Plant and Soil 201: 17, 1998. 1998 Kluwer Academic Publishers. Printed in the Netherlands. 1
Black alder (Alnus Glutinosa (L.) Gaertn.) trees mediate methane andnitrous oxide emission from the soil to the atmosphere
H. Rusch1;2 and H. Rennenberg1;31Fraunhofer-Institute for Atmospheric Environmental Research, Kreuzeckbahnstrasse 19, 82467 Garmisch-Partenkirchen, Germany. 2Present address: Krieseby 1, 24354 Rieseby, Germany. 3Corresponding author;permanent address: Institut fr Forstbotanik und Baumphysiologie, Professur fr Baumphysiologie, Albert-Ludwigs-Universitt Freiburg, Am Flughafen 17, 79085 Freiburg i. Br., Germany
Received 30 October 1997. Accepted in revised form 11 February 1998
Key words: alder, Alnus glutinosa, methane, nitrous oxide, trace gas flux
Three-year-old seedlings of black alder (Alnus glutinosa (L.) Gaertn.), a common European wetland tree species,were grown in native soil taken from an alder swamp. Fluxes of methane (CH4) and nitrous oxide (N2O) betweenthe tree stem and the atmosphere were determined under controlled conditions. Both CH4 and N2O were emittedthrough the bark of the stem into the atmosphere when the root zone exhibited higher-than-ambient CH4 and N2Ogas mixing ratios. Flooding of the soil caused a decreased N2O emission but an increased CH4 efflux from the stem.Immediately after flooding of the soil, N2O was emitted from the seedlings bark at a rate of 350 mol N2O m2h1 whereas CH4 flux could not be detected. After more than 40 days of flooding CH4 fluxes up to 3750 molCH4 m2 h1 from the stem were measured, while N2O emission had decreased below the limit of detection. Gasefflux decreased with increasing stem height and correlated with gas mixing ratios in the soil, indicating diffusionthrough the aerenchyma as the major path of gas transport. From these results it is assumed that woody specieswith aerenchyma can serve as conduits for soil-derived trace gases into the atmosphere, to date only shown forherbaceous plants. This, yet unidentified, woody plant pathway contributes to the total greenhouse gas sourcestrength of wetlands.
As a morphological adaptation to flooding conditions,wetland plants possess aerenchyma. This air-filled tis-sue is required to aerate the roots of wetland plantsand constitutes the major pathway for the flux of O2through plants (Grosse and Schrder, 1986; Justin andArmstrong, 1987; Perata and Alpi, 1993; Schrder,1989). This pathway is also a means for soil-bornegases to escape in the opposite direction, i.e. from theroot zone to the shoot and into the atmosphere.
Gases that have been shown to escape into the at-mosphere via aerenchyma of wetland plants are: (1)methane (e.g. Sebacher et al., 1985; Schimel, 1995),(2) nitrous oxide and (3) dinitrogen (Mosier et al.,
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1990; Reddy et al., 1989), (4) carbon monoxide (Con-rad et al., 1988), (5) hydrogen (Schtz et al., 1988),and (6) carbon dioxide (e.g. Thomas et al., 1996). Theplant pathway may make up more than 90% of totalemission of soil-borne gases as has been shown forCH4 emission from rice paddies (Holzapfel-Pschornand Seiler, 1986; Schtz et al., 1989) and wetlandsdominated by Carex spec. (Whiting and Chanton,1992). Other pathways of CH4 exchange between thesoil and the atmosphere, i.e. the release of gas bub-bles (Keller and Stallard, 1994) and diffusion at thesoil/water-atmosphere interface (Barber et al., 1988;Schtz et al., 1991), appear to be of minor importance.
Whereas the role of wetland plants as conduits forCH4 has been investigated for numerous herbaceousspecies (for a review see Schtz et al., 1991; Singhand Singh, 1995), the flux of nitrogen both as N2 and
2N2O from the root zone into the atmosphere has sofar been shown for only a few species including rice(Oryza sativa L.) (Mosier et al., 1990; Reddy et al.,1989), Pontederia cordata L. and Juncus effusus L.(Reddy et al., 1989). Plant-mediated efflux of soil-derived gases is assumed to be of minor importancefor total gas emissions of ecosystems with low vegeta-tion like Sphagnum spec. dominating bogs or heathwhere plants have no or only small roots (Thomaset al., 1996). Thus, depending on the soil and theplant community, the plant pathway may or may notcontribute to the total gas source strength of wetlandecosystems. For most natural ecosystems the contri-bution of plant-mediated gas efflux to total emission isunknown.
So far only herbaceous wetland species have beenidentified as paths for gas transport from the root zoneinto the atmosphere. Schtz et al. (1991) discussed apotential gas efflux out of the trunks of woody speciesvia the lenticels, but actual data on trees as conduitsfor soil-borne trace gases have not been published todate. Trees and shrubs make up a large percentageof wetland biomass, especially in riverine ecosystemspermanently or temporarily flooded. Similar to wet-land herbs, these woody plants exhibit morphologicaland physiological adaptations to wetland conditions,among them aerenchyma formation (Perata and Alpi,1993; Schrder, 1989).
Therefore, in the present study the question hasbeen addressed as to whether aerenchymous woodyplants mediate gas transport from the root zone viathe shoot into the atmosphere. We decided to inves-tigate fluxes of the greenhouse gases CH4 and N2O(IPCC, 1994) because wetlands are considered strongsources of CH4 and, to a lesser extent, N2O (Ciceroneand Oremland, 1988; Martikainen et al., 1993, 1995;Schtz et al., 1990). Black alder (Alnus glutinosa(L.) Gaertn.) was selected for these studies becauseaerenchyma formation and oxygen transport of thisEuropean wetland species is well documented (Grosseand Schrder, 1986; Schrder, 1989).
Material and methods
Two-year-old seedlings of black alder were obtainedfrom a tree nursery (Baumschule Handel, Tutzing,Germany) and planted in 15-L pots containing soilfrom an alder swamp (top layer, 030 cm) on Lake Be-lau in Northern Germany (5460 N, 10140 E) taken in
Figure 1. Root fumigation chamber used for N2O and CH4 trans-port studies.
spring. Plants were left in the field and were kept well-watered during the summer, using tap water withoutaddition of nutrients. The seedlings were not wateredduring winter, and the roots were protected from freez-ing damage by covering the pots with wood chips.Watering was resumed in April of the following year.At this time the 3-year-old seedlings had reached astem height of approximately 2 m and a diameterof 2.5 cm at soil level. Plants were moved to thelaboratory a few days before the experiments werestarted to allow adaptation to indoor conditions. Af-ter holes in the bottom of the pots had been closed,the plants were watered to achieve waterlogged con-ditions. Twigs were cut off at their base up to a stemheight of 40 cm, on average four twigs per plant, tofacilitate mounting of the chambers on the plants. Toprevent effects on gas flux, the cut ends were sealedwith grease. Light intensity was maintained at 200mol m2s1 PPFD (photosynthetic photon flux den-sity) at the top of the canopy and at about a tenth ofthis value at soil level, representing the conditions of aforest understorey. Air temperature was kept at 20 2C.
3Figure 2. Static chamber for the determination of N2O and CH4efflux from the stem.
Root fumigation chamber
In order to expose bare roots of intact plants to N2Oor CH4, a root fumigation chamber similar to that de-scribed by Huss-Danell (1978) was construced (Figure1). For this purpose two perspex lids were fitted ona perspex cylinder (35 cm high, i.d. 12 cm) with O-rings. A hole was drilled in the centre of the top lidwide enough to hold the stem base, and the top lid wascut in two pieces. Edges of the lid were smoothed andfitted with rubber seals. The entire root was rinsed withtap water to remove the soil and was inserted into thechamber in such a way that the stem base was lined upwith the top lid. The two parts of the top lid were in-serted from left and right, and the remaining openingsin the top lid were sealed with an inert sealing material(Teroson, Teroson GmbH, Heidelberg, Germany). Thebottom of the chamber was covered with water beforefumigation started to achieve a saturated atmosphereonce the chamber was sealed. Up to several mL of pureCH4 and N2O (Messer-Griesheim, Olching, Germany)were injected into the chamber through the septum ofa 1/4" fitting.
Gas flux out of stems of alder seedlings was measuredwith static chambers. For their design perspex cylin-ders (i.d. 12 cm) and lids fitted with O-rings were used(Figure 2). Two holes were drilled wide enough to fitthe stem, and the cylinder was cut in two parts acrossthe holes to attach the chamber horizontally to the stem(Figure 2). The cut edges of the cylinder were fitted
with rubber seals, and the two parts of the chamberwere held in place with four steel springs attached tosmall screws. The remaining openings of the cham-ber were sealed with Teroson. One of the lids wasequipped with a fan and two 1/4" fittings. The fan waspowered by a 24 V electric motor with a magnet oneither side of the lid. Each chamber enclosed 12 cm ofthe stem. Three chambers were attached on top of eachother, starting at the stem base, thus enclosing stem ar-eas between 012 cm, 1426 cm, and 2840 cm abovesoil, respectively. In control experiments high mixingratios of N2O and CH4 had been injected into emptychambers. Mixing ratios had remained constant over afew hours, showing that the chambers were gas-tight.
Soil air samples
The mixing ratios of CH4 and N2O were monitoredwith the sampling device developed by Bodenbender(1997). This device is essentially a rectangular-shapedpost made from perspex (35 cm long, 9 cm wide,5 cm deep) with small holes (18 mm diameter) ascha