Soil properties, C fractions and their dynamics in land use conversion from native forests to croplands in northern Iran

Land use conversion from forests to agricultural lands may have a major influence on soil organic carbon (SOC) and total nitrogen (TN) stocks and dynamics in northern Iran. The objectives of this research were to investigate the effects of lowland and upland farming in forest ecosystems on bulk soil chemical and physical properties, SOC and TN fractions, and their dynamics in northern Iran. Soil samples (0–0.2 m and 0.2–0.4 m depths) from adjacent cultivated and forest lands were obtained at Paresar site with lowland rice cultivation and at Gorgan site with upland cultivation. The study sites differed in rainfall, annual tillage frequency and cultivation practices. Land use changes from forests to agricultural fields resulted in a significant alteration in soil properties, and SOC fractions and their dynamics at both study sites. However, the magnitude and direction of changes depended upon the land use type replacing forests and the soil depth. Results indicate that after 50–70 years of cultivation, SOC and TN declined by 29–66% in the 0–40 cm layer, and that 23–62% of the decline occurred in the sand-size fraction C, particularly in the 0–20 cm layer. Forest clearing and subsequently intensive tillage practices resulted in soil degradation by declining SOC and TN, decreasing aggregate stability, increasing dispersible clay (DC) and stimulating SOC mineralization in northern Iran. Our study shows that losses in SOC and soil structure are more evident in forest ecosystems with low rainfall and more frequent tillage practices (i.e., upland crops) than in forest ecosystems with high rainfall and less tillage practices (i.e., lowland rice). Therefore, the natural forest ecosystems of northern Iran may represent a significant carbon (C) source after conversion to agricultural cropping systems. In brief, significant losses in SOC, TN and soil structure following land use conversion would affect CO2 emission from the soil and nutrient dynamics in the natural forests of northern Iran.

► Soil C and N declined following cultivation on forest soils.
► Most of C and N losses occurred in the sand-size fraction C and N.
► Forest conversion degraded soil structure and stimulated C mineralization.
► Changes in soil properties were more pronounced in low than high rainfall climate.
► Conversion of forests to croplands represents a significant atmospheric C source.