Vegetation controls on soil organic carbon dynamics in an arid, hyperthermic ecosystem

Abstract

The large land area occupied by arid lands, roughly 36% to 40% globally, underscores the importance for understanding how these ecosystems function in the global carbon cycle. Few studies have directly examined soil organic carbon (SOC) dynamics and the effect of vegetation on SOC and microbial community structure in arid ecosystems. The objective of this study was to determine the effect of vegetation type on SOC dynamics in an arid, hyperthermic Sonoran Desert ecosystem. We specifically examined the influence of Prosopis velutina (mesquite), Larrea tridentata (creosote), and a combination of Bouteloua barbata, Bouteloua aristidoides, Aristida adscensionis, and Cynodon dactylon (mixed grass) vegetation types on SOC dynamics by quantifying: (i) local scale SOC stocks; (ii) soil aggregate stability; (iii) SOC turnover; and (iv) soil microbial community composition. There was significantly greater SOC in mesquite A-horizons relative to creosote and grass sites with values of 46.7, 30.4, and 24.4 g m − 2 , respectively. Subsurface SOC content did not vary significantly between vegetation types. Aggregate stability determined using an ultrasonic dispersion technique was found to be similar among vegetation types. The only significant difference noted was greater energy required to disperse stable aggregates in mesquite relative to grass soils, 1500 and 735 J g − 1 soil, respectively. Laboratory incubations were performed to determine SOC dynamics, pool sizes, and active pool mean residence times (MRT) for each vegetation type. Incubation results indicated significant variation in the cumulative respired CO 2 under mesquite, creosote, and grasses with 151, 186, and 207 mg C g − 1 soil C respired from each respective vegetation type. The incubation data indicated that 7–11% of total SOC was highly labile across all vegetation types with modeled active pool SOC MRT averaging 17 days. Bacterial community analysis by Terminal Restriction Fragment Length Polymorphism (TRFLP) indicated significant differences in microbial community structure among vegetation types. Microbial composition was highly correlated with soil pH and electrical conductivity. Furthermore, community composition was correlated with cumulative respired CO 2, suggesting an interaction among vegetation type, soil properties and microbial community control SOC dynamics in this ecosystem. The combined results indicated significant variation in SOC dynamics within a specific ecosystem by vegetation type. Understanding local-scale vegetation controls of soil carbon cycling may improve efforts to model regional carbon dynamics in arid environments.