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Soil Biology & Biochemistry 41 (2009) 1577–1579

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Soil Biology & Biochemistry

journal homepage: www.elsevier .com/locate/soi lb io

Short communication

Substantial rewetting phenomena on soil respiration can be observed at lowwater availability

Thomas Fischer*

Brandenburg University of Technology at Cottbus, Faculty of Environmental Sciences and Process Engineering, Central Analytical Laboratory, Konrad-Wachsmann-Allee 6,03046 Cottbus, Germany

a r t i c l e i n f o

Article history:Received 22 December 2008Received in revised form7 April 2009Accepted 16 April 2009Available online 3 May 2009

Keywords:Soil rewettingSoil respirationSoil water potential

* Tel./fax: þ49 355 692840.E-mail address: fischer@zal.tu-cottbus.de

0038-0717/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.soilbio.2009.04.009

a b s t r a c t

Although metabolic activity of soil organisms is determined by water accessibility, little attention wasgiven to rewetting with different water potentials. Rapid water potential increase induced a respirationpulse in organic layers in laboratory experiments and significant effects could be observed when soilbelow �6300 hPa was rewetted.

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Soils are substantially rewetted by rainfall events at the end ofthe drought period, and soil organic matter mineralization isstimulated. Phenomena associated with soil drying and rewettingcycles have become known as the Birch effect. Jarvis et al. (2007)summarized four main hypotheses to explain the nutrient pulseand, by implication, the CO2 pulse: (1) drying and rewetting shat-ters soil aggregates and exposes previously unavailable organicsubstrates for decomposition (Denef et al., 2001); (2) microorgan-isms killed by soil drying are decomposed on rewetting to releasetheir nutrients (Bottner, 1985); (3) there is a spontaneous rapidincrease in microbial biomass and fungal hyphae in response to theavailability of water (Griffiths and Birch, 1961; Jager and Bruins,1974; Orchard and Cook, 1983; Scheu and Parkinson, 1994); and (4)there is a microbial hypo-osmotic stress response (Kieft et al., 1987;Fierer and Schimel, 2003). However, ‘‘dry’’ and ‘‘wet’’ or ‘‘rewetted’’soils have been quantified in the literature mostly with respect tovolumetric or gravimetric water content increase, and little atten-tion was paid to water potential change. Xiang et al. (2008) relatedrewetting effects to a water potential increase from �15 MPa to�60 kPa. Orchard and Cook (1983) investigated the relationshipbetween the change in water potential and the ratio of basalrespirations after and before rewetting, but they excluded the flushof respiration, thus neglecting the Birch effect. The question arisesas to how dry a soil should be before expressing a respiration pulse

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after rewetting. Because accessibility of water to living organisms isdetermined by water potential, this concept should be used forquantification.

In general the global pool of C in soils exceeds that in vegetationby about 4:1, the pool of carbon in soils of the mid-latitudinal beltexceeds that in vegetation by about 2:1 (Dixon et al., 1994; Jarviset al., 2007). Here, 80–90% of the soil carbon reside in the organicOF and OH horizons (Fischer et al., 1995). Thus, the understandingof ecosystem carbon budgets requires knowledge about soil organicmatter storage and breakdown, and organic soil horizons are ofparticular significance in that processes. In this study I determinedthe relation between rapid water potential increase (DJ) andrespiration flush.

The main characteristics of the stands are shown in Table 1. Afterthe vegetation cover, litter and roots had been removed, the OFhorizons were sampled from a 1�1 m2 sampling plot and stored at4 �C for later use.

The samples have been equilibrated to standard matric suctionssmaller than or equal to �500 hPa using a ceramic suction plateassembly and to standard matric suctions greater than �500 hPawith pressure pots (Weisdorfer, 1999). Water retention curves didnot differ significantly (p ¼ 0.14 in ANOVA of residues) between thesites (Fig. 1).

Best fit for all sites could be achieved using the equation

log10ð �JÞ ¼ 4:82e�0:014$GWC; r2 ¼ 0:89***

J is the water tension in hPa, GWC is the gravimetric water contentin 100 g/g dry matter.

Table 1Experimental sites.

Roesa Taura Neuglobsow

Location 12�260 E 51�380 N 13�20 E 51�280 N 13�20 E 53�80 NVegetation Scots pine Scots pine Scots pineStand age 60 yrs. 45 yrs. 60 yrs.Annual precipitation 566.0 mm 564.6 mm 594.8 mmMean annual

temperature8.87 �C 8.87 �C 8.18 �C

Soil type Leptic Podzol Spodi-dystricCambisol

DystricCambisol

Texture Sand Silty sand Sand

OF horizonThickness [cm] 6 5 4Carbon [%] 34 39 44Nitrogen [%] 1.68 1.54 1.61pH H2O 5.2 4.1 3.8

1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

log(|ΔψΔψ| [hPa])

RC

(m

g C

O2−

C g

−1 C

)

NeuglobsowTauraRoesa95% confidence interval95% prediction interval

RC=0.323 log(|Δψ|)>3.20−1.01 r2=0.76***

Fig. 2. Relation between water potential increase and rewetting effect RC.

T. Fischer / Soil Biology & Biochemistry 41 (2009) 1577–15791578

CO2 production was determined at 22 �C using IR detection,where CO2 accumulated in sealed jars with known volumes whichwere consecutively connected to a closed gas cycling system witha known volume (Klimanek, 1994) according to the following timeschedule. Amounts of field moist samples (J > �10 hPa) thatcorresponded to 10 g of dry weight were brought to basal respi-ration and then dried successively by opening the incubation jarsfor 2–8 h daily until they reached a pre-defined water potential. Thewater potential was controlled gravimetrically. In the meantime,the jars were sealed and CO2 production was determined. Afterrewetting, the drying procedure was omitted and CO2 productionwas determined every 6 h during the respiration pulse and every24 h until the end of the experiment. The CO2 concentration in thejars never exceeded 2000 ppm. The use of sealed incubation jarsensured controlled drying and constant water potentials duringincubation. After measurement, the gas system and incubation jarswere aerated with water saturated air for gas exchange andremoval of accumulated CO2.

Respiration maxima of the organic horizons occurred atJ > �10 hPa and amounted to 0.442 � 0.166 (mean value � 95%C.I.), 0.429 � 0.081 and 0.418 � 0.072 mg CO2-C g�1 C d�1 atNeuglobsow, Taura and Roesa, respectively. As could be expected,decreasing soil water potential resulted in decreasing respiration(Moore, 1986). However, at the permanent wilting point(J ¼ �1.5 MPa) respiration amounted to approx. 5–8% of maximalrespiration, indicating still ongoing microbial activity under

0

1

2

3

4

5

0 30 60 90 120gravimetric water content (100 g/g dry matter)

lo

g(-Ψ

)

NeuglobsowTauraRoesa

Fig. 1. Water retention curve of the OF horizons of the study sites.

conditions of plant wilting. These values are comparable to thosereported by Moore (1986) for leaf and needle litter who found thatdecomposition will cease at �38 MPa. Swift et al. (1979) concludedthat bacteria are inactive below �1.0 to �1.5 MPa in soils, whereassoil fungi are active down to �15 MPa, and that some Aspergillusand Penicillium are active to �40 MPa and below.

After the samples reached a constant basal respiration at theirpre-defined water potential, distilled water was sprayed on theirtop. The amount of water added to adjust J > �10 hPa wascontrolled gravimetrically. The samples were not mixed afterrewetting, but water was allowed to infiltrate spontaneously. CO2

production was determined until the samples reached theirmaximal basal respiration rates (see above) for 3 successive days.The water potential increase at rewetting was expressed as thelogarithm of the positive difference between soil water potentialsin hPa before and after rewetting.

The rewetting effect RC was defined as

RC ¼Xn

i¼0

CminðiÞ � nCminðJ>�10hPaÞ

Cmin(i) is the daily CO2-C production at day i after rewetting, n is thenumber of successive days after rewetting and Cmin(J>�10hPa) is themaximal basal respiration.

The relation between water potential increase and rewettingeffect RC is illustrated in Fig. 2.

Rewetting effects increased with increasing water potentialchanges. Irrespective of the location, significant rewetting effectswere predicted at log(jDJj [hPa]) � 3.2. Hence, remarkablerewetting effects on soil respiration were detected when the soilwater potential was below �6300 hPa.

Acknowledgements

This study was funded by the German Ministry of Education andResearch. I am grateful to Oliver Dilly and an anonymous reviewerfor the truly helpful comments.

References

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