Variability in the C Isotopes in Plants Soil and Soil CO2 Efflux in a Boreal ForestA Case Study

To illustrate the variability in SI3C of soil-respired CO2 and ecosystem components in boreal forests, we here present original data from a 90-m-long gradient in northern Sweden (Fig. 4.6). The gradient starts in a nutrient-poor forest in a groundwater recharge area (at 0 m) and ends in a nutrient-rich groundwater discharge area (at 90 m). Soil conditions and plant and microbial communities along this gradient have been described in detail by Giesler et al. (1998) and Hogberg et al. (2003). 'Bomb-C' estimates of the age of the C in the SOM suggest that the mor-layer does not vary substantially in age along the gradient; the 14C enrichment in percent relative to the modern pre-bomb value (100%)

62 4. Factors Determining the^ C Abundance of Soil-Respired CO% in Boreal Forests -22

0 20 40 60 80 100

Position along gradient (m)

Figure 4.6 The <S13C of ecosystem compartments and soil CO2 efflux along a 90-m-long gradient from dry nutrient-poor (0 m) to moister nutrient-rich conditions (90 m) at Betsele, northern Sweden (Hogberg and Hogberg, unpublished). Note that n = 8 for each understorey plant species, while the soil CO2 efflux was measured in the field on three occasions, May, June, and August (n = 10 replicates per sub-site and occasion).

varied between 126 and 118% and indicated a minor tendency of older SOM at the richer and moister end of the gradient. The presence of charcoal fragments at the bottom of the mor-layer throughout the gradient and the relatively uniform age-distribution of the dominant trees suggests that the forest regenerated after a fire less than 150 years ago.

We sampled and partitioned the organic mor-layer into three horizons: the superficial (S) layer with litter and ground mosses and lichens, the fermentation (F) layer with decomposing plant remains still identifiable, and the humus (H) layer with highly decomposed amorphous organic matter (cf. Fig. 4.5). Leaves of understorey plants were sampled as described previously (Giesler et al, 1998). The trees displayed considerable variations in <513C between sun and shade needles (c. 2%o), and to get representative samples of needles we sampled (in 1996) newly fallen specimens on top of the soil surface. Soil respiration was sampled (n = 10) at 0, 60, and 90 m, using a static headspace (cf. Hogberg and Ekblad, 1996) in May, June, and August 2002, and the ¿13C of the soil C02 efflux was calculated using

Keeling plots as described above.

Contrary to the idea that the SOM at wetter sites should have lower <513C because their plants should be less drought-stressed (Balesdent et al., 1993), we found increases in <513C in the direction of the discharge area

62 4. Factors Determining the^ C Abundance of Soil-Respired CO% in Boreal Forests -22

SOM Aa

0

+ _ ▲

♦ ; OOO OOO ► \<

O, Understorey plants +, Litter of overstorey trees A, S-layer of SOM ▼ , H-layer of SOM O, Soil-respired C02

in both the dominant trees and the SOM (Fig. 4.6). We agree, however, with Balesdent et al. (1993) that the S13C of the SOM is strongly correlated with that of the long-term C inputs from plants. Note that the isotopic signature of the surface of the organic mor-layer rather closely followed that of needle litter of the dominant trees (Fig. 4.6), suggesting that dominant trees contribute most C to SOM. We suspect that the unexpected increase in <513C of dominant plants and SOM in the direction of the discharge area is caused by an effect of an improved nutrient (N) supply on the ratio Q/Ca in the dominant plants, and that this is more important than effects of variations in soil moisture under these generally humid conditions. Plant-available N increases several times along the gradient, and this should have a large effect on the rate of photosynthesis (Field and Mooney, 1986) and, hence, on the ratio Q/Ca. Understorey plants had very low <513C values (down to —33%o) in the dense nutrient-rich forest (Fig. 4.6), but the 13C data on SOM indicated that they did not contribute substantially to SOM.

Relevant to the discussion above, the 513C of CO2 respired from root-free soil samples did not differ from that of the SOM (Ekblad, unpublished), while the CO2 efflux in the field, as measured in 2002, was 2-5 %o enriched in 13C relative to the SOM (Fig. 4.6). We suggest that this enrichment of the efflux in the field indicates a large influence of the root respiratory component (cf. Ekblad and Hogberg, 2001). Note that the opposing trends in soil CO2 efflux and SOM most likely reflect the fact that 2002 was a dry year; the high average S13C of the soil CO2 efflux at 0 m is probably due to the high value for May, which was unusually dry. Note also that the difference in <513C between soil-respired CO2 and SOM decreases in the direction of higher nutrient supply, which probably reflects a decrease in root respiration (Hogberg et al, 2003) .

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