Soil seed bank and mapping Chromolaena odorata an invasive weed in agro-ecosystems of Serengeti district, Tanzania

The spread of Chromolaena odorata in some parts of Serengeti district has stimulated interest in the prediction of their soil seed bank and mapping its distribution. Thirty clusters from each agro-ecosystem namely fallow land, grazing land, and cultivated land were sampled for soil seed bank assessment. Each cluster was randomly demarcated into five 20 m2 plots. Soils were collected from 0-5 cm, 5-10 cm, and 10-15 cm depths using a 10 cm diameter and 5 cm length cylinder. Coordinates were taken using a hand held GPS (Map76 cx) along transects. Each soil sample was soaked for a minimum of 30 minutes in a solution of sodium hexametaphosphate (50 g/L) and sodium bicarbonate (25 g/L) and poured over a set of three different sieve sizes to remove debris. The results were converted to number of weed seeds per square meter, and statistically analysed using R software version 3.5.1. Quantum GIS (1.8.0) was used to map the distribution of the weed. The soil seed bank in all agro-ecosystems decreased significantly (p <0.05) from 0-5 cm to 10-15 cm soil depths. Mean weed seed densities varied depending on land use but significantly highest in fallow land followed by cultivated land (p <0.05). In cultivated land the top soil had more than twice Siam weed seed densities (1254 seeds/m2) than in the bottom soil layers (597 seeds/m2). The Siam weed was found to have ability to thrive in a wide variety of soils, hence increase its invasion success. Distribution maps of C. odorata in Serengeti district and surrounding areas provide insight on the aggressiveness of the weed according to its spread and infestation levels. Future research work should be carried out to study fallow age in relation to soil seed bank of Chromolaena odorata and physio-chemical properties of the soil and their influence on distribution of the weed.