Mercury uptake by Paspalum distichum L. in relation to the mercury distribution pattern in rhizosphere soil

Paspalum distichum L. was tested to evaluate their phytoremediation capacity for Hg contaminated soil through analyzing the dissipation of Hg in soil through a greenhouse study by using self-made rhizos box. Original soil samples were collected at Hg mining site with serious Hg contamination and a control site, respectively. Planting of P. distichum. L last for 60 days. Soil and plant samples were collected from four periods (0 d, 20 d, 40 d and 60 d) and soil samples were collected from five different rhizosphere distance in horizontal direction (0–2 cm, 2-4cm, 4-6cm, 6-8cm,8-10cm). The results showed that the presence of P. distichum. L significantly accelerated the Hg dissipation in soil compared with control. Hg concentration in the rhizospheric soil was affected by the plant growth period and the distance to the plant roots. The closer of soil to the root of P. distichum. L, the lower mercury concentration in soil. During the 60-day growing period, the concentrations of total Hg (THg) and methylmercury (MeHg) reduced by 45% and 64%, respectively, in the rhizosphere (0-2cm) of Hg contaminated soil. However, MeHg concentration was increased near the roots (0–4 cm) during the initial growing period (0–20 d), which may be attributed to the influence of root exudates. Root is the major part for Hg accumulation in P. distichum. L. The low ratio between Hg concentrations in underground and aboveground tissues indicated that it seemed difficult for Hg translocation from root to shoot. The highest THg (9.71 ± 3.09 µg·g− 1) and MeHg (26.97 ± 0.98 ng·g− 1) value in root of P. distichum. L were observed at the 20th day when P. distichum. L grown in Hg contaminated soil. The results of chemical fractions analyses showed that elemental Hg and residual Hg were the two major speciations followed by organic bound Hg in the Hg contaminated soil, which indicated the high bioavailability and ecological potential risk of Hg in Hg contaminated soil.

Paspalum distichum (knotgrass).

P. distichum is a fast-growing rhizomatous grass of wet areas. It has become a major weed of rice and many other crops, as well as occurring in uncropped wetlands in both its native and introduced regions. Its introduction to Europe, Asia and the Pacific is not well documented but apparently occurred many years ago. New records are reported in e.g. Indonesia, Spain and Croatia, suggesting that it continues to spread in countries to which it has been introduced.

Weed composition study on wheat and boro rice in research and farmers’ fields

A weed survey was conducted in Agronomy Field Laboratory, BAUFarm and farmers’ fields of Sutiakhali Natunchar village in Mymensingh district during the period from December 2015 to March 2016 to study the weed vegetation in wheat and boro rice. Ten fields were selected for each crop from each location. A quadrat of 0.25 m2 was placed randomly at four places in each plot and the weeds within the quadrat were identified and counted species-wise. The results revealed that the composition of weed species in two different crops was different and the composition of weed flora in each crop under different locations was also different. In wheat, a total number of 29 weed species belonging to 16 families were found from all three locations. The infesting weed species were 18 in Agronomy Field Laboratory, 16 species in BAU Farm and 19 species in farmers’ fields. From the five most dominant species, Polygonum hydropiper had the highest relative abundance value in both Agronomy Field Laboratory and BAU Farm and on the other hand, in farmers’ field, a new weed species, Eleusine indica was in the top most of the dominant list. The other weed species which were present in the dominant list of three locations were Cynodon dactylon, Cyperus rotundus, Paspalum distichum, Chenopodium album, Marsilea quadrifolia and Leucas aspera. In boro rice fields, among the three locations, 25 weed species were observed in total belonging to 13 families. Of which, 20 weed species were found in both Agronomy Field Laboratory and BAU Farm whereas in farmers’ fields, 21 weed species were identified. In both Agronomy Field Laboratory and BAU Farm, Eleocharis atropurpurea had the highest abundance value but in farmers’ field, Cyperus difformis occupied the highest value. The other dominant weed species were Leersia hexandra, Monochoria vaginalis, Echinochloa crusgalli, Paspalum distichum and Alternanthera philoxeroides found in three locations. Many common weeds were present in three locations in a crop but their frequency, uniformity, density and relative abundance value were different. The similarity index of weed species between wheat and boro rice indicated that the infesting weed species between the two crops was less associated. Thus the weed divergence in boro rice with wheat was high. From this study it was clear that the weed infestation was changed by crop and locations. In this study a little bit divergence and ranking of five most abundant weed species and low similarity index value of weed species in different crops under a specific area or among the area indicated that weed control measure should be adopted on crop basis in a specific area considering the dominant weed species.J. Bangladesh Agril. Univ. 15(2): 148-157, December 2017

Diversities of soil weed seed bank and rice yield performance in response to tillage method and weeding regime

Rice field was infested with eight weed species. Among the species found in conventionally tilled plots, the five dominant species ranked in order of importance value were Cynodon dactylon (L.), Paspalum distichum (L.), Cyperus rotundus (L.), Azolla pinnata (L.) and Cyanotis axillaris (L.) while in stale seedbed the rank was Paspalum distichum (L.), Cynodon dactylon (L.), Cyanotis axillaris (L.), Leersia hexandra (L.) and Cyperus rotundus (L.). There were 15 weed species found in the soil weed seed bank studied at the net house. Among them five species were common of rice field. Ten new species were found in the seed bank. Azolla pinnata (L.), Cyperus difformis (L.) and Paspalum commersonii (L.) of field rice were not found in net house seed bank. In the field, Cynodon dactylon (L.) was the dominant over Paspalum distichum (L.) but at the net house Paspalum distichum (L.) was dominant over Cynodon dactylon (L.). In seed bank under conventional tillage, 11 species were found consisting of 3559 heads count and in stale seedbed 12 species consisting 3826 heads were counted. Among the identified species, nine were common in both tillage practice. Parapholis strigose (Dumort.) and Phyllanthus niruri (L.) were present in the soil of conventional tillage but not in stale bed while Echinochloa colonum (L.), Fimbristylis miliaceae (L.) and Eragrotis gangetica (Roxb.) were found in stale bed but not in conventional tillage. At the rice field grass weeds were dominant over sedges and broad leaf under the both types of tillage. At the seed bank of net house, grass weeds were dominant over sedges and broadleaf under conventional tillage while under stale seedbed, sedges were dominant over broad leaf and grass. Around 7% higher grain was recorded from stale seedbed compared to conventional tillage which attributed from higher number of effective tillers hill-1, higher number of fertile grains panicle-1, and lower number of sterile grains panicle-1. Weed free condition yield the highest grains followed by the combination of pre and post-emergence herbicides and unweeded control yielded the lowest. Stale seedbed kept weed free yielded the highest followed by stale seedbed sprayed the two herbicides. Conventional tillage remained unweeded yielded the lowest grain.Progressive Agriculture 27 (1): 39-47, 2016