The Effects of Long-Term Fertilization on the Accumulation of Organic Carbon in the Deep Soil Profile of an Oasis Farmland

Soil organic carbon (SOC) has a profound effect on soil quality, ensuring the sustainability of an agroecosystem, and can also be a significant sink for atmospheric CO₂. The accumulation of SOC in agricultural soils can be achieved by adopting efficient management practices.

Most long-term fertilizer experiments have demonstrated that the combined application of inorganic and organic fertilizer can improve SOC content and soil nutrient status. However, previous studies have focused on the topsoil or the top 1 m of the soil at most. Few attempts have been made to determine the effect of management practices on organic carbon turnover in deep soil or soil below 1 m. SOC content is low in deep soil and its change is also slow compared to topsoil. The soil profile can be many meters deep and the volume of these deep soil layers can be very large. As a result, deeper soil horizons could contain large quantities of sequestered organic C.

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, a field experiment was conducted started in 1990 in an oasis farmland cropped with winter wheat.

The results indicated that 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 soil layer (0.6–3.0 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 (0.25–2.00 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.

The study indicated that applying straw with inorganic fertilizer is the best practice for carbon sequestration, which occurred mainly in the deep soil layer. The result was published in Plant and Soil in August 2013.