Distribution and storage of crop residue carbon in aggregates and its contribution to organic carbon of soil with low fertility

• Soils added 13C-labelled maize straw residue were incubated in-situ for 720 days.
• Contribution of residue C to organic C in soil aggregates.
• Distribution of residue C to macroaggregate was stable but to microaggregate increased.
• More than 50% of fine particulate organic C was derived from residue C.
• Residue C mainly stores in mineral-associated organic C fraction in the long-term.

Long-term intensive cultivation leads to the decrease of soil organic carbon (SOC) and soil fertility. Crop residue amendment to soil is documented as an effective measure to increase SOC and improve soil productivity. However, there is limited information on the turnover and storage of crop residue carbon (C) in soil aggregates after the residue is added to soil with low fertility. The objectives of this research were to investigate the distribution and storage of residue C in soil aggregates and its contribution to different physical fractions of SOC, and to quantify the turnover of residue C in soil with low fertility. Soil samples added with 13C-labelled maize straw residue were put into carborundum tubes for two-year long in-situ incubation. Soil aggregates were separated by wet sieving and then physically fractionated. During the whole incubation process, 12–15% of residue C was stably distributed to 2000–250 μm aggregates, while the percentage of residue C distributed to microaggregates (<250 μm) increased with incubation time. The contribution of residue C to particulate organic C (POC) fractions decreased from average 63% on day 60 to average 43% on day 720 and that to mineral-associated organic C (mSOC) fraction increased from average 23% on day 60 to average 28% on day 720. More than 50% of fine POC (fPOC) was derived from residue C, especially 71% in microaggregates on day 360. Within aggregates, the percentages of residue C distributed to free light organic C (fLOC) and coarse POC (cPOC) reduced and these to fPOC and mSOC strengthened with incubation time. Mean residence time (MRT) of residue C was shortened with the increase of the aggregate sizes. MRT of mSOC was longer compared to other SOC physical fractions. These results suggest that microaggregates could provide favorable conditions for microbial activities and conduce to fPOC accumulation in a low fertility soil amendment with crop residue.