Is black carbon a long-term C pool in
Ethiopian forest soils ?
Bruno Glaser1, Korbinian Freier1 und Wolfgang Zech2
1 Abteilung Bodenphysik, Universität Bayreuth, 95440 Bayreuth
2 Lehrstuhl Bodenkunde und Bodengeographie, Universität Bayreuth, 95440 Bayreuth
Abstract
Soil organic carbon (SOC) plays a major role in the global carbon budget. Therefore, in finding
strategies to reduce CO2 emissions into the atmosphere, a better understanding of carbon
sequestration into soils is of special interest. Theoretically, black carbon has a high potential for
long-term C sequestration due to its highly aromatic structure leading to a certain resistance
against degradation in the terrestrial environment. Also its positive effect on soil fertility
especially in highly weathered soils of the humid tropics is well known. However, conflicting
results on black carbon turnover and thus, its potential for long-term C sequestration has been
reported with mean residence times ranging between 300 and 3000 years. Therefore, the
objective of this study was a comparison of black carbon turnover in natural compared to
plantation forests.
For this aim, a Podocarpus falcatus dominated natural forest and an Eucalyptus saligna
plantation from the highlands of the Ethiopian rift valley were sampled in three-fold replication
down to 1 m soil depth. Compound-specific 13C and 14C analyses were carried out on black
carbon. Mean residence times were calculated using three different mathematical models. In the
first model, changes in 13C values due to vegetation change (C3 – C4) were used. In the second
model, concentration changes of black carbon with depth were applied. In the third model, both
black carbon turnover and transport caused by diffusion and advection were taken into account.
Additionally, the isotope signature of black carbon allowed us to trace its origin.
13C values of bulk soil were similar in both stands being lowest at the topsoil with -27‰ and
increased to values of -14‰ at a one meter depth being clearly caused by vegetation change from
savanna to forest. Within this shift, the main increase in 13C values was observed at a soil depth
between 10 and 40 cm corresponding to 14C ages between 600 and 900 a BP. 13C values of black
carbon followed the trend of bulk 13C values.
Black carbon comprised between 10 and 28% of total organic carbon in natural and plantation
forests. Litter layer and top 10 cm of mineral soil of forest plantations contained 50% less black
carbon compared to natural forests while 14C ages of black carbon were comparable (107 and 110
years, respectively). In 60 – 70 cm soil depth, black carbon exhibited 14C ages of 3,070 and 7,480
a BP in natural and plantation forests, respectively.
Mean residence time of black carbon varied between 34 and 255 years in the topsoil while mean
residence times of black carbon in deeper parts of the soil were mainly influenced by factors
which can be modelled with an exponential decrease in turnover with depth. With this, between 3
and 8 % of the investigated fractions reached mean residence times of more than 1500 years and
can therefore, be assigned to the stable C pool.