Root turnover as determinant of the cycling of C,N, and P in a dry heathland ecosystem

Root production and turnover were studied using sequential core sampling and observations in permanent minirhizotrons in the field in three dry heathland stands dominated by the evergreen dwarfshrub Calluna vulgaris and the grasses Deschampsia flexuosa and Molinia caerulea, respectively. Root biomass production, estimated by core sampling, amounted to 160 (Calluna), 180 (Deschampsia) and 1380 (Molinia) g m^-2 yr^-1, respectively. Root biomass turnover rate in Calluna (0.64 yr^-1) was lower compared with the grasses (Deschampsia: 0.96 yr^-1; Molinia 1.68yr^-1)). Root length turnover rate was 0.75–0.77 yr^-1 (Deschampsia) and 1.17–1.49 yr^-1 (Molinia), respectively. No resorption of N and P from senescing roots was observed in either species. Input of organic N into the soil due to root turnover, estimated using the core sampling data, amounted to 1.8 g N m^-2 yr^-1(^Calluna), 1.7 g N m^-2 yr^-1 (Deschampsia) and 19.7 g N m^-2 yr^-1 (Molinia), respectively. The organic P input was 0.05, 0.07 and 0.55 g P M^-2 yr^-1, respectively. Using the minirhizotron turnover estimates these values were20–22% (Deschampsia) and 11–30% (Molinia) lower.When the biomass turnover data were used, it appeared that in the Molinia stand root turnover contributed 67% to total litter production, 87% to total litter nitrogen loss and 84% to total litter phosphorus loss. For Calluna and Deschampsia these percentages were about three and two times lower, respectively.This study shows that (1) Root turnover is a key factor in ecosystem C, N, and P cycling; and that (2) The relative importance of root turnover differs between species.     

The effect of increased nutrient availability on vegetation dynamics in wet heathlands

A three year fertilization experiment was conducted in which nitrogen (N series: 20 g N m^–2 yr^–1), phosphorus (P series: 4 g P m^–2 yr^–1) and potassium (K series: 20 g K m^–2 yr^–1) were added to a mixed vegetation of Erica tetralix and Molinia caerulea. At the end of each growing season the percentage cover of each species was determined. At the end of the experiment percentage cover of each species was found to be positively correlated with the harvested biomass. In the unfertilized control series the cover of Erica and Molinia did not change significantly during the experiment. In all fertilized series however, especially in the P series, cover of Erica decreased significantly. The cover of Molinia increased significantly in the P series only.In the fertilized series the biomass of Erica and total biomass per plot did not change significantly compared with the control series. In the P series the biomass of Molinia increased significantly.It is suggested that with increasing phosphorus or nitrogen availability Molinia outcompetes Erica because the former invests more biomass in leaves which in turn permits more carbon to be allocated to the root system, which thereupon leads to a higher nutrient uptake.     

Root distribution of a Mediterranean shrubland in Portugal

The distribution of roots of an Erica (Erica scoparia and Erica lusitanica) dominated Mediterranean maquis was studied using three different approaches: root counts on trench walls (down to 120 cm), estimation of the maximum rooting depth using an allometric relationship and estimation of fine root biomass and fine root length using soil cores (down to 100 cm). Roots were classified according to diameter (fine, 1.0 mm; small, 1.1–5.0 mm; medium, 5.1–10.0 mm; coarse, >10.0 mm) and species (Erica sp., Pteridium aquilinum, Rubus ulmifolius and Ulex jussiaei). The depth corresponding to 50% of all roots (D ₅₀) was determined by fitting a new model to the cumulative root distribution. Fine roots represented 96% of root counts. Root counts of Erica represented 59%, Ulex 34%, Rubus 6% and Pteridium 1%. Overall root counts showed a D ₅₀ of 26 cm. D ₅₀ was higher for Ulex (40 cm) and Erica (22 cm), than for Pteridium (9 cm) and Rubus (3 cm). D ₅₀ for fine roots was 27 cm, for small roots 11 cm, for medium roots 6 cm and for coarse roots 4 cm. The estimated average maximum rooting depth of the 28 deepest Erica roots was 222 cm. The deepest Erica root was estimated to reach 329 cm. A total of 82% of roots growing deeper than 125 cm were not reaching more than 175 cm. The overall fine root length density ranged from 4.6 cm/cm³ at 10 cm to 0.8 cm/cm³ at 80 cm. The overall fine root biomass ranged from 7.7 mg/cm³ at 10 cm to 0.6 mg/cm³ at 40 cm. D ₅₀ for root biomass was 12 cm and D ₅₀ for root length was 14 cm. Fine root biomass was estimated as 1.6 kg/m² and the respective root length as 18.7 km/m².     

The relation between above- and belowground biomass allocation patterns and competitive ability

Summary In a 2-year experiment, the evergreen shrubsErica tetralix andCalluna vulgaris (dominant on nutrient-poor heathland soils) and the perennial deciduous grassMolinia caerulea (dominant on nutrient-rich heathland soils) were grown in replacement series in a factorial combination of four competition types (no competition, only aboveground competition, only belowground competition, full competition) and two levels of nutrient supply (no nutrients and 10 g N+2 g P+10 g K m⁻² yr⁻¹). Both in the unfertilized and in the fertilized treatmentsMolinia allocated about twice as much biomass to its root system than didErica andCalluna. In all three species the relative amount of biomass allocated to the roots was lower at high than at low nutrient supply. The relative decrease was larger forMolinia than forErica andCalluna. In the fertilized monocultures biomass of all three species exceeded that in the unfertilized series.Molinia showed the greatest biomass increase. In the unfertilized series no effects of interspecific competition on the biomass of each species were observed in either of the competition treatments. In the fertilized mixtures where only belowground competition was possibleMolinia increased its biomass at the expense of bothErica andCalluna. When only aboveground competition was possible no effects of interspecific competition on the biomass of the competing species were observed. However, in contrast with the evergreens,Molinia responded by positioning its leaf layers relatively higher in the canopy. The effects of full competition were similar to those of only belowground competition, so in the fertilized series belowground competition determined the outcome of competition. The high competitive ability ofMolinia at high nutrient supply can be attributed to the combination of (1) a high potential productivity, (2) a high percentage biomass allocation to the roots, (3) an extensive root system exploiting a large soil volume, and (4) plasticity in the spatial arrangement of leaf layers over its tall canopy. In the species under study the allocation patterns entailed no apparent trade-off between the abilities to compete for above- and belowground resources. This study suggests that this trade-off can be overcome by: (1) plasticity in the spatial arrangement of leaf layers and roots, and (2) compensatory phenotypic and species-specific differences in specific leaf area and specific root length.     

A comparative study on nutrient cycling in wet heathland ecosystems

Summary The term relative nutrient requirement is introduced in order to measure and to compare the nutrient losses from different perennial plant populations and the amount of nutrient that they need to absorb to compensate these losses. The relative nutrient requirement (L) is defined as the amount of the growth-limiting nutrient that must be taken up to maintain or replace each unit of biomass during a given time interval (e.g., mgN g^-1 biomass year^-1). It is derived that in a plant community with two competing perennial plant populations, species¹ will become dominant if the relative competition coefficient k ₁₂ (sensu De Wit 1960) exceeds the ratio between the relative nutrient requirements of the two species (L ₁/L ₂), whereas species 2 will become dominant, if k ₁₂ is below this critical ratio. The above-ground litter production was measured inwet heathland communities dominated by Erica tetralix or by Molinia caeruleain order to estimate N and P losses from theaboveground biomass and to calculate the relative N and P requirements of these species. Molinia lost during one year 63% and 34%, respectively, of the amount of N and P present in the above-ground biomass at the end of the growing season. These losses were in Erica 27% and 31%, respectively. The relative N requirements of the two species show the same difference: 7.5 and 2.6 mg N g^-1 yr^-1, respectively, in Molinia and in Erica. Also the relative P requirement of Molinia is higher as well as that of Erica (0.18 versus 0.08 mg P g^-1 yr^-1). The relative competition coefficient of Molinia with respect to Erica (k _me ) is equal to unity under unfertilized conditions but increases with increasing nutrient supply. Under nutrient-poor conditions k _me is below the critical ratio of the relative nutrient requirements of the two species (L _m /L _e =2.9 or 2.3), so that Erica will be the dominant species. After an increase in nutrient availability k _me increases and exceeds this critical limit which results in Molinia replacing Erica. During the last 20 years this replacement of Erica-dominant communities by monocultures of Molinia has been observed in almost all wet heathlands in The Netherlands along with a strong increase in nitrogen availability.     

Longevity and turnover of roots in the shortgrass steppe: influence of diameter and depth

We used minirhizotrons to determine patterns of root longevity andturnover for the perennial bunchgrass Bouteloua gracilisinthe shortgrass steppe of eastern Colorado, USA. We hypothesized that rootlongevity would be partially controlled by root diameter, following previouslyobserved patterns in woody plants. In addition, we hypothesized that rootturnover would be greatest in surface soil horizons and decrease with depth dueto variation in soil moisture availability and temperature. Root longevity wascorrelated with root diameter. Median life span of roots > 0.4mm was approximately 320 days, while roots < 0.2mmhad a median life span of 180 days. There was approximately a 6%decreasein the likelihood of mortality with a 0.10-mm increase inroot diameter, controlling for the effect of depth in the soil profile. Rootlength production and mortality were highest in the upper20 cm of the soil profile and decreased with depth.However,because root length density also decreased with depth, there were nosignificantdifferences in turnover rate of root length among sampling intervals. Turnoverwas approximately 0.86 yr^–1 based on root length production,while turnover was 0.35 yr^–1 using root length mortality as ameasurement of flux. The imbalance between turnover estimates may be aconsequence of the time the minirhizotrons were in place prior to imaging or mayresult from our lack of over-winter measures of mortality. Our worksuggests that Bouteloua gracilis roots have complex lifehistory strategies, similar to woody species. Some portion of the root systemishighly ephemeral, while slightly larger roots persist much longer. Thesedifferences have implications for belowground carbon and nitrogen cycles in theshortgrass steppe.     

Biomass production by alders on four abandoned agricultural soils in Québec

The production of aboveground tissue of three alder species (Alnus crispa (Ait.) Pursh,A. rugosa (Du Roi) Spreng. andA. glutinosa (L) Gaertn.) on four sites ranged from 0.4 t ha^–1 yr^–1 to 4.0 t ha^–1 yr^–1 after four growing seasons. Large differences were observed among the four sites studied and among species. Soil nutrient levels affected the biomass production and foliar symptoms of P and Mg deficiency occurred withA. crispa andA. rugosa. Because of their poor aboveground biomass production (0.4–1.4 t ha^–1 yr^–1),A. crispa andA. rugosa should be used mainly as nurse trees. For its higher potential for biomass production (up to 4.0 t ha^–1 yr^–1), and its apparent higher ability to use P and Mg on deficient sites,A. glutinosa should be used preferably toA. crispa andA. rugosa for the production of biomass.     

Biomass and aboveground production of four angiosperms in Cantabrian (N. Spain) salt marshes

New data of aboveground biomass and production of four angiosperms over a 12 month period for the Cantabrian Sea salt marshes (Bay of Biscay, N. Spain) are presented. Based on harvest methods, maximum aboveground total biomass values for Spartina maritima (Curtis) Fernald, Spartina alterniflora Loisel, Salicornia ramosissima J. Woods and Halimione portulacoides (L.) Aellen were 628, 1109, 480 and 1267 gm^-2, respectively. We conclude that although a slight latitudinal gradient in biomass is revealed in the data compiled with reference to some of the species studied, more work is neccesary in order to assess the potential productivity of these ecosystems on the coasts of Europe and/or to make comparisons with salt marshes of the American coasts. Annual net aerial primary production estimates using Smalley's method were: 296, 1160, 486 and 952 gm^-2yr^-1, for Spartina maritima, Spartina alterniflora, Salicornia ramosissima and Halimione portulacoides, respectively. These results together with turnover rate estimates point to the lack of vigour of the native S. maritima, while the exotic S. alterniflora, which seems to be spreading along the Cantabrian estuaries, behaves like a veritable pionner throughout the low marshes in this region.     

Estimation of fine root production, mortality and turnover with Minirhizotron in Larix gmelinii and Fraxinus mandshurica plantations

Fine root turnover is a major pathway for carbon and nutrient cycling in forest ecosystems. However, to estimate fine root turnover, it is important to first understand the fine root dynamic processes associated with soil resource availability and climate factors. The objectives of this study were: (1) to examine patterns of fine root production and mortality in different seasons and soil depths in the Larix gmelinii and Fraxinus mandshurica plantations, (2) to analyze the correlation of fine root production and mortality with environmental factors such as air temperature, precipitation, soil temperature and available nitrogen, and (3) to estimate fine root turnover. We installed 36 Minirhizotron tubes in six mono-specific plots of each species in September 2003 in the Mao’ershan Experimental Forest Station. Minirhizotron sampling was conducted every two weeks from April 2004 to April 2005. We calculated the average fine root length, annual fine root length production and mortality using image data of Minirhizotrons, and estimated fine root turnover using three approaches. Results show that the average growth rate and mortality rate in L. melinii were markedly smaller than in F. mandshurica, and were highest in the surface soil and lowest at the bottom among all the four soil layers. The annual fine root production and mortality in F. mandshurica were significantly higher than in L. gmelinii. The fine root production in spring and summer accounted for 41.7% and 39.7% of the total annual production in F. mandshurica and 24.0% and 51.2% in L. gmelinii. The majority of fine root mortality occurred in spring and summer for F. mandshurica and in summer and autumn for L. gmelinii. The turnover rate was 3.1 a⁻¹ for L. gmelinii and 2.7 a⁻¹ for F. mandshurica. Multiple regression analysis indicates that climate and soil resource factors together could explain 80% of the variations of the fine root seasonal growth and 95% of the seasonal mortality. In conclusion, fine root production and mortality in L. gmelinii and F. mandshurica have different patterns in different seasons and at different soil depths. Air temperature, precipitation, soil temperature and soil available nitrogen integratively control the dynamics of fine root production, mortality and turnover in both species. Transtlated from Journal of Plant Ecology, 2007, 31(2): 333–342 [译自: 植物生态学报]     

Root proliferation in perennial grasses of low and high palatability

Root proliferation of desirable (Stipa clarazii andS. tenuis) and undesirable (S.ambigua)perennial grasses was studied in semiarid rangelands of Central Argentina(40°39S, 62°54W) in 1998. On 17 September, soil coreswereremoved from the edge of the plant, metal structures lined with screen mesh(hereafter called bags) were buried in the holes, and root-free soil was placedinto these structures. Numbers of green tillers and circumference per plant hadpreviously been determined. Since plants were of unequal size among species,root length and root dry weight data are reported on a per green tiller basis.Half of the plants was defoliated to 5 cm stubble height on 17September and/or 12 October, while the other half remained undefoliated(controls). Bags were destructively harvested either 20 days after the firstdefoliation (first sampling) or 56 days after the second defoliation (secondsampling) by digging out soil very carefully around each bag. Roots were washedfrom soil, root length estimated by the line intercept method, root dry weightdetermined after oven-drying, and root length per unit root dry weightcalculated from the two measured variables. Root length and dry weight weremorethan 96% greater on defoliated and undefoliated plants ofS. clarazii than on those of S.tenuisor S. ambigua for both sampling dates. Root length perunitroot dry weight, however, was more than 43% greater (p < 0.05) inS. tenuis than in S. clarazii andS. ambigua during the second sampling. Defoliated plantshada similar root length and root dry weight than undefoliated plants in all threespecies, although plants of S. tenuis defoliated twiceshowed a greater (p < 0.05) root length than undefoliated controls. Rootlength and root dry weight were similar between sampling periods, except onundefoliated plants of S. tenuis which had a greater (p<0.05) root length and root dry weight at the first than at the second sampling.Although root length per unit root dry weight may be greater inS. tenuis than in S. clarazii andS. ambigua, greater root length and dry weight increasesinS. clarazii after defoliation appear determinant incontributing to explain its greater competitive ability and defoliationtolerance when compared with the other two species.Nomenclature of taxa followed.     

Nutrient use efficiency in evergreen and deciduous species from heathlands

Summary The nutrient (N, P) use efficiency (NUE: g g^–1 nutrient), measured for the entire plant, of field populations of the evergreen shrubs Erica tetralix (in a wet heathland) and Calluna vulgaris (in a dry heathland) and the deciduous grass Molinia caerulea (both in a wet and a dry heathland) was compared. Erica and Calluna are crowded out by Molinia when nutrient availability increases. NUE was measured as the product of the mean residence time of a unit of nutrient in the population (MRT: yr) and nutrient productivity (A: annual productivity per unit of nutrient in the population, g g^–1 nutrient yr^–1. It was hypothesized that 1) in low-nutrient habitats selection is on features leading to a high MRT, whereas in high-nutrient habitats selection is on features leading to a high A; and that 2) due to evolutionary trade-offs plants cannot combine genotypically determined features which maximize both components of NUE.Both total productivity and litter production of the Molinia populations exceeded that of both evergreens about three-fold. Nitrogen and phosphorus resorption from senescing shoots was much lower in the evergreens compared with Molinia. In a split-root experiment no nutrient resorption from senescing roots was observed. Nutrient concentrations in the litter were equal for all species, except for litter P-concentration of Molinia at the wet site. Both Erica and Calluna had a long mean residence time of both nitrogen and phosphorus and a low nitrogen and phosphorus productivity. The Molinia populations showed a shorter mean residence time of N and P and a higher N- and P-productivity. These patterns resulted in an equal nitrogen use efficiency and an almost equal phosphorus use efficiency for the species under study. However, when only aboveground NUE was considered the Molinia populations had a much higher NUE than the evergreens.The results are consistent with the hypotheses. Thus, the low potential growth rate of species from low-nutrient habitats is probably the consequence of their nutrient conserving strategy rather than a feature on which direct selection takes place in these habitats.     

The effect of phytohormones on growth and artemisinin production in <Emphasis Type="Italic">Artemisia annua</Emphasis> hairy roots

Summary Few studies have focused on the effect of a broad range of phytohormones on growth and secondary metabolism of a single hairy root species. We measured growth, development, and production of the antimalarial drug, artemisinin, in Artemisia annua hairy roots in response to the five main hormones: auxins, cytokinins, ethylene, gibberellins (GA), and abscisic acid (ABA). Single roots grown in six-well plates in medium B5 with 0.01 mgl⁻¹ (0.029 μM) GA₃ produced the highest values overall in terms of the number of lateral roots, length of the primary root, lateral root tip density, total lateral root length, and total root length. When the total root lengths are compared, the best conditions for stimulating elongation appear to be: GA 0.01 mgl⁻¹ (0.029μM)> ABA 1.0 mgl⁻¹ (3.78μM)=GA 0.02 mgl⁻¹ (0.058μM). Bulk yields of biomass were inversely proportional to the concentration of each hormone tested. All cultures provided with ABA yielded the highest amount of biomass. Both 6-benzylaminopurine and 2-isopentenyladenine inhibited root growth, however, only 2-isopentenyladenine stimulated artemisinin production, more than twice that of the B5 controls, and more than any other hormone studied. These results will prove useful in increasing hairy root growth and artemisinin production.     

不同施肥方法对马来沉香和土沉香苗期根系生长的影响

以珍贵树种马来沉香、土沉香1年生播种苗木为材料进行试验,研究指数施肥、平均施肥对2种沉香苗期根系生长动态及对N的响应特征。结果表明,经氮素处理的马来沉香、土沉香的根系生物量、根系长度、根系表面积、根系平均直径、根体积等指标均显著高于对照处理(P<0.05)。经指数施肥处理的苗木根系生长及各形态指标均高于平均施肥处理。同一时期在相同施肥处理方式下,马来沉香苗根系生长及根系形态指标值均高于土沉香。洛伦兹模型对不同施肥方法处理下马来沉香、土沉香苗木生长指标与根系生物量进行拟合,具有较高的R²(0.95-0.99)和较低的RSMD(0.538-2.352);抛物面模型对不同施肥方法处理下马来沉香、土沉香苗木生长指标与比根长进行拟合,具有较高的R²(0.92-0.99)和较低的RSMD(3.218-6.692)。     

Nitrogen mineralization in heathland ecosystems dominated by different plant species

Net N mineralization rates were measured in heathlands still dominated by ericaceous dwarf shrubs (Calluna vulgaris or Erica tetralix) and in heathlands that have become dominated by grasses (Molinia caerulea or Deschampsia flexuosa). Net N mineralization was measuredin situ by sequential soil incubations during the year. In the wet area (gravimetric soil moisture content 74–130%), the net N mineralization rates were 4.4 g N m^–2 yr^–1 in the Erica soil and 7.8 g N m^–2 yr^–1 in the Molinia soil. The net nitrification rate was negligibly slow in either soil. In the dry area (gravimetric soil moisture content 7–38%), net N mineralization rates were 6.2 g N M-2 yr^–1 in the Calluna soil, 10.9 g N m^–2 yr^–1 in the Molinia soil and 12.6 g N m^–2 yr^–1 in the Deschampsia soil. The Calluna soil was consistently drier throughout the year, which may partly explain its slower mineralization rate. Net nitrification was 0.3 g N m^–2 yr^–1 in the Calluna soil, 3.6 g N m^–2 yr^–1 in the Molinia soil and 5.4 g N m^–2 yr^–1 in the Deschampsia soil. The net nitrification rate increased proportionally with the net N mineralization rate suggesting ammonium availability may control nitrification rates in these soils. In the dry area, the faster net N mineralization rates in sites dominated by grasses than in the site dominated by Calluna may be explained by the greater amounts of organic N in the soil of sites dominated by grasses. In both areas, however, the net amount of N mineralized per gram total soil N was greater in sites dominated by Molinia or Deschampsia than in sites dominated by Calluna or Erica. This suggests that in heathlands invaded by grasses the quality of the soil organic matter may be increased resulting in more rapid rates of soil N cycling.     

Fine root turnover of irrigated hedgerow intercropping in Northern Kenya

Fine root turnover (<2 mm) was determined from repeated measurements of root distribution up to 120 cm soil depth by core sampling in four month intervals. Sole cropped Sorghum bicolor and Acacia saligna were compared with the agroforestry combination in an alley cropping system in semiarid Northern Kenya. Three methods for the calculation of root production were used: the max-min, balancing-transfer and compartment-flow method. The highest root biomass was found in the topsoil for all cropping systems, though trees had a deeper root system. Trees and crops had a similar amount of below-ground biomass during the vegetation period (0.3 and 0.4 Mg DM ha^-1 120 cm^-1), but in the agroforestry combination root biomass was more than the sum of the sole cropped systems (1.1 Mg DM ha^-1 120 cm^-1). The tree system showed a very static root development with little fluctuation between seasons, whereas root biomasses were very dynamic in the crop and tree + crop systems. Root production was highest in the tree + crop combination with 2.1 Mg DM ha^-1 a^-1, with about 50% less in sole cropped trees and crops. Root N input to soil decreased in the order tree + crop>tree>crop system with 13.5, 11.0 and 3.2 kg N ha^-1 a^-1, and cannot be estimated from total below-ground biomass or carbon turnover, as N is accumulated in senescing roots. Such low N input to soil stresses the need for investigating other processes of nutrient input from roots to soil. Areas of highest N input were identified in the topsoil under the tree row in the tree system. Resource utilisation and C and N input to soil were highest with a combination of annual and perennial crops.     

Climate and species affect fine root production with long-term fertilization in acidic tussock tundra near Toolik Lake,Alaska

Long-term fertilization of acidic tussock tundra has led to changes in plant species composition, increases in aboveground production and biomass and substantial losses of soil organic carbon (SOC). Root litter is an important input to SOC pools, although little is known about fine root demography in tussock tundra. In this study, we examined the response of fine root production and live standing fine root biomass to short- and long-term fertilization, as changes in fine root demography may contribute to observed declines in SOC. Live standing fine root biomass increased with long-term fertilization, while fine root production declined, reflecting replacement of the annual fine root system of Eriophorum vaginatum, with the long-lived fine roots of Betula nana. Fine root production increased in fertilized plots during an unusually warm growing season, but remained unchanged in control plots, consistent with observations that B. nana shows a positive response to climate warming. Calculations based on a few simple assumptions suggest changes in fine root demography with long-term fertilization and species replacement could account for between 20 and 39% of the observed declines in SOC stocks.     

Fine Root Production and Turnover in a Norway Spruce Stand in Northern Sweden: Effects of Nitrogen and Water Manipulation

Fine root length production, biomass production, and turnover in forest floor and mineral soil (0–30 cm) layers were studied in relation to irrigated (I) and irrigated-fertilized (IL) treatments in a Norway spruce stand in northern Sweden over a 2-year period. Fine roots (<1 mm) of both spruce and understory vegetation were studied. Minirhizotrons were used to estimate fine root length production and turnover, and soil cores were used to estimate standing biomass. Turnover was estimated as both the inverse of root longevity (RTL) and the ratio of annual root length production to observed root length (RTR). RTR values of spruce roots in the forest floor in I and IL plots were 0.6 and 0.5 y⁻¹, respectively, whereas the corresponding values for RTL were 0.8 and 0.9 y⁻¹. In mineral soil, corresponding values for I, IL, and control (C) plots were 1.2, 1.2, and 0.9 y⁻¹ (RTR) and 0.9, 1.1, and 1 y⁻¹ (RTL). RTR and RTL values of understory vegetation roots were 1 and 1.1 y⁻¹, respectively. Spruce root length production in both the forest floor and the mineral soil in I plots was higher than in IL plots. The IL-treated plots gave the highest estimates of spruce fine root biomass production in the forest floor, but, for the mineral soil, the estimates obtained for the I plots were the highest. The understory vegetation fine root production in the I and IL plots was similar for both the forest floor and the mineral soil and higher (for both layers) than in C plots. Nitrogen (N) turnover in the forest floor and mineral soil layers (summed) via spruce roots in IL, I, and C plots amounted to 2.4, 2.1, and 1.3 g N m⁻² y⁻¹, and the corresponding values for field vegetation roots were 0.6, 0.5, and 0.3 g N m⁻² y⁻¹. It was concluded that fertilization increases standing root biomass, root production, and N turnover of spruce roots in both the forest floor and mineral soil. Data on understory vegetation roots are required for estimating carbon budgets in model studies.     

Evaluation of some common aquatic macrophytes cultivated in enriched water as possible source of protein and biogas

The biomass production of three common aquatic macrophytes,viz. Azolla pinnata, Eichhornia crassipes andHydrilla verticillata, was high at the prevailing environmental conditions and by the enriched water of River Ganga. The biomass production ofAzolla andEichhornia was positively correlated with the orthophosphate phosphorus and nitrate-nitrogen concentrations of the enriched water. The biomass ofAzolla andHydrilla was positively correlated with the electrical conductivity of the water. The average yield of crude protein was highest in Azolla (8,520 kg.ha^–1.yr^–1), and somewhat lower inEichhornia (6,520 kg.ha^–1.yr^–1). The annual biogas production was highest inEichhornia (44,381 litres), and somewhat lower inAzolla (17,186 litres).     

外来红树植物拉关木对乡土种桐花树和正红树的化感作用研究

为了探究外来红树植物拉关木对乡土红树植物的化感作用,该研究观察了不同浓度(0.1、0.3、0.5g·mL~(-1))的拉关木根、叶水浸提液对乡土红树植物桐花树和正红树的胚轴(种子)萌发、幼苗生长及叶片抗氧化酶活性的影响。结果表明:(1)拉关木水浸提液对桐花树种子的成苗率、萌发指数和根长均存在抑制作用,其中对根长的抑制作用随水浸提液浓度的提高而增强。(2)根水浸提液对桐花树幼苗的根长、苗高、生物量等生长指标的影响总体上均表现为低浓度促进,高浓度抑制。(3)拉关木水浸提液对正红树胚轴的萌发率、萌发指数、生长指标均表现为促进作用,且根水浸提液0.1、0.3 g·mL~(-1)处理组的芽长以及根、叶水浸提液0.1、0.3 g·mL~(-1)处理组的生物量显著大于对照组;拉关木水浸提液对正红树幼苗的生物量也表现为促进作用。(4)抗性生理方面,随着拉关木水浸提液浓度的升高,桐花树和正红树幼苗SOD活性降低,正红树幼苗POD活性在根水浸提液0.3 g·mL~(-1)和叶水浸提液0.1 g·mL~(-1)处理组显著高于对照组。以上结果表明,不同乡土植物对拉关木化感作用的敏感性不同,拉关木水浸提液抑制了桐花树的生长,而对正红树的生长则表现出一定程度的促进作用。     

Root turnover in a beech and a spruce stand of the Belgian Ardennes

The theoretical basis of fine root turnover estimation in forest soils is discussed, in relation to appropriate experimental techniques of measurement. After sequential coring, the correct expression is the sum of significant positive increments of live and dead roots of the various diameter categories, to which the transfer of dead roots to organic matter derived from roots, OMDR, has to be added. This should not be confounded with dead root mineralization. The transfer rates should first be estimated in root dimensions and not in weight of dry matter. The measurements were carried out in a 120 year old beech (Fagus sylvatica L.) stand and a 35 year old Norway spruce (Picea abies Karst) stand, in the Eastern Ardennes, Belgium. The turnover rate of fine roots (diam. <5 mm) was 4393 kg ha⁻¹ year⁻¹ (root dry weight), including 711.2 kg ha⁻¹ year⁻¹ for dead root transfer to OMDR, for beech. For spruce, turnover rate was 7011 kg ha⁻¹ year⁻¹ (root dry weight), including 1498 kg ha⁻¹ year⁻¹ for dead root transfer to OMDR. Under beech, there was a slight root density increase in spring. No seasonal fluctuations were observed under spruce, but a strong irreversible drop in live root growth was found in the later season 1980–1981, corresponding to a decrease of tree height growth and trunk radius increment. Turnover rates were further expressed in dry weight and in amounts of elements (kg ha⁻¹ year⁻¹) (Ca, Mg, K, Na, Al, N, P, S). Correlative relations between root dimensions and dry weight and element concentrations show that the derived values, and in particular root specific density (dry weight volume⁻¹) vary according to species, root category, and seasonal sampling. Various schemes of seasonal variations of root growth, described in Europe, show that the major dependance on general climate is obscured by environmental factors (soil, exposure, species). It is suggested that root density fluctuation approach the steady state on an annual basis under mild Atlantic conditions.