Low-Temperature Respiration in Arctic Soils

G.J. Michaelson and C.L. PingUniversity of Alaska Fairbanks,

School of Natural Res. and Agric. Sciences

SUMMARYWith fall snow cover, cold-season soil temperatures in arctic soils hover around 0oC for an extended period during the early winter and then as soil bulk water freezes, soil temperatures drop to subzero levels. Soil respiration under conditions representing these two phases of the cold-season was studied using cryobath incubation experiments at 0oC and –2oC. The study included a total of 98 fresh-frozen soils from the ATLAS and LAII flux study sites along the western Alaska transect and from the Kuparuk River Basin transect extending from Prudhoe Bay to Toolik Lake. Organic surface horizons [O-horizons] respired at average rates 2.5 times higher than mineral horizons at both temperatures. Results indicate that as bulk soil water freezes, respiration rates fall an average of 91% in tundra O-horizons and 70% in corresponding shrub/forest soils, as compared to 85% in mineral horizons. At 0oC before bulk soil water freezes, soil respiration rates were more closely related to soil water-soluble organic carbon [OCws] stocks in O-horizons and to soil total organic carbon [TOC] stocks for mineral horizons. As bulk soil water freezes at –2oC these apparently different associations for organic and mineral horizons were strengthened. While OCws stocks are good predictors of respiration in the surface O-horizons, soil TOC stocks are better than OCws as predictors of low-temperature respiration for mineral horizons. These data indicate that with information on soils and soil organic carbon contents, it should be possible to

predict CO2-respiration responses of arctic soils under changed winter-season conditions.

IntroductionIt has been found that respiration in arctic soils can continue until soil temperatures drop to –10oC or lower [Flanagan and Veum, 1974]. Significant amounts of CO2 gas flux have been observed during the cold-season, and it is estimated that this can account for up to 60-80% of the net ecosystem exchange [Oechel et al., 1997]. High CO2-efflux rates have been observed particularly during the shoulder-seasons around September-October-November and again in May. Soil temperatures in the active layer during these months are often nearly isothermal at around 0oC [see example, Figure 1]. The duration of these periods of moderately cold temperatures is highly dependent on seasonal snow conditions and air temperature, and thus there is considerable inter-annual variation. With climate change over a decadal time-period, changes in average duration of these isothermal periods could have significant effects in net ecosystem function and C-dynamics.

Soil temperature during the cold-season can be divided with regard to the physical state of bulk soil water i.e. frozen or unfrozen [Figure 1]. Soil temperatures remain at or near 0oC when bulk soil water is cooled but still in a liquid state and then as temperatures drop below 0oC, bulk soil water freezes leaving only brine-films around soil particles. Presumably soil biological respiration must occur in a liquid water medium, therefore at the time when soil bulk water freezes there should be a major impact on soil respiration. At this point of freezing there is a drastic change in the amount of soil liquid water. It

follows at there should also be a change in the access to and availability of soil organic matter [SOM] as substrates for soil respiration. In order to understand and predict cold-season respiration activity, more must be known about the factors controlling respiration across this major dividing point in soil water and temperature conditions as it occurs twice a year at onset and conclusion of the cold-season.

ObjectivesThis study was undertaken to investigate cold-season soil respiration and the interaction of soils and SOM substrates with temperature, as bulk soil water freezes under cold-season soil conditions. The three main questions were:

1. How do soil respiration rates vary across the threshold temperature of 0oC below which most of bulk soil water freezes?

2. Does soil respiration rate respond to temperature drop across the OoC mark with different sensitivity in surface organic soils compared with mineral subsurface soil?

3. When bulk soil water freezes, is there an apparent change in the SOM substrates important for respiration such that water-soluble substrates become more closely related to observed soil respiration rates?

Laboratory Study MethodsSoils and Incubations – Samples were collected from the genetic horizons representing the active layers (or upper 1 meter) at the LAII-ATLAS C-flux study sites at Oumalik, Ivotuk, Council, Mauze Creek, and at Kougarok in western Alaska and at the LAII-Flux sites at Betty Pingo, Deadhorse, Sagwon, Happy Valley and Toolik [Figure 2]. Samples were kept frozen at –15oC and at field moisture content until just before incubations began. Two separate sets of 98 soil samples were incubated in constant temperature cryobaths [see Figure 3], one at 0oC and another at –2oC. Samples were incubated in 250 cm3 jars containing 150 cm3 cores packed to the field-bulk densities for individual soil. Carbon dioxide respiration rates were measured weekly using a Columbus Instruments Mico-oxymax respirometer and the 2-week rates used for this study.

Soil Characteristics – Soil moisture contents were determined on parallel samples taken at incubation, one to represent 0oC water content by oven-drying at 105oC and one at –2oC by Vitel sensors placed in incubated samples. Total OC was determined on a LECO 1000 CHN Analyzer. Soil water-soluble organic carbon [OCws] was indexed by determination of the dissolved organic carbon in solution [.45 m filtered] after a 4 hr. equilibration at 25oC using 150 cm3 of thawed field-moist soil with an equal volume of deionized water. Organic horizons had TOC contents of >20% while mineral soils were 20% TOC.

Figure 1. An example of the annual soil temperature cycle [R. F. Paetzold, C.L. Ping and V.E. Romanovsky, unpublished data].

Active Layer < 0oC

Much of Active Layer @ 0oC

< 0oC

0oC 0oC

Toolik Lake, Alaska 2000

Figure 3. Cryobath incubations and respirometer.

ResultsRespiration Rates [Table 1]

Unfrozen water content at 0oC averages higher [66%] as would be expected in organic horizons [O-horizons] compared to mineral horizons [31%]. At –2oC much of soil bulk water freezes and average unfrozen water contents fall to about the same level [6-7%] for both Organic and mineral horizons. However, O-horizons lose about 90% of their liquid water compared to only an average 77% for mineral horizons. There is no apparent direct or simple relationship between soil liquid water and respiration rate.

Respiration rates at both temperatures averaged higher in organic compared to mineral horizons.

O-horizons from tundra sites had the highest average respiration rates at 0oC but their rates were more sensitive to temperature or freezing of soil bulk water than organics from shrub and forest sites. As temperature dropped from 0oC to –2oC, respiration in tundra organic soils fell 90-92%, while respiration in the shrub and forest O-horizons fell only 62-75%. At –2oC

O-horizons from shrub and forest sites had average respiration rates almost twice as high as corresponding soils from tundra sites.

Mineral horizons from the group including permafrost, lichen cover, or nonacidic tundra sites, and the group of Bg horizons, each had respiration rates that averaged highest among mineral horizons at both temperatures. The Council B-horizons respired at lower rates than most mineral horizons.

Soil OC and Respiration [Table 2]

Organic Horizons Respiration rates in O-horizons were more closely related to OCws than

TOC at both temperatures. Relationships between respiration and OCws were improved with dropping temperature. The only significant relationship for respiration and TOC among O-horizons was for the Council forest sites and this was only due to a high negative correlation [r=-0.85] between TOC and OCws among those soils. The forest surface O-horizons are less dense and high in OCws [and respiration] but are lower in TOC compared with the denser more decomposed O-horizons immediately below them [forest Oi versus Oa horizons respectively].

As data are grouped by site or vegetative-cover within site, relationships improve [Figure 4]. Respiration in O-horizons from shrub and forest sites

was very highly related to OCws at –2oC [R2=0.90-0.97] and at OoC but with more scatter in data. However, respiration responds differently in forest organic soils and any significant relationship between respiration and OCws is lost at 0oC.

Mineral Horizons When all mineral horizons were grouped together, respiration was not

related to OCws and only weakly related to TOC [Table 2, and Figure 5]. Data grouped by soil horizon type and site vegetative cover improved some relationships.

Respiration rates were not related to OCws or TOC for the Bw or Bg all-inclusive groups, but when B-horizons from these were selected for their close proximity to the surface O-horizons, TOC was highly related to respiration at 0oC [R2=0.86]. Stocks of OCws were low and –2oC respiration rates low for this group [Table 1] and there were no significant relationships for OCws or TOC.

The Bf/Lichen/NATundra-group had the highest overall respiration rates [Table 1]. In this group, OCws and TOC were significantly correlated with each other [r=0.73, p<0.01]. Therefore, respiration at –2oC was significantly related to both OCws and TOC [Table 2] but only TOC related to respiration at 0oC.

Respiration at –2oC in mineral B-horizons from Council forests was highly related to TOC but the relationship was significantly reduced at 0oC.

Conclusions1. As soil bulk water freezes, respiration in soil surface O-horizons of arctic

Alaska can be expected to drop [on average 86% in this study]. During the early cold-season when soil temperatures are at 0oC, tundra organic layers are respiring at higher general rates than shrub and forest organic layers. But as bulk soil water freezes, respiration rates in tundra organic layers will drop more drastically than those of the shrub and forest soils and be respiring at rates averaging approximately half that of the shrub and forest soils.

2. As soil temperature drops from 0oC to –2oC, soil respiration in mineral horizons can be expected to drop on average about the same as for the surface O-horizons [about 85%]. Overall respiration response to temperature is less variable among mineral horizons compared with O-horizons.

3. Respiration rates in O-horizons relate reasonably well to index soil stock levels of soil water-soluble organic carbon (OCws), this relationship is especially strong at subzero temperatures after bulk soil water has frozen. Mineral horizons however, appear to be more complex and apparently a variety of factors are significant in determining respiration rates at low temperatures. However, low-temperature respiration related reasonably well to total organic carbon [TOC] present for groups such as those mineral horizons close to the surface, those associated with higher levels of

activity [i.e. permafrost, lichen, and nonacidic tundra associated mineral horizons], and those found in more specific locations or groups as under forest at Council.

4. Given the apparent strength of the relationships between CO2-respiration and either OCws or TOC stocks it seems that prediction of soil respiration responses under changed cold-season soil temperature conditions should be possible as an aid in modeling winter CO2-fluxes. These results point to the importance of soil temperature and snow-cover on the duration of the 0oC soil isothermal period and its potential for significant impacts to cold-season CO2-fluxes from arctic soils.

Acknowledgements:

The authors wish to thank Vladimir Romanovsky for his assistance with measurement of soil unfrozen water content and the Palmer AFES-Laboratory for soil chemical analysis.

References

Flanagan, P. W., and A.K. Veum, Relationship between respiration, weight loss, temperature and moisture in organic residues on tundra, in Soil Organisms and Decomposition in Tundra, edited by A.J. Holding, O.W. Heal, S.F. MacClean Jr., and P.W. Flanagan, pp. 249-277, IBP Tundra Biome Steering Committee, Stockholm, Sweden, 1974.

Oechel, W. C., G. Vourlitis, and S.J. Hastings, Cold season CO2 emission from arctic soils, Global Biogeochem. Cycles, 11, (2), 163-172, 1997.

Table 1. Selected data averages for 98 study soils (37 organic and 61 mineral horizons).

Unfrozen Water Resp. RateSoil Horizon TypeGroup by Site or Veg. n 0oC -2oC

OCws TOC0oC -2oC

0 to-2oCChange

Vol. % g m-2 cm-1 gCO2 cm-3 da-1 %

Organic 37 66 6 2.0 680 57.3 8.3 -86 Tundra (All) 25 70 7 1.8 730 60.8 6.3 -90 Tundra (Acidic) 12 75 7 1.8 740 66.0 5.3 -92 Shrub (All) 8 54 5 2.1 470 44.5 11.0 -75 Shrub (Council) 5 68 7 2.7 620 35.3 13.3 -62 Forest (Council) 4 63 6 2.5 640 47.7 13.0 -73

Mineral 61 31 7 0.74 500 22.0 3.3 -85 Bg 10 27 7 0.86 770 27.6 3.6 -87 Bw 26 27 5 0.81 500 18.8 2.3 -88 B-hor. (near surface)1 11 29 8 0.46 390 18.1 2.6 -86 Bf, Lichen, NATundra2 13 37 9 0.78 450 27.8 5.6 -80 B-hor. (Council ) 8 42 10 0.30 300 14.8 2.5 -83

1 B-horizons immediately beneath surface organic horizons.2 B –horizons of permafrost (Bf), from lichen vegetated sites, and from nonacidic tundra sites (NAT).

Table 2. Regression coefficients (R2) for respiration rate as a function of water-soluble organic carbon index (OCws) and total organic carbon (TOC).

Soil Horizon Type OCws TOCGroup by Site or Veg. n 0oC -2oC 0oC -2oC

Organic 37 0.21** 0.69*** 0.03 0.15 Tundra (All) 25 0.32** 0.45*** 0.01 0.14 Tundra (Acidic) 12 0.50** 0.64** 0.02 0.01 Shrub (All) 8 0.64* 0.90*** 0.10 0.14 Shrub (Council) 5 0.93*** 0.96*** 0.09 0.03 Forest (Council) 4 0.11 0.97*** 0.02 0.84***

Mineral 61 0.15 0.04 0.23*** 0.29*** Bg 10 0.14 0.01 0.15 0.02 Bw 26 0.19 0.01 0.05 0.01 B-hor. (near surface)1 11 0.18 0.01 0.86*** 0.06 Bf,Lichen, NATundra2 13 0.11 0.73*** 0.60** 0.69*** B-hor. (Council) 8 0.01 0.02 0.45* 0.86***

1 B-horizons immediately beneath surface organic horizons.2 B –horizons of permafrost (Bf), from lichen vegetated sites, and from nonacidic tundra sites (NAT). ***, **, * F-test of regression significant at the p>0.001, 0.01, and 0.10 levels respectively.

y = 0.19x + 20.6R2 = 0.21

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Figure 4. Soil water-soluble organic carbon [OCws] and soil respiration for selected groups of organic soil horizons.

y = 0.06x - 1.94R2 = 0.90 p>.001

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y = 0.40x + 6.21R2 = 0.60 p>..01

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Figure 5. Soil total organic carbon [TOC] and respiration for selected groups of mineral soils.