Unveiling the impact of winter cover crops and weedy fallow on the soil seedbank

Abstract

Cover crops (CCs) are recognised as valuable for weed management, while fallow soil between cash crop seasons likely increases weed presence. Weeds may offer similar ecosystem services as CCs, although they pose a risk of seedbank buildup. This study evaluated the impact of two winter CC systems (3-year triticale cultivation, TRIT; and a 3-year succession of rye, clover, and mustard, RCM) compared to weedy fallow (WF) on weed seedbank size and composition in a 3-year ‘maize (Zea mays L.)–maize–soybean (Glycine max (L.) Merr)’ crop succession. After 3 years, seed density of spring/summer weeds reduced in all treatments, potentially stemming from herbicide use during cash crop seasons and tillage operations. Triticale had the lowest seedbank density (9,487 seeds m⁻²) and higher diversity (Shannon Index 6.9) compared to WF (28,543 seeds m^-² and 4.1, respectively). Furthermore, stochastic analysis revealed a lower risk of enlarging weed seedbanks in TRIT compared to WF (for seed densities above 900 seeds m⁻²). Moreover, management practices (CCs, cash crop sowing, termination/harvest) synchronised with weed seed production and germination likely contributed to the decreasing seed density of species including Portulaca oleracea and Chenopodium album, which were reduced by 90 and 80 %, respectively, by the study’s end. Over three years, autumn/winter and indifferent weed seed densities increased 4.2 times more in WF and RCM (22,638 seeds m⁻²) than in TRIT. This may be due to the varying growth rates among CC species in RCM, whereas TRIT consistently established rapidly, potentially outcompeting weeds until termination. Fallow periods between cash crops may increase weed species linked to that season and future crop–weed interference in varied crop rotations. Introducing CCs can mitigate this effect, although the choice of CC species may influence the extent of the impact.