Temporal dynamics of carbon and nitrogen in the surface soil and forest floor under different prescribed burning regimes

Prescribed burning has been widely used in the management of forests for reducing wildfire risk, and can have significant effects on soil carbon (C) and nitrogen (N) pools and their temporal changes. This study aimed to investigate the impacts of different burning frequency regimes on the temporal dynamics of C and N in the topsoil and forest floors. The experimental site was a 39 year old wet scherophyll forest prescribed burning trial at Peachester, southeast Queensland, Australia, with treatments of no burning (NB) since 1969, 2 yearly burning (2yrB) and 4 yearly burning (4yrB) since 1972. Each of three burning treatments had four replications and these plots were randomly distributed over an area of the forest with similar vegetation and soil characteristics. Soil (0–10 cm) and forest floors were sampled monthly for 6 months prior to the next scheduled burning for both burning treatments to minimize the effects of fire recency. Prescribed burning significantly (P < 0.01) affected most C and N variables in both soils and forest floors. The 2yrB treatment had significantly lower (P < 0.01) soil total C, total C:N ratio, microbial biomass C (MBC) and N (MBN), MBC:MBN ratio, dissolved organic C (DOC) and N (DON), NO3−-N, inorganic N and L layer total N, DON and NO3−-N, compared with the NB and 4yrB treatments. However, there were no overall significant differences in these variables between the NB and 4yrB treatments. Sampling month significantly (P < 0.01) affected C and N variable in both soils and forest floors except for soil total C and N and F layer MBC. Temporal dynamics of most of these labile C and N variables were highly related to soil and forest floor moisture content, seven day mean air temperature (MAT) and cumulative rainfall prior to sampling date. However, fire effects were independent of sampling month, as there were no significant interactions between them for most response variables measured. This study has clearly demonstrated that more frequent burning (2 yr burning) had negative impact on soil and forest floor C and nutrient pools. Insignificant differences observed in soil C and N and forest floor N pools between less frequent burning (4yrB) and NB treatments highlighted that the prescribed burning at four year interval gave sufficient time for recovery of these soil and forest floor nutrients to pre-burn levels.