Measuring short-term soil health additionality of increased crop diversity in consolidated no-till: a case study from subtropical South America

The widespread adoption of no-till in South America has established high baselines of soil health indicators, creating challenges for demonstrating additionality. In this study, we evaluated 13 soil health indicators across a crop diversity gradient in Carambeí, Southern Brazil, where long-term no-till has been practiced for over three decades. Treatments included soybean–oats monoculture, diversified crop rotations, and cover crop mixtures. Linear mixed models and radar plots were used to assess management effects on physical, chemical, and biological indicators, and to interpret their functional implications. Results revealed that most physical indicators, including bulk density and water-stable aggregates, showed no significant response to crop diversification, reflecting structural recovery already achieved under long-term no-till. In contrast, active carbon and nutrient availability (P and K) increased significantly under diversified systems, while soil organic carbon and protein showed positive but non-significant trends. Radar plots highlighted that crop rotation enhanced water regulation and carbon turnover, while cover crops provided complementary benefits for nutrient cycling. These findings illustrate the “Law of Diminishing Returns” in mature conservation systems where further improvements are incremental, but functionally important. For agricultural extension, highlighting these incremental benefits is critical to sustain farmer-led innovation in Brazil and Argentina, where adoption historically spread through perceived agronomic and economic advantages. For policy, incorporating sensitive indicators such as active carbon into soil health frameworks will ensure that long-term conservation farmers are recognized within regenerative agriculture and incentive programs.