In Soil Profiles

The main sources for soil organic matter in natural systems are leaf litter input to the top of the soil and root litter input down to the rooting depth. We evaluated both—the relative distribution of soil carbon and root biomass with depth—from a global dataset of 2721 and 117 samples, respectively (Jobbagy and Jackson, 2001). The samples in this dataset were from all major biomes of the earth, i.e., boreal forest, crops, desert, sclero-phyllous shrubs, temperate deciduous forest, temperate evergreen forest, temperate grassland, tropical deciduous forest, tropical evergreen forest, tropical grassland/savanna, and tundra. Over 60% of the root biomass (as a global average) was found in the top 20 cm of soil and it declined in a logarithmic pattern with depth (Fig. 3.5). Only 14% of root biomass was found below 40 cm. In contrast, only 40% of the soil carbon was located

Distribution (%)

Figure 3.5 Global summary of the distribution of soil carbon and root biomass in depth profiles of the world's major ecosystems: y error bars indicate sampling interval; x error bars indicate standard deviation from 11 biomes summarizing 2721 soil samples and 117 root biomass samples. Data from Jobbagy and Jackson (2001).

Distribution (%)

Figure 3.5 Global summary of the distribution of soil carbon and root biomass in depth profiles of the world's major ecosystems: y error bars indicate sampling interval; x error bars indicate standard deviation from 11 biomes summarizing 2721 soil samples and 117 root biomass samples. Data from Jobbagy and Jackson (2001).

in the top 20 cm of soil. Soil carbon also declined logarithmically; however, 36% of it was found at a depth below 40 cm. The strong correlation between root biomass distribution and soil carbon distribution supports the importance of root-derived carbon for the formation of soil carbon (y = 0.0199x2 I81,R2 = 0.9991). Relative to the distribution of root biomass less carbon is found in the top 20 cm of soils and more carbon in the subsoil. This underlines the importance of (1) microbial degradation of biomass in the upper 20 cm, (2) water movement for the downward transport of dissolved organic carbon, and (3) the sorption of carbon in deeper soil horizons. These findings suggest that the distribution of root carbon to the soil, which is influenced by the plants, might be a factor for controlling carbon storage. However, in the upper 20 cm of soil profiles the decomposition (Cebrian and Duarte, 1995; Cebrian, 1999), and hence the community of soil organisms, might control carbon storage, whereas in deeper soil horizons intrinsic soil factors might be more important for carbon storage. However, a further distinction between these different processes is not possible based only on analyses of the bulk carbon content.

Coinciding with the decreasing carbon concentration with soil depth is a correlated change in the concentration of soil nitrogen. However, the decline in soil nitrogen content is less pronounced and consequently shifts occur in the C/N ratio of soil organic matter along a depth profile. The C/N ratio changes from values of above 30 ± 15, which are characteristic for plant litter, to values of 10 ± 2, which are characteristic for microbial biomass (Fig. 3.6). The effect was stronger for independent replicates from

-10

0

10

E

20

£

SZ

30

Q.

C

T>

40

o

w

50

60

10 15 20 25 C/N Ratio

30 35

Figure 3.6 C/N ratio of soil organic matter from different depth intervals of 100 independent replicates from an old-growth beech stand in the Hainich National Park, Germany (unpublished). Similar data are also presented for 4 beech stands and 6 spruce stands from a latitudinal gradient in Europe. Data from Schulze (2000).

Figure 3.7 Difference between the <513C and values of soil organic matter in various soil depths and the <5^C and <5^N values of litter from 100 independent samples from an old-growth beech stand in the Hainich National Park, Germany (unpublished). Similar data are also presented for 4 beech stands and 6 spruce stands from a latitudinal gradient in Europe. Data from Schulze (2000).

Figure 3.7 Difference between the <513C and values of soil organic matter in various soil depths and the <5^C and <5^N values of litter from 100 independent samples from an old-growth beech stand in the Hainich National Park, Germany (unpublished). Similar data are also presented for 4 beech stands and 6 spruce stands from a latitudinal gradient in Europe. Data from Schulze (2000).

an old growth beech forest in the Hainich National Park, Germany, than for different beech and spruce stands from a European latitudinal gradient (Schulze, 2000). The decrease in C/N ratios suggests that, in the upper few centimeters of soil profiles, root and leaf litter may be a substantial part of the SOM pool, whereas in deeper horizons microbial-derived carbon structures may dominate the SOM pool.

In order to prove the microbial origin of carbon in deeper soil horizons, we compared the enrichment of 13C and 15N values with depth to the enrichment of both isotopes in trophic networks. This enrichment is known to be between 0 and l%o for C and between 3 and 4%o for N (Rothe and Gleixner, 2000). We used <513C and <515N values of 10 different beech and spruce stands over a latitudinal gradient in Europe (Schulze, 2000) and analyzed in addition S13C and S15N values of 100 independent depth profiles from the Hainich National Park, Germany. Interestingly, for both cases the 13C and 15N values were highly correlated (Fig. 3.7) indicating that both the 13C and the 15N values of soil organic matter increased with depth at a slope of between 3.7 and 4.6. These values are in good agreement with the trophic level shift expected from food chains, and this suggests that soil carbon in deeper horizons derives mainly from soil organisms.

Was this article helpful?

0 0

Post a comment