UCLA

Understanding the mechanisms of soil carbon (C) formation and loss is essential for predicting the C storage capacity of soils under ongoing global change scenarios. Climatic variables, vegetation structure, microbial activity, soil mineralogy, and tissue C chemistry each have the potential to affect the fate of C in soils, and the interactions among these controls vary in different environments. Our mechanistic understanding of how these factors interact with each other to determine soil C storage is still rudimentary. This dissertation used a series of field and laboratory studies to assess the interacting roles of vegetation, soil mineralogy, microbial activity, C chemistry, and temperature in regulating the fate of C in soils.

In the first experiment, we sought to understand how soil mineralogy, soil nutrient and C status, and C chemistry interact to determine warming effects on the fate of newly added soil C using a 13C isotopic tracing approach. By tracking the added 13C label in soil pools at 4 days and 255 days in tropical forest soils with differing weathering and mineralogical conditions, we found that initial microbial uptake of 13C and average carbon use efficiency (CUE) by microbes were strongly correlated with longer-term C retention in mineral soils. Overall, warming had a negative effect on 13C retention in soil in the youngest, least-weathered soil only, with no warming effect on moderately to strongly weathered soils. Thus, soil C stocks in less weathered soils, and with lower microbial CUE, may be most vulnerable to C loss with a warming climate.

Our second study assessed the fate of newly added organic 13C-labeled compounds in soils of differing fertility along weathering gradients. Comparing additions of two low molecular weight compounds, 2.9x greater retention occurred for 13C-labeled glucose versus 13C-labeled glycine after two years, suggesting that glucose may be a better precursor for soil organic matter formation. Soil mineralogy and nutrient availability were not significant factors in 13C retention in soil. Soil spectra from 13C NMR revealed an increase in the proportion of alkyl C in glucose and glycine amended soil relative to control soils, and alkyl C are commonly associated with relatively stable organic C. Thus, our results indicate that microbial incorporation of labile organic compounds like glucose into biomass may be associated with greater C retention in stable soil components.

Our third study estimated the long-term effect of grass cover loss on soil organic C (SOC) and total nitrogen (TN) storage, and the spatial heterogeneity of SOC and TN in two arid grasslands. The nine years of experimental grass removal resulted in soil deflation and 30% and 35% declines in SOC and TN respectively in 100% grass removal plots (TU100). Grass removal also led to soil deposition in downwind areas of the plot (TD100). Soil organic C and TN concentrations in the deposition plot (TD100) was variable, and likely depended on the structure of the vegetation community trapping wind-blown particulates. Geostatistical analysis showed that weaker and smaller fertile islands, compared to the control, developed in TD100 plots over nine years of aeolian transport.

The outcomes of this dissertation will add to the current body of knowledge about mechanisms of soil C stabilization across environmental conditions and with warming.