Soil organic matter in restored rangelands following cessation of rainfed cropping in a mountainous semi-arid landscape

Agricultural abandonment is known to influence plant cover composition and C inputs into the soil with a consequence for changes in soil organic matter (SOM) storage and dynamics in rangeland ecosystems. This study was conducted on a chronosequence of high altitude rangelands (1) cultivated with rainfed wheat (CR0), (2) abandoned for 4 (AR4), 12 (AR12) and 45 (AR45) years and (3) uncultivated (reference) rangelands (UR) with three replicates in Zagros Mountains, Central Iran. We studied the changes in the concentrations and stocks of bulk soil organic carbon (OC), total N, particulate organic C (POC) and N (PON), dissolved organic C (DOC), microbial biomass C (MBC), and potentially mineralizable C (Min-C) at 0-0.15 and 0.15-0.3 m soil depths. Results showed that the concentrations and stocks of OC, N, and labile fractions increased with the abandonment of agriculture at both soil sampling depths. After 4-45 years of agricultural abandonment, soil OC and N stocks increased logarithmically by 3.8-46 % and 2.8-32 % in the whole 0-0.3 m, respectively. Although, the stocks of labile fractions decreased slightly 4 years after agricultural abandonment, there were considerable increases (logarithmic) in these fractions after 12-45 years of abandonment (POC, 65-148 %; PON, 68-147 %; DOC, 76-139 %; MBC, 24-62 %). The study shows that rangelands abandoned for 45 years contained lower soil OC and N concentrations and stocks compared to uncultivated rangelands, reflecting 45 years of abandonment would not be sufficient for SOM to attain the level of uncultivated rangelands. The present study provided evidence that the rate of increases in POC and DOC stocks was greater than that of OC and MBC stocks, demonstrating POC and DOC fractions of total SOM pool may be suitable and sensitive indicators for detecting the effects of agricultural abandonment on soil OC changes and storage in these restored semi-arid rangelands. Soil bulk density decreased, while the mean weight diameter (MWD) and aggregate ratio as measures of aggregate stability increased considerably within the abandoned rangelands with increasing time of abandonment. Results from a multivariate analysis suggested that soil variables such as bulk density, OC, TN, DOC, POC, PON, MBC, MWD and metabolic quotient (qCO(2)) were successful in separating land uses. In brief, the abandonment of agricultural activities in previously cultivated high altitude rangelands can potentially lead to an increase of total and labile SOM and also sequestration of C in these semi-arid rangelands.