Hydroperiod Influences on Nutrient Dynamics in Decomposing Litterof a Floodplain Forest

B. G. Lockaby,* A. L. Murphy, and G. L. Somers

ABSTRACTLack of clarity regarding the relationship between hydro period and

decomposition dynamics has long been a barrier to the developmentof a clear understanding of floodplain biogeochemistry. Relationshipsbetween hydroperiod and decomposition processes were investigatedusing a controlled, field approach on the Ogeechee River floodplainin south Georgia. The study intent was to develop cause-effect relation-ships between specific flooding regimes and decomposition parameters.Microcosms designed to manipulate flooding regimes were installedand used in combination with litterbags containing abscised foliage.Treatments were designed to mimic realistic hydroperiod-nutrientinflow scenarios and included: a nonflooded control, flooded for 6 mo,flooded for 3 mo, flooded intermittently for 4 mo, flooded for 3 mowith elevated P inflow, and flooded for 3 mo with elevated N inflow.Mass, C, N, and P dynamics were studied for a 106-wk period.Flooding stimulated mass, C, N, and P loss. The data suggest thatsingle, brief flooding regimes may stimulate mass and C loss to thegreatest extent. The proportion of N and P remaining after 106 wk wasonly marginally different among flooding regimes. However, temporalpatterns of immobilization-mineralization were strongly influencedby the nature of the flooding regime.

ALTHOUGH DECOMPOSITION in floodplains representsa critical pathway for energy flow and nutrient

exchange between aquatic and terrestrial systems, thefactors that regulate the decomposition process in for-ested wetlands remain poorly understood (Brown, 1990;Mitsch and Gosselink, 1993; Sharitz and Mitsch, 1993).The lack of understanding is primarily caused by the highlyvariable nature of hydroperiod, which affects the processboth through litter quality and in defining the nature ofthe decomposition environment.

Many studies have documented an influence of floodingon decomposition rates (Bell and Sipp, 1975; Brinson,1977; Yarbro, 1979; Peterson and Rolfe, 1982; Briggsand Maher, 1983; Shure et al., 1986; Cuffney and Wal-lace, 1987). Much of that work has sought to correlatelitter decomposition with hydroperiod and microsite char-acteristics. In general, flooding has been shown to pro-mote decomposition, although some exceptions havebeen found (Irmler and Furch, 1980; Duever et al.,1984). While Mitsch and Gosselink (1993) noted thatinconsistencies exist in reports of decomposition-flood-ing relationships, they theorized that rates are probablymost rapid in microsites that are aerobic but moist,become slower in sites that are continually dry, and areslowest in permanently anaerobic areas.

Brinson et al. (1981) suggested that increased floodingwill not uniformly lead to increased rates of decomposi-tion and that alternating wetting-drying cycles may havethe greatest stimulatory effect on decomposition rates.

School of Forestry, Auburn Univ., Auburn, AL 36849-5418. Received24 July 1995. *Corresponding author (lockaby@forestry.auburn.edu).

Published in Soil Sci. Soc. Am. J. 60:1267-1272 (1996).

The suggestions of Brinson et al. (1981) and Mitsch andGosselink (1993) indicate a strong need to provide betterdescriptions of the specific nature of hydroperiod priorto making predictions regarding its impact on the decom-position process.

Previous investigations of decomposition withinfloodplain systems have utilized a correlative approachto compare forest floor standing crops or mass-nutrientdynamics using litter bags across an inundation gradient(Bell and Sipp, 1975; Brinson, 1977; Bell et al., 1978;Peterson and Rolfe, 1982; Shure et al., 1986). Thisapproach is valuable but the results are potentially con-founded by simultaneous changes in litter quality andmicroenvironment that occur across a flooding gradient.

This confounding prevents a clear interpretation ofinundation gradient data in terms of microenvironmentalone, as is sometimes attempted. Also, the nature ofhydroperiod variation across the gradient has rarely beendenned. Thus, interpretations of the effects of floodingare usually vague because the timing, duration, andfrequency of the flooding is poorly defined.

In other cases, controlled approaches have been usedto separate the interactions of litter quality and microenvi-ronment and to better define flooding regimes (Yarbro,1979; Cuffney and Wallace, 1987; Taylor and Parkinson,1988). However, some of these efforts occurred in labora-tory environments and may not be directly applicable tofield conditions.

As a result, we developed a field approach for manipu-lation of flooding regimes to study the influence of hy-droperiod on the decomposition of foliar litter in thefield environment. We used replicated treatments to testcause-effect relationships between variation in floodingduration, periodicity, and nutrient inflow on mass andnutrient dynamics.

METHODSStudy Area

The study area lies in the eastern floodplain of the OgeecheeRiver approximately 50 km north of Savannah, GA. The siteis occupied by an unevenaged forest of primarily deciduousspecies, including laurel oak (Quercus laurifolia Michx.),sweetgum (Liquidamber styraciflua L.), water oak (Quercusnigra L.), black gum (Nyssa sylvatica Marshall var. sylvatica),and swamp tupelo (Nyssa sylvatica var. biflora [Walter] Sarg.).The dominant species of lower vegetation is switch cane (Arun-dinaria gigantea subsp. tecta [Walter] McClure). Soils areclassed as Typic Endoaquults.

Flooding usually occurs intermittently between Novemberand June. The Ogeechee is a low-gradient, blackwater riverwith low nutrient and sediment loads relative to those ofredwater systems. Total N and P in the river average 1.0and 0.1 mg/L, respectively (Georgia Department of NaturalResources, 1992, personal communication). Extractable P insurface soils using the double-acid method (Nelson et al.,1953) is approximately 7 mg/kg, which is near the deficiency

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1268 SOIL SCI. SOC. AM. J., VOL. 60, JULY-AUGUST 1996

level for many deciduous species of southern floodplain forests(H. Kennedy, Soil Scientist, U.S. Forest Service, 1990, per-sonal communication).

ApproachA microcosm system was designed to manipulate hydro-

period in terms of duration, periodicity, and nutrient inflow.Individual microcosms consisted of plastic cylinders (0.45 min diameter and 1.0 m in height) that were open on both ends.These were inserted into the soil to a depth of 45 cm withminimal disturbance to the soil core within the cylinder. Inflowwater (pumped from the Ogeechee River approximately 50 maway) entered the cylinders through a faucet placed 40 cmabove the soil surface. The height of the outflow was establishedat 30 cm above groundline and controlled the depth of waterwithin the cylinder. This design allowed continuous watermovement through the cylinder to ensure that the water re-mained well oxygenated (5-6 mg/L of O2, a level very similarto that within the river). Cylinders remained open on the upperend to expose litter to throughfall as in natural conditions.

Twenty-four microcosms were installed in a randomizedcomplete block design (four blocks and six treatments) under-neath the forest canopy. Treatments were initiated in May1992 and consisted of the following:(NF) no flooding(F6) flooded continuously for 6 mo(F3) flooded continuously for 3 mo(F2/2) flooded for 2 mo, drained for 1 mo, and reflooded for

2 mo(FP) flooded continuously for 3 mo with elevation of P(FN) flooded continuously for 3 mo with elevation of N

Treatments F6, F3, and F2/2 reflected flooding scenariosthat might be commonly observed in riverine forests of thesoutheastern USA. Treatments FP and FN were accomplishedby drip application of concentrated phosphate (KH2PO4) andammonium (NH4C1) solutions, respectively, using peristalticpumps to maintain constant drip rates. Target concentrationswithin the chambers were 10 and 1 mg/L for N and P, respec-tively.

Fourteen nylon litterbags, containing a common species mixof air-dried, abscised foliage, were placed in each microcosmprior to treatment initiation. The initial C/N, C/P, N/P, lignin/N, and lignin/P ratios in the litter were 47.1, 348.0, 8.1, 19.8,and 153.6, respectively. The nylon bags were 12.7 by 12.7cm in size and had 6- and 2-mm openings on the upper andlower sides, respectively. The collection schedule was 0, 1,

2, 4, 8, 12, 21, 31, 39, 51, 65, 79, 94, and 106 wk. Onebag was collected from each microcosm per collection period.

Litter was removed from bags upon collection, gentlywashed in distilled water if there appeared to be sedimentaccumulation, oven dried at 70°C for 48 h, weighed, andground to pass a 20-mesh sieve. Total N and C were determinedusing thermal combustion with a Leco CN Analyzer (LecoCorp., St. Joseph, MI). Total P was determined using thevanadomolybdate procedure on a HC1 extract following dryashing at 500° C for 4 h (Jackson, 1958). Lignin was determinedaccording to forage fiber analysis (Van Soest and Wine, 1968).All weights were expressed on an ash-free basis.

Statistical analyses consisted of analysis of variance compari-sons of the percentage of mass, N, P, and C remaining at 106wk. Mean comparisons were accomplished using Duncan'snew multiple-range test at the 0.05 probability level. Nonlinearregressions (mass percentage remaining = bO [weeks]bl) wereused to calculate rates of mass loss (k) for each treatment-replication combination, and these were then compared usinganalysis of variance. Temporal patterns of N and P immobiliza-tion-mineralization were also developed and qualitatively com-pared.

RESULTS AND DISCUSSIONMass and Carbon Dynamics

Flooding, with the exception of the FN treatment,stimulated mass loss compared with the NF controls(Table 1). Carbon loss closely followed that of mass,although the NF treatment was also statistically differentfrom the FN treatment. Although there was no statisticalseparation among flooded treatments, F3 exhibited theleast mass and C remaining. Temporal patterns of massloss revealed that the NF treatment consistently exhibitedgreater mass throughout the study period. Similarly,treatment F3 had greater mass than the other floodingtreatments from 30 wk until study termination.

Comparisons of rates of mass loss (k) indicate similarresults (Table 1). The NF and F3 treatments exhibitedthe slowest and most rapid rates of mass loss, respec-tively. The addition of N to the F3 flooding regime (i.e.,FN) significantly slowed mass loss. Treatments FP, F2/2, and F6 had similar mass loss rates.

Mass loss in flooded treatments was similar after 1 yrto that reported by Conner and Day (1991) where the

Table 1. Percentage of mass, C, N, and P remaining, C/N, C/P, N/P ratios, and decomposition rates (k) associated with litterbags inflooding microcosms after 106 wk on Ogeechee River floodplain.

Treatment! Mass N C/N C/P N/P

NF

FN

FP

F2/2

F6

F3

38.9 a^(8.0)23.7 ab(4.9)21.9 b(5.7)21.5 b(4.7)18.4 b(1.7)13.1 b(1.7)

39.4 a(12.2)6.7 b(1.7)6.2 b(1.7)4.8 b(0.6)4.1 b(0.3)3.9 b(0.8)

39.4 a(12.2)20.4 ab(4.5)21.9 ab(6.5)11.8 b(2.6)11.7 b(1-3)8.4 b(1.6)

38.7 a(15.5)38.9 ab(11.9)36.0 ab(9.8)27.5 ab(5.4)27.6 ab(3.6)16.0 b(1.4)

22 a(0.02)16 ab(0.01)15 b(0.01)22 ab(0.03)18 ab(0.01)24 a(0.03)

115 a(0.25)71 a(0.04)68 a(0.03)73 a(0.07)60 a(0.04)

103 a(0.30)

4.90 a(0.67)4.37 a(0.50)4.69 a(0.66)3.36 a(0.27)3.37 a(0.22)4.31 a(1.19)

0.10 a(0.01)0.14 b(0.00)0.16 b(0,01)0.15 b(0.01)0.15 b(0.01)0.17 c(0.01)

t NF, not flooded; FN, flooded 3 mo with elevated N; FP, flooded 3 mo with elevated P; F2/2, flooded 2 mo, drained 1 mo, reflooded 2 mo; F6, flooded6 mo; F3, flooded 3 mo.

t Column means followed by the same letter are not significantly different at the 0.05 level.§ Standard error of the mean in parentheses.

LOCKABY ET AL.: HYDROPERIOD INFLUENCES IN DECOMPOSING LITTER 1269

most rapid loss occurred in well-oxygenated areas withhigh loss of floodwaters. Although Day (1982) foundthat the more extensively flooded forest communities inthe Great Dismal Swamp exhibited higher decompositionrates, differences among the communities in terms oflitter quality were thought to control rates to a greaterextent than degree of flooding. In our study, the degreeof oxygenation and litter quality were very similar amongthe treatments so that other hydroperiod characteristicsdominated.

We saw no indication that alternating wet-dry cycles(treatment F2/2), known to be so critical in upland sys-tems (Swift et al., 1979), drove decomposition to thegreatest extent, as hypothesized by Brinson et al. (1981).This could be due to inherent differences in the degreeto which moisture is limiting to microbes during dryconditions in an upland vs. floodplain system. In thelatter, during some noninundated periods, floodplain sur-face soils may remain moist in comparison to an uplandwhere soils are more likely to become very dry. Conse-quently, microbial populations may not move throughexpansion-mortality cycles to the same degree.

Similarly, prolonged flooding (i.e., treatment F6) didnot stimulate decomposition more than single, shorterevents (i.e., treatment F3). We suggest that decompo-sition is promoted by any form of periodic, well-oxygenated flooding. However, drawdown must occurbefore the full influence of the stimulatory effect can berealized, because the interactive supply of moisture andO2 will be more conducive for microbial activity in amoist, aerobic, yet nonflooded environment than underflooded, aerobic conditions.

In our study, there is evidence that enhanced N inflow(i.e., treatment FN) reduced or interfered with decompo-sition. Some studies have found that N additions stimulatedecomposition (Howarth and Fisher, 1976; Dierbergand Ewel, 1984). However, Fog (1988) cited numerousexamples of reduced decomposition associated with ele-vated N. Reduced decomposition is thought to be associ-ated with interference between N and lignin degradation(Berg and Tamm, 1991). This explanation is supported bythe suppression of lignin degradation in the FN treatment(Lockaby et al., 1996). Elevated P inflow had littlediscernible effect on patterns of mass loss and, thus, ourresults agree with those of Howarth and Fisher (1976).However, Elwood et al. (1981) found that P additionsstimulated decomposition in streams.

Nitrogen and Phosphorus DynamicsThe proportion of original N remaining at 106 wk

followed similar trends as those for mass loss (Table 1).The NF treatment had the most N remaining and the F3treatment the least. The latter treatment reduced theproportion of N in bags by approximately 30%. Allflooding treatments increased N loss by at least 20%relative to that of the NF treatment.

Phosphorus loss was stimulated by flooding to a lesserextent than that of N (Table 1). The NF treatment retainednearly 40% of its P, while the F3 treatment retainedonly 16%. Other flooding treatments were intermediate

and were not statistically different from the NF treatmentat the 0.05 level.

Although comparisons of N and P remaining at 106 wkrevealed minimal difference among the different floodingregimes, the temporal patterns (Fig. la-If and 2a-2f)indicated that differences in flooding induced shifts inN and P dynamics. In the NF treatment, there was asteady mineralization trend that proceeded more slowlythan those of the flooding treatments (Fig. la). Initially(first 8 wk), N in flooding treatments behaved in muchthe same fashion as in the NF treatment. Thereafter, amuch sharper decline in N content occured in the floodedtreatments. Although the F3 and F6 treatments continuedto exhibit N decreases, the F2/2, FP, and FN treatmentsdisplayed a tendency to immobilize N around 51 wk,followed by a second mineralization phase.

Although the initial N/P ratio in the litter was 8.1(i.e., a level where P is supposedly not limiting todecomposition; Vogt et al., 1986), P was immobilizedto a greater extent than N. In the NF treatment, afteran initial decrease, an immobilization phase occurredduring 31 to 51 wk, followed by mineralization. Ingeneral, flooding stimulated the onset of immobilization,presumably in response to greater P availability in inflow,whether elevated or natural. There appeared to be twoperiods of immobilization in each flooding treatment,although this tendency was clearer in treatments F6,F2/2, and FP. All flooding treatments induced an immo-bilization phase near 4 through 8 wk, exhibited a mineral-ization pulse minimum in 31 wk, and display a secondimmobilization phase near 51 wk (with the exception ofF3). The FP and, to a lesser extent, the FN treatmentsshowed larger initial immobilization pulses in responseto enhanced nutrient inflow.

Treatments also induced variation in C/N ratios butnot in C/P and N/P ratios (Table 1). The F3 regimeexhibited the highest C/N ratios, which differed signifi-cantly from the FP treatment. Although little informationis available on temporal patterns of N and P immobiliza-tion-mineralization in periodically flooded environ-ments, Brinson (1990) observed a greater immobilizationtendency for P than N in an alluvial swamp. In contrast,Day (1982) found that both N and P were accumulatedin decomposing litter in the Great Dismal Swamp. Inour study, flooding in general induced major changes in Nand P dynamics within decomposing leaf litter comparedwith nonflooded conditions. The degree to which miner-alization or immobilization occurs for particular elementsmay be a key determinant of the nature of the biogeo-chemical transformation process associated with CoastalPlain blackwater riverine systems in particular (Elder,1985).

We suggest that temporal patterns of N and P dynamicsin litter could reflect an important interactive mechanismbetween hydroperiod, nutrient availability, and otherprocesses in nutrient-deficient floodplains such as black-water systems. In many floodplains, the phenology offine root initiation during the early growing season isclosely linked to floodwater drawdown (Jones et al.,1996). Consequently, N and P mineralization pulsesinduced shortly after flooding cessation may reflect in-

1270 SOIL SCI, SOC. AM. 1., VOL. 60, JULY-AUGUST 1996

Trt FN Trt FP

1 2 4 8 12 21 31 39 51 65 79 94 106

120

0 1 2 4 8 12 21 31 39 51 65 79 94 106

Trt F2/2 Trt F3

0 1 2 6 8 12 21 31 39 51 65 79 94 106

120

0 1 2 4 8 12 21 31 39 51 65 79 94 106

Trt F6 Trt NF

0 1 2 6 8 12 21 31 39 51 65 79 94 106 0 1 2 6 8 12 21 31 39 51 65 79 94 106

Fig. 1. Percentage of N remaining in microcosms on Ogeechee River floodplain. Treatments were: FN, flooded 3 mo with elevated N; FP, flooded3 mo with elevated P; F2/2, flooded 2 mo, drained 1 mo, reflooded 2 mo; F3, flooded 3 mo; F6, flooded 6 mo; and NF, not flooded.

LOCKABY ET AL.: HYDROPERIOD INFLUENCES IN DECOMPOSING LITTER 1271

Trt FN Trt FP120

2 4 8 12 21 31 39 51 65 79 94 106Week

8 12 21 31 39 51 65 79 94 106

120

100

80a>c

oE0>>-

CL

20

Trt F2/2

ob

m PI

I III I III M i l l0 1 2 6 8 12 21 31 39 51 65 79 94 106

Week

Q.

Trt F3

0 1 2 4 8 12 21 31 39 51 65 79 94 106

Trt F6

0 1 2 6 8 12 21 31 39 51 65 79 94 106

120

100

fr?80

cnc.9 60O

40

20

Trt NF

0 1 2 6 8 12 21 31 39 51 65 79 94 106

Fig. 2. Percentage of P remaining in microcosms on Ogeechee River floodplain. Treatments were: FN, flooded 3 mo with elevated N; FP, flooded3 mo with elevated P; F2/2, flooded 2 mo, drained 1 mo, reflooded 2 mo; F3, flooded 3 mo; F6, flooded 6 mo; and NF, not flooded.

1272 SOIL SCI. SOC. AM. J., VOL. 60, JULY-AUGUST 1996

creased availability during periods of very active rootgrowth. Such a synchroneity has been observed in therainforests of the Amazon River Basin where the timingof P mineralization pulses is thought to be critical, sincecommunity species composition tends to track spatialvariation in the timing of P mineralization (Swift andAnderson, 1992).

CONCLUSIONSOur findings indicate that, within the generally moist

environments associated with floodplain forests of thetemperate zone, mass and C loss from decomposinglitter will be stimulated by temporary, aerobic flooding.Among the range of flooding regimes imposed here,differential effects on mass and C dynamics were notobserved. However, the data suggest that single, briefflooding events, as opposed to the wetting-drying cyclesassociated with rapid decomposition in uplands, maystimulate rates of degradation to the greatest extent.Also, elevation of N levels within the decompositionenvironment may suppress degradation.

When compared with nonflooded conditions, effectsof flooding on N and P dynamics after 106 wk weregenerally similar to those for mass and C. The differentregimes had similar effects on the losses of both elements.However, there were major differences in immobiliza-tion-mineralization patterns among the regimes tested.The latter may serve as a biogeochemical bridge infloodplain forest communities between nutrient availabil-ity and growth initiation following floodwater drawdownin spring.