Advances in the positional cloning of nodulation genes in soybean

The effect of liming on the decomposition of Norway spruce needle litter was studied in 40–60-year-old Norway spruce stands. Finely-ground limestone had been spread about 30 years ago at a dose of 2 t ha^–1 and reliming was carried out about 20 yr later at a dose of 4 t ha^–1. Needle litter was collected from both control and limed plots, and it was placed in litter bags in the middle of the humus layer of the plot from which they originated, and similarly to the other plot in May. Litter bags were sampled after 4, 12 and 16 months. The site of origin of the needle litter, whether from control plot or from limed plot, affected mainly the early stages of decomposition. Initially the effect of liming was seen as decreased concentration of water soluble material and then, during decomposition, as decreased mass loss and decreased degradation of lignin, and increased C/N ratio. The incubation site, whether the control or the limed plot, did not affect decomposition significantly.Decomposition of Scots pine needles in a young Scots pine plantation was also studied. The treatments were: 2 t ha^–1 of finely-ground limestone and 2.5 t ha^–1 of bark ash spread 8 months before this study. The treatments did not affect decomposition much, but some stimulation of the treatments on decomposition was observed. Compared to spruce needles, the C/N ratio of pine seedles was lower, they contained less lignin and more water soluble material, and decomposed faster in the first summer.     

Mixed leaf litter effects on decomposition rates and soil microarthropod communities in an oak–pine stand in Japan

Rates of decomposition, and soil faunal abundance and diversity associated with single-species and mixed-species litters were studied in a litter bag experiment in an oak–pine forest. We used two canopy species of leaf litter, pine and oak, and one shrub species, Sasa, and compared decomposition rates, and soil microarthropod abundance and community structure of oribatid mites in the litter bags. Mass loss of single species decreased in the order: oak > pine > Sasa. While the total mass loss rates of mixed litter were intermediate between those of the constituent species, enhancement of mass loss from the three-species mixture and from mixed slow-decomposing litters (pine and Sasa) was observed. Faunal abundance in litter bags was higher in mixed-species litter than in those with single-species litter, and species richness of oribatid mites was also higher in the three-species mixed litter. Faunal abundance in single-species litter bags was not correlated with mass loss, although enhancement of mass loss in mixed litter bags corresponded with higher microarthropod abundance. Habitat heterogeneity in mixed litter bags seemed to be responsible for the more abundant soil microarthropod community.     

The influence of invertebrates on the breakdown ofPotamogeton pectinatus L. in a coastal marina (Zandvlei,South Africa)

The influence of invertebrates upon the decomposition ofPotamogeton pectinatus L. in a coastal Marina system was examined over 112 days using litter bags. Invertebrate inclusion bags (2 mm mesh, 5 mm holes) registered a dry mass loss of 1% d^–1, while exclusion litter bags (80 µm mesh) produced a 0.4% mass loss d^–1 (a 2.5 fold difference). Losses of ash and N from inclusion bags were greater than those from exclusion bags (p < 0.05). There was a three fold difference between the two treatments in the time taken for litter to breakdown to half the initial stock: T_1/2 for inclusion bags = 43 d, exclusion bags = 130 d. In both treatments, minerals showed an expected rapid loss, due to leaching, with a subsequent slow increase relative to the dry material remaining. A total of nine invertebrate taxa was recorded from inclusion bags, with a peak biomass of 64 mg g^–1 dry massPotamogeton bag^–1 reached at 64 days after immersion. Grazing amphipods,Melita zeylanica Stebbing andAustrochiltonia subtenuis (Barnard), numerically dominated the litter bag fauna, whileM. zeylanica and nymphs of the zygopteran predatorIschnura senegalensis (Rambur) formed most of the biomass. Scanning Electron Microscopy indicated heavy grazing of micro-organisms by invertebrates, with major qualitative differences occurring 112 days after immersion. Invertebrates significantly accelerated the rate of litter breakdown through their feeding activities, assisting fragmentation and thus contributing to plant losses and also by increasing the surface area for microbial colonisation and attack.     

The Effects of Exotic Grasses on Litter Decomposition in a Hawaiian Woodland: The Importance of Indirect Effects

Exotic grasses and grass-fueled fires have altered plant species composition in the seasonal submontane woodlands of Hawaii Volcanoes National Park. These changes have altered both structural and functional aspects of the plant community, which could, in turn, have consequences for litter decomposition and nitrogen (N) dynamics. In grass-invaded unburned woodland, grass removal plots within the woodland, and woodland converted to grassland by fire, we compared whole-system fluxes and the contributions of individual species to annual aboveground fine litterfall and litterfall N, and litter mass and net N loss. We assessed the direct contribution of grass biomass to decomposition and N dynamics, and we determined how grasses affected decomposition processes indirectly via effects on native species and alteration of the litter layer microenvironment. Grasses contributed 35% of the total annual aboveground fine litterfall in the invaded woodland. However, total litterfall mass and N were not different between the invaded woodland and the grass removal treatment because of compensation by the native tree Metrosideros polymorpha, which increased litter production by 37% ± 5% when grasses were removed. The 0.3 g N m^–2/y^–1 contained in this production increase was equal to the N contained in grass litter. Litter production and litterfall N was lowest in the grassland due to the loss of native litter inputs. Decomposition of litterfall on an area basis was highest in the grass-invaded woodland. We attributed this effect to increased inherent decomposability of native litter in the presence of grasses because (a) the microenvironment of the three vegetation treatments had little effect on decomposition of common litter types and (b) M. polymorpha litter produced in the invaded woodland decomposed faster than that produced in the grass removal plots due to higher lignin concentrations in the latter than in the former. Area-weighted decomposition was lowest in the grassland due to the absence of native litter inputs. Across all treatments, most litter types immobilized N throughout the incubation, and litter net N loss on an area basis was not different among treatments. Our results support the idea that the effects of a plant species or growth form on decomposition cannot be determined in isolation from the rest of the community or from the direct effects of litter quality and quantity alone. In this dry woodland, exotic grasses significantly altered decomposition processes through indirect effects on the quantity and quality of litter produced by native species.     

Production,decomposition, and nitrogen dynamics ofMyrica gale litter

Summary Myrica gale litter deposition and decomposition were studied in a central Massachusetts peatland to determine the amount of N made available to the ecosystem by these processes. Leaf litter added 114–140 g biomass m^–2 annually and contained 2.12–2.59 g N m^–2 returning about 70% as much N to the ecosystem as was fixed annually byMyrica gale. During the first five years of decomposition, the leaf liter lost only 40% of its initial biomass and released only 10% of its initial N content. About 60% of its original N mass was still present when the litter reached the permanently waterlogged zone, and thus was effectively lost to the vegetation. The low decomposition rate was due primarily to the chemical content of the litter because similarly low rates were observed in an upland forest where the native litter decayed rapidly. The initial lignin content (40%) ofM. gale litter may be largely responsible for its slow decomposition in spite of its relatively high (1.69%) initial N content.M. gale litter decayed substantially more slowly and had a much higher initial lignin content than the litter of other woody N₂-fixing plants which have been examined.     

Decomposition process in Negev ecosystems

Summary The effects of supplemental water and natural rainfall on decomposition were studied in the Negev Highland desert, Israel. There was a mass loss of approximately 40% in Hammada scoparia leaves and Salsola inermis litter placed on the soil surface and buried in fine mesh bags. There was an annual mass loss of 80% in S. inermis litter buried in large fiberglass mesh bags. Supplemental water provided during the wet season (January to March) did not result in more rapid decomposition of litter of the annual grass Stipa capensis but irrigation during the dry season (August to September) produced a marked increase in the decomposition rate of S. capensis. These data suggest that rain events, not water quantity, are the most important regulators of decomposition in the Negev. Annual rates of decomposition were higher than predicted by models utilizing actual evapotranspiration and lignin content as regulating variables. Rates of decomposition were equal to those reported for tropical wet forests.     

Grain legumes and green manures as pre-rice crops in Northeast Thailand

The loss of dry matter (ash corrected), nitrogen (N) and carbon (C) from residues of several tropical legume species was monitored using litter bags in the field over a three-month period in Northeast Thailand. This work was linked to an experiment in a farmers' field where the residual benefits of the same legume species grown before flooded rice were measured. Litter bags were incorporated in the flooded rice plots at the same time as residue incorporation in the field experiment. The species studied were Sesbania rostrata, Aeschynomene afraspera and a multi-purpose cowpea variety (Vigna unguiculata cv KVC-7). In the case of S. rostrata the breakdown of fresh and oven-dried residues and of residues buried at depths of 2–3 cm and 15 cm was also compared.Although the initial N and C concentrations were similar for all the residues they exhibited differing dry matter, N and C loss patterns. With Sesbania rostrata, 80% of the N was lost from the residues after 20 days, however, there was only a 40% decline in C and weight during the same period. The rate and amount of N loss from Aeschynomene afraspera residues was much less than with S. rostrata, declining by approximately 35% during the first 40 days. There were marked differences in rates of N loss from stem and leaves of A. afraspera indicating that monitoring the decomposition of stem and leaves combined can be misleading. In multi-purpose cowpea, loss patterns of dry matter, N and C were all similar and 50–65% was lost after 40 days burial. There was little difference between breakdown of fresh and oven-dried S. rostrata residues nor were there noticeable differences between residues incorporated superficially (2–3 cm) and buried at 15 cm. Although both %N and lignin:N ratios correlated well with weight and N loss from the residues, this was only the case when leaf and stem material were analyzed separately for A. afraspera.Despite the slower rate and smaller total amount of N released from the A. afraspera residues compared with the S. rostrata residues, a similar amount and proportion (around 20 kg N ha^-1 or 22–28%) of the N was recovered from both residues by a crop of rice planted at the time of residue incorporation. This suggests a considerably higher use efficiency by rice of the N released from the A. afraspera residues (approximately 40%) compared with that for S. rostrata (30%).     

Intraspecific variability in leaf traits strongly affects alder leaf decomposition in a stream

This study assessed the intraspecific variability of senescent leaves of alder (Alnus glutinosa Gaertn.) and the effects of this variability on leaf decomposition in streams. Leaves were collected at five geographically distant locations in Europe. We analyzed 10 batches of leaf samples for seven quantitative leaf traits as well as leaf decomposition rate in coarse and fine mesh bags exposed in a single stream. The geographic origin of leaf samples largely explained the observed variation in litter quality and decomposition rate. Phosphorus (0.034–0.187%) and lignin (3.9–18.7%) concentrations in leaves varied widely. Together, these two traits accurately predicted leaf decomposition rate (r²=84.1%). Intraspecific variation in leaf decomposition rate was within a range similar to that reported for interspecific variation among co-occurring riparian plant species in Europe. Our study demonstrates extensive intraspecific variability in leaf traits on a continental scale, which can have enormous effects on major ecosystem processes such as leaf decomposition.     

Decomposition of roots of black alder and hybrid poplar in short-rotation plantings: Nitrogen and lignin control

The decomposition of the roots (0–2 mm, 2–5 mm and 5–10 mm) of black alder (Alnus glutinosa (L.) Gaertn.) and hybrid poplar (Populus nigra L. X Populus trichocarpa Torr & Gray) was followed over a 462-day period in pure and mixed plantings in southern Quebec. Small roots of alder had the highest initial concentrations of nitrogen and lignin, and lost 9 and 10% less mass than medium and large roots, respectively. Large roots of poplar had the highest lignin-to-nitrogen ratio and showed the smallest loss of mass over the total incubation period. Slow root decomposition of black alder and hybrid poplar was characterized by a greater proportion of initial root nitrogen immobilized per unit of carbon respired. Lignin concentration in roots of alder and poplar increased rapidly at the beginning of the incubation. Our results suggest that high levels of nitrogen in roots of alder could contribute in slowing the rate of decomposition by allowing the formation of nitrogen-lignin derivatives and low levels of nitrogen in roots of poplar may limit the growth of microorganisms and the rate of root decomposition. A multiple regression was developed using initial nitrogen, lignin concentration and the ratio of lignin to nitrogen to produce an index of the rate of root decomposition. The correlation between the index values and the percentage of residual root mass was significant (r=0.98, p<0.01).     

Decomposition of roots of three plant communities in a Dutch salt marsh

Results of the first published study on root decomposition in a West European salt marsh are presented. In situ decomposition of roots of Spartinetum, Puccinellietum and Halimionetum communities were investigated using litter bags. Both the temporal pattern of decomposition and decomposition rate of belowground tissues of the three communities differed during 30 weeks in the marsh; Puccinellietum root litter lost 30–45% ash-free dry weight, Halimionetum root litter 17–26% and Spartinetum root litter 7–17%. Compared to aboveground decomposition in salt marshes these rates are low, however they are in the range of results reported for American and Australian salt marshes. Decomposition rates of root material buried at depths of 10 and 20 cm differed and there was a community × depth interaction. Initial content of structural components was highest in Halimionetum root litter and lowest in Puccinellietum root litter. Integrated soil temperature was highest in the Puccinellietum habitat, while flooding frequency was lowest in the Halimionetum habitat. Results indicate that environmental conditions can cause irregular fluctuations in belowground decomposition rates.     

Root growth and litter decomposition in a coffee plantation under shade trees

In a non-fertilized coffee plantation under shade trees the root biomass was excavated to estimate its distribution in the soil profile. One third of total fine (less than 1 mm) roots was found in the first 10 cm of soil; the cumulative total to 30 cm reached 73%. A highly variable and transient amount of fine roots colonized the litter layer. Root production both in the litter and in the first 7.5 cm of mineral soil was estimated from sequential samplings and was 10 g m^–2 yr^–1 and 660 g m^–2 yr^–1 respectively. The decomposition rate of weighed averages of litter fractions in the coffee plantation, calculated as the ratio of litter fall rate to the amount found in the soil was k=4.8. Shade tree leaves, the major component of litter descomposed slower than coffee leaves and these slower than flowers and fruits. Litter bag experiments showed considerable slower rates when mesh was 0.03 mm than 0.5 mm. Nitrogen and phosphorous showed increases in concentrations as decomposition progressed while potassium, calcium and magnesium followed a decrease in concentration that paralleled that of dry weight loss. In comparing the decomposition rate for litter with or without coffee roots growing in the bags, a tendency to show faster decomposition rates was found for the treatment with roots. These differences were however, only significant for one month for shade tree leaves litter. Nitrogen amounts remaining in shade tree leaves litter was lower in the treatment with roots that without roots. Potassium concentration in roots was positively correlated with potassium concentration in decomposing leaf litter where roots were growing. These results suggest that while roots growing attached to decomposing litter had little or no effect in speeding the decomposition process, the superficial roots seem to play an important role in absorbing very efficiently the mineralized nutrients from litter. The anatomical study of roots showed that the plantation is intensely infected with V-A mycorrhiza. External mycorrhizal hyphae did not to play a role in attachment of roots to decomposing litter while root hairs were found to grow in profusion on root surfaces oriented toward litter.     

The influence of different litter bag designs on the breakdown of leaf material in a small mountain stream

Leaf breakdown of two riparian tree species, Cunonia capensis L. and Ilex mitis (L.) Radlk. was investigated in vitro at Window Stream, Table Mountain, using three different designs of litter bag. Breakdown of Cunonia and Ilex in coarse-mesh (5 mm) litter bags was very rapid (respectively 14.79 and 13.93% loss d^–1), and was significantly greater than the loss of leaf material of 1% d^–1 for both species from fine-mesh bags (180 µm). Differences recorded between fine-mesh and composite-mesh bags (180 µm mesh with 5 mm mesh top) represented macro-invertebrate ingestion, and at t = 28 d, amounted to 67.57% material loss in Cunonia and 62.58% in Ilex. The losses due to microbial activity and leaching, 31.28% in Cunonia and 29.17% in Ilex were not significantly different. Weight loss of Cunonia in coarse-mesh bags (14.79% loss d^–1) and in composite-mesh bags (13.93% loss d^–1) did not differ, but this was not the case for Ilex, where a significantly higher rate of loss in coarse-mesh bags (13.93% loss d^–1) than in composite-mesh bags (7.69% loss d^–1) was observed. This difference was used to quantify fragmentation losses. It was concluded that future leaf breakdown experiments in mountain streams must take cognisance of differential fragmentation losses before inferences can be made as to both invertebrate feeding preferences and biological decomposition of leaves.     

Decomposition of litter of three mudflat annual species in a northern prairie marsh during drawdown*

The loss of dry mass, nitrogen and phosphorus from shoot and root litter of mudflat annuals was examined in a series of experimental marshes in the Delta Marsh, Manitoba, Canada. Litter bags containing shoot material of three mudflat annuals (Aster laurentianus Fern., Atriplex patula L., and Chenopodium rubrum L.) were placed on the sediment surface of the marshes under drawdown conditions. In addition, litter bags containing root material of these three species were shallowly buried. Approximately 70% and 50% of both shoot and root litter, respectively, was still present after one year in the field. During the second year when the marshes were flooded, shoot and root litter lost an additional 20% and 0% of their mass, respectively. Except for Chenopodium roots, which accumulated nitrogen and phosphorus during both years, shoot and root litter lost from 0 to 50% of their nitrogen and phosphorus early in the first year, with levels generally remaining constant through the remainder of the study period. Our results indicate that mudflat annual litter decomposed slowly and would provide abundant habitat for aquatic invertebrates when these marshes were reflooded. However, most nutrient loss took place in the first year when the litter was unflooded, with little loss occurring in the second year when flooded.     

Decomposition patterns of leaf litter of seven common canopy species in a subtropical forest: N and P dynamics

Litter decomposition, governing nutrient and C cycling, is strongly influenced by the chemical litter quality. In order to determine the interspecific variation in leaf decomposition rates and to understand the chemical basis for such variation, decomposition dynamics of seven common canopy species was investigated over 2year using the litterbag technique in a subtropical evergreen broad-leaved forest on Okinawa Island, Japan. The species studied are representatives of the vegetation in the study area and differed significantly in their chemical litter quality. Dry mass loss at the end of study varied in the order: Distylium racemosum< Quercus miyagii< Rapanea neriifolia< Symplocos confusa< Castanopsis sieboldii< Schima wallichii< Daphniphyllum glaucescens. All species showed a pattern characterized by a rapid initial decomposition followed by lower rates except for D. glaucescenswhich decomposition rate appeared to be rather constant. In the late phase, decomposition rates were correlated positively to initial N and ash contents and negatively to lignin content, lignin:N, C:N, and C:P ratios. The effects of N and lignin content or lignin:N ratio were stronger than other quality parameters. There was a wide range in patterns of N and P concentrations, from a net accumulation to a rapid loss in decomposition. The correlation between N and P release suggests that N and P dynamics may have influenced each other during litter decomposition. Analysis of initial quality for species showed that the C:P ratios were extremely high (range 1639–3811) but the N:P ratios were from 28 to 56, indicating a likely P-limitation for this forest. Our results suggest that P is an important control of litter decomposition and N and P dynamics.     

Leaf litter decomposition of Piper aduncum,Gliricidia sepium and Imperata cylindrica in the humid lowlands of Papua New Guinea

No information is available on the decomposition and nutrient release pattern of Piper aduncum and Imperata cylindrica despite their importance in shifting cultivation systems of Papua New Guinea and other tropical regions. We conducted a litter bag study (24 weeks) on a Typic Eutropepts in the humid lowlands to assess the rate of decomposition of Piper aduncum, Imperata cylindrica and Gliricidia sepium leaves under sweet potato (Ipomoea batatas). Decomposition rates of piper leaf litter were fastest followed closely by gliricidia, and both lost 50% of the leaf biomass within 10 weeks. Imperata leaf litter decomposed much slower and half-life values exceeded the period of observation. The decomposition patterns were best explained by the lignin plus polyphenol over N ratio which was lowest for piper (4.3) and highest for imperata (24.7). Gliricidia leaf litter released 79 kg N ha^–1, whereas 18 kg N ha^–1 was immobilised in the imperata litter. The mineralization of P was similar for the three species, but piper litter released large amounts of K. The decomposition and nutrient release patterns had significant effects on the soil. The soil contained significantly more water in the previous imperata plots at 13 weeks due to the relative slow decomposition of the leaves. Soil N levels were significantly reduced in the previous imperata plots due to immobilisation of N. Levels of exchangeable K were significantly increased in the previous piper plots due to the large addition of K. It can be concluded that piper leaf litter is a significant and easily decomposable source of K which is an important nutrient for sweet potato. Gliricidia leaf litter contained much N, whereas imperata leaf litter releases relatively little nutrients and keeps the soil more moist. Gliricidia fallow is more attractive than an imperata fallow for it improves the soil fertility and produces fuelwood as additional saleable products.     

Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities?

Though field data for naturally senesced leaf litter are rare, it is commonly assumed that rising atmospheric CO₂ concentrations will reduce leaf litter quality and decomposition rates in terrestrial ecosystems and that this will lead to decreased rates of nutrient cycling and increased carbon sequestration in native ecosystems. We generally found that the quality of␣naturally senesced leaf litter (i.e. concentrations of C, N and lignin; C:N, lignin:N) of a variety of native plant species produced in alpine, temperate and tropical communities maintained at elevated CO₂ (600–680 μl l⁻¹) was not significantly different from that produced in similar communities maintained at current ambient CO₂ concentrations (340–355 μl l⁻¹). When this litter was allowed to decompose in situ in a humid tropical forest in Panama (Cecropia peltata, Elettaria cardamomum, and Ficus benjamina, 130 days exposure) and in a lowland temperate calcareous grassland in Switzerland (Carex flacca and a graminoid species mixture; 261 days exposure), decomposition rates of litter produced under ambient and elevated CO₂ did not differ significantly. The one exception to this pattern occurred in the high alpine sedge, Carex curvula, growing in the Swiss Alps. Decomposition of litter produced in situ under elevated CO₂ was significantly slower than that of litter produced under ambient CO₂ (14% vs. 21% of the initial litter mass had decomposed over a 61-day exposure period, respectively). Overall, our results indicate that relatively little or no change in leaf litter quality can be expected in plant communities growing under soil fertilities common in many native ecosystems as atmospheric CO₂ concentrations continue to rise. Even in situations where small reductions in litter quality do occur, these may not necessarily lead to significantly slower rates of decomposition. Hence in many native species in situ litter decomposition rates, and the time course of decomposition, may remain relatively unaffected by rising CO₂. Received: 12 September 1996 / Accepted: 30 November 1996     

Controls on long-term root and leaf litter decomposition in neotropical forests

Litter decomposition represents one of the largest annual fluxes of carbon (C) from terrestrial ecosystems, particularly for tropical forests, which are generally characterized by high net primary productivity and litter turnover. We used data from the Long-Term Intersite Decomposition Experiment (LIDET) to (1) determine the relative importance of climate and litter quality as predictors of decomposition rates, (2) compare patterns in root and leaf litter decomposition, (3) identify controls on net nitrogen (N) release during decay, and (4) compare LIDET rates with native species studies across five bioclimatically diverse neotropical forests. Leaf and root litter decomposed fastest in the lower montane rain and moist forests and slowest in the seasonally dry forest. The single best predictor of leaf litter decomposition was the climate decomposition index (CDI), explaining 51% of the variability across all sites. The strongest models for predicting leaf decomposition combined climate and litter chemistry, and included CDI and lignin ( R ²=0.69), or CDI, N and nonpolar extractives ( R ²=0.69). While we found no significant differences in decomposition rates between leaf and root litter, drivers of decomposition differed for the two tissue types. Initial stages of decomposition, determined as the time to 50% mass remaining, were driven primarily by precipitation for leaf litter ( R ²=0.93) and by temperature for root litter ( R ²=0.86). The rate of N release from leaf litter was positively correlated with initial N concentrations; net N immobilization increased with decreasing initial N concentrations. This study demonstrates that decomposition is sensitive to climate within and across tropical forests. Our results suggest that climate change and increasing N deposition in tropical forests are likely to result in significant changes to decomposition rates in this biome.     

荒漠草原4种典型植物群落枯落物分解速率及影响因素

测定荒漠草原甘草、赖草、蒙古冰草以及黑沙蒿等植物群落枯落物分解过程中质量损失量分析荒漠草原枯落物分解速,同时通过枯落物自身化学成份、含水率的测定,结合气候因子进行偏相关分析,探讨荒漠草原枯落物分解的影响因素。结果表明:荒漠草原4种植物群落枯落物的质量累积损失率随分解时间的延长而增加,但枯落物分解的质量损失量与时间并不呈线性相关;4种群落枯落物质量损失量大小依次均为:甘草群落赖草群落蒙古冰草群落黑沙蒿群落;荒漠草原枯落物分解采用单指数衰减的Olson模型拟合效果较好,4种植物群落中甘草群落枯落物分解最快,黑沙蒿群落分解最慢;蒙古冰草、赖草和甘草群落枯落物中N、P、K的含量显著高于黑沙蒿群落,但是C、木质素、纤维素、C/N、木质素/N和纤维素/N值则显著低于黑沙蒿群落枯落物,蒙古冰草群落、甘草群落、赖草群落和黑沙蒿群落4种群落枯落物分解速率(k)与枯落物初始N、P、K含量均呈显著正相关;偏相关分析表明,4种植物群落枯落物分解速率与降雨量、枯落物自身含水量的偏相关系数达显著水平,其余因子偏相关系数均未达显著水平。结合上述研究可以确定荒漠草原枯落物分解50%所需时间为2—5a,分解95%需8—24a。     

Dry weight loss and colonization of plant litter by macroinvertebrates: plant species and lake types compared

Rates of dry weight loss and of colonization of litter samples of different plants by macro-invertebrates were estimated using litter bags submerged in different types of lake. Samples of Carex rostrata litter were submerged for 3 months, in summer, in two lakes, one with an acidic and nutrient-poor water and one with near-neutral and nutrient-rich water. More plant litter remained (75%) in the former lake than in the latter (51–62%, varying with the mesh size of the litter bags used). The biomass of invertebrates in the litter samples from the acid lake was about 10 times less than in those from the near-neutral. Oligochaeta and Chironomidae were the dominant taxa found in the litter bags from the acidic lake, whereas Hirudinea and Isopoda predominated in those from the near-neutral lake.Litter samples from four different plants (Carex rostrata, Typha latifolia, Triticum sp. and Phleum pratense) were immersed in litter bags in another near-neutral lake further south for 12 months. When recovered, about 45% of the Triticum litter remained compared with only 7% of the Phleum. The values for Carex and Typha fell between these two extremes. When expressed as per g dry weight of residual plant litter, the greatest biomass of invertebrates was recorded in the Phleum litter samples and the least in the Typha samples. The faunal composition of the invertebrates in the different types of litter was approximately the same, with Isopoda and Hirudinea predominating.     

Litter Decomposition in a California Annual Grassland: Interactions Between Photodegradation and Litter Layer Thickness

In annual grasslands that experience a mediterranean-type climate, the synchrony between plant senescence and peak solar radiation over summer results in high litter sun exposure. We examined the decomposition of both shaded and sun-exposed litter over summer and inferred the effects of photodegradation from changes in mass loss and litter chemistry. The carry-over effects of summer litter exposure on wet season decomposition were also assessed, and the attenuation of photodegradation with litter layer thickness was used to estimate the proportion of grass litter lignin susceptible to photodegradation under different treatments of a factorial global change experiment. Over summer, mass loss from grass and forb litter exposed to ambient sunlight ranged from 8% to 10%, whereas lignin decreased in grass litter by approximately 20%. After one year of decomposition, mass losses from grass leaves exposed to sunlight over summer were more than double the mass losses from summer-shaded leaves. When shade litter layer thickness was varied, mass losses over summer for all treatments were also approximately 8%; however, lignin decreased significantly only in the low shade treatments (0–64 g m⁻² of shade litter). Aboveground production of annual grasses nearly quadrupled in response to the combined effects of N addition, elevated atmospheric CO₂, increased precipitation and warming. The estimated proportion of grass litter lignin experiencing full photodegradation ranged from 100% under ambient conditions to 31–62% in plots receiving the combined global change treatments. These results reveal an important role of sun exposure over summer in accelerating litter decomposition in these grasslands and provide evidence that future changes in the quantity of litter deposition may modulate the influence of photodegradation integrated across the litter layer.