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Nutrient resorption response to fire and nitrogen addition in a semi-arid grassland

Ecological Engineering 37 (2011) 534–538

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Ecological

Engineering

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e c o l e n

Short communication

Nutrient resorption response to ?re and nitrogen addition in a semi-arid grassland

Xiao-Tao Lüa ,b ,Qiang Cui b ,Qi-Bing Wang b ,Xing-Guo Han a ,b ,?

a Institute of Applied Ecology,Chinese Academy of Sciences,Shenyang 110016,China

State Key Laboratory of Vegetation and Environmental Change,Institute of Botany,Chinese Academy of Sciences,Beijing 100093,China

a r t i c l e i n f o Article history:

Received 29June 2010Received in revised form 29November 2010

Accepted 12December 2010

Available online 15 January 2011Keywords:

Nitrogen deposition Prescribed burning Resorption ef?ciency Resorption pro?ciency Temperate steppe

a b s t r a c t

Fire and nitrogen (N)addition,both widely used grassland restoration strategies,strongly in?uence com-munity composition and ecosystem functioning.However,little is known about their effects on plant nutrient resorption from senescing leaves,especially in semi-arid ecosystems.We evaluated the effects of ?re,N addition (5.25g N m ?2yr ?1)and their potential interactions on nutrient resorption in ?ve plant species in a semi-arid grassland in northern China.Foliar nutrient concentrations and resorption pro?-ciencies and ef?ciencies varied substantially among species and functional groups.Fire increased green leaf N concentration ([N]g)and decreased N resorption pro?ciency (N RP),P resorption pro?ciency (P RP)and P resorption ef?ciency (P RE).N addition led to higher [N]g and lower N resorption,whereas it did not affect P related responses.There was no interaction between ?re and N addition to affect all response variables except for green leaf P concentration ([P]g).These results suggest that ?re and N addition can in?uence ecosystem nutrient cycling directly by changing resorption patterns and litter quality.Given the substantial interspeci?c variations in nutrient content and resorption and the potentially changing com-munity composition,both ?re and N addition may have indirect impacts on ecosystem nutrient cycling in this semi-arid grassland.

1.Introduction

Nutrient resorption,one of the most important nutrient conservation mechanisms,makes plants less dependent on envi-ronmental nutrient availability and has important consequences for species interaction at population level and nutrient cycling at ecosystem level (Aerts and Chapin,2000).Global change factors and human activities such as ecosystem management strategies may all have strong impacts on the plant nutrient resorption pro-cess (Aerts et al.,2007;Blanco et al.,2009).

Prescribed ?re is a widely used grassland restoration strat-egy.Fire-induced changes in soil nutrient availability (Marcos et al.,2009)and moisture (Henry et al.,2006)potentially have great implications for plant growth and community structure.The relationship between plant nutrient status and ?re is elusive,with positive (Van De Vijver et al.,1999),negative (Blair,1997)and neutral effects (Bennett et al.,2002)having been reported.Understanding the responses of plant nutrient status and resorp-

?Corresponding author at:State Key Laboratory of Vegetation and Environmental Change,Institute of Botany,The Chinese Academy of Sciences,Beijing 100093,China.Tel.:+861062836283;fax:+861082595771.

E-mail address:xghan@https://www.wendangku.net/doc/ee16437797.html, (X.-G.Han).tion to ?re is critical to understanding ecosystem functioning of grasslands,because these changes play an important role in sev-eral plant-mediated element cycling processes (Aerts and Chapin,2000).Many studies have added N to various ecosystems to simu-late increasing N availability resulting from both N deposition and climate change (Makipaa,1998;Henry et al.,2006).Increased N availability can lead to higher [N]g (Xia and Wan,2008).Generally,N RE usually decreases with increasing [N]g (Kobe et al.,2005).Thus,it is expected that N addition can lead to decreased N RE.The effects of soil nutrient availability on nutrient RE are,however,reported to be inconsistent and unclear (Aerts,1996;Güsewell,2005).On the other hand,the responses of ecosystem processes to N addition are modulated by natural and anthropogenic distur-bances,such as ?re (Henry et al.,2006).Given that both ?re and N addition can change soil N and P availability (Blair,1997;Henry et al.,2006),it is reasonable to assume a corresponding change in plant nutrient status and resorption.

In this study,we investigated how annual burning and N addi-tion affect plant leaf nutrient status and nutrient resorption of ?ve dominant species in a semi-arid grassland in northern China.We experimentally carried out a combination of N addition and ?re frequency treatments on the ecosystem since 2006.We expected that:(1)annual burning would not affect leaf nutrient concen-tration and resorption,as ?re had no effects on plant-available

X.-T.Lüet al./Ecological Engineering 37 (2011) 534–538535

Table 1

Results of three-way ANOVAs for leaf nutrient variables and nutrient resorption parameters as dependent on species (S),?re (F),N addition (N)and their interactions.The F -ratios are presented,together with their level of signi?cance.[N]g and [P]g,N and P concentration in green leaves;[N]s and [P]s,N and P concentrations in senesced leaves.For all variables error df =96.

[N]g [P]g [N]s [P]s N RE P RE S 465.61***27.57***206.68***240.73***116.80***107.10***F 127.30***0.0120.77***14.86*** 3.71o 11.17***N 118.84***0.0037.20*** 1.029.53**0.43S ×F 4 2.24o 2.23o 0.120.490.190.95S ×N 4 1.60 3.47* 1.180.40 3.58** 1.66F ×N 1 1.74 4.40* 1.24 3.74o 0.600.21S ×F ×N

0.40

2.48*

0.48

2.43o

0.18

1.33

o 0.05

P <0.001.

nutrient concentration during growing seasons in this ecosystem (Zhou et al.,2009);(2)N addition would increase [N]g and reduce N RE and N RP but would lead to lower [P]g and higher P RE and P RP.2.Methods 2.1.Study site

The study was carried out in a semi-arid grassland near the Inner Mongolia Grassland Ecosystem Research Station (IMGERS,116?42 E,43?38 N).At this site,mean annual precipitation is approximately 345mm and mean annual temperature is 0.3?C.The length of the growing season is about 150days.The sandy

soil of this site is classi?ed as Haplic Calcisols according to the Food and Agriculture Organization classi?cation.Mean bulk den-sity is 1.3g cm ?3and pH is 7.4.The vegetation is dominated by Leymus chinensis (Trin.)Tzvel.,Stipa grandis P.Smirn.,Cleistogenes squarrosa (Trin.)Keng.,Caragana microphylia Lam.,and Potentilla bifurca L.

2.2.Experimental design

This study was conducted as part of the GFE (Grassland Fire Experiment)experimental setup near IMGERS.The design consists of 9blocks distributed across a grassland ?eld,with each block containing sets of 10m ×10m plots representing fully crossed

L.c.

S.g.P.b.T.s. C.m.

G r e e n l e a f N c o n c e n t r a t i o n (m g g -1)

10203040

50Unburned

Burned Amb N

Enr N

L.c.S.g.P.b.T.s. C.m.G r e e n l e a f P c o n c e n t r a t i o n (m g g -1)

0.0

.51.01.52.02.5

3.0L.c.S.g.P.b.T.s.

C.m.

Unburned Burned

Fig.1.Adjusted means plots of the effects of species identity,?re,N addition,and their possible interactions on green leaf N (A–C)and P concentration (D–F).Letters above each column indicate signi?cance according to a Tukey’s multiple comparisons test.Error bars indicate ±1SE.L.c.,Leymus chinensis ;S.g.,Stipa grandis ;P.b.,Potentilla bifurca ;T.s.,Thalictrum squarrosum ;C.m.,Caragana microphylia .Amb N,ambient N condition;Enr N,enriched N condition.

536X.-T.Lüet al./Ecological Engineering 37 (2011) 534–538

L.c.S.g.P.b.T.s. C.m.S e n e s c e d l e a f N c o n c e n t r a t i o n (m g g -1)

25Unburned

Burned Amb N

Enr N

L.c.S.g.P.b.T.s. C.m.S e n e s c e d l e a f P c o n c e n t r a t i o n (m a g -1)

0.0

1.0

1.5

2.0Unburned

Burned

Fig.2.Adjusted means plots of the effects of species identity,?re,N addition,and their possible interactions on senesced leaf N (A–C)and P concentration (D and E).Letters above each column indicate signi?cance according to a Tukey’s multiple comparisons test.Error bars indicate ±1SE.L.c.,Leymus chinensis ;S.g.,Stipa grandis ;P.b.,Potentilla bifurca ;T.s.,Thalictrum squarrosum ;C.m.,Caragana microphylia .

treatments of ?re frequency,N addition,and mowing frequency.Blocks were separated by a 2m walkway and plots within each block were separated by 1m buffers.Plots were established in 2005,and burning,N addition and mowing started in 2006.Burn-ing was carried out in early or late April each year before the start of growing season,depending on snow melt.Nitrogen in the form of NH 4NO 3was added in the rainy days of late June every year.Only the treatments of two burning frequencies (never burned vs annual burning)and two N addition levels (0vs 5.25g N m ?2yr ?1,representing a low level of N fertilization)that without mowing in 6blocks were used in this study,with 2treatments ×2levels ×6replications =24plots in all.

2.3.Sample collection and chemical analysis

We chose ?ve dominant plant species:two perennial grasses (L .chinensis and S .grandis ),two non-legume forbs (P .bifurca and Thalictrum squarrosum Stephan ex Willd.)and one legume shrub (C .microphylia ).Representative mature leaves (the third or fourth visible leaf from the top of the shoot)of these species were sampled during the second half of August 2008.In each plot,we randomly collected 20mature leaves from each species for the grasses and forbs and 10leaves from the shrub,avoiding plot edges.In the third week of September 2008,we collected a similar number of senescing leaves from the shrub by gently removing them from

the branch.We considered the leaves ready to abscise according to the same criterion used by Wright and Westoby (2003).For the grass and forbs,20recently senesced leaves without obvious leaf area losses were collected in the second half of October 2008.For the treatment of unburned plus N addition,both P .bifurca and T .squarrosum existed in 5of the 6replicated blocks used in this study;while for the annually burned plus N addition treatment,T .squarrosum existed in 4of the 6replications.

In the laboratory,all samples were counted,oven dried at 65?C for 48h,weighed,and then ?nely ground in a ball mill (Retsch,Germany).Plant material was re-dried (65?C)prior to digestion in H 2SO 4–H 2O 2.Total N concentration was analyzed by a Kjeltec 2300Analyzer Unit (Foss,Sweden).Total P content was determined colorimetrically with molybdenum blue.2.4.Calculations and statistical analysis

Calculation based on the total N pool in green and senesced leaves was an appropriate option for determination of RE (Aerts et al.,2007).Given that we collected the same number of green and senesced leaves for each species in each plot.N RE was calculated as:N RE =

Ng ?Ns

×100%

X.-T.Lüet al./Ecological Engineering 37 (2011) 534–538537

L.c.S.g.P.b.T.s. C.m.

N r e s o r p t i o n e f f i c i e n c y (%)

20406080

100

Amb N

Enr N L.c.S.g.P.b.T.s. C.m.

L.c.

S.g.

P.b.

T.s.

C.m.

P r e s o r p t i o n e f f i c i e n c y (%)

Unburned

Burned

Fig.3.Adjusted means plots of the effects of species identity,?re,N addition,and their possible interactions on N (A–C)and P resorption ef?ciency (D and E).Letters above each column indicate signi?cance according to a Tukey’s multiple comparisons test.Error bars indicate ±1SE.L.c.,Leymus chinensis ;S.g.,Stipa grandis ;P.b.,Potentilla bifurca ;T.s.,Thalictrum squarrosum ;C.m.,Caragana microphylia .Amb N,ambient N condition;Enr N,enriched N condition.

where Ng is the N pool of green leaves sampled in August 2008;Ns is the N pool of senesced leaves collected in September and October 2008.P RE was calculated with the same method.

Resorption pro?ciency is the level to which nutrient concentra-tions are reduced in senesced leaves.Unlike ef?ciency,RP is not subject to the temporal variations in green leaf nutrient concentra-tions and the timing of sampling (Killingbeck,1996;Ratnam et al.,2008).Nutrient RP was quanti?ed by the senesced leaf nutrient concentration (Killingbeck,1996),with lower leaf nutrient concen-tration indicating higher nutrient RP.

Data were tested for normality using Levene’s test.The main and interactive effects of species identity,?re and N addition treat-ments on plant green and senesced leaf nutrient concentrations and resorption variables were determined by three-way ANOVAs.Sig-ni?cant differences among treatment means were analyzed by the post hoc Tukey’s multiple comparison test.Adjusted means plots are shown for signi?cant main and interaction effects.All statisti-cal analyses were performed using SPSS V13.0(SPSS,Chicago,IL,USA).

3.Results and discussion

3.1.Green leaf nutrient concentrations

Green leaf N and P concentrations varied signi?cantly among species (Table 1;Fig.1).Both ?re and N addition signi?cantly

increased [N]g by 10.47%and 8.37%,respectively (Fig.1B,C).Fire can elevate [N]g through increasing soil nutrient supply,relocat-ing nutrients from roots to new shoots,rejuvenating plant organs,and allocating a similar amount of nutrients over less aboveground biomass (Van De Vijver et al.,1999).As soil inorganic N concen-tration did not change after burning indicated by a previous study conducted in the same ecosystem (Zhou et al.,2009),we conclude that variation of soil nutrient availability was not the primary fac-tor driving foliar nutrient responses to burning in this semi-arid grassland ecosystem.

Fire and N addition interacted to affect [P]g,in that N addition tended to enhance [P]g in unburned plots and reduce [P]g in burned plots (Fig.1E).We suppose the response of soil phophatase activ-ities to N addition and ?re may contribute a lot to the variation of [P]g.Nitrogen addition could stimulate root-surface phosphatase activities (Phoenix et al.,2003)which may increase soil P avail-ability.In contrast,?re may exert negative effect of phosphatase activities (Autunes et al.,2009).Species showed different responses to N addition in terms of [P]g (Fig.1F).

3.2.Leaf nutrient resorption pro?ciency

Species differed signi?cantly in [N]s and [P]s (Table 1;Fig.2).Following the criterion suggested by Killingbeck (1996),the grasses showed complete or intermediate N resorption and intermediate P resorption,whereas forbs and the legume showed incomplete

538X.-T.Lüet al./Ecological Engineering37 (2011) 534–538

resorption,indicating that the grasses may be limited by both N and P in this ecosystem.Both?re and N addition caused a signi?-cant increase in[N]s by15.40%and19.80%,respectively(Table1; Fig.2B,C),indicating that?re and N addition signi?cantly reduced plant N RP.Fire increased[P]s by9.31%(Fig.2E),whereas N addition did not affect[P]s,suggesting that?re reduced while N addi-tion did not affect P RP.The reduced NRP and PRP following?re have implications for nutrient cycling dynamics in this grassland ecosystem,as higher nutrient concentrations in litter generally pro-duce a faster decomposition rate(Hefting et al.,2005;Güsewell and Gessner,2009).Fire thus has the potential to increase lit-ter decomposition and stimulate nutrient cycling in this semiarid grassland.

3.3.Nutrient resorption ef?ciency

For both N RE and P RE,there were signi?cant differences among species(Table1),indicating that plant species have distinguished resorption patterns.The two grasses generally showed higher N RE and P RE than the forbs and the legume(Fig.3A,D).Stronger resorption contributes to tight nutrient conservation and higher nutrient use ef?ciency for the grasses(Aerts and Chapin,2000). Thus,this gives further support in explaining the dominant posi-tion of perennial grasses in the temperate steppe(Bai et al.,2008). Across all the species,N addition caused a signi?cant decrease in N RE by8.28%(Table1;Fig.3B),while?re had a marginally signi?cant effect.The decreased N RE may be accounted for by the increased soil inorganic N availability after N addition,because N addition even as low as4g N m?2yr?1could signi?cantly enhance soil inor-ganic N availability in this area(Zhang et al.,2008).The effect of N addition on N RE interacted with species identity(Table1),as three species(L.chinensis,S.grandis,and T.squarrosum)decreased but P.bifurca and C.microphylia increased N RE under enriched N conditions(Fig.3C).

Annual burning signi?cantly reduced P RE across all?ve species (Fig.3E),indicating that annual burning would lead to a non-conserved P use strategy for the plants in this ecosystem.It is expected that plants would increase P RE under N enriched envi-ronment due to the potential P-limitation.However,P RE showed no responses to N addition(Table1).We attribute this to the low level of N addition(5.25g N m?2yr?1)in our study.In fact,even under a higher N addition level(10g N m?2yr?1),Van Heerwaarden et al.(2003)showed that PRE of most species did not respond to increased N supply.

4.Conclusions

Our study showed that3-year annual burning increased[N]g and decreased nutrient RP and RE,whereas N addition increased [N]g and decreased N RE and N RP but showed no effects on [P]g and P resorption.Lower nutrient resorption,and thus higher [N]s and[P]s,would favor a higher litter decomposition rate and potentially a higher nutrient cycling rate in the ecosystems fol-lowing burning and/or N addition.Results from this study indicate that prescribed?re and N addition,as two widely used grassland restoration strategies,would stimulate plant growth by elevating green leaf nutrient concentrations and enhance ecosystem nutrient cycling through increasing senesced leaf nutrient concentrations. Furthermore,our results showed large variations among plant species in terms of leaf nutritional status and resorption patterns, indicating that?re and N addition may have indirect effects on plant-mediated nutrient cycling via changing community compo-sition.Acknowledgements

We thank Hans Cornelissen,Jayashree Ratnam,and two anonymous reviewers for constructive comments and language improvement on the earlier version of this manuscript.This study was supported by fund from the State Key Labora-tory of Vegetation and Environmental Change,The Ministry of Science and Technology of China(2007CB106801),and The National Natural Science Foundation of China(30821062and 30830026).

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