ungulate herbivory of regenerating conifers in relation to foliar nutrition and terpenoid production

Ungulate browsing greatly influences regeneration dynamics of some forest ecosystems, yet the relationship between browse susceptibility and foliar chemistry of forest tree seedlings is not well understood. We applied field fertilization (15N-9P-10K controlled-release fertilizer at 0, 20, 40, and 60 g per seedling) and investigated how subsequent changes in terpenoid production and foliar nutrition influence ungulate browse preference for Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco), western hemlock (Tsuga heterophylla Raf. Sarg.), and western redcedar (Thujaplicata Donn ex D. Don) seedlings across four sites in northwestern Oregon, USA. Fertilization increased foliar N concentration of all three species, but above-ground growth of only Douglas-fir and western hemlock. Foliar monoterpene concentrations for western hemlock and western redcedar also increased at higher fertilization rates, while Douglas-fir monoterpene production was not affected by fertilization. Regardless of monoterpene levels, ungulate browse preference was greater for fertilized western hemlock seedlings. The opposite response, however, was observed for western redcedar at two of the four study sites where the likelihood of browse was greater for non-fertilized than fertilized seedlings. Differences in browse preference among species may depend on the type and amount of individual monoterpenes manufactured in response to fertilization. Western redcedar produce α- and β-thujone, oxygenated monoterpenes known to promote gastroenteritis and possibly inhibit microbial rumen activity, which were absent from Douglas-fir or western hemlock. Higher concentrations of α- and β-thujone associated with increasing fertilizer rate provide a plausible explanation as to why ungulates preferred non-fertilized western redcedar. Our results illustrate species-specific adaptation in browse avoidance and selective ungulate browsing behavior of individual trees as linked to foliar chemistry.     

Steady-state nutrition of hybrid poplar grown from un-rooted cuttings

Fertilizing plantations of fast-growing tree crops, in manners which supply nutrients at rates that match plant demand and maintain stable internal plant nutrient ratios, can maximize biomass production and carbon sequestration while reducing fertilizer wastage and pollution. Our objectives were to determine nutrient ratios of common hybrid poplar (Populus trichocarpa Torr. and Gray × Populus deltoides Marsh.) (T × D) clones under steady-state nutrition, and to determine if incremental additions of fertilizer were more successful than conventional (evenly-split) fertilizer additions in maximizing biomass production and inducing stable nutrient ratios. Un-rooted cuttings of three T × D clones (49–177, DTAC-7, 15–29) were grown under a conventional regime and a modified-exponential fertilization regime at three application rates (1.8, 3.7 or 7.4 g N plant⁻¹). Above- and below-ground biomass and nutrient concentrations were measured after one growing season. There were few differences in total plant biomass between conventional and modified-exponential fertilization regimes, but for one clone, biomass accumulation equivalent to the highest rate under the conventional regime was achieved with the medium rate of the modified-exponential regime. Stable nutrient ratios (at conditions consistent with steady-state nutrition) were: 100N:14P:50K (49–177), 100N:13P:49K (DTAC-7) and 100N:12P:60K (15–29).     

Natural abundance of <Superscript>15</Superscript>N in two cacao plantations with legume and non-legume shade trees

Natural abundance of ¹⁵N was sampled in young and mature leaves, branches, stem, and coarse roots of trees in a cacao (Theobroma cacao) plantation shaded by legume tree Inga edulis and scattered non-legumes, in a cacao plantation with mixed-species shade (legume Gliricidia sepium and several non-legumes), and in a tree hedgerow bordering the plantations in Guácimo, in the humid Caribbean lowlands of Costa Rica. The deviation of the sample ¹⁵N proportion from that of atmosphere (δ¹⁵N) was similar in non-legumes Cordia alliodora, Posoqueria latifolia, Rollinia pittieri, and T. cacao. Deep-rooted Hieronyma alchorneoides had lower δ¹⁵N than other non-N₂-fixers, which probably reflected uptake from a partially different soil N pool. Gliricidia sepium had low δ¹⁵N. Inga edulis had high δ¹⁵N in leaves and branches but low in stem and coarse roots. The percentage of N fixed from atmosphere out of total tree N (%N_f) in G. sepium varied 56–74%; N₂ fixation was more active in July (the rainiest season) than in March (the relatively dry season). The variation of δ¹⁵N between organs in I. edulis was probably associated to ¹⁵N fractionation in leaves. Stem and coarse root δ¹⁵N was assumed to reflect the actual ratio of N₂ fixation to soil N uptake; stem-based estimates of %N_f in I. edulis were 48–63%. Theobroma cacao below I. edulis had lower δ¹⁵N than T. cacao below mixed-species shade, which may indicate direct N transfer from I. edulis to T. cacao but results so far were inconclusive. Further research should address the ¹⁵N fractionation in the studied species for improving the accuracy of the N transfer estimates. The δ¹⁵N appeared to vary according to ecophysiological characteristics of the trees.     

Foliar δ15N variations with stand ages in temperate secondary forest ecosystems,Northeast China

Abstract

Natural abundance of ¹⁵N changes strongly with gradients in soil or environmental conditions across large spatial and temporal scales, but variation in δ¹⁵N with stand age at intermediate scales is poorly understood. We analyzed soil and foliar δ¹⁵N along a secondary forest chronosequence in Northeast China in mid-growing seasons 2008 and 2009, to address two questions: how does foliar δ¹⁵N vary with stand age; and are the variations driven by compositional differences in species among stands, or by consistent changes in δ¹⁵N with plant growth forms, and within species. The results showed that community-level foliar δ¹⁵N depleted as stand age increased, and these responses were remarkably consistent within three contrasting plant growth forms (herbs, shrubs, and trees), and within individual species. In spite of the three plant growth forms sharing similar responses to the stand age, tree species showed smaller variations in foliar δ¹⁵N along stand ages than herbs and shrubs. Soil δ¹⁵N also significantly depleted with increasing stand age, which may drive the variations of foliar δ¹⁵N. In addition, mycorrhizal fungi discrimination against ¹⁵N may also partly affect the patterns of foliar δ¹⁵N along stand ages. The results clearly indicate that differences in foliar δ¹⁵N among different stand ages are dominantly driven by the δ¹⁵N variations at the species levels, which reflects the variations of soil δ¹⁵N and mycorrhizal association intensity and association type, not by compositional difference in species among stands.     

Modeling mid-rotation fertilizer responses using the age-shift approach

Growth and yield modelers have incorporated mid-rotation fertilizer responses by: modifying site index; developing new models to include fertilizer responses directly; using multipliers or additional terms to scale existing models. We investigated the use of age-shifts to model mid-rotation fertilizer responses. Age-shift prediction models were constructed from 43 installations of a nitrogen (0, 112, 224 and 336 kg ha⁻¹ elemental) by phosphorus (0, 28 and 56 kg ha⁻¹ elemental) factorial experiment established in mid-rotation loblolly (Pinus taeda L.) pine stands in the southeastern US. Age-shifts for dominant height and basal area increased with time after fertilization, to a maximum and then either remained fairly constant, or declined. The initial rate of increase, maximum age-shift and decline were functions of the rate and combinations of fertilizers applied, as well as stand density and age at fertilization. Volume age-shifts increased linearly throughout the 10-year measurement period for most treatments with the rate of increase being a function of the elements applied, stocking, site index and age at fertilization. A mid-rotation fertilizer application of 224 and 28 kg ha⁻¹ elemental N and P, respectively, resulted in age-shifts of 1.1, 1.9 and 2.4 years for dominant height, basal area and volume, respectively, 10 years after fertilization. The age-shifts were incorporated into growth and yield models.     

Pasture production and tree growth in a young pine plantation fertilized with inorganic fertilizers and milk sewage in northwestern Spain

The use of organic waste materials such as milk sewage as an organic fertilizer could have the dual advantages of organic-waste disposal and reduced dependence on inorganic fertilizers. The effects of fertilization with (1) conventional mineral fertilization, (2) milk sewage sludge at 40 kg N ha⁻¹ target rate and (3) no fertilization on pasture production and tree growth were examined in an experiment consisting of two pasture mixtures under a one-year-old Pinus radiata plantation with a density of 2500 trees ha⁻¹. The two pasture mixtures were: (1) Dactylis glomerata L. var. saborto (25 kg ha⁻¹) + Trifolium repens L. group Ladino (4 kg ha⁻¹) + Trifolium pratense L. var. Marino (1 kg ha⁻¹); (2) Lolium perenne L. var. Tove (25 kg ha⁻¹) + Trifolium repens L. group Ladino (4 kg ha⁻¹) + Trifolium pratense L. var. Marino (1 kg ha⁻¹). The experiment began in the spring of 1995 using a randomized block design with three replicates in Castro Riberas de Lea (Lugo, Galicia, north-western Spain). Plot size was 12 × 8 m², with a 1 m buffer strip between plots. Two-year data showed that fertilization with either material had a positive effect on pasture production, with no significant difference between the two fertilization treatments. Tree growth in the milk sewage sludge plot was significantly higher than in the control plots. Inorganic fertilization increased pasture production, but affected tree growth negatively. The results show that milk sewage sludge could be used as a fertilizer in silvo-pastoral systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogen-fixation dynamics in a cut-and-carry silvopastoral system in the subhumid conditions of Guadeloupe, French Antilles

This paper summarizes several studies on N recycling in a tropical silvopastoral system for assessing the ability of the system to increase soil fertility and insure sustainability. We analyzed the N₂ fixation pattern of the woody legume component (Gliricidia sepium), estimated the recycling rate of the fixed N in the soil, and measured N outputs in tree pruning and cut grass (Dichanthium aristatum). With this information, we estimated the N balance of the silvopastoral system at the plot scale. The studies were conducted in an 11-year-old silvopastoral plot established by planting G. sepium cuttings at 0.3 m × 2 m spacing in natural grassland. The plot was managed as a cut-and-carry system where all the tree pruning residues (every 2-4 months) and cut grass (every 40-50 days) were removed and animals were excluded. No N fertilizer was applied. Dinitrogen fixation, as estimated by the ¹⁵N natural abundance method, ranged from 60-90% of the total N in aboveground tree biomass depending on season. On average, 76% of the N exports from the plot in tree pruning (194 kg [N] ha^–1 yr^–1) originated from N₂ fixation. Grass production averaged 13 Mg ha^–1 yr^–1 and N export in cut grass was 195 kg [N] ha^–1 yr^–1. The total N fixed by G. sepium, as estimated from the tree and grass N exports and the increase in soil N content, was about 555 kg [N] ha^–1 yr^–1. Carbon sequestration averaged 1.9 Mg [C] ha^–1 yr^–1 and soil organic N in the 0-0.2 m layer increased at a rate of 166 kg [N] ha^–1 yr^–1, corresponding to 30% of N₂ fixation by the tree. Nitrogen released in nodule turnover (10 kg [N] ha^–1 yr^–1) and litter decomposition (40 kg [N] ha^–1 yr^–1) contributed slightly to this increase, and most of the recycled N came from the turnover or the activity of other below-ground tree biomass than nodules. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phosphorus form and bioavailability in the pine rotation following fertilization: P fertilization influences P form and potential bioavailability to pine in the subsequent rotation

Soils in some parts of the world are known to be severely phosphorus (P) deficient. As little as 20 kg P ha⁻¹ have produced large growth responses in forest stands. If increased tree growth and augmented P cycling are caused by a one-time P application, a valid question is whether the effect of the initial fertilization will be evident in the regenerating forest, thereby reducing the need for P fertilization early in the second rotation. The objective of this study was to evaluate the P status of the forest floor and inorganic P status of mineral surface soil of two long-term fertilizer trials in order to determine if there were increases in soil P amount and bioavailability that benefits the next rotation. Long-term fertilizer trials in Georgia, USA and New Zealand (29 and 22 years after fertilization) were the study sites. In Georgia, forest fertilization increased the mass of the F horizon by 239% and its P content by 318%. Anion exchange membranes measured up to a 47% increase in bioavailable P in the mineral soil in a fertilized treatment. While bioassays did not show significant residual P in the mineral soil under the fertilized plots, soil from an adjacent operationally fertilized stand, which had the forest floor bedded into its planting rows, showed a 100% increase in seedling P content. At the New Zealand site, the mineral soil contained 142% more P in the anion exchange membrane form than in the unfertilized soils. Bioassays indicated a 224% increase in seedling P content when grown in fertilized soil at the highest fertilization rate. Operational levels of fertilization did not yield a significant bioassay result. The data show that there can be a residual effect of fertilizer P in the rotation following fertilization. The bioassays, anion membrane exchangeable P and forest floor P from Georgia suggest that there is enough P in the soil system to support the needs of a regenerating stand of loblolly pine, while residual bioavailable P in the mineral soil from operational levels of fertilization at New Zealand site would require residual P in the forest floor or immediate fertilization to meet the demands of newly planted seedlings. Conservation of the forest floor is an important part of P management in these P deficient sites.     

Growth and nutrition of Pinus radiata in response to fertilizer applied after thinning and interaction with defoliation associated with Essigella californica

Infestations of Essigella californica following the installation of post-thinning fertilizer trials in Pinus radiata plantations provided an opportunity to examine the impact of repeated defoliation over a period of 8 years (1997–2005). Replicated treatments (n = 4) of nil fertilizer (control), N (300 kg ha⁻¹) as urea, P (80 kg ha⁻¹) and S (45 kg ha⁻¹) as superphosphates were applied immediately after thinning at three sites and this was followed by a second application of NPS fertilizers 6 years later with N applied at 300 kg ha⁻¹ as urea and ammonium sulphate and P at 80 or 120 kg ha⁻¹. Defoliation of untreated P. radiata gradually increased to 50% over a period of 8 years. Basal area growth was negatively correlated with average defoliation for two consecutive post-fertilizer periods of 6 and 2 years. Growth responses to fertilizer varied considerably between sites but the largest improvement in growth was due to NPS fertilizer, this increased basal area by 30–80%. Application of N fertilizer raised total N levels in foliage and increased defoliation with a commensurate loss in growth under conditions of deficiencies of S or P. Repeated infestations gradually increased the percentage of trees with severe defoliation (>80% loss of foliage) indicating that nutrient-deficient trees have a reduced capacity for foliage recovery between episodes of peak infestation. In contrast, treatment with N fertilizer in combination with S- and P-corrected deficiencies of these nutrients, raised levels of total N in foliage and reduced defoliation to approximately 20%. Basal area growth responses to NPS fertilizers reflected improved nutrition as well as reduced insect damage. The reduction in defoliation under conditions of balanced tree nutrition was most likely due to enhanced needle retention following correction of P deficiency as well as greater availability of nutrients enabling a more vigorous recovery of P. radiata after an episode of E. californica activity. Treatment with fertilizer therefore reduced the long-term impact of aphid damage and improved growth of P. radiata.     

Seasonal changes of understorey herbage yield in relation to light intensity and soil moisture content in a Pinus pinaster plantation

The relationship of understorey herbage yield with light intensity and soil moisture content was investigated in a 20-year-old Pinus pinaster plantation in Kilkis of Macedonia, northern Greece, in three seasons (fall, winter and spring). The plantation was thinned at three density levels, high, medium and low (2500, 1750 and 1000 trees/ha respectively) and it was seeded with Dactylis glomerata. In addition, two fertilization levels (0 and 110 N+150 P₂O₅ kg/ha) and grazing intensity levels (0 and 0.8 sheep/ha/yr) were applied in a split-split plot experimental design. It was found that herbage yield was highest in the ungrazed and fertilized low tree density stands in all seasons. In the medium tree density stand, although light was lacking, fertilized plots produced higher herbage yield as compared to unfertilized low tree density stands while fertilization is of no use in close stands if herbage production is the goal. Animals seemed to prefer fertilized and medium density stands. Third degree polynomial equations were the best to explain herbage variation through the seasons both in fertilized and unfertilized plots.     

Nitrogen-15 Uptake by Pinus contorta Seedlings in Relation to Phenological Stage and Season

This study measured the amount of uptake of labeled nitrogen (¹⁵N) of lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm) seedlings, at three different phenological stages, in a growth chamber experiment. Thirty days after ¹⁵N application, the amount of ¹⁵N recovered in seedlings as a percentage of the total ¹⁵N fertilizer applied was 4% in early spring, 43% in summer and 33% in autumn. The total ¹⁵N recovered in the plant–pot system ranged from 80 to 96%, and is higher than reported in other studies. Total ¹⁵N recovered from the pot compartment alone ranged from 48 to 95%, suggesting that substantial pools of N remain in the soil. Results suggest that low ¹⁵N uptake in the spring was associated with limited development of new root as a result of low spring soil temperatures. The lack of unsuberized roots in spring could be a key factor decreasing the effectiveness of early spring fertilization in the boreal forest.     

交趾黄檀幼林施肥效应分析

开展不同施肥量对交趾黄檀幼树生长影响的试验。结果表明,5年生交趾黄檀胸径、树高、材积、冠幅生长量最大的为处理3(350 g/株),分别为7.12 cm、5.96 m、0.012 8 m³、3.20 m,分别比林分平均胸径6.24 cm、树高5.23 m、材积0.009 2 m³、冠幅3.06 m提高14.1%、13.96%、39.13%、4.58%。应用层次分析法得到综合得分最高的为处理2(250 g/株),表明在合理范围内增加施肥量可加速幼树生长,但交趾黄檀冠幅大,侧枝发达,易受风害,不是施肥量越大越好,应适当控制施肥量。     

Production and quality of senesced and green litterfall in a pine–oak forest in central-northwest Mexico

Litterfall is an important ecological process in forest ecosystems, influencing the transfer of organic matter, carbon (C), nitrogen (N), phosphorous (P) and other nutrients from vegetation to the soil. We examined the production of different litterfall fractions as well as nutrient content and nutrient inputs by senesced and green leaf-litter in a semiarid forest from central Mexico. From September 2006 to August 2007, monthly litter sampling was carried out in monospecific and mixed stands of Quercus potosina and Pinus cembroides. Litterfall displayed a marked bimodal pattern with the largest annual amount (5993 ± 655 kg ha⁻¹ yr⁻¹) recorded in mixed stands, followed by Q. potosina (4869 ± 510 kg ha⁻¹ yr⁻¹), and P. cembroides (3023 ± 337 kg ha⁻¹ yr⁻¹). Leaves constituted the largest fraction of total litterfall reaching almost 60%, while small branches contributed with 20–30%. Overall, N content in leaf-litter was higher while lignin content was significantly lower for Q. potosina than for P. cembroides. Thus, greater litter quality together with higher litter production caused the largest C, N and P inputs to forest soils to occur in monospecific Q. potosina stands. Green leaf fall displayed significantly lower lignin:N and C:N ratios in Q. potosina than P. cembroides suggesting faster decomposition and nutrient return rates by the former. Although we recorded only two green leaf fall events, they accounted for 18% and 11% of the total N and P input, respectively, from leaf-litter during the study period. Apart, from the large spatiotemporal heterogeneity introduced by differences in litter quantity and quality of evergreen, deciduous and mixed stands, green litterfall appears to represent a much more important mechanism of nutrient input to semiarid forest ecosystems than previously considered.     

Nitrogen dynamics during ecosystem development in tropical forest restoration

We considered whether ecological restoration using high diversity of native tree species serves to restore nitrogen dynamics in the Brazilian Atlantic Forest. We measured δ¹⁵N and N content in green foliage and soil; vegetation N:P ratio; and soil N mineralization in a preserved natural forest and restored forests of ages 21 and 52 years. Green foliage δ¹⁵N values, N content, N:P ratio, inorganic N and net mineralization and nitrification rates were all higher, the older the forest. Our findings indicate that the recuperation of N cycling has not been achieved yet in the restored forests even after 52 years, but show that they are following a trajectory of development that is characterized by their N cycling intensity becoming similar to a natural mature forest of the same original forest formation. This study demonstrated that some young restored forests are more limited by N compared to mature natural forests. We document that the recuperation of N cycling in tropical forests can be achieved through ecological restoration actions.     

杉木幼林施肥效应研究

在由粉砂员岩坡积母质发育而成的硅质黄红壤上,经不同施肥量及配比12个处理、不同施肥时间10个处理5年试验研究表明,杉木幼林施氮肥和钾肥无显著效应,施磷肥效应显著。造林当年及第3年各施钙镁磷肥50g/株,与对照比较,杉木幼林地径、胸径、树高、蓄积分别增加15％、18％、15％、53％,取得生长效应与经济效益的最佳水平。配比施肥效应以造林当年与第3年每株各施尿素30g、钙镁磷肥,100g最佳,但配比施肥效应仍以磷肥效应为主。施肥时间以造林当年与第3年隔年施肥效果最佳。