Kauser A. Malik

Nitrogen Fixation with Non-Legumes

Acetobacter diazotrophicus, a diazotrophic endophyte of sugarcane, carries a cluster of nifgenes similar to those characterized in other Proteobacterial diazotrophs. These include, in order, nifA, nifB, nifHDKEN, nifUSVW andfixABC. This order of nifand fix gene organization is most similar to that found in Azospirillum brasilense, although the deduced gene products of each gene are generally more similar to those characterized in species of Rhizobium, Bradyrhizobium or Azorhizobium. Downstream of fixABC is a gene encoding a protein with similarity to MCPA, a methyl-accepting chemotaxis protein which is involved in chemotatic responses to extracellular signals. mcpA gene may play an important role in symbiosis and plant-microbe interaction by allowing colonization of sugarcane tissues far distant from the site of A. diazotrophicus infection or entry.

Transformations in soil and availability to plants of15N applied as inorganic fertilizer and legume residues

SummaryA pot experiment was conducted to study the transformations of organic and inorganic N in soil and its availability to maize plants. Inorganic N was in the form of15N labelled ammonium sulphate (As) and15N labelledSesbania aculeata (Sa), a legume, was used as organic N source. Plants utilized 20% of the N applied as As; presence of Sa reduced the uptake to 14%. Only 5% of the Sa-N was taken up by the plants and As had no effect on the availability of N from Sa. Losses of N from As were found to be 40% which were reduced to 20% in presence of Sa. Losses of N were also observed from Sa which increased in the presence of As. Application of As had no effect on the availability of soil or Sa-N. However, more As-N was transported into microbial biomass and humus components in the presence of Sa.Plants derived almost equal amounts of N from different sourcesi.e., soil, Sa and As. However, more As-N was transported into the shoots whereas the major portion of nitrogen in the roots was derived from Sa.

Transformation of 14C labelled plant components in soil in relation to immobilization and remineralization of 15N fertilizer

SummaryUniformly14C labelled glucose, cellulose and wheat straw and specifically14C labelled lignin component in corn stalks were aerobically incubated for 12 weeks in a chernozem soil alongwith15N labelled ammonium sulphate. Glucose was most readily decomposed, followed in order by cellulose, wheat straw and corn stalk lignins labelled at methoxyl-, side chain 2-and ring-C. More than 50% of14C applied as glucose, cellulose and wheat straw evolved as CO2 during the first week. Lignin however, decomposed relatively slowly. A higher proportion of14C was transformed into microbial biomass whereas lignins contributed a little to this fraction.After 12 weeks of incubation nearly 60% of the lignin14C was found in humic compounds of which more than 70% was resistant to hydrolysis with 6N HCl. Maximum incorporation of15N in humic compounds was observed in cellulose amended soil. However, in this case more than 80% of the15N was in hydrolysable forms.Immobilization-remineralization of applied15N was most rapid in glucose treated soil and a complete immobilization followed by remineralization was observed after 3 days. The process was much slow in soil treated with cellulose, wheat straw or corn stalks. More than 70% of the newly immobilized N was in hydrolysable forms mainly reepresenting the microbial component.Serial hydrolysis of soil at different incubation intervals showed a greater proportion of 6N HCl hydrolysable14C and15N in fractions representing microbial material.14C from lignin carbons was relatively more uniformly distributed in different fractions as compared to glucose, cellulose and wheat straw where a major portion of14C was in easily hydrolysable fractions.

Effect of salinity and inoculation on growth, nitrogen fixation and nutrient uptake ofVigna radiata (L.) Wilczek

This study reports the effect of salinity and inoculation on growth, ion uptake and nitrogen fixation byVigna radiata. A soil ECe level of 7.5 dS m−1 was quite detrimental causing about 60% decline in dry matter and grain yield of mungbean plants whereas a soil ECe level of 10.0 dS m−1 was almost toxic. In contrast most of the studied strains of Rhizobium were salt tolerant. Nevertheless, nodulation, nitrogen fixation and total nitrogen concentration in the plant was drastically affected at high salt concentration. A noticeable decline in acetylene reduction activity occurred when salinity level increased to 7.5 dS m−1.

Effect of humic acid on wheat (Triticum aestivum L.) seedling growth

Abstract The effect of humic acid on the growth of wheat seedlings in the presence and absence of nitrogen (N) was studied. Small concentrations (54 mg/l) of humic acid in the water medium resulted in a 500% increase in root length. Fresh and dry weight of roots also increased significantly due to humic acid. Dry matter yield of shoots increased by 22% in the presence of 54 mg/l humic acid. Moisture uptake and N content increased significantly. In N-free medium, humic acid caused a significant increase in the growth of roots and shoots as well as moisture uptake and N content of the seedlings. A maximum growth effect was obtained at 54 mg/l humic acid. In the presence of N, root and shoot growth was retarded. However, N uptake by the seedlings was enhanced by 22% in the presence of 54 mg/l humic acid.

Microbial biomass and mineralization-immobilization of nitrogen in some agricultural soils

SummaryThe chloroform fumigation-incubation method (CFIM) was used to measure the microbial biomass of 17 agricultural soils from Punjab Pakistan which represented different agricultural soil series. The biomass C was used to calculate biomass N and the changes occurring in NH4+-N and NO3−-N content of soils were studied during the turnover of microbial biomass or added C source. Mineral N released in fumigated-incubated soils and biomass N calculated from biomass C was correlated with some N availability indexes.The soils contained 427–1240 kg C as biomass which represented 1.2%–6.9% of the total organic C in the soils studied. Calculations based on biomass C showed that the soils contained 64–186 kg N ha−1 as microbial biomass. Immobilization of NCO3−-N was observed in different soils during the turnover of microbial biomass and any net increase in mineral N content of fumigated incubated soils was attributed entirely to NH4+-N.Biomass N calculated from biomass C showed non-significant correlation with different N availability indexes whereas mineral N accumulated in fumigated-incubated soils showed highly significant correlations with other indexes including N uptake by plants.

Inmobilization-remineralization of NO3-N and total N balance during the decomposition of glucose, sucrose and cellulose in soil incubated at different moisture regimes

A laboratory incubation experiment was conducted to study the effect of organic amendment and moisture regimes on the immobilization-remineralization of NO3-N and total N balance in soil fertilized with KNO3. Immobilization of NO3-N was very rapid in soil amended with glucose and sucrose followed by a remineralization of organic N and accumulation of mineral N. Cellulose caused a slow but continued immobilization and did not show net accumulation of mineral N during 8 weeks of incubation. At the end of incubation, a significant increase in total N and organic N content of the soil was observed which is perhaps attributable to the activity of free living N2 fixers. Although N losses seemed to have occurred at 100% WHC through denitrification in soil amended with glucose and sucrose, main cause of NO3 elimination was microbial immobilization.

Seed germination and salinity tolerance in plant species growing on saline wastelands.

Seven plant species including three chenopods:Suaeda fruticosa, Kochia indica, Atriplex crassifolia and four grasses:Sporobolus arabicus, Cynodon dactylon, Polypogon monspeliensis, Desmostachya bipinnata, varied greatly in their seed germination and growth responses to soil moisture or salinity. The germination percentage of each species was significantly lower at soil moisture level of 25 % of water holding capacity than at the levels ranging from 50 to 125 %. Increase in salinity resulted in gradual decrease in seed germination of each species. Growth responses of species to salinity varied widely from significant decrease with slight salinity to stimulation up to salinity levels of 20 dS m-2. Higher K+Na+ratios in plant shoots of all species compared to that in the root medium indicated selective K+uptake. Higher tolerance in chenopod species seems to be attendant on their ability for internal ion regulation.

Amelioration of a saline sodic soil through cultivation of a salt-tolerant grass Leptochloa fusca

SUMMARY Reclamation of saline lands seems difficult for climatic and economic reasons, but cultivation of salttolerant plants is an approach to increasing productivity and improvement of salt-affected wastelands. A five-year field study was conducted to evaluate the effects of growing a salt-tolerant species Leptochloa fusca (L.) Kunth (kallar grass) on chemical properties of a saline sodic soil irrigated with poor quality groundwater. Soil salinity, sodicity and pH decreased exponentially by growing kallar grass as a result of leaching of salts from surface (0‐20 cm) to lower depths (� 100 cm). Concentrations of soluble cations (Na � ,K � ,C a 2� and Mg 2� ) and anions (Cl � ,S O 4 2� and HCO 3 � ) were reduced through to greater soil depths. A significant decline in soil pH was attributed to release of CO 2 by grass roots and solublization of CaCO 3 . Both soil salinity and soil pH were significantly correlated with Na � ,C a 2� ,M g 2� ,K + ,C l � , HCO 3 � and sodium adsorption ratio (SAR). Significant correlations were found between soluble cations (Na � ,C a 2� and K � ), soluble anions (Cl � ,S O 4 2� and HCO 3 � ) and the SAR. In contrast, there were negative correlations between soil organic matter content and all chemical properties. The ameliorative effects on the soil chemical environment were pronounced after three years of growing kallar grass. Cultivation of kallar grass enhanced leaching and interactions among soil chemical properties and thus restored soil fertility. The soil maintained the improved characteristics with further growth of the grass up to five years suggesting that growing salt-tolerant plants is a sustainable approach to biological amelioration of saline wastelands.

Attachment, colonization and proliferation ofAzospirillum brasilense andEnterobacter spp. on root surface of grasses

Root colonization studies, employing immunofluorescence and using locally isolated strains, showed thatEnterbacter sp. QH7 andEnterobacter agglomerans AX12 attached more readily to the roots of most plants compared withAzospirillum brasilense JM82. Heat treatment of either root or inoculum significantly decreased the adsorption of bacteria to the root surface. Kallar grass and rice root exudates sustained the growth ofA. brasilense JM82,Enterobacter sp. QH7 andE. agglomerans AX12 in Hoagland and Fahraeus medium. All the strains colonized kallar grass and rice roots in an axenic culture system. However, in studies involving mixed cultures,A. brasilense JM82 was inhibited byEnterobacter sp. QH7 in kallar grass rhizosphere and the simultaneous presence ofEnterobacter sp. QH7 andE. agglomerans AX12 suppressed the growth ofA. brasilense JM82 in rice rhizosphere. The bacterial colonization pattern changed from dispersed to aggregated within 3 days of inoculation. The colonization sites corresponded mainly to the areas where root mucigel was present. The area around the point of emergence of lateral roots usually showed maximum colonization.