Evaluation of Weed Biodiversity on Rice (Oryza sativa L) Agro-ecosystems (Organic and High-input)

Introduction Weeds are one of the complementary components of agricultural ecosystems and inseparable part of them. So, it is essential to understand the composition and structure of them in agricultural ecosystems, to conservation and optimal utilization of ecosystems components. The agro-ecosystems are special case of secondary succession, so regular and consecutive disturbance to them, causing the compatibility of specific weeds. This study was conducted to comparison the biodiversity of rice agro-ecosystems' weeds in Mazandaran province. Material and Methods Three fields inside the ecosystems (range from 0.30 to 0.50 ha) were replications of the experiment. In each field nine quadrates (1m ×1m) was randomly determined. The weeds species and their frequency in each quadrate were counted in four phases (tillering, stem elongation, grain filling and post-harvest).Identification was performed using reliable sources, as much as possible at the species level. The diversity and evenness of weeds in the studied fields and its average in management systems were determined using relevant indicators. Data analysis was performed by comparing the mean of biodiversity indices of the two communities with method of one-way ANOVA using SAS 9.1 software. Results and Discussion Based on the results of this study, weed species belong to four plant families (Poaceae, Cyperaceae, Plantaginaceae and Asteraceae). From this collection, there were eight weed species which belong to monocotyledone (Gramineae, Cyperaceae and Plantaginaceae) and two dycotyledone (Asteraceae). Of the 10 species available, four weed species (Xanthium strumarium, Eclipta prostrata, Digitaria spp., and Cyperus spp.) only exist in the organic management system and the Alisma plantagoaquatica species only in the high input system was observed. Comparison of the mean of biodiversity indicators of the two management systems showed that despite increasing the amount of diversity indices in the organic system, there was no significant difference between the two systems. Confirmations suggest that the combination of plant and animal communities and the structure of food webs in rice agro-ecosystems are very similar in different parts of the world. Comparison of different stages of sampling based on the average of the two systems revealed a decline in diversity and evenness of weeds in the fourth stage (after harvest). The evaluation of biodiversity indicators of both organic and high input systems also showed a significant reduction in weed biodiversity indices in the fourth stage compared to the previous stages. This result could be due to the disturbance of rice agro-ecosystems affected by harvesting and soil preparation for the emergence of various weed species. As regards that evidences suggest that in these conditions (disturbance and stress) only certain species (especially resistant species) are dominant. Despite the anticipation of increasing diversity in these conditions, because of drought stress at the harvesting stage, resistant species to stress emerged and established. In the tolerance model, despite the existence of opportunistic species, low-expected species (low R star ) are grown and mature at lower resource levels. The superiority of diversity in the organic system, especially in the fourth stage, can be related to the emergence and establishment of resistant weeds to disturbance and stress conditions such as Digitaria spp. X. strumarium, and E. prostrata (species of non-flooding conditions) in addition to common weeds of the rice agro-ecosystems, such as Echinochloacrussgulli and Cyperus spp. (species of flooding conditions). Conclusion The results showed that in general, biodiversity (diversity and evenness) indices of weeds were higher in the organic system compared to the conventional one. From this viewpoint, it can be said that the sustainability of organic systems rather than the conventional ones is mainly due to their greater diversity. Because of the greater diversity of weeds, the higher the diversity of other living organisms, including arthropods, is also a measure of sustainability