Red Dirt and Forest Management - Oxides, Treatment and Altered SOM Stability.

Ultisols are an important component of forest soils in California from the central Sierra Nevada through the southern Cascade. The ‘red dirt’ belt crosses a range of parent materials (felsic through ultra mafic) while supporting a variety of pedogenic oxides (short range order (SRO) through crystalline minerals). These soils naturally support mixed conifer forests but a majority of this area is managed as plantation forests. Common management of these stands enhances fertility and reduces vegetation competition to focus site potential on tree growth. This focus on conifers alters litter inputs (quantity and quality/diversity) often producing confounding affects to C storage and turnover.

We investigated the response of soil C to silvicultural treatments (fertilization (F) and herbicide (H) and a cross (HF)) on similarly classified soils (parasesquic, mesic, Andic/Xeric Haplohumults) formed on two parent material ‘end members’ in this belt (silica rich quartzite (Q) vs intermediate to mafic volcanic mudflows (MF)). Analysis with X-ray diffraction indicates that site Q has abundant crystalline gibbsite and kaolinite with limited SRO materials while site MF possessed abundant SRO and kaolinite. Treatment affects on bulk soil C were not consistent across sites with increased C storage in surface soils at MF for F and HF (vs Control) and reduced C storage for H. There was little increase (F) to reduction (H and HF) of soil C at site Q. Differences are attributed to the capacity of SRO materials to store a portion of enhanced litter inputs related to fertilization while the more crystalline oxides at site Q could not capture these increases. This talk will report on the partitioning of C into organo-metallic complexes and associated with poorly and high crystalline iron and aluminum oxides extracted or remaining after selective dissolution analysis. Better understanding of site-treatment interactions based on soil oxide content could have interesting implications to soil C dynamics in managed forest ecosystem in California and around the world.