Evaluating taxonomic adjacency as a source of soil map uncertainty

Summary Soil types or map units are considered to be taxonomically adjacent if they differ in only one criterion, defined by an arbitrary threshold value. By treating soil types as nodes of a graph and taxonomic adjacency as the graph edges connecting nodes, algebraic graph theory can be used to produce a measurement of the uncertainty in a soil map associated with arbitrary classification boundaries between soil types. The largest eigenvalue of the adjacency matrix of a graph, the spectral radius, is an indication of network complexity. A larger spectral radius indicates a more complex network, and a greater degree of uncertainty or potential error associated with taxonomic adjacency. Benchmark values of spectral radius for cases of no taxonomic adjacency, including a single pair of adjacent soils, a chain or cycle‐type graph structure and a fully connected graph, are established so that taxonomic adjacency indices based on the spectral radius can be established. Examples are shown from two contrasting USA soil landscapes in the O uachita M ountains, A rkansas, and the coastal plain of N orth C arolina, using both US S oil T axonomy and the world reference base. The taxonomic adjacency indices are also useful in assessing soil richness and pedodiversity, with smaller values indicating a greater likelihood that identified soils represent distinct entities.