Plants are closely associated with soil, hence plant microbe interactions are not only essential for the plant in terms of their nutrition, growth promotion, bio-control, stress alleviation etc., but they also influence soil physical, chemical and biological properties through biogeochemical cycles (Velmourougane et al., 2017). Several reports related to agriculturally important microbial biofilms are available from India and Srilanka (Velmourougane et al., 2017). Extensive studies were performed with cyanobacteria based bacterial and fungal biofilms and their phytopathogenic activity (Prasanna et al., 2013). The agronomic potential of leguminous crops was improved due to application of biofilmed preparations of Anabaena/Trichoderma with different agriculturally useful bacteria/fungi (Prasanna et al., 2014). Swarnalakshmi et al. (2013) reported that the soil biological and chemical properties were enhanced due to application of cyanobacterial-based biofilms in the wheat rhizosphere. According to Prasanna et al. (2015) microbial biofilm inoculation in flooded and SRI (System of Rice Intensification) rice recorded differential effect on plant growth and soil nutrient dynamics. The results of our study revealed that, all the sixteen yeast isolates possessed the ability to produce biofilm. Four isolates produced biofilm in higher amounts than the reference strains Pseudomonas sp. and one isolate produced in equal amounts with that of the reference strain. From the results of our study it has been concluded that a better understanding of yeast biofilm formation in crop rhizosphere and development of more effective biofilmed biofertilizers are potential areas for future research to sustain the agricultural productivity.
The regular application of organic manure increases the soil microbial count, improves soil characteristics and involved in mineral transformation (Fernández et al., 1997; Bastida et al., 2008). The overall outcome of the experimental results showed some improvement in physical properties of soil like mean diameter, macro and micro-aggregates, dry aggregation ratio and water stable aggregates with the application of yeast isolates than the control (uninoculated) soil. Investigation of results revealed that the application of yeast inoculants contribute more to micro-aggregate formation. The possible mechanism underlying for this may be cementation of soil micro-aggregates due to EPS production by the inoculated yeast isolate which led to increase in micro-aggregate formation. Garcia-Franco et al. (2015) have stated that formation of micro-aggregates within macro-aggregates may increase soil stability. Microaggregates (< 250 µm) are formed from organic molecules i.e., polysaccharides attached to clay (Cl) and polyvalent cations (P) to form compound particles (Cl–P–OM) (Tisdall, 1996). According to Le Bissonnais, (1996) unstable aggregations formed in soil led to low infiltration rate of soil. He also stated that the Mean Weight Diameter (MWD) of the soil aggregate at the range of 0.7 mm was considered as very unstable aggregates but 3.4 mm indicates very stable aggregates. In the present study, inoculation of OT3 8 and RT2 4 yeast isolates increased the MWD upto1.8 mm at 30 DAI and it is expected that it will be further improved with increase in incubation period.
Medina et al. (2004) reported that amendment of Yarrowia lipolytica is a useful tool to modify soil physico-chemical, biological and fertility properties that enhance the plant performance probably by making nutrients more available to plants.
Soil labile carbon makes up a fraction of the total carbon pool, but sensitive with turnover times of a few days to months. Soil labile carbon (SLC) (Parton et al., 1987) is one of the biological indicator for the soil biological quality index scaling. Li et al., (2018) reported that addition of organic manures increased the pool of stable carbon along with increased concentration and proportion of soil labile carbon in the soil surface layer. In our study, inoculation of OT3 12 yeast isolate and OT3 8 recorded significant increase in soil labile carbon status on 15 and 30 days of incubation period. Literatures support that labile carbon pool influences the decomposition rate and acts as a major source of nutrient for soil microbes. Increase in soil labile carbon in the present investigation revealed that, the microbial activity is higher in the soils inoculated with soil yeast isolates when compared to the uninoculated control.
Other than organic carbon, some amount of protein and protein like substances present in the soil is referred as soil protein index. Organically bound nitrogen present in the soil organic carbon which is known as protein index influences the storage of nitrogen and makes it available to plants. It is also associated with soil aggregation and make the availability of water to plant growth (Moebius-Clune, 2016). From the present investigation, in the yeast isolates inoculated soils the protein content was reduced over the period of incubation and this indicates that yeast isolates have utilized the proteins present in the soil as a source of nitrogen for its growth.
The soil organic carbon, representing total soil carbon content is a measure of all carbonaceous material derived from living organisms. It is also known as the representation of soil aggregation, water holding capacity, nutrient stocking and also the energy and carbon source for the microbial diversity of the soil. The Indian agricultural soils are generally recognized as low in soil organic matter (Lal, 2002). Soil organic carbon (SOC) serves as a source and sinks for nutrients and plays a major role in maintenance of soil fertility.
Diverse microorganisms present in soil ecosystem are responsible for decomposition of the organic carbon fraction like cellulose, lignin, hemicelluloses, chitin and lipids present in soil organic matter (Khatoon et al., 2017). Microbial biomass carbon and soil organic carbon ratio was highest in soil receiving continuous poultry manure, due to increased activity of microorganisms (Kaur et al., 2005). Moreover, green manuring is considered a good agricultural practice because of its positive effect on soil fertility, quality and biodiversity (Stark et al., 2007), because green manuring increases the abundance and diversity of microbes. The soil organic carbon was found to be increased in the soil inoculated with the soil yeast isolate OT3 8. The increase in soil organic carbon due to inoculation of soil yeasts reveals the increased activity of yeast isolates in the soil. Production of polysaccharides by the inoculated yeast isolates may possibly enhance water retention in the microbial environment and regulates the diffusion of carbon sources such as glucose.
Soil pH is one of the most influential factors that affect the soil microbial community. It affects the carbon availability, nutrient availability, and the solubility of metals.
Soil pH also controls many biotic factors like fungal and bacterial biomass
(Rousk et al., 2009). In the present study, there was a slight reduction in the pH of the soil after inoculation with the yeast isolates. Medina et al. (2004) reported that application of yeast strain Yarrowia lipolytica changed the pH of the soil from 8.90 to 8.75. This is in correspondence with the findings of our study. The slight reduction in the soil pH may be due to the production of organic acids by the soil yeast isolates.
Organic content of the soil is the representation of soil enzyme activity
(Beare and Bruce, 1993). Soil dehydrogenase activity is a good measure of microbial activity in the soil as it acts as the function of oxidative activity and microbial count
(Nannipieri et al., 2003). The dehydrogenase activity was directly proportional to microbial population, nutrient and organic content (Masto et al., 2007; Kumar et al., 2013) of the soil (Masto et al., 2006). The dehydrogenase activity was found to be positively influenced by the inoculation of soil yeasts. Our findings are in accordance with Medina et al. (2004),
who reported that in Yarowia lipolytica amended soil the dehydrogenase activity was increased. Hence, in the present study, the increase in dehydrogenase activity indicates the increment in microbial population and promotion of organic content by microbial activity. The dehydrogenase activity was also positively correlated with the other biological variables used in the study.
Microbial biomass carbon can act as a potential indicator to soil quality by responding to management practices. Application of farmyard manure increases the microbial biomass carbon of the soil as reported by (Marschner et al., 2003). Organic management increases the microbial biomass carbon and microbial activity in soil which tends to make nutrients available for crop growth (Zaman et al., 1999; Marinari et al., 2006; Tu et al., 2006; Wang et al., 2007). The application of organic manures in soil was found to increase the MBC, because the manures supply the essential nutrients for microbial growth. In our present study, a slight reduction in MBC was observed and this may be due to the exploitation of organic carbon source present in the soils for the growth of the yeast isolates.
Soil microorganisms produce various intra and extracellular polysaccharides,
which play an important role in the life of microorganisms and have great practical application. The properties of the exopolysaccharides contained in the extracellular polymeric substances (EPS) have been well documented, but the properties of colloidal polysaccharides produced by certain microorganism still remain unexplored (Markosyan et al., 2017). Wherever the multiplication of microorganisms is high, those soils contain high amount of soil polysaccharides and these have a major contribution to stabilize the soil aggregate formation. Microbiota of soil is highly responsible for soil polysaccharide synthesis. Hence it is very tedious process to separate the pure polysaccharide from the soil. The extraction of these polymers was related to soil stability and aggregation. In the present study it was evident that due to inoculation of yeast isolate OT3 8, the soil colloidal polysaccharide was increased. From the results of the present study it is evident that the soil yeast isolates possess the ability to produce soil colloidal polysaccharides. Markosyan et al. (2017) investigated the properties of a colloidal polysaccharide of the iron oxidizing chemolithotrophic bacteria Leptospirillum ferriphilum newly isolated in Armenia and reported that it consists of three monomers viz., glucose, fructose, mannose.