N2-fixing Tree Research Articles

Nitrogen fixation rate of Acacia mangium Wild at mid rotation in Brazil is higher in mixed plantations with Eucalyptus grandis Hill ex Maiden than in monocultures

Inter-specific interactions with eucalypts in mixed plantations increased N 2 fixation rate of acacia trees compared to monocultures. N 2 fixation was higher during the wet summer than during the dry winter both in acacia monocultures and in mixed plantations. Introducing N-fixing trees in fast-growing tropical plantations may contribute to reducing the long-term requirements of N fertilizers. Management practices established in forest monocultures should be revisited in mixed-species plantations. This field experiment aimed to compare N2 fixation rates of Acacia mangium Wild in monospecific stands and in mixed-species stands with Eucalyptus grandis W. Hill ex Maiden. A secondary objective was to gain insight into the seasonal variations of N2 fixation. 15N was applied to acacia and eucalypt monocultures and mixed-species with a 1:1 ratio at mid rotation. Leaves were collected in autumn, winter, spring, and summer to determine the foliar N concentrations and 15N atom fraction. The N content in the above-ground biomass was estimated as well as the percentage of N derived from atmospheric N2 (%Ndfa) using eucalypts in monoculture as reference plants. %Ndfa values averaged over the year were 14% in monoculture and 44% in mixed-species stands. While the stocking density of acacia trees was twice as high in monoculture as in mixture, the amounts of N fixed in above-ground biomass of acacia trees were close (35–39 kg N ha−1) at 39 months after planting. %Ndfa values were higher during the wet summer than the dry winter both in acacia monocultures and in mixed plantations. The stocking density of acacia trees can be reduced in mixed plantations with eucalypts in comparison to acacia monocultures with a low influence on the input of N to soil through biological fixation.

赤杨对辽东落叶松人工林土壤氨基糖积累的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 赤杨对辽东落叶松人工林土壤氨基糖积累的影响 DOI: 10.5846/stxb201705150897 作者: 作者单位: 沈阳农业大学;沈阳农业大学,中国林业科学研究院森林生态环境与保护研究所,沈阳农业大学,沈阳农业大学,沈阳农业大学,沈阳农业大学,沈阳农业大学 作者简介: 通讯作者: 中图分类号: 基金项目: 东北低山丘陵区大流域尺度防护林体系景观格局调控技术（2015BAD07B030103） Effects of N-fixing tree species (Alnus sibirica) on amino sugars in the soils of a Larix kaempferi plantation in eastern Liaoning Province, China Author: Affiliation: Shenyang Agricultural University,,,,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:微生物残留物是土壤稳定性碳库的重要组成部分，然而其对固氮树种的响应还不清楚。以辽东山区日本落叶松（Larix kaempferi）人工纯林和落叶松-赤杨混交林为研究对象，以氨基糖作为微生物残留物的生物标识物，研究了赤杨对落叶松人工林根际和非根际土壤氨基糖积累的影响。结果表明，混交林中落叶松根际和非根际各氨基糖单体含量均显著高于纯林，说明赤杨引入有利于土壤微生物残留物的积累。其中，混交林落叶松根际及非根际土壤氨基糖葡萄糖含量分别比纯林高出99.5%（P<0.01）和154%（P<0.01）；胞壁酸含量分别比纯林高出66.1%（P<0.01）和132.3%（P<0.01）。赤杨引入对氨基葡萄糖/胞壁酸比值的影响不显著，但显著增加氨基糖对土壤有机碳的贡献。冗余度分析表明全氮是驱动土壤中氨基糖发生变化的主要因子。表明赤杨引入显著提高微生物固持的土壤碳库和有机碳稳定性，这对落叶松人工林合理经营具有重要意义。 Abstract:Microbial residues are an important component of soil stable carbon (C) pools. However, their response to N-fixing tree species is still unclear. The present study investigated the effects of alder (Alnus sibirica) on microbial residues (as indicated by amino sugars) in the rhizosphere and non-rhizosphere soils of a larch (Larix kaempferi) plantation in eastern Liaoning Province, China. The contents of individual amino sugars significantly increased in the larch rhizosphere and non-rhizosphere soils in the mixed plantation relative to the pure plantation, indicating that alder benefits microbial residue accumulation. Specifically, the glucosamine (GluN) contents were 99.5% (P<0.01) and 154% (P<0.01) higher in the larch rhizosphere and non-rhizosphere soils in the mixed plantation than in the pure plantation, respectively, while the muramic acid (MurN) contents in the rhizosphere and non-rhizosphere soils were 66.1% (P<0.01) and 132.3% (P<0.01) higher in the mixed plantation than in the pure plantation, respectively. We found that alder had no effect on the GluN/MurN ratio, but significantly enriched the amino sugar proportion of soil organic C. Redundancy analysis revealed that total nitrogen was the dominant factor driving changes in amino sugars. This study indicates that alder would significantly stimulate the microbial fixation of C and the stability of soil organic C, which means that it plays a vital role in managing larch plantations. 参考文献 相似文献 引证文献

A stoichiometric perspective of the effect of herbivore dung on ecosystem functioning.

Ungulate herbivores play a prominent role in maintaining the tree–grass balance in African savannas. Their top‐down role through selective feeding on either trees or grasses is well studied, but their bottom‐up role through deposition of nutrients in dung and urine has been overlooked. Here, we propose a novel concept of savanna ecosystem functioning in which the balance between trees and grasses is maintained through stoichiometric differences in dung of herbivores that feed on them. We describe a framework in which N2‐fixing trees and grasses, as well as ungulate browsing and grazing herbivores, occupy opposite positions in an interconnected cycle of processes. The framework makes the testable assumption that the differences in dung N:P ratio among browsers and grazers are large enough to influence competitive interactions between N2‐fixing trees and grasses. Other key elements of our concept are supported with field data from a Kenyan savanna.

Why are nitrogen-fixing trees rare at higher compared to lower latitudes?

Symbiotic nitrogen (N) fixation provides a dominant source of new N to the terrestrial biosphere, yet in many cases the abundance of N-fixing trees appears paradoxical. N-fixing trees, which should be favored when N is limiting, are rare in higher latitude forests where N limitation is common, but are abundant in many lower latitude forests where N limitation is rare. Here, we develop a graphical and mathematical model to resolve the paradox. We use the model to demonstrate that N fixation is not necessarily cost effective under all degrees of N limitation, as intuition suggests. Rather, N fixation is only cost effective when N limitation is sufficiently severe. This general finding, specific versions of which have also emerged from other models, would explain sustained moderate N limitation because N-fixing trees would either turn N fixation off or be outcompeted under moderate N limitation. From this finding, four general hypothesis classes emerge to resolve the apparent paradox of N limitation and N-fixing tree abundance across latitude. The first hypothesis is that N limitation is less common at higher latitudes. This hypothesis contradicts prevailing evidence, so is unlikely, but the following three hypotheses all seem likely. The second hypothesis, which is new, is that even if N limitation is more common at higher latitudes, more severe N limitation might be more common at lower latitudes because of the capacity for higher N demand. Third, N fixation might be cost effective under milder N limitation at lower latitudes but only under more severe N limitation at higher latitudes. This third hypothesis class generalizes previous hypotheses and suggests new specific hypotheses. For example, greater trade-offs between N fixation and N use efficiency, soil N uptake, or plant turnover at higher compared to lower latitudes would make N fixation cost effective only under more severe N limitation at higher latitudes. Fourth, N-fixing trees might adjust N fixation more at lower than at higher latitudes. This framework provides new hypotheses to explain the latitudinal abundance distribution of N-fixing trees, and also provides a new way to visualize them. Therefore, it can help explain the seemingly paradoxical persistence of N limitation in many higher latitude forests.

Nitrogen-fixing trees inhibit growth of regenerating Costa Rican rainforests

More than half of the world's tropical forests are currently recovering from human land use, and this regenerating biomass now represents the largest carbon (C)-capturing potential on Earth. How quickly these forests regenerate is now a central concern for both conservation and global climate-modeling efforts. Symbiotic nitrogen-fixing trees are thought to provide much of the nitrogen (N) required to fuel tropical secondary regrowth and therefore to drive the rate of forest regeneration, yet we have a poor understanding of how these N fixers influence the trees around them. Do they promote forest growth, as expected if the new N they fix facilitates neighboring trees? Or do they suppress growth, as expected if competitive inhibition of their neighbors is strong? Using 17 consecutive years of data from tropical rainforest plots in Costa Rica that range from 10 y since abandonment to old-growth forest, we assessed how N fixers influenced the growth of forest stands and the demographic rates of neighboring trees. Surprisingly, we found no evidence that N fixers facilitate biomass regeneration in these forests. At the hectare scale, plots with more N-fixing trees grew slower. At the individual scale, N fixers inhibited their neighbors even more strongly than did nonfixing trees. These results provide strong evidence that N-fixing trees do not always serve the facilitative role to neighboring trees during tropical forest regeneration that is expected given their N inputs into these systems.

Shifts in the bacterial community composition along deep soil profiles in monospecific and mixed stands of Eucalyptus grandis and Acacia mangium.

Our knowledge of the rhizosphere bacterial communities in deep soils and the role of Eucalyptus and Acacia on the structure of these communities remains very limited. In this study, we targeted the bacterial community along a depth profile (0 to 800 cm) and compared community structure in monospecific or mixed plantations of Acacia mangium and Eucalyptus grandis. We applied quantitative PCR (qPCR) and sequence the V6 region of the 16S rRNA gene to characterize composition of bacterial communities. We identified a decrease in bacterial abundance with soil depth, and differences in community patterns between monospecific and mixed cultivations. Sequence analysis indicated a prevalent effect of soil depth on bacterial communities in the mixed plant cultivation system, and a remarkable differentiation of bacterial communities in areas solely cultivated with Eucalyptus. The groups most influenced by soil depth were Proteobacteria and Acidobacteria (more frequent in samples between 0 and 300 cm). The predominant bacterial groups differentially displayed in the monospecific stands of Eucalyptus were Firmicutes and Proteobacteria. Our results suggest that the addition of an N2-fixing tree in a monospecific cultivation system modulates bacterial community composition even at a great depth. We conclude that co-cultivation systems may represent a key strategy to improve soil resources and to establish more sustainable cultivation of Eucalyptus in Brazil.

Global climate change will increase the abundance of symbiotic nitrogen-fixing trees in much of North America.

Symbiotic nitrogen (N)-fixing trees can drive N and carbon cycling and thus are critical components of future climate projections. Despite detailed understanding of how climate influences N-fixation enzyme activity and physiology, comparatively little is known about how climate influences N-fixing tree abundance. Here, we used forest inventory data from the USA and Mexico (>125,000 plots) along with climate data to address two questions: (1) How does the abundance distribution of N-fixing trees (rhizobial, actinorhizal, and both types together) vary with mean annual temperature (MAT) and precipitation (MAP)? (2) How will changing climate shift the abundance distribution of N-fixing trees? We found that rhizobial N-fixing trees were nearly absent below 15°C MAT, but above 15°C MAT, they increased in abundance as temperature rose. We found no evidence for a hump-shaped response to temperature throughout the range of our data. Rhizobial trees were more abundant in dry than in wet ecosystems. By contrast, actinorhizal trees peaked in abundance at 5-10°C MAT and were least abundant in areas with intermediate precipitation. Next, we used a climate-envelope approach to project how N-fixing tree relative abundance might change in the future. The climate-envelope projection showed that rhizobial N-fixing trees will likely become more abundant in many areas by 2080, particularly in the southern USA and western Mexico, due primarily to rising temperatures. Projections for actinorhizal N-fixing trees were more nuanced due to their nonmonotonic dependence on temperature and precipitation. Overall, the dominant trend is that warming will increase N-fixing tree abundance in much of the USA and Mexico, with large increases up to 40° North latitude. The quantitative link we provide between climate and N-fixing tree abundance can help improve the representation of symbiotic N fixation in Earth System Models.

Nutrient feedbacks to soil heterotrophic nitrogen fixation in forests

Multiple nutrient cycles regulate biological nitrogen (N) fixation in forests, yet long-term feedbacks between N-fixation and coupled element cycles remain largely unexplored. We examined soil nutrients and heterotrophic N-fixation across a gradient of 24 temperate conifer forests shaped by legacies of symbiotic N-fixing trees. We observed positive relationships among mineral soil pools of N, carbon (C), organic molybdenum (Mo), and organic phosphorus (P) across sites, evidence that legacies of symbiotic N-fixing trees can increase the abundance of multiple elements important to heterotrophic N-fixation. Soil N accumulation lowered rates of heterotrophic N-fixation in organic horizons due to both N inhibition of nitrogenase enzymes and declines in soil organic matter quality. Experimental fertilization of organic horizon soil revealed widespread Mo limitation of heterotrophic N-fixation, especially at sites where soil Mo was scarce relative to C. Fertilization also revealed widespread absence of P limitation, consistent with high soil P:Mo ratios. Responses of heterotrophic N-fixation to added Mo (positive) and N (negative) were correlated across sites, evidence that multiple nutrient controls of heterotrophic N-fixation were more common than single-nutrient effects. We propose a conceptual model where symbiotic N-fixation promotes coupled N, C, P, and Mo accumulation in soil, leading to positive feedback that relaxes nutrient limitation of overall N-fixation, though heterotrophic N-fixation is primarily suppressed by strong negative feedback from long-term soil N accumulation.

Spatial heterogeneity can resolve the nitrogen paradox of tropical forests.

Many tropical forests are characterized by large losses of plant-available forms of nitrogen (N), indicating that they are N rich, and by an abundance of plants capable of symbiotic N fixation. These N-fixing plants can fix enough N to drive N richness. However, biological N fixation (BNF) is more expensive than using plant-available N, so sustained BNF in N-rich soils appears to be a paradox. Here, we use spatially explicit ecosystem models to analyze the conditions under which spatial heterogeneity can induce simultaneous BNF and loss of plant-available N (hereafter, we call this combination "N-rich BNF"). Spatial movement of litter to neighboring plants' rooting zones can maintain N-rich BNF under a variety of conditions. For example, when N-fixers have higher N demand than non-fixers, N-fixers export N-rich litter to non-fixers, inducing large losses of plant-available N from the ecosystem, and receive N-poor litter from non-fixers, inducing BNF. BNF and N loss fluxes increase in proportion to the ratio of N-fixer litter N:P to non-fixer litter N:P, and also in proportion to the fraction of litter transferred out of a tree's rooting zone. Stoichiometric variability augments N-rich BNF, as does increasing the fraction of the landscape occupied by N-fixers, at least when they are rare. On the contrary, greater root overlap between neighbors and clumping of N-fixers diminish N-rich BNF. Finally, we examined how spatial litter transfer interacts with another mechanism that can sustain N-rich BNF, incomplete down-regulation of BNF. Spatial transfer and incomplete down-regulation can both sustain N-rich BNF, but they are compensatory rather than additive. These mechanisms can be distinguished by examining where N losses occur. Incomplete down-regulation of BNF leads to greater N loss under N-fixing trees, whereas spatial litter transfer leads to greater N loss under non-fixing trees. Along with time lags in regulating BNF, these results comprise a series of hypotheses that could help understand the N paradox of tropical forests.

Demography of Symbiotic Nitrogen-Fixing Trees Explains Their Rarity and Successional Decline in Temperate Forests in the United States.

Symbiotic nitrogen (N) fixation is the major N input to many ecosystems. Although temperate forests are commonly N limited, symbiotic N-fixing trees (“N fixers”) are rare and decline in abundance as succession proceeds–a challenging paradox that remains unexplained. Understanding demographic processes that underlie N fixers’ rarity and successional decline would provide a proximate answer to the paradox. Do N fixers grow slower, die more frequently, or recruit less in temperate forests? We quantified demographic rates of N-fixing and non-fixing trees across succession using U.S. forest inventory data. We used an individual-based model to evaluate the relative contribution of each demographic process to community dynamics. Compared to non-fixers, N fixers had lower growth rates, higher mortality rates, and lower recruitment rates throughout succession. The mortality effect contributed more than the growth effect to N fixers’ successional decline. Canopy and understory N fixers experienced these demographic disadvantages, indicating that factors in addition to light limitation likely contribute to N fixers’ successional decline. We show that the rarity and successional decline of N-fixing trees in temperate forests is due more to their survival disadvantage than their growth disadvantage, and a recruitment disadvantage might also play a large role.

N-fixing trees in wetland restoration plantings: effects on nitrogensupply and soil microbial communities.

To investigate the impact of an exotic Frankia nodulated tree (Alnus trabeculosa) on soil nitrogen content, soil microbial composition, and the abundance of N turnover-related functional microorganism community, we compared the community structure and abundance of key functional genes (nifH, bacterial/archaeal amoA, and nosZ) in the rhizosphere and nonrhizosphere of monoculture of Phragmites australis and A.trabeculosa-P.australis mixed communities by MiSeq Illumina sequencing and real-time PCR, respectively. The introduction of Frankia nodulated tree to recover degraded wetland was effective in the accumulation of soil organic carbon and nitrogen, which was the key factor to impact on the bacterial community composition revealed by canonical correspondence analysis. Acidobacteria and Proteobacteria were the dominant bacterial phylums while seven rare phyla appeared the most phylogenetically different among the investigated soil of two vegetations, including Chlorobi, Cyanobacteria, OD1, OP11, TM6, TM7, and GN02. The gene copy numbers of nifH were ranged from 2.28×108 to 2.96×109copies g-1 dry soil in the wetland, and which were significantly higher in soil samples from P. australis than that from A.trabeculosa. While the abundance of nosZ in both rhizosphere and nonrhizosphere soils of A.trabeculosa-P.australis mixed communities was significantly lower compared with P.australis monoculture. The potential nitrification (PNA) (0.15-0.41mg NOx-Nkg-1 drysoild-1) in the rhizosphere of A. trabeculosa was significantly higher than that of P. australis, and the soil denitrification enzyme activity (DEA) (0.42-0.90nmol N2O-Ng-1drysoilh-1) was lower in the mixed community compared with monoculture of P. australis. The introduced planting of Frankia nodulated tree effectively accumulated soil organic carbon and nitrogen and reduce the relative abundance and activity of nitrogen-fixing bacteria and denitrification bacteria.

Differences in nitrogen cycling and soil mineralisation between a eucalypt plantation and a mixed eucalypt and Acacia mangium plantation on a sandy tropical soil

Sustainable wood production requires appropriate management of commercial forest plantations. Establishment of industrial eucalypt plantations on poor sandy soils leads to a high loss of nutrients including nitrogen (N) after wood harvesting. An ecological intensification of eucalypt plantations was tested with the replacement of half of the Eucalyptus urophylla × E. grandis by Acacia mangium in the eucalypt monoculture to sustain soil fertility through enhancement of the N biological cycle. A randomised block design was set up on ferralitic arenosol in the Congolese coastal plains to assess differences in soil N mineralisation, N fluxes in litterfall, and N stocks in forest floor litter and soil between pure acacia (100A), pure eucalypt (100E) and mixed-species treatments (50A50E). Soil N mineralisation was enhanced under acacia, reaching on average 0.17 and 0.15 mg kg−1 soil d−1 in 100A and 50A50E, respectively, compared with 0.09 mg kg−1 soil d−1 in 100E. Higher amounts of N returning to the soil through harvest residues and litterfall were observed under acacia than under eucalypt. However, N stock in mineral soil was not increased in 100A and exhibited a limited increase only in the top soil layer of 50A50E. Our results suggest a much faster N turnover under acacia than under eucalypt. Although A. mangium is an exotic N2-fixing tree in central Africa, it appears to be well adapted to the climatic and edaphic conditions of the Congo, showing an efficient growth strategy. Eucalypt trees could benefit from the increase in soil N availability in mixed-species stands.

GROWTH OF NATIVE TREES IN TWO AGROFORESTRY SYSTEMS

ABSTRACT Agroforestry systems with eucalyptus prevail in Central and Southeast Brazil, and little information is available about systems using native trees. The aim of the present study was to evaluate the development of seven native tree species grown under two agroforestry systems. The experiment was conducted starting in 2007 in 12-hectare area in the municipality of São Carlos, São Paulo state, Brazil. The tree species planted in the two systems (a silvopastoral system and an agrisilvicultural system) were: 'capixingui' (Croton floribundus) and 'mutambo' (Guazuma ulmifolia) (tutors), 'jequitibá-branco' (Cariniana estrellensis), 'canafistula' (Peltophorum dubium) and 'ipê felpudo' (Zeyheria tuberculosa) (timber trees), and 'angico-branco' (Anadenanthera colubrina) and 'pau-jacaré' (Piptadenia gonoacantha) (N-fixing trees). Data were collected for 48 months. The results show differences among tree development, which was evaluated as growth in height and diameter, as well as sensitivity to insect and disease damage. The overall results show that the agrisilvicultural system allowed better tree development. The species with best performance in the two systems were capixingui, mutambo and canafístula. Ipê-felpudo and jequitibá-branco showed the worst results. The high variability among individuals of the same species indicates the possibility of high production advances with selective breeding of these species.

Can evolutionary constraints explain the rarity of nitrogen-fixing trees in high-latitude forests?

Contents 1195 I. 1195 II. 1196 III. 1196 IV. 1200 1200 References 1200 SUMMARY: The rarity of symbiotic nitrogen (N)-fixing trees in temperate and boreal ('high-latitude') forests is curious. One explanation - the evolutionary constraints hypothesis - posits that high-latitude N-fixing trees are rare because few have evolved. Here, we consider traits necessary for high-latitude N-fixing trees. We then use recent developments in trait evolution to estimate that >2000 and >500 species could have evolved from low-latitude N-fixing trees and high-latitude N-fixing herbs, respectively. Evolution of N-fixing from nonfixing trees is an unlikely source of diversity. Dispersal limitation seems unlikely to limit high-latitude N-fixer diversity. The greater number of N-fixing species predicted to evolve than currently inhabit high-latitude forests suggests a greater role for ecological than evolutionary constraints.

Nutrient and Organic Matter Inputs to Hawaiian Anchialine Ponds: Influences of N-Fixing and Non-N-Fixing Trees

Invasive nitrogen-fixing plants often increase energy and nutrient inputs to both terrestrial and aquatic ecosystems via litterfall, and these effects may be more pronounced in areas lacking native N2-fixers. We examined organic matter and nutrient inputs to and around anchialine ponds on Hawai‘i Island's leeward coast from an invasive, N2-fixing tree, Prosopis pallida, and a non-N2-fixing tree, Thespesia populnea. On a monthly basis we quantified: (1) tree basal area and stem density surrounding the ponds, and (2) nutrient content and quantity of P. pallida and T. populnea litter inputs. Tree density and basal area significantly predicted litterfall mass inputs for both species, but there was no difference in mean total annual litterfall mass, foliar N concentration, or litterfall N between the pond types. Time did not significantly influence litterfall mass, C, or P but did affect litterfall N, likely due to the production of P. pallida pods. In contrast, P concentrations were three times greater in T. populnea litter than in P. pallida litter, resulting in significantly higher P deposition where T. populnea was the prevalent tree species. Because this deposition of labile P by the non-N2-fixing T. populnea has the potential to strongly influence water quality and soil chemistry, its replacement by P. pallida in tree communities of lowland Hawai‘i may alter the functioning of anchialine pond ecosystems.

FAUNA DO SOLO AO LONGO DO PROCESSO DE SUCESSÃO ECOLÓGICA EM VOÇOROCA REVEGETADA NO MUNICÍPIO DE PINHEIRAL – RJ

O objetivo do trabalho foi avaliar a diversidade, riqueza e composição da comunidade da fauna do solo epígea em uma voçoroca revegetada no ano de 2000 com diferentes espécies de leguminosas arbóreas. A voçoroca está localizada no Instituto Federal de Educação (IFRJ) campus Nilo Peçanha no município de Pinheiral – RJ. Foram avaliadas as seguintes áreas: plantio de Acacia mangium Willd. (PA), plantio de Mimosa caesalpiniifolia Benth. (PM), plantio de leguminosas arbóreas no terço superior da encosta (PTS), plantio de leguminosas no interior da voçoroca (IVR2000). Os plantios de A. mangium e M. caesalpiniifolia foram realizados antes da revegetação. Para efeito comparativo também foi avaliada uma área de floresta secundária com 20 anos, próxima ao local de coleta. Na coleta da fauna do solo epígea foram utilizadas 10 armadilhas do tipo pitfall (MOLDENKE, 1994) por área. Foram realizadas três coletas, uma no início da revegetação, em maio de 2000, em junho de 2006 e abril de 2008. Houve um aumento no número de grupos e na diversidade da fauna do solo no plantio no terço superior, no interior da voçoroca, e no plantio de Mimosa caesalpiniifolia após 6 anos da intervenção. Nas coletas realizadas em 2000 e 2006, a APC mostrou que a maioria dos grupos estava associada ao ano de 2006, mostrando total antagonismo com o ano de 2000. O plantio de Acacia mangium foi a área com o menor número de grupos. Na coleta realizada em 2008, a pastagem foi a área com menor número médio de grupos coletados e na ACP do interior da voçoroca os grupos estavam associados ao ano de 2008 com total antagonismo com os anos de 2000 e 2006. Os grupos Formicidae e Collembola (Entomobryomorpha) foram os mais dominantes em todas as áreas amostradas no ano de 2000 e 2008 e no interior da voçoroca ano de 2006.

Effects of soil type and water saturation on growth, nutrient and mineral content of the perennial forage shrub Sesbania sesban

Sesbania sesban (L.) Merr is a perennial N2-fixing tree with high potential for use in agricultural production systems as a green manure and livestock forage. We studied the interactive effects of soil type and water level on the growth, biomass allocation, nutrient and mineral content of S. sesban. Four-week old seedlings of S. sesban were grown for 49 days (n = 5) in a factorial mesocosm set-up with six soil types (sediment, sand, alluvial, acid-sulfate, saline and clay) and three water levels (drained, water-saturated and flooded). The soils tested represent the predominant alluvial soil types of the Mekong delta, Vietnam. Sesbania sesban grew well with relative growth rates (RGR) around 0.08 g g−1 d−1 in all studied soil types, except the saline soil where plants died. In the low-pH (3.9) acid sulfate soil, that constitute more than 40 % of the Mekong delta, the RGR of the plants was slightly lower (0.07 g g−1 d−1), foliar concentration of calcium was 3–6 times lower, and concentrations of iron and sodium up to five times higher, than in other soils. The nutrient and mineral contents of the plant tissues differed between the soils and were also affected by the flooding levels. Foliar concentrations of nitrogen (50–74 mg N g−1 dry mass) and phosphorus (5–9 mg P g−1 dry mass) were, however, generally high and only slightly affected by water level. The results show that S. sesban can grow well and with high growth rates on most wet soils in the Mekong delta, except saline soils where the high salt content prevents establishment and growth. The nutrient and mineral contents of the plants, and hence the nutritional value of the plants as e.g. fodder or compost crops, is high. However, soil type and water level interactively affect growth and tissue composition. Hence, optimal growth conditions for S. sesban differ in the different regions of the Mekong delta.

Leucaena-KX2 mulch additions increase growth, yield and soil C and N in a managed full-sun coffee system in Hawaii

One option to sustain full-sun coffee plantation is the use of a “cut-and-carry” system in which a stand of N-fixing trees is grown outside the coffee field and utilized as a source of mulch to capture the benefits of organic matter, nutrient additions, and continuous soil cover. The objectives of this study were to evaluate the potential of a cut-and-carry system of mulch additions from a multipurpose N-fixing tree for open-grown coffee in Hawaii to support high crop yield as well as improved soil organic matter and nitrogen. A stand of Leucaena variety KX2 trees were pollarded every 6 months, and the pollarded material was chipped and added to open-grown coffee plots. Approximately 65 Mg ha−1 of mulch dry matter was added over a 3-year period, including ~27.5 Mg ha−1 of C and ~530 kg ha−1 of total N. Plots without mulch addition were fertilized with equivalent amounts of inorganic N for comparison. Mulch decomposition averaged 64 % (weight basis) over the first year and followed first-order decay dynamics. Net N mineralization was positive by 3 months after addition. There was significant loss of all major biochemical components during the decomposition process. Mulching increased soil CO2 efflux by 2.89 Mg ha−1. Total soil C and N increased by 2.90 and 1.42 Mg ha−1, respectively. Mulch addition significantly increased all measures of coffee growth and yield over 2 years, except for average bean weight. Where space is available or shade is undesirable, a cut-and-carry mulching system using Leucaena-KX2 can increase soil C and N and achieve coffee yields similar to or greater than other full-sun systems.

Effects of nitrogen addition on litter decomposition and nutrient release in two tropical plantations with N2-fixing vs. non-N2-fixing tree species

Atmospheric nitrogen (N) deposition has elevated rapidly in tropical regions where N2-fixing tree species are widespread. However, the effect of N deposition on litter decomposition in forests with N2-fixing tree species remains unclear. We examined the effect of N addition on litter decomposition and nutrient release in two tropical plantations with Acacia auriculiformis (AA, N2-fixing) and Eucalyptus urophylla (EU, non-N2-fixing) in South China. Three levels of N additions were conducted: control, medium-N (50 kg N ha−1 yr.−1) and high-N (100 kg N ha−1 yr.−1) in each plantation. Initial decomposition rate (k a ) for the control plots was faster in the AA plantation than in the EU plantation, but later in decomposition, larger fraction of slowly decomposing litter (A) remained in the former. N addition increased the slow fraction (A), decreasing soil microbial biomass and reducing acid-unhydrolyzable residue (AUR) degradation in the AA plantation. In the EU plantation, however, N additions significantly increased initial decomposition rate (k a ) and soil N availability. Furthermore, N addition decreased litter carbon and N release (in the AA plantation), while litter phosphorus release also decreased in both plantations. With ongoing N deposition in future, tropical plantations with N2-fixing tree species would potentially increase carbon accumulation and nutrient retention in forest floor by slowing litter decomposition.

Shifts in soil microbial community structure, nitrogen cycling and the concomitant declining N availability in ageing primary boreal forest ecosystems

Plant growth in boreal forests is commonly limited by a low supply of nitrogen, a condition that may be aggravated by high tree below-ground allocation of carbon to ectomycorrhizal (ECM) fungi and associated microorganisms. These in turn immobilise N and reduce its availability to plants as boreal ecosystems develop. Here, we studied a boreal forest ecosystem chronosequence created by new land rising out of the sea due to iso-static rebound along the coast of northern Sweden. We used height over the ocean to estimate ecosystem age and examined its relationship to soil microbial community structure and the gross turnover of N. The youngest soils develop with meadows by the coast, followed by a zone of N2-fixing alder trees, and primary boreal conifer forest on ground up to 560 years old. The young soils in meadows contained little organic matter and microbial biomass per unit area. Nitrogen was turned over at low rates when expressed per area (m−2), but specific rates (per gram soil carbon (C)) were the highest found along the transect. In the zone with alder, the amounts of soil C and microbial biomass were much higher (bacterial biomass had doubled and fungal biomass quadrupled). Rates of gross N mineralisation (expressed on an area basis) were highest, but the retention of added labelled NH4+ was lowest in this soil as compared to other ages. The alder zone also had the largest extractable pools of inorganic N in soil and highest N % in plant foliage. In the older conifer forest ecosystems the amounts of soil C and N, as well as biomass of both bacteria and fungi increased. Data on organic matter 14C suggested that the largest input of recently fixed plant C occurred in the younger coniferous forest ecosystems. With increasing ecosystem age, the ratio of microbial C to total soil C was constant, whereas the ratio of microbial N to total soil N increased and gross N mineralization declined. Simultaneously, plant foliar N % decreased and the natural abundance of 15N in the soil increased. More specifically, the difference in δ15N between plant foliage and soil increased, which is related to relatively greater retention of 15N relative to 14N by ECM fungi as N is taken up from the soil and some N is transferred to the plant host. In the conifer forest, where these changes were greatest, we found increased fungal biomass in the F- and H-horizons of the mor-layer, in which ECM fungi are known to dominate (the uppermost horizon with litter and moss is dominated by saprotrophic fungi). Hence, we propose that the decreasing availability of N to the plants and the subsequent decline in plant production in ageing boreal forests is linked to high tree belowground C allocation to ECM fungi, a strong microbial sink for available soil N.