Influence of Farming Systems (Organic vs. Conventional) With Crop Rotation and Intercropping on Weed Seedbank in Central Kenya

Weed seedbanks are a key driver of long‐term weed pressure and are strongly shaped by farming systems, crop rotation, and intercropping. This study evaluated the influence of organic (ORG) and conventional (CONV) farming systems, each managed under high (113–145 kg N·ha −1 , 60–83 kg P·ha −1 ) and low (31–45 kg N·ha −1 and 13–27 kg P·ha −1 ) fertilization levels, on weed seedbank density, composition, dominance, and diversity in two long‐term trials (established in 2007) at Chuka and Thika in the Central Highlands of Kenya. From 2017 to 2019, across six cropping seasons under a three‐year rotation, soil samples (0–20 cm) were collected after each season from four systems: CONV‐high, CONV‐low, ORG‐high, and ORG‐low. Weed seedbanks were quantified using the seedling emergence method. Under these conditions, weed species were identified, and density, Shannon diversity ( H ′ ), evenness (), and principal component analysis were used to assess treatment effects. Crop rotation affected densities of all weed species, with responses dependent on crop sequence and farming system. High‐fertilization systems (ORG‐high and CONV‐high) generally supported higher total weed densities, linked to greater organic and mineral fertilizer inputs and favored high‐fertility indicator species such as Bidens pilosa , Amaranthus hybridus , Galinsoga parviflora , Stellaria media , and Commelina benghalensis . Low‐fertilization systems were associated with low‐fertility species such as Portulaca oleracea , Tagetes minuta , and Schkuhria pinnata . Dominant species included B. pilosa , G. parviflora , S. pinnata , Eleusine indica , and Setaria verticillata at Chuka and A. hybridus , B. pilosa , E. indica , and S. media at Thika, whose dominance generally declined over successive rotations. Intercropping baby corn/ Mucuna and maize/ Mucuna in ORG‐high and maize/beans and kales/Swiss chard in ORG‐low substantially reduced weed densities compared with corresponding monocrops and CONV‐low. Organic systems supported higher weed diversity and evenness ( H′ up to ∼2.0; E H  > 0.8) than CONV systems, indicating more functionally diverse, less dominance‐driven weed communities. These results highlight that integrating crop rotation, strategic intercropping, and tailored nutrient management can reduce weed seedbank build‐up and dominance of problematic species while maintaining diverse weed communities compatible with sustainable, low‐herbicide smallholder farming systems.