Methane and nitrous oxide emissions from mature forest stands in the boreal forest, Saskatchewan, Canada

Despite the spatial significance of Canada's boreal forest, there is very little known about CH₄ and N₂O emissions from non-peatlands within it. The primary objective of this project was to study the atmosphere–soil exchange of CH₄ and N₂O at three sites in the boreal forest of central Saskatchewan. In the summers of 2006 and 2007, CH₄ and N₂O emissions were measured along transects in three different mature forest stands (aspen, black spruce and jack pine) using a sealed chamber method. At the aspen site, the gross rates of mineralization and nitrification, and the relative contribution of nitrification and denitrification to N₂O emissions, were also measured using the ¹⁵N isotope dilution technique. Results indicated that the jack pine and black spruce sites were slight sinks of CH₄ (−0.123 g CH₄–C m⁻² yr⁻¹and −0.017 g CH₄–C m⁻² yr⁻¹ respectively in 2006 and −0.095 g CH₄–C m⁻² yr⁻¹and 0.045 g CH₄–C m⁻² yr⁻¹ respectively in 2007), whereas the aspen site was a net source (4.40 g CH₄–C m⁻² yr⁻¹ in 2006 and 19.60 g CH₄–C m⁻² yr⁻¹ in 2007). The high CH₄ emissions at the aspen site occurred at depressions that were water-filled due to above-average precipitation levels in 2005–2007. All three sites had very low cumulative N₂O emissions, ranging from −0.002 to 0.014 g N₂O–N m⁻² yr⁻¹ in both years. The ¹⁵N results indicated that N cycling at the aspen site was very conservative, allowing little N to escape the system as N₂O; the emissions that did occur were due primarily to a nitrification-related process.     

Inorganic sediment budget in the mangrove-fringed Fly River Delta,Papua New Guinea

Six oceanographic moorings were maintained for 8 weeks across the mouth of the mangrove-fringed Fly River estuary from April to June 1995 in the southeast trade wind season. A further 4 moorings were deployed for 8 weeks along the estuary channel in 1992, also in the southeast trade wind season. These data were used to estimate net exchange of suspended sediment between the estuary and the Gulf of Papua. A net inflow of fine sediment into the estuary from the coastal ocean was found to be considerable, about 40 tonnes s^-1 or about 10 times the riverine inflow rate, resulting in a calculated, spatially averaged vertical accretion rate of 2 mm year^-1. Mangroves may account for trapping 6% of the riverine sediment inflow or about 1/4 of the riverine clay inflow. If this sediment was distributed only over the observed accumulation zones near islands the local accumulation rates in these zones would reach 4 cm year^-1. Estimates of soft sediment mass accumulation rates (1–10 kg m^-2 year^-1) in the channel from Pb-210 and C-14 measurements from cores of deltaic mangrove mud cannot account for this accumulation rate on a 100–1000 year time scale. The fate of the remaining sediment is unknown, it may be exported from the estuary in the monsoon season.     

Crop yields of rice and wheat grown in rotation as intercrops with three tree species in the outer hills of Western Himalaya

Studies on tree crop interaction under rainfed condition in Dehradun valley were conducted for 13 years during 1977 to 1990. Grewia optiva (Bhimal), Morus alba (Mulbery) and Eucalyptus hybrid were tried along with rice (CV: Akashi) — wheat (CV: RR-21) rotation. One-year-old tree seedlings of the above tree species were planted in line, 5 m apart in N-S direction, in July 1977, in the middle of the plot (size 20 m × 20 m). Eucalyptus was first harvested in 1987. Grewia optiva, Morus alba and coppice of Eucalyptus were harvested in 1990. All tree species had depressing effect on crop yields. Eucalyptus had maximum effect in depressing crop yield till the first harvest and had least effect thereafter. From 1987 onwards, Morus alba affected rice most, while wheat was mostly affected by Grewia optiva. The depressing effect on an average varied from 28 to 34% depending upon the species.Distance of tree line from the crop significantly affected the crop yield upto a distance of 5 m and there was 39% decrease in crop yield upto 1 m, 33% from 1–2 m, 25% from 2–3 m and 12% from 3–5 m distance. Annual removal of lops and tops from trees partly compensated the deficit. Grewia optiva could produce 1.08 t ha^–1 yr^–1 of branches and 0.26 t ha^–1 yr^–1 of leaves (air dry) and 1.28 t ha^–1 yr^–1 of branches and 0.28 t ha^–1 yr^–1 of leaves were obtained from Morus alba. Wood (ADT) produced by the trees was 33.6 t ha^–1 from Eucalyptus, 9.5 t ha^–1 from Grewia optiva and 11.6 t ha^–1 from Morus alba.     

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.     

Seasonal variation in biomass and primary productivity of para grass (Brachiaria mutica) under a mixed tree stand and in an adjacent open area in northern India

Seasonal changes in biomass, net primary productivity and turnover of dry matter of para grass (Brachiaria mutica) under a mixed tree stand and in an adjacent open stand in northern India are presented. Both stands attained peak values of live shoot biomass in September with a higher value under mixed tree stand (665 g m^–2) than in the open stand (522 g m^–2). The net aboveground production was 590 and 527 g m^–2 yr^–1 under mixed tree stand and in the open, respectively. The belowground net primary production was also greater under mixed tree stand (100 g m^–2 yr^–1) than in the open (76 g m^–2 yr^–1). Maximum aboveground and belowground net primary productions in both stands were obtained during the rainy season. The total net primary production for para grass was about 15% higher under mixed tree stand than in the open. The turnover rates of total plant biomass were greatest in the rainy season and the least during the summer season. The system transfer functions showed that the production of para grass on both stands was aboveground-oriented, accounting for 85–87% of annual total net primary production.     

Vertical accretion and shallow subsidence in a mangrove forest of southwestern Florida,U.S.A.

Simultaneous measurements of vertical accretion from artificial soilmarker horizons and soil elevation change from sedimentation-erosion table(SET) plots were used to evaluate the processes related to soil building infringe, basin, and overwash mangrove forests located in a low-energy lagoonwhich receives minor inputs of terrigenous sediments. Vertical accretionmeasures reflect the contribution of surficial sedimentation (sedimentdeposition and surface root growth). Measures of elevation change reflectnot only the contributions of vertical accretion but also those ofsubsurface processes such as compaction, decomposition and shrink-swell. Thetwo measures were used to calculate amounts of shallow subsidence (accretionminus elevation change) in each mangrove forest. The three forest typesrepresent different accretionary environments. The basin forest was locatedbehind a natural berm. Hydroperiod here was controlled primarily by rainfallrather than tidal exchange, although the basin flooded during extreme tidalevents. Soil accretion here occurred primarily by autochthonous organicmatter inputs, and elevation was controlled by accretion and shrink-swell ofthe substrate apparently related to cycles of flooding-drying and/or rootgrowth-decomposition. This hydrologically-restricted forest did notexperience an accretion or elevation deficit relative to sea-level rise. Thetidally dominated fringe and overwash island forests accreted throughmineral sediment inputs bound in place by plant roots. Filamentous turfalgae played an important role in stabilizing loose muds in the fringeforest where erosion was prevalent. Elevation in these high-energyenvironments was controlled not only by accretion but also by erosion and/orshallow subsidence. The rate of shallow subsidence was consistently3–4 mm y^–1 in the fringe and overwash island forests butwas negligible in the basin forest. Hence, the vertical development ofmangrove soils was influenced by both surface and subsurface processes andthe processes controlling soil elevation differed among forest types.The mangrove ecosystem at Rookery Bay has remained stable as sea levelhas risen during the past 70 years. Yet, lead-210 accretion data suggest asubstantial accretion deficit has occurred in the past century (accretionwas 10–20 cm < sea-level rise from 1930 to 1990) in the fringe andisland forests at Rookery Bay. In contrast, our measures of elevation changemostly equalled the estimates of sea-level rise and our short term estimatesof vertial accretion exceeded the estimates by the amount of shallowsubsidence. These data suggest that (1) vertical accretion in this system isdriven by local sea-level rise and shallow subsidence, and (2) the mangroveforests are mostly keeping pace with sea-level rise. Thus, the vulnerabilityof this mangrove ecosystem to sea-level rise is best described in terms ofan elevation deficit (elevation change minus sea-level rise) based on annualmeasures rather than an accretion deficit (accretion minus sea-level rise)based on decadal measures.     

The role of groundwater flow in controlling the spatial distribution of soil salinity and rooted macrophytes in a southeastern salt marsh, USA

Groundwater flow is an important factor in governing botanical zonation in the salt marsh at North Inlet, SC. Areas of the marsh adjacent to upland forest are characterized by upward flow of fresh groundwater. This inhibits the infiltration and evapoconcentration of saline tidal water and the development of a habitat for hypersaline-tolerant fugitive species such as Salicornia europaea. Areas of high marsh that are not adjacent to extensive upland forest are characterized by downward gradients in hydraulic head. This allows the infiltration and evapoconcentration of tidal water and the development of hypersaline conditions that are suitable for salt-tolerant fugitives.     

Stem respiration in leafy cuttings of Prosopis juliflora during the rooting process

An oxygen electrode was used to estimate the respiration rate of basal 1 cm sections of Prosopis juliflora cuttings. The respiration rate per unit dry mass decreased linearly with increasing diameter at a rate of 0.42 nmol0₂g^–1s^–1mm^–1 between the range of 1–3 mm diameter. However, as a result of increased mass, total respiration for the whole 1 cm section of tissue increased linearly with increasing diameter. The average respiration rate of the basal 1 cm of stem tissue, measured 14 days after insertion in the propagator, was almost double that of the 1 cm section of tissue above it (1.01 nmol0₂g^–1s^–1 compared to 0.57 nmol0₂g^–1s^–1). The value of this technique as an aid to our understanding of the development of adventitious roots is discussed.     

Comparative study of organic solvent-soluble and water-soluble lipophilic extractives from wheat straw 2: spectroscopic and thermal analysis

As a continuing study of six organic solventsoluble lipophilic extractives and one water-soluble lipophilic extract from wheat straw, seven extracts were further compared using Fourier transform-infrared (FT-IR) analysis,¹H and¹³C nuclear magnetic resonance (NMR) spectroscopy, and thermal analysis. In the FT-IR spectra of two-thirds chloroform/one-third methanol, methyltertbutyl ether (MTBE), hexane, petroleum ether, and dichloromethane extracts, a strong band at 1746cm^–1 is indicative of the ester carbonyl of triglycerides or steryl esters or waxes, whereas the occurrence of carboxylic carbonyl of free fatty acids or resin acids at 1712–1719 cm^–1 is verified in the spectra of two-thirds toluene/one-third ethanol and hot water extracts. Signals at 187.9,178.4, and 173.3 ppm in the¹³C NMR spectrum of MTBE extract correspond to carbonyl groups of resin acids, fatty acids, and fatty acid/steryl esters, respectively. All the extractives were to varying degrees thermally unstable at a temperature of about 200°C, and the melting temperature occurred at about 54°C.     

A novel approach to optimize management strategies for carbon stored in both forests and wood products

We present a new approach to maximize carbon (C) storage in both forest and wood products using optimization within a forest management model (Remsoft Spatial Planning System). This method was used to evaluate four alternative objective functions, to maximize: (a) volume harvested, (b) wood product C storage, (c) forest C storage, and (d) C storage in the forest and products, over 300 years for a 30,000 ha hypothetical forest in New Brunswick, Canada. Effects of three initial forest age-structures and a range of product substitution rates were tested. Results showed that in many cases, C storage in product pools (especially in landfills) plus on-site forest C was equivalent to forest C storage resulting from reduced harvest. In other words, accounting for only forest, and not products and landfill C, underestimates true forest contributions to C sequestration, and may result in spurious C maximization strategies. The scenario to maximize harvest resulted in mean harvest for years 1–200 of 3.16 m³ ha⁻¹ yr⁻¹ and total C sequestration of 0.126 t ha⁻¹ yr⁻¹, versus 0.98 m³ ha⁻¹ yr⁻¹ and 0.228 t ha⁻¹ yr⁻¹ for a scenario to maximize forest C. When maximizing total (forest + products) C, mean harvest and total C storage for years 1–200 was 173% and 5% higher, respectively, than when maximizing forest C; and 218% and 6% higher, respectively, when maximizing substitution benefits (0.25 t of avoided C emissions per m³ of lumber used) in addition to total C. Initial forest age-structure affected harvest in years 1–50 < 34% among the four alternative management objective scenarios, and resulted in mean C sequestration rates of 0.31, 0.10, and −0.14 t ha⁻¹ yr⁻¹ when maximizing total C storage for young, even-aged, and old forests, respectively. Our results reinforce the importance of including products in forest-sector C budgets, and demonstrate how including product C in management can maximize forest contributions toward reduced atmospheric CO₂ at operational scales.     

Seasonal dynamics of mineral N pools and N-mineralization in soils under homegarden trees in South Andaman,India

Agroforestry trees are now well known to play a central role in the build up of nutrients pools and their transformations similar to that of forest ecosystem, however, information on the potential of homegarden trees accumulating and releasing nitrogen (mineralization) is lacking. The present study reports seasonal variations in pool sizes of mineral N (NH₄⁺-N and NO₃⁻-N), and net N-mineralization rate in relation to rainfall and temperature under coconut (Cocos nucifera L.), clove (Eugenia caryophyllata Thunb) and nutmeg (Myristica fragrans Houtt. Nees) trees in a coconut-spice trees plantation for two annual cycles in the equatorial humid climate of South Andaman Island of India. Concentration of NH₄⁺-N was the highest during wet season (May–October) and the lowest during post-wet season (November–January) under all the tree species. On the contrary, concentration of NO₃⁻-N was the lowest in the wet season and the highest during the post-wet season. However, concentrations of the mineral N were the highest under the nutmeg and the lowest under the coconut trees. Like the pool sizes, mean annual mineralization was the highest under the nutmeg (561 mg kg⁻¹ yr⁻¹) and the lowest under the coconut trees (393 mg kg⁻¹ yr⁻¹). Rate of mineralization was the highest during the post-wet season and the lowest during the dry season (February–April) under all the tree species. High rainfall during the wet season, however, reduced the rate of nitrification under all the tree species. The mean annual mineralization was logarithmically related with rainfall amount and mean monthly temperature.     

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.     

A comparison of agrisilvicultural systems with plantation forestry in the Atlantic Lowlands of Costa Rica

Survival and growth data (ages 0–5 years) are presented for two timber species (Acacia mangium, Cordia alliodora) planted in monocultures or in association with a sequence of agricultural cropsZea mays, Zingiber officinale andEugenia stipitata (a fruit shrub) in the tropical humid Caribbean lowlands of Costa Rica. Average annual height and diameter growth rates were 3.2 m and 4.0 cm (C. alliodora), 3.5 m and 3.8 cm (A. mangium).C. alliodora associated with crops gave the greatest productivity with an average total stem volume increment of 19 m³ ha^–1 yr^–1. Root rot ofA. mangium (mainlyRosellinia sp.), leading to tree mortality, was greater in pure plots compared to associated plots.A. mangium can not be recommended for similar sites because of this problem.     

Gross N transformations were little affected by 4 years of simulated N and S depositions in an aspen-white spruce dominated boreal forest in Alberta, Canada

The effects of 4 years of simulated nitrogen (N) and sulfur (S) depositions on gross N transformations in a boreal forest soil in the Athabasca oil sands region (AOSR) in Alberta, Canada, were investigated using the ¹⁵N pool dilution method. Gross NH₄⁺ transformation rates in the organic layer tended to decline (P < 0.10, marginal statistical significance, same below) in the order of control (CK, i.e., no N or S addition), +N (30 kg N ha⁻¹ yr⁻¹), +S (30 kg S ha⁻¹ yr⁻¹), and +NS treatments, with an opposite trend in the mineral soil. Gross NH₄⁺ immobilization rates were generally higher than gross N mineralization rates across the treatments, suggesting that the studied soil still had potential for microbial immobilization of NH₄⁺, even after 4 years of elevated levels of simulated N and S depositions. For both soil layers, N addition tended to increase (P < 0.10) the gross nitrification and NO₃⁻ immobilization rates. In contrast, S addition reduced (P < 0.001) and increased (P < 0.001) gross nitrification as well as tended (P < 0.10) to reduce and increase gross NO₃⁻ immobilization rates in the organic and mineral soils, respectively. Gross nitrification and gross NO₃⁻ immobilization rates were tightly coupled in both soil layers. The combination of rapid NH₄⁺ cycling, negligible net nitrification rates and the small NO₃⁻ pool size after 4 years of elevated N and S depositions observed here suggest that the risk of NO₃⁻ leaching would be low in the studied boreal forest soil, consistent with N leaching measurements in other concurrent studies at the site that are reported elsewhere.     

Modelling agroforestry systems of cacao (Theobroma cacao) with laurel (Cordia alliodora) and poro (Erythrina poeppigiana) in Costa Rica II. Cacao and wood production,litter production and decomposition

During 7 years (1979–1985) cacao harvests (beans and husks) have been recorded for the agroforestry systems ofTheobroma cacao underCordia alliodora andErythrina poeppigiana shade trees. The mean oven dry cacao yields were 626 and 712 kg.ha^–1.a^–1 cocoa beans underC. alliodora andE. poeppigiana respectively. Harvests have gradually increased over the years and the plantation has now reached maturity.Annual extraction of N, P, K, Ca and Mg in fruits, which is relatively small, was calculated on the basis of chemical analyses. The following average values were found (kg.ha^–1.a^–1): At the age of 8 years, theC. alliodora trees have reached 26.7 cm diameter (DBH) and 14.0 m in height. Mean annual growth (from age 5 to 7) is 14.6 m³.ha^–1.a^–1.Natural plant residue production has been measured for 4 years (Nov. 1981–Oct. 1985). UnderE. poeppigiana it has reached a value of 8.91 t.ha^–1.a^–1 and underC. alliodora 7.07 t.ha^–1.a^–1. The shade trees have contributed 57 and 47% respectively. Transference and decomposition rates are high and important in the nutrient cycles.The nutrient content of the litter was analysed and corresponding average yearly transfers were (kg.ha^–1.a^–1): For part I see Vol. 4, No. 3, 1986.Agroforestry Project, CATIE/GTZ (Tropical Agricultural Research and Training Center/Gesselschaft für Technische Zusammenarbeit), Turrialba, Costa Rica     

Role of weeds in the management of nitrogen in a young Pinus radiata plantation

Pinus radiata trees were grown on a podzolized sandy soil at a second rotation site under the following treatments: total weed control, total weed control plus ammonium nitrate, strip weed control and no weed control. During the first two summers after planting the differences in needle water potential between trees under no, strip or total weed control were very small. Despite similar rates of net N-mineralization in strip and total weed control treatments, which averaged 64 kg ha^–1 yr^–1 in the 0–15 cm soil depth, weeds in the strip weed control treatment reduced soil mineral-N concentrations by 50–80%, leaching of N by the end of the first growing season by 45%, foliar-N concentrations by 4–14% and stem biomass at 20 months after planting by 46%. Although N-uptake by above-ground vegetation (trees plus weeds) was 49% higher in the strip weed control treatment, the amount of N apportioned to trees during the first 20 months after planting was reduced from 15.5 to 9.0 kg ha^–1. These effects of weeds were even more pronounced in the no weed control treatment. Since weeds had little effect on the needle water potential of trees and the annual rates of N-mineralization, but adversely affected N-uptake by trees, results indicate that weeds directly competed with trees for N, and thereby aggravated N-deficiency in trees. Application of ammonium nitrate after complete weed control increased foliar-N concentrations, and N-uptake and growth of trees, but also induced severe stem deformation.     

Seasonal variation of inorganic nitrogen and net mineralization in a salt marsh ecosystem

Inorganic nitrogen pools and net mineralization were estimated in three sites of a Tagus estuary salt marsh in Portugal throughout 1 year. Ammonium (NH₄ ⁺) was the major form of inorganic nitrogen found in the salt marsh soil. Extractable NH₄ ⁺ concentrations showed a marked seasonal pattern with a concentration peak during the hotter months of July/August. The great majority (>99%) of the total nitrogen in the soil was found in sedimented organic matter, not readily available for plant uptake. Net nitrogen mineralization, determined using a field incubation method, showed a peak during the months of June/July which resulted in an increase on nitrogen availability. With the exception of the lower salt marsh, estimated rates of in situ net nitrogen mineralization in the soil during summer were well related to the increase in plant aboveground biomass and plant nitrogen pools, indicating that the process is an important source of available nitrogen for plant uptake and growth. Annual net nitrogen mineralization ranged between 2.4 and 4.5gNm^–2yr^{– 1} being significantly higher for the lower salt marsh site. Rates of net nitrogen mineralization were relatively low during most of the year with a particularly active period from June to August, possibly due to an effect of temperature on soil microbial activity.     

Effects of soil flooding on leaf gas exchange and growth of two neotropical pioneer tree species

Schinus terebinthifolius Raddi (Anacardiaceae) and Rapanea ferruginea (Ruiz & Pavon) Mez (Myrsinaceae) are two neotropical pioneer trees with wide geographical distribution in South America, highly degree of adaptation to different soil conditions and intense regeneration in areas with anthropic activities. With the aim to recommend the use Schinus and Rapanea in gallery forest restoration programs, we conducted an experiment with the objective to analyze the capacity of these two pioneer trees to tolerate soil flooding, mainly by accessing the effects of flooding on leaf gas exchange, growth and dry matter partitioning. Seedling survival throughout the 56-day flooding period were 100 and 90% for Schinus and Rapanea, respectively. The mean values of stomatal conductance (g_s) and net photosynthesis (A) observed in the control seedlings were, respectively, 0.4 mol m^–2s^–1 and 14 mmolm^–2s^–1, for Schinus, and 0.5 mol m^–2s^–1 and 14 mmolm^–2s^–1, for Rapanea. On day 20 flooding reduced gs and A by 36 and 29% in Schinus, and 81 and 61% in Rapanea. At the end of the experiment, significant decreases were also observed for root and whole plant biomass, in both species. Based on the results, we concluded that seedlings of Schinus and Rapanea can survive and grow throughout a medium period of soil waterlogging, in spite of the alterations observed in their physiological behavior, such as the decreases in stomatal conductance and in whole plant biomass.     

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.