Applied Vegettion Science ]]: 1 11, 29 DOI: 1.1111/j.1654-19X.29.17.x & 29 Interntionl Assocition for Vegettion Science Nutrient limittion in species-rich Clthion grsslnds in reltion to opportunities for restortion in pet medow lndscpe B.P. Vn de Riet, A. Brendregt, K. Brouns, M.M. Hefting & J.T.A. Verhoeven Abstrct Questions: Which nutrient(s) limit(s) vegettion productivity in Clthion grsslnds? Is phosphorus relese bottleneck for restortion of species-rich Clthion grsslnds on rewetted diry medows? Loction: Three species-rich Clthion grsslnds in the Western Pet District in the Netherlnds. Methods: We conducted field fertiliztion experiment with nitrogen (N), phosphorus (P) nd potssium (K) in three existing Clthion grsslnds to evlute the potentil for restortion on rewetted diry medows. Responses of bove-ground biomss, tissue nutrient concentrtions nd nutrient rtios were determined fter 2 yr of fertiliztion. Results: Biomss incresed with fertiliztion with N- only nd K-only but did not rect to P-only dditions. Comprisons of tissue nutrient concentrtions nd nutrient rtios lso gve indictions of N nd K limittion. Conclusions: The strong P relese expected fter rewetting should not necessrily interfere with restortion of Clthion communities on rewetted diry medows. It is concluded tht for successful restortion mngement mesures should focus on reducing N nd/or K vilbility. Potssium might be n overlooked bottleneck in the restortion of species-rich grsslnds. Keywords: Field fertiliztion; Nitrogen; Pet medow restortion; Phosphorus; Potssium. Nomenclture: Vn der Meijden (25), Schminée et l. (1996). Vn de Riet, B. P. (corresponding uthor; B.P.vnde Riet@uu.nl), Brouns, K. (K.Brouns@uu.nl), Hefting, M. M. (M.M.Hefting@uu.nl) & Verhoeven, J. T. A. (J.T.A.Verhoeven@uu.nl): Utrecht University, Lndscpe Ecology, P.O. Box 8 84, NL 358 TB Utrecht, the Netherlnds. Brendregt, A. (A.Brendregt@geo.uu.nl): Utrecht University, Environmentl Sciences, P.O. Box 8 115, NL 358 TC Utrecht, the Netherlnds. Introduction In Europe, species-rich grsslnds hve become restricted in re since intensive griculture strted to expnd t the beginning of the twentieth century (Fuller 1987; Ellenberg 1996; Grootjns et l. 1996; Prch 1996; Schrutzer et l. 1996). Improvement of griculturl production hs gone together with the extinction of rre plnt species nd gret losses of biodiversity (Ellenberg 1996). Smll relicts of species-rich grsslnd communities survive in nture reserves. Nevertheless, species composition continues to be negtively ffected by high nutrient concentrtions in surfce wter nd by tmospheric nitrogen (N) deposition (Ellenberg 1996; Bobbink et l. 1998). The Western Pet District in the Netherlnds is n exmple of n re where dringe nd excessive fertilizer ppliction hve trnsformed lrge complexes of wet species-rich grsslnds into highly productive species-poor grsslnds (Scheygrond 1931; Zuidhoff et l. 1996). As consequence of long-term dringe nd the ssocited lnd subsidence, most pet polders (res with controlled wter levels) re now situted 1-2 m below se level. The combintion of subsidence rtes of 1-2 cm yr 1 (Vn den Akker et l. 28) nd totl predicted se-level rise of 35-85 cm by the end of this century (KNMI 26; IPCC 27) require dpttion of current lnd-use nd wter-mngement policies. Rewetting medows to prevent or slow down soil subsidence cused by pet oxidtion hs been proposed by uthorities in the Netherlnds. The Dutch government currently tkes inititives in mjor lnd-use trnsitions; in mny of these projects lrge res re designted to be restored s wet species-rich grsslnds. However, rising wter tbles often hs not led to successful restortion of species richness owing to high soil fertility (Klimkowsk et l. 27; Vn Dijk et l. 27). It is likely tht the long history of intensive diry frming is mjor constrint for restoring species richness becuse diry medows contin fr more nutrients thn the trgeted
2 VAN DE RIET, B.P. ET AL. semi-nturl grsslnd communities. Moreover, rewetting itself strongly influences the nutrient vilbility nd stoichiometry in soils. In prticulr, phosphorus (P) relese is considered to be mjor constrint becuse its vilbility is dependent on soil redox potentil nd strongly increses fter rewetting, prticulrly in soils rich in iron (Young & Ross 21; Vn Dijk et l. 24; Zk & Gelbrecht 27). In ddition, Zk & Gelbrecht (27) found incresed mmonium concentrtions in the initil phses fter rewetting. It is cler tht under these nutrient-rich circumstnces the restortion of reltively eutrophic fen medow communities belonging to the Clthion plustris llince will be more fesible thn plnt communities from nutrient-poor ecosystems. The Clthion plustris llince hs been common in the Western Pet District till the 196s nd occurs on pet or pety cly soils tht, compred with other species-rich grsslnds, hve reltively high nutrient sttus (Kyl 1965; Ellenberg 1996). For successful restortion of species-rich Clthion grsslnds on rewetted former griculturl lnd it is importnt to crete conditions such tht vegettion productivity is limited by the nutrient tht will be most esily reduced fter rewetting. Knowledge bout which nutrient controls plnt growth in existing Clthion stnds in the Western Pet District cn provide us with vluble informtion. If Clthion stnds would be limited by P, the potentil for restortion of these communities on diry grsslnds would be miniml s phosphorus is often relesed in lrge quntities fter rewetting. If they were N- or potssium (K)-limited, there would be better opportunities. Knowledge of nutrient limittion would lso be vluble for choosing the pproprite restortion mesures, s reducing the vilbility of, for exmple N, requires other mesures thn for P or K (Koerselmn & Meulemn 1996; Vn Duren & Pegtel 2). We hypothesize tht N, rther thn P is limiting growth in Clthion grsslnds, becuse N frequently hs been found limiting, especilly in reltively undisturbed herbceous wetlnd communities (Vn Duren & Pegtel 2) such s in vrious wet medow types (Vermeer 1986, b; Vn der Woude et l. 1994; Boeye et l. 1997) nd fens (Verhoeven et l. 1983; Verhoeven & Schmitz 1991; Wssen et l. 1995, 1998). Phosphorus limittion seems mostly confined to low productive ecosystems tht hve hd long history of mowing nd no fertilizer use (Koerselmn et l. 199; Verhoeven & Schmitz 1991) or to sites tht re rich in iron, luminium (De Mrs et l. 1996; Boeye et l. 1997) or clcium (Boyer & Wheeler 1989). We lso wnted to test the possibility of potssium limittion nd included it s tretment, becuse K limittion, lthough not frequently found in wetlnd vegettion (Verhoeven et l. 1996), hs been demonstrted for Clthion grsslnd in the Pleistocene District in the Netherlnds (Vn Duren et l. 1997b). A field fertiliztion experiment ws crried out to investigte which (combintion) of the mcronutrients N, P, or K increses biomss production in existing Clthion medows in the Western Pet District. The following reserch questions were ddressed: (1) Wht type of nutrient limittion controls biomss production in Clthion grsslnds? (2) Is phosphorus relese fter rewetting bottleneck for restortion of species-rich Clthion medows? Methods Study sites A fertiliztion experiment ws crried out on three species-rich Clthion grsslnds in the Pet District in the western prt of the Netherlnds (Fig. 1). The grsslnd Hzeleger is situted in the Nture Reserve De Nieuwkoopse Plssen (5218 25 N, 4148 4 E). The grsslnds Kttendijk (51158 47 N, 414 19 E) nd Veerstlblok (51159 52 N, 4142 16 E) re both smll nture reserves locted in the Polder Krimpenerwrd long the River Hollndse IJssel. The grsslnd Hzeleger hs been in extensive griculturl use t lest since the seventeenth century, but probbly since the region ws reclimed in the Middle Ages. It is reltively poor in soil nitrogen nd soil phosphorus (Tble 1). The other two grsslnds functioned until the end of the nineteenth century s wter storge bodies in which wter from the surrounding polder ws collected nd subsequently dischrged into the river. Around 1885 the vegettion ws dominted by Phrgmites ustrlis nd Scirpus lcustris, but fter losing their initil wter storge function both sites were drined nd used for hymking (Scheygrond 1931). Both grsslnds re reltively eutrophic compred with Hzeleger; the Veerstlblok soil is reltively rich in phosphorus, while the Kttendijk soil is rich in potssium (Tble 1). Both grsslnds re currently isolted from the river nd re never flooded. The medow Kttendijk, however, is receiving some seepge wter from the river, s cn be seen in the high concentrtions of sulphte, bicrbonte nd potssium in pore wter (Tble 1).
NUTRIENT LIMITATION IN SPECIES-RICH CALTHION GRASSLANDS 3 HL Nieuwkoopse Plssen Woerden River Oude Rijn Bodegrven Goud River Hollndse IJssel VE KD Krimpenerwrd River Lek 5 km Fig. 1. Loction the Clthion grsslnds (%) tht were included in the field fertiliztion experiment. HL 5 Hzeleger, KD 5 Kttendijk nd VE 5 Veerstlblok. All three medows re nnully mown in June-July. A second mowing tkes plce in the grsslnds Veerstlblok nd Kttendijk in cse of strong regrowth of the vegettion. Occsionlly, in Kttendijk low-intensity sheep grzing is introduced in utumn. Wter tbles vry from close to the ground surfce in winter to 15-3 cm below surfce in summer. However, the grsslnd Kttendijk is hydrologiclly connected to the surrounding polder nd groundwter levels my decrese significntly in dry summers. All three medows hve high conservtion vlue, becuse they hrbour plnt communities tht belong to the most vluble in the Western Pet District in terms of species richness of higher plnts. Syntxonomiclly, the vegettion belongs to the Clthion plustris llince nd is chrcterized by the presence of, for exmple, Clth plustris, Lychnis flos-cuculi nd Rhinnthus ngustifolius (Zuidhoff et l. 1996). Dominnt grminoids re Holcus lntus, Anthoxnthum odortum nd Crex cut. Species richness is high in ll three medows with pproximtely 19 species per squre metre (Tble 2). Field fertiliztion At ech grsslnd five replicte blocks of homogeneous vegettion were selected. Ech block
4 VAN DE RIET, B.P. ET AL. Tble 1. Soil nitrogen (N), phosphorus (P) nd potssium (K) pools in the -2 cm soil lyer (g m 2 ; n 5 5), in the bove-ground biomss (g m 2 ; n 5 5), nd the nutrient concentrtions in pore wter (mg l 1 ; n 5 3) (verge vlues 1 SE) in the three Clthion grsslnds Hzeleger (HL), Kttendijk (KD) nd Veerstlblok (VE). contined five 11 m plots for fertiliztion with N, P, K, NPK-combined nd control tretment, respectively. Between the tretment plots there ws 5 cm wide buffer nd within ech block the tretments were rndomly ssigned to the plots. For the nutrient solutions NH 4 NO 3 (N tretment), NH 2 PO 4 (P tretment), KCl (K tretment) were dissolved in deminerlized wter. Fertiliztion ws pplied in four dressings per growing seson; ech plot received 2 l of fertilizer solution t 3-wk intervls, strting in Mrch. The control tretment received only deminerlized wter. The quntities pplied mounted to 15 kg N h 1 yr 1, 6 kg P h 1 yr 1 nd 15 kg K h 1 yr 1, respectively. The NPK tretment received combintion of ll nutrients in quntities similr to the single tretments. Fertiliztion ws crried out for two successive growing sesons, in 27 nd 28. Biomss HL KD VE Soil nd vegettion Soil N.4 M KCl.3 (.4).47 (.7) 1.42 (.34) N in stnding stock 6.16 (.59) 8.63 (.51) 5.64 (.64) Soil P lctte 2.17 (.19) 4.6 (.8) 12.38 ( 2.17) P in stnding stock.6 (.9) 1.16 (.6) 1.3 (.21) Soil K demi.45 (.28) 2.12 ( 1.26).21 (.18) K in stnding stock 1.83 (.13) 5.6 (.98) 1.53 (.16) Pore wter NO 3.15 (.4).28 (.7).13 (.3) NH 4.15 (.4).32 (.5).24 (.5) PO 4.11 (.3).21 (.4).21 (.7) K.66 (.14) 1.62 (.59).81 (.1) Fe 1.47 (.24).88 (.1) 1.69 (.32) Al.17 (.2).8 (.1).9 (.1) C 2. ( 1.8) 69.5 ( 12.6) 12.5 ( 1.5) Mg 4.32 (.34) 5.11 (.97) 1.74 (.25) HCO 3 28.4 ( 4.2) 88.7 ( 17.9) 27.7 ( 4.8) SO 4 42.6 ( 5.2) 117.3 ( 24.3) 1.7 ( 1.) Cl 7.17 ( 1.18) 17.2 ( 3.2).67 (.32) N 7.97 (.45) 14.5 ( 2.3) 1.26 (.19) Pore wter smples were collected in Apr 27, soil nutrients were determined in Sep 28 following the procedures described by Houb et l. (1989). In July 28 the bove-ground biomss ws hrvested t pek stnding crop by clipping the vegettion in subplots of 5 cm5 cm to ground level. Vegettion smples were stored t 21C. The hrvested vegettion of Veerstlblok nd Hzeleger Tble 2. Synoptic tble summrizing species richness (verge 1 SE) nd both the frequency (1-5) nd verge cover clss (I-VII) for the higher plnt species in the control plots (n 5 5) in the Clthion grsslnds Hzeleger (HL), Kttendijk (KD) nd Veerstlblok (VE) in June 28. HL KD VE Species richness (m 2 ) 18.8 1.4 18.8 1.3 19.4.8 Species Holcus lntus 5 VI 5 VI 5 IV Anthoxnthum odortum 5 V 5 VI 5 VII Rhinnthus ngustifolius 4 I 5 II 5 V Rumex cetos 4 I 3 IV 5 I Rnunculus cris 3 I 5 III 4 IV Festuc rubr 5 VI 2 I 4 V Crdmine prtensis 5 I 2 I 4 I Plntgo lnceolt 4 II 1 II 5 VII Lychnis flos-cuculi 4 II 2 I 4 I Lotus peduncultus 5 VI 2 II 3 V Crex cut 1 V 3 VII 5 VII Rnunculus repens 2 II 3 IV 3 II Glium plustre 3 I 2 I 3 I Juncus conglomertus 3 I 2 I 2 V Filipendul ulmri 3 V 2 IV 1 I Cerstium fontnum subsp. 1 I 2 I 1 I vulgre Trifolium prtense 2 II 1 I 1 I Cirsium plustre 4 II 5 IV Lythrum slicri 2 I 3 I Vlerin officinlis 1 I 2 I Vici crcc 4 I Phrgmites ustrlis 4 I Glechom hederce 3 I Crex nigr 3 VI Luzul multiflor subsp. 2 I multiflor Crex ripri 2 I Crex distich 3 VI 5 VII Equisetum fluvitile 5 I Cynosurus cristtus 2 VI Crex hirt 2 I Po prtensis 4 IV 4 II Lolium perenne 4 IV 3 II Festuc prtensis 5 IV 2 I Clth plustris subsp. plustris 5 VI 1 I Trxcum officinle 4 I 1 I Juncus effusus 4 V Chrcter species of the Clthion llince re shown in bold type (ccording to Schmine e et l., 1996). Infrequent species re not listed (i.e. frequency 5 1 in complete dtset). Averge cover clsses: I 5-1%; II 5 1-2%; III 5 2-3%; IV 5 3-4%; V 5 4-8%; VI 5 8-18%; VII 5 18-38%. ws sorted into different species groups (Pocee, Cypercee, Junccee, Fbcee, Equisetcee), stnding ded nd herbs. The hrvested vegettion of Kttendijk ws not sorted nd only totl dry weight ws mesured. For tht purpose smples were dried until mss constncy (71C, for t lest 48 h) nd weighed. Per plot vegettion ws ground nd 15 mg per smple ws used to determine totl N, P nd K concentrtion of bove-ground biomss with slicylic cid thiosulphte modifiction of the
NUTRIENT LIMITATION IN SPECIES-RICH CALTHION GRASSLANDS 5 Kjeldhl digestion (Bremner & Mulvney 1982). The N nd P concentrtions were mesured colorimetriclly on continuous flow Auto Anlyser (Sklr SA-4; Sklr Anlyticl, Bred, the Netherlnds) nd K concentrtions by flme emission spectroscopy (IL 543; Instrumenttion Lbortory, IJsselstein, the Netherlnds). Sttisticl nlyses All sttisticl nlyses were crried out using SPSS 16. for Windows. For the overll effect of fertiliztion the biomss dt were nlysed using two-wy ANOVA with block s rndom fctor. For ech site biomss dt were nlysed seprtely using one-wy ANOVA. If necessry, dt were logrithmiclly trnsformed to stbilize vrinces between groups. Post-hoc Tukey tests were performed to test for significnt differences between tretments nd P-vlues were corrected for the multiple comprisons of the different tretments with the control tretment. Differences in nutrient concentrtions nd nutrient rtios mong sites were tested using one-wy ANOVA. One pired t-test ws used to nlyse whether nutrient concentrtions nd nutrient rtios differed from criticl nutrient concentrtions nd criticl rtios, respectively. Results Above-ground biomss Overll, totl biomss production ws significntly higher in the single N nd single K tretments thn in the control tretment (P 5.24 nd P 5.8, respectively). The combined NPK tretment hd the highest biomss production (P.1) nd fertiliztion with P hd no significnt effect (Fig. 2). When nlysed seprtely per site, the vegettion in Hzeleger, Kttendijk nd Veerstlblok incresed strongly fter the combined NPK tretments (P 5.8, P 5.36 nd P.1, respectively; Fig. 2b d). Responses to single-nutrient tretments were less strong or bsent, which indicted tht biomss production in ll three Clthion grsslnds is co-limited by two or more nutrients. In Veerstlblok, K incresed production significntly (Fig. 2d; P 5.24), but neither N nor P ddition hd significnt effects. In both Hzeleger nd Kttendijk single-nutrient tretments did not revel ny significnt effects. Biomss increse fter fertiliztion ws minly the effect of significntly incresed production of Pocee (dt not shown). In Veerstlblok, the biomss of Pocee incresed significntly fter N fertiliztion (P 5.8) nd fter the combined NPK tretment (P.1). Stnding ded mteril incresed in the combined NPK tretment (P 5.32). Moreover, in the N tretment C. cut tended to be more productive (P 5.84). In Hzeleger, no significnt effects on different species groups were observed; however, production of Pocee tended to increse in the combined NPK tretment (P 5.6) nd stnding ded tended to be incresed (P 5.96). In other species groups no significnt effects of the different fertilizer tretments were observed. Chnges in the dominnce of individul plnt species In ll three grsslnds N ddition obviously incresed the dominnce nd the number of inflorescences of H. lntus, A. odortum nd Festuc rubr. Crex distich, Crex nigr nd Filipendul ulmri lso incresed in dominnce in response to N. In plots fertilized with K n increse in cover nd number of inflorescences ws observed for H. lntus. The chrcter species of the Clthion llince (i.e. L. flos-cuculi nd R. ngustifolius nd other wet medow species such s Rnunculus flmmul, Rnunculus cris, Cirsium plustre nd Juncus effusus) showed more bundnt flowering in the K-treted plots. Lotus peduncultus, legume species cpble of fixing N, responded positively to potssium nd in Hzeleger lso to phosphorus. Nutrients in plnt tissue In ddition to mesuring the biomss response fter field fertiliztion, there re two methods tht cn give indictions for the type of nutrient limittion tht plnts hve experienced during growth. First, De Wit et l. (1963) showed tht if the tissue concentrtion of prticulr nutrient is lower thn certin criticl vlue, there is strong indiction tht plnt growth hs been limited by tht prticulr nutrient (Verhoeven et l. 1983). Second, rtios of tissue nutrient concentrtions proved to be indictive of nutrient limittion t the vegettion level (Koerselmn & Meulemn 1996; Gu sewell & Koerselmn 22; Olde Venterink et l. 23; Gu sewell 24). Figure 3-c depict the N, P nd K concentrtions in plnt tissue in the control plots. The dotted lines indicte the criticl nutrient concentrtions.
6 VAN DE RIET, B.P. ET AL. totl boveground biomss (g DW/m 2 ) 15 125 1 75 5 25 A overll totl boveground biomss (g DW/m 2 ) 15 125 1 75 5 25 B HL N P K NPK CTR N P K NPK CTR totl boveground biomss (g DW/m 2 ) 15 125 1 75 5 25 C KD totl boveground biomss (g DW/m 2 ) 15 125 1 75 5 25 D VE N P K NPK CTR N P K NPK CTR Fig. 2. Totl bove-ground biomss for the overll dtset nd for ech medow seprtely. Indictes significnt differences from control tretment (CTR, Po.5). overll 5 three medows combined, HL 5 Hzeleger, KD 5 Kttendijk, VE 5 Veerstlblok. Error brs indicte 11 SE. Dt from ll grsslnds combined show tht tissue P concentrtions were significntly bove, nd tissue K concentrtions significntly below this criticl vlue. Tissue N concentrtions were close to the criticl vlue indicting N limittion. We found the sme results for ech site except for Kttendijk where K ws not limiting (Fig. 3c). Nevertheless, K concentrtions were significntly lower thn the criticl concentrtion in the sites Hzeleger nd Veerstlblok (Fig. 3c). Figure 3b clerly shows tht P ws not limiting plnt growth in ny of the Clthion medows investigted. Nutrient rtios in the bove-ground plnt tissue re shown in Fig. 3d-f. In ll three grsslnds the N:P rtio ws significntly below the criticl vlue of 14.5, which is strong indiction tht N, rther thn P is limiting. Both the N:K rtios nd the K:P rtios in Hzeleger nd Veerstlblok indicted tht in ddition to N lso K ws limiting. Only in Kttendijk ws no indiction for K limittion found. Discussion Nutrient limittion in Clthion grsslnds The primry gol of this reserch ws to study the nutrient limittion in existing species-rich Clthion grsslnds in order to explore possible bottlenecks in the restortion success of this plnt community on bndoned diry medows. A field fertiliztion experiment is considered to be the most strightforwrd method for ssessing the type of nutrient limittion in herbceous vegettion (Vn Duren & Pegtel 2). In our study, high nutrient doses (comprble to the mounts used in intensive frming
NUTRIENT LIMITATION IN SPECIES-RICH CALTHION GRASSLANDS 7 N concentrtion (mg g 1 DW) 2 15 1 5 4 12 A B C b 1 b b b 3 8 2 6 4 1 2 overll HL KD VE overll HL KD VE overll HL KD VE P concentrtion (mg g 1 DW) K concentrtion (mg g 1 DW) N:P 2 15 1 5 5 7 D E F 6 4 5 b c N:K 3 b 2 1 K:P 4 3 2 1 b overll HL KD VE overll HL KD VE overll HL KD VE Fig. 3. (-c). Concentrtion of nitrogen (N), phosphorus (P) nd potssium (K) in bove-ground biomss in the control plots. Dotted lines represent criticl nutrient concentrtions ccording to De Wit et l. (1963) nd Verhoeven et l. (1983). (df) Nutrient rtios bsed on totl nutrient concentrtions in bove-ground biomss. Dotted lines represent criticl vlues for the different nutrient rtios derived from Olde Venterink et l. (23). % t the bse of the brs indictes tht brs differ significntly from the criticl vlues (P.5). Letters bove the tinted brs indicte differences between sites. Overll 5 ll three medows combined, HL 5 Hzeleger, KD 5 Kttendijk, VE 5 Veerstlblok. Error brs indicte 11 SE. prctice: 15 kg N h 1 yr 1,6kg P h 1 yr 1 nd 15 kg h 1 yr 1 ) were pplied during two growing sesons, which is considered sufficiently long period to test current nutrient limittion, without the effects occurring fter prolonged periods of fertiliztion when the nutrient dynmics of the stnd will chnge s result of shifts in the species composition (Güsewell 24; Vn der Hoek et l. 24). We hve lso used nutrient rtios nd criticl nutrient concentrtions for investigting indictions of nutrient limittion, which re bsed on extensive reserch reviews (Koerselmn & Meulemn 1996; Verhoeven et l. 1996; Gu sewell & Koerselmn 22; Olde Venterink et l. 23; Gu sewell 24). From the overll significnt growth responses to the fertiliztion tretments we conclude tht both N nd K re controlling biomss production in Clthion communities. In ddition, both the nutrient concentrtions nd the nutrient rtios in bove-ground biomss support this conclusion. Vn Duren et l. (1997b) found comprble results in fertiliztion experiments in Clthion communities in brook vlleys on minerl soils, where they occurred under quite different conditions with respect to hydrology nd geomorphology. The observed differences in responsiveness between the three sites, lthough not investigted in detil, might be explined by differences in locl hydrology, wter level nd lnd-use history. For exmple, the bsence of K limittion in Kttendijk might be cused by dischrge of groundwter originting from the river, rich in K, contrry to the other two sites which lck these inputs (Tble 1). Nitrogen limittion is frequently found in wetlnds (Vn Duren & Pegtel 2). It cn occur when N inputs vi fertiliztion, eutrophic surfce wter nd tmospheric deposition re low nd N loss is substntil: high wter tbles reduce N vilbility by slowing down minerliztion (Grootjns et l. 1985; Berendse et l. 1994; Schrutzer et l. 1996) nd by stimulting denitrifiction (Olde Venterink et l. 22; Hefting et l. 24). Potssium limittion occurs only occsionlly nd is mostly relted to dringe in combintion with long-term hymking (Verhoeven et l. 1983; Vermeer 1986b; De Mrs et l. 1996; Boeye et l.
8 VAN DE RIET, B.P. ET AL. 1997; Vn Duren et l. 1997). Kyser & Isselstein (25) mention K limittion to occur minly on sndy soil with low cly contents nd it is therefore ll the more surprising tht it ws found to be limiting in these Clthion grsslnds on pety-clyey soils in the Western Pet District. However, K is mobile ion nd severl ppers suggest tht significnt mounts cn be lost by leching fter dringe (De Mrs et l. 1996; Vn Duren et l. 1997b; Alfro et l. 24). Comprison of the soil nutrient pools in the three Clthions with the mount of nutrients nnully removed by hymking shows tht the vilble mounts of soil N nd K could grdully become depleted by nnul mowing (shortges of 5 7 g N m 2 nd 1 4 g K m 2 ), while the soil P pool is lrger thn the mount removed by mowing (surplus of 2 11 g P m 2 ; Tble 1). Although minerliztion probbly will replenish these nutrient pools prtly, these dt support the observtion tht the vegettion is not P-limited, while both N nd K might be in short supply. In mny herbceous plnt communities, reltively low biomss productivity is essentil for mintining species richness (Vermeer & Berendse 1983; Oomes 1992). Eutrophiction of species-rich herbceous stnds leds to incresed biomss production of competitive plnt species. Often, grminoid species become more dominnt, which leds to monotonous stnds, poor in plnt species (Bobbink et l. 1998; Gu sewell 24). We indeed did find n increse in the dominnce of grss species in the sites where productivity incresed s result of the ddition of N or combined NPK, lthough the setup of the experiment is not suitble for predicting long-term chnges in species composition. These generlly described shifts in species composition imply tht the nutrient vilbility of rewetted diry grsslnds should not be too high nd t lest one of the mjor nutrients N, P or K should be limiting, preferbly the sme element s in the reference sites where the vegettion is still intct. Implictions for restortion of Clthion vegettion on former diry medows The results of this study indicte tht it is importnt to strive for N nd/or K-limited plnt growth in restortion efforts to restore Clthion grsslnds on rewetted diry medows. Rewetting will ffect the vilbility of soil nutrients. Nitrogen vilbility is expected to decrese grdully in the long term becuse of slower N minerliztion (Grootjns, 1985; Berendse et l. 1994; Olde Venterink et l. 22) nd fster denitrifiction (Olde Venterink et l. 22; Hefting et l. 24). Little is known bout the effects of rewetting on K vilbility. In phytometer experiment, rewetting reduced the extent of K limittion for H. lntus grown on soil cores tken from K-limited site (Vn Duren & Vn Andel 1997). In soil cores, Koerselmn et l. (1993) showed K ws relesed in rewetted pet soil, but others found tht K ws not ffected by rewetting (Olde Venterink et l. 22). Despite these inconsistent results we know tht the K vilbility in drined diry medows is high nd cn be found to be incresed fter rewetting (B. P. vn de Riet, unpubl. dt). For P it is cler tht rewetting will increse its vilbility, becuse iron-bound P becomes mobilized t low redox potentils fter rewetting (Vn Dijk et l. 27; Zk & Gelbrecht 27). In mny cses, the success of restortion fter rewetting hs proven to be very limited (Klimkowsk et l. 27) nd P relese fter rewetting is often ssumed to be mjor constrint (Vn Dijk et l. 27). However, n importnt result of this reserch with respect to rewetting in the pet medow re is tht existing Clthion communities re not limited by P becuse the ppliction of high doses (6 kg P h 1 yr 1 ) did not cuse ny chnges in vegettion productivity. Our results imply tht the expected strong P relese fter rewetting, estimted to be up to 4 kg P h 1 yr 1 in rewetted griculturl petlnds in Germny (Zk et l. 28), should not necessrily interfere with restortion of Clthion communities, s long s N nd/or K vilbility cn be sufficiently reduced. For the restortion of Clthion grsslnd on bndoned diry medows, mngement mesures should be optimized to chieve N nd/or K limittion. We therefore hve the following recommendtions for mngement: (1) Extr nutrient inputs need to be prevented s much s possible. Fertiliztion should be cesed nd restortion sites should be isolted from surrounding griculturl res. (2) To ccomplish net removl of N nd K it is probbly necessry to mow multiple times per yer, s ws found for bndoned pet grsslnds nd eutrophic Clthions in northern Germny (Schrutzer et l. 1996). The sme ws indicted by nutrient budget study in fens in the Netherlnds, which reveled tht mowing once yer only blnced the tmospheric input of N (Koerselmn et l. 199). Oomes et l. (1996) reported tht period of 9 yr of two cuttings nnully resulted in such low K content in the biomss tht it ws likely to be limiting plnt productivity on n bndoned
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NUTRIENT LIMITATION IN SPECIES-RICH CALTHION GRASSLANDS 11 mesure of nutrient limittion in mire ecosystems. Journl of Vegettion Science 6: 5 16. Wssen, M.J., vn der Vliet, R.E. & Verhoeven, J.T.A. 1998. Nutrient limittion in Biebrz fens nd floodplin (Polnd). Act Botnic Neerlndic 47: 241 253. Young, E.O. & Ross, D.S. 21. Phosphte relese from sesonlly flooded soils: lbortory microcosm study. Journl of Environmentl Qulity 3: 91 11. Zk, D. & Gelbrecht, J. 27. The mobiliztion of phosphorus, orgnic crbon nd mmonium in the initil stge of fen rewetting ( cse study from NE Germny). Biogeochemistry 85: 141 151. Zk, D., Augustin, J., Wgner, C. & Gelbrecht, J. 28. Prediction of P mobiliztion in rewetted fens nd the risk of P exchnge to djcent surfce wter (in Germn). Leipnitz-Institute of freshwter ecology nd inlnd fisheries. Berichte des IGB 26: 129 141. Zuidhoff, A.C., Schminée, J.H.J. & vn t Veer, R. 1996. Molinio-Arrhentherete. Klsse der mtig voedselrijke grslnden. In: Schmine e, J.H.J., Stortelder, A.H.F. & Weed, E.J. (eds.) De Vegettie vn Nederlnd. Deel 3. Plntengemeenschppen vn grslnden, zomen en droge heiden. pp. 163 226. Opulus Press, Leiden, NL. Supporting Informtion Additionl supporting informtion my be found in the online version of this rticle: Fig. S1. Mrsh Mrigold (Clth plustris) bundntly flowering in erly spring in Clthion grsslnd in Nture Reserve Kttendijkblokboezem in polder Krimpenerwrd, the Netherlnds. Fig. S2. Mrsh Mrigold (Clth plustris) bundntly flowering in erly spring in Clthion grsslnd in Nture Reserve Veerstlblokboezem in polder Krimpenerwrd, the Netherlnds. Fig. S3. Vegettion structure in Clthion grsslnd with Mrsh Mrigold (Clth plustris), Rgged Robin (Lychnis flos-cuculi), Slender-tufted Sedge (Crex cut), nd Sweet Vernl-grss (Anthoxnthum odortum). Picture tken in April 27 in control plot of fertiliztion experiment in Nture Reserve Veerstlblokboezem in polder Krimpenerwrd, the Netherlnds. Fig. S4. Species-rich Clthion grsslnd in Nture Reserve Veerstlblokboezem in polder Krimpenerwrd, the Netherlnds. Erly summer spect with Slender-tufted Sedge (Crex cut), Rgged Robin (Lychnis flos-cuculi), nd Greter Yellowrttle (Rhinnthus ngustifolius). Fig. S5. Species-rich Clthion grsslnd in Nture Reserve Veerstlblokboezem in polder Krimpenerwrd, the Netherlnds. Erly summer spect with Slender-tufted Sedge (Crex cut), Rgged Robin (Lychnis flos-cuculi), Greter Yellow-rttle (Rhinnthus ngustifolius), Common Sorrel (Rumex cetos) nd Medow Buttercup (Rnunculus cris). Plese note: Wiley-Blckwell is not responsible for the content or functionlity of ny supporting mterils supplied by the uthors. Any queries (other thn missing mteril) should be directed to the corresponding uthor for the rticle. Received 2 July 29; Accepted 13 November 29. Co-ordinting Editor: Prof. Mrtin Hermy