Distribution of soil nutrients under and outside tree and shrub canopies on a revegetated loessial slope

Studies of soil nutrients in revegetated land have often not provided the sampling positions on a scale of individual trees and shrubs, suggesting that nutrients were assumed to not vary substantially at fine scales. This assumption, however, conflicts with the "fertile island" theory for arid and semi-arid areas. We assessed the importance of sampling position on nutrient contents in 0-100 cm soil profiles by examining differences between soils under and outside the canopies of Armeniaca sibirica and Caragana korshinskii on a slope on the Loess Plateau, China. Soil organic carbon, total nitrogen (TN), total phosphorus, ammonium N, and extractable P did not differ significantly under and outside the canopies, except for nitrate N (NO3--N). The differences between these two canopy positions were significantly larger for C. korshinskii than A. sibirica for TP, significantly larger on upper than middle and lower slope sections for TN and NO3--N. The NO3--N content varied with sampling position around individual trees and shrubs, and trail tests about sampling position can be conducted around individual leguminous plants, in flatter areas, and in topsoil