What are the primary factors controlling the light fraction and particulate soil organic matter content of agricultural soils?

Particulate organic matter (POM) and light fraction organic matter (LFOM) are the fractions of soil organic matter (SOM) considered most active in terms of nutrient cycling and maintenance of soil structure. They respond quickly to changes in management and may offer insights into the long-term effect of management on SOM. However, the literature provides contradictory evidence regarding the factors which influence the amount of POM and LFOM, and there is little evidence to differentiate the relative importance of factors. Utilising data from over 150 experiments reported in the literature, we employed multiple regression to produce separate models quantifying the effect of management factors and environmental variables on POM, LFOM and total SOM; 29.3 % of the variance in the response variables was explained for POM, 28.3 % for LFOM, and 29.3 % for total SOM. Climate, organic amendments and inclusion of fallow periods were significant terms for all fractions. Climate had a larger influence on total SOM than POM or LFOM, whilst POM and LFOM were more strongly influenced by factors related to the recent history of organic matter addition; organic amendments and inclusion of fallows. Factors that were not significant variables for any of the fractions included tillage and application of N fertiliser, whilst soil texture was only a significant factor for SOM. General agreement between the total SOM, POM and LFOM models on the most important factors supports the idea that both POM and LFOM are good predictors of long-term changes to total SOM.