D. R. Biswas

An Environmentally Friendly Engineered Azotobacter Strain That Replaces a Substantial Amount of Urea Fertilizer while Sustaining the Same Wheat Yield

ABSTRACT In our endeavor to improve the nitrogen fixation efficiency of a soil diazotroph that would be unaffected by synthetic nitrogenous fertilizers, we have deleted a part of the negative regulatory gene nifL and constitutively expressed the positive regulatory gene nifA in the chromosome of Azotobacter chroococcum CBD15, a strain isolated from the local field soil. No antibiotic resistance gene or other foreign gene was present in the chromosome of the engineered strain. Wheat seeds inoculated with this engineered strain, which we have named Azotobacter chroococcum HKD15, were tested for 3 years in pots and 1 year in the field. The yield of wheat was enhanced by ∼60% due to inoculation of seeds by A. chroococcum HKD15 in the absence of any urea application. Ammonium only marginally affected acetylene reduction by the engineered Azotobacter strain. When urea was also applied, the same wheat yield could be sustained by using seeds inoculated with A. chroococcum HKD15 and using ∼85 kg less urea (∼40 kg less nitrogen) than the usual ∼257 kg urea (∼120 kg nitrogen) per hectare. Wheat plants arising from the seeds inoculated with the engineered Azotobacter strain exhibited far superior overall performance, had much higher dry weight and nitrogen content, and assimilated molecular 15N much better. A nitrogen balance experiment also revealed much higher total nitrogen content. Indole-3-acetic acid (IAA) production by the wild type and that by the engineered strain were about the same. Inoculation of the wheat seeds with A. chroococcum HKD15 did not adversely affect the microbial population in the field rhizosphere soil. IMPORTANCE Application of synthetic nitrogenous fertilizers is a standard agricultural practice to augment crop yield. Plants, however, utilize only a fraction of the applied fertilizers, while the unutilized fertilizers cause grave environmental problems. Wild-type soil diazotrophic microorganisms cannot replace synthetic nitrogenous fertilizers, as these reduce atmospheric nitrogen very inefficiently and almost none at all in the presence of added nitrogenous fertilizers. If the nitrogen-fixing ability of soil diazotrophs could be improved and sustained even in the presence of synthetic nitrogenous fertilizers, then a mixture of the bacteria and a reduced quantity of chemical nitrogenous fertilizers could be employed to obtain the same grain yield but at a much-reduced environmental cost. The engineered Azotobacter strain that we have reported here has considerably enhanced nitrogen fixation and excretion abilities and can replace ∼85 kg of urea per hectare but sustain the same wheat yield, if the seeds are inoculated with it before sowing.

Role of Phosphate-Solubilizing Microbes in the Enhancement of Fertilizer Value of Rock Phosphate Through Composting Technology

Preparation and use of enriched compost has become an important component of sustainable agriculture and received much interest in recent years as a means of alternative utilization of crop residues and low-grade minerals like rock phosphate (RP) by composting technology, thus reducing the ill effects of residue burning as well as improving the P content. The process has many advantages including sanitation, mass and bulk reduction, and decrease in carbon-to-nitrogen (C/N) ratio of crop residues. The performance of compost depends on the quality of the substrate. However, composts prepared from farm wastes have low nutrient content, particularly phosphorus (P) and potassium (K), and are considered poor suppliers of nutrients to crops. A possible means of improving the nutrient content, after prepare enriched compost by addition of low-grade RP and microbial techniques for RP solubilization. It will not only help farmers in supplying plant nutrients at a very low investment but also keep the environment pollution-free. The mineralization of insoluble forms of P present in RP by organic acids released during decomposition of organic matter is the major advantage of preparation of enriched compost. Organic acids released from the various organic materials used in the composting help in breaking down the RP faster during the composting period by pushing the dissolution reaction to the forward direction. The availability of P also increases due to inoculation with PSM, which are also known to produce organic acids. These acids are able to dissolve the mineral phosphate and make it available for the plant. The composting of organic wastes with RP has been demonstrated to be an effective method for producing end-products which are ensuring their maximum benefit for agriculture. Here, we emphasize pre-application techniques, especially the co-composting of RP with various organic by-product materials that include crop residues. A range of laboratory incubations have demonstrated the underlying mechanisms involved with solubilization. The significance of microbial induced production of organic acids and acidity during composting is particularly important in this respect. Inoculation with Aspergillus awamori into the composting mass increased the content of total P (2.35%) as well as water-soluble P (0.05% P) and citrate-soluble P (0.85% P) significantly. While co-composting with RP offers a great potential that could be developed for use at the individual farm scale, the key controlling factors and underlying mechanisms are far from being fully understood. Studies on synchronization of nutrient mineralization and crop demand will provide information on the efficient use of RP-enriched compost in a more integrated manner with inorganic fertilizers for achieving sustainable higher crop productivity.

Use of mineralization kinetics to estimate the potentially mineralizable nitrogen of rock phosphate-enriched composts-amended soil

ABSTRACT Predicting nitrogen (N) mineralization has been one of the greatest challenges to improving N management in agriculture. A laboratory incubation experiment was conducted to study the N mineralization of soil amended with rock phosphate (RP)-enriched composts. The RP-enriched rice straw compost amended soil mineralized highest N as compared to compost prepared from mustard stover and tree leaves. The first-order model was found to be the most suitable for N because it provided the best fit to the experimental data and for its simplicity. The model predicted that potentially mineralized N (N0) was varied from 4.0 to 52.1 mg kg−1 and the mineralization rate k varied from 0.015 to 0.066 day−1. The rice straw compost amended soil had higher N0 value than mustard stover and tree leaves compost amended soil. This study demonstrated the importance of application of rock phosphate-enriched composts in improving N supplying capacity of soil.

Depth dynamics of soil N contents and natural abundances of 15N after 43 years of long-term fertilization and liming in sub-tropical Alfisol

ABSTRACT The aim of this study was to understand impacts of long-term (43 years) fertilization on soil aggregation, N accumulation rates and δ15N in surface and deep layers in an Alfisol. Soil samples from seven treatments were analysed for N stocks, aggregate-associated N in 0–30 cm and the changes in δ15N in 0–90 cm depths. The treatments were: unfertilized control (control); recommended N dose (N); recommended N and phosphorus doses (NP); recommended N, P and potassium doses (NPK); 150% of recommended N, P and K doses (150% NPK); NPK + 10 Mg FYM ha−1 (NPK + FYM) and NPK + 0.4 Mg lime ha−1 (NPK + L). Results revealed that plots under NPK + FYM had ~39% higher total N concentrations than NPK + L in 0–30 cm soil layers. In NPK + L, macro-aggregates had 35 and 11% and microaggregates had 20 and 9% lower δ15N values than NPK + FYM in 0–15 and 15–30 cm soil layers, respectively. However, plots receiving NPK + FYM had ~39% greater deep soil (30–90 cm) N accumulation than NPK + L. These results would help understanding N supplying capacity by long-term fertilization and assist devising N management strategies in sub-tropical acidic Alfisols.

Citric acid loaded nano clay polymer composite for solubilization of Indian rock phosphates: a step towards sustainable and phosphorus secure future

ABSTRACT Non-renewable nature of rock phosphate (RP) reserves coupled with open ended nature of P cycle makes it imperative for maximum utilization of available P resources. In this context, use of Indian RPs from Purulia and Udaipur along with citric acid loaded nanoclay polymer composite (CA-NCPC) as P source to costly diammonium phosphate (DAP) was investigated through an incubation experiment followed by a greenhouse experiment with wheat-rice cropping sequence in a Luvisol (pH 5.14, available P 13.5 mg kg−1). Soil available P, crop yield parameters and dynamics of soil P fractions were taken to judge the efficacy of CA-NCPC in solubilizing RPs. Application of CA-NCPC and DAP resulted in 82% and 69% increase in available P over control, respectively under incubation study. Direct effect of treatment receiving CA-NCPC + RP on yield and P uptake by wheat was comparable with DAP but residual impact of CA-NCPC + RP (16.7 g pot−1) was better than DAP (13.8 g pot−1) in rice. The changes in inorganic P fractions were also significant as inclusion of RP increased calcium-P from 16.1 to 61.5 mg kg−1. Results indicated potentiality of RPs treated with CA-NCPC as an alternate P source which could prove promising amidst P scarcity.

Synchronization of Nitrogen Supply with Demand by Wheat Using Sewage Sludge as Organic Amendment in an Inceptisol

Nitrogen (N) is the most important essential nutrient for plant growth. Intensive agriculture requires large quantities of N mainly applied through commercial fertilizers like urea. However, the N use efficiency (NUE) hardly exceeds 50% under best management practices. Use of organics as N source to increase NUE is a very promising approach. The present investigation was carried out to study the mineralization and subsequent release of N from sewage sludge for synchronization with crop demand and to evaluate its effectiveness for maximization of NUE by wheat. Sewage sludge was collected from Okhla sewage sludge treatment plant, Delhi, characterized and incubated in an Inceptisol along with fertilizer N to monitor the release pattern of N. A pot culture experiment was also conducted to evaluate the efficiency of sludge for synchronization of N supply with the demand of wheat crop. Incubation experiment revealed that the sludge amended soil released significantly higher amounts of NH4+-N as well as NO3−-N than that of control. Release of NH4+-N from the sludge treated soil was slow during the first 15 days of incubation, then it increased and reached its maxima at around 45 days, after which it decreased sharply. Release of NO3−-N was slow during the first 15 days of incubation, followed by a high release rate up to 45 days and a slow release rate beyond 45 days. Pot culture experiment revealed that maximum yield, N content and uptake by wheat was obtained when the soil was treated with sludge along with fertilizer N @ 100 mg kg−1 soil but those were statistically at par with the results obtained using sludge along with fertilizer N @ 50 mg kg−1 soil. The same treatment i.e. sludge along with fertilizer N @ 50 mg kg−1 soil, also showed the highest N recovery (50%). This treatment also showed significantly higher yield and N uptake over the sole fertilizer treatment i.e. N @ 100 mg kg−1 soil along with recommended dose of P and K. Hence, with sludge amendment fertilizer N requirement could be curtailed by 50% and might be implicative for better NUE.

Nutrient Transformations in Soil Amended with Rock Phosphate Enriched Composts for Improving Productivity of Wheat-Green Gram Sequence

ABSTRACT The present study focuses on the transformations of nitrogen (N), phosphorus (P), and sulfur (S) in soil amended with rock phosphate (RP) enriched composts prepared using different crop residues under wheat-green gram sequence. Data emanated from the field study revealed that significant buildup in available pools of potassium permanganate (KMnO4-N), Olsen-P, and calcium chloride (CaCl2-S) as well as different fractions of N, P, and S were maintained higher under RP enriched composts treated plot than control. The total N, P, and S varied from 979 to 1776, 604 to 1165, and 273 to 464 kg ha−1, respectively in different treatments. The yield of wheat and green gram was significantly correlated with different fractions of N, P, and S in soil. Results conclude that RP enriched compost could be an alternative and cost-effective option for mitigating the shortage of chemical fertilizers for crop production.

Changes in the soil properties and availability of micronutrients after six-year application of organic and chemical fertilizers using STCR-based targeted yield equations under pearl millet-wheat cropping system

ABSTRACT Field experiments were carried out to assess the effect of nutrient management on soil properties and available micronutrients using Soil Test Crop Response (STCR) based targeted yield equations under a six-year old pearl millet-wheat cropping system. After six years, results showed that soil pH and bulk density decreased, while cation exchange capacity and organic carbon increased in farmyard manure (FYM) as compared to control and nitrogen, phosphorus and potassium (NPK) treated plots in both surface and sub-surface soil depths. Higher values of available zinc (Zn) (1.54 mg kg−1) and iron (Fe) (5.68 mg kg−1) were maintained in FYM+NPK treated plots, while higher values of manganese (Mn) (6.16 mg kg−1) and copper (Cu) (1.07 mg kg−1) were found in FYM alone at surface soil as compared to sub-surface soil. This study demonstrated the importance of application of FYM in improving soil properties and maintaining micronutrients availability in soil and their uptake by wheat for sustainable crop production.