1. Introduction of RW Cropping System

The rice–wheat cropping system occupies an area of 10.0 million ha in the Indo-Gangetic plains of India and plays a crucial role in the food security of the region^[1]. In this region, rice is traditionally established as puddling, followed by hand transplanting of rice seedling and consecutive flooding^[2] whereas ploughing is the dominant tillage practice for establishment of wheat^[1]. During the 1990s, due to scarcity of water, energy and labor resources vis-à-vis enhanced production cost and diminishing farm profits, initiatives were taken for adoption of resource conservation technologies^[3] such as Direct Seeding of Rice (DSR) in Conventional-Till (CT)/Zero-Till (ZT) field, and ZT establishment in case of wheat. Adoption of DSR overcame the problems of labor and water scarcity associated with traditional rice cultivation; further, it matured seven to ten days earlier, facilitating timely sowing of succeeding wheat crop^[4]. However, the change in land preparation practices i.e., CT to ZT not only influenced the weed species composition and level of infestation, which subsequently emerged with DSR crop, but also affected the Weed Seed Bank (WSB) dynamics. Actually, in India, in economic terms, weeds in rice field cause a loss of 4420 million USD^[5]. This too differs with the Crop Establishment (CE) method; for example, uncontrolled weed growth reduces the yield of transplanted rice by 12 per cent, whereas dry seeded rice sown without soil tillage leads to 98 per cent reduction in crop yield^[6].

The WSB, comprised of the weed seeds in different soil profile as well as lying on soil surface, is the principal source of annual weed infestation in field crops^[7]. The literature reveals that most of the studies on the influence of crop establishment methods on weed dynamics in rice were based only on the above ground emerged weed flora^[8], rather than giving due consideration to the WSB dynamics in different soil profiles. In fact, WSB regulates the weed communities and better indicator of long term influence of agronomical practices on weeds rather than the above ground vegetation^[9]. The prime objective of weed management in arable crop production is to reduce the WSB in long-term^[7]. In general, in the tropical region of India, only limited studies have been conducted on the WSB dynamics of agroecosystems^[9].

2. Weed Seed Bank Changes in RW Cropping System

Change in the crop establishment methods alters the soil ecology, while affecting the soil nutrient, soil structure and temperature, as well as the depth of burial of weed seeds, which ultimately affects the germination of weed species and its composition^[10]. In fact, level of soil disturbance affects the weed species richness, abundance and density of weeds^[11]. Species diversity within weed communities and the nature of their relationship are of agronomic importance^[12]. Gallandt et al.^[13] are also of the opinion that No-Till (NT) system do not enhance the microbial decay of weed seeds; moreover, proportions of dead or decayed seeds are similar under NT and CT system.

the abundance of predominant weeds under CTPTR-CTW it is likely to suppose the inherent biology of these weeds to survive under PTR condition. Secondly, due to repeated puddling operations over the year in CTPTR-CTW, weed seed, particularly grassy weeds, are distributed in the deeper soil horizon and remain unaffected. Moreover, after the harvest of rice, for the sowing of CTW, the tillage operation is performed only up to the depth of 15 cm; thus, seeds lying in surface soil (i.e., only up to 15 cm) are vulnerable to predation by resident insects, rodents, birds and other organisms, while the seeds in deeper horizon remain unaffected. Carabid beetles are highly active on the soil surface and may be particularly important as weed seed predators^[14]. Furthermore, in CTDSR-CTW, in general, the density of all the weeds seeds are reduced, and interestingly the seed of DISA was absent. It might be due to DISA’s natural short-term persistence seed bank. It means seeds remain viable in the soil only for one to five years^[15] and are lost thereafter. Due to the absence of the puddling process in CTDSR-CTW, all the weed seeds including DISA, remain lying on the top layer of the soil and are exposed during conventional tillage operation of wheat sowing, thus resulting in rapid loss of the seeds. Punia et al.^[16] also noticed higher weed seed density under CTPTR-CTW as compared to ZTR-CTW, CTR-ZTW, ZTR-ZTW and minimum tillage rice (MTR)-ZTW; further, higher numbers of seeds are distributed up to 10 cm soil depth.

CTDSR-CTW and CTDSR-ZTW show similar WSB, this might be due to the fact that tillage operations before the sowing of DSR exhume buried weed seed to the shallow layers, thus forcing the weed seed suffer from harsh atmospheric conditions like high humidity and moist soil condition, and favors the microbial decay of weed seeds, as well as it exposes them to predation; this might be the reasons for the enhanced mortality and reduced density of WSB. Robert^[17], Robert and Feast^[18] also observed that weed seeds in shallow soil layers or exhumation of the buried seeds to the shallow layers increase the chances of mortality of the weed seeds as compared to the deeper ones. Actually, the burial of weed seeds in deeper layers provide more or less constant soil conditions, and longevity of these seeds may be more closely related to the adaptations by the seeds to this condition itself ^[19].

In fact, higher biodiversity is a sign of healthy ecosystem, and will not only increase the stability and productivity of the system but also reduce the susceptibility of invasion to the non-native species. In near future the agronomic implications of lower diversity under CTDSR-CTW have led to dominance of a few competitive, highly adapted, widely distributed weed species, having similar resource requirements to the crop^[20] that may pose difficulty in its management as compared to the system being dominated by a few problematic weeds^[21]. Moreover, under situation of increased population of dominant weed species, the farmers are more reliant on use of fewer selective herbicides, which would lead to increase in selection pressure and development of herbicide resistant dominant weeds^[22]. Currently, many of the world’s cropping system showed herbicide resistance, such that as weed diversity has declined^[20] that pose threat to the ongoing sustainability of the whole system. Overall, diversity of the crop, cropping system, weeds and weed management practices, result in better exploitation of available resources ^[21].

This entry is adapted from 10.3390/agronomy10020292