Decomposition of emergent macrophytes in a Wisconsin marsh

Loss of both dry weight and nutrients during decomposition was measured using litter bags, both in a natural marsh and in controlled experiments. At 348 days dry weight remains of Typha latifolia, Sparganium eurycarpum, Scirpus fluviatilis shoot litter in the marsh were 47.5, 26.9, 51.4% respectively, and for the rootrhizome litter were 59.1, 42.1, 27.8% (Scirpus > Sparganium > Typha). Under controlled conditions both temperature and type of water produced significant effect on dry weight loss of Typha leaves. Sterilization and antibiotics effectively inhibited the growth and activities of decomposers. Initial weight, N, P, Ca, and Mg losses resulted chiefly from leaching. These elements accumulated in spring and summer; N exhibited the highest accumulation. In the laboratory, N accumulation occurred within 15 days, as a result of microorganisms inhabiting the litter. Increase in P, Ca, Mg in later stages of decomposition were attributed to microorganisms, epiphytes, and precipitation from solution. High C : N ratios and relatively low P, Ca, Mg in original standing crop may be the cause of low herbivore consumption, whereas the relative increases in N, P, Ca, Mg in decomposed litter provide a more nutrient-rich substrate for detritivores. Much of the nutrient uptake in the annual cycle is via microbial and detritivore growth rather than by macrophyte producers.     

Decomposition and nutrient dynamics of marsh litter in the Sanjiang Plain, Northeast China

The litterbag technique was used to study the decomposition and nutrient dynamics of marsh litter in the four communities, Carex pseudocuraica (C.pa), C. lasiocarpa (C.la), Deyeuxia angustifolia (D.aa), and D. angustifolia-Shrub (D.aa-Srb), in Sanjiang Plain, Northeast China. Decomposition was divided into two periods in the first year, with the mass loss ranging from 11.7% to 31.4% of the initial mass during summer and autumn, accounting for more than 75% of the annual loss. The decomposition rates ranged from 0.000 612 to 0.000 945 d^-1 depending on the depth of the flooding and its duration, and differed significantly in each community. The litter decomposed faster in communities with deeper and perennial flooding than in those with shallow and seasonal flooding. The initial ratios of C:N and C:P were also different among the four litter types, but these differences had no impact on the decomposition rates, suggesting that the main factor influencing the decomposition rates of marsh litter was the flooding status rather than the litter quality. The N concentrations in C.pa and C.la almost continuously increased over time, with their final values being 2.8 and 2.4 times higher than the initial ones, respectively. However, the nutrient dynamics in D.aa and D.aa-Srb offered another pattern, sharply falling in the first month and then gradually rising, with the values at the end of the experiment being close to those at the beginning. The litter accumulated substantial amounts of N in C.pa and C.la, while net N release from the litter was observed in both D.aa and D.aa-Srb. The difference may be caused by microorganisms' demand for nutrition, and then limited by the C:N ratios of litter and the availability of nitrogen from the soil and marsh water. In contrast with N dynamics, P concentrations of all the litter types apparently decreased during the first month, and then continued to decline in C.pa, remained constant in C.la and D.aa and increased slightly in D.aa-Srb. At the end of the experiment, the P concentrations decreased, respectively, by 56%, -5%, 47% and 24% of the initial values of C.la, C. pa, D.aa and D.aa-Srb. The net P release was observed in all marsh litter over 480 days of decomposition and the intensity of the P release was different amongst communities, which may be regulated by ratios of initial C:P. The results suggested that in the marsh with the N limitation, litter tended to accumulate N and release P during decomposition and the intensity of accumulation or release was closely related to the initial C:N and C:P ratios.     

Decomposition of Spartina anglica,Elytrigia pungens and Halimione portulacoides in a Dutch salt marsh in association with faunal and habitat influences

Decomposition of Spartina anglica, Elytrigia pungens and Halimione portulacoides was studied for 20.5 months in situ in two habitats on a salt marsh in The SW Netherlands. Litter bags of three different mesh sizes were used to exclude meio- and/or macrofauna. The middle-marsh habitat was flooded more frequently than the plant-debris habitat in the highest marsh zone. Decomposition of the three species followed an exponential pattern of decay: instantaneous decay rates varied from 0.0026 to 0.0054 per day. Decay rates were significantly influenced by habitat factors and fauna, while there was a significant interaction between plant species and habitat. In case of a significant meio- and/or macrofauna effect, this became noticeable 12–16 weeks after the start of decomposition and resulted in a difference of 5–10% ash-free dry weight remaining after 20.5 months. Nematodes were the dominant microfaunal group in the plant litter. Densities were influenced by habitat conditions but not by resource quality, season and meio- and/or macrofauna. Only initial C/N and C/P ratios were correlated with differences in decomposition rates between the plant species. During the later stages of decomposition N and P concentrations of the plant litter were higher in the plant-debris habitat than in the middle-marsh habitat, probably as a result of fluctuating detritivores densities. The course of the decomposition process differed per plant species and per habitat. The results of this study underline the importance of knowledge of long-term decomposition rates.     

Wetland plant decomposition under different nutrient conditions: what is more important, litter quality or site quality?

Plants in nutrient poor environments are often characterized by high nutrient resorption resulting in poor litter quality and, consequently, slow decomposition. We used oligotrophic, P-limited herbaceous wetlands of northern Belize as a model system, on which to document and explain how changes in nutrient content along a salinity gradient affect decomposition rates of macrophytes. In 2001 we established a nutrient addition experiment (P, N, and N&P) in 15 marshes of a wide range of water conductivities (200–6000 μS), dominated by Eleocharis spp. To determine what is more important for decomposition, the initial litter quality, or site differences, we used reciprocal litter placement and cellulose decomposition assay in a combined “site quality” and “litter quality” experiment. Our prediction of the positive effects of P-enrichment on decomposition rate due to both the quality of litter and the site was confirmed. The site effect was stronger than the litter quality although both were highly significant. Strong site quality effect was apparently the result of more active decomposer community in P-enriched plots as supported by finding of higher microbial biomass in litter decomposing there. The strong effect of site quality on decomposition was further confirmed by the cellulose assay. The cellulose decomposition was significantly slower at high salinity sites indicating lower decomposer microbial activity. Litter nutrient N and P content and nutrient ratios were well correlated with decomposition with the best fit found for log C/P. At C/P mass ratio of >4000 decomposition processes were extremely slow. We hypothesize that in a long run, the increased decomposition will compensate the increase in primary production resulting from increased nutrient loading and there will be no differences in accumulation of organic material between the controls and nutrient enriched plots.     

Effect of legume understorey on decomposition and nutrient content of eucalypt forest litter

Summary In Jarrah (Eucalyptus marginata Donn ex Sm.) forest of south-western Australia dense germination and regeneration of the native legumeAcacia Pulchella R. Br. can occur following moderate to high intensity fire. The effect of this legume understorey on rate of decomposition and change in nutrient content ofE. marginata litter was investigated using the mesh bag techniques and by examining four components of forest floor litter representing increasing stages of decomposition. E. marginata leaf litter confined in mesh bags lost 37% of its initial dry weight in the first 8 months on the forest floor and 44% of its initial dry weight after 20 months. During this period weight loss was similar for leaf litter located in forest without legume understorey and for leaf litter placed under dense stands ofA. pulchella. MixingA. pulchella litter withE. marginata litter had no significant effect on rate ofE. marginata litter breakdown. The presence of understorey vegetation had a marked effect on chemical composition of decomposingE. marginata leaves. After 8 and 20 months exposure on the forest floor, leaf litter in mesh bags placed underA. pulchella understorey had significantly (P<0.001) higher concentration and contained significantly (P<0.001) greater amounts of N, P, K, S, Ca and Mg than leaf litter placed in areas without legume understorey. This effect was particularly marked for N and P. In forest without legume understorey the amounts of these two nutrients inE. marginata leaf litter changed little during the first 20 months of decomposition, but forE. marginata leaf litter in mesh bags underA. pulchella there were absolute gains of up to 68% in the amount of N and 109% in the amount of P during this period. This represents accumulation of N and P from sources outside the litter bags. The concentration of N, P, S, Ca and Mg were higher at each of the four stages of decomposition in eucalypt leaf litter collected from the forest floor beneathA. pulchella compared to eucalypt leaf litter collected in forest without understorey. Concentrations of N, P and S increased with stage of decomposition. Levels of these three nutrients in eucalypt litter from under the legume were 1.5 to 2.9 fold higher than in the same component of litter from forest without understorey. The effect of legume understorey on nutrient concentrations in the forest floor and on Cielement ratios in decomposing litter is discussed in relation to long term rates of litter breakdown and net mineralisation of litter nutrients.     

Effects of sediment-borne nutrient and litter quality on macrophyte decomposition and nutrient release

Macrophyte decomposition is a critical process that affects carbon and nutrient cycling, and energy flow, although the majority of the details involved in the process remain unclear. For the present study, a litter bag experiment was conducted to investigate the effects of sediment-borne nutrient and litter quality on the decomposition rates and nutrient release of four macrophyte life forms (emergent macrophyte: Phragmites australis, free-floating macrophyte: Hydrocharis dubia, floating-leaved macrophyte: Nymphoides peltata, submerged macrophyte: Ceratophyllum demersum), and a species mixture. Our results indicated that litter quality significantly influenced macrophyte decomposition and nutrient release. High-quality litter species (high initial nitrogen and phosphorus contents, as well as low C:N, C:P, and N:P ratios) decomposed more rapidly than low-quality litter species, and the initial C:N and C:P ratios, rather than the initial N and P contents, were effective indicators of the decomposition rate of macrophytes. Sediment-borne nutrients had little effect on the decomposition rate, yet a strong effect on the release of N and P, although the interactions between litter quality and sediment-borne nutrients significantly affected the decomposition rate. Three-way ANOVA analysis revealed that the litter quality imparted a more potent effect on the macrophyte decomposition rate and release of N and P than sediment-borne nutrients. These results implied that litter quality interacts with sediment-borne nutrients and may control macrophyte decomposition in shallow lakes.     

Decomposition and nutrient dynamics of marsh litter in the Sanjiang Plain, Northeast China

The litterbag technique was used to study the decomposition and nutrient dynamics of marsh litter in the four communities, Carex pseudocuraica (C.pa), C. lasiocarpa (C.la), Deyeuxia angustifolia (D.aa), and D. angustifolia-Shrub (D.aa-Srb), in Sanjiang Plain, Northeast China. Decomposition was divided into two periods in the first year, with the mass loss ranging from 11.7% to 31.4% of the initial mass during summer and autumn, accounting for more than 75% of the annual loss. The decomposition rates ranged from 0.000 612 to 0.000 945 d^?1 depending on the depth of the flooding and its duration, and differed significantly in each community. The litter decomposed faster in communities with deeper and perennial flooding than in those with shallow and seasonal flooding. The initial ratios of C:N and C:P were also different among the four litter types, but these differences had no impact on the decomposition rates, suggesting that the main factor influencing the decomposition rates of marsh litter was the flooding status rather than the litter quality. The N concentrations in C.pa and C.la almost continuously increased over time, with their final values being 2.8 and 2.4 times higher than the initial ones, respectively. However, the nutrient dynamics in D.aa and D.aa-Srb offered another pattern, sharply falling in the first month and then gradually rising, with the values at the end of the experiment being close to those at the beginning. The litter accumulated substantial amounts of N in C.pa and C.la, while net N release from the litter was observed in both D.aa and D.aa-Srb. The difference may be caused by microorganisms' demand for nutrition, and then limited by the C:N ratios of litter and the availability of nitrogen from the soil and marsh water. In contrast with N dynamics, P concentrations of all the litter types apparently decreased during the first month, and then continued to decline in C.pa, remained constant in C.la and D.aa and increased slightly in D.aa-Srb. At the end of the experiment, the P concentrations decreased, respectively, by 56%, ?5%, 47% and 24% of the initial values of C.la, C. pa, D.aa and D.aa-Srb. The net P release was observed in all marsh litter over 480 days of decomposition and the intensity of the P release was different amongst communities, which may be regulated by ratios of initial C:P. The results suggested that in the marsh with the N limitation, litter tended to accumulate N and release P during decomposition and the intensity of accumulation or release was closely related to the initial C:N and C:P ratios.     

闽江河口湿地植物枯落物立枯和倒伏分解主要元素动态

采用分解袋法,对闽江河口湿地2种挺水植物——芦苇(Phragmites australis)和互花米草(Spartina alterniflora)花和叶枯落物的立枯和倒伏分解过程及C、N、P元素动态进行研究。结果表明:(1)立枯分解是2种湿地盐沼植物重要的分解阶段,干物质损失率在13.26%—31.89%之间。多项式模型能较好描述2种植物花和叶的枯落物分解残留率动态。(2)立枯分解阶段,芦苇花和叶的C含量主要为波动下降,互花米草较为稳定;倒伏阶段后期,2种植物都以升高为主。立枯分解阶段2种植物枯落物N含量略有下降,而倒伏阶段逐渐上升。分解过程中枯落物P含量的波动较大。(3)2种植物花和叶C、N的NAI值在分解过程中<100%。芦苇的花和叶中P的NAI值在立枯和倒伏分解阶段都经历了明显下降和升高的过程,而互花米草在立枯阶段变化不大,倒伏阶段下降较为明显。(4)与芦苇相比,互花米草的花和叶枯落物C库较高,N库较低,P库差异不大。     

Tree Species Effect on Litter Decomposition and Nutrient Release in Mediterranean Oak Forests Changes Over Time

Tree species can affect the decomposition process through the quality of their leaf fall and through the species-specific conditions that they generate in their environment. We compared the relative importance of these effects in a 2-year experiment. Litterbags containing leaf litter of the winter-deciduous Quercus canariensis, the evergreen Q. suber and mixed litter were incubated beneath distinct plant covers. We measured litter carbon loss, 9 macro- and micronutrients and 18 soil chemical, physical and biological parameters of the incubation environment. Tree species affected decay dynamics through their litter quality and, to a lesser extent, through the induced environmental conditions. The deciduous litter showed a faster initial decomposition but left a larger fraction of slow decomposable biomass compared with the perennial litter; in contrast the deciduous environment impeded early decomposition while promoting further carbon loss in the latter decay stages. The interaction of these effects led to a negative litter–environment interaction contradicting the home-field advantage hypothesis. Leaf litter N, Ca and Mn as well as soil N, P and soil moisture were the best predictors for decomposition rates. Litter N and Ca exerted counteractive effects in early versus late decay stages; Mn was the best predictor for the decomposition limit value, that is, the fraction of slowly decomposable biomass at the later stage of decomposition; P and soil moisture showed a constant and positive relation with carbon loss. The deciduous oak litter had a higher initial nutrient content and released its nutrients faster and in a higher proportion than the perennial oak litter, significantly increasing soil fertility beneath its canopy. Our findings provide further insights into the factors that control the early and late stages of the decomposition process and reveal potential mechanisms underlying tree species influence on litter decay rate, carbon accumulation and nutrient cycling.     

Functional Group Identity Does not Predict Invader Impacts: Differential Effects of Nitrogen-fixing Exotic Plants on Ecosystem Function

The introduction of exotic plants can have large impacts on ecosystem functions such as soil nutrient cycling. Since these impacts result from differences in traits between the exotic and resident species, novel physiological traits such as N cycling may cause large alterations in ecosystem function. It is unclear, however, whether all members of a given functional group will have the same ecosystem effects. Here we look at a within functional group comparison to test whether an annual (Lupinus luteus) and a perennial (Acacia saligna) N-fixing exotic species cause the same effects on soil N cycling in the fynbos vegetation of South Africa. We measured litterfall quantity and quality, and soil total nitrogen and organic matter for each vegetation type as well. Available nitrogen was quantified using ion exchange resin bags monthly for 1 year. We used microcosms to evaluate litter decomposition. Although both exotic species increased the available nitrogen in the soil, only Acacia increased the total soil N and organic matter. This could be explained by the slow decomposition of Acacia litter in the microcosm study, despite the fact that Acacia and Lupinus litter contained equivalent N concentrations. Presumably, low carbon quality of Acacia litter slows its decomposition in soil, resulting in retention of organic nitrogen in Acacia stands after clearing for restoration purposes. The differences in long term impacts of these annual and perennial species highlight the fact that not all N-fixing exotic species exert equivalent impacts. Ecologists should consider multiple traits rather than broadly defined functional groups alone when predicting invader impacts.     

Leaf litter decomposition and nutrient dynamics in a subtropical forest after typhoon disturbance

Decomposition of typhoon-generated and normal leaf litter and their release patterns for eight nutrient elements were investigated over 3 yr using the litterbag technique in a subtropical evergreen broad-leaved forest on Okinawa Island, Japan. Two common tree species, Castanopsis sieboldii and Schima wallichii, representative of the vegetation and differing in their foliar traits, were selected. The elements analyzed were N, P, K, Ca, Mg, Na, Al, Fe and Mn. Dry mass loss at the end of study varied in the order: typhoon green leaves > typhoon yellow leaves > normal leaves falling for both species. For the same litter type, Schima decomposed faster than Castanopsis. Dry mass remaining after 2 yr of decomposition was positively correlated with initial C:N and C:P ratios. There was a wide range in patterns of nutrient concentration, from a net accumulation to a rapid loss in decomposition. Leaf litter generated by typhoons decomposed more rapidly than did the normal litter, with rapid losses for N and P. Analysis of initial quality for the different litter types showed that the C:P ratios were extremely high (range 896 – 2467) but the P:N ratios were < 0.05 (range 0.02 – 0.04), indicating a likely P-limitation for this forest. On average 32% less N and 60% less P was retranslocated from the typhoon-generated green leaves than from the normal litter for the two species, Castanopsis and Schima. An estimated 2.13 g m^–2 yr^–1 more N and 0.07 g m^–2 yr^–1 more P was transferred to the soil as result of typhoon disturbances, which were as high as 52% of N and 74% of P inputted from leaf litter annually in a normal year. Typhoon-driven maintenance of rapid P cycling appears to be an important mechanism by which growth of this Okinawan subtropical forest is maintained.     

Nutrient dynamics and lignocellulose degradation in decomposing Quercus serrata leaf litter

The litter mass loss, concentration and mass of some major nutrient elements, degradation of lignin and cellulose in decomposing Quercus serrata Murray leaf litter were monitored for 3 years using the litterbag method. The mobility of elements during the course of the study was in the order of: K > P > C > Mg > Ca > N. Three patterns of nutrient dynamics were observed: (i) concentration increased while mass decreased (N, Mg and Ca); (ii) concentration and nutrient mass decreased (K and C); and (iii) both concentration and mass had fluctuated (P). The C to element ratio tended to increase as the element was released, and decreased as the element was retained. Nitrogen mobility in relation to carbon was characterized by three phases: (i) initial release; (ii) accumulation and (iii) final release. The decay rate (k) calculated from 0–6 months period was overestimated for an average annual rate while those of 0–36 months fit the negative single exponential model (Adj. r² = 0.99) better than shorter periods. For lignin, the concentration had increased then decreased but tended to stabilize after 1 year while the lignin mass had continuously decreased throughout the study period. During the first 9 months, both the concentrations and mass of cellulose had fluctuated but declined thereafter. The amounts of N had initially increased but declined after 1 year; P had fluctuated while K, Ca, Mg and C had decreased throughout the study. N and C/N ratio exerted strong influence on mass loss during the first24 months but the influence of lignin emerged after 24 months.     

Litter production,nutrient recycling and litter accumulation inPinus caribaea Morelet var.hondurensis stands in the northern Guniea savanna of Nigeria

Summary The seasonal pattern and quantity of litterfall were studied during a two-year period in two unthinned stands ofPinus caribaea Morelet var. hondurensis Barr. and Golf. in Nigeria. Although pine needles were cast continuously throughout the year, the peak period of litterfall occurred in the dry months of March and April. Mean values of annual litterfall were 3068 and 3665 kg/ha in the two stands aged 7–9 and 9–11 years respectively. Nutrient returns in litterfall in the stands had mean values of 15.0, 0.6, 17.3, 18.2 and 6.3 kg/ha of N, P, K, Ca and Mg respectively. Comparatively low amounts of N and P returned in litterfall were attributed to soil deficiencies of the two elements.Measurements of ground litter showed considerable dry matter accumulation (11378 kg/ha) in the litter layers. Estimates of litter decomposition rate and recycling time showed that it would take 3 to 4 years for the organic matter in annual litterfall to decompose completely as contrasted to about 2 to 5 months often reported under mixed nautral savanna vegetation in the same climatic environment. Similar estimates of nutrient recycling time also showed that between 2 to 4 years were required to mineralize nutrient elements in the annual litterfall; the relative mobilities of the elements were in the order K>Mg>P>NCa.     

Hydrologic control of litter decomposition in seasonally flooded prairie marshes

The effect of seasonal inundation on the decomposition of emergent macrophyte litter (Scolochloa festucacea) was examined under experimental flooding regimes in a northern prairie marsh. Stem and leaf litter was subjected to six aboveground inundation treatments (ranging from never flooded to flooded April through October) and two belowground treatments (nonflooded and flooded April to August). Flooding increased the rate of mass loss from litter aboveground but retarded decay belowground. Aboveground, N concentration decreased and subsequently increased earlier in the longer flooded treatments, indicating that flooding decreased the time that litter remained in the leaching and immobilization phases of decay. Belowground, both flooded and nonflooded litter showed an initial rapid loss of N, but concentration and percent of original N remaining were greater in the nonflooded marsh throughout the first year. This suggested that more N was immobilized on litter under the nonflooded, more oxidizing soil conditions. Both N concentration and percent N remaining of belowground litter were greater in the flooded than the nonflooded marsh the second year, suggesting that N immobilization was enhanced after water-level drawdown. These results suggest different mechanisms by which flooding affects decomposition in different wetland environments. On the soil surface where oxygen is readily available, flooding accelerates decomposition by increasing moisture. Belowground, flooding creates anoxic conditions that slow decay. The typical hydrologic pattern in seasonally flooded prairie marshes of spring flooding followed by water-level drawdown in summer may maximize system decomposition rates by allowing rapid decomposition aboveground in standing water and by annually alleviating soil anoxia.     

不同林龄杉木人工林植物-凋落叶-土壤C、N、P化学计量特征及互作关系

以贵州8年、16年、28年生杉木人工林为研究对象,分析植物-凋落叶-土壤的C、N、P化学计量特征及其内在联系,探讨林龄对杉木人工林生态化学计量的影响,为杉木人工林可持续经营提供参考。结果表明:(1)杉木人工林植物-凋落叶-土壤均呈高C低N、P元素格局,两两组分间差异显著(P0.05);成熟叶C/N(38.58)、C/P(376.67)偏低,其养分利用效率较低;与成熟叶相比,凋落叶N、P偏低,C/N、C/P偏高;土壤C/P、N/P偏低,C/N较高,说明土壤P素分解较快而N保存较好,反映了凋落叶分解不利。(2)成熟叶C、P以及根、凋落叶、土壤的C、N、P、C/N、C/P、N/P均受林龄的显著影响;从8年到28年,C、N、P含量在植物体呈先增后减趋势,而在土壤中相反,呈先减后增趋势,但在凋落物中C、P显著减小,且C/P,N/P显著增加,反映杉木林早期对养分需求旺盛,随年龄增大需求减小,凋落物分解受制于P素,加剧中幼期杉木生态系统养分供需矛盾。(3)成熟叶与凋落叶N、C/N、N/P之间显著正相关,凋落叶养分源自成熟叶;成熟叶重吸收率P(0.518—0.645)N(0.292—0.488),即对P的利用效率高于N。凋落叶与土壤C、C/N之间显著负相关,表明土壤C、N来源于凋落叶分解,但凋落叶分解缓慢,导致大量元素滞留于凋落叶,土壤损耗元素得不到补给,两者间养分循环缓慢。土壤与根C、P、C/N、C/P、N/P之间均显著正相关,土壤与成熟叶的C、N、P均不相关,表明土壤养分是杉木生长养分的主要来源,但土壤C、N、P含量对成熟叶C、N、P含量影响不大。     

马尾松人工林林窗大小对四种凋落叶质量损失和养分释放的影响

马尾松人工林乔木层植物凋落物的分解对林地养分平衡和系统物质循环具有重要意义,并可能受不同大小林窗下微环境差异的影响。采用凋落物袋分解法,以马尾松(Pinus massoniana)人工林人为砍伐形成的7个不同大小林窗(G1:100 m~2、G2:225 m~2、G3:400 m~2、C4:625 m~2、G5:900 m~2、G6:1225m~2、G7:1600 m~2)为研究对象,林下(G0)为对照,研究林窗大小对红椿(Toona ciliata)、桢楠(Phoebe zhennan)、香樟(Cinnamomum camphora)和马尾松4种乡土树种凋落叶质量损失及养分释放的影响。结果显示:1)林窗大小(G0-G7)显著影响林窗中心放置的红椿和桢楠凋落叶N和P释放率、香樟凋落叶失重率和N、P、K释放率以及马尾松凋落叶P和K释放率。相对于林下,中小型林窗(G1-G4)的凋落叶失重率和N、P释放率明显较大,而大型林窗(G6-G7)的凋落叶K释放率明显较大。2)林窗内放置位置显著影响红椿、桢楠和马尾松凋落叶的K释放率及香樟凋落叶的P释放率。红椿和桢楠的凋落叶K释放率从林窗中心到边缘显著减少,而马尾松凋落叶K释放率及香樟P释放率从林窗中心到边缘显著增加。3)4种凋落叶类型中红椿凋落叶分解最快,其分解50%和95%所需时间分别为5.29和23.14个月。上述结果表明,林窗大小和林窗内位置对凋落物质量损失及其养分释放具有显著影响,但影响大小及趋势随物种初始基质质量的差异具有明显变化,研究结果为亚热带低山丘陵区马尾松人工低效林的科学经营及管理提高了一定的科学依据。     

Decomposition and nutrient release in halophytes of a Mediterranean salt marsh

This study dealt with the decomposition and nutrient release from the halophytes Atriplex portulacoides, Arthrocnemum macrostachyum, Limoniastrum monopetalum, and Spartina densiflora, the dominant species in the Castro Marim salt marsh, Portugal. Environmental effects on decomposition were also assessed. The study was carried out for one year using the in situ litterbag technique. S. densiflora showed a lower decomposition rate (k = 0.003 day⁻¹) than the other study species (k = 0.005-0.009). Study species showed similar decomposition patterns, that is, the weight loss mostly occurred during the autumn-winter period (study beginning in November). This indicates that temperature in this period did not hamper the decomposition process. The decomposition rate was positively affected by the initial N concentration (r² = 0.87, P < 0.05) and negatively by the C:N ratio (r² = 0.86, P < 0.05) in decomposing materials. At the end of the study, S. densiflora and L. monopetalum, the species with lower initial N concentrations, retained much higher proportion of initial N (89-109%) than the others (5-14%). Also, S. densiflora with the lowest P concentration retained higher proportion of initial P (48%) than the others (5-20%). Release of K and Mg were also slower from S. densiflora and was associated with their initial low concentration in this species. The lowest Mn release was observed from A. macrostachyum and also in relation to the lowest initial concentration. Our study supports the hypothesis that decomposition patterns of marsh species are mostly associated with differences regarding their morphology and chemical composition. Given the higher resistance of S. densiflora to decomposition, its progressive spreading may result in accumulation of organic detritus overtime in invaded salt marshes.     

Morphology and response of roots of pasture species to phosphorus and nitrogen nutrition

The root morphology of ten temperate pasture species (four annual grasses, four perennial grasses and two annual dicots) was compared and their responses to P and N deficiency were characterised. Root morphologies differed markedly; some species had relatively fine and extensive root systems (Vulpia spp., Holcus lanatus L. and Lolium rigidum Gaudin), whilst others had relatively thick and small root systems (Trifolium subterraneum L. and Phalaris aquatica L.). Most species increased the proportion of dry matter allocated to the root system at low P and N, compared with that at optimal nutrient supply. Most species also decreased root diameter and increased specific root length in response to P deficiency. Only some of the species responded to N deficiency in this way. Root morphology was important for the acquisition of P, a nutrient for which supply to the plant depends on root exploration of soil and on diffusion to the root surface. Species with fine, extensive root systems had low external P requirements for maximum growth and those with thick, small root systems generally had high external P requirements. These intrinsic root characteristics were more important determinants of P requirement than changes in root morphology in response to P deficiency. Species with different N requirements could not be distinguished clearly by their root morphological attributes or their response to N deficiency, presumably because mass flow is relatively more important for N supply to roots in soil.Section editor: H. Lambers     

Nutrients and mass in litter and top soil of ten tropical tree plantations

The importance of litter to nutrient and organic matter storage and the possible influence of species selection on soil fertility in ten stands each consisting of a separate tree species were examined in this study. The plantations had been grown under similar conditions in an arboretum in the Luquillo Experimental Forest, Puerto Rico. The species involved were: Anthocephalus chinensis, Eucalyptus × patentinervis, E. saligna, Hernandia sonora, Hibiscus elatus, Khaya nyasica, Pinus caribaea var. hondurensis, P. elliottii var. densa, Swietenia macrophylla, and Terminalia ivorensis. After 26 yr, litter mass ranged from 5 mg ha^-1 in the H. sonora stand to 27.2 Mg ha^-1 in the P. caribaea stand. Nutrients in the litter (N, P, K, Ca, and Mg) also varied widely, but stands were ranked in different order when ranked by nutrients in the litter than then ranked according to accumulation of mass. Only E. saligna and A. chinensis stands were ranked similarly in accumulation of both nutrients and mass, and the stand of H. elatus was ranked higher with respect to nutrient accumulation than to accumulation of mass. The nutrient concentration in standing leaf litter generally increased in the order of recently fallen <old intact< fragmented. Nutrient concentration of standing leaf litter appears to increase with age and depth in the litter layer. The amount of nutrients stored in the litter compartment of these plantations was in the same order of magnitude as the quantity of available nutrients in the top 10-cm of mineral soil. Total litter mass was negatively correlated with the mass-weighted concentration of N, K, and Mg. The same relationship was found for Ca in the leaf litter and N in the fine wood litter compartments. In some stands (notably P. caribaea, P. elliottii, and E. saligna), leaf litter derived from species other than the species planted in that particular stand had higher nutrient concentration than leaf litter from the planted species. Soils of the 10 stands were classified in the same soil series and had similar texture (clay soils). However, significantly different chemical characteristics were found. Results obtained by analysis of covariance and by limiting comparisons to adjacent stands with similar soil texture, indicate that different species have had different influences on the concentration of available nutrients in soil.     

Holcus lanatus invasion slows decomposition through its interaction with a macroinvertebrate detritivore, Porcellio scaber

Detritus based food webs may mediate the impacts of invasive species on ecosystem processes. Holcus lanatus (L.) is an invasive perennial grass that is rapidly spreading in the coastal prairie of California. We used litterbags to determine if H. lanatus altered the rate of litter decomposition through its interaction with the dominant macroinvertebrate detritivore at our study site, the isopod Porcellio scaber (Latreille). Over the course of a year, H. lanatus litter loss was 15% less than annual grasses, a difference that was directly attributable to P. scaber. Although there was no effect of isopods on litter loss during the winter, when most decomposition occurred, isopods had substantial effects on litter loss of annual grasses during the summer. P. scaber had no effect on litter loss of H. lanatus litter. Our findings suggest that H. lanatus invasion slows decomposition in the coastal prairie because it is unusually refractory to an important detritivore. The decreased decomposition of H. lanatus litter, along with increased production during the growing season, lead to a doubling of the end-of-season litter biomass where H. lanatus had invaded.