Reviews and syntheses: Agropedogenesis – humankind as the sixth soil-forming factor and attractors of agricultural soil degradation
Authors: Kuzyakov Yakov , Zamanian Kazem 
Published: December 2019

Abstract. Agricultural land covers 5.1×109 ha (ca. 50 % of potentially suitable
land area), and agriculture has immense effects on soil formation and
degradation. Although we have an advanced mechanistic understanding of
individual degradation processes of soils under agricultural use, general
concepts of agropedogenesis are absent. A unifying theory of soil
development under agricultural practices, of agropedogenesis, is urgently needed. We introduce a theory of anthropedogenesis – soil development under the main factor “humankind” –
the sixth factor of soil formation, and deepen it to encompass agropedogenesis as the most important direction of anthropedogenesis. The
developed theory of agropedogenesis consists of (1) broadening the classical
concept of factors→processes→properties→functions along with their feedbacks to the processes, (2) a new concept of attractors
of soil degradation, (3) selection and analysis of master soil properties,
(4) analysis of phase diagrams of master soil properties to identify
thresholds and stages of soil degradation, and, finally, (5) a definition of
the multidimensional attractor space of agropedogenesis. The main feature
of anthropedogenesis is the narrowing of soil development to only one
function (e.g. crop production for agropedogenesis), and this function is
becoming the main soil-forming factor. The focus on only one function and
the disregard of other functions inevitably lead to soil degradation. We show
that the factor humankind dominates over the effects of the five natural
soil-forming factors and that agropedogenesis is therefore much faster than
natural soil formation. The direction of agropedogenesis is largely opposite
to that of natural soil development and is thus usually associated with soil
degradation. In contrast to natural pedogenesis leading to divergence of soil
properties, agropedogenesis leads to their convergence because of the efforts to
optimize conditions for crop production. Agricultural practices lead soil
development toward a quasi-steady state with a predefined range of measured
properties – attractors (an attractor is a minimal or maximal value of a
soil property toward which the property will develop via long-term
intensive agricultural use from any natural state). Based on phase diagrams
and expert knowledge, we define a set of “master properties” (bulk density
and macroaggregates, soil organic matter content, C:N ratio, pH and electrical conductivity – EC,
microbial biomass and basal respiration) as well as soil depth (A and B horizons). These master properties are especially sensitive to land use and
determine the other properties during agropedogenesis. Phase diagrams of
master soil properties help identify thresholds and stages of soil
degradation, each of which is characterized by one dominating process.
Combining individual attractors in a multidimensional attractor space
enables predicting the trajectory and the final state of agrogenic soil
development and developing measures to combat soil degradation. In
conclusion, the suggested new theory of anthro- and agropedogenesis is a prerequisite for merging various degradation
processes into a general view and for understanding the functions of
humankind not only as the sixth soil-forming factor but also as an
ecosystem engineer optimizing its environment to fulfil a few desired
functions.