Biotoxic effects of pesticides on symbiotic properties of Rhizobial sps

Microbial and genetic ecology of tropical Vertisols under intensive chemical farming

There are continued concerns on unscientific usage of chemical fertilizers and pesticides, particularly in many developing countries leading to adverse consequences for soil biological quality and agricultural sustainability. In farmers' fields in tropical Vertisols of peninsular India, "high" fertilizer and pesticide usage at about 2.3 times the recommended rates in black gram (Vigna mungo) did not have a deleterious effect on the abundance of culturable microorganisms, associative nitrogen fixers, nitrifiers, and 16S rRNA gene diversity compared to normal rates. However, "very high" application at about five times the fertilizers and 1.5 times pesticides in chilies (Capsicum annuum) adversely affected the populations of fungi, actinomycetes, and ammonifiers, along with a drastic change in the eubacterial community profile and diversity over normal rates. Actinobacteria were dominant in black gram normal (BG1) (47%), black gram high (BG2) (36%), and chili normal (CH1) (30%) and were least in chili very high (CH2) (14%). Geodermatophilus formed 20% of Actinobacteria in BG1 but disappeared in BG2, CH1, and CH2. Asticcacaulis dominated at "very high" input site (CH2). Diversity of nitrogen fixers was completely altered; Dechloromonas and Anaeromyxobacter were absent in BG1 but proliferated well in BG2. There was reduction in rhizobial nifH sequences in BG2 by 46%. Phylogenetic differences characterized by UniFrac and principal coordinate analysis showed that BG2 and CH2 clustered together depicting a common pattern of genetic shift, while BG1 and CH1 fell at different axis. Overall, there were adverse consequences of "very high" fertilizer and pesticide usage on soil microbial diversity and function in tropical Vertisols.

In vitro sensitivity of Rhizobium and phosphate solubilising bacteria to herbicides

Nitrogen fixing bacteria, rhizobia and phosphate solubilizing bacteria (PSB) are the commonly applied microbial inoculants in grain legumes (Pulses). It is important to apply herbicides to control weeds in order to augment yield of the crop. The herbicides may however, be incompatible with the microbial inoculants. This study compared the effect of the recommended pre-plant incorporated herbicide, fluchloralin (20.25 × 10(4) ppm) and pre-emergence herbicide, pendimethalin in two doses (9 × 10(4) and 15 × 10(4) ppm) on the growth and survival of mungbean Rhizobium and PSB, under laboratory conditions. These herbicides were also used under field conditions in conjunction with biofertilizers (R, PSB) to improve grain yield of mungbean. It was found that fluchloralin (20.25 × 10(4) ppm) and the lower dose of pendimethalin (9 × 10(4) ppm) had no adverse effect on growth of Rhizobium and PSB. The higher dose of pendimethalin (15 × 10(4) ppm) was safe on PSB but it imposed a retarding effect on the growth of Rhizobium.

Herbicide use, productivity, and nitrogen fixation in field pea (Pisum sativum)

Grain legumes grown in low-rainfall (<300 mm per annum) cropping regions of southern Australia have at times failed to provide the rotational benefits observed in other regions, such as improved cereal yields in the season following a legume. ‘In-crop’ herbicides were identified as one possible factor that may have been negatively affecting the legume–rhizobia symbiosis. To test this hypothesis and identify possible mechanisms behind any observed effects, field trials were conducted at Waikerie (South Australia) in 2001, 2003, and 2004. Field pea (Pisum sativum L.) was grown and treated with one of several herbicides 5 weeks after sowing. Crop yellowing, biomass, nodulation, and nitrogen (N2) fixation were assessed 3 weeks after spraying, and biomass, yield, percent nitrogen derived from fixation (%Ndfa), and N2 fixation (2003, 2004) were assessed at the end of the season. Some herbicides stunted plant growth and caused crop yellowing 3 weeks after application; however, none of the herbicides affected N nutrition of peas. Despite this, in 2003, half of the herbicides assessed reduced the %Ndfa by 34–60% relative to unsprayed control plots. Herbicide effects on the measured parameters followed similar trends over each year of the 3-year study. However, effects were rarely significant in 2004 as the trials were primarily affected by low rainfall, indicating that environmental parameters play a key role in determining the severity of herbicide effects on symbiotic N2 fixation. The possible mechanisms behind herbicide-induced damage to the pea–rhizobium symbiosis are discussed, including reduced photosynthetic capacity of plants exposed to herbicides.

In vitro studies on the effects of herbicides on the growth of rhizobia

AIMS

To study the possible adverse effect of herbicides on nodulation and nitrogen fixation in legumes by affecting the nitrogen-fixing rhizobia.

METHODS AND RESULTS

Experiments were conducted to study the effect of four herbicides (terbutryn/terbuthylazine, trietazine/simazine, prometryn and bentazone) on the growth of nitrogen-fixing pea rhizobia (Rhizobium leguminosarum) in vitro by measuring optical density. Terbutryn/terbuthylazine, trietazine/simazine and prometryn had an adverse effect on the growth of rhizobia whereas bentazone was safe to rhizobia.

CONCLUSIONS

The above herbicides could be used in pea at the recommended rates.

SIGNIFICANCE AND IMPACT OF THE STUDY

The adverse effects of herbicides on rhizobia were observed at concentrations not normally expected to occur under field conditions. Further, previously observed adverse effects of these herbicides on nodulation and nitrogen fixation of peas were, possibly, not due to their effects in rhizobia but to their adverse effects on the plant growth itself.