Sixty Years of Sugarcane Monoculture Alters Carbon Preservation in Large Soil Macroaggregates in Tropical Soil

Abstract

Frequent and intensive tillage in conventional agriculture disrupts soil aggregates, engendering significant depletion of soil organic carbon (SOC) stocks. Long-term studies are essential for assessing the impact of sustainable practices on aggregate dynamics and carbon sequestration in sugarcane monocropping in tropical ecosystems. The following hypothesis was first, conventional sugarcane management would degrade soil, thereby reducing C stocks and breaking down aggregates. Second, conservation management that involves reduced disturbance could restore C storage, reaching levels similar to those observed in native vegetation (NV) over time. Third, macroaggregation could be a key mechanism for C storage under conservation management. This study evaluated the distribution of SOC among distinct aggregate classes under conventional (CC) and minimum (MC) cultivation in a 60-year-old sugarcane system, comparing results with undisturbed NV. The monitored properties included aggregate mass, SOC content and stock, weighted average diameter (WAD), aggregate and C stability indexes (ASI and CSI), soil C–CO₂ emissions, and C preservation capacity (CPC). The long-term CC had a significant impact on aggregate dynamics, manifesting in a reduction in macroaggregate mass and SOC content, an increase in CO₂ emissions, and a decrease in CPC. While the total SOC stock remained comparable in NV and MC (76 Mg C ha⁻¹), CC led to a 20% SOC loss. Although CC led to an increase in microaggregate C, it failed to counterbalance the loss of macroaggregate C. The ASI, with a reliability of approximately 99% in NV, exhibited a decline of 1% at 10 cm and 2% at 20 cm under MC and CC conditions. Concurrently, WAD reduced by 16% at 10 cm. CSI exhibited higher levels in NV and MC compared to CC at 10 cm depth. CPC in large macroaggregates was highest in NV (21 g kg⁻¹), decreasing by 50% in CC (7 g kg⁻¹) and by 33% in MC (14 g kg⁻¹). At a depth of 20 cm, large macroaggregate mass was 28% lower in CC and 14% lower in MC compared to NV, resulting in a 29% and 21% reduction in macroaggregate C stock, respectively. In summary, the application of MC resulted in the preservation of SOC stocks at levels comparable to those observed in NV, thereby underscoring its capacity to maintain crop sustainability and augment carbon sequestration in tropical sugarcane systems.