Plant effects on the soil community: A microcosm experiment

Eur. J. Soil Biol., 1999, 35 (I), S1164556399001041/FLA

17-21 0 1999 editions

Plant effects

scientif’iques

et mkdicales

SAS. All rights reserved

on the soil community: Juha

Department of Biological

Elsevier

and Environmental

Katajisto,

A microcosm

experiment

Veikko

Jouni

Huhta*,

Laakso

Science, University OfJyviiskylii, Box 35, 40351 Jyvtiskylti, Finland. * Corresponding

author (fax: +358 14 602321; e-mail:[email protected]

ReceivedJanuary29, 1999;acceptedSeptember28, 1999.

- An experimentwascarriedout in microcosms for testingthe hypothesisthat a higherlevel of primaryproductionshould maintaina decomposer communitywith higherbiomassandactivity. Microcosmswith coniferousforesthumusanda diversemicrobial and fauna1communityweredivided into threesets:(1) control without plants,(2) with birch seedlingsin full illumination,and (3) with birch seedlings,shadedto reducethe netprimary production.During 16weeksof incubationat +16 “C, no treatmenteffects werefound in numbersor biomassof taxonomicor functional groupsof soil organisms,nor in the systemrespirationin darkness. The communitystructureof the shadedsystemsdifferedfrom that of the othertwo treatments(CCA analysis).High organiccontent of the soil and long time-lagin the systemresponsewere consideredthe main reasonsfor the smalldifferencebetweenthe treatments.0 1999Editionsscientifiqueset medicalesElsevierSAS Abstract

Decomposer

community

/ root effects / forest soil

R&urn6 - Influence des plantes sur les communauth du sol : une expbimentation en microcosme. Uneexperimentationa CtC conduitedansdesmicrocosmes pour testerl’hypotheseconsiderantque le niveauClevCde la productionprimairedevrait maintenir uneactivite et unebiomasse Cleveesdansunecommuna.utC de decomposeurs. Desmicrocosmes renfermantdeshumusde coniferes et unecommunautemicrobienneet fauniquediversifieesont CtCrepartisentrois series: (1) temoinsansplantes,(2) avecbouleauen plein Cclairage,et (3) avec bouleauet ombragepour reduire la productionprimairenette. Durant les 16 semaines d’incubationa +16 “C, aucuneffet destraitementsn’a Cteobserveni ence qui concernele nombre,la biomasse ou le fonctionnementdesgroupes d’organismes du sol, ni en ce qui concernel’intensid respiratoirea l’obscurite.La structurede la communautedu systemeombrage differe de celle desdeux autrestraitements(analysecanoniquedescorrespondances). La teneurCleveeen matiereorganiquedu sol et le longdelaide reponsesontconsider&commelesprincipalesraisonsdu peude differenceobserveeentrelestraitements.0 1999 Editionsscientifiqueset medicalesElsevierSAS CommunautC

de dkcomposeurs

/ influence

des racines / sol forestier

been realized. Due to the increasedamount of available energy [9, 14, 18, 193,the narrow zone surrounding the root, the rhizosphere, is considered a ‘hot spot’ of biological activity [4]. However, experimental evidence on plant effects on the soil fauna is scarce, especially concerning soils with a high organic content. In addition to considerably increased bacterial and protozoan populations [5], positive ‘rhizosphere effects’ have been reported on nematodes[ 10, 1 l] and collembolans [7]. There is a feedback loop between primary production and soil processes[3]. The presence of soil fauna generally enhancesthe rate of decomposition and min-

1. INTRODUCTION The amount of net primary production supports and regulates the heterotrophic soil community, both directly via root exudation, and indirectly by determining the amount of dead organic matter entering the decomposer food web. According to Persson [23], Scats pine (Pinus sylvestris) may transport up to 60 % of the assimilated carbon into the root system, where it is used for growth and replenishment of roots and mycorrhizae, and for root exudation. The important role of root exudates in the soil processeshas only recently Eur. J. Soil Biol.,

1164-5563/99/01/O

1999 fiditions

scientifiques

et mCdicales

Elsevier

SAS. All

rights reserved

18 eralization [26-28, 331, which further exerts a positive influence on primary production [ 13, 17, 261. This in turn is expected to benefit the soil community. The aim of this study was to assess the impact of primary production on decomposer biomass and community structure, N mineralization and system respiration in a soil of high organic content (mor). We carried out the experiment in microcosms with coniferous forest humus and birch seedlings. One set of them was kept in full illumination in a climate chamber, in another set the primary production was reduced by shading, and both were compared with systems without plants. Our main hypothesis was that a higher level of primary production should maintain a decomposer community with higher biomass and activity.

2. METHODS The experiment was conducted in 1.3-L transparent plastic bottles. Twenty seven microcosms were divided into three treatments: (1) control, no plants, (2) unshaded with birch seedlings, and (3) shaded with birch seedlings. A thin, white plastic cylinder was inserted inside the shaded variants. This reduced the photosynthetically active illumination (400-700 nm; PRM-14 recorder) by 61 % in comparison with the unshaded ones, but did not affect the temperature (digital thermometer, accuracy + 0.1 “C). Forest soil organic layer was taken from a Myrtillustype spruce stand near the town of Jyvaskyla, central Finland, sieved (10 mm) and mixed (living plant parts, twigs and large soil animals were removed), and 116.6 g portions (fresh; 36.0 g DM) were divided into the microcosms. To compensate for losses in the mechanical treatment, microarthropods were re-introduced from additional soil samples, from which the animals were extracted using two large (60 x 60 cm) Tullgren funnels suspended above a rotating disc with thirty collecting jars. Each microcosm received the contents of one jar, after removing large invertebrates. A small (average 2 cm) seedling of silver birch (Betula pen&la), inoculated with mycorrhizae, was planted in the centre of the nine shaded and nine unshaded microcosms. Soil moisture was adjusted to 69 % of fresh mass, and the soil in all microcosms was covered with a disc of black plastic to prevent growth of mosses and weeds. The bottles were closed with cotton plugs and incubated in a climate chamber at +16 “C with a daily illumination cycle of 19 h light (228 pE.rn-‘.s-’ PAR) + 5 h darkness. During the first weeks, a few seedlings were killed by a curculionid beetle Otiorhynchus scriber, and were later substituted. The microcosms were rewatered at ca. 2-week intervals, when 2 to 3 mL water had evaporated. Eventual weeds were killed but left in the microcosms.

J. Katajisto

et al.

Carbon dioxide evolution from the microcosms was measured both in full illumination and in darkness at weeks 2, 4, 8, 10, 12, 14 and 16 using an infrared carbon analyser (EQ 92 Easy Quant). Air samples (1 mL) were taken with injection syringes, at first through the cotton plugs, then after sealing the bottles and incubating for 30 min. The experiment was terminated after 16 weeks. Some replicates were abandoned because the seedlings had died or were severely suffering; six shaded and eight unshaded with plants and nine controls were included in the analyses. Soil dry weights were determined without the aboveground parts of plants. Roots were separated under water from 40-g sectors of soil. Leaves, stems and roots were separately dried at +80 “C and weighed. Soil pH (H,O), water content and loss on ignition were measured. KCl-extractable (2 mol) NH:-N was determined from 5 g (fresh) soil samples according to SFS standard 3032. Substrate-induced respiration (SIR) was measured from 10 g (FM) samples using the method of Anderson and Domsch ([I]; slightly modified). Before mixing the soil, 35.5 g (FM) sectors of soil were cut for extracting microarthropods (high gradient extractor), and 25.0 g for enchytraeids (wet funnel by O’Connor [21]). Then the soil was mixed and 5.0 g subsamples were weighed for nematodes and tardigrades extracting (wet funnel by Sohlenius [31]). The animals were counted and identified: Nematoda (subsamples of 100) to genera, Acarina to species, genera or families, Collembola (subsamples of 100) to species, Coleoptera to families and Enchytraeidae to species (Cognettia splzagnetorm only). Biomasses of Nematoda were estimated according to Andrassy [2], those of microarthropods after Persson et al. [24], and those of enchytraeids after Huhta and Koskenniemi [ 151. Microbial carbon was transformed into microbial biomass assuming C to be 49 % of dry mass. The organisms were classified into trophic groups at three resolutions: (1) twelve groups according to feeding habits [6, 25, 29, 30, 34, 351 and size: microbes, bacterivores, fungivores, microbi-detritivores, herbivores, omnivores, predators; (2) three major groups: microbes, 2nd level consumers, 3rd level consumers; and (3) total biomass. Effects of treatments on abiotic variables and biomass of trophic groups were compared using ANOVA, when the assumptions of parametric analyses were fulfilled. Treatment effects on the community structure were analysed using canonical ordination (CCA; CANOCO programme), which is suited for community data 1321. The treatments were set as nominal environmental variables. Significance of treatment/ community structure relationship was tested using the Monte Carlo permutation [32]. Relationships between abiotic variables and trophic structure were tested similarly after removing the treatment effects (treatments as covariants). Eur. J. Soil Bid

Plant effects on soil community

19

1.2

-$ ’ 0.8 0.8

Figure

1. System

respiration:

in darkness subtracted during illumination.

CO, evolved

by

CO,

evolved

4

8

10

12

14

16 Week

3. RESULTS The birch seedlings in the unshaded microcosms produced 30 % more biomass than those in the shaded ones. At week 16, the birch mass in the unshaded systems was 2.8 %, and in the shaded ones, 2.0 % of the total dry mass of the systems. The effect of shading on the photosynthetic activity was estimated by calculating the difference between CO, evolution in darkness and in full illumination (fisure Z). During the last quarter of the experiment, the microcosms without birches assimilated the least carbon, and the shaded birch seedlings over 50 % less effectively than the unshaded seedlings (ANOVA for the last quarter: F,,,, = 6.9, P = 0.002; log-transformed data). Thus, the experimental design was successful in this respect. However, the respiration in darkness and the cumulative respiration did not differ between the three treatments (fisure Z), which indicates that the systems were equal with respect to decomposer activity. The soil pH was ca. 0.5 units lower (4.5-4.6) in the microcosms with plants than in those without plants (ANOVA: F = 25.9, P < O.OOl), but no difference was found between the shaded and unshaded systems. Loss on ignition was 1 % lower in the shaded microcosms (82 %) than in the unshaded ones with plants (ANOVA: F = 4.93, P = 0.018). The amount of KCIextractable NH:-N was highest (470 yg.g-l DM) in the

control microcosms without plants, and lowest in those with birch seedlings (100 in shaded, 40 pg-g-’ in unshaded systems; ANOVA: F = 75.09, P < 0.001). (The amount of nitrate nitrogen in this soil is negligible, and was not measured). The abiotic factors had no influence on the soil community structure (Monte Carlo permutation; F = 2.00, P = 0.139). Microbes formed the bulk (average 93 %; SIR method) of the total decomposer biomass, which was about equal in all treatments. Total fauna1 biomass in the shaded systems was 13 % lower than in unshaded with plants, and 12 % lower than in controls. More than half of the animal biomass was composed of earthworms (Dendvobaena octuedru). No trophic group showed a significant difference between the treatments (ANOVA). CCA analyses, both on taxa and on trophic groups, revealed that the shaded systems differed significantly from the other two treatments (figure 3) (taxa, eigenvalues for the first three axes: 0.03, 0.08 and 0.045; Monte Carlo permutation, axis 1: F = 2.01, P = 0.038; trophic groups, eigenvalues: 0.018, 0.061 and 0.015; Monte Carlo permutation, axis 1: F = 3.78, P = 0.020). A total of 61 taxa were recorded in the shaded microcosms, 57 in the controls, and 56 in the unshadedones with plants; this difference in speciesdiversity was not statistically significant.

4. DISCUSSION 0.6

T I-O-No

plant

0 2

4

6

10

12

1 16

14 Week

Figure

2. Cumulative

Vol. 35.

no 1 - 1999

system respiration

in darkness.

It has been shown that root exudates stimulate microbial growth in the rhizosphere [4, 121which can further increasethe populations of microbial grazers in comparison with the bulk soil [ 11, 12, 221. However, the results of the present experiment do not support the hypothesis that a higher level of primary production should maintain a decomposer community with higher biomass and activity. The reason for the weak system response in our experiment may be due to the high organic content of raw humus soil. Parmelee et al. [22] observed that an

J. Katajisto

20

et al.

+ Oribjuv

Hepao IPar&t Tardic ra + + Plecttot +Eudlatot r$kor+ +Stegaca En cfl Fol&ad+ ‘Phthsp Plapptot* Prioto t’ l

‘Isono

Figure 3. CCA analysis on taxonomic units, including seventeen toxa with the highest weights in the ordination. X axic. treatment.

.Eosttot l

Lumbr ic

Coleopt

increase in pine (Pinus rigida) root biomass decreases the amount of NH:-N and retards microbial growth in soil with high organic matter content (42 %, which is half of that in our experiment). The positive influence of the root system on the soil biota became evident only in mineral soil, where the rhizosphere supported higher populations of microbes, nematodes and microarthropods. In addition, Fisher and Gosz [8] showed that the biomass of soil organisms increases in mineral soil with the growth of root biomass. The high C:N ratio of soil may also lead to a long time lag in the system response. Early in the experiment, energy is easily available for soil microbes (dead tine roots, etc.), and concentration of mobile nutrients is also high [ 161. As the seedlings were still small, root exudation probably formed only a small part of the total available energy, and the decomposer system at this stage was not nutrient-limited either. By the end of the experiment (week 16, after one growing season), the plants in the unshaded microcosms had extracted

most of the mineral N from the soil, but maybe this had not yet started to limit the microbial production. Laakso et al. [20] observed that decomposer populations in microcosms started to decline after 9 months, when no fresh organic matter was added. Acknowledgements We wish to thank the Soil Ecology Group at the Department of Biological and Environmental Science, University of Jyv&skyl& for pleasant co-operation. The study was financed by the Academy of Finland.

REFERENCES [I] Anderson J.P.E., Domsch K.H., A physiological method for the quantitative measurement of microbial biomass in soils. Soil Biol. Biochem. 10 (1978) 215-221. Eur. J. Soil Biol.

Plant effects on soil community [2] Andrassy I., The determination of volume and weight of nematodes, Acta Zool. (Hung. Acad. Sci.) 2 (1956) l-15. [3] Bengtsson J., Setill H., Zheng D.W., Food webs and nutrient cycling in soils: Interactions and positive feedbacks, in: Polis G.A., Winemiller K.O. (Eds.), Food Webs, Chapman & Hall, New York, 1996, pp. 30--38. [4] Curl E.A., Truelove B., The Rhizosphere, Springer, New York, 1986, 288 p. [5] Darbyshire J., Soil Protozoa, CAB International, London, 1994,209 p. [6] Didden W.A.M., Ecology of terrestrial Enchytraeidae, Pedobiologia 37 (1993) 2-29. [7] Finlay R.D., Interactions between soil micro-arthropods and endomycorrhizal associations of higher plants, in: Fitter A.H. (Ed.), Ecological Interactions in Soil, Blackwell, Oxford, 1985, 319-331. [8] Fisher EM., Gosz J.R., Effects on trenching on soil processes and properties in a New Mexico mixed-conifer forest, Biol. Fertil. Soils 2 (1986) 35-42. [9] Grayston S.J., Vaughan D., Jones D., Rhizosphere carbon flow in trees, in comparison with annual plants: the importance of root exudation and its impact on microbial activity and nutrient availability, Appl. Soil E:col. 5 (1996) 29-56. [lo] Grifflths B.S., A comparison of microbial-feeding, nematodes and protozoa in the rhizosphere of different plants, Biol. Fertil. Soils 9 (1990) 83-88. [ 111 Griftiths B.S., Young I.M., Boag B., Nematodes associated with the rhizosphere of barley (Hordeum vulgure), Pedobiologia 35 (199 1) 265-272. [12] Grifflths B.S., Welschen J.J., van Arendonk C.M., Lambers H., The effect on nitrate-nitrogen supply on bacteria and bacterial-feeding fauna in the rhizosphere of different grass species, Oecologia 91 (1992) 253259. [13] Haimi J., Huhta V., Boucelham M., Growth increase of birch seedlings under the influence of earthworms - A laboratory study, Soil Biol. Biochem. 24 (1992) 15251528. [14] Hendricks J.J., Nadelhoffer K.J., Aber J.D., Assessing the role of fine roots in carbon and nutrient cycling, Tree 8 (1993) 174-178. [15] Huhta V., Koskenniemi A., Numbers, biomass and community respiration of soil invertebrates in spruce forests at two latitudes in Finland, Ann. Zool. Fenn. 12 1(1975) 164-182. [16] Huhta V., Sulkava P., Viberg K., Interactions between enchytraeid (Cognettia sphagnetorum), microarthropod and nematode populations in forest soil at different moistures, Appl. Soil Ecol. 9 (1998) 53-58. [17] Ingham R.E., Trofymow J.A., Ingham E.R., Coleman D.C., Interactions of bacteria, fungi, and their nematode grazers: effects on nutrient cycling and plant growth, Ecol. Monogr. 55 (1985) 119-140. [ 181 Johansson G., Release of organic C from growing roots of meadow fescue (Fesrucu prutensis L.), Soil Biol. Biochem. 24 (1992) 427433. [19] Keith H., Oades J.M., Martin J.K., Input of carbon to soil from wheat plants, Soil Biol. Biochem. 18 (1986) 445-449.

Vol. 35. no 1

1999

21 [20] Laakso J., Salminen J., Settil% H., Effects of abiotic conditions and microarthropod predation on the structure and function of soil animal communities, Acta Zool. Fenn. 196 (1995) 162-167. [21] O’Connor F.B., The extraction of Enchytraeidae from soil, in: Murphy P.W. (Ed.), Progress in Soil Zoology, Chapman & Hall, London, 1962, pp. 279-285. [22] Parmelee R.W., Ehrenfeld J.G., Tate R.L., Effects of pine roots on microorganisms, fauna, and nitrogen availability in two horizons of coniferous forest spodosol, Biol. Fertil. Soils 15 (1993) 113-l 19. [23] Persson H., Root dynamics in a young Scats pine stand in central Sweden, Oikos 30 (1978) 508-5 19. [24] Persson T., B%th E., Clarholm M., Lundkvist H., SbderstrGm B.E., Sohlenius B., Trophic structure, biomass dynamics and carbon metabolism of soil organisms in a Scats pine forest, Ecol. Bull. (Stockholm) 32 (1980) 419-459. [25] Ponge J.F., Food resources and diets of soil animals in a small area of Scats pine litter, Geoderma 49 (1991) 33-62. [26] Set%12 H., Huhta V., Soil fauna increase Beth pendula growth: Laboratory experiments with coniferous forest floor, Ecology 72 (1991) 665-671. [27] Set212 H., Haimi J., Huhta V., A microcosm study on the respiration and weight loss in birch litter and raw humus as influenced by soil fauna, Biol. Fertil. Soils 5 (1988) 282-287. [28] SettilB: H., Martikainen E., Tyynismaa M., Huhta V., Effects of soil fauna on leaching of nitrogen and phosphorus from experimental systems simulating coniferous forest floor, Biol. Fertil. Soils 10 (1990) 17@177. 291 Siepel H., Maaskamp F., Mites of different feeding guilds affect decomposition of organic matter, Soil Biol. Biochem. 26 (1994) 1389-1394. 301 Siepel H., de Ruiter-Dijkman E.M., Feeding guilds of oribatid mites based on their carbohydrase activities, Soil. Biol. Biochem. 11 (1993) 1491-1497. . 31 ] Sohlenius B., A carbon budget for nematodes, rotifers and tardigrades in a Swedish coniferous forest soil, Holarctic Ecol. 2 (1979) 30-40. 32!] Ter Braak C.J.F., CANOCO - a Fortran program for Canonical Community ordination by partial detrended canonical correspondence analysis, principal components analysis and redundancy analysis (version 2.1), TN0 Institute of Applied Computer Science, Wageningen, 1987. [33] Verhoef H.A., Brussaard L., Decomposition and nitrogen mineralization in natural and agroecosystems: the contribution of soil animals, Biogeochemistry 11 (1990) 175-211. [34] Walter D.E., Hunt H.W., Elliot E.T., Guilds or functional groups? An analysis of predatory arthropods from a shortgrass steppe soil, Pedobiologia 31 (1988) 247-260. [35] Yeates G.W., Bongers T., de Goede R.G.M., Freckman D.W., Georgieva S.S., Feeding habits in soil nematode families and genera - An outline for soil ecologists, J. Nematol. 25 (1993) 315-331.