Use of Carbon‐13 and Carbon‐14 to Measure the Effects of Carbon Dioxide and Nitrogen Fertilization on Carbon Dynamics in Ponderosa Pine

Soil C sequestration in predicted, future elevated CO 2 environments will be important to atmospheric CO 2 levels, soil tilth, and fertility. An elevated CO 2 study with ponderosa pines ( Pinus ponderosa Laws) grown in chambers produced above ground vegetation with a δ 13 C of −44‰ and roots with −42‰. This together with carbon dating made it possible to follow soil C dynamics. Fifty percent of the California upland soil C, resistant to acid hydrolysis, was designated as the resistant fraction. Carbon dating showed the mean residence times of this fraction to be 400 to 1500 yr greater than the total soil C for the horizons sampled. Young ponderosa pines grown in CO 2 chambers produced negligible leaf litter. There were 32% more roots in the presence of either added N or double CO 2 but 77% more in the presence of both. Root‐derived soil C was equivalent to 10% of the root C after the 6‐yr growth period. Analysis of laboratory CO 2 evolution during extended incubation showed the active soil C pool represented 1 to 2% of the soil C with a field‐equivalent mean residence time (MRT) of 24 to 53 d. The slow pool represented 46 to 52% of the C with MRT of 24 to 67 yr depending on treatment and soil depth. Analysis of the 13 CO 2 label during incubation from the elevated CO 2 treatments, showed the root‐derived 13 C of the active fraction to have residence times similar to those of the total soil non labeled C at ≈35 d. Root‐derived C of the slow pool at 10 yr MRT turned over three to four time as fast as the general soil C. The 13 C of the light fraction (LF), showed it to be most closely associated with the active pool. The particulate organic matter (POM) was part of the slow pool as determined with incubation.