Evaluation of Soil Organic Matter Content under Topographic Influences in Agroforestry Ecosystems: a Study in Central Vietnam

We investigate the key predictors that influence the soil organic matter (SOM) content in higher altitude plantations. This is to study the potential of carbon sequestration for recapturing CO₂ from the atmosphere, which is an important consideration in mitigating global climate change. This research applied distance-based linear models to evaluate and predict the influence of topography and types of land use on soil carbon storage in the tropical mountainous areas of Central Vietnam. Three predictors plantation, altitude, and slope together explained 73.8% of the SOM variation. We focused on four types of plantations including acacia, rubber tree, cassava plantations, and plantations with rotation system between acacia and cassava. We found that they had different carbon accumulation, which depended on various characteristics of the terrain. The acacia plantation forest stored the highest amount of SOM and was less sensitive to changes in altitude. The soil carbon content in cassava plantations was the lowest, and altered considerably with altitude. SOM content in rubber tree and acacia-cassava rotation plantations were similar to each other. Altitude was the most critical driver in affecting the capacity of soil carbon sequestration in each plantation. In higher elevation zones, soil carbon stock was greater, which was probably related to decreasing decomposability by microorganism, and increasing carbon content in foliage and leaf litter. The carbon retention tended to increase gradually over an elevation range of 133–300 m a.s.l. The effects of the terrain slope were observed, when considering certain types of land use, and ones with small altitude disparity. In the slope range of 0° to 20°, the carbon pool showed a tendency to increase its capacity. Higher slope gradients, however, could hinder carbon retention due to organic material loss through erosion. In particular, when the slope was above 30°, the soil organic matter content decreased substantially.