Assessing Cover Crop and Intercrop Performance Along a Farm Management Gradient

Abstract

The impacts of crop diversity on ecosystem functioning may vary depending on farms’ baseline environmental conditions and management histories. We conducted a two-year experiment to assess the effects of two diversification practices—vetch-oat cover cropping and cucumber-snow pea intercropping—on nitrogen (N) cycling and productivity across a farm management gradient. The field experiment spanned 14 farms in southern Brazil with different long-term management histories, from conventional tobacco to agroecological mixed crop-livestock systems. As predicted, soils with longer histories of agroecological management had higher levels of soil organic carbon, potentially mineralizable carbon, and extractable phosphorus, captured by a principal component reflecting “biological soil fertility”. In the study’s second year, the vetch-oat cover crop doubled N inputs to soil compared to fallows across all farms. Nitrogen mineralization following cover crop incorporation was twice as high on agroecological farms as on transitioning or conventional farms. Structural equation modeling revealed that after accounting for variation in background fertility, cover crop mixtures explained a further 67% of the variation in soil N availability at vegetable planting. Consequently, benefits of diversification practices for soil N cycling were ecologically relevant across farms within the span of our experiment, with the greatest performance overall on agroecological farms. Low soil pH was the strongest constraint on vetch N fixation and on intercrop yield (Land Equivalent Ratio, LER). Overall, the vegetable intercrop overyielded relative to monocrops across farms and years (mean LER=1.19), with stronger overyielding in the second year (mean LER=1.27). Though prior research has emphasized facilitation with greater crop diversity in lower fertility soils, our results suggest that benefits for nutrient cycling and productivity are robust to varied soil conditions and accrue over the course of transitions to agroecological management.