Fine Root Production and Belowground Soil Processes Across a Dryland Ecotone

Drylands cover 30% of terrestrial land area and can influence global C (carbon) and N (nitrogen) cycles. Due to recent changes in dryland vegetation from grasslands to shrublands (shrub encroachment), the soil C pool has undergone changes in C loss or gain, but the effects of shrub encroachment are difficult to predict and the role of roots in this change remain understudied.  However, plant C allocation to roots can also be affected by increased concentrations of atmospheric carbon dioxide (CO₂), and in drylands this can affect soil C where the relative importance of root derived C compared to aboveground C inputs may be enhanced.  Research suggests root derived carbon from root decomposition and exudation is an important component of the soil matrix and may play a larger role in nutrient dynamics and soil processes than previously thought.  Moreover, soil respiration can be root or heterotrophic yet the relative importance of root carbon remains unclear. This study will examine correlations between fine root production (>2 mm diameter) and soil respiration as an index of the relative contribution of root derived C to soil C in drylands where belowground biomass can exceed aboveground biomass in many species. 

We will examine root derived C via soil CO₂ ­efflux measurements under shrub canopy/grass patch microsite and interspaces along a grassland to shrub dominated ecotone at the Jornada LTER.  The study aims to 1) investigate the relationship between root production and soil respiration in drylands, and 2) determine if differences in shrub and grass root production account for differences in soil respiration.  We hypothesize that root production will be positively correlated with soil respiration rate because root derived C can significantly contribute to the soil C, and shrub sites will have greater root production and higher soil respiration rates than grasses because shrub root mass may be greater than grass root mass in this system.  To test the hypothesis, root ingrowth cores were deployed at shrub and grass microsites to measure fine root production at 20 cm depth.  Soil respiration measurements were taken after rainfall events at the same microsite location as root ingrowth cores.  We expect to find a positive correlation between fine root mass collected and soil respiration.  The results of the study will have implications for ecosystems experiencing caused by land degradation and shrub encroachment.