Strip Tillage and Crop Residue Retention Decrease the Size but Increase the Diversity of the Weed Seed Bank under Intensive Rice-Based Crop Rotations in Bangladesh

Abstract

Cropping under conservation agriculture (CA) has become increasingly attractive among farmers in recent years. However, weed control may be more difficult during the transition to CA from conventional establishment methods due to the reduction in tillage intensity. Conversely, CA changes to weed dynamics can alter the weed seed bank in the longer run. In Bangladesh’s intensively cropped rice-based rotations, the nature of weed seed bank shifts over time after adopting CA are poorly known. Two 2-year studies were sampled from on-farm CA experiments under wheat-mungbean-winter rice and monsoon rice-mustard-winter rice rotations. We investigated the effects of reduced soil disruption in the form of strip-tillage (ST) combined with increased deposition of standing reside from previous crops (0 vs. 50%). The weed seed bank in 0–5, 5–10, and 10–15 cm depths of soil were quantified in a shade-house experiment by measuring weed emergence over 12 months in seedling trays. After 2 years of field study, the year-round count of emerged weeds from the seed bank showed that ST plus 50% mulch had a lower weed abundance and biomass and fewer weed species than that of conventional tillage (CT) without residue. The perennial weeds Ageratum conyzoides L., Alternanthera philoxeroides L., Cynodon dactylon L., Cyperus rotundus L., Jussia decurrence Walt., Leersia hexandra L., Scirpus mucronatus (L.) Palla., and Solanum torvum Sw. were enriched in the smaller-sized ST seed banks in terms of both density and biomass. The CT, on the other hand, was dominated by annual weeds: Cyperus difformis L., Cyanotis axillaris Roem., Echinochloa crus-galli (L.) Beauv., Eleusine indica L., Fimbristylis miliacea (L.) Vahl., and Rotala ramosior L. Overall, ST plus 50% residue had a more diverse seed bank than CT without mulch. The majority of weed seeds were amassed in the 0–5 cm soil depth of the ST, while most of them were accumulated in the 10–15 cm layer of the CT. The wheat-mungbean-winter rice rotation had a more diverse floristic composition with many more weed species than the monsoon rice-mustard-winter rice rotation.

Continuous Practice of Conservation Agriculture for 3–5 Years in Intensive Rice-Based Cropping Patterns Reduces Soil Weed Seedbank

“…Hence, the present study may have overestimated the likely abundance of germinable weed seeds, by creating suitable germination conditions for seed that are normally buried too deep in CT, and especially in SP, to germinate and/or emerge. However, in another study, we found a lower weed density and biomass, and fewer weed species in the weed seedbank in soils sampled to only 0-5 cm under the SP + 50% residue retention, compared to CT after only 2 years [16]. Dormant seeds in CT become viable to germinate by scarification, ambient CO 2 concentrations, and higher nitrate concentrations, which may lead to the production of a higher weed emergence of new weed species in plowed soils [17].…”

“…The seedbank of the CT + 20% residue was more diversified (values 3.09 and 0.41, respectively) than the SP + 50% residue. In a similar study, which lasted only 2 years, Hossain et al [16] reported a more diversified seedbank of CT without residue (with diversity and dominance indices of 2.93 and 0.54, respectively), relative to the SP + 50% residue (indices of 2.44 and 0.90, respectively), even though that study sampled soils to 5 cm depth rather than 15 cm. In another study, Cardina et al [37] found a decrease in species diversity with the increasing number of tillage operations; their diversity indices were 0.6, 0.5, and 0.2 in NT, RT, and CT, respectively).…”

“…Similarly, in Digram, the diversity and dominance indices of 50% residue, along with BP, SP, and CT, were 3.20 and 0.48, 2.83 and 0.59, and 3.06 and 0.39, respectively, while with 20% residue, the indices were 3.39 and 0.50, 3.01 and 0.47, and 3.09 and 0.41, respectively. Similarly, Hossain et al [16] discovered that 50% residue with CT and SP had a lower diversity index (2.89 and 2.44, respectively) than no-residue (2.93 and 2.49, respectively). However, they found a lower dominance index of 50% residue (0.90) than no-residue (0.91) in RT, but the reverse in CT (0.57 and 0.54, respectively).…”

“…In our study, the seedbank of 50% and 20% residue had 86 and 77% similar species compositions at the Beluapara and Digram sites, respectively. Hossain et al [16] found 75% similar species compositions (qualitative values) in RT and CT, and 86% similar species compositions in 50% residue and no-residue.…”

Weed Responses to Crop Residues Management in a Summer Maize Cropland in the North China Plain

“…Many studies observed that reduced or zero tillage increased weed density and weed aboveground biomass [24,45]. In addition, strip-tillage with crop mulch could decrease the size, but increase the diversity of the soil seed bank when comparing with conventional tillage with no crop mulching [46]. Therefore, further studies with long term observation are needed in the future to determine the mechanisms of integrated weed management related to the soil physicochemical properties, nutrient uptake mechanisms of both crops and weeds, soil microbial community, and soil enzyme activities on the weed community composition and structure.…”