Short-term wildfire effects on the spatial pattern and scale of labile organic-N and inorganic-N and P pools

The spatial heterogeneity of essential plant resources plays a crucial role in the structure, composition and productivity of many terrestrial ecosystems. Fires may affect both the availability and spatial pattern of soil nutrients. However, little is known about the effect of fire on the spatial pattern of soil resources. We hypothesized that shortly after a wildfire, the spatial patterns of soil mineral-N, organic labile-N (microbial biomass-N and dissolved organic-N) and extractable-P pools would become more clumped because of ash accumulation and post-fire deposition of litter around individual adult trees. To test this hypothesis, we used plots within a Pinus canariensis forest (with both Pinus canariensis and Adenocarpus viscosus present) and sampled them one month before and one month after a wildfire. Using geostatistical analyses, we examined the spatial patterns of soil mineral-N (NH₄-N and NO₃-N), dissolved organic-N (DON), microbial biomass-N (MB-N) and soil extractable-P (PO₄-P). Burned plots of P. canariensis and A. viscosus both had values that were significantly greater than the unburned plots for all variables, except for DON in both cases, and the N:P ratio in the case of A. viscosus, which showed significantly lower values. Except for DON, we observed an increased spatial dependence and range after a fire for all studied variables in the P. canariensis plots (large individuals). However, in plots with A. viscosus (smaller individuals), we only found differences before and after the fire for the PO₄-P and DON spatial patterns. Our results confirm the changes in the spatial structure of soil variables with fire, and suggest that, on a short-term basis, the physical structure of the plant community may determine the new spatial structure after fire, with a more clumped distribution around large surviving trees and shrubs. The spatial patch size of limiting resources has important consequences for the success of restoration of forest communities on burned areas.