Seasonal and altitudinal variation in decomposition of soil organic matter inferred from radiocarbon measurements of soil CO2 flux

The rate of carbon (C) cycling in soils is controlled by an array of processes and conditions. It has been widely accepted that an increase in temperature would accelerate microbial decomposition of soil organic matter (SOM) and provide a positive feedback to global warming, other factors being equal. However, soil moisture has received little attention in C cycle studies. In this project, we developed a technique for sampling soil-respired CO2 for isotopic measurements and a model that relates the radiocarbon (C-14) content of soil respired CO2 to the rate of C cycling in soils. We measured soil CO2 flux, carbon isotopic content (both C-13 and C-14) of soil-respired CO2, soil temperature, and soil moisture on a monthly basis along an elevation transect in the Sierra Nevada in an attempt to determine the relationship between the rate of soil C cycling and soil environmental conditions. Both soil CO2 flux and its C-14 content displayed significant variations (spatially and temporally), which reflect natural variations in the rate of SOM decomposition and in the relative amount of SOM-derived CO2 versus root-respired CO2 caused by seasonal changes in soil temperature, moisture, and plant activity. The relative contribution of SOM decomposition to total soil CO2 production changed throughout the year from similar to 20 - 50% at the peak of the growing season to close to 100% in the nongrowing season. The apparent decay rate of SOM determined from the C-14 content of soil-respired CO2 varied from similar to 0.2 yr(-1) in the spring to similar to 0.01 yr(-1) in the fall at the lowest-elevation site and from 0.1 yr(-1) in the summer to - 0.01 yr(-1) in the late fall at the highest-elevation site. It appears that the apparent decay rate of SOM increased with increasing temperature when soil moisture was adequate but decreased with increasing temperature when soil moisture became limited. The apparent decay rate of SOM also varied with soil moisture. Higher soil moisture content accelerated decomposition of SOM until it reached an optimal level of similar to 14 - 25 wt % water content and then inhibited decomposition when more pores in soils became saturated with water and perhaps oxygen availability (for microbes) became limited. Although the rate of SOM decomposition varied throughout the year in response to fluctuations in soil temperature and moisture, the maximum apparent decay rate was higher at the low-elevation site (i.e., maximum apparent decay rate = 0.22 yr(-1)) than at the high-elevation sites (i.e., maximum apparent decay rate = 0.10 yr(-1)). Litter decomposition simulated by measuring changes in mass of litter in litter bags placed in the field also showed a similar decomposition pattern with decreasing decomposition rate with elevation. Multivariable regression analyses including various terms of soil temperature, moisture, and site variability suggest that soil moisture was a major factor, but not the only factor, controlling the rate of SOM decomposition and soil CO2 flux in the Sierra Nevada soils. Both decay rate and total soil CO2 flux are related significantly to soil moisture, temperature, and site effects.