Controls On the Stability of Soil Humic Substances.

Previous research has established that soil and plant management likely effect affects a range of soil characteristics ranging from the size of the microbial community to the rate of soil carbon accumulation. The importance of microbial processes in controlling the humification process and its’ affect on the stability of soil C is receiving increasing attention.   We present a series of studies and analytical methods to reveal the fractions and stability of soil organic matter (SOM) contributing to the stability of soil C.  The studies range from the use of isotopically labeled litter to determine the sources of new soil C to pyrolysis-Gas Chromatograph/Mass Spectrometry/Isotope Ratio Mass Spectrometry (PyGC/MS/IRMS) to determine the conservation and transformation of new C inputs to Differential Scanning Calorimetry (DSC) to determine changes in energetics of soil C under various treatments.  The studies were located at various sites in the Ponderosa Pine (Pinus Ponderosa) biome in the Sierra Nevada of California. The understory typically contains ceanothus a N fixing shrub and manzanita an aggressive shrub.  Treatments for the first study included comparing removing the understory to leaving it intact. Removal of the understory is typically done when plantations are established to remove competition for water and nutrients. In this talk we show that mineralogy exerts controls on soil C stability beyond the effect of temperature, mineralogy controls the priming of older SOM in the presence of fresh litter additions, litter diversity promotes soil C gain over monoculture forest plantations and litter diversity and addition of N creates organic matter varying enthalpic character. Specifically, N fertilization and diverse litter inputs increase the energy stored in SOM.  Our results show that mineralogy and vegetation management strongly influences the stability of SOM and is likely important to sustaining soil C and long-term soil forest productivity.