Carbon and nitrogen mineralization in Sacramento River restored riparian forests
California riparian forests are a very threatened habitat. For the past 20 years, these areas have been restored along the middle Sacramento River. As these restored forests become established, it is important to know if they are returning to their native function, suggesting successful re-establishment. This experiment looks at the carbon and nitrogen mineralization in the forest soils as an indicator of the restored function. We looked at mineralization in soil samples from 15 sites ranging in age from <5 to 21 years old. We also collected samples from 5 remnant forest sites, which are old growth forests serving as our baseline for native function. All of the sites share the same mixed riparian forest type, soil type and climate, so they were assembled into age classes which, together, serve as a chronosequence. This allows for assesing mineralization in the forests over time. In order to test for carbon mineralization, we measured the rate at which soil microbes produced CO2 at different intervals during a 60-day incubation. In order to test for mineralized nitrogen, nitrate and ammonium were extracted from both incubated soils and non-incubated samples via KCl extraction.
Mineralized carbon, measured as CO2 emitted per mass of soil (mols/g), varied across age classes of sites during Day 1 and Day 60 of incubations. At day 1, soils from remnant sites significantly had the highest amount of mineralized carbon. Soils in age class “19 & 21 years old” had the closest mineralization in comparison. Amount of mineralized carbon decreased with age class, with the exception of soils in the “9 & 10 y.o.” class having a higher amount on average than those in the “15 & 16 y.o.” class. The data from Day 60 showed a similar pattern. These patterns suggest that, overall, carbon mineralization does increase as forest sites age, but not linearly. Mineralized nitrogen after the 60-day incubation differed significantly between “19 & 21” class and the “5 & 6” class. This suggests that nitrogen mineralization does not significantly change in restored riparian forests in the long term, but does increase at around 20 years.