Soil Organic Carbon Density Dynamics after Forestation Driven by Old Carbon Loss and New Carbon Gain Processes in the Dryland Region of the Loess Plateau in Northwest China

Abstract

Forestation has been implemented and increasingly encouraged worldwide to mitigate climate change by enhancing carbon sequestration. Compared with vegetation biomass, the soil organic carbon (SOC) pool is dominant and varies with a greater complexity, but it remains poorly understood in dryland plantations. Thus, it is necessary to understand and quantify the effects of the influencing factors on the SOC pool variation after forestation. This study aimed to investigate the dynamics of SOC density (SOCD) of plantations and to develop a model integrating the SOC input and output processes under the main driving factors in the dryland region of the Liupan Mountains and their surroundings on the Loess Plateau in Northwest China. This study showed different SOCD variation patterns among forestation types, i.e., a continuous increase, a slight decrease then an increase, and an obvious decrease then an increase, for plantations converted from croplands, grasslands, and natural forests, respectively; the corresponding net average increases in the SOCD in 40-year-old forests were 136.6, 70.7, and 52.7 t·ha-1 for the three forestation types, respectively. The diverse SOCD variation patterns and magnitudes could be well interpreted by the differences in the mineralization process of the initial SOCD before forestation and the accumulation process of the new SOC from litter biomass. In addition to the initial SOCD and forest age, the other main driving factors for the mineralization of old SOC were mean annual temperature, mean annual precipitation and altitude. A satisfactory SOCD dynamics model was developed by integrating the offsetting SOC input and output processes and the effects of the initial SOCD, tree biomass, forest age, and environmental factors. Based on these findings, both SOC input and output processes and their site-specific influencing factors should be considered for the precise assessment and management of dryland plantations with varying SOCDs.

Keywords: Soil Organic Carbon, carbon sequestration, forest, plantation, forestation type, dryland, Loess Plateau