Recovery of organic matter and microbial biomass after abandonment of degraded agricultural soils: the influence of climate

After abandonment of agricultural lands (ongoing on 220 million hectares worldwide), degraded arable soils undergo self‐restoration and development towards natural ecosystems. We studied the linkage between microbial properties and density fractions of soil organic matter (SOM) during post‐agricultural restoration of former arable Phaeozems and Chernozems. The chronosequence study was conducted in two contrasting bioclimatic zones of European Russia: deciduous forest ( Luvic Phaeozem ) and dry steppe ( Calcic Chernozem ). Each chronosequence included an arable soil, 3–4 soils with increasing periods after abandonment (from 7 to 35 years), and reference sites with native soils. The basal respiration (BR) and microbial biomass ( C mic ) were closely correlated with the soil organic carbon ( C org ) content as well as SOM density fractions: free particulate organic matter (fPOM), occluded particulate organic matter (oPOM), and mineral–SOM. The greatest increase in most properties was common in the top 0–5 cm and was maximal for fPOM and oPOM fractions (by 1.5–2.5‐times), C mic (1.9‐times), and BR (1.5–2.5‐times). For the first time, the duration of full recovery of soil properties depending on climate were estimated. Generally, ~40–120 and 20–30 years in the forest and steppe, respectively, are required to restore C org , total nitrogen (TN), and C mic contents in the 0‐ to 5‐cm layer after the abandonment of agricultural lands. The maximal restoration rates of all properties are common in the first 15–20 years after abandonment.