Weed Seed Bank and Seedling Dynamics in Response to Crop Rotation in Conventional Agroecosystems

Cropping history can affect our knowledge about the compositions and diversity of weed communities in the soil. Weed Species composition and density are influenced by farming practices and vary from field to field and among areas within fields. Plants that escape control and produce seeds within the field can be considered as a major source of seed entering the soil. Crop rotation is an effective weed management tool which can change weed distribution pattern by increasing selection pressures. Two types of rotation including corn-winter wheat and fallow-winter wheat are the most common cropping systems in arid and semi-arid areas of Iran. The multiple tillage operations can affect the vertical distribution, germination and emergence of weed seeds in the soil. Weed seed bank density, species composition and diversity will change when crop management practices are altered.

A field experiment was conducted to evaluate the effects of corn-wheat and fallow-wheat crop rotations on weed seed bank dynamics and seedling population during 2016-2018 growing season at Shiraz University. The fields divided into 10 by 10 meter grids. Soil samples were taken from 0-15 and 15-30 cm depths by soil sampler (auger) with 10 cm diameter. These samples collected after seedbed preparation and before crop sowing from 144 points. The samples of each depth were mixed together, placed in black plastic bags, and transferred to the laboratory. Then, 250g of the total soil was weighed and separated. These samples was placed in silk bags and washed with low water pressure. Finally, weed seeds dried, identified and counted to the level of species using a binocular stereomicroscope. Those seeds that were resistant to forceps pressure assumed as healthy seeds. Weed seedling population were calculated using a quadrat before and after application of herbicide at the same points were seed bank was carried out. Geostatistics technique was used to investigate density and spatial distribution of weed seedlings in two different crop rotations.

The highest frequency of weed seed bank belongs to Portalaca oleracea, Amaranthus retroflexus, Chenopodium album and Lamium amplexicaule. Relative density of P. oleracea and A. retroflexus seeds increased in corn-wheat rotation compared with fallow-wheat. The results showed that weed seed frequency was significantly affected by crop rotation and depth of plowing. As corn-wheat rotation had 33% increasing and fallow-wheat 19.44% decreasing of weed seed in 15-30 cm soil depth. The size of the total weed seed bank in corn-wheat caused a greater seed accumulation in the surface layer of the soil (0-15 cm). Weed seed density in corn-wheat rotation increased 89.79% and 62.85% in 0-15 and 15-30 cm, respectively during two years. Shannon diversity index increased by 12% in corn-wheat and decreased by 5.4% in fallow-wheat compared with the first year. Margalf index of corn-wheat rotation at 0-15 and 15-30 cm of soil depths decreased 33.70% and 38.25%, respectively, compared to the fallow-wheat rotation in the first year. Sorenson similarity index of corn-wheat and fallow-wheat at 0-15 and 15-30 soil depths was 0.82% and 0.80% during two years. The slope of linear regression also indicates that fallow-wheat weed seed bank (0.79) decreased more than corn-wheat rotation (0.47). Spatial distribution of weed seedlings showed that there is a large similarity in distribution patterns of total weed population between before and after herbicide application in two different rotations during the second year. However, distribution pattern of seedlings in corn-wheat rotation at the first year herbicide application was different after application of herbicide. Therefore, herbicide application can affect spatial distribution and number of weed seedling species.

Corn-wheat rotation as a high input level cropping system with deep tillage increases the size of the weed seed bank, especially in the soil surface layer (0-15 cm). While fallow-wheat rotation as a low input level cropping system causes a decreasing in weed seed bank dynamics, increasing in the diversity and richness of the seed bank and improving the soil structure by wheat residual on the soil during fallow system. The results of this study will be valuable in aiding the prediction of likely weed infestations in rotation systems. This ability to predict the size of weed seed bank, pouplation, diversity and emergence would also provide valuable input to population dynamics models that can be used in weed management.