Soil respiration activity and thermal stability of organic matter under the post-agricultural evolution of Haplic Luvisol

The aim of the study was to investigate the effects of post-agricultural soil development on the respiratory activity and thermal stability of accumulated organic matter. A post-agricultural chronosequence formed on Haplic Luvisols and including currently arable land, 7- and 25-year-old post-agricultural abandoned land and grassland was studied. Soils were studied using thermogravimetric analysis aimed to investigate pools of thermally labile, stable and persistent soil organic matter (SOM). Elemental analysis was used to determine organic carbon and total nitrogen in bulk soil and water extracts. Basal and substrate-induced respiration rates were determined by incubating soils and measuring the amount of CO2 released. The content and pool of soil microbial carbon were calculated based on the measured substrate-induced respiration. Organic, dissolved and microbial carbon, total and dissolved nitrogen and basal respiration rates increased during the post-agricultural development of soils. The specific respiration of microbial biomass was lowest in arable soil and increased after its abandonment. The availability of soil carbon for microbial decomposition increased in abandoned soils compared to arable ones. The SOM was dominated by the thermally labile pool (54–68%). The size of the thermally stabile pool of SOM was 19–25%, persistent SOM was 13–21%. The accumulation rate of the thermally labile pool in the upper 30 cm of the soil, calculated for the 25-year period of the abandoned land use, was equal to 3.9 mg lab-SOM per year, stable –0.97 mg stab-SOM per year and persistent – 0.52 mg pers-SOM per year. As arable land was converted to abandoned, the proportion of the thermally labile pool in SOM increased. The post-agricultural increase in dissolved organic carbon was associated with the rise of the thermally labile pool of SOM, indicating the relationship between the availability of organic matter for microbial decomposition and its thermal stability.