Effects of long-term prescribed burning on soil organic carbon and total nitrogen content in the Missouri Ozarks

Background/Question/Methods

Wildfires and prescribed burns are a critical natural disturbance and management tool, respectively, for maintaining ecosystem function in many forests. Soil organic carbon (OC) and total nitrogen (TN) are important determinants for soil physical and chemical characteristics (e.g., water holding capacity, cation exchange capacity) and biological properties (e.g., microbial nutrient transformations). While a wealth of studies exists on short and intermediate term fire effects on OC and TN, very few studies have examined the long-term effects of repeated prescribed burning. We investigated the effects of 60 years of spring prescribed burning on OC and TN content of litter, soil organic and mineral horizons in oak-hickory forest of the Ozark Mountains in SE Missouri, USA. Litter and soil were collected from eight locations in each of six 40m×40m experimental plots. Two of the six plots have been burned annually, periodically (every four years), and never, respectively, since 1949. Forest floor and the soil organic horizon were collected from within circular areas of 30 cm diameter. Mineral soil was collected at depths of 0-5 cm, 5-10 cm, 10-20 cm, and 20-30 cm using soil cores. 

Results/Conclusions

In annually burned plots, litter OC and TN content averaged 100 g/m² and 3 g/m², respectively, while contents were approximately two times higher in periodically burned plots and three to five times higher in unburned controls. In mineral soil, TC and TN concentrations were lowest in annually burned soil at 0-10 cm depth (9 g/kg and 0.6 g/kg, respectively) and at 10-20 cm depth relative to the other treatments (P<0.05). TC and TN were highest in periodically burned soil (14 g/kg and 0.9 g/kg, respectively) at 0-10 cm depth relative to the other treatments (P<0.05). The low TC and TN concentrations in annually burned plots relative to the control can be explained by annual loss of the forest floor to combustion and subsequent lack of C and N inputs into the soil. The unexpectedly high TC and TN concentrations in periodically burned plots may be due to a vigorous woody understory (absent in other treatments) supplying C and N through belowground litter. It is apparent that even low-severity spring prescribed burning carried out repeatedly over decades at two different frequencies can have significant effects on soil C and N content and subsequently on physical and biological properties of the ecosystem.