δ 15N of soil nitrogen pools and their dynamics under decomposing leaf litters in a suburban native forest subject to repeated prescribed burning in southeast Queensland, Australia

Abstract

Purpose: Global environmental changes could affect forest productivity and thus organic matter input to soil via litterfall. We conducted a 9-month litter decomposition experiment to examine the effect of litter removal and addition on soil nitrogen (N) dynamics in a subtropical eucalypt forest subject to prescribed burning.

Materials and methods: Two litter treatments were applied: addition of double litter rates and without any litter addition. In situ 15N pool dilution method was used to determine soil gross N transformation rates, and δ 15N of soil inorganic N pools and their dynamics were also measured.

Results and discussion: The results showed that the addition of decomposing litters had no significant effect on soil net and gross N mineralization and nitrification rates. Soil N rates showed a pronounced seasonal pattern with higher rates in summer and lower rates in winter, which could be explained by the seasonal variation of environmental conditions such as temperature but not soil water content in this study. The soils had significantly higher δ 15N in NO3 −-N than in NH4 +-N after removing the forest floor, which might result from gaseous N losses via nitrification, denitrification and NO3 −-N leaching during the experimental period.

Conclusions: Soil N transformations were not significantly affected by decomposing litters during the 9-month of decomposition. Our findings demonstrate that long-term studies of litter decomposition combined with repeated measurement of soil N transformation rates are needed to fully understand the seasonal patterns of soil N cycling and its response to decomposing litters in forest ecosystems.