Spatial pattern and scale of soil N and P fractions under the influence of a leguminous shrub in a Pinus canariensis forest

Nitrogen-fixing plants alter the chemical properties of the soil beneath plant canopies, particularly by concentrating nitrogen-rich organic matter. We hypothesize that the presence of a legume canopy inside a plot will more greatly influence the spatial structure of soil nitrogen (N) than phosphorus (P). We also investigated whether the effects of legume individuals on the soil properties beneath their canopies might be mediated by soil texture and water availability. Thus, we expected that the local effect of a legume canopy would be more conspicuous in nutrient-poor sandy soils than in nutrient-rich loamy soils. Moreover, the spatial pattern should differ during the wet (winter) and dry seasons (summer) because the microbial processes driving nutrient cycling are sensitive to water availability. To test these hypotheses, square plots (4 m×4 m or 3 m×3 m) were placed around isolated mature individuals of Adenocarpus viscosus in two pine forest stands of the Canary Islands (Spain) with contrasting soil textures (loamy and sandy soil). The spatial pattern and scale of microbial biomass-N (MB-N), dissolved organic-N (DON), and inorganic-N and -P fractions (NH4–N, NO3–N and PO4–P) were analyzed with geostatistical methods for two sampling dates (summer and winter). Soil variables with spatial structure demonstrated a greater spatial dependence in the loamy than sandy soil, with the exception of MB-N during summer. Except for NH4–N and NO3–N in winter plots, the spatial range was also lower in the sandy than the loamy soil. The legume canopy only had a clear effect on the spatial pattern of winter NH4–N, NO3–N, and DON in the sandy soil; no dependence was observed for PO4–P on the legume canopy in both soil types. Our results suggest that the presence of A. viscosus individuals may be an important source of spatial heterogeneity in the N content of the soil in these forests. However, soil texture and water content modulated the magnitude of the legume canopy effect on the spatial distribution of these N forms beneath canopies.