Use of turnip as green manure in a rainfed forage maize-grass ley rotation system: evolution of several soil biochemical properties

Our research team is participating in the ReCROP project, funded by the PRIMA-Med program, which focuses on developing and evaluating sustainable soil management practices. The project aims to reduce reliance on inorganic fertilizers and pesticides, prevent soil erosion, maintain or restore soil organic matter, and support the restoration of soil biodiversity.

The temperate-humid zone of Spain is predominantly focused on livestock production, with forage maize being one of the main crops. An alternative practice proposed for this crop is its rotation with turnips (Brassica rapa L. var. rapa) as green manure. Turnips might offer multiple benefits: i) prevent the soil from remaining bare between harvest and sowing, ii) enrich the soil with minerals and organic matter, and iii) provide biofumigant properties through the release of thioisocyanates during the decomposition of the glucosinolates contained in their biomass.

In 2021, a field experiment was established to assess the impact of turnip green manure on soil functioning and soil health. Six plots were established: three with conventional forage maize cultivation and three with maize planted after turnip green manure incorporation. The traditional crop rotation system of the region, consisting of one year of maize followed by three years of ley grassland, was followed. Soil properties were monitored over three years—during maize cultivation in 2021 and ley grassland in 2022 and 2023. Soil samples (0-10 cm) were collected in late spring each year and analyzed for biochemical, physical, and chemical properties. This study presents the results on hydrolytic enzyme activities (C, N, P, and S cycles), dehydrogenase activity, and hot-water soluble carbon forms (carbohydrates and phenolic compounds).

In the first year, when the soils were under maize cultivation, all biochemical properties showed higher values in the soil under conventional cultivation compared to the soil amended with green manure. Unexpectedly, the same was observed for organic matter content. However, over time, and as the soils transitioned to grassland cultivation, all biochemical properties tended to level out. When activities are expressed per unit of carbon, they initially follow the same pattern, being higher in conventionally managed soil than in soil amended with green manure (except for phosphomonoesterase and cellulase, which exhibit similar values in both soils). However, over time, this trend tends to reverse. This could reflect differences in agricultural practices resulting from the incorporation of green manure and the distinct practices between the maize and grassland cultivation phases, highlighting the sensitivity of the measured parameters to variations in agricultural soil use and management. However, to better understand the underlying causes of these differences, the study will continue into the next phase, when the soils will return to maize cultivation.

Acknowledgements: The research was funded by the project PRIMA ReCROP Bioinocula and CROPping systems: an integrated biotechnological approach for improving crop yield, biodiversity and REsilience of Mediterranean agro-ecosystems. Ref PCI2021-121981/ AEI /10.13039/501100011033