Soil phosphorus and microbial response to a long-term wildfire chronosequence in northern Sweden

摘要

In the prolonged absence of major disturbances, ecosystems may enter a stage of retrogression, which is characterized by decreased ecosystem process rates both above and belowground, and often reduced availability of phosphorus (P). Disturbance through wildfire can increase soil P losses through leaching or erosion, but in the long-term absence of fire, soil P could potentially become increasingly bound in more stable forms that are less available to microbes. We studied forms of P and microbial respiration kinetics in the humus layer of a group of islands that vary considerably in wildfire frequency (40-5,300 years since last fire), and which are known to enter retrogression in the prolonged absence of fire. We found a decrease in labile P with decreasing fire frequency but no change in total P. Soil microorganisms responded more strongly to N than to P addition, and microbial biomass N:P ratios remained unchanged across the gradient. However, the concentration of labile P was the best predictor of microbial respiration responses across the islands, and this provides some evidence that declining access to P could contribute to the decline in soil microbial activity during retrogression. Our results show that even though N is arguably the main limiting nutrient during retrogression in this chronosequence, long term absence of fire also causes a decline in P availability which negatively affects microbial activity. This in turn could potentially impair microbially driven processes such as decomposition and mineralization and further contribute to the reduced availability of soil nutrients during retrogression.