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Plant-microbial interaction in rhizosphere plays vital role in shaping plant’s growth and ecosystem function. Most of the rhizospheric microbial diversity studies are restricted to bacteria. In natural ecosystem, archaea also constitutes a major component of the microbial population. However, their diversity is less known compared to bacteria. Experiments were carried out to examine diversity of bacteria and archaea in the rhizosphere of bioenergy crop Jatropha curcas (J. curcas). Samples were collected from three locations varying widely in the soil physico-chemical properties. Diversity was estimated by terminal restriction fragment length polymorphism (TRFLP) targeting 16S rRNA gene of bacteria and archaea. Fifteen bacterial and 17 archaeal terminal restriction fragments (TRFs) were retrieved from J. curcas rhizosphere. Bacterial indicative TRFs were Actinobacteria, Firmicutes, Acidobacteria, Verrumicrobiaceae, and Chlroflexi. Major archaeal TRFs were crenarchaeota, and euryarchaeota. In case of bacteria, relative fluorescence was low for TRF160 and high for TRF51, TRF 420. Similarly, for archaea relative fluorescence of TRF 218, and TRF 282 was low and high for TRF 278, TRF468 and TRF93. Principal component analysis (PCA) of bacterial TRFs designated PC 1 with 46.83% of variation and PC2 with 31.07% variation. Archaeal TRFs designated 90.94% of variation by PC1 and 9.05% by PC2. Simpson index varied from 0.530 to 0.880 and Shannon index from 1.462 to 3.139 for bacteria. For archaea, Simpson index varied from 0.855 to 0.897 and Shannon index varied from 3.027 to 3.155. Study concluded that rhizosphere of J. curcas constituted of diverse set of both bacteria and archaea, which might have promising plant growth promoting activities.

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The processes regulating nitrification in soils are not entirely understood. Here we provide evidence that nitrification rates in soil may be affected by complexed nitrate molecules and microbial volatile organic compounds (mVOCs) produced during nitrification. Experiments were carried out to elucidate the overall nature of mVOCs and biogenic nitrates produced by nitrifiers, and their effects on nitrification and redox metabolism. Soils were incubated at three levels of biogenic nitrate. Soils containing biogenic nitrate were compared with soils containing inorganic fertilizer nitrate (KNO 3 ) in terms of redox metabolism potential. Repeated NH 4 –N addition increased nitrification rates (mM NO 3 1- produced g -1 soil d -1 ) from 0.49 to 0.65. Soils with higher nitrification rates stimulated ( p < 0.01) abundances of 16S rRNA genes by about eight times, amoA genes of nitrifying bacteria by about 25 times, and amoA genes of nitrifying archaea by about 15 times. Soils with biogenic nitrate and KNO 3 were incubated under anoxic conditions to undergo anaerobic respiration. The maximum rates of different redox metabolisms (mM electron acceptors reduced g -1 soil d -1 ) in soil containing biogenic nitrate followed as: NO 3 1- reduction 4.01 ± 0.22, Fe 3+ reduction 5.37 ± 0.12, SO 4 2- reduction 9.56 ± 0.16, and CH 4 production (μg g -1 soil) 0.46 ± 0.05. Biogenic nitrate inhibited denitrificaton 1.4 times more strongly compared to mineral KNO 3 . Raman spectra indicated that aliphatic hydrocarbons increased in soil during nitrification, and these compounds probably bind to NO 3 to form biogenic nitrate. The mVOCs produced by nitrifiers enhanced ( p < 0.05) nitrification rates and abundances of nitrifying bacteria. Experiments suggest that biogenic nitrate and mVOCs affect nitrification and redox metabolism in soil.

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Biochar (BC) application to agricultural soil has been proposed as an effective countermeasure to mitigate climate change. A laboratory incubation experiment was carried out to gain insight into the effectiveness of BC on methane (CH4) consumption in a tropical clayey vertisol. Except for the control treatment, BC of two different sizes (<0.25 or 0.25–2.00 mm) was mixed with vermicompost (VC), poultry manure (PM) or farmyard manure (FYM). BC and organic amendment were added to soil at 10% w/w and 80 kg N/ha, respectively. BC increased CH4 consumption rate, k, in soil, irrespective of organic amendment type. The CH4 consumption potential of soil was greater with the smaller size BC (<0.25 mm). Of the three organic amendments, VC exhibited the highest k (0.105) followed by FYM (0.093) and PM (0.072). BC (<0.25 mm) + PM was the most effective of the organic amendments in enhancing CH4 consumption (k = 0.242). The lag phase varied between 7.3 day (control) and 1.0 day (soil + VC). Results revealed that there was a significant (P < 0.0001) effect of organic amendment and BC on CH4 consumption, CO2 production and microbial abundance. Cumulative CO2 production (mg/g soil) varied between 2.15 (control) and 8.77 (soil + PM + BC < 0.25 mm). Pearson's correlation analysis showed significant correlation between CH4 consumption and methanotrophs abundance (P < 0.001). The study shows that BC enhanced CH4 consumption potential in agricultural land on a tropical vertisol, particularly using the smaller size (<0.25 mm), and could be an effective strategy to mitigate atmospheric CH4.

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