亚热带杉木纯林生态系统中营养元素的积累、分配和循环的研究

本文是1980--1983年本所湖南会同森林生态实验站成年杉木纯林中进行定位研究根据所取得的观测资料,分析和论述了营养元素在林内的积累与分配;以及从降雨中的输入量,计算了营养元素的年吸收量和归还量。研究表明,杉木纯林在21--23年生时,即达到主伐年龄期,营养元素年吸收量仍大于归还量,整个林分还处在养分消耗阶段。     

天然枫桦红松林凋落量动态及养分归还量

三年定位研究表明小兴安岭天然枫桦红松林年均凋落量有5.8t／ha(干重)。凋落量的季节变化格局是随着气候变冷有一明显秋季凋落高峰期(9—10月)。凋落物每年养分的归还量：Ca、N、K、Mg、P,相应为67.0、56.9、14.8、9.5、和6.6kg／ha,总计155.0kg／ha。据测定阔叶树落叶养分含量明显高于所有针叶的含量。尽管阔叶树的年凋落量仅占该混交林的年总凋落量的三分之一,阔叶树落叶仍有相当高的养分比例(43.4%)归还土壤。因此,红松林分的经营管理中,保留适当比重的阔叶树有利于土壤改良和促进林分生长。     

Is primary production in holm oak forests nutrient limited?

Correlations between primary production and patterns of nutrient use and nutrient availability were investigated in 18 plots in closed holm oak (Quercus ilex L.) stands in the Montseny mountains (NE Spain), searching for evidence of nutrient limitation on primary production. The plots spanned a range of altitudes and slope aspects within a catchment. Nutrients considered were nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg) in plant samples, and the above plus calcium (Ca) and sodium (Na) in the soil. Primary production was found by summing the annual aboveground biomass increment to the annual litterfall. Across plots, primary production was correlated with the annual return of nutrients in litterfall, but this relationship probably arose from the common effects of the amount of litterfall on both primary production and nutrient return, and not from any nutrient limitation. Primary production was not significantly correlated with nutrient concentrations in mature leaves nor leaf litterfall, nor with absolute or relative foliar retranslocation of nutrients before leaf abscission, nor with the concentration and content (kg/ha) of total N, extractable P, and exchangeable K, Mg, Ca and Na in the upper mineral soil. We conclude that there is no correlational evidence that primary production is nutrient limited in this holm oak forest.     

马尾松林杆材阶段生物产量和经级分化及密度效应初探

马尾松(Pinus massoniana)是我国松树中分布最广、数量最多的主要用材和薪炭材树种。它具有适应性强、生长快、用途广,造林成本低和易于成林等优点,是我国南方荒山造林的重要先锋树种,其经济意义、环境防护效能及美学价值,都是为人们所公认的。马尾松的干材成长阶段,是林木胸径和树高旺盛生长时期,林木与生长空间的矛盾及树木间的竞争较剧烈,天然整枝、林木分化和自然稀疏都很强烈。如何调节和控制这个时期的立木密度,使其形成合理的群体结构,对于保证林木成熟期的材积生长和取得最大生物量将具有十分重要意义。本试验目的就在于探明不同密度松树杆材林的生物量、林木分化与产量结构以及不同密度下的生态效应,为确定合理的造林密度和制定疏伐措施提供科学依据。     

Phosphorus cycling in a Mexican tropical dry forest ecosystem

The study was conducted in five contiguous small watersheds (12–28 ha) gauged for long-term ecosystem research. Five 80 × 30 m plots were used for the study. We quantified inputs from the atmosphere, dissolved and particulate-bound losses, throughfall and litterfall fluxes, standing crop litter and soil available P pools. Mean P input and output for a six-year period was 0.16 and 0.06 kgha^–1yr^–1, respectively. Phosphorus concentration increased as rainfall moved through the canopy. Annual P returns in litterfall (3.88 kg/ha) represented more than 90% of the total aboveground nutrient return to the forest floor. Phosphorus concentration in standing litter (0.08%) was lower than that in litterfall (0.11%). Phosphorus content in the litterfall was higher at Chamela than at other tropical dry forests. Mean residence time on the forest floor was 1.2 yr for P and 1.3 yr for organic matter. Together these results suggest that the forest at Chamela may not be limited by P availability and suggest a balance between P immobilization and uptake. Comparison of P losses in stream water with input rates from the atmosphere for the six-year period showed that inputs were higher than outputs. Balances calculated for a wet and a dry year indicated a small P accumulation in both years.     

Biogeochemical cycles in natural forest and conifer plantations in the high mountains of Colombia

Plant litter production and decomposition are two important processes in forest ecosystems, since they provide the main organic matter input to soil and regulate nutrient cycling. With the aim to study these processes, litterfall, standing litter and nutrient return were studied for three years in an oak forest (Quercus humboldtii), pine (Pinus patula) and cypress (Cupressus lusitanica) plantations, located in highlands of the Central Cordillera of Colombia. Evaluation methods included: fine litter collection at fortnightly intervals using litter traps; the litter layer samples at the end of each sampling year and chemical analyses of both litterfall and standing litter. Fine litter fall observed was similar in oak forest (7.5 Mg ha/y) and in pine (7.8 Mg ha/y), but very low in cypress (3.5 Mg ha/y). Litter standing was 1.76, 1.73 and 1.3 Mg ha/y in oak, pine and cypress, respectively. The mean residence time of the standing litter was of 3.3 years for cypress, 2.1 years for pine and 1.8 years for oak forests. In contrast, the total amount of retained elements (N, P, S, Ca, Mg, K, Cu, Fe, Mn and Zn) in the standing litter was higher in pine (115 kg/ha), followed by oak (78 kg/ha) and cypress (24 kg/ha). Oak forests showed the lowest mean residence time of nutrients and the highest nutrients return to the soil as a consequence of a faster decomposition. Thus, a higher nutrient supply to soils from oaks than from tree plantations, seems to be an ecological advantage for recovering and maintaining the main ecosystem functioning features, which needs to be taken into account in restoration programs in this highly degraded Andean mountains.     

不同坡位对琉球松人工林凋落物及其养分归还量的影响

对日本冲绳岛北部不同坡位的琉球松人工林凋落物量及其养分归还量进行了为期3年的观测研究,结果表明,琉球松成林年凋落物量为6．54～8．05Mg·hm^-2·年^-1,其凋落高峰出现在6、7月份．台风干扰对年凋落物量及其季节动态影响显著．凋落物不同组分的养分含量差异明显．年均养分归还总量为113．4～154．6kg·hm^-2·年^-1,其中氮素归还量最大,占42．2％;各养分归还量的大小依次为N>Ca>K>Mg>Na>P．两调查林分由于立地条件的显著差异,年均凋落物量及其相应的养分归还量亦存在明显差异．在立地条件较好的下坡,林分P1的年均凋落物量达8．05Mg·hm^-2·年^-1,高于立地条件相对较差的上坡的林分P2的23．1％;其相应的年养分归还量为：N66、04,P1.63,K17．42,Ca48.31,Mg14.65和N。6、57kg·hm^-2·年^-1,分别高于林分P2的39.7％、48.8％、39．4％、32.9％、24.8％和49.3％．两林分凋落物的养分利用效率分别是N为122和138,P为4934和5945,K为462和523,林分P1明显低于林分P2,这与林分P2的立地条件较差相关．可见,所调查的琉球松林是一种高效的养分利用系统。     

鼎湖山南亚热带季风常绿阔叶林凋落物的养分动态

本文探讨鼎湖山季风常绿阔叶林调落物养分特征,分析了其动态、季节间与种间变异、构成特征等。结果表明,该群落各种群含氮量的季节变化较小,各个种的变异系数多在15％以下,但磷、钾和钠的变异较大。无论在群落还是在种的水平上,各种营养成份间均没有显著的相关性。除磷外,不同种问养分含量有显著差异,季节间则除镁外无显著差异,磷在红壤地区土壤中难于利用的主要原因可能是不同植物在磷的利用上趋同．全年通过凋落物回归土壤的养分量为氮99．88kghm-2,磷15．03kghm-2．钾27．01kghm-2,钙37．11kghm-2;镁12．19kghm-2。在该群落中,椎栗是对落叶养分贡献最大的种,荷木其次。8月是当年凋落物养分量最高的月份,主要是由于台风作用产生大量的枝叶折落,12月则是最低月份,两者在构成上的主要区别是8月份枝条所持养分量极大,而12月份枝条极少。全年凋落物养分出现两次高峰,4月和8月,而以较寒冷的1、2、11、12月份最低。     

Impacts of the exotic,nitrogen-fixing black locust (Robinia pseudoacacia) on nitrogen-cycling in a pine–oak ecosystem

We investigated the influence of the exotic nitrogen-fixing black locust (Robinia pseudoacacia) on nitrogen cycling in a pitch pine (Pinus rigida) −scrub oak (Quercus ilicifolia, Q. prinoides) ecosystem. Within paired pine-oak and adjacent black locust stands that were the result of a 20-35 year-old invasion, we evaluated soil nutrient contents, soil nitrogen transformation rates, and annual litterfall biomass and nitrogen concentrations. In the A horizon, black locust soils had 1.3-3.2 times greater nitrogen concentration relative to soils within pine-oak stands. Black locust soils also had elevated levels of P and Ca, net nitrification rates and total net N-mineralization rates. Net nitrification rates were 25-120 times greater in black locust than in pine-oak stands. Elevated net N-mineralization rates in black locust stands were associated with an abundance of high nitrogen, low lignin leaf litter, with 86 kg N ha^–1 yr^–1 in leaf litter returned compared with 19 kg N ha^–1 yr^–1 in pine-oak stands. This difference resulted from a two-fold greater litterfall mass combined with increased litter nitrogen concentration in black locust stands (1.1% and 2.6% N for scrub oak and black locust litter, respectively). Thus, black locust supplements soil nitrogen pools, increases nitrogen return in litterfall, and enhances soil nitrogen mineralization rates when it invades nutrient poor, pine-oak ecosystems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Survival ofSclerotinia minor Jagger sclerotia in solarized soil

Calcium, magnesium and potassium dynamics in decomposing litter of three tree species were measured over a two-year period. The speices studied were flowering dogwood (Cornus florida), red maple (Acer rubrum) and chestnut oak (Quercus prinus). The order of decomposition was:C. florida>A. rubrum>Q. prinus.Calcium concentrations increased following any initial leaching losses. However, there were net releases of Ca from all three litter types since mass loss exceeded the increases in concentration. Net release of Ca by the end of two years from all three species combined was 42% of initial inputs in litterfall. Magnesium concentrations increased in the second year, following decreases due to leaching during the first year inC. florida andA. rubrum litter. Net release of Mg by the end of two years was 58% of initial inputs. Potassium concentrations decreased rapidly and continued to decline throughout the study. Net release of K by the end of two years was 91% of initial inputs.These data on cation dynamics, and similar data on N, S and P dynamics from a previous study, were combined with annual litterfall data to estimate the release of selected nutrients from foliar litter of these tree species at the end of one and two years of decomposition. The relative mobility of all six elements examined in relation to mass loss after two years was; K>Mg>mass>Ca>S>P>N.     

Effect of typhoon disturbance on fine litterfall and related nutrient input in a subtropical forest on Okinawa Island, Japan

Typhoons are frequent on Okinawa Island, southwestern Japan. The effects of typhoon disturbance on the patterns of fine litterfall and related nutrient inputs in a subtropical evergreen broad-leaved forest were studied over 5 years from May 1996 to April 2001. Annual fine litterfall averaged 7558 kg ha⁻¹ (range from 6188 to 9439 kg ha⁻¹) for six sampling plots over 5 years, which differed significantly among years (p<0.001) but not among plots (p=0.122). A seasonal maximum was most evident for leaf litter component. Woody litter fell more irregularly through the year, and peak fall varied with typhoon and windstorm. The mean ratio of annual litterfall mass of sexual organs to leaves was 0.06, much lower than that in other tropical and subtropical rain forests. Nutrient concentrations varied in litterfall components, but were not significantly different among plots. The lowest concentrations of N and P in leaf litter were observed in March, which is also the month with the greatest leaf fall. However, the highest concentrations were recorded in typhoon season. Nitrogen and P concentrations were 34% and 106% greater in the green leaves that fell during typhoons than in senescent leaves. Mean nutrient inputs by litterfall were: N 83, P 3.2, K 25, Ca 71, Mg 19, Al 12, Na 10, Fe 0.86 and Mn 3.9 kg ha⁻¹ yr⁻¹, and differed significantly among years for all elements (p<0.0005) and among plots only for K (p<0.05) and Mn (p<0.0001). Typhoon disturbance strongly affected annual fine litterfall and related nutrient inputs, which contributed an average of 30% of the annual litterfall mass, and from 30% to 39% (for different nutrient elements) of annual total nutrient inputs. The results from this study suggest that typhoon-driven maintenance of rapid cycling of P and N and their high availability in soil appears to be an important mechanism to maintain productivity in the subtropical forest on Okinawa Island.     

Effects of water and nutrient supply on amount and on nutrient concentration of litterfall and forest floor litter in Eucalyptus globulus plantations

A field experiment was initiated in March 1986 in central Portugal to evaluate the influence of water and nutrient supply on the productivity of E. globulus. The treatments applied were, irrigation plus fertiliser, irrigation and the application of fertilisers in rainfed plots. The control received neither fertilisers nor irrigation. The annual pattern litterfall was measured over a period of 5 years and the litter layer was quantified 6 years after planting. The amount of litterfall varied with the treatments. Simultaneous water and nutrient supply increased significantly litterfall, in respect to control. In rainfed conditions the timing of the maximum of litterfall was anticipated relative to the irrigated plots. However, the time of maximum litterfall did not coincide with the dry season but rather with period of maximum growth in each treatment. The N and P concentration was higher in the litterfall of the two fertilised treatments than in the others. The lowest concentrations of N and P in the leaf litter were coincident with the summer period in all treatments. The withdrawal of N and P ranged between 32 and 65% according to treatment and season. The deliverance of nutrients through litterfall was strongly increased by simultaneous water and nutrient supply. The supply of fertilisers in rainfed conditions promoted higher deliverance of nutrients than in the plots irrigated only. The mass of the litter layer was significantly increased by simultaneous water and nutrient supply. Application of fertilisers induced an increase in N and P concentration and a decrease in C/N ratio of the litter layer. Treatments and C/N values did not influence apparently the proportion of carbon, N and P released through mineralization from the litter layer. The non-fertilized treatments showed a more efficient N cycling than the others.     

Leaf-litter and changing nutrient levels in a seasonally dry tropical hardwood forest,Belize, C.A.

Summary Total above ground plant biomass in a 45 year old seasonally dry tropical hardwood forest was estimated to be approximately 56,000 kg/ha oven dry weight. Nutrients immobilized in the standing vegetation were: N, 203 kg/ha; P, 24 kg/ha; K, 234 kg/ha; Ca, 195 kg/ha; Mg, 47 kg/ha; Na, 9 kg/ha; Mn, 1 kg/ha; Cu, 0.5 kg/ha; Zn, 3 kg/ha; Fe, 4 kg/ha. Total nutrients returned each year through the litter were: N, 156 kg/ha; P, 9 kg/ha; K, 59 kg/ha; Ca, 373 kg/ha; Mg, 32 kg/ha; Na, 5 kg/ha; Mn, 1 kg/ha; Al, 21 kg/ha; Zn, 0.3 kg/ha; Fe, 9 kg/ha. Half of the nutrients immobilized in the standing vegetation were found in the leaves and are returned annually to the soil. Although litter fall is interrupted during the year, the mean nutrient content of the litter was high –5.2%.A decomposition rate of 0.48 percent per day was considered high for a seasonally dry tropical hardwood forest. Fluctuations in soil nutrient levels showed a sharp increase at the start of the rainy season. Later during the dry season nutrient levels decreased to concentrations similar to what they were just prior to the rainy season. Soil organic matter levels were very high –20% in the top 12 cm.     

Litter fall and nutrient cycling in karri (Eucalyptus diversicolor F. Muell.) forest in relation to stand age

Annual litter fall, nutrient concentrations in litter components and annual weight of nutrients in litter fall have been estimated for karri forest stands aged 2, 6, 9 and 40 years and in mature forest. The weight of litter falling annually increases with stand age, ranging from 1.13 t/ha in 2-year-otd regeneration to 9.45 t/ha in mature forest. This increase is due mainly to greater amounts of twigs, bark and fruit falling in older stands. Leaf fait is relatively independent of stand age once the canopy of regenerating stands closes and the understorey has developed. Concentrations of N, P, K, S and Mn in karri leaf litter differ significantly between sites and the differences appear to be related to stand age. Highest levels of these elements are found in karri leaf litter from the youngest stand and the concentrations decrease with increasing stand age. The amounts of annual litter fall and of nutrients cycling in litter are among the largest reported for Australian forests. In particular cycling of Ca, K and Mg in mature karri forest is greater than has been reported for any other eucalypt forest. Karri forest understorey plays a key rote in nutrient cycling in these ecosystems, contributing 30–70% of the weight of many of the nutrients in the leaf component of titter. Understorey leaf material is particularly important in the cycling of N, S and the micro-nutrients Cu and Zn.     

Litterfall and nutrient return in a semi-arid southern African savanna woodland dominated by <Emphasis Type="Italic">Colophospermum mopane</Emphasis>

The objective of this study was to investigate litter production, litter standing crop and nutrient return to soil in a semi-arid southern African savanna in Bulawayo, Zimbabwe. We used a randomized block design with five blocks of 100 × 100 m demarcated in a 10-ha pocket of Colophospermum mopane-dominated open woodland protected from grazing and fire. Litter traps were installed beneath large (8.3 m crown diameter) and small mopane trees (2.7 m crown diameter) and in the intercanopy area, representing 27, 3 and 62% of the woodland area, respectively. Mean annual total litterfall over 2 years of observations was 197, 83 and 35 g m⁻² yr⁻¹ beneath large and small trees as well as in the intercanopy area, respectively. Leaf proportions of total litterfall beneath large and small trees and in the intercanopy area were 68.6, 73.0 and 75.3%, respectively. Litterfall followed a uni-modal distribution pattern and was much higher during the period of May–September (dry period) compared to other months. The total potential annual element inputs via litterfall beneath large trees were 2 and 5 times greater than beneath small trees and in the intercanopy area, respectively. Total litter standing crop was 405, 177 and 67 g m⁻² beneath large and small trees and in the intercanopy area, respectively. Concentrations of N, P and K in litterfall and surface soil were closely correlated with each other. At all sampling sites, element accession to soil through litterfall followed the decreasing sequence C > Ca > N > Mg > K > P. These results suggest that litterfall is a major process responsible for soil organic matter and nutrient enrichment beneath isolated trees in semi-arid savannas.     

Nutrient cycling in Pinus sylvestris stands in eastern Finland

Nutrient cycling within three Pinus sylvestris stands was studied in eastern Finland. The aim of the study was to determine annual fluxes and distribution of N, P, K, Ca, Mg, Zn, Fe, B, and Al in the research stands. Special emphasis was put on determining the importance of different fluxes, especially the internal cycle within the trees in satisfying the tree nutrient requirements for biomass production. The following nutrient fluxes were included, input; free precipitation and throughfall, output; percolation through soil profile, biological cycle; nutrient uptake from soil, retranslocation within trees, return to soil in litterfall, release by litter decomposition. The distribution of nutrients was determined in above- and belowground tree compartments, in ground and field vegetation, and in soil.The nitrogen use efficiencies were 181, 211 and 191 g of tree aboveground dry matter produced per g of N supplied by uptake and retranslocation in the sapling, pole stage and mature stands, respectively. Field vegetation was more efficient in nitrogen use than trees. Stand belowground/aboveground and fine root/coarse root biomass ratios decreased with tree age. With only slightly higher fine root biomass, almost three times more nitrogen had to be taken-up from soil for biomass production in the mature stand than in the sapling stand.The annual input-output balances of most nutrients were positive; throughfall contained more nutrients than was lost in mineral soil leachate. The sulphate flux contributed to the leaching of cations, especially magnesium, from soil in the mature stand.Retranslocation supplied 17–42% of the annual N, P and K requirements for tree aboveground biomass production. Precipitation and throughfall were important in transferring K and Mg, and also N in the sapling stand. Litterfall was an important pathway for N, Ca, Mg and micro nutrients, especially in the oldest stands.     

Nutrient stocks in litterfall and litter in cocoa agroforests in Brazil

Aim

To compare the internal balances of nutrients and the rates of nutrient cycling across nine cocoa agroforestry systems consisting of various combination of soil types (Latosols and Cambisols), production systems (cabruca and Erythrina glauca-shade) and fertilization regimes in southern Bahia, Brazil.

Methods

We measured nutrient stocks in litter fall production, in the accumulated litter and fruits. The internal nutrient balance for various simulations was obtained by the following expressions: (1) Balance 1?=?litter – fruit (seeds and husks) and (2) Balance 2?=?(litter?+?husks) – seeds. Annual litter decomposition coefficients (k) and subsequent potential of nutrient release were also investigated. The data were analyzed by principal components analysis and by Pearson correlations.

Results

There was a high degree of dissimilarity among the cocoa agrosystems in relation to the nutrient cycling and the internal nutrient balance. The mean annual litterfall production ranged from 4.6 to 8.5 Mg/ha, and the amount of accumulated litter ranged from 7.7 to 16.8 Mg/ha. The results showed significant differences in quality among litter from cocoa agroforests; the decomposition coefficient of litter and the subsequent nutrient release were regulated by the litter quality. In general, the cocoa-erythrina system presented a higher capacity to recycle nutrients compared to the cocoa-cabruca system, with the cocoa-erythrina system having the largest transfer rate of nutrients through litterfall, high values for the decomposition coefficient of litter and the lowest values for the Mean Residence Time of nutrients. Cocoa tree leaves functioned as a sink of nutrients, while shade tree leaves functioned predominantly as a source. The nutritional reserves of litter?+?cocoa fruit husks, with respect only to the nutrients exported in the seeds, the balance was positive for all nutrients (N, P, K, Ca and Mg) in all agroforests, which emphasizes the potential productive capacity of these agroforests to sustain the estimated production in different harvest cycles.

Conclusions

The internal balance of nutrients reflects an agroforests’s productive capacity, which accumulated litter and cocoa fruit husks may be important nutrient sources that could enable the development of fertilizer recommendation systems aimed at increasing the efficiency of fertilizer use and at maintaining soil fertility in cocoa agroforests. Therefore, further research is needed to develop nutritional balance systems integrating litter?+?fruits stock and other nutrient pathways (e.g., soil quality, biological N fixation, leaching), which were not measured, for making recommendations regarding liming and fertilizers that are suitable for highly complex biological agrosystems, such as cocoa agroforests that have low levels of elements exported during seed production.     

Litterfall,leaf decomposition and litter colonization of Tessaria integrifolia (compositae) in the Parana river floodplain

Litterfall and leaf decomposition rates were measured in Choui Island, 45 km downstream from the confluence of the Paraná and Paraguay rivers. The material was collected biweekly from April 1985 through September 1986. Decomposition was measured in situ by the litter bag technique.Annual litterfall of Tessaria integrifolia gallery forest measured in the period April 1985 to March 1986 was 8.15 t ha^-1. Leaf litterfall was seasonal, i.e. significantly less leaf litter was shed during the high water phase than during the low water phase. The half life of the T. integrifolia litter over 38 days of decomposition was 20 days. At the beginning of the experiment, 15 and 38 days subsamples of remaining detritus were analyzed in order to determine changes in the nutrient content. After 38 days of incubation, the order of nutrient disappearance was Ca > K > N > Mg > Na > P.The number of invertebrates per g remaining litter of Tessaria integrifolia increased between incubations days 7 and 31. Collector-gatherers were more abundant after 38 days incubation; there were no shredders colonizing the leaf litter bags.     

Litter Interception on Astrocaryum mexicanum Liebm. (Palmae) in a Tropical Rain Forest1

The goal of this study was to quantify litter interception on Astrocaryum mexicanum Liebm., an abundant and efficient litter–capturing palm. The study was done in a forest at Los Tuxtlas, Veracruz, on the Gulf of Mexico coast. Palms were randomly selected and total litter retention was quantified by determining dry weight over two years. Litter interception by palms was 4.4 and 27.1 Mg/ha/yr for the first and second years, respectively, and accounted for 47.9 and 239.4 percent of the reported annual litterfall in the same area. Results showed that litter interception by plants is related to litterfall and should therefore be added to total litterfall in order to calculate the net primary productivity.     

湖南省会同县两个森林群落的生物生产力

本文就我国亚热带分布最广、最有代表性的两个森林群落——杉木人工林和马尾松天然次生林的生物生产力进行了测定,并从群落的现存量、营养元素和叶绿素含量以及群落中光照分布和消光系数等方面进行分析和比较。