microbial biomass dynamics in tallgrass prairie

Microbial Biomass Dynamics in Tallgrass PrairieFernando O. Garcia and Charles W. Rice*

ABSTRACTThe temporal dynamics and effects of burning, mowing, and N

fertilization on microbial biomass (MBM) in tallgrass prairie werestudied in a field experiment established in 1986. Microbial C (MC)and microbial N (MN), determined by the fumigation-incubationprocedure during the growing seasons of 1989 through 1991, averaged217 mg C kg-1 and 32.6 mg N kg-', respectively, for the 0- to 30-cntdepth. Accumulation of litter and greater production of roots nearthe surface resulted in stratification of MBM. Seasonally, MBM washigher in early spring, decreased with the initiation of plant growth,and then recovered by late summer or early fall. Decreases of MNbetween March and July coincided with plant N uptake. The increaseof MC and decrease of MN during the 3 yr of the study were relatedto increased plant production. Burning had a short-term and variableeffect on MC. Burning tended to reduce MC during dry years andincrease it in normal to wet years. Mowing and raking decreased MCand MN, probably because of reduced root biomass and removal ofstanding vegetation. Nitrogen addition resulted in higher MN andtended to reduce MC, possibly by modifying the composition of themicrobial population. Microbial biomass seems to play a critical rolein conserving N in the tallgrass prairie ecosystem.

M ICROBIAL BIOMASS is considered a transformationagent of soil organic materials and a labile reser-

voir of nutrients such as C, N, P, and S (Jenkinson andLadd, 1981). Because of its role in the N cycle, MBMhas been thoroughly studied in agroecosystems (Carterand Rennie, 1984; McGill et al., 1986). The size of theMBM is regulated by substrate and water availability,protection capacity of the soil, and temperature (vanVeen et al., 1984; McGill et al., 1986). Its role in con-

F.O. Garcia, Departamento Agronomia, E.E.A. INTA, C.C. 276, (7620)Balcarce, Argentina; and C.W. Rice, Dep. of Agronomy, Kansas StateUniv., Throckmorton Hall, Manhattan, KS 66506-5501. Received 6 Aug.1992. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 58:816-823 (1994).

serving mineral nutrients and transforming organic nutri-ents into plant-available forms could be critical in thetallgrass prairie ecosystem in which external additionsor losses of nutrients, such as N, are minimal (Wood-mansee, 1978). Clark (1977) indicated that N require-ments in the shortgrass prairie ecosystem are met byinternal translocation by plants and mineralization of soilorganic N by MBM.

In tallgrass prairie, which is a major ecosystem in theU.S.A., fire is a common disturbance. Burning increasesthe photosynthetic capacity of postburn plant growth(Knapp and Seastedt, 1986) and results in changes insoil temperature, water, and nutrient status. Ojima (1987)reported that annual burning resulted in lower soil organicmatter but higher plant productivity compared with noburning. This apparent contradiction may be explainedby (i) synchronization of nutrient release with plant up-take and microbial activity; (ii) extension of the growingseason because of earlier soil warming; (iii) changes inthe rate of ecosystem processes that allow for recoveryof volatilized nutrients (i.e., N2 fixation); (iv) changesin the utilization of available nutrients; or (v) a combina-tion of all these reasons (Knapp and Seastedt, 1986;Ojima, 1987). Although MC and MN were also reducedby long-term annual burning (>40 yr), they were notaffected after only 1 to 2 yr of burning (Ojima, 1987).

Grazing is an important element in most tallgrassprairie ecosystems and interacts with burning in de-termining the structure and composition of vegetation(Anderson, 1990; Hobbs et al., 1991). Grazing tends toreduce root growth and rhizome carbohydrate reserves(Rains et al., 1975; Turner et al., 1993). The decreaseof belowground C inputs could result in a reducedC/N ratio of belowground plant biomass, reduced micro-

Abbreviations: MC, microbial C; MN, microbial N; MBM, microbialbiomass; LSD, least significant difference.

GARCIA & RICE: MICROBIAL BIOMASS DYNAMICS 817

Table 1. Microbial C at 0- to 30-cm depth under burning, mowing, and N fertilization treatments during the 1989,1990, and 1991 growingseasons in tallgrass prairie.

Unburned prairieUnmowed

Date

15 Apr. 19898 June 198925 July 198929 Sep. 198920 Mar. 199023 Apr. 199025 May 199021 June 199017 July 199025 Aug. 19906 Oct. 199029 Mar. 19914 June 19912 July 19917 Aug. 199127 Sep. 1991Mean

-Nt

253235211223214216205187200242258240253240267348237

+ Nt

269192209214171179212181207225234241249203243256218

Mowed-N

206189184202203207195163200219228233234193245272211

+ N

————— mg C kg-1225198184186186190205184175210231223230197239257208

BurnedUnmowed

-N

241185227209189206209195200231232289230234277283228

+ N

241161199198179188194176222214230264228230263271217

prairieMowed

-N

197157171222192198191167199238225238237215256249210

+ N

248157195170189199213175210203193246231207234260209

t - N = control treatment.t + N = N-fertilized treatment.

bial growth, and reduced potential for N immobilization(Holland and Detiing, 1990). The objectives of our re-search were to determine the temporal dynamics of MCand MN in tallgrass prairie and to evaluate the effectsof annual burning, mowing, and N fertilization on MBM.

MATERIALS AND METHODSThe experimental site is located in the Konza Prairie Re-

search Natural Area, 12 km south of Manhattan, KS. KonzaPrairie is a remnant of the unplowed tallgrass prairie withvegetation dominated by €4 grasses such as big bluestem(Andropogon gerardii Vitman), little bluestem (A. scopariusMichaux.), indiangrass [Sorghastrum nutans (L.) Nash], andswitchgrass (Panicum virgatum L.). The experiment began in1986 on an area mapped as Irwin silty clay loam (fine, mixed,

mesic, Pachic Argiustoll). Soil total C and N contents at0- to 30-cm depth were 19.2 g C kg'1 and 1.64 g N kg"1.The experimental design was a split-split plot with main plotsarranged in a randomized complete-block design with fourblocks (Milliken and Johnson, 1984). Main plots correspondedto the burning treatment (annually burned and unburned),subplots to the mowing treatment (annually mowed and un-mowed), and sub-subplots to the N fertilization treatment (con-trol and N fertilized). Burning was performed in late spring,usually the last week of April, and 10 g N m"2 as NItiNOawas applied 7 to 10 d later. Simulated grazing plots weremowed and raked once each year in early June.

We sampled the plots at 0- to 5-, 5- to 15-, and 15- to30-cm depths between March and September of 1989, 1990,and 1991; sampling dates are shown in Tables 1 and 2. Allsamples were immediately sieved to pass a 6-mm mesh and

Table 2. Microbial N at 0- to 30-cm depth under burning, mowing, and N fertilization treatments during the 1989,1990, and 1991 growingseasons in tallgrass prairie.

Unburned prairieUnmowed

Date

15 Apr. 19898 June 1989

25 July 198929 Sep. 198920 Mar. 199023 Apr. 199025 May 199021 June 199017 July 199025 Aug. 19906 Oct. 1990

29 Mar. 19914 June 19912 July 19917 Aug. 1991

27 Sep. 1991Mean

-Nt

43.526.727.438.038.239.237.332.534.631.230.228.636.328.924.628.932.9

+ N*

45.543.238.950.746.141.846.639.940.934.537.637.430.732.932.529.739.4

Mowed-N

31.125.321.628.641.133.232.626.528.426.327.025.627.122.619.123.927.6

+ N

————— mg N kg'141.540.136.040.333.546.244.137.335.133.435.330.629.426.929.531.035.7

BurnedUnmowed

-N

32.424.524.626.733.233.033.329.224.325.427.032.526.625.023.721.627.8

+ N

37.432.234.141.041.542.641.434.933.534.335.433.031.131.033.632.235.6

prairieMowed

-N

29.720.621.235.736.728.231.827.624.326.922.925.325.222.826.523.326.8

+ N

44.232.233.439.244.146.741.035.033.529.031.134.729.529.028.327.034.9

t — N = control treatment.$ + N = N-fertilized treatment.

818 SOIL SCI. SOC. AM. J., VOL. 58, MAY-JUNE 1994

stored at 4°C for analysis. Microbial C and N were determinedby the fumigation-incubation method (Jenkinson and Powlson,1976) using 25 g of each sample in each of two 125-mLErlenmeyer flasks. When the gravimetric soil water contentwas <0.28 kg kg'1, water was added to this level. Both sampleswere preincubated at 25 °C for 5 d, after which one of thesamples was fumigated with chloroform in a vacuum desiccatorcontaining a wet paper towel and a beaker with approximately50 mL of ethanol-free chloroform and nonvolatile granulesfor distillation. Vacuum was applied three times for approxi-mately 30 s each to boil the chloroform. Immediately afterthe third evacuation, the desiccator was tightly closed to allowthe chloroform to diffuse into the soil. After 20 to 24 h,the beaker and towel were removed and the desiccator wasevacuated eight times for 3 min each time. The flasks wereplaced in 940-mL mason jars containing enough water tomaintain a highly humidified environment. Jars were tightlyclosed and incubated for 10 d at 25°C, after which the head-space COi-C concentration was measured using a gas chroma-tograph (Shimadzu GC-8A, Shimadzu Scientific Instruments,Columbia, MD) with a 2-m Porapak Q column (70°C) andHe carrier (14 mL min~'). Subsequently, 100 mL of 1 M KC1was added to each flask, and the flasks were shaken for 1 hon an orbital shaker at 300 rpm. The suspensions were trans-ferred to 250-mL centrifuge bottles, centrifuged at 16 000 Xg for 10 min, filtered through a nylon mesh (10 um), andstored in the freezer until analyzed for NHi"-N and NO3~-N.Nitrate-N was determined by the Griess-Qosvay technique(Keeney and Nelson, 1982), and Nrtf-N by the salicylate-hypochlorite method (Crooke and Simpson, 1971), both imple-mented on an Alpkem Autoanalyzer (Alpkem Corp., Clacka-mas, OR).

We expressed MC and MN as the flush of MC and MN,the difference in CO2-C evolved and N mineralized betweenfumigated and unfumigated samples, to avoid the confusion ofusing different conversion factors (kc and &N). When comparingwith other published data, we calculated MC and MN assuggested by Voroney and Paul (1984):

MCMicrobial biomass C = —— [1]0.41

Microbial biomass N =

where *N = -0.014(Cf/Nf) + 0.39

MN[2]

Cf = CO2-C evolved from the fumigated sampleNf = NrLt-N + NOf-N mineralized from the fumigated

sample.Inorganic N (NHj"-N + NOf-N) was determined by ex-

tracting 20 g of soil with 100 mL of 1 M KC1 as indicated abovefor MN determination. Soil water content was determined byoven drying at 105 °C for 24 h.

Data were analyzed as a repeated-measures design thatincluded the split-split plot arrangement (Milliken and Johnson,1984). Analysis of variance and separation of means by the leastsignificant difference test were performed using SAS procedures(SAS Institute, 1988).

RESULTSMicrobial C in the tallgrass prairie was 217 mg C

kg"1 for the 0- to 30-cm depth averaged across all treat-ments and sampling dates (Table 1). This value is equiva-lent to 529 mg C kg"1 as MBM C computed accordingto Eq. [1]. Microbial N (Table 2) averaged 32.6 mg Nkg"1, which is equivalent to 116 mg N kg"1 as MBMN (Eq. [2]). The concentrations of MC and MN weregreatest at the soil surface (Fig. 1), but the fraction oftotal MC in the 0- to 5-cm layer (24%) was smaller thanin the 5- to 15- and 15- to 30-cm depth increments (35and 41%, respectively). Total MN contents of the threelayers were similar (35, 31, and 34% for the 0- to 5-,5- to 15-, and 15- to 30-cm depths, respectively). TheC/N ratio of the MBM increased with depth from 4.75at the surface to 8.05 at 15- to 30-cm depth.

Dynamics of Microbial BiomassThe temporal dynamics of MC and MN were highly

statistically significant (Table 3, Fig. 2). Both parameterswere generally higher in the spring, decreased followinginitiation of plant growth, and (hen recovered in latesummer and fall. The magnitude of the spring decreaseaveraged 9% for MC and 59% for MN. In both 1990and 1991, the decrease in MN was accompanied by atransitory increase in inorganic N (Fig. 2), which wasrapidly assimilated by the actively growing plants. Inor-

wi

OB

°o

400

300

200

100

n

-

-

-

1 •1 -

-

80

60

20

0

no

z

it2:'a• •—i

ooS

0-5 5-1515-30 0-5 5-1515-30

Soil Depth (cm)Fig. 1. Microbial C (MC) and N (MN) at different sampling depths in tallgrass prairie. Least significant differences (P < 0.05) were 17.4 mg C

kg"1 for MC and 3.64 mg N kg-' for MN.


Table 3. Summary of analysis of variance for microbial C (MC)and N (MN) at 0- to 30-cm depth in tallgrass prairie.

F values

Source of variationBurning (B)BlockMowing (M)M x BN Fertilization (N)N x BN x MN x B x MDate (D)D x BD x MD x ND x B x MD x B x ND x M x ND x B x M x N

df13111111

1515151515151515

MC0.35

10.85*4.55t0.134.24t0.332.320.13

21.48**2.57t0.831.160.320.600.730.59

MN

1.990.22

12.18*5.92*

45.08**0.090.170.11

22.88**1.310.282.69**1.141.151.511.13

t, *, ** Significant at the 0.10,0.05, and 0.01 probability levels, respectively.

ganic N was significantly but weakly correlated to MC(r = 0.204, P < 0.10) and MN (r = 0.369, P < 0.05).In addition to within-year changes, MC tended to increaseduring the course of the study from 205 mg C kg"1 in1989 to 246 mg C kg"1 in 1991, whereas MN significantlydecreased from 34 mg N kg"1 in 1989 to 28 mg N kg"1

in 1991.

The temporal dynamics of MC and MN may be relatedto changes in substrate availability, water, or tempera-ture. Precipitation during the growing season was belownormal in 1989 and near normal in 1990 and 1991. Soilwater content was not correlated with MC (r = 0.002)and only weakly correlated with MN (r = 0.168, P <0.10). Although temperature was not significantly corre-lated to MC and MN dynamics, lowest values occurredwith high temperatures during summer. Plant productionfollowed precipitation with low plant biomass in 1989and high plant biomass in 1990 and 1991 (Table 4).

Effects of Burning, Mowing, and N FertilizationBurning tended to decrease MC and MN, but this effect

was not statistically significant (Table 3). The significantinteraction of burning with sampling date reflects that burn-ing significantly decreased MC on one sampling date inthe spring of 1989 but increased MC on one samplingdate in the spring of 1991 (LSD = 19 mg C kg"1, P <0.10) (Fig. 3).

Mowing and raking significantly decreased MC andMN in tallgrass prairie (LSD = 12 mg C kg"1, P <0.10 for MC, and LSD = 1.9 mg N kg"1, P < 0.05for MN), probably because this treatment reduced Cinput to the soil through aboveground biomass removaland root growth suppression (Garcia, 1992). There was

GO^ 300o2f 250

" 200132 150s_o

bfl

0)-»J(0

oCO

Zao

Zo

(0M

"*• Microbial NO Microbial C

i i i i i i i i i i i i i i i i I I I

40 j?Z

35 M

30 ^13

25 2o

20 |

A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

1989 1990 1991Fig. 2. Temporal dynamics of microbial C and N, soil water content, and soil inorganic N at 0- to 30-cm depth in unfertilized tallgrass prairie.

Data are means of four replications across burning and mowing treatments.

820 SOIL SCI. SOC. AM. }., VOL. 58, MAY-JUNE 1994

Table 4. Average aboveground biomass production and precipita-tion in 1989, 1990, and 991 in Konza Prairie.

Year

198919901991

Abovegroundbiomass production

g m"2

438587613

Totalprecipitation

614742846

PrecipitationAprfl-July

166325333

a significant interaction of mowing with burning for MN(LSD = 2.7 mg N kg-1, P < 0.05). Microbial N wasgreater in unburned unmowed prairie than in unburnedmowed or burned prairie regardless of the mowing treat-ment (Fig. 4). In contrast, inorganic N was significantlygreater in the burned mowed prairie than in the burnedunmowed or unburned prairie regardless of the mowingtreatment (LSD = 0.20mgNkg-', P<0.10). InorganicN averaged 2.07 mg N kg"1 in the burned mowed prairieand 1.80 mg N kg"1 for the other three burning xmowing combinations.

Nitrogen fertilization increased MN but reduced MC(LSD = 7 mg C kg"1, P < 0.10 for MC, and LSD =2.5 mg N kg"1, P < 0.05 for MN). The increase in MNwas expected, and its significance depended on the dateof sampling (sampling date x N fertilization interaction,LSD = 3.6 mg N kg-1, P < 0.05) (Fig. 5).

DISCUSSIONOur estimates of MC were 10 to 15% lower and

estimates of MN were 5 to 10% greater than reported

for other grasslands (Schimel, 1986; Woods, 1989; Tateet al., 1991). The resulting smaller C/N ratios of MBMat our tallgrass prairie site suggest that its turnover maybe more rapid. The increasing C/N ratio with depthprobably resulted from the low C/N ratio of abovegroundbiomass compared with that of root biomass. The strati-fication of MC and MN probably reflects the distributionof litter and roots. Our estimates of MC and MN intallgrass prairie were 10 to 50% higher than those re-ported for agricultural ecosystems (Carter and Rennie,1982; McGill et al., 1986; Schimel, 1986; Patra et al.,1990; Omay et al., 1992).

Variations hi MC and MN between years could berelated to plant production hi Konza Prairie. Averageaboveground plant production was 438, 587, and 613 gm"2 hi 1989, 1990, and 1991, respectively. In the latter2 yr, which had normal or above normal precipitation,MC tended to increase while MN followed the oppositetrend. The occurrence of low MN in years with highplant production suggests transfer of limited resourcessuch as N from the microbial pool to the plant pool.

Seasonal variation hi MBM also has been observedhi other ecosystems (Ross, 1988; Jordan et al., 1989;Wheatly et al., 1990, Hassink et al., 1991). McGill etal. (1986) suggested that the seasonal dynamics of soilMBM are controlled by environmental conditions, suchas drying and rewettuig. Although measured changes hiMC and MN generally reflected changes hi soil water,we did not find a significant correlation between thesevariables. The increases hi MBM observed hi Augustto September could be explained by the occurrence of

00

o&0a

«s

300

250

200

150ouS 100

I I I I i I I I I I I I I I I I I I I I I I I I I I I I I I I


Qfl

£2;13OSHO

O Unburned• Burned


Fig. 3. Temporal dynamics of microbial C and N at 0- to 30-cm depth in burned and unburned tallgrass prairie. Asterisks indicate significantdifferences between treatments (P < 0.10).


ouu

250

O 200Qfla

150O

.£ 1000

. 50

n

I ]

_

-

-

.

._

-

-

-

T

I-

30

- 20

- 10

UBUM UBM BUM BM UBUM UBM BUM BM

iao

^ttf£

&

'ed

O

fnO

Fig. 4. Microbial C and N at 0- to 30-cm depth in tallgrass prairie under burning and mowing treatments; UB = unburned, B = burned,UM = unmowed, and M = mowed.

the maximum seasonal litterfall (Seastedt, 1988), rootdecay (Hayes and Seastedt, 1987), and the translocationof N and carbohydrates from aboveground to below-ground organs (Owensby et al., 1970; McKendrick etal., 1975). Belowground reserves of carbohydrates andN decrease in late spring (May-June) (Owensby et al.,

1970; McKendrick et al., 1975), when MC and MNalso decrease. Both belowground and aboveground bio-mass are increasing at this time, however, and usuallywater is not limiting (Fig. 2). Short-term MBM dynamicscould be associated with root dynamics, which respondto drying-rewetting cycles (Hayes and Seastedt, 1987).

o

oi—i(0

of-lO

00

!Z

oS-H

o

250

200

150

100

O Control• N fertilized



Fig. 5. Temporal dynamics of microbial C and N at 0- to 30-cm depth in unfertilized and N-fertilized tallgrass prairie. Asterisks indicate significantdifferences between treatments (P < 0.05).

822 SOIL SCI. SOC. AM. J., VOL. 58, MAY-JUNE 1994

Decreases in MN between March and July averaged7.3 and 9.2 g N m~2 for unburned and burned prairie,respectively. These decreases occurred while abovegroundplant biomass and N uptake were increasing rapidly(McKendricketal., 1975; Hayes, 1985; Seastedt, 1988).Average aboveground plant N in this study was approxi-mately 4.8 and 6.9 g N m~2 for unburned and burnedprairie, respectively, and average belowground plant Nwas estimated at 5.8 and 6.6 g N m~2. Previous researchhas shown that N translocation from root and rhizomereserves may account for 18 to 58% of aboveground Nneeds (McKendrick et al., 1975; Hayes, 1985). Thus,approximately 0.9 to 2.8 and 1.2 to 4.0 g N m~2 couldbe supplied by translocation from belowground reservesin unburned and burned prairie, respectively. The de-crease in MN may account for the remainder ofaboveground plant N needs: 2.0 to 3.9 and 2.9 to 5.7 gN m~2 in unburned and burned prairie, respectively.These calculations are a first approximation of the roleof MBM in N cycling in tallgrass prairie; they supportthe hypothesis that the release of MN may be synchro-nized with plant N uptake in the tallgrass prairie. Clark(1977) indicated that belowground N translocation couldaccount for 33% of the N needed for plant productionin shortgrass prairie, with the remainder provided bymineralization of soil organic N.

Ojima (1987) found no differences in MBM betweenunburned and recently burned prairie (1-2 yr) but didfind smaller MBM in long-term burned (>40 yr) thanin unburned prairie. In this study, the apparent reductionin MC and MN due to burning during an intermediateperiod was not statistically significant. Greater plant Nuptake in burned than in unburned prairie was sustainedby a decrease in MN between March and July. The MNsupply was then replenished in fall and winter. Thus, agreater proportion of MN must be recycled in burnedthan in unburned prairie to sustain greater plant N uptake.

The response of MC to fire was similar to that ofplant production in the Konza Prairie (Knapp andSeastedt, 1986; Knapp and Fahnestock, 1990). In a dryyear like 1989, both MC and plant productivity werelower in the burned prairie. Under normal to wet condi-tions such as in 1990 or spring of 1991, MC and plantproduction were higher with burning. The coincidentresponse of plant production and MC to burning undercontrasting water availability could indicate a direct effectof water on both variables or an indirect effect of in-creased substrate availability because of increased plantgrowth on MC.

Mowing tends to decrease root production in the tall-grass prairie (Garcia, 1992; Turner et al., 1993). Lowerroot biomass and removal of clippings resulted in lowersubstrate availability and, thus, lower MBM. The sig-nificantly greater MN found under unburned, unmowedprairie may be a result of the high immobilization poten-tial of unmowed prairie (Holland and Detling, 1990) andthe lower N limitation of unburned prairie (Seastedt etal., 1991).

Nitrogen fertilization increased MN as expected. Thesimultaneous decrease in MC and increase in MN causedby N fertilization may reflect a change in the composition

of the microbial population. Evaluation of the proportionsof microbial activity due to bacteria and fungi showedthat microbial activity of N-fertilized prairie was domi-nated by bacteria that have a lower C/N ratio than dofungi (Garcia, 1992).

In conclusion, the amount of MC and MN in thetallgrass prairie reveals its importance in the cycling ofboth nutrients. Microbial biomass may act as a regulatorof N dynamics, allowing conservation of N in tallgrassprairie. Release of MN at the beginning of the growingseason appears to be in synchrony with plant N uptake,whereas N returned to the soil at the end of the growingseason is conserved by microbial immobilization. Con-servation of N also is demonstrated by the low levelsof soil inorganic N susceptible to loss and the low levelsof denitrification and leaching losses measured in otherstudies (Woodmansee, 1978; Knapp and Seastedt, 1986;Ojima et al., 1990; Groffman et al., 1993).

ACKNOWLEDGMENTSWe are grateful to R. Ramundo and the personnel of Konza

Prairie Research Natural Area for help in the field experiment.We also thank T. Seastedt for helpful discussions. This researchwas supported by NSF grants BSR-8514327 and BSR-9011662to Kansas State Univ. Contribution no. 93-17-J from the KansasAgric. Exp. Stn., Manhattan.

XIE ET AL.: LIGNOSULFONATE CARBON AND UREA NITROGEN TRANSFORMATION 823

microbial biomass dynamics in tallgrass prairie

Documents