Soil microbial and nutrient dynamics under different sowings environment of Indian mustard (Brassica juncea L.) in rice based cropping system

Farmers are not growing diversified crops and applying huge amounts of agrochemicals and imbalanced fertilizers in the rice-wheat cropping system (RWCS), since the 1960s. The objective of this study was to evaluate the microbial and nutrient dynamics in Indian mustard (Brassica juncea L.) under various sowing environments and nutrient sources during Rabi season (October–March), 2015–2016. The experiment was laid out in the split-plot design with three sowing dates in main-plots, and eight nutrient sources in sub-plots. The maximum bacteria, fungi, and actinomycetes population, soil microbial biomass carbon (SMBC), dehydrogenase activities, and available nitrogen, phosphorus, potassium, and sulphur (NPKS) were recorded on November 17 sown crop, and the lowest was observed on December 7 sowing during both the years, and in the pooled analysis. Furthermore, applied nutrient sources, highest bacteria, fungi, and actinomycetes population, available NPKS, SMBC, and dehydrogenase activity were observed in 75% recommended dose of fertilizers (RDF) + 25% N through pressmud (PM) + Azotobacto + phosphorus solubilizing bacteria (PSB) than other nutrient sources. In conclusion, high demand and cost of chemical fertilizers can be replaced by 25% amount easily and locally available organic manures like PM compost to sustain the soil health and crop productivity. It will be helpful to restore the soil biodiversity in the RWCS and provide a roadmap for the researchers, government planners, and policymakers for the use of PM as a source of organic matter and nutrients.

www.nature.com/scientificreports/ productivity of the system. PM compost is low cost in nature and easily available in sugarcane growing areas, which may be the best alternative of organic matter to farmyard manure (FYM). The current study was conducted with objectives: (1) to find out the effect of sowing dates and nutrient sources on microbial dynamics in the diversified cropping system, and (2) to find the effect of sowing dates and integrated nutrient sources on available nutrients i.e., NPKS dynamics after crop harvest. It is hypothesized that sowing dates and nutrients management can positively influence the microbial dynamics of the rhizosphere.

Results
Effect of sowing dates. The 6 CFU g −1 soil at harvest stage) were observed on December 7 sowing on the pooled basis. Among the sowing dates, the maximum dehydrogenase activity in Table 4 (48.66 µg TPF g −1 soil day −1 ) was recorded on November 17 sown crop followed by November 27 (47.02 µg TPF g −1 soil day −1 ), and lowest dehydrogenase activity (44.94 µg TPF g −1 soil day −1 ) was observed on December 7 sowing in the pooled analysis. The data (Table 4) was recorded significantly maximum SMBC (204.09 mg kg −1 soil) on November 17 sown crop followed by November 27 (197.22 mg kg −1 soil), and lowest SMBC (188.47 mg kg −1 soil) was observed on December 7 sowing in the pooled analysis. It is apparent from the data (  www.nature.com/scientificreports/     www.nature.com/scientificreports/ TPF g −1 soil day −1 ) was observed in control in the pooled analysis. Moreover, results showed in Table 4 that the highest SMBC (212.72 mg kg −1 soil) was observed in 75% RDF + 25% N through PM + Azotobactor + PSB, which was (31.21 mg kg −1 soil) higher than control in the pooled analysis. While, the application of 75% RDF + 25% N through PM + PSB and 75% RDF + 25% N through PM + Azotobactor response on SMBC (204.86 and 203.65 mg kg −1 soil) found at par with each other on a pooled basis, respectively. However, application of 100% RDF + Azotobactor + PSB was observed (Table 5) higher SMBC than 100% RDF + PSB, 100% RDF + Azotobactor, 100% RDF, and control during both the years and in the pooled analysis. It is apparent from the data presented in Table 4 that all the nutrient sources could not bring variation in organic C status of soil after harvest of Indian mustard up to the level of significance. A critical examination of the data indicated that among the nutrient sources, the highest available N (211.60 kg ha −1 ), available P (20.43 kg ha −1 ), available K (256.46 kg ha −1 ), and available S (10.79 mg kg −1 ) was observed in 75% RDF + 25% N through PM + Azotobactor + PSB in the pooled analysis, respectively than other nutrient sources. While, the application of 75% RDF + 25% N through PM + PSB and 75% RDF + 25% N through PM + Azotobactor was found at par with each other, respectively. However, application of 100% RDF + Azotobactor + PSB was observed the higher available N (15.74 kg ha −1 ), available P (1.52 kg ha −1 ), available K (19.08 kg ha −1 ), and available S (0.8 mg kg −1 ) than control in the pooled analysis, respectively. Moreover, the 100% RDF + PSB and 100% RDF + Azotobactor effect on available NPKS was found at par with each other during both the years and in the pooled analysis, respectively.

Discussion
Effect of sowing dates. Suitable soil environmental conditions with sustained soil fertility are the key tools that help in catching the full potential of the growing crops in terms of higher growth and production. The data of microbial population and dehydrogenase activity (Tables 1, 2, 3, 4) revealed that different sowing dates had a significant effect on microbial population. Among the sowing dates, the maximum population of bacteria, fungi, actinomycetes, and dehydrogenase activity were recorded on November 17 sown crop during both the growing periods and after harvest of Indian mustard, which significantly differed from other sowing dates. The bacterial population (Table 3) showed a gradual decreased with an increased gap of sowing date from November 17 to December 7. The microbial population peaked during winter followed by spring and autumn, respectively. Tables 1, 2, 3 indicated that the microbial population was significantly increased in the first sown crop. It may be due to the microbial population was significantly influenced by temperature and OM content of the soil, which was found suitable in the first sowing. At the time of harvesting lower bacterial population was observed in the last sowing due to initially very low decomposition of OM in the soil which results in the low microbial population in later stages and also decreases in C food to the microbes 10,25 . Enzymatic activity (Table 4) was found to be higher in the soil of the first sown crop due to soil enzymes providing insights into biogeochemical cycling of C and other nutrients to the microbial functions 16,26 . The data of available nutrients presented in Figs. 1, 2, 3 and 4, revealed that different sowing dates significantly affected the available nutrients after harvest of Indian mustard. Among the sowing dates, the maximum available NPKS and SMBC were recorded on November 17 sown crop in the pooled analysis that may be due to the longer growth period of the crop in which more mineralization of soil organic matter takes place as compared to other sowing dates. First sown crop increased the level of available N (Fig. 1), this may be due to high microbial population in soil (Tables 1, 2, 3) results in high mineralization of organic manure, which improves the physicochemical properties of soil and also improves the availability of the nutrients to the crop to fulfilling the crop requirement and ultimately improved the soil fertility. Available P of soil (Fig. 2) significantly improved with the first sown crop after harvest may be due to the higher rate of mineralization and favourable condition for microbial activity as well as chemical activity 25 . Available K of soil was (Fig. 4) improved with the application of nutrients might be due long growth period providing more opportunities to more mineralization of potassium in soil on first sown crop 20 . The higher available S content in soil (Fig. 4) could be attributed to greater mineralization and release of S as SO 4 2− ions from organic material after its oxidation due to suitable soil temperature and soil health in the first sown crop 27 . Likewise, the increase in the SMBC in soil (Table 4) with first sowing was probably due to the favourable moisture and temperature conditions and increased decomposition of SOM providing more source of utilization for microbes, which was attributed to greater SMBC 16,28 . Effect of nutrient sources. The data on microbial population (Tables 2, 3, 4) indicated that the significantly highest population of bacteria, fungi, actinomycetes, and dehydrogenase activity were observed in 75% RDF + 25% N through PM + Azotobactor + PSB in the pooled analysis. It may be due to the bacterial population was influenced significantly by PM application with inorganic fertilizers and microbial inoculation, which significantly increased the microbial population over all other nutrient sources. At harvest lower microbial population was observed as compared to the 90 DAS stage of Indian mustard due to decomposition of OM more rapidly at peak growth stages and reduced availability of nutrients compared to the flowering stage of Indian mustard 29,30 . It may be due to the microbial breakdown debris and help to fasten the decomposition process, and enhanced content of total N from the PM, ultimately multiplying very fast. Soil enzymes (dehydrogenase) function as a measurement of the metabolic condition of soil microbes by relating it to the occurrence of feasible microorganisms and their oxidative capability 31 . Data on maximum dehydrogenase activities after harvesting of the crop (Table 3) observed with 75% RDF + 25% N through PM + Azotobactor + PSB. It may be due to higher OM supplied with PM, and fast decomposition. The dehydrogenase activity increased with the addition of PM could be attributed to increased microbial activities, which also encourage the dehydrogenase activity 32 .
The data on available nutrients were significantly influenced by nutrient sources. The available N significantly increased after harvest in soil (Fig. 1). It may be due to the mineralization of PM in soil and the initial availability of N through chemical fertilizers and N fixation with Azotobactor. The PM is a store house of all the essential www.nature.com/scientificreports/ plant nutrients required for crop growth. PM application improved the soil environment by improving the physicochemical and biological properties of soil 33 . The availability of most of the essential plant nutrients increased owing to the improvement in pH and the cation exchange capacity (CEC) of soil. The available P content (Fig. 2) of soil increased significantly with the application of 75% RDF + 25% N through PM + Azotobactor + PSB might be due to the greater solubilization and mobilization of fixed native soil P by micro-organisms (PSB), vigorous root proliferation and contribution through biomass. The available K content of soil increased significantly with the application of 75% RDF + 25% N through PM + Azotobactor + PSB. The buildup of available K in soil under PM application resulted in additional K supplied through it. The solubilizing action of various organic acids released during the decomposition of PM, and it has a better capacity to hold K in the soluble form. The availability of S content (Fig. 4) in soil significantly increased because PM is a good source of S in soil and the slow release nature of PM as OM resulted in higher residual S availability. Blending of 25% N through Pressmud and 75% NPKS through fertilizers could only meet the nutrient demand of the crop resulted in higher uptake and yield of Indian mustard. The SMBC (Table 4) of soil significantly increased with the application of 75% RDF + 25% N through PM + Azotobactor + PSB. It may be due to the microbial biomass which depends on the type of OM added, and the organic source being able to supply essential nutrients to soil fauna and to the plants 34,35 . The addition of organic C with the PM application stimulated microbial population and growth thus mineralization of nutrients in the PM [36][37][38] . Although plants and microbes may at the start compete for nutrients, the organic source supplied adequate nutrients for plants and soil microbes, particularly PM which had high nutrients concentration [39][40][41][42] . Further, microbial decomposition could release essential nutrients for plant uptake when the readily available organic C from the PM [43][44][45][46] .

Conclusion
Based on the two years experiment on diversified Indian mustard, it is concluded that the best treatment combination response on the microbial population, enzymatic activities, SMBC and available nutrients after harvest of the crop as follows: • Diversified Indian mustard sowing in the first fortnight of November sowing is a novel approach in the intensive rice-based cropping system. • Results reveal that the crop sowing in the first fortnight of November and applied nutrient source as 75% RDF + 25% N through PM + Azotobactor + PSB significantly increased all the recorded parameters except organic C content. • High demand and cost of chemical fertilizers can be replaced 25% amount easily and locally available organic manures like PM compost to sustain the soil health and crop productivity. • This study will help the scientific society, producers, and policymakers for planning to convert the industrial waste as a nutrient source with bio-inoculants to restore the soil health in RWCS.  47 . The total rainfall of 61.3 mm was received during the experimentation cropping period of the first year was higher than the second year of the experiment having 52.1 mm. The weekly mean maximum temperature (19 to 41.4 °C and 20.1 to 42.2 °C) and minimum temperature (7.2 to 29.9 °C and 8.2 to 27.3 °C) were recorded during both the years of experimentation, respectively. The average sunshine hours during the 2015-16 (6.43 h) was comparatively higher as compared to the 2016-17 (5.92 h). The texture of experimental soil was sandy clay loam, which was well-drained. Among the major nutrients, the soil was low in available N and P and medium in available K.

Materials and methods
Treatments and treatments and experimental design. The experiment was designed in a split-plot design (SPD) with three replications. In main-plots three sowing dates (November 17, 27 and December 7) and in sub-plots eight nutrient sources (control, 100%RDF (NPKS), 100%RDF + Azotobactor, 100%RDF + PSB, 100%RDF + Azotobactor + PSB, 75% RDF + 25% N through PM + Azotobactor, 75% RDF + 25% N through PM + PSB, 75% RDF + 25% N through PM + Azotobactor + PSB) were planned. RDF @100-50-50-40 NPKS kg ha −1 is recommended for this region, and urea, Di-ammonium phosphate (DAP), muriate of potash (MOP), and elemental S were used as a source of fertilizers, respectively. The experimentation field was cross ploughing by tractor-drawn cultivator with a rotavator and divided into blocks and plots according to the plan of the layout.
A full dose of PKS and half dose of N after adjusting with DAP was applied as a basal and the remaining half dose of N was top-dressed at 40 DAS through urea. Applied PM chemical composition is presented in Fig. 5.
In PM compost treatments 25% N was applied through PM compost which was thoroughly mixed in soil one week before sowing and the remaining 75% N was applied through fertilizer. The seed @ 5 kg/ha was used. Biofertilizers i.e. Azotobacter and PSB were used for seed treatment as per the standard procedure. The remaining agronomic practices are followed as per the crop requirement.
Soil analysis and recording the observations. Five soil samples were collected from each treatment to assess the nutrient and biological properties of the experimental soil. The samples were brought to the laboratory, processed, and subjected to standard chemical and biological analyses as presented in Table 5.  www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.