Rhizosphere Dehydrogenase Activity, Soil Organic Carbon and Post-Harvest Nutrient Status of Soil as Influenced by Hybrids, Planting Density and Fertility Levels of Maize in Irrigated Eco-System of Northern Karnataka

The soil sample of effective root zone of the maize crop with respect to soil dehydrogenase activity...


Introduction
Maize was the untouched crop of Indian green revolution; today it occupied a pride place among the cereals in India, because it is considered as promising option for diversifying agriculture in both rainfed and irrigated eco-systems. Its adoptability in various cropping systems as component crop made it to stand in third position after rice and wheat. In India, maize related research programs have now given focus on single cross hybrids. Since 1990, large number of single cross hybrids of normal and quality protein maize have been released and they made the spectacular difference in maize production. Further, it was well documented by earlier worker [1] that single cross hybrids were flexible to planting density and fertility levels. Increased response of recently developed maize hybrids to high density planting and elevated fertility levels encourages the grower to habituate mono-culturing of maize even in the nontraditional areas of maize, as a resultant of continuous cultivation of high biomass producing maize there is an apprehension of deteriorating soil resource base due to lack of diversity and nutrient mining.
Hence, to know the probable changes of soil chemical and biological properties present study was undertaken because, oil dehydrogenases are the major representatives of the Oxidoreductase enzymes class [2]. Among all enzymes in the soil environment, dehydrogenases are one of the most important and are used as an indicator of overall soil microbial activity [3] because they occur intracellular in all living microbial cells [4]. Moreover, they are tightly linked with microbial oxidoreduction processes [5]. Dehydrogenases play a significant role in the biological oxidation of soil organic matter by transferring hydrogen from organic substrates to inorganic acceptors [6]. Further, available nutrient status is another index which decides the crop growth and productivity. Hence, a field experiment and concurrent laboratory study was undertaken to assess the cause and effects of maize hybrids, elevated plant population and fertility levels on soil health and fertility was studies for two cropping seasons (Kharif 2013 and 2014).  Table 1. The soil of the experimental site was deep black soil. The soil was alkaline in reaction and low in organic carbon. The soil nutrient status of soil was low in available nitrogen (237 kg ha -1 ) and phosphorous (23 kg ha -1 ) and high in available potassium (427 kg ha -1 ) ( Table 1)

Soil Analysis
Soil samples were drown from the effective root zone of the maize crop (0-45 cm), freshly collected soil samples was subjected to quantification of dehydrogenase activity and was estimated by using 2, 3, 5 triphenyal tetrazolium chloride using 1 g of field moist soil (<2 mm particle size) and expressed as μg of triphenyl formazan (TPF) formed per gram of oven dry soil per 24 hours [8]. Further, for the post-harvest soil analysis for organic carbon and available nitrogen, phosphorous and potassium soil samples

Statistical Analysis
For the proper presentation of data all soil factors was analyzed statistically by standard analysis of variance (ANOVA) and differences were separated by standard error means (S.Em) using M-STAT-C software. To find out error mean sum of squares (EMSSq), Microsoft-Excel software (Microsoft Corporation, USA) was used and significant differences were determined at LSD (p = 0.05).

Dehydrogenase Activity
Dehydrogenase activity is the manifestation of total microbial load, which indicate the mineralization capacity of plant nutrient in soil. Hence, assessment of its activity at peak nutrient demanding stages of the maize crop like beginning silking (45 DAS) and grain filling stage (90 DAS) is necessary. Dehydrogenase activity at 45 and 90 days after sowing did not influenced by hybrids and planting density ( Table 2)  , respectively) these findings are in the close agreement with the findings of Nath et al. [9].

Soil Organic Carbon
Irrespective of the treatments imposed, the soil organic carbon content decreased over the years of experimentation when compared to initial soil status (Table 1 & 2). It could be due to no residue retention in the field after harvest of maize under intensive cropping practices. Particularly among the maize hybrids and between the plant population levels organic carbon content of the soil not differed significantly (Table 1)    Note: Means followed by same letter (S) with in a column have not significantly differed each other at 5 per cent probability level.

Available Nitrogen, Phosphorous and Potassium
Laboratory analysis on dynamics of available nitrogen, phosphorous and potassium status of the soil after harvest of maize is presented in the Variation in dry matter production by maize hybrids (Table 3) created clear gradients in available nutrient status of the soil which is in close agreement with the findings of Mohan Kumar et al. [10]. Similar opinion of elevated fertility levels increasing the available nutrient status of the soil after harvest of crop was also reported by Paramasivan et al. [11]. Note: Means followed by same letter (S) with in a column have not significantly differed each other at 5 per cent probability level.

Conclusion
Soil enzymes are the most sensitive part of the soil resource base, hybrids and planting density did not influence the dehydrogenase activity of the soil, whereas, successive elevation in fertility level decreases the soil dehydrogenase activity of soil. Cultivation of inefficient maize hybrids at lower planting density with application of higher levels of fertility levels buid up the soil nutrient status substantially.