Investigating priming of soil organic matter decomposition with litter extract in genetic horizons of two harvested red spruce podzols

Soil organic matter (SOM) represents a significant store of carbon (C), and factors that influence its stability are essential to understand, particularly since rising greenhouse gases such as CO2 play an important role in exacerbating climate change. Several studies have documented losses of SOM as a result of forest harvesting, particularly in mineral soils below 20 cm, but the specific mechanism for this loss has not been identified. Priming of SOM decomposition has been observed in some soils with the addition of labile C sources, so it is important to consider whether leaching of organic matter from litter through soil profiles is a mechanism that might explain these observed harvesting-related losses. Here we present preliminary results of a study whose objective is to quantify changes in respiration rates from paired soils differing only in their harvest history, through depth and under C amendment conditions that closely mimic those found in the field setting. Composite field moist soil samples from Ae, Bf and BC (transitional) horizons were incubated to assess respiration rates (2.5 g dry weight, in triplicate) at 15 C for paired sites representing 110 yr and 35 yr post-harvest temperate forest soils. The approach combined descriptive measurements (elemental composition and C and N stable isotope ratios), with soil headspace CO2 and δ13C-CO2 measurements for soils experiencing additions of litter extract or deionized water over a 28 day period. Results indicate an order of magnitude difference in CO2 evolution rates between the shallow horizons (Ae and Bf) and the deeper soil (BC). The respired 13C-CO2 ratio was higher in the 110 yr old site soils, with a more depleted signature released from the 35 yr old site soils. The stable isotope signature of respired CO2 matched that of the bulk soil in the Ae horizon, but signatures that emerged from the Bf horizon did not necessarily follow that trend. Litter extracts produced variable results between sites, yielding insight into the role a labile C source derived from the upper profile may play in the stability of C in the profiles of harvested soils. Studies of this nature have important implications for understanding key processes governing SOM stability.