Belowground carbon sequestration in a mature planted mangroves (Northern Viet Nam)

Mangroves play a key role in carbon sequestration along tropical and subtropical coastlines, mainly as a result of their high primary productivity and the anoxic character of their soils, in which organic matter decay processes are slow. Unfortunately, these forests are disappearing worldwide, notably due to aquaculture and the need for space on the shoreline. The mangrove area in Viet Nam decreased substantially until late 90's, which was a concern for the Vietnamese government. Then, replantation programs were developed in order to protect the shoreline against the erosion; like in northern Viet Nam, where monospecific stands were planted from the mid 90's. Within this context, our objectives were to determine carbon stocks and carbon burial rates in the soil of a 18 years old Kandelia obovata (Sheue, Lui & Yong) mangrove forest. Three plots were set up in the planted mangroves and three in adjacent bare land (without any mangrove) at the mouth of Red River. Three geo-slicers up to 100 cm depth were taken in each plot; and in each core, ten soil samples were collected with a 10 cm depth interval. Later on, dead and live roots were sorted. Carbon contents in soils and in roots were determined using the Walkley-Black method and loss on ignition (LOI) method, respectively. Then, the total belowground carbon sequestration was estimated. There was a depth related distribution of organic carbon in soil with the highest concentrations measured between 20 and 60 cm depth. Accumulated carbon up to depth of 100 cm in soil and roots of the planted K. obovata were 146.78 ± 3.87 Mg OC ha−1 and 12.67 ± 0.14 Mg OC ha−1, respectively. The value was 87.59 ± 1.08 Mg OC ha−1 for the adjacent bare land. Consequently, the mean carbon burial rate for this system was approximately 6.94 Mg OC ha−1 yr−1, which is higher than most of the mangroves and may be related to a low export of the net primary productivity or most likely to a high sedimentation rate. These results were compared with previously published results on carbon stocks in younger planted forests of the same species in the same region, a positive and linear relationship was determined between the age of the planted stand and the belowground carbon stocks.