Weed seed bank dynamics: soil organic carbon dynamics and weed seed bank modulation through residue, tillage and weed management: A Review

Weeds are unwanted plants playing a very important role in different eco-systems and many of them cause enormous direct and indirect losses. The losses include interference with cultivation of crops, loss of bio-diversity, loss of potentially productive lands, loss of grazing areas and livestock production, erosion following fires in heavily invaded areas, choking of navigational and irrigation canals and reduction of available water in water bodies. Weed management takes away nearly one third of total cost of production of field crops. In India, the manual method of weed control is quite popular and effective. Of late, labour has become non-availability and costly, due to intensification, diversification of agriculture and urbanization. The usage of herbicides in India and elsewhere in the world is increasing due to possible benefits to farmers and continuous use of the same group of herbicides over a period of time on a same piece of land leads to ecological imbalance in terms of weed shift and environmental pollution. The complexity of these situations has resulted in a need to develop a holistic sustainable eco-friendly weed management programme throughout the farming period. Weed infestation is one of the major biotic constraints in crop production. Field crops are infested with diverse type of weed flora, weed density and weed diversity as it is grown under diverse agro-climatic conditions, different cropping sequence, and tillage intensities and weed management strategies. The yield losses due to weeds vary depending on the weed species, their density and environmental factors. Knowledge about how the type, timing, and arrangement of cultural practices influence weed species composition is important for understanding the ecological results of control strategies and designing alternative crop management systems. Studies of soil weed seed banks are of relatively recent origin considering their importance as sources of diversity and continued occupation of many types of habitats, including agro-ecosystems. The management of weed seed banks is based on knowledge and modification of the behaviour of seeds within the soil seed bank matrix. The behaviour of seeds defines the phenotypic composition of the floral community of a field. Selection and adaptation over time have led to the highly successful weed populations that exploit resources unused by crops. The weed species infesting agricultural seed banks are those populations that have found successful trait compromises within and between the five roles of seeds: dispersal and colonization, persistence, embryonic food supply, display of genetic diversity, and as a means of species multiplication. Tillage system was more important determinant of weed seed density than the weed management practices. Movement pattern of weed seeds by all tillage treatments differ significantly over three weeding management practices at 0-5 cm soil core. Zero tillage system promoted infestation of some broadleaf weeds. The lowest weed mass was determined for the conventional tillage plots, compared to minimum tillage, and especially zeros tillage plots. Because herbicide and cultivation efficacy is generally density independent, seedling density following these weed control practices will be proportional to the density of germinal seeds in the seed bank. Most farmers would therefore benefit from management practices that reduce seed inputs, increase seed losses, and reduce the probability that remaining seeds establish. Germination, predation, and decay are the primary sources of loss to the seed bank that may respond to management. Although seeds would seem to be an ideal carbon source for soil microorganisms. To reduce seed bank inputs and increase losses is to reduce the size of the effective seed bank through manipulation of residues and disturbance to reduce the probability of establishment. Incorporation of green manures generally reduces weed establishment, whereas larger-seeded or transplanted crops may better tolerate the residue-mediated changes in the chemical, biological, and physical properties of the soil surface environment. Evidence from no-till systems further support the changes in soil surface conditions may regulate the abundance of “safe sites” for weed establishment, thereby modulating the size of the effective seed bank. Crop residues on the soil surface reduce weed seedling establishment in no-till systems, but tillage eliminates this effect. This provides useful information to improve methods for maintaining plant population balance.