Decay rate and substrate quality of fine roots and foliage of two tropical tree species in the Luquillo Experimental Forest,Puerto Rico

Decomposition rates, initial chemical composition, and the relationship between initial chemistry and mass loss of fine roots and foliage were determined for two woody tropical species, Prestoea montana and Dacryodes excelsa, over a gradient of sites in two watersheds in the Luquillo Experimental Forest, Puerto Rico. At all locations, fine roots decayed significantly more slowly than foliage during the initial 6 months.Substrate quality of the initial tissue showed marked differences between roots and foliage when using cell wall chemistry, secondary chemistry and total elemental analysis as indices. Quantity of acid detergent fiber (ADF) (non-digestible cell wall fiber) and lignin content were higher for roots than leaves: D. excelsa roots had 55.3% ADF and 28.7% lignin while leaves had 36.2% ADF and 11.8% lignin; P. montana roots had 68.0% ADF and 26.8% lignin while leaves had 48.5% ADF and 16.1% lignin. Aluminum concentrations were higher in fine roots (843 mg kg^–1 in D. excelsa, 1500 mg kg^–1 in P. montana) than leaves (244 mg kg^–1 in D. excelsa, 422 mg kg^–1 in P. montana), while calcium concentrations were higher in foliage (5.5 mg g^–1 in D. excelsa, 7.8 mg g^–1 in P. montana) than roots (3.4 mg g^–1 in D. excelsa, 3.1 mg g^–1 in P. montana). Nitrogen did not show any trend with tissue or species type. A linear model between mass remaining after 6 months and initial tissue chemistry could be developed only for calcium (r²=0.64).     

模拟氮沉降对杉木幼苗细根的生理生态影响

细根对氮沉降的生理生态响应将显著影响森林生态系统的生产力和碳吸存。为了揭示氮沉降对杉木细根的生理生态影响,对一年生杉木(Cunninghamia lanceolata)幼苗进行了模拟氮沉降试验,并测定施氮1年后杉木幼苗细根生物量、细根形态学特征(比根长、比表面积)、元素化学计量学指标(C、N、P、C/N、C/P、N/P)、细根代谢特征(细根比呼吸速率、非结构性碳水化合物)。结果表明:(1)杉木细根生物量随氮添加水平的升高而显著降低,尤其是0—1 mm细根生物量;细根比根长和比表面积随氮添加水平升高而显著增大。(2)氮添加后杉木细根C含量、C/N、C/P显著降低,高氮添加导致1—2 mm细根N含量和N/P显著升高,而低氮添加导致1—2 mm细根P含量显著升高、N/P显著降低,而0—1 mm细根的N、P含量则保持相对稳定。(3)氮添加后杉木细根比呼吸速率无显著变化,细根可溶性糖含量随氮添加增加而显著增加,而淀粉含量和NSC显著降低。综合以上结果表明:氮添加后用于细根形态构建的碳分配减少,这可能会减少土壤中有机碳的保留,0—1 mm细根的形态更易发生变化,但是其内部N、P养分含量相对更稳定以维持生理活动,细根NSC对氮添加的响应表明施氮可能导致细根受光合产物的限制。     

Rapid Degradation of Isolated Lignins by Phanerochaete chrysosporium

Phanerochaete chrysosporium degraded purified Kraft lignin, alkali-extracted and dioxane-extracted straw lignin, and lignosulfonates at a similar rate, producing small-molecular-weight (~1,000) soluble products which comprised 25 to 35% of the original lignins. At concentrations of 1 g of lignin liter⁻¹, 90 to 100% of the acid-insoluble Kraft, alkali straw, and dioxane straw lignins were degraded by 1 g of fungal mycelium liter⁻¹ within an active ligninolytic period of 2 to 3 days. Cultures with biomass concentrations as low as 0.16 g liter⁻¹ could also completely degrade 1 g of lignin liter⁻¹ during an active period of 6 to 8 days. The absorbance at 280 nm of 2 g of lignosulfonate liter⁻¹ increased during the first 3 days of incubation and decreased to 35% of the original value during the next 7 days. The capacity of 1 g of cells to degrade alkali-extracted straw lignin under optimized conditions was estimated to be as high as 1.0 g day⁻¹. This degradation occurred with a simultaneous glucose consumption rate of 1.0 g day⁻¹. When glucose or cellular energy resources were depleted, lignin degradation ceased. The ability of P. chrysosporium to degrade the various lignins in a similar manner and at very low biomass concentrations indicates that the enzymes responsible for lignin degradation are nonspecific.     

Growth and maintenance respiration of roots of clonal Eucalyptus cuttings: scaling to stand-level

Root respiration consumes an important part of the daily assimilated carbon but the magnitude of this component of forest net ecosystem exchange and its partitioning among the different energy demanding processes in roots are still poorly documented. 5-month old Eucalyptus cuttings were grown in a greenhouse in pot filled with coarse sand. They were fertilized with three different amounts of a slow-release fertilizer with the doses of 8, 24 and 48 g of nitrogen per plant. Root respiration was measured using an infrared gas analyser by perfusing air through the pot on 9 plants per treatment on three dates 14 days apart. Measure of root respiration of the three treatments over time was made in order to obtain a large range of growth and nutrient uptake. Root respiration normalized at 22°C ranged from 0.09 to 0.23 g_C d^?1 for the three treatments during all the experiment. It was well predicted with a model that includes root growth rate and root nitrogen content.The nitrogen related maintenance coefficient was negatively correlated to the root nitrogen concentration suggesting a decrease in protein turnover with increasing fertility. Growth rate of fine root in a virtual stand was simulated using age-related allometric equations and further used to estimate root respiration in the field. Simulated root respiration increased over time from 0.39 to 3.14 g_C m^?2 d^?1 between 6 and 126 months assuming a turnover of 2 yr^?1 for fine roots. The major fraction of simulated root respiration in the field (78–92%) was used for the maintenance of the existing biomass.     

Use of decreasing foliar carbon isotope discrimination during water limitation as a carbon tracer to study whole plant carbon allocation

Foliar carbon isotope discrimination (Δ) of C₃ plants decreases in water‐deficit situations as discrimination by the photosynthetic primary carboxylation reaction decreases. This diminished Δ in leaves under water deficit can be used as a tracer to study whole plant carbon allocation patterns. Carbon isotope composition (δ¹³C value) of leaf hot water extracts or leaf tissue sap represents a short‐term integral of leaf carbon isotope discrimination and thus represents the δ¹³C value of source carbon that may be distributed within a plant in water‐deficit situations. By plotting the δ¹³C values of source carbon against the δ¹³C values of sink tissues, such as roots or stems, it is possible to assess carbon allocation to and incorporation into sink organs in relation to already present biomass. This natural abundance labelling method has been tested in three independent experiments, a one‐year field study with the fruit tree species Ziziphus mauritiana and peach (Prunus persica), a medium‐term drought stress experiment with Ziziphus rotundifolia trees in the glasshouse, and a short‐term drought stress experiment with soybean (Glycine max). The data show that the natural abundance labelling method can be applied to qualitatively assess carbon allocation in drought‐stressed plants. Although it is not possible to estimate exact fluxes of assimilated carbon during water deficit the method represents an easy to use tool to study integrated plant adaptations to drought stress. In addition, it is a less laborious method that can be applied in field studies as well as in controlled experiments, with plants from any developmental stage.     

Soil fertility and fine root dynamics in response to 4 years of nutrient (N,P, K) fertilization in a lowland tropical moist forest,Panama

The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m^?2 year^?1), phosphorus (P as superphosphate, 5 g P m^?2 year^?1) and potassium (K as KCl, 5 g K m^?2 year^?1) within 38 ha of old‐growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg^?1), low concentrations of Bray‐extractable phosphate (PO₄ = 2.2 mg kg^?1), and modest concentrations of KCl‐extractable nitrate (NO₃ = 5.0 mg kg^?1) and KCl‐extractable ammonium (NH₄ = 15.5 mg kg^?1). Added N increased concentrations of KCl‐extractable NO₃ and acidified the soil by one pH unit. Added P increased concentrations of Bray‐extractable PO₄ and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m^?2. The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species‐rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.     

Influence of CO2 enrichment and nitrogen fertilization on tissue chemistry and carbon allocation in longleaf pine seedlings

One-year old, nursery-grown longleaf pine (Pinus palustris Mill.) seedlings were grown in 45-L pots containing a coarse sandy medium and were exposed to two concentrations of atmospheric CO₂ (365 or 720 mol^-1) and two levels of nitrogen (N) fertility (40 or 400 kg N ha^-1 yr^-1) within open top chambers for 20 months. At harvest, needles, stems, coarse roots, and fine roots were separated and weighed. Subsamples of each tissue were frozen in liquid N, lyophilized at -50°C, and ground to pass a 0.2 mm sieve. Tissue samples were analyzed for carbon (C), N, nonpolar extractives (fats, waxes, and oils = FWO), nonstructural carbohydrates (total sugars and starch), and structural carbohydrates (cellulose, lignin, and tannins). Increased dry weights of each tissue were observed under elevated CO₂ and with high N; however, main effects of CO₂ were significant only on belowground tissues. The high N fertility tended to result in increased partitioning of biomass aboveground, resulting in significantly lower root to shoot ratios. Elevated CO₂ did not affect biomass allocation among tissues. Both atmospheric CO₂ and N fertility tended to affect concentration of C compounds in belowground, more than aboveground, tissues. Elevated CO₂ resulted in lower concentrations of starch, cellulose, and lignin, but increased concentrations of FWO in root tissues. High N fertility increased the concentration of starch, cellulose, and tannins, but resulted in lower concentrations of lignin and FWO in roots. Differences between CO₂ concentrations tended to occur only with high N fertility. Atmospheric CO₂ did not affect allocation patterns for any compound; however the high N treatment tended to result in a lower percentage of sugars, cellulose, and lignin belowground.Joseph W. Jones     

Food resources of shredders and other benthic macroinvertebrates in relation to shading conditions in tropical Hong Kong streams

1. Gut content analyses (GCA) of benthic macroinvertebrates, supplemented by carbon and nitrogen stable isotope analyses (SIA), were used to determine the relative contribution of leaf litter and autochthonous food sources to consumer biomass in five shaded and five unshaded streams in tropical Hong Kong. 2. Only four obligate shredders and two facultative shredders were identified out of 58 morphospecies dissected. Non‐shredder taxa consumed little (<23% food eaten) coarse particulate organic matter (CPOM) in spite of its abundance in streams, and GCA revealed that fine particulate organic matter was the major food (25–99%) of most primary consumers. 3. Stable isotope analysis results were in general agreement with the findings of GCA, and confirmed that three of the four obligate shredders had a high dependence (55–78% of assimilated carbon) on CPOM. 4. Autochthonous energy sources were important in all streams: non‐shredding primary consumers examined, which accounted for 72% of total macroinvertebrate abundance in shaded streams, derived (on average) 61% of their biomass from autochthonous foods; the equivalent values for unshaded streams were 72% (abundance) and 71% (biomass).     

Distribution of assimilated carbon in plants and rhizosphere soil of basket willow (Salix viminalis L.)

Willow is often used in bio-energy plantations for its potential to function as a renewable energy source, but knowledge about its effect on soil carbon dynamics is limited. Therefore, we investigated the temporal variation in carbon dynamics in willow, focusing on below-ground allocation and sequestration to soil carbon pools. Basket willow plants (Salix viminalis L.) in their second year of growth were grown in pots in a greenhouse. At five times during the plants growth, namely 0, 1, 2, 3 and 4 months after breaking winter dormancy, a subset of the plants were continuously labelled with ¹⁴CO₂ in an ESPAS growth chamber for 28 days. After the labelling, the plants were harvested and separated into leaves, first and second year stems and roots. The soil was analysed for total C and ¹⁴C content as well as soil microbial biomass. Immediately after breaking dormancy, carbon stored in the first year stems was relocated to developing roots and leaves. Almost half the newly assimilated C was used for leaf development the first month of growth, dropping to below 15% in the older plants. Within the second month of growth, secondary growth of the stem became the largest carbon sink in the system, and remained so for the older age classes. Between 31 and 41% of the recovered ¹⁴C was allocated to below-ground pools. While the fraction of assimilated ¹⁴C in roots and root+soil respiration did not vary with plant age, the amount allocated to soil and soil microbial biomass increased in the older plants, indicating an increasing rhizodeposition. The total amount of soil microbial biomass was 30% larger in the oldest age class than in an unplanted control soil. The results demonstrate a close linkage between photosynthesis and below-ground carbon dynamics. Up to 13% of the microbial biomass consisted of carbon assimilated by the willows within the past 4 weeks, up to 11% of the recovered ¹⁴C was found as soil organic matter.     

Flow of photosynthesized carbon from rice plants into the paddy soil ecosystem at different stages of rice growth

To elucidate the flow of C assimilated by rice plants into soil C, soil biomass C, and emitted CH₄ at different rice growth stages, ¹³C pulse-labeling was conducted at the maximum-tiller-number (first experiment), booting (second experiment), and milky stages (third experiment) for potted rice grown outdoors under flooded conditions. The distribution of the assimilated C into each fraction was traced during a 14-day period. The atom-¹³C% excess of shoots was the highest just after the 6 h feeding of ¹³CO₂ and decreased until days 14, 7, and 3 in the first, second, and third experiments, respectively. Translocation of the assimilated C into roots was largest in the first experiment, 13%, while that into ears was more than 50% in the third experiment. The proportion of the rice-assimilated C recovered in soil organic matter increased with time after labeling and reached 3.4%, 3.0%, and 1.7% on day 14 in the first, second, and third experiments, respectively. Incorporation of the rice-assimilated C was faster into soil microbial biomass than into gross soil organic matter or the 0.5 M K₂SO₄-extractable fraction. Although the percent of labeled soil C that is in the microbial biomass on day 0 was much larger at the maximum-tiller-number stage (42%) than at the milky stage (5%), its variation among growth stages was small on day 14 (10 to 15%). The percent of the rice-assimilated C emitted as CH₄ during the 14-day period at the maximum-tiller-number, booting, and milky stages was 0.003%, 0.26%, and 0.30%, respectively.     

Plant growth, biomass partitioning and soil carbon formation in response to altered lignin biosynthesis in Populus tremuloides

We conducted a glasshouse mesocosm study that combined (13)C isotope techniques with wild-type and transgenic aspen (Populus tremuloides) in order to examine how altered lignin biosynthesis affects plant production and soil carbon formation. Our transgenic aspen lines expressed low stem lignin concentration but normal cellulose concentration, low lignin stem concentration with high cellulose concentration or an increased stem syringyl to guaiacyl lignin ratio. Large differences in stem lignin concentration observed across lines were not observed in leaves or fine roots. Nonetheless, low lignin lines accumulated 15-17% less root C and 33-43% less new soil C than the control line. Compared with the control line, transformed aspen expressing high syringyl lignin accumulated 30% less total plant C - a result of greatly reduced total leaf area - and 70% less new soil C. These findings suggest that altered stem lignin biosynthesis in Populus may have little effect on the chemistry of fine roots or leaves, but can still have large effects on plant growth, biomass partitioning and soil C formation.     

Anthropogenic N deposition alters soil organic matter biochemistry and microbial communities on decaying fine roots

Fine root litter is a primary source of soil organic matter (SOM), which is a globally important pool of C that is responsive to climate change. We previously established that ~20 years of experimental nitrogen (N) deposition has slowed fine root decay and increased the storage of soil carbon (C; +18%) across a widespread northern hardwood forest ecosystem. However, the microbial mechanisms that have directly slowed fine root decay are unknown. Here, we show that experimental N deposition has decreased the relative abundance of Agaricales fungi (?31%) and increased that of partially ligninolytic Actinobacteria (+24%) on decaying fine roots. Moreover, experimental N deposition has increased the relative abundance of lignin‐derived compounds residing in SOM (+53%), and this biochemical response is significantly related to shifts in both fungal and bacterial community composition. Specifically, the accumulation of lignin‐derived compounds in SOM is negatively related to the relative abundance of ligninolytic Mycena and Kuehneromyces fungi, and positively related to Microbacteriaceae. Our findings suggest that by altering the composition of microbial communities on decaying fine roots such that their capacity for lignin degradation is reduced, experimental N deposition has slowed fine root litter decay, and increased the contribution of lignin‐derived compounds from fine roots to SOM. The microbial responses we observed may explain widespread findings that anthropogenic N deposition increases soil C storage in terrestrial ecosystems. More broadly, our findings directly link composition to function in soil microbial communities, and implicate compositional shifts in mediating biogeochemical processes of global significance.     

Coexistence of a C4 grass and a leaf succulent shrub in an arid ecosystem. The relationship between rooting depth, water and nitrogen

Here we aim to demonstrate that in arid environments the competitive balance between species can be determined by niche separation with either nitrogen or water as the relevant niche axis. To do this we sampled roots <2 mm in diameter for 5 soil pits equidistant between two coexisting species, a shrub and a grass. Using stable carbon and nitrogen isotope ratios of fine roots we determine both photosynthetic pathway and rooting depth. We also examine the distribution of soil moisture and nitrogen relative to root biomass. Our results for root biomass and stable isotope ratios of fine roots demonstrate both niche separation and competition for resources. Root biomass is highest at the top of the profile where soil nitrogen is highest and soil moisture is lowest. We conclude that while there is competition for resources in the middle of the profile, competition is mitigated by photosynthetic pathway. The facultative CAM shrub grows whenever the soil at the surface is wet enough. The C₄ photosynthetic pathway of the grass is more nitrogen and water use efficient making it better adapted to the low nitrogen in the middle of the profile and low summer rainfall.     

模拟氮沉降对中亚热带杉木幼树根系生物量的影响

植物根系是全球陆地生态系统碳储量的重要组成部分,在全球生态系统碳循环中起着重要作用,日益加剧的氮沉降会影响根系生物量在空间和不同径级的分配,进而影响森林生态系统的生产力和土壤养分循环。以杉木幼树为研究对象,通过野外氮沉降模拟实验,研究氮沉降四年后对不同土层、不同径级根系生物量的影响。结果发现：（1）低氮和高氮处理总细根生物量较对照均无显著差异（P > 0.05）,高氮处理粗根生物量及总根系生物量较对照分别增加45%和40%（P < 0.05）;（2）与对照相比,施氮处理显著增加20-40 cm与40-60 cm土层细根和粗根生物量,且在低氮处理下,20-40 cm土层细根、粗根在总土层细根与粗根生物量的占比显著提高。（3）与对照相比,高氮处理显著增加了2-5 mm、5-10 mm及10-20 mm径级的根系生物量,低氮处理显著增加2-5 mm、5-10 mm径级根系生物量,且显著降低20-50 mm径级根系生物量。综上所述表明：氮沉降后杉木幼树通过增加较粗径级根系来增加对养分及水分的输送,同时通过增加深层根系生物量及其比例的策略来维持杉木幼树的快速生长;而根系生物量的增加,在一定程度上会增加根系碳源的输入,影响土壤碳循环过程。     

NH4 : NO3 nutrition influence on biomass productivity and root respiration of poplar and willow clones

Nitrogen fertilization often improves the yield of intensively managed, short‐rotation coppices. However, information of N nutrition form on the growth of common species and clones used for biomass production is limited. Thus, this study aims at evaluating N form effects on the growth of two Salicaceae clones. Cuttings of the poplar clone Max 4 (Populus maximovizcii × P. nigra) and the willow clone Inger (Salix triandra × S. viminialis) were fertilized in a pot experiment with four ratios of nitrate (NO₃^?) to ammonium (50%, 62.5%, 75% and 87.5% NO₃^? balanced with ammonium (NH₄⁺) to constant total N) for one growing season and under stable soil pH. Plants were harvested for analysis of biomass and morphology of leaves, stem and roots. Respiration of fine and coarse roots (RR) was determined and related to biomass growth. Salix cv. Inger accumulated more total dry matter than Populus cv. Max 4. In both Salicaceae clones, the total biomass was significantly influenced by the nitrate ratio and greatest in plants fertilized with 50% NO₃^? of the total N supply. Both clones possess a different leaf and root morphology, but no significant influence of the NO₃^? ratio on the morphology was found. Fine RR rates differed significantly between clones, with significantly greater fine RR in Max 4; 87.5% NO₃^? fertilization increased the fine RR. Fine RR and total accumulated plant biomass were closely related. Our study is the first to show the tremendous influence of fine root respiration, especially including the carbon‐intensive reduction of NO₃^? to NH₄⁺, on the aboveground growth of Salicaceae clones. Ways to improve yield in SRC are thus to lower the assimilate consumption by fine roots and to match fertilization regimes to the used clones or vice versa.     

Fine root branch orders respond differentially to carbon source-sink manipulations in a longleaf pine forest

Fine roots are a key component of carbon (C) flow and nitrogen (N) cycling in forest ecosystems. However, the complexity and heterogeneity of the fine root branching system have hampered the assessment and prediction of C and N dynamics at ecosystem scales. We examined how root morphology, biomass, and chemistry differed with root branch orders (1–5 with root tips classified as first order roots) and how different root orders responded to increased C sink strength (via N fertilization) and reduced carbon source strength (via canopy scorching) in a longleaf pine (Pinus palustris L.) ecosystem. With increasing root order, the diameter and length of individual roots increased, whereas the specific root length decreased. Total root biomass on an areal basis was similar among the first four orders but increased for the fifth order roots. Consequently, total root length and total root surface area decreased systematically with increasing root order. Fine root N and lignin concentrations decreased, while total non-structural carbohydrate (TNC) and cellulose concentrations increased with increasing root order. N addition and canopy disturbance did not alter root morphology, but they did influence root chemistry. N fertilization increased fine root N concentration and content per unit area in all five orders, while canopy scorching decreased root N concentration. Moreover, TNC concentration and content in fifth order roots were also reduced by canopy scorching. Our results indicate that the small, fragile, and more easily overlooked first and second order roots may be disproportionately important in ecosystem scale C and N fluxes due to their large proportions of fine root biomass, high N concentrations, relatively short lifespans, and potentially high decomposition rates.Electronic Supplementary Material Supplementary material is available for this article at     

间伐对川西亚高山粗枝云杉人工林细根生物量及碳储量的影响

以川西亚高山50年生粗枝云杉(Picea asperata)人工林为研究对象, 探讨了间伐对粗枝云杉人工林1-5级细根生物量及碳储量的影响。结果表明: 粗枝云杉人工林细根生物量和碳储量随根序等级的增加而显著增加(p < 0.05), 5级根序中1级根生物量及碳储量最小, 5级根生物量及碳储量最大。与对照(间伐0%)相比, 间伐对粗枝云杉人工林林分细根生物量及碳储量有显著影响(p < 0.05); 而对单株细根生物量影响不一, 间伐10%和20%与对照没有显著性差异(p > 0.05)。间伐显著影响生物量在各根序中的分配, 随着间伐强度的增加, 1、2级细根中生物量分配比例增加, 1级细根的生物量增加幅度最大; 3-5级细根的生物量分配比例减小, 5级细根减少幅度最大。其中, 间伐50%显著减少了细根在下层(20-40 cm)土壤中的生物量比例(p < 0.05), 但与间伐20%和30%无显著差异(p > 0.05)。     

Lignin and cellulose concentrations in roots of Douglas fir and European beech of different diameter classes and soil depths

Comparing two tree species, we tested the effects of root diameter (up to 30 mm) and soil depth (down to 1.2 m) on the concentrations of lignin, cellulose and nitrogen (N) in roots of approximately 50-year-old Douglas fir and European beech growing in a temperate forest in South-western Germany. Fine roots (diameter 0.5–2 mm) exhibited significantly higher lignin concentrations, but lower cellulose concentrations than medium or coarse roots (diameter >5 mm). The cellulose and lignin concentrations of the roots as well as their lignin:cellulose ratios did not differ significantly among soil depths. In the Douglas fir, there was a tendency of decreasing N concentrations and increasing lignin:N ratios with increasing soil depth. This trend was absent or less pronounced in the beech. Beech roots displayed significantly higher cellulose and N concentrations and lower lignin:cellulose and lignin:N ratios than roots of the Douglas fir. Generally, the lignin concentrations of the roots did not differ between the tree species. Cellulose and lignin concentrations exhibited a significantly negative correlation. As several studies have demonstrated that plant litter decomposition is governed by the lignin:cellulose and lignin:N ratios more than by the lignin concentration of the detritus, the fraction of individual tree species in the stand composition might affect the decomposability of roots in beech–Douglas fir forests, and might also have an influence on soil carbon sequestration.     

Fine-root biomass and soil properties in a semideciduous and a lower montane rain forest in Panama

The distribution of root biomass and physical and chemical properties of the soils were studied in a semideciduous and in a lower montane rain forest in Panama. Roots and soil samples were taken by means of soil cores (25 cm deep) and divided into five, 5-cm deep sections. Soils were wet-sieved to retrieve the roots that were classified in four diameter classes: very fine roots (<1 mm), fine roots (1–2 mm), medium roots (2–5 mm) and coarse roots (5–50 mm). Soil samples were analyzed for organic carbon, total nitrogen, available phosphorus, exchangeable bases, cation exchange capacity, pH, aluminium and exchangeable acidity. Total root biomass measured with the soil corer (roots <50 mm in diameter) was not different between the forests (9.45 t ha^-1), while biomass of very fine roots was larger in the mountains (2.00 t ha^-1) than in the lowlands (1.44 t ha^-1). The soils in the semideciduous forest were low in available phosphorus, while in the mountains, soils had low pH, high exchangeable aluminium and exchangeable acidity, and low concentration of exchangeable bases. Phosphorus was in high concentration only in the first 5 cm of the soil. In both forests, there was an exponential reduction of root biomass with increasing depth, and most of the variation in the vertical distribution of roots less than 2 mm in diameter was explained by the concentration of nitrogen in the soils. The results of this study support the hypothesis that a large root biomass in montane forests is related to nutrients in low concentration and diluted in organic soils with high CEC and low bulk density, and that fine root biomass in tropical forests in inversely related to calcium availability but not a phosphorus as has been suggested for other forests.