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Deeper soils represent a poorly understood, but potentially important, sink for carbon sequestration. The objective of this study was to determine the effects of long-term fertilization on soil organic carbon (SOC), its labile fractions and aggregate-associated carbon throughout a 0-3 m soil profile.

The investigation was conducted in a field experiment started in 1990 in an oasis farmland cropped with winter wheat. The following treatments were compared with the desert from which the oasis was created: CK (no fertilizer), NPK, N2P2K, NPKR, and N2P2R2 ("2" for double fertilizer and "R" for straw residue)

SOC contents increased by 14-56 % in the topsoil (0-0.2 m), but decreased by 15-22 % in the subsoil (0.2-0.6 m) under all fertilizer treatments. In the deep layer (0.6-3 m) there were significant differences between the treatments: SOC decreased by 5-9 % in treatments without straw, but increased by 4-9 % in treatments with straw. Labile fractions (particulate organic carbon and light fraction organic carbon) also showed similar trends. Both the fertilizer and CK treatments led to an increase in the amount of macro-aggregates (> 0.25 mm), especially small macro-aggregates (0.25-2 mm), throughout the soil profile. SOC content was highest in the macro-aggregates, intermediate in the silt + clay fraction (< 0.053 mm), and lowest in the micro-aggregates (0.25-0.053 mm). However, 44-87 % of total SOC was stored in the silt + clay fraction, especially in the deep layer (at least 80 %).

After 20 years of fertilizer applications, difference in SOC mainly occurred in the deep layer, and preservation of SOC in the silt + clay fraction appeared to be a prerequisite for soil-carbon sequestration. Applying inorganic fertilizer alone decreased SOC content in the silt + clay fraction in the deep layer, while the combined applications with straw resulted in higher SOC content in the silt + clay fraction in that layer, which turned out to be the main mechanism for increasing SOC content. Our study indicated that applying straw with inorganic fertilizer is the best practice for carbon sequestration, which occurred mainly in the deep soil layer.