共查询到20条相似文献,搜索用时 31 毫秒
1.
We measured nitrogenase activity (acetylene reduction) of asymbiotic, heterotrophic, nitrogen-fixing bacteria on leaf litter
from the tree Metrosideros polymorpha collected from six sites on the Hawaiian archipelago. At all sites M. polymorpha was the dominant tree, and its litter was the most abundant on the forest floor. The sites spanned a soil chronosequence
of 300 to 4.1 million y. We estimated potential nitrogen fixation associated with this leaf litter to be highest at the youngest
site (1.25 kg ha-1 y-1), declining to between 0.05 and 0.22 kg ha-1 y-1 at the oldest four sites on the chronosequence. To investigate how the availability of weathered elements influences N fixation
rates at different stages of soil development, we sampled M. polymorpha leaf litter from complete, factorial fertilization experiments located at the 300-y, 20,000-y and 4.1 million–y sites. At
the youngest and oldest sites, nitrogenase activity on leaf litter increased significantly in the plots fertilized with phosphorus
and “total” (all nutrients except N and P); no significant increases in nitrogenase activity were measured in leaf litter
from treatments at the middle-aged site. The results suggest that the highest rates of N fixation are sustained during the
“building” or early phase of ecosystem development when N is accumulating and inputs of geologically cycled (lithophilic)
nutrients from weathering are substantial.
Received 4 February 1999; accepted 29 March 2000. 相似文献
2.
Exotic grass invasion promotes fire which drives the conversion of native woodlands to exotic grasslands in the seasonally
dry submontane forests of the island of Hawai'i. We compared potential rates of N fixation in an unburned forest site and
a converted grassland site using the acetylene reduction assay. In addition to measuring rates of N fixation on separate and
mixed substrates in each site, we tested the effect of abiotic factors on rates of N fixation of specific substrates. We hypothesized
that rates of N fixation would be higher in the converted grassland site. N fixation estimates were 4.9 kg N ha−1 year−1 for the unburned forest, and 0.10 kg N ha−1 year−1 for the grassland site, so our hypothesis was rejected. The N fixation in the unburned forest occurs mostly on the leaf litter
of native woody species. These substrates are absent from the grassland site, except for wood debris which was not consumed
during the fires. No nitrogenase activity was detected in the rhizosphere and litter of grasses, the rhizospheres of shrubs
or in soil. Although wood debris is not a significant contributor to the N fixed in the unburned forest, it contributes the
majority of N fixed in the grassland. The response of nitrogenase activity to varying conditions of moisture and temperature
suggests that microclimatic differences between sites do not control differences in N fixation activity; rather, these differences
are due to the abundance of N-fixing substrates. The substantial decrease in N fixation activity after the conversion from
woodland to grassland implies that ecosystem-level rates of N accretion are decreased by fire in these sites so much that
the N lost during volatilization due to fire is not replenished over the long term by N fixation.
Received: 10 January 1997 / Accepted: 7 August 1997 相似文献
3.
The above-ground accumulation of N,N uptake and litter quality resulting from improved or deteriorated availability of water
and nutrients in a 25 year old Norway spruce stand in SW Sweden (as part of the Skogaby project) is presented. Treatment include
irrigation; artificial drought; ammonium sulphate addition; N-free-fertilisation and irrigation with liquid fertilisers including
a complete set of nutrients according to the Ingested principle (fertigation).
At start of the experiment the stand contained 86.5 t dry mass and 352 kg N ha−1. The following three years the annual N uptake in untreated trees was 32 kg N ha−1 to be compared with the annual N throughfall of 17 kg ha−1. Simultaneously, the treatment with ammonium sulphate and liquid fertilisation resulted in 48 and 56 kg ha−1 y−1, respectively, in treatment specific N-uptake following an application of 100 kg N ha−1 y−1. Addition of a N-free fertiliser resulted in improved N-uptake by 19 kg N ha−1 y−1 and irrigation by 10 kg N ha−1 y−1, compared to control. A linear relation between total above-ground dry mass production and N-uptake was found for trees growing
with similar water availability. Dry mass production increased with increased water availability given the same N-uptake.
It is concluded that the studied stand this far is not N saturated', as N fertilisation resulted in both increased N uptake
and increased growth. Addition of a N-free-fertiliser resulted in increased uptake of N compared to the control, indicating
an increased mineralisation rate or uptake capacity of the root system. The linear relation between N uptake and biomass production
shows that at this study site N is a highly limiting factor for growth. 相似文献
4.
Improvement of dinitrogen fixation in beans (Phaseolus vulgaris L.) will depend on the selection of superior plant genotypes and the presence of efficient rhizobial strains. This study
was conducted to evaluate diverse bean lines for N2 fixation potential using the15N-depleted dilution technique under field conditions in Wisconsin, USA. Plants of 21 bean lines and three non-nodulating isolines
of soybean received appliin Wisconsin, USA. Plants of 21 bean lines and three non-nodulating isolines of soybean received
applications of15N-depleted ammonium sulphate. Shoots harvested at the V6, R3 and R7 stages and dry seeds were analyzed for total N using the Kjeldahl procedure, and the ratio of15N to14N was determind on a MAT 250 mass spectrometer. Nodule occupancy of the applied strain ofR. leguminosarum biovarphaseoli, CIAT 899, was determined in five of the bean lines. Total shoot N content showed a pattern of accumulation similar to shoot
dry weight and fixed N2 in the shoot. Based on shoot total N, N2 fixed in the shoot and shoot dry weight Riz 30 and Preto Cariri were identified as being as good fixers as Puebla 152 and
Cargamanto appear to begin N2 fixation early. Furthermore, some bean lines that fixed considerable N2 did not translocate a large amount of N to the grains. Preto Cariri accumulated 21.2 kg N ha−1 in the seeds compared to Puebla 152 which accumulated 43.8 kg N ha−1 of the fixed N2 into the grains. At the early sampling, Puebla 152 and 22–27 had a considerable higher percentage of their crown nodules
formed by the inoculant strain CIAT 899, than did Rio Tibagi which has been considered a poor N2 fixer. 相似文献
5.
In their review of 24 studies of forest nitrogen (N) budgets, Binkley and others (2000) found that only one of them supported
the conclusion that an N accumulation of more than 25 kg N ha−1 y−1 is possible without known symbiotic N2–fixing plants. They contended that, given how well the N cycle is known, new N accumulation pathways are unlikely. They also
concluded that the Hubbard Brook sandbox study (Bormann and others 1993) was insufficiently replicated and had low precision
in vegetation and soil estimates. Here we reevaluate and extend the sandbox analysis and place the findings in a broader context.
Using multiple methods of estimating vegetation N accumulation in pine sandboxes, we arrived at results that differed from
the reported rates but still strongly supported large biomass N accumulation. The original study's conclusions about soil
N changes were strengthened when new evidence showed that N accumulated in lower horizons and that the sandboxes were successfully
homogenized at the beginning of the experiment. Unexplained ecosystem N accumulation ranged from about 40 to 150 kg ha−1 y−1, with 95% confidence intervals that did not include zero. No evidence was found that could balance the sandbox ecosystem
N budgets without adding unexplained N. Unreplicated experiments, such as the sandboxes, can explore the possibility that
N can accumulate in ways not explainable by mass balance analysis, but they cannot quantify the frequency and extent of the
phenomenon. New studies should combine substantive microbiological, mass balance, and process research using multiple direct
measures of N2 fixation.
Received 4 September 2001; accepted 3 April 2002. 相似文献
6.
Differences in Englemann Spruce Forest Biogeochemistry East and West of the Continental Divide in Colorado, USA 总被引:5,自引:2,他引:3
We compared Englemann spruce biogeochemical processes in forest stands east and west of the Continental Divide in the Colorado
Front Range. The divide forms a natural barrier for air pollutants such that nitrogen (N) emissions from the agricultural
and urban areas of the South Platte River Basin are transported via upslope winds to high elevations on the east side but
rarely cross over to the west side. Because there are far fewer emissions sources to the west, atmospheric N deposition is
1–2 kg N ha−1 y−1 on the west side, as compared with 3–5 kg N ha−1 y−1 on the east side. Species composition, elevation, aspect, parent material, site history, and climate were matched as closely
as possible across six east and six west side old-growth forest stands. Higher N deposition sites had significantly lower
organic horizon C:N and lignin:N ratios, lower foliar C:N ratios, as well as greater %N, higher N:Ca, N:Mg, and N:P ratios,
and higher potential net mineralization rates. When C:N ratios dropped below 29, as they did in east-side organic horizon
soils, mineralization rates increased linearly. Our results are comparable to those from studies of the northeastern United
States and Europe that have found changes in forest biogeochemistry in response to N deposition inputs between 3 and 60 kg
ha−1 y−1. Though they are low by comparison with more densely populated and agricultural regions, current levels of N deposition,
have caused measurable changes in Englemann spruce forest biogeochemistry east of the Continental Divide in Colorado.
Received 22 January 2001; accepted 11 June 2001. 相似文献
7.
A field study was carried out near Zürich (Switzerland) to determine the yield of symbiotically fixed nitrogen (15N dilution) from white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L) and from red clover (Trifolium pratense L.) grown with Italian ryegrass (Lolium multiflorum Lam.). A zero N fertilizer treatment was compared to a 30 kg N/ha per cut regime (90 to 150 kg ha−1 annually).
The annual yield of clover N derived from symbiosis averaged 131 kg ha−1 (49 to 227 kg) without N fertilization and 83 kg ha−1 (21 to 173 kg) with 30 kg of fertilizer N ha−1 per cut in the seeding year. Values for the first production year were 308 kg ha−1 (268 to 373 kg) without N fertilization and 232 kg ha−1 (165 to 305 kg) with 30 kg fertilizer N ha−1 per cut. The variation between years was associated mainly with the proportion of clover in the mixtures. Apparent clover-to-grass
transfer of fixed N contributed up to 52 kg N ha−1 per year (17 kg N ha−1 on average) to the N yield of the mixtures.
Percentage N derived from symbiosis averaged 75% for white and 86% for red clover. These percentages were affected only slightly
by supplemental nitrogen, but declined markedly during late summer for white clover. It is concluded that the annual yield
of symbiotically fixed N from clover/grass mixtures can be very high, provided that the proportion of clover in the mixtures
exceeds 50% of total dry mass yield. 相似文献
8.
E. S. Jensen 《Plant and Soil》1987,101(1):29-37
The seasonal patterns of growth and symbiotic N2 fixation under field conditions were studied by growth analysis and use of15N-labelled fertilizer in a determinate pea cultivar (Pisum sativum L.) grown for harvest at the dry seed stage.
The patterns of fertilizer N-uptake were almost identical in pea and barley (the non-fixing reference crop), but more fertilizer-N
was recovered in barley than in pea. The estimated rate of N2 fixation in pea gradually increased during the pre-flowering and flowering growth stages and reached a maximum of 10 kg N
fixed per ha per day nine to ten weeks after seedling emergence. This was the time of early pod-development (flat pod growth
stage) and also the time for maximum crop growth rate and maximum green leaf area index. A steep drop in N2 fixation rate occurred during the following week. This drop was simultaneous with lodging of the crop, pod-filling (round
pod growth stage) and the initiation of mobilization of nitrogen from vegetative organs. The application of fertilizer-N inhibited
the rate of N2 fixation only during that period of growth, when the main part of fertilizer-N was taken up and shortly after. Total accumulation
of fixed nitrogen was estimated to be 244, 238 and 213 kg N ha−1 in pea supplied with nil, 25 or 50 kg NO
3
−
−N ha−1, respectively. About one-fourth of total N2 fixation was carried out during preflowering, one fourth during the two weeks of flowering and the remainder during post-flowering.
About 55% of the amount of N present in pods at maturity was estimated to be derived from mobilization of N from vegetative
organs. “Starter” N (25 or 50 kg NO
3
−
−N ha−1) did not significantly influence either dry matter and nitrogen accumulation or the development of leaf area. Neither root
length and root biomass determined 8 weeks after seedling emergence nor the yield of seed dry matter and nitrogen at maturity
were influenced by fertilizer application. 相似文献
9.
A detailed fixed nitrogen (N) mass balance was constructed for the Central Arizona–Phoenix (CAP) ecosystem. Input of fixed
N input to the ecosystem was 98 Gg y−1. Of this, humans deliberately imported or mediated the fixation of 51 Gg N y−1; combustion processes added another 36 Gg y−1. Fixation by desert plants, wet deposition, and surface water input accounted for 11% of total N input. Total fixed N output
was 78 Gg N y−1, a large component of which was gaseous N products of combustion and denitrification. Computed accumulation of N was 21 Gg
y−1 (total input minus total output) or alternatively, 17 Gg y−1 (summing individual accumulation fluxes). Key uncertainties include dry deposition of atmospheric N and changes in soil storage.
Inputs to the urban and agricultural components of the ecosystem were an order of magnitude higher than inputs to the desert.
Human hydrologic modifications in this ecosystem promote the accumulation and volatilization of N while keeping riverine export
low (3% of input). Interplay among the form and amount of N inputs, edaphic and climatic characteristics of the system, hydrologic
modifications, and deliberate efforts to reduce N pollution controls the fate of N in human-dominated ecosystems.
Received 24 September 1999; accepted 7 March 2001. 相似文献
10.
Toshiyuki Ohtsuka Wenhong Mo Takami Satomura Motoko Inatomi Hiroshi Koizumi 《Ecosystems》2007,10(2):324-334
Biometric based carbon flux measurements were conducted over 5 years (1999–2003) in a temperate deciduous broad-leaved forest
of the AsiaFlux network to estimate net ecosystem production (NEP). Biometric based NEP, as measured by the balance between
net primary production (including NPP of canopy trees and of forest floor dwarf bamboo) and heterotrophic respiration (RH),
clarified the contribution of various biological processes to the ecosystem carbon budget, and also showed where and how the
forest is storing C. The mean NPP of the trees was 5.4 ± 1.07 t C ha−1 y−1, including biomass increment (0.3 ± 0.82 t C ha−1 y−1), tree mortality (1.0 ± 0.61 t C ha−1 y−1), aboveground detritus production (2.3 ± 0.39 t C ha−1 y−1) and belowground fine root production (1.8 ± 0.31 t C ha−1 y−1). Annual biomass increment was rather small because of high tree mortality during the 5 years. Total NPP at the site was
6.5 ± 1.07 t C ha−1 y−1, including the NPP of the forest floor community (1.1 ± 0.06 t C ha−1 y−1). The soil surface CO2 efflux (RS) was averaged across the 5 years of record using open-flow chambers. The mean estimated annual RS amounted to
7.1 ± 0.44 t C ha−1, and the decomposition of soil organic matter (SOM) was estimated at 3.9 ± 0.24 t C ha−1. RH was estimated at 4.4 ± 0.32 t C ha−1 y−1, which included decomposition of coarse woody debris. Biometric NEP in the forest was estimated at 2.1 ± 1.15 t C ha−1 y−1, which agreed well with the eddy-covariance based net ecosystem exchange (NEE). The contribution of woody increment (Δbiomass + mortality)
of the canopy trees to NEP was rather small, and thus the SOM pool played an important role in carbon storage in the temperate
forest. These results suggested that the dense forest floor of dwarf bamboo might have a critical role in soil carbon sequestration
in temperate East Asian deciduous forests. 相似文献
11.
Nitrogen fixation was measured in monocropped sweet-blue lupin (Lupinus angustifolius), lupin intercropped with two ryegrass (Lolium multiflorum) cultivars or with oats (Avena sativa) on an Andosol soil, using the 15N isotope dilution method. At 117 days after planting and at a mean temperature below 10°C, monocropped lupin derived an average
of 92% or 195 kg N ha−1 of its N from N2 fixation. Intercropping lupin with cereals increased (p<0.05) the percentage of N derived from atmospheric N2 (% Ndfa) to a mean of 96%. Compared to the monocropped, total N fixed per hectare in intercropped lupin declined approximately
50%, in line with the decrease in seeding rate and dry matter yield. With these high values of N2 fixation, selection of the reference crop was not a problem; all the cereals, intercropped or grown singly produced similar
estimates of N2 fixed in lupin. It was deduced from the 15N data that significant N transfer occurred from lupin to intercropped Italian ryegrass but not to intercropped Westerwoldian
ryegrass or to oats. Doubling the 15N fertilizer rate from 30 to 60 kg N ha−1 decreased % Ndfa to 86% (p<0.05), but total N fixed was unaltered. These results indicate that lupin has a high potential for N2 fixation at low temperatures, and can maintain higher rates of N2 fixation in soils of high N than many other forage and pasture legumes. 相似文献
12.
Intercropping cotton (Gossypium hirsutum L.) and cowpea (Vigna unguiculata (L.) Walp) is one of the ways to improve food security and soil fertility whilst generating cash income of the rural poor. A study was carried out to find out the effect of cotton–cowpea intercropping on cowpea N2-fixation capacity, nitrogen balance and yield of a subsequent maize crop. Results showed that cowpea suppressed cotton yields but the reduction in yield was compensated for by cowpea grain yield. Cowpea grain yield was significantly different across treatments and the yields were as follows: sole cowpea (1.6 Mg ha−1), 1:1 intercrop (1.1 Mg ha−1), and 2:1 intercrop (0.7 Mg ha−1). Cotton lint yield was also significantly different across treatments and was sole cotton (2.5 Mg ha−1), 1:1 intercrop (0.9 Mg ha−1) and 2:1 intercrop (1.5 Mg ha−1). Intercropping cotton and cowpea increased the productivity with land equivalence ratios (LER) of 1.4 and 1.3 for 1:1 and 2:1 intercrop treatments, respectively. There was an increase in percentage of N fixation (%Ndfa) by cowpea in intercrops as compared to sole crops though the absolute amount fixed (Ndfa) was lower due to reduced plant population. Sole cowpea had %Ndfa of 73%, 1:1 intercrop had 85% and 2:1 intercrop had 77% while Ndfa was 138 kg ha−1 for sole cowpea, 128 kg ha−1 for 1:1 intercrop and 68 kg ha−1 for 2:1 intercrop and these were significantly different. Sole cowpea and the intercrops all showed positive N balances of 92 kg ha−1 for sole cowpea and 1:1 intercrop, and 48 kg ha−1 for 2:1 intercrop. Cowpea fixed N transferred to the companion cotton crop was very low with 1:1 intercrop recording 3.5 kg N ha−1 and 2:1 intercrop recording 0.5 kg N ha−1. Crop residues from intercrops and sole cowpea increased maize yields more than residues from sole cotton. Maize grain yield was, after sole cotton (1.4 Mg ha−1), sole cowpea (4.6 Mg ha−1), 1:1 intercrops (4.4 Mg ha−1) and 2:1 intercrops (3.9 Mg ha−1) and these were significantly different from each other. The LER, crop yields, %N fixation and, N balance and residual fertility showed that cotton–cowpea intercropping could be a potentially productive system that can easily fit into the current smallholder farming systems under rain-fed conditions. The fertilizer equivalency values show that substantial benefits do accrue and effort should be directed at maximizing the dry matter yield of the legume in the intercrop system while maintaining or improving the economic yield of the companion cash crop. 相似文献
13.
Soil Phosphorus Fractions and Symbiotic Nitrogen Fixation across a Substrate-Age Gradient in Hawaii 总被引:1,自引:0,他引:1
We evaluated soil phosphorus (P) fractions, other soil characteristics, and rates of symbiotic N2 fixation across a substrate-age gradient in Hawaii that was dominated by the leguminous tree Acacia koa (koa). Patterns of soil P observed on this gradient were compared to those on a slightly wetter gradient dominated by the
nonfixer Metrosideros polymorpha (ohia). Along both gradients, concentrations of primary-mineral P fell sharply between the young and intermediate-aged sites,
while labile inorganic P declined most steeply between the intermediate-aged and old sites. The most marked difference between
the two gradients was that total soil carbon (C), nitrogen (N), and P, as well as nonoccluded organic P, were more variable
across the ohia gradient, increasing to the intermediate-aged sites, then declining sharply at the old site. On the koa gradient,
specific nitrogenase activity, measured by the acetylene-reduction (AR) assay, decreased three- to eightfold between the young
site and the intermediate-aged and old sites, respectively. Nodule biomass showed no clear pattern. N2 fixation rates, estimated by combining AR activity and nodule biomass measurements, were up to 8 kg N · ha−1 · y−1 at the young site and no more than 2 kg N · ha−1 · y−1 at the older sites, suggesting that koa may be a modest source of N in these Hawaiian forests.
Received 26 September 2000; accepted 15 February 2002 相似文献
14.
Chris van Kessel 《Plant and Soil》1994,164(1):69-76
Although wheat (Triticum aestivum L.) is the dominant crop of the semi-arid plains of Canada and the western United States, lentil (Lens culinaris Medik.) has become an important alternative crop. Sources and seasonal accumulation of N must be understood in order to identify
parameters that can lead to increased N2-fixing activity and yield. Inoculated lentil was grown in a sandy-loam soil at an irrigated site in Saskatchewan, Canada.
Wheat was used as the reference crop to estimate N2 fixation by the A-value approach. Lentil and wheat received 10 and 100 kg N ha−1 of ammonium nitrate, respectively. Crops were harvested six times during the growing season and plant components analyzed.
During the first 71 days after planting the wheat had a higher daily dry matter and N accumulation compared to lentil. However,
during the latter part of the growing season, daily dry matter and N accumulation were greater for lentil. The maximum total
N accumulation for lentil at maturity was 149 kg ha−1. In contrast, wheat had a maximum N accumulation of 98 kg ha−1 in the Feekes 11.1 stage, or 86 days after planting. The maximum daily rates of N accumulation were 3.82 kg N ha−1 day−1 for lentil and 2.21 kg N ha−1 day−1 for wheat. The percentage of N derived from N2 fixation (% Ndfa) ranged from 0 at the first harvest to 92 % at final harvest. Generative plant components had higher values
for % Ndfa than the vegetative components which indicates that N in the reproductive plant parts was derived largely from
current N2 fixation and lentil continued to fix N until the end of the pod fill stage. At final harvest, lentil had derived 129 kg N
ha−1 from N2 fixation with maximum N2-fixing activity (4.4 kg N ha−1 day−1) occurring during the early stages of pod fill. Higher maximum rates of N2-fixing activity than net N accumulation (3.82 kg N ha−1 day−1) may have been caused by N losses like volatilization. In addition, lentil provided a net N contribution to the soil of 59
kg ha−1 following the removal of the grain. 相似文献
15.
Response of Litter Decomposition to Simulated N Deposition in Disturbed, Rehabilitated and Mature Forests in Subtropical China 总被引:16,自引:1,他引:15
The response of decomposition of litter for the dominant tree species in disturbed (pine), rehabilitated (pine and broadleaf
mixed) and mature (monsoon evergreen broadleaf) forests in subtropical China to simulated N deposition was studied to address
the following hypothesis: (1) litter decomposition is faster in mature forest (high soil N availability) than in rehabilitated/disturbed
forests (low soil N availability); (2) litter decomposition is stimulated by N addition in rehabilitated and disturbed forests
due to their low soil N availability; (3) N addition has little effect on litter decomposition in mature forest due to its
high soil N availability. The litterbag method (a total of 2880 litterbags) and N treatments: Control-no N addition, Low-N:
−5 g N m−2 y−1, Medium-N: −10 g N m−2 y−1, and High-N: −15 g N m−2 y−1, were employed to evaluate decomposition. Results indicated that mature forest, which has likely been N saturated due to both long-term high N deposition in the region
and the age of the ecosystem, had the highest litter decomposition rate, and exhibited no significant positive and even some
negative response to nitrogen additions. However, both disturbed and rehabilitated forests, which are still N limited due
to previous land use history, exhibited slower litter decomposition rates with significant positive effects from nitrogen
additions. These results suggest that litter decomposition and its responses to N addition in subtropical forests of China
vary depending on the nitrogen status of the ecosystem. 相似文献
16.
Dinitrogen fixation in white clover (Trifolium repens L.) grown in pure stand and mixture with perennial ryegrass (Lolium
perenne L.) was determined in the field using 15N isotope dilution and harvest of the shoots. The apparent transfer of clover N to perennial ryegrass was simultaneously assessed.
The soil was labelled either by immobilizing 15N in organic matter prior to establishment of the sward or by using the conventional labelling procedure in which 15N fertilizer is added after sward establishment. Immobilization of 15N in the soil organic matter has not previously been used in studies of N2 fixation in grass/clover pastures. However, this approach was a successful means of labelling, since the 15N enrichment only declined at a very slow rate during the experiment. After the second production year only 10–16% of the
applied 15N was recovered in the harvested herbage. The two labelling methods gave, nonetheless, a similar estimate of the percentage
of clover N derived from N2 fixation. In pure stand clover, 75–94% of the N was derived from N2 fixation and in the mixture 85–97%. The dry matter yield of the clover in mixture as percentage of total dry matter yield
was relatively high and increased from 59% in the first to 65% in the second production year. The average daily N2 fixation rate in the mixture-grown clover varied from less than 0.5 kg N ha−1 day−1 in autumn to more than 2.6 kg N ha−1 day−1 in June. For clover in pure stand the average N2 fixation rate was greater and varied between 0.5 and 3.3 kg N ha−1 day−1, but with the same seasonal pattern as for clover in mixture. The amount of N fixed in the mixture was 23, 187 and 177 kg
N ha−1 in the seeding, first and second production year, respectively, whereas pure stand clover fixed 28, 262 and 211 kg N ha−1 in the three years. The apparent transfer of clover N to grass was negligible in the seeding year, but clover N deposited
in the rhizosphere or released by turnover of stolons, roots and nodules, contributed 19 and 28 kg N ha−1 to the grass in the first and second production year, respectively.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
17.
Large Loss of Dissolved Organic Nitrogen from Nitrogen-Saturated Forests in Subtropical China 总被引:5,自引:0,他引:5
Yunting Fang Weixing Zhu Per Gundersen Jiangming Mo Guoyi Zhou Muneoki Yoh 《Ecosystems》2009,12(1):33-45
Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially
under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest
soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very
high long-term ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry
under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/broadleaf forest
in subtropical southern China. All three forests have previously been shown to have high nitrate (NO3−) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest
specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results
showed that under ambient deposition (35–50 kg N ha−1 y−1 as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5–16.9 kg N ha−1 y−1). DON leaching increased 35–162% following 2.5 years of experimental input of 50–150 kg N ha−1 y−1. The fertilizer-driven increase of DON leaching comprised 4–17% of the added N. A concurrent increase in DOC loss was observed
only in the pine forest, even though DOC:DON ratios declined in all three forests. Our data showed that DON accounted for
23–38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving
toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had
the highest DON loss under ambient conditions. DON leaching was highly correlated with NO3− leaching in all three forests. We hypothesize that abiotic incorporation of excess NO3− (through chemically reactive NO2−) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the
consistent and large DON loss in the N-saturated subtropical forests of southern China.
Dr. YT Fang performed research, analyzed data, and wrote the paper; Prof. WX Zhu participated in the initial experimental
design, analyzed data, and took part in writing the paper; Prof. P Gundersen conceived the study and took part in writing;
Prof. JM Mo and Prof. GY Zhou conceived study; Prof. M Yoh analyzed part of the data and contributed to the development of
DON model. 相似文献
18.
A field experiment was conducted at the Bangladesh Rice Research Institute, Joydebpur, Dhaka during the late wet season. Basal
application of P at both 5 and 10 kg ha−1 significantly increased total biomass production and nitrogen fixation byAzolla pinnata R. Brown (local strain). Addition of both 5 and 10 kg P ha−1 in equal splits at inoculation and at six day intervals thereafter during growth periods of 12, 24 and 36 days increased
biomass production and nitrogen fixation by Azolla over that attained with the basal application. Biomass and nitrogen fixation
using a split application of 5 kg P ha−1 exceeded that attained with basal application of 10 kg P ha−1 and split application of 10 kg P ha−1 resulted in 0.58, 11.2, and 18.3 t ha−1 more biomass, and 0.47, 18.9, and 18.3 more kg fixed N ha−1 at 12, 24 and 36 days, respectively, than the same amount applied as a basal application. Analyses indicated that the critical
level of dry weight P in Azolla for sustained growth was in the range of 0.15–0.17%. Compared with the control, where no P
was added, and additional 30 and 36 kg N ha−1 were fixed after 24 and 36 days, respectively, when P was provided at 10 kg ha−1 using a split application. A separate field study showed that flooded rice plants received P from incorporated Azolla with
about 28% of the P present in the supplied Azolla being incorporated into the rice plants. 相似文献
19.
Water hyacinth productivity and detritus accumulation 总被引:2,自引:1,他引:1
Water hyacinth [Eichhornia crassipes (Mart) Solms] productivity and detritus accumulation were evaluated in eutrophic lake water with and without added nutrients
(fertilized and control reservoirs, respectively). Seasonal changes in plant productivity and detritus accumulation were determined
at monthly intervals for one year.
Significant differences were observed in plant productivity between seasons and nutrient additions. Seasonal plant productivity
ranged from 1.9 to 23.1 mg (dry wt) ha−1 for the fertilized reservoir and −0.2 to 10.2 mg ha−1 for the control reservoir. Detritus accumulation was not significantly different between seasons or nutrient additions. Seasonal
N assimilation by plants ranged from 34 to 242 kg N ha−1 for plants in the fertilized reservoir and < 0 to 104 kg N ha−1 for plants in the control reservoir. Annual net N recovered in detritus represented 21 and 28% of the total N removed by
plants in the fertilized and control reservoirs, respectively. Net N loading to the reservoirs from detritus was 92 to 148
kg N ha−1 yr−1. 相似文献
20.
Old growth forest soils are large C reservoirs, but the impacts of tree-fall gaps on soil C in these forests are not well
understood. The effects of forest gaps on soil C dynamics in old growth northern hardwood–hemlock forests in the upper Great
Lakes region, USA, were assessed from measurements of litter and soil C stocks, surface C efflux, and soil microbial indices
over two consecutive growing seasons. Forest floor C was significantly less in gaps (19.0 Mg C ha−1) compared to gap-edges (39.5 Mg C ha−1) and the closed forest (38.0 Mg C ha−1). Labile soil C (coarse particulate organic matter, cPOM) was significantly less in gaps and edges (11.1 and 11.2 Mg C ha−1) compared to forest plots (15.3 Mg C ha−1). In situ surface C efflux was significantly greater in gaps (12.0 Mg C ha−1 y−1) compared to edges and the closed forest (9.2 and 8.9 Mg C ha−1 y−1). Microbial biomass N (MBN) was significantly greater in edges (0.14 Mg N ha−1) than in the contiguous forest (0.09 Mg N ha−1). The metabolic quotient (qCO2) was significantly greater in the forest (0.0031 mg CO2 h−1 g−1/mg MBC g−1) relative to gaps or edges (0.0014 mg CO2 h−1 g−1/mg MBC g−1). A case is made for gaps as alleviators of old growth forest soil C saturation. Relative to the undisturbed closed forest,
gaps have significantly less labile C, significantly greater in situ surface C efflux, and significantly lower decreased qCO2 values. 相似文献