Calculating Organic Carbon Stock from Forest Soils

Abstract

The organic carbon stock (SOC) (t/ha) was calculated in different approaches in order to enhance the differences among methods and their utility regarding specific studies. Using data obtained in Romania (2000-2012) from 4,500 profiles and 9,523 soil horizons, the organic carbon stock was calculated for the main forest soils (18 types) using three different methods: 1) on pedogenetical horizons, by soil bulk density and depth class/horizon thickness; 2) by soil type and standard depths; 3) using regression equations between the quantity of organic C and harvesting depths. Even though the same data were used, the differences between the values of C stock obtained from the three methods were relatively high. The first method led to an overvaluation of the C stock. The differences between methods 1 and 2 were high (and reached 33% for andosol), while the differences between methods 2 and 3 were smaller (a maximum of 23% for rendzic leptosol). The differences between methods 2 and 3 were significantly lower especially for andosol, arenosol and vertisol. A thorough analysis of all three methods concluded that the best method to evaluate the organic C stock was to distribute the obtained values on the following standard depths: 0 - 10 cm; 10 - 20 cm; 20 - 40 cm; > 40 cm. For each soil type, a correlation between the quantity of organic C and the sample harvesting depth was also established. These correlations were significant for all types of soil; however, lower correlation coefficients were registered for rendzic leptosol, haplic podzol and fluvisol.

Silvicultural Interventions Drive the Changes in Soil Organic Carbon in Romanian Forests According to Two Model Simulations

“…For the Romanian forests, the biomass input to soils in Picea abies (PA) forests was less than half of the amount in other forest types (to which Picea abies tree species contributes, like for coniferous broadleaved mixtures), while both models simulated similar C stock values for both initialization and simulated period. As far C stock estimated by NFI is accurate [38], it seems there was a failure to reasonably simulate either the living biomass compartments or the turnover rates that allows litterfall inputs. In our case, the underestimation of soil C stock in PA forests by both models, it is most likely linked to the amount of litterfall simulated, so further linked to the compartmentation of the living biomass and/or turnover rates for this forest type as implemented into the CBM.…”

“…The total soil C stock seems rather realistically simulated in our study when looking to other studies. Dincă et al [38] estimated similar stock values based on the soil parameters regularly sampled as part of the Romanian forest management planning (the majority of the forest experiences a planning every 10 years). In Hernández et al [56], the country-wide averaged C stock was estimated as about 57 ) tC ha −1 , which is rather half or less, compared to the NFI estimates in this study.…”

“…Additionally, NFI only collected data on C concentration and skeleton content while soil apparent density was obtained from model-based procedure [49], being known that soil density has a significant influence on the C stock [24]. Scarce and nonsystematically sampled data on soil apparent density exists nevertheless in Romania, having the same order of magnitude as those used here [38,63].…”

“…Skeleton content was estimated in the field. The soil apparent density was extracted from digital maps available [49] and checked against existing national references on soil types [38]. Litter pool was sampled from the same plot as the soil, in a subset of 1158 NFI plots.…”

“…Romanian forests have a strong altitudinal distribution, which is reflected in the climate, vegetation and soil properties [38]. Most of the existing studies in Romania are focused on soils' spatial and geographical distribution [39] and few on the C stocks [40,41].…”