Conversion of tropical forests to croplands greatly reduces stock size of soil organic resources. Low-impact agriculture may curtail this loss in soil fertility, and favor long-term agroecosystem functioning. In east tropical Mexico, indigenous smallholders have been managing sugarcane plots without burning, fertilizing, tillaging, and removal of crop residues for the last 70 years. This study examined the long-term effects of such low-input sugarcane agriculture on soil aggregate structure and soil organic carbon (SOC) and soil nitrogen (N) stocks along a 50-year chronosequence of cultivation including differently aged sugarcane plots and tropical forests as a reference site. Soils were physically fractioned in four aggregate size classes and in four soil organic matter (SOM) fractions and changes in C concentrations and C/N ratios of these fractions were assessed. Long-term cultivation did not alter soil aggregate structure, or litter C content; however it reduced significantly litter N content in sugarcane compared to forest plots. After 20 years of cultivation, SOC and soil N stocks dropped by 25% at 0-10. cm soil depth compared to current C stocks in forest soils. After 50 years of cultivation, pooled over 0-20. cm, SOC stocks were similar in sugarcane and forest plots, while soil N stocks remained 12-19% lower in sugarcane than in forest plots. The mineral-associated organic C fraction remained unaffected by land use change. Forest conversion to sugarcane, depleted the free light particulate organic matter (POM) C fraction at 0-10. cm depth. Forest intra-aggregate POM C concentration declined after 20 years of cultivation and then recovered after 50 years of cultivation at 0-10. cm depth. This study demonstrates that traditional sugarcane cropping may be a long-term sustainable alternative to sequester SOC, while maintaining moderate soil N fertility levels without trading off on sugarcane production.
