Soil carbon allocation, composition, and sequestration changes induced by cropping diversification in tropical systems

Physical protection by carbon (C) occlusion inside soil aggregates is assumed to be a key mechanism for C stabilization. Nevertheless, little is known about how cropping diversification strategies affect C allocation and protection in soil aggregates and their stability, especially in highly weathered tropical soils. This study evaluated how cover crops and no-tillage affect soil organic matter (SOM) storage and aggregation in a tropical clayey Oxisol in the Brazilian Cerrado region. Changes in soil C and nitrogen (N) stocks, soil aggregate stability, C allocation within aggregates, and C speciation within aggregate classes using spectroscopy analysis were quantified. Soil C and N stocks under the most complex crop systems with cover crops increased by similar to 20 % and 29 %, respectively, compared to soybean-maize double cropping. The systems including cover crops increased soil aggregate stability, doubling large macroaggregation in the 5-10 and 10-20 cm depths. Increased aggregation under cover crops enhanced C contents within aggregates, i.e., similar to 48 %, 32 %, and 34 % higher for large macroaggregates, macroaggregates, and microaggregates, respectively, compared to soybean-maize double cropping (average considering different layers and cover cropping systems). Spectroscopy analysis showed that the diverse cropping systems increased the aromatic C (i.e., C=C/C-C) in large macroaggregates at the 0-5 cm layer while reducing oxidized groups, i.e., C=O/O=CO and C-O. Overall, our results revealed that cropping diversification is an effective strategy for increasing soil aggregate stability and physical protection of C (especially aromatic), and therefore can promote soil C storage in tropical soils. (AU)