Seedling emergence vigor, establishment success, and biomass yield stability of cover crop mixtures compared to pure stands

Context

Cover cropping has the potential to enhance the sustainability of cropping systems across temperate regions. However, poor establishment of cover crops (CCs), often driven by unfavorable weather conditions, remain a major barrier to their adoption. Sowing CC mixtures may mitigate the risks of poor establishment compared to pure stands, thereby lowering spatio-temporal variability in biomass production, which is critical for consistent ecosystem service provision. Yet, it is unclear whether CCs establish more successfully in mixtures than in pure stands, and whether a better establishment results in greater biomass production that enhances ecosystem services.

Objective

We aimed to: i) analyze seedling emergence dynamics, final emergence rates, and emergence vigor (i.e., speed of emergence) in CC mixtures vs. pure stands; ii) determine the relationship between seedling establishment success and final biomass production; and iii) evaluate the effect of CC mixtures vs. pure stands on soil cover, weed suppression, nitrogen (N) catch crop, and N green manure services.

Methods

A 2-year field experiment (2020–2022) was conducted in Southwestern France, testing 11 pure stands and six two-species mixtures of brassicas, legumes, and grasses. CCs were sown in autumn and grown for eight months, with a fallow treatment as control. Data were analyzed using analysis of variance.

Results

Significant intra- (p < 0.001) and inter-annual (p < 0.001) variability was observed in CC seedling emergence dynamics. Brassica CCs showed the highest emergence vigor (77 ± 21°Cd), and their establishment success was positively correlated with final biomass yield (0.4 ± 0.2–8.6 ± 0.6 t.ha⁻¹; r = 0.61, p < 0.001). In contrast, legumes, such as faba bean, showed the lowest emergence vigor (147 ± 52°Cd), and their biomass yield (3.1 ± 0.1–6.8 ± 0.4 t.ha⁻¹) depended mainly on post-establishment climatic conditions. Seedling emergence vigor and establishment success did not differ significantly between CC mixtures and pure stands. Single species CC yielded more variable biomass between years, mixtures produced more stable yields (2.1 ± 0.1–9.9 ± 0.3 t.ha⁻¹). Soil cover was similar between CC pure stands (75.0 ± 0.0–100.0 ± 0.0 %) and mixtures (66.7 ± 5.3–100.0 ± 0.0 %), whereas poor establishment reduced soil cover (25.0 ± 7.9–50 ± 0.0 %). All CCs effectively suppressed weeds (up to 100 % reduction in biomass) and supplied N through scavenging and/or green manuring (up to 203 ± 18 kgN.ha⁻¹) compared to fallow.

Implications

In systems dominated by monocultures or short rotations under high-input management and climatic uncertainty, Brassicaceae-Fabaceae mixtures represent a promising option to ensure good establishment, stable biomass production, and delivery of key ecosystem services.