Variability of the Content and Stock of Soil Organic Matter in Time and Space: An Analytical Review
N. B. Khitrov, D. A. Nikitin, Е. А. Иванова, M. V. Semenov
Abstract
Abstract The aim of this analytical review is to systematize information on the quantitative characteristics of variation in the content and stock of organic carbon (Corg) in soils. The review considers the estimates for validity and reproducibility of determined Corg values, as well as the spatial variation and heterogeneity of Corg at different hierarchical levels of the soil cover composition and their changes in time. The most powerful factor of Corg changeability in soils is spatial variation. The absolute standard deviation and coefficient of variation for Corg content and stock in soil shows the trend of an increase with the logarithm of the examined plot area. This trend is observed on the background of a wide range of the values of the indicators of spatial Corg variation in each narrow range of the plot area, which leads to a high uncertainty of the estimates with an increase in the area coverage. Direct dry combustion is considered the preferable method among the methods used to determine Corg content. This method gives valid (i.e., with the least biases) and well reproducible data. The indirect Tyurin and Walkley–Black methods systematically underestimate the Corg content and their reproducibility is comparable to the amplitude of seasonal dynamics and minimum spatial variation indices within an elementary soil area. The estimates for the long-term trend of Corg content require strict adherence to stringent monitoring conditions over the time intervals longer than 15 years. The spatial variation in Corg stock is more pronounced as compared with Corg content, which further increases the requirements to monitoring.