Changes in physical and chemical soil characteristics and their impact on soil CO2 efflux during secondary succession

Background/Question/Methods

Approximately one fifth of cultivated land in the eastern US has been abandoned since the 1950s. Reforestation on these lands has been linked to carbon sequestration in eastern forests. The carbon sink potential for these lands depends in part on soil CO₂ efflux (F_Soil). Both CO₂ production in soils and its transport through the soil are functions of gas and water transport in surface soils. By governing the movement of water and gasses through soils, soil characteristics such as bulk density, porosity, hydraulic conductivity and soil organic matter (SOM) can influence F_Soil. This study has two questions: 1) How do the aforementioned soil characteristics change during secondary succession? 2) How do these characteristics relate to growing season F_Soil? We employed a chronosequence approach at the Blandy Experimental Farm near Boyce, VA. We classified six fields ranging in age from 6 to >100 years as early, mid or late successional, with two age-matched fields in each stage. We measured F_Soil during the growing season (May to September) for two years. Hydraulic conductivity was assessed in situusing a mini disk infiltrometer. Soil samples to a depth of 20 cm were analyzed in the laboratory to determine bulk density, porosity, and SOM. 

Results/Conclusions

Bulk density significantly decreased with stand age (p < 0.0001); soil organic matter and soil porosity both increased significantly with stand age (p = 0.039; p < 0.0001, respectively). Hydraulic conductivity increased with stand age, but not significantly (p = 0.076). There was a trend of increasing total growing season soil CO₂ efflux with increasing hydraulic conductivity and decreasing porosity. The fraction of water filled pore space was relatively low during the growing season – approximately 20%. Previous research at this site has shown that F_Soil during the growing season is highly dependent on fraction water filled pore space, especially in the early successional fields that exhibit rapid biomass accrual. By controlling rain water infiltration, hydraulic conductivity may influence water availability to roots and subsequently root and rhizopshere respiration. Because the dynamics of soil CO₂ efflux during secondary succession have the potential to impact whether a system is a net source or sink of carbon, enhanced understanding of changes in physical and chemical soil characteristics during secondary succession and their impact on soil CO₂ efflux can improve numerical modeling efforts.