Temporal Asynchrony in Soil Nutrient Dynamics and Plant Production in a Semiarid Ecosystem

A central goal of ecosystem ecology is to understand how the cycling of nutrients and the growth of organisms are linked. Ecologists have repeatedly observed that nutrient mineralization and plant production are closely coupled in time in many terrestrial ecosystems. Typically, mineralization rates of limiting nutrients, particularly of nitrogen, during the growing season determine nutrient availability while pools of mineral nutrients remain low and relatively constant. Although several previous reports suggest nitrogen mineralization has the potential to vary seasonally and out of phase with plant production, such a phenomenon has been poorly documented. Here we report results from a semiarid savanna ecosystem characterized by distinct temporal asynchrony in rates of soil nitrogen cycling and plant production. Periods of positive plant growth following the onset of rains coincide with periods of low N turnover rates, whereas higher rates occur late in the wet season following plant senescence and throughout dry seasons. Plant uptake from the substantial mineral N pool present early in the growing season is sufficient to explain most of the N allocation to aboveground plant biomass during the growing season, even in the absence of any wet-season mineralization. The mineral N pool is subsequently recharged by late wet- and dry-season mineralization, plus urine inputs at sites with high levels of ungulate activity. These findings suggest fundamental changes in the quality of substrates available to decomposers over a seasonal cycle, with significant implications for the partitioning of limiting nutrients by plant species, the seasonal pattern of nutrient limitations of aboveground production, and the effective use of N fertilizers in semiarid ecosystems.     

Effects of Herbivory,Fire and N<Subscript>2</Subscript>-fixation on Nutrient Limitation in a Humid African Savanna

The quantities and spatial distribution of nutrients in savanna ecosystems are affected by many factors, of which fire, herbivory and symbiotic N₂-fixation are particularly important. We measured soil nitrogen (N) pools and the relative abundance of N and phosphorus (P) in herbaceous vegetation in five vegetation types in a humid savanna in Tanzania. We also performed a factorial fertilization experiment to investigate which nutrients most limit herbaceous production. N pools in the top 10 cm of soil were low at sites where fires were frequent, and higher in areas with woody legume encroachment, or high herbivore excretion. Biomass production was co-limited by N and P at sites that were frequently burnt or heavily grazed by native herbivores. In contrast, aboveground production was limited by N in areas receiving large amounts of excreta from livestock. N₂-fixation by woody legumes did not lead to P-limitation, but did increase the availability of N relative to P. We conclude that the effects of fire, herbivory and N₂-fixation upon soil N pools and N:P-stoichiometry in savanna ecosystems are, to a large extent, predictable. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author Contributions  P.C., H.O.V. and P.E. designed the study and wrote the paper. P.C. and T.K. performed the research and analyzed the data.     

一个农牧结合生态系统营养循环的源,库,流

一个农牧结合生态系统营养循环的源、库、流曾江海,张玉铭（中国科学院石家庄农业现代化研究所０５００２１）Ｓｏｕｒｃｅ,ＰｏｏｌａｎｄＦｌｕｘｏｆＮｕｔｒｉｅｎｔＣｙｃｌｉｎｇｉｎａＣｏｍｂｉｎｅｄＡｇｒｏ－ＡｎｉｍａｌＨｕｓｂａｎｄｒｙＥｃｏｓｙｓｔｅ...     

Interactive Effects of Fire, Elevated Carbon Dioxide, Nitrogen Deposition, and precipitation on a California Annual Grassland

Although it is widely accepted that elevated atmospheric carbon dioxide (CO₂), nitrogen (N) deposition, and climate change will alter ecosystem productivity and function in the coming decades, the combined effects of these environmental changes may be nonadditive, and their interactions may be altered by disturbances, such as fire. We examined the influence of a summer wildfire on the interactive effects of elevated CO₂, N deposition, and increased precipitation in a full-factorial experiment conducted in a California annual grassland. In unburned plots, primary production was suppressed under elevated CO₂. Burning alone did not significantly affect production, but it increased total production in combination with nitrate additions and removed the suppressive effect of elevated CO₂. Increased production in response to nitrate in burned plots occurred as a result of the enhanced aboveground production of annual grasses and forbs, whereas the removal of the suppressive effect of elevated CO₂ occurred as a result of increased aboveground forb production in burned, CO₂-treated plots and decreased root production in burned plots under ambient CO₂.The tissue nitrogen–phosphorus ratio, which was assessed for annual grass shoots, decreased with burning and increased with nitrate addition. Burning removed surface litter from plots, resulting in an increase in maximum daily soil temperatures and a decrease in soil moisture both early and late in the growing season. Measures of vegetation greenness, based on canopy spectral reflectance, showed that plants in burned plots grew rapidly early in the season but senesced early. Overall, these results indicate that fire can alter the effects of elevated CO₂ and N addition on productivity in the short term, possibly by promoting increased phosphorus availability.     

TOC, TON, TOS and TOP in Rainfall, Throughfall, Litter Percolate and Soil Solution of a Montane Rainforest Succession at Mt. Kilimanjaro, Tanzania

Organic nutrients have proven to contribute significantly to nutrient cycling in temperate forest ecosystems. Still, little is known about their relevance in the tropics. In the present study, organic C, N, S and P were analysed in rainfall, throughfall, litter percolate and soil solution of a montane rainforest at Mt. Kilimanjaro, Tanzania. The aim was to determine the amounts of organic nutrients in different water pathways and to assess the influence of forest disturbance on organic nutrients by comparing mature forests, secondary forests and shrub vegetation in clearings. Concentrations of all studied elements increased from rainfall to throughfall and litter percolate and then exhibited a rapid decrease in the mineral soil. Concentrations of organic P were above the detection limit only in the litter percolate. Organic N (ON) as a fraction of total N increased from 50% in rainfall (0.19 mg l⁻¹) to 66% (0.45 mg l⁻¹) in throughfall followed by a decline to 39% in the litter percolate (0.77 mg l⁻¹) of the mature forest. Similarly, proportions of organic S and P amounted to 43 and 34%, respectively, in the litter percolate in mature forest. For ON, this proportion further decreased to less than 10% in the soil solution. The latter was probably attributable to a high sorption capacity of the studied Andisols, which led to overall low organic element concentrations in the soil solution (OC: 1.2 mg l⁻¹, ON: 0.05 mg l⁻¹ at 1 m soil depth) as compared to other temperate and tropical forest ecosystems. Organic element concentrations were higher in litter percolate and soil solution under the clearings, but there were no differences in the relative contribution of these elements. Organic nutrient forms at Mt. Kilimanjaro appeared to be much less susceptible to leaching than their inorganic forms.     

Effects of Elatobium abietinum on nutrient fluxes in Sitka spruce canopies receiving elevated nitrogen and sulphur deposition

1 In a field acid mist simulation experiment, Sitka spruce (Picea sitchensis) was sprayed with different pollutant treatments: N, NH₄NO₃; S, Na₂SO₄; NS Acid, NH₄NO₃ + H₂SO₄ and control, no spray. Treatment effects on the abundance of the green spruce aphid Elatobium abietinum and honeydew production were assessed. In addition, needles were sampled for phyllosphere micro‐organisms. In a manipulative experiment, shoots were established and maintained as with or without E. abietinum infestation in order to determine the effects of infestation on needle loss and throughfall nutrient fluxes. 2 Aphid numbers were highest during the end of May and early June, with almost twice as many needles infested in the NS Acid treatment compared with the other treatments. Honeydew production was not affected by the treatments. 3 On infested shoots, increasing numbers of yellowing and dead needles were recorded above the throughfall collectors as the season progressed. The numbers of dead needles falling into the collectors were significantly higher beneath infested shoots. There were strong positive correlations between aphid numbers above the throughfall collectors, the number of yellowing and dead needles on the shoots and the number of needles in the funnels of the throughfall collectors. Litter production was more affected by aphid number than by pollutant treatment. 4 Bacteria, yeasts and filamentous fungi were more prolific on infested needles and treatment effects on colony forming units (CFUs) were most pronounced in the NS Acid treatment. 5 Fluxes of inorganic nitrogen beneath infested shoots were generally lower than beneath uninfested shoots. This effect was more pronounced in those treatments that supplied N i.e. N, NS Acid. The combination of aphid infestation and N‐addition exerted the strongest influence on nutrient fluxes. The fluxes of potassium and of organic carbon (DOC) were higher beneath infested shoots in all treatments, through most of the survey period.     

The Effects of Crop Rotation and Nitrogen Fertilization on Soil Chemical and Microbial Properties in a Guinea Savanna Alfisol of Nigeria

The impacts of crop rotation and inorganic nitrogen fertilization on soil microbial biomass C (SMBC) and N (SMBN) and water-soluble organic C (WSOC) were studied in a Guinea savanna Alfisol of Nigeria. In 2001, fields of grain legumes (soybean and cowpea), herbaceous legume (Centrosema pascuorum) and a natural fallow were established. In 2002, maize was planted with N fertilizer rates of 0, 20, 40 and 60 kg N ha⁻¹ in a split-plot arrangement fitted to a randomized complete block design with legumes and fallow as main plots and N fertilizer levels as subplots. Surface soil samples were taken at 4 weeks after planting and tasselling stage of the maize. Inorganic N fertilization had no significant (P>0.05) effect on SMBC, SMBN and WSOC, while crop rotation significantly (P<0.0001) affected both SMBC and WSOC. These results demonstrate that crop rotation do not necessarily influence the gross soil microbial biomass, but may affect physiologically distinct subcomponent of the microbial biomass. The soils under the various rotations had a predominance of fungi community as indicated by their wide biomass C/N ratio ranging from 9.2 to 20.9 suggesting fungi to be mainly responsible for decomposition in these soils. Soil microbial biomass and WSOC showed significant (P<0.05) correlation with both soil pH and organic carbon but no relationship with total N. Based on these results, it appears that the soil pH and organic carbon determined the flux of the soil microbial biomass and amount of WSOC in these soils.     

Survival, Growth, and Ecosystem Dynamics of Displaced Bromeliads in a Montane Tropical Forest1

Epiphytes generally occupy arboreal perches, which are inherently unstable environments due to periodic windstorms, branch falls, and treefalls. During high wind events, arboreal bromeliads are often knocked from the canopy and deposited on the forest floor. In this study, we used a common epiphytic tank bromeliad, Guzmania berteroniana (R. & S.) Mez, to determine if fallen bromeliads can survive, grow, and reproduce on the forest floor and evaluate the potential impact of adult dispersal on plant and soil nutrient pools. Bromeliads were transplanted to and from tree stems and the forest floor and monitored intensively for six months; survival, growth, and impacts on ecosystem nutrient pools were followed on a subset of plants for 16 months. Six months after transplanting, bromeliad mortality was low (3%), and 19 percent of study individuals had flowered and produced new juvenile shoots. Mortality on the subset of plants followed for 16 months was 14–30 percent. Although survival rates were relatively high in all habitats, bromeliads transplanted to trees grew significantly more root length (x?± SE: 189 ± 43 cm) than those moved to the forest floor (53 ±15 cm) and experienced lower rates of leaf area loss. All transplanted bromeliads rapidly altered the substrate they occupied. Individuals transplanted to and among trees rapidly decreased base cation concentrations but significantly increased P concentrations of their underlying substrate. On the ground, bromeliads increased C, N, and P concentrations within nine months of placement. Our results suggest that in this montane tropical forest, bromeliads respond rapidly to displacement, locally modify their substrates, and can access the resources needed for survival regardless of habitat.     

Effects of Holocene Alnus Expansion on Aquatic Productivity, Nitrogen Cycling, and Soil Development in Southwestern Alaska

Numerous pollen records provide evidence for the widespread range expansion of Alnus throughout Alaska and adjacent Canada during the middle Holocene. Because Alnus can fix atmospheric N₂, this vegetational change probably had a profound effect on N availability and cycling. To assess this effect, we analyzed a sediment core from Grandfather Lake in southwestern Alaska for a suite of geochemical indicators, including elemental composition, biogenic silica (BSi) content, and carbon (C) and nitrogen (N) isotopes of organic matter. These data, in conjunction with a pollen record from the same site, are used to infer biogeochemical processes associated with the mid-Holocene Alnus expansion. The increase in Alnus pollen percentages from 10% to 70% circa 8000-7000 BP (¹⁴C years before present) suggests the rapid spread of Alnus shrub thickets on mountain slopes and riparian zones in the Grandfather Lake region. Coincident with this vegetational change, the mean value of the sediment BSi content increases from 20.4 to 106.2 mg/g, reflecting increased diatom productivity within the lake as a result of Alnus N₂ fixation in the watershed soils and the associated N flux to the lake. Elevated aquatic productivity at this time is also supported by increased percentages of organic C and N, decreased C:N ratios, and decreased values of δ ¹³C. Furthermore, the δ ¹⁵N values of sediments increase substantially with the establishment of Alnus shrub thickets, suggesting enhanced N availability and accelerated N cycling within the lake and its watershed. Superimposed on a general trend of soil acidification throughout the postglacial period, soil acidity probably increased as a result of the Alnus expansion, as can be inferred from decreasing ratios of authigenic base cations to allogenic silica (Si) and increasing ratios of authigenic aluminum (Al) to allogenic Si. The ultimate cause of these mid-Holocene ecosystem changes was an increase in effective moisture in the region. Received 21 July 2000; accepted 3 January 2001.     

The effects of quantity and duration of simulated pollutant nitrogen deposition on root-surface phosphatase activities in calcareous and acid grasslands: a bioassay approach

A field and laboratory based bioassay has been developed to investigate the effects of the quantity and duration of simulated pollutant nitrogen (N) deposition on root-surface phosphomonoesterase (PME) activities in calcareous and acid grasslands. Seedlings of Plantago lanceolata were transplanted to a calcareous grassland and Agrostis capillaris seedlings were grown in microcosms containing soil from an acid grassland that had received either 7 yr (long-term) N additions or 18 months (short-term) N and phosphorus (P) additions. The bioassay revealed that short-term N treatments had little effect on the enzyme activity, whereas long-term N additions significantly increased PME activity within 7 d of transplanting into the field plots. Root-surface PME activity of A. capillaris was significantly reduced in soil that received additions of P. In the plots receiving long-term additions of N, a strong relationship was observed between extractable soil ammonium and root-surface PME activity. Soil ammonium concentrations accounted for 67% of the variation in PME activity of P. lanceolata in the calcareous grassland, and 86% of the variation in PME activity of A. capillaris in the acid grassland. These results provide evidence that N deposition may have considerable effects on the demand and turnover of P in ecosystems that are approaching or have reached N saturation.     

The Influence of Altered Rainfall Regimes on Early Season N Partitioning Among Early Phenology Annual Plants,a Late Phenology Shrub,and Microbes in a Semi-arid Ecosystem

In Mediterranean-type ecosystems, nitrogen (N) accumulates in soil during dry summer months and rapidly becomes available during early season rain events. The availability of early season N could depend on the size of rainfall events, soil microbial activity, and phenology of the plant community. However, it is poorly understood how precipitation patterns affect the fate of early season N. Microbes and plants with early phenology may compete strongly for early season N but theory suggests that microbial N storage can meet plant N demands later in the season. Using a ¹⁵N tracer and rainfall manipulation we investigated the fate of early season N. N allocation patterns differed substantially between microbes, early and late phenology plants. As expected early phenology annuals and microbes took up ¹⁵N, within 1 day, whereas a late-phenology shrub allocated ¹⁵N to leaves later in the season. We saw no evidence for microbial storage of early season N; the peak of ¹⁵N in shrub leaves did not coincide with detectable levels of ¹⁵N in the microbial biomass or labile soil pool. This suggests that shrubs were able to access early season N, store and allocate it for growth later in the season. Although we saw no evidence of microbial N storage, N retention in soil organic matter (SOM) was high and microbes may play an important role in sequestering N to SOM. Plant N uptake did not respond significantly to 1 year of rainfall manipulation, but microbes were sensitive to dry conditions. 1 year after ¹⁵N addition shrubs had resorbed up to half of the N from leaves whereas N in annuals remained as dead leaf litter. Differences in end-of-season N partitioning between dead and living biomass in the two vegetation types suggest that plant species composition could affect N availability in the following growing season, but it may take several years of altered precipitation patterns to produce rainfall-dependent changes.     

Effects Of Vegetation and Soil on Species Diversity of Soil Dwelling Diptera in a Heathland Ecosystem

The role of vegetation and soil factors on the biodiversity of two soil dwelling, saprophagous, fly families (Sphaeroceridae and Lonchopteridae) in a heathland ecosystem were investigated. The fly community is primarily affected by soil humidity and the amount of organic matter while the vegetation structure and species composition only indirectly influence the fly communities. There was no correlation between plant species richness and the fly diversity indices. Based on our results and literature data, we hypothesise that the direct effects of the vegetation is more evident for herbivorous insects than for species that do not feed on plants. The investigated families show a clear response to microhabitat differences in the soil factors, which makes them promising indicators for soil health and as tool for monitoring environmental changes.     

The Effects of Land-use History on Soil Properties and Nutrient Dynamics in Northern Hardwood Forests of the Adirondack Mountains

Soil nutrient pools and nitrogen dynamics in old-growth forests were compared with selectively logged stands and stands that were selectively logged and then burned approximately 100 years ago to test the hypothesis that land-use history exerts persistent controls on nutrient capital and nitrogen (N) transformation rates. We provide estimates of net N mineralization and nitrification rates for old-growth forests from the northeastern United States, a region in which few old-growth forests remain and for which few published accounts of mineralization rates exist. At the plot level, no effects of the dominant tree species were observed on any measured soil properties or N-cycling rates. Effects of alternate disturbance histories were detected in soil carbon (C) and N pools. Old-growth forest soils had higher total C (67 Mg·ha^–1) and N capital (3.3 Mg·ha^–1) than that of historically logged then burned soils (C = 50 Mg·ha^–1 and N = Mg·ha^–1), with intermediate values (C = 54 Mg·ha^–1 and N = 2.7 Mg·ha^–1) in the stands that were historically logged. Despite these differences in C and N content, corresponding differences in C–N ratio, net N mineralization rates, and net nitrification rates were not observed. The N concentration in the green foliage of American beech trees (Fagus grandifolia) was also highest from canopy trees growing in old-growth stands (3.0%), followed by logged stands (2.6%), and lowest in the logged/burned stands (2.2%). These data suggest that some legacies of light harvesting on ecosystem processes may be detected nearly 100 years following the disturbance event. These results are discussed in the context of how multiple forest disturbances act in concert to affect forest dynamics.     

The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha^–1 y^–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha^–1 y^–1 below the litter layer to around 50 kg C kg C ha^–1 y^–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO₃⁻). The dominance of NO₃^– relative to the total amount nitrate of N leaching from the soil shows that NO₃^– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.     

模拟放牧斑块与氮素添加对半干旱草原群落植物生长的影响(英文)

放牧时,动物采食及其排泄物会影响植物的生长,但动物采食及其排泄物的空间异质性可能会影响这种效应。在位于我国北方典型农牧交错区的内蒙古多伦县,我们研究了模拟放牧斑块和施氮肥对植物生长的影响,实验采用模拟放牧采食斑块(刈割半径分别为0、10、20、40和80 cm)和土壤施氮(分别为0、5、10、20 g N/m2)两种处理,植物地上部收获后分为绿体和立枯两部分,并分析其含氮量。结果表明,刈割降低了植物的生物量(41.5％),而施氮可增加生物量(57.8％)。刈割对植物生长的抑制作用在面积最小又施肥的斑块上表现更明显。土壤施氮可以促进植物生长并且影响刈割效应。同时植物的绿-枯比随施氮水平的增加而增加,因此氮会延迟植物的衰老。以上结果表明,刈割(模拟动物采食)斑块的大小会影响草原植物的生长,土壤施氮(模拟动物尿氮)可以提高草原生态系统的初级生产力,并且影响刈割效应。     

Effects of Elevated Atmospheric CO2 on Soil Microbial Biomass, Activity, and Diversity in a Chaparral Ecosystem

This study reports the effects of long-term elevated atmospheric CO₂ on root production and microbial activity, biomass, and diversity in a chaparral ecosystem in southern California. The free air CO₂ enrichment (FACE) ring was located in a stand dominated by the woody shrub Adenostoma fasciculatum. Between 1995 and 2003, the FACE ring maintained an average daytime atmospheric CO₂ concentration of 550 ppm. During the last two years of operation, observations were made on soil cores collected from the FACE ring and adjacent areas of chaparral with ambient CO₂ levels. Root biomass roughly doubled in the FACE plot. Microbial biomass and activity were related to soil organic matter (OM) content, and so analysis of covariance was used to detect CO₂ effects while controlling for variation across the landscape. Extracellular enzymatic activity (cellulase and amylase) and microbial biomass C (chloroform fumigation-extraction) increased more rapidly with OM in the FACE plot than in controls, but glucose substrate-induced respiration (SIR) rates did not. The metabolic quotient (field respiration over potential respiration) was significantly higher in FACE samples, possibly indicating that microbial respiration was less C limited under high CO₂. The treatments also differed in the ratio of SIR to microbial biomass C, indicating a metabolic difference between the microbial communities. Bacterial diversity, described by 16S rRNA clone libraries, was unaffected by the CO₂ treatment, but fungal biomass was stimulated. Furthermore, fungal biomass was correlated with cellulase and amylase activities, indicating that fungi were responsible for the stimulation of enzymatic activity in the FACE treatment.     

Decadal-scale Dynamics of Water, Carbon and Nitrogen in a California Chaparral Ecosystem: DAYCENT Modeling Results

The Mediterranean climate, with its characteristic of dry summers and wet winters, influences the hydrologic and microbial processes that control carbon (C) and nitrogen (N) biogeochemical processes in chaparral ecosystems. These biogeochemical processes in turn determine N cycling under chronic N deposition. In order to examine connections between climate and N dynamics, we quantified decadal-scale water, C and N states and fluxes at annual, monthly and daily time steps for a California chaparral ecosystem in the Sierra Nevada using the DAYCENT model. The daily output simulations of net mineralization, stream flow and stream nitrate (NO₃⁻) export were developed for DAYCENT in order to simulate the N dynamics most appropriate for the abrupt rewetting events characteristic of Mediterranean chaparral ecosystems. Overall, the magnitude of annual modeled net N mineralization, soil and plant biomass C and N, nitrate export and gaseous N emission agreed with those of observations. Gaseous N emission was a major N loss pathway in chaparral ecosystems, in which nitric oxide (NO) is the dominant species. The modeled C and N fluxes of net primary production (NPP), N uptake and N mineralization, NO₃⁻ export and gaseous N emission showed both high inter-annual and intra-annual variability. Our simulations also showed dramatic fire effects on NPP, N uptake, N mineralization and gaseous N emission for three years of postfire. The decease in simulated soil organic C and N storages was not dramatic, but lasted a longer time. For the seasonal pattern, the predicted C and N fluxes were greatest during December to March, and lowest in the summer. The model predictions suggested that an increase in the N deposition rate would increase N losses through gaseous N emission and stream N export in the chaparral ecosystems of the Sierra Nevada due to changes in N saturation status. The model predictions could not capture stream NO₃⁻ export during most rewetting events suggesting that a dry-rewetting mechanism representing the increase in N mineralization following soil wetting needs to be incorporated into biogeochemical models of semi-arid ecosystems.     

植被覆盖度和综合治理对纸坊沟流域土壤氮素流失的影响

同小流域土壤侵蚀一样,小流域土壤氮素随洪流流失也受到植被覆盖度的影响,通常经过调整小流域内土地利用结构以达到控制水土流失。该研究以8.27 km2纸坊沟流域和1:400比例流域模型为研究对象,研究植被覆盖度和综合治理对纸坊沟流域土壤氮素流失的影响。结果表明:在模拟降雨下,当流域植被覆盖度分别为60%、40%、20%和0时,流域模型铵态氮流失量分别为87.08、44.31、25.16和13.71 kg/km2,硝态氮为85.50、74.05、63.95和56.23 kg/km2,全氮为0.81、1.18、1.98和7.51 t/km2;在自然降雨下,1998年与1992年相比,全流域年土壤侵蚀量为1 086 t/km2和1 119 t/km2,氮素流失量为8 758.5和7 562.2 kg,减少了15.8%,其中农地减少了52.0%。流域对降水中的矿质氮具有过滤作用,硝态氮的过滤作用明显高于铵态氮。洪流泥沙中<20 mm微团聚体富集造成了泥沙有机质和全氮的富集。植被覆盖虽能有效地减少流域土壤侵蚀和全氮的流失,却能增加土壤矿质氮的流失。坡地退耕还林草可显著减少流域土壤氮素流失。     

Residual effects of natural bush,Cajanus cajan and Tephrosia candida on the productivity of an acid soil in southeastern Nigeria

An experiment was established in 1986 to examine the contribution of Tephrosia candida and Cajanus cajan shrubs to improving the productivity of an acid soil. The main treatments were N levels (0 and 60 kg ha^-1) with subplots of maize/natural bush, maize/Tephrosia candida, maize/Cajanus cajan, maize + cassava/natural bush, maize + cassava/Tephrosia candida, and maize + cassava/Cajanus cajan. In 1988, all plots were cleared and maize uniformly planted to study the residual effects of the treatments. No residual effects of N application were observed. Tephrosia candiada and Cajanus cajan increased surface soil organic carbon and total N levels over the natural bush. However, only Tephrosia candida plots produced improved maize grain and stover yield. Highly significant correlations were found between maize grain yield and earleaf N (r=0.73^**), grain N (r=0.51^**), and stover N (r=0.54^**) contents. These results suggest that Tephrosia candida increased N availability in the soil. Therefore, the shrub has potential for improving the productivity of acid soils under traditional systems, where N is limiting due to the absence of N₂-fixing legumes in the natural bush fallow.