Spatial variability of soil properties under Pinus canariensis canopy in two contrasting soil textures

Knowledge of the spatial pattern and scale of plant resources is important to aid in understanding the causes of this spatial pattern and their consequences on process at the population, community, and ecosystem levels. We tested whether the effect of individual plants on the soil properties beneath their canopies might be mediated by soil texture, since this soil property has great influence on the soil organic matter protection, the soil cation exchange capacity, and the nutrients diffusion rate. We hypothesize that variables directly related to organic matter (microbial biomass-N [MB-N] or dissolved organic-N [DON]), as well as soil nutrients interacting with soil secondary minerals (PO4-P and NH4-N), should more closely follow the plant canopy projection in sandy soils than loamy ones. We also expected a higher spatial range and dependence of NO3-N in sandy soils, although the spatial distribution should not necessarily be affected by the plant position. To test these hypotheses, we used square plots (8 m x 8 m or 6 m x 6 m) placed around isolated mature individuals of Pinus canariensis in both loamy and sandy soils in P. canariensis forests, with replicates in summer and winter. Spatial pattern and scale of MB-N, DON, and inorganic-N and -P were analyzed with geostatistical methods. In the summer sampling, all soil variables had lower spatial ranges in the loamy soil than the sandy soil. However, no clear trend was observed in the winter. The spatial dependence of NO3-N from the two sampling dates was higher for the sandy soil than the loamy soil. Kriged maps in the sandy soil revealed that the spatial distributions of the summer soil moisture, MB-N, DON, and PO4-P were all dependent on pine location. Our results suggested that the presence of P. canariensis individuals may be an important source of spatial heterogeneity in these forests. Soil texture may determine the magnitude of the pine canopy's effect on the spatial distribution of chemical and biological soil properties when water content is scant, but it may have negligible effects under conditions of higher water availability.