Plant traits mediate agroecological outcomes in restored agricultural lands

Despite being cultivated with annual crops, marginal agricultural lands are usually more suitable for grasses, trees, or other perennial vegetation with persistent roots that are better adapted to unfavorable growth conditions. Replacing crops with perennial grassland vegetation (i.e., grassland restoration) on such marginal land can also contribute to realizing positive agroecological outcomes. However, while agroecological principles can guide multiple pathways and management practices, little work has explored the role of marginal agricultural land restoration in soil-based agroecological outcomes. Here we analyzed how soil properties linked with achieving three positive agroecological outcomes (enhanced soil health, higher recycling, and input reduction) are affected by marginal-land restoration and the plant traits of the established vegetation. To do this, we focused on changes in three indicators of these agroecological outcomes: active soil C (soil health), diversity of soil bacterial communities (recycling), and nutrient acquisition via total root biomass (input reduction) measured in three plant-cover types (forest, cropland, restored grassland) in 13 farms distributed in an agricultural landscape in southern Ontario, Canada. Active C and root biomass were highest in forests, intermediate in grasslands and lowest in croplands, while there were no differences in terms of soil bacterial diversity, indicating that restoring land can help achieve some positive agroecological outcomes. Using path analysis, we also found that two plant traits, leaf mass per area (LMA) and leaf dry matter content (LDMC), were key in mediating the link between grassland restoration and agroecological outcomes. Specifically, grassland restoration helped improve indicators of soil health and input reduction via the promotion of vegetation with conservative traits (high LMA and LDMC). Our analytical approach enabled the assessment of the agroecological outcomes after implementing restoration practices, while identifying the most important mechanistic paths mediated by plant traits.