Accumulation of organic carbon and its association with macro-aggregates during 100 years of oasis formation

The maintenance and accumulation of soil organic carbon (SOC) is critical to the agricultural sustainability and environmental stability in oasis-desert belts. The objective of this study was to determine the effects of oasis formation on SOC and aggregate structure, as well as the linkage between them, throughout a 0-200 cm soil profile. The investigation was conducted in five oases at the northern foot of the Tianshan Mountains in Central Asia. Oasis farmlands reclaimed 3, 5, 10, 20, 50, and >100 years ago were compared with the desert pairs they originated from. The SOC content significantly increased throughout the whole profile after 20 years of reclamation, despite a loss in the first 10 years of reclamation. The values reached maxima at 50 years of reclamation, which increased by 67-135% compared to initial values. The macro-aggregate (diameter > 0.25 mm) fraction with high carbon (C) concentration significantly increased throughout the soil profile after reclamation, and showed the greatest variation during oasis formation, compared with the other aggregate fractions (0.25-0.053 and <0.053 mm). These changes were significantly correlated with increases in soil nutrients and microbial biomass and decreases in soil pH and salt during oasis formation. In conclusion, the oasis formation enhanced SOC accumulation not only in topsoil but also in deep soil, and soil aggregate structure was improved by increased macro-aggregates. The formation of macro-aggregates and the increase in their associated C had significant correlations with SOC accumulation. Fertilization, especially inorganic nitrogen, very likely promoted the SOC accumulation and soil aggregation in concert with annual input of crop residues into the originally poor desert soils during oasis formation.