Does Management Intensity Affect Soil Carbon and Nitrogen Pools at Depth in Northern California Ponderosa Pine Plantations?.

In the late 1980’s, the Garden of Eden Study was established to test effects of repeated fertilizer applications, competing vegetation control using herbicide, and the combined effects of these two treatments on productivity of planted ponderosa pine in northern California. This ongoing study provides an opportunity to investigate effects of plantation management on belowground carbon (C) and nitrogen (N) storage. We selected three plantations on contrasting soils across the site quality range found in the California Westside Ponderosa Pine Region. We assessed total belowground C and N pools 18 years after plantation establishment to one m depth. Across treatments and sites, an average of half of the N pool and over 40% of the C pool were below 30 cm with a larger portion of the total soil pool found deeper in the profile at the higher quality sites than the poorest site. Treatment impacts on mineral soil C and N storage were small compared to aboveground biomass and forest floor responses. However, soil C storage on the poorest and intermediate site qualities averaged 35% and 38% higher on treated plots than on untreated controls, equivalent to gains of 20 and 24 Mg C/ha, respectively. Competing vegetation control had no significant effect on mineral soil C and N storage. Mineral soil C and N response to fertilization varied across sites, with no effects found at the highest quality site. At the intermediate quality site, fertilization increased C and N pools at all depths, but the effect was more pronounced in the top 30 cm. At the poorest quality site, fertilization increased N in the top 15 cm only. Deep storage may be important for quantifying pools and assessing site and soil quality. However, short-term responses of deep soil pools to management practices such as these are likely to be minimal.