Performance evaluation of native plant growth-promoting rhizobacteria for paddy yield enhancement in the jhum fields of Mokokchung, Nagaland, North East India

Abstract

The present study was carried out to evaluate the performance of native plant growth-promoting rhizobacteria (PGPR) on jhum paddy yield enhancement in Nagaland, Northeast India. Three indigenous PGPR isolates (Bacillus cereus MKGB, Pseudomonas fluorescens MKGPf, and Azospirillum oryzae MKGAz) were tested in the soil microcosm and jhum fields of Longkhum and Ungma villages in Mokokchung, Nagaland. The maximum 78.44% seed germination, 165 cm plant height, 30 leaves, 5 tillers, and 5 panicles per plant were recorded in the PGPR consortium inoculated pot soil. Similarly, maximum 151 grains per panicle, 21.66 g grain yield per plant, and 33.50 g of straw biomass were recorded in the same treatment. The observations from the field trials revealed a maximum of 4.67 t ha⁻¹ paddy yield in the Longkhum village jhum field inoculated with the PGPR consortium which was significantly different from the control (T1) at a p value of ≤0.05%. Similarly 4.74 t ha⁻¹ paddy yield was obtained from the PGPR consortium applied jhum plots in Ungma village. The PGPR consortium was found more effective and promising than the single culture inoculation in paddy yield enhancement. The study suggests the application of tested PGPR consortium in jhum fields for soil health and crop productivity improvement and achieving agricultural sustainability as well as social prosperity in the rural areas of North East India.

1. Introduction

Shifting cultivation locally known as jhum farming is a traditional land use practice in the hill states of North East (NE) India. Jhum farming is practiced in a cyclic manner involving slash and burn of natural forest landscapes, land preparation followed by crop cultivation for a period of 1–3 years on the same piece of land [1]. Jhum cultivation is an integral part of ethnic communities and a source of subsistence in the region. The farming communities across the region cultivate paddy maize and millets as principal crops in the jhum land [2,3]. Besides, tuber crops, cereals, legume pulses, oilseeds, cucurbits, and green leafy vegetables are grown in the jhum fields [4]. Jhum cultivation is a scientific system, which tries to capture soil fertility nurtured by forest growth. The fertility is released in one single flush through slash and burn and the resultant nutrients get washed out very quickly because of the steep slopes and heavy rainfall. The farmers cultivate a large variety of crops at times as many as 30–35 to make the best use of the nutrients. Through the diverse cropping system, farmers simultaneously ensure protein, cereal, and fiber requirements from a single crop field [5]. Jhum cultivation in the past has been a successful practice that provided livelihood support to the poorest sections of society. The traditional jhum cycle of 15–30 years has been found effective to restore soil fertility and maintain sustainability [6] because the longer cycle was aligned with the natural regeneration cycle of the forest. However, the legacy of the traditional farming system passed on from generation to generation is now getting disrupted due to accelerated soil erosion, land degradation, increasing population, and decreased land accessibility. These changes have resulted in a short time cycles of 2–5 years or sometimes even less in the last decade [2]. Due to the short rotation fallow period, this traditional farming system has become an unsustainable land use practice.

Nagaland is a hill state having a 16579 km² geographical area, comprising 6.32% of NE India. The state accounts for 0.504% of India's geographical area and supports around 0.16% of the country's total population. The state is inhabited by 16 major ethnic tribes, six sub-tribes, and is known as the land of festivals in India [7]. The climate of Nagaland is largely monsoon with high humidity, 21 to 40 °C annual mean temperature, and 1800–2500 mm annual mean rainfall. The state has an agrarian economy as 73% population is engaged in agriculture with a high degree of agrobiodiversity and distinct jhum cultivation practices.

Soil fertility and crop productivity management are the major challenges in jhum farming system due to soil erosion and rapid nutrient runoff from hill agroecosystems. Maintenance of agroecosystem functioning, crop diversity, and productivity requires an adequate amount of limiting nutrients viz., nitrogen, and phosphorous. Unlike modern agricultural practices that receive fertilizer inputs, jhum agroecosystem gains nutrients from natural inputs through slash and burn of biomass, atmospheric nitrogen (N) deposition, organic N decomposition, and biological N fixation (BNF) where BNF is driven by a diverse group of soil microorganism [8]. In NE states the soil pH below 5.5 and between 7.5 and 8.5 limit phosphate availability to plants. Low soil fertility during jhum cultivation and post-jhuming soil infertility can be addressed using chemical fertilizers, but it has adverse effects on the agroecosystems [9], environment and physicochemical and biological properties of soil [10]. Moreover, jhum farming is a low input organic like crop cultivation system in the region [11].

Crop rhizosphere harbours diverse group of plant growth promoting rhizobacteria (PGPR) which includes free-living, symbionts, endophytes colonizing plant tissues, and Cyanobacteria promoting plant growth directly or indirectly [12,13]. BNF, organic P and Potassium (K) solubilization, nodulation, siderophore, and phytohormone production are the direct action of PGPR, while the production of hydrolytic enzymes, exopolysaccharides, hydrogen cyanide, development of induced systemic resistance, and heavy metal detoxification is the indirect functions [14,15]. The plant root exudates released in the rhizosphere act as chemical signals for microorganisms [16] and perform plant-microbe interaction in the environment. Banerjee et al. [17] reported that the native microbes to the jhum fields exhibited enhanced seed germination, plant growth promotion, and production of Indole-3-acetic acid (IAA) in upland paddy crop fields of NE India. Further, PGPR exhibits synergistic and antagonistic interactions with the soil microbiota and offers an array of activities of ecological and economic significance [18]. Genus Azospirillum, Azotobacter Acetobacter, Bacillus, Paenibacillus, Pseudomonas. Serratia, Kosakonia sacchari [14,19], etc. are very common plant growth promoting bacteria that help in enhancing crop yield and overall plant growth substantially. The present study was carried out to evaluate the performance of native PGPRs (Bacillus, Pseudomonas, Azospirillum) isolated from the paddy crop rhizosphere for productivity enhancement in the jhum fields of Nagaland.

2. Materials and methods

2.1. Study site

The present study was carried out in Longkhum (N 26° 31' 30.7", E 94° 40' 5.2"; 1025 m) and Ungma (N 26° 19' 24.2", E 94° 33' 9.1"; 1038 m) village jhum paddy fields of Mokokchung district in Nagaland. The study was carried out during the Kharif season for two consecutive years (2017 and 2018) and five farmers from each village were selected based on the prior discussion with the village headman and participating farmers. All the selected farmers provided their jhum plots for field trials and participated in the experimental work. Post-PGPR application field operations were carried out by the concerned farmers in a participatory manner under the guidance of the project implementation team.

2.2. Isolation and identification of PGPRs

Soil samples from jhum paddy crop rhizosphere were collected aseptically from different locations and mixed properly to obtain representative composite samples. The collected samples were brought to the laboratory and stored at 4 °C before the isolation of plant growth-promoting rhizobacteria. Selective artificial culture media viz., Nutrient agar (NA), King's medium B, and Azospirillum Medium with 0.17% agar was used for the isolation of Bacillus, Fluorescent pseudomonads, and Azospirillum strains, respectively. The serial dilution spread plate method was used for PGPR isolation and ∼10⁶ fold diluted samples were inoculated on agar plates. The inoculated NA and King's B plates were incubated for 24 h at 28 °C while the Azospirillum medium plates were incubated for 96 h at 28 °C. The discrete colonies from each Petri dish were streaked on fresh agar medium plates for the purification of bacterial isolates. The pure cultures of PGPR isolates were preserved on the agar slants for further study.

The screened PGPR isolates were identified using 16S rDNA partial gene sequencing from Chromous Biotech Pvt. Ltd. Bangaluru, India. The universal primers specific to the 16S rRNA spacer region were used to amplify the target bacterial DNA. The Basic Local Alignment Search Tool (BLAST) was used for homology search, and identification using NCBI online database (https://www.ncbi.nlm.nih.gov) and the processed sequences were submitted to the NCBI Genbank database. The PGPR isolates designated as MKGB, MKGPf and MKGAz were thus identified to be Bacillus cereus (MG196047), Pseudomonas fluorescens (MG020682), and Azospirillum oryzae (MG196056) respectively (Fig. 1).

Fig. 1.

Neighbour-joining phylogenetic tree constructed using 16S rDNA sequences of the PGPR strains and their nearest type strains (T). Bootstrap values expressed as a percentage of 1000 resampling are shown at the nodes.

2.3. Soil microcosm (pot) experiment

All three PGPRs were first tested in soil microcosm for their potential to enhance the growth and yield of paddy crop. The soil was collected from degraded jhum plots of Longkhum and Ungma villages and basic soil properties (pH, available nitrogen, phosphorous and exchangeable potassium) were determined as per the method described earlier [20,21]; soil organic carbon (SOC) [22]. The soil used for the microcosm experiment was sieved, sterilized, and filed in plastic poly pots. Paddy seeds were surface sterilized with 0.1% mercuric chloride (HgCl₂) for 5 min and washed with sterilized distilled water. These seeds were soaked with bacterial cultures for 10 min having a cell density of ∼10⁶ CFU/ml and air-dried at room temperature [23]. Each pot was sown with five seeds treated with PGPR cultures while paddy seeds soaked in distilled water were sown in the control. Since the soil medium used for the experiment was deficient in available plant nutrients as it was collected from the degraded jhum fields, thus inoculated with PGPR cultures having a cell density of ∼10⁶ CFU/ml at the time of sowing. The experiments were conducted following completely randomized design (CRD) method in five treatments and three replications. Observation on seed germination, plant height, number of leaves, tillers/plant and panicles per tiller were recorded at the one-month interval and presented mean values of all the observations. Grain yield and biomass were recorded at the time of harvest.

2.4. Field trials in jhum land

For field trials, bacterial isolates were mass-multiplied in the media broth at 28 ^°C with continuous shaking at 150 rpm. The final population density of mass culture was adjusted to ∼10⁶ CFU/ml. The mass culture of each isolate was transferred into sterilized containers and transported to the field for application in the paddy crop. Five experimental sites (S1–S5) were selected in the same piece of jhum land cultivated by five farmers in both villages. The experimental jhum plots were divided into 10 m × 10 m sub-plots with proper bunding. Five plots were prepared in each farmer's field and randomized using online statistical design software. Randomized complete block design (RCBD) was followed for field experiments.

The PGPR inoculum alone and in combination of each strain was applied in the soil at a flow rate of 200 L ha⁻¹ having a population density of ∼10⁶ CFU/ml viable cells mL⁻¹ [24]. The PGPR consortium was prepared using an equal volume of mass cultures. The productivity of the paddy crop was estimated using the harvest method. For this purpose, 3 subplots of 1 m² were laid in each plot of 10 m², and paddy samples were harvested. Grains of harvested samples were manually separated from the straw and air-dried till constant weights were obtained. The yield was estimated in terms of g per m² and extrapolated as t ha⁻¹ [25].

2.5. Data analysis

The data obtained from the soil microcosm was analyzed and presented as mean values with standard error. Further, one-way analysis of variance (ANOVA) was performed by using the Statistica analytics software package. The significant differences in means were tested using Duncan's test. The data are presented as the mean values of three replicates with standard error (SEM) at a 5% significance level and the ranking of treatments denoted by alphabets.

3. Results

3.1. Soil microcosm experiment

The selected PGPR isolates were initially tested in a soil microcosm to study the effect on paddy growth and yield attributes. The pot soil pH, available NPK (kg ha⁻¹), and organic carbon (%) values were determined to be 5.11, 106.67, 9.56, 56.50, and 1.72 respectively. After PGPR inoculation, all the pots were kept inside the agro-shade net house in a randomized manner as per a completely randomized design (CRD). The bacterial population count in the sterilized pot soil was performed after 15 days of inoculation. The observations revealed promising progression with an active population of inoculated PGPR in the sterilized soil (Fig. 2).

Fig. 2.

Colony forming units (CUF) in the soil microcosm experiment T1- Control, T2-mass culture of Bacillus cereus, T3-mass culture of Pseudomonas fluorescens, T4-mass culture of Azospirillum oryzae, T5-consortium all the three PGPRs in equal ratio.

Paddy seeds were treated with PGPR culture following the bacterization process and the pot soil was also enriched at the time of seed sowing. Watering of pot soil was done regularly and paddy growth and yield parameters were recorded periodically. The observations showed measurable differences in the growth and development of paddy plants in different treatments. The highest 78.44% germination was observed in consortium-treated seed followed by MKGAz (65.71%), MKGPf (58.21%), and MKGB (55.34%) as compared to the 50.12% seed germination in control. The growth parameters viz., plant height, number of leaves, tillers, and panicles per tiller were recorded at one-month intervals and the mean values of all observations along with standard errors are present. The maximum 21.66 ± 0.61 g grain yield per plant was recorded in consortium applied pots (T5) followed by14.19 ± 1.08 g in (T4) and 13.18 ± 1.31g in T3. Similarly, the paddy straw biomass (g) was in the order of T5>T4>T2>T3>T1 while root biomass (g) was observed to be T5>T3>T4 & T2>T1 (Table 1). However, the average number of grains per panicle, grain yield per plant, straw, and root biomass was recorded at the time of harvest. The observations varied in consortium-applied pots as compared to the control while at par with each other in single culture inoculated pots i.e. T2, T3 & T4. The paddy crop ripening in the control was observed one week before the PGPR-treated pots. The observation revealed a remarkable improvement in plant growth and yield attributes due to PGPR inoculation (Table 1). The tested PGPR isolates were evaluated for paddy yield enhancement in the jhum fields of Nagaland.

Table 1.

Effect of PGPR inoculation on paddy growth and yield attributes in the soil microcosm experiment.

Treatments	Parameters
	Germination (%)	plant height (cm)	No. of leaves per plant	No of tillers per plant	No. of panicles per plant	Average grain per panicle	Grain yield per plant (g)	Straw biomass (g)	Root biomass (g)
T1 (Control)	50.12	143.33 ± 4.33	11 ± 1.15	3.0 ± 0.08	2.03 ± 0.03	98.67 ± 7.80	06.68 ± 0.59	17.07 ± 0.58	09.20 ± 0.64
T2 (MKGB)	55.34	157.33 ± 4.18	24.33 ± 1.86	3.67 ± 0.33	3.10 ± 0.34	131 ± 14.60	11.26 ± 0.56	24.77 ± 1.39	15.52 ± 0.72
T3 (MKGPf)	58.21	154.67 ± 7.31	25.61 ± 3.53	4.61 ± 1.20	3.00 ± 0.11	128 ± 3.53	13.18 ± 1.31	23.51 ± 1.30	16.58 ± 1.45
T4 (MKGAz)	65.71	149.67 ± 0.67	26.57 ± 4.26	4.33 ± 0.33	3.50 ± 0.21	140 ± 19.65	14.19 ± 1.08	25.07 ± 0.39	15.55 ± 0.90
T5 (Consortium)	78.44	165.00 ± 3.79	30.33 ± 1.20	5.33 ± 0.54	4.50 ± 0.20	151 ± 6.08	21.66 ± 0.61	33.58 ± 2.77	21.42 ± 0.76

(T1- Control, T2-mass culture of MKGB, T3-mass culture of MKGPf, T4-mass culture of MKGAz, T5-consortium of MKGB + MKGPf + MKGAz in 1:1:1 ratio, n = 3, ±SEM-Standard Error of mean).

3.2. Field trials and paddy yield enhancement

The first field trial laid in crop year 2017 (Karif season) showed a maximum 4.67 t ha⁻¹ paddy yield in PGPR consortium (T5) treated plots at experimental site-5 which was significantly different (p ≤ 0.05) from the grain yield of 2.33 t ha⁻¹ in control plots. The mean values of grain yield in different treatments across the experimental sites compared using one-way ANOVA revealed that the paddy yield in PGPR consortium applied jhum plots differed significantly from the untreated plots (control) (p ≤ 0.05). However, paddy yield in the treatments with single culture inoculation was at par with each other with no significant differences except in a few sites. Minimum 1.27 and 2.30 t ha⁻¹ paddy yields were recorded in control and PGPR consortium applied jhum plots respectively at experimental site-2 (Table 2). The net maximum paddy yield enhancement of 2.33 t ha⁻¹ in Longkhum village jhum fields was recorded in the first trial.

Table 2.

Paddy grain yield (t ha⁻¹) in Longkhum village jhum fields during crop year-2017.

Treatments	Sites
	S1	S2	S3	S4	S5
T1 (Control)	2.23 ± 0.09c	1.27 ± 0.07c	2.83 ± 0.23b	2.17 ± 0.23c	2.33 ± 0.30c
T2 (MKGB)	2.90 ± 0.06b	1.93 ± 0.13b	3.00 ± 0.06ab	2.87 ± 0.33b	3.53 ± 0.12bc
T3 (MKGPf)	3.03 ± 0.24ab	1.80 ± 0.15b	3.03 ± 0.32ab	3.00 ± 0.21ab	3.60 ± 0.50abc
T4 (MKGAz)	3.07 ± 0.18ab	1.82 ± 0.06b	3.30 ± 0.25ab	3.23 ± 0.2b	3.47 ± 0.07ab
T5 (Consortium)	3.50 ± 0.21a	2.30 ± 0.06a	3.67 ± 0.23a	3.70 ± 0.10a	4.67 ± 0.50a
CD (5%)	0.535	0.283	0.271	0.972	0.780
SEM	0.162	0.085	0.062	0.293	0.235

(CD- Critical Difference, SEM- Standard Error of mean, values indicated with same letter are not statistically different (p < 0.05), T1- Control, T2-mass culture of MKGB, T3-mass culture of MKGPf, T4-mass culture of MKGAz, T5-consortium of MKGB + MKGPf + MKGAz in 1:1:1 ratio)).

Similarly, the minimum and maximum paddy yield during crop year 2018 in Longhkhum village were recorded to be 1.81 and 2.1 t ha⁻¹, respectively at site-1 (control) whereas the maximum 4.02 t ha⁻¹ paddy yield in PGPR consortium applied jhum plots was observed at experimental site-4. The lowest paddy yield in consortium-applied jhum plots was 3.29 t ha⁻¹ at site-1. The yield estimates in control and consortium-applied jhum plots were statistically different (p ≤ 0.05) from each other (Table 3). Paddy yield in single culture inoculated jhum plots at S1 was at par with each other with no significant differences in their mean values. However, in some of the sites, the grain yield differed significantly even in the single culture inoculation but was not exceeded that of PGPR consortium inoculated jhum sites (Table 3).

Table 3.

Paddy grain yield (t ha⁻¹) in Longkhum village jhum fields during crop year-2018.

Site/treatments	Sites
	S1	S2	S3	S4	S5
T1 (Control)	2.04 ± 0.07c	1.81 ± 0.04d	1.99 ± 0.06d	2.10 ± 0.03e	2.05 ± 0.07d
T2 (MKGB)	2.55 ± 0.11b	2.04 ± 0.04cd	2.39 ± 0.08c	2.54 ± 0.19d	2.71 ± 0.17c
T3 (MKGPf)	2.66 ± 0.10b	2.21 ± 0.12c	2.72 ± 0.06c	2.67 ± 0.11c	2.74 ± 0.22c
T4 (MKGAz)	2.82 ± 0.20b	2.77 ± 0.12b	3.23 ± 0.12b	2.90 ± 0.10b	3.50 ± 0.19b
T5 (Consortium)	3.29 ± 0.38a	3.37 ± 0.28a	3.90 ± 0.18a	3.83 ± 0.06a	4.02 ± 0.04a
CD (5%)	0.352	0.302	0.291	0.193	0.182
SEM	0.106	0.091	0.088	0.058	0.055

(CD- Critical Difference, SEM- Standard Error of mean, values indicated with same letter are not statistically different (p < 0.05), T1- Control, T2-mass culture of MKGB, T3-mass culture of MKGPf, T4-mass culture of MKGAz, T5-consortium of MKGB + MKGPf + MKGAz in 1:1:1 ratio)).

The paddy yield estimates from 5 experimental sites in Ungma village jhum field exhibited a maximum of 4.30 t ha⁻¹ in PGPR consortium applied plots at site-3. The net yield enhancement of 2.48 t ha⁻¹ was recorded at this site as a result of PGPR inoculation as compared to the 1.82 t ha⁻¹ grain yield in un-inoculated (control) jhum plots. The yield data in control and consortium-applied jhum plots was found statistically significant (p ≤ 0.05). The paddy yield at S4 was 4.10 and 1.44 t ha⁻¹ in PGPR consortium applied plots and control respectively where a net increase in yield was recorded to be 2.66 t ha⁻¹ during Kharif season 2017 (Table 4). Similar to the Longkhum village experimental sites, the paddy yield in jhum plots at Ungma village inoculated with MKGB (T2), MKGPf (T3), and MKGAz (T4) was at par with each other except in a few sites where significant differences in the average yield were recorded (Table 4).

Table 4.

Paddy grain yield (t ha⁻¹) in Ungma village jhum fields during crop year-2017.

Site/treatments	Sites
	S1	S2	S3	S4	S5
T1 (Control)	2.04 ± 0.08c	2.05 ± 0.04d	1.82 ± 0.08d	1.72 ± 0.04d	1.44 ± 0.12d
T2(MKGB)	2.93 ± 0.09b	3.14 ± 0.03c	2.36 ± 0.13c	2.82 ± 0.04c	2.49 ± 0.18c
T3(MKGPf)	3.02 ± 0.05b	3.37 ± 0.09bc	3.12 ± 0.07b	3.44 ± 0.03b	3.13 ± 0.04b
T4 (MKGAz)	3.42 ± 0.06b	3.57 ± 0.04b	3.31 ± 0.08b	3.69 ± 0.06b	3.67 ± 0.15a
T5 (Consortium)	3.97 ± 0.35a	4.07 ± 0.25a	4.30 ± 0.32a	4.15 ± 0.26a	4.10 ± 0.17a
CD (5%)	0.575	0.404	0.530	0.405	0.450
SEM	0.174	0.122	0.160	0.122	0.136

(CD- Critical Difference, SEM- Standard Error of mean, values indicated with same letter are not statistically different (p < 0.05), T1- Control, T2-mass culture of MKGB, T3-mass culture of MKGPf, T4-mass culture of MKGAz, T5-consortium of MKGB + MKGPf + MKGAz in 1:1:1 ratio)).

The paddy yield in the Kharif season 2018 estimated from all 5 experimental sites in Ungma village was found to be 4.74 t ha⁻¹ at S-5 inoculated with the PGPR consortium (T5). The net increase in grain yield by 2.09 t ha⁻¹ was statistically significant from 2.65 t ha⁻¹ recorded in the control plots (Table 5). This was followed by the site (S2) where the highest 4.11 t ha⁻¹ yield was obtained in consortium-applied jhum plots with an increase of 1.19 t ha⁻¹ over the control. These results revealed that the decline in crop yield in the second year of cultivation was very less while in some of the sites, it was slightly increased as a result of PGPR inoculation.

Table 5.

Paddy grain yield (t ha⁻¹) in Ungma village jhum fields during crop year-2018.

Treatments	Sites
	S1	S2	S3	S4	S5
T1 (Control)	2.15 ± 0.10d	2.91 ± 0.14d	2.86 ± 0.28e	1.97 ± 0.11d	2.65 ± 0.31d
T2(MKGB)	3.13 ± 0.17b	3.48 ± 0.27b	3.30 ± 0.23c	2.83 ± 0.06c	3.54 ± 0.14c
T3(MKGPf)	3.04 ± 0.09bc	3.56 ± 0.13b	3.07 ± 0.08d	2.87 ± 0.12c	3.56 ± 0.16c
T4 (MKGAz)	2.93 ± 0.15c	3.27 ± 0.21c	3.57 ± 0.13b	3.18 ± 0.06b	3.82 ± 0.16b
T5 (Consortium)	3.54 ± 0.13a	4.11 ± 0.19a	3.91 ± 0.07a	3.90 ± 0.15a	4.74 ± 0.16a
CD (5%)	0.129	0.160	0.098	0.119	0.183
SEM	0.039	0.048	0.030	0.036	0.055

(CD- Critical Difference, SEM- Standard Error of mean, values indicated with same letter are not statistically different (p < 0.05), T1- Control, T2-mass culture of MKGB, T3-mass culture of MKGPf, T4-mass culture of MKGAz, T5-consortium of MKGB + MKGPf + MKGAz in 1:1:1 ratio)).

4. Discussion

The traditional jhum farming system is the major source of subsistence in the hilly region of NE India. The average state rice productivity in Nagaland is ≈ 1.7 t ha⁻¹, though; there has been an increase in productivity from 1.7 to 1.93 t ha⁻¹ through farmers’ traditional management practices [26]. However, this increase does not match the population growth and actual food requirement in the state. The scope of crop productivity improvement in jhum farming system through scientific interventions was identified to be the major objective of the present study. Jhum cultivation is an age-old farming system while terrace cultivation is another intervention and operates in a few rural pockets in Nagaland. Wet terrace cultivation is predominantly practiced in Kohima and Phek districts while jhum paddy cultivation is practiced in the entire state. As soil fertility is compromised and water balance is disturbed owing to excess deforestation, people are now becoming increasingly concerned and looking for alternative means of efficient cultivation. Terrace cultivation has received wide appreciation and people are beginning to embrace terrace cultivation in most areas along with conventional jhum farming [27]. The state is having about 56.50% area under jhum cultivation which contributes about 49.26% of rice production. Previous studies have revealed higher paddy productivity in wet terrace cultivation systems than in conventional jhum [28]. This could be due to the proper management of paddy fields with sufficient water availability and less nutrient loss from the system contrary to the lack of efficient water management, more weeds, surface runoff, and nutrient washout from jhum fields, and lack of necessary plant protection measures. PGPR are known to exert one or more direct and indirect mechanisms to improve plant growth and health, although the major mode of action of many PGPRs is facilitated through increasing the availability of nutrients in the rhizosphere [29,30].

In the present study, it was observed that the paddy yield in jhum fields of Longkhum and Ungma villages enhanced significantly as a result of native PGPR inoculation. A combination of three PGPR isolates yielded more fruitful and promising results than the single culture inoculation. The paddy yield in both villages was recorded to be higher than the state average of 1.7 t ha⁻¹. This is attributed to the multiple traits of the PGPR consortium and their direct benefits to crops. Rathore and Bhatt [30] conducted a field experiment on the integrated farming system and evaluated the economics of different cropping systems in Nagaland. The study reported 1.5 t ha⁻¹ yield in jhum rice whereas in rice-vegetables- bean, rice-cauliflower, and rice-garlic-maize cropping systems, the yield was recorded to be 5.0, 4.8, and 1.2 t ha⁻¹ respectively. The yield estimates in the present study are in agreement with the findings of [30]. The highest yield in the rice-vegetable bean cropping system is attributed to the biological nitrogen fixation by legume crop and sufficient nutrient availability to paddy crop while in the present study the PGPR consortium enriched soil fertility and crop productivity. The findings of past studies and our experiments revealed a huge scope of yield improvement in jhum farming system as the average productivity in traditional jhum is very low. Similarly, plant growth promotion abilities of microbial strains viz., Curtobacterium oceanosedimentum, Bacillus methylotrophicus, Bacillus cereus, Penicillium virgatum, Metarhizium pinghaense, and Penicillium stratisporum native to jhum cultivation system were evaluated for yield improvement of upland paddy crop and the findings were similar to our observations from the soil microcosm study, where shoot length, root length, plant height, the number of leaves, tillers, panicles, biomass, and grain yield was significantly increased by PGPR inoculation in the pot soil [17]. The early crop ripening in control (un-inoculated) pots in the present study could be due to the abiotic stress in nutrient-deficient conditions. Moreover, under stressed environmental conditions, the increased ethylene levels also trigger the early ripening of crops which is otherwise controlled by PGPR through the synthesis of 1-amino-cyclopropane-1-carboxylate (ACC) deaminase enzyme that hydrolyzes the ethylene precursor ACC [10,31].

Kumar et al. [32] conducted an on-farm trial in the Longleng district of Nagaland to improve the yield and profitability of paddy crops and to find out the most suitable nutrient management practices among farmer's traditional practices, recommended doses of fertilizers (RDF), and locally available weed biomass addition. The study reported that RDF and weed biomass application was more effective than farmers' practice to improve the growth and yield of rice crop with a maximum 3.59 t ha⁻¹ grain yield in RDF applied plots followed by 3.27 t ha⁻¹ in weed biomass amendments and the least 2.37 t ha⁻¹ in traditional jhum. The study recommended suitable interventions such as weed biomass incorporation or RDF in traditional jhum farming practices for yield improvement and maximum profitability. These observations support the scientific interventions of our study for crop yield improvement in jhum farming system.

On-farm trials in jhum fields of the Zunheboto district of Nagaland were conducted to evaluate the response of Rhizobium, PSB, and farmyard manure (FYM) in soybean crop at nine demonstration sites in 3 villages. The study recorded a 1.21 t ha⁻¹ soybean yield in the demonstration plots amended with FYM, Rhizobium, and PSB as compared to a 1.01 t ha⁻¹ yield in farmer's practice alone. The technology intervention reported improved yield, net return, B: C ratio and available NPK of the soil after harvest as compared to traditional practice [33]. A recent study on soil fertility and rice productivity in the Lengpui district of Mizoram reported 2.26 to 2.69 t ha⁻¹ year⁻¹ yield using rhizospheric microbial inoculants responsible for phosphate solubilization, plant growth promotion, and N fixation [34]. The paddy yield in the present study was higher than the yield in Mizoram which may be due to the application of native soil bioinoculants from the same jhum fields.

The study on BioGro formulation of PGPR strains reported a 20–30% increase in rice growth and yield under variable soil conditions in Vietnam [35,36], while Mia et al. [37] reported significantly higher root hairs in rice seedlings inoculated with Rhizobium strain. Ecomonas-based PGPR formulation developed in India was found to increase 2.0 t ha⁻¹ rice yields with a 37.7% reduction in rice sheath blight caused by Rhizoctonia solani [38]. The rice yield enhancement by Ecomonas-based formulation strongly supports the findings of the present investigation where significant improvements were recorded at all the experimental sites in Nagaland. Application of organic fertilizer and Pseudomonas strains in maize crop was found to improve 39% growth, 16% plant height, 11.7% grains per spike, and 39% grain yield compared to the uninoculated plants [39]. Similarly, Hafeez et al. [40] reported 50–70% savings in nitrogen fertilizer and a 20% increase in rice yield using biofertilizers in Pakistan. Ashrafuzzaman et al. [41] characterized ten PGPR isolates from the rice crop rhizosphere among which six were able to produce IAA and one was capable of phosphate solubilization. The characterized isolates were then applied in the pot soil and observed a significant enhancement in seed germination, plant height, shoot, and root length, and biomass. The study suggested that the PGPR inoculants were effective for the rice cultivation system. A similar trend in plant growth and yield attributes in our soil microcosm experiment was recorded upon PGPR inoculation. Application of Pseudomonas fluorescens, Bacillus subtilis, and Azospirillum brasilens in rice crop by Elekhtyar [42] reported enhanced plant growth, yield, and its components as well as grain quality characters and chemical properties with a 25% reduction in N fertilizer requirements as a result of integrated use of inorganic nitrogen fertilizer with PGPR strains as biofertilizers. In the present study, promising results were recorded in jhum fields of Nagaland with enhanced paddy yield and a very slight decline in productivity in the second year of cropping.

It is evident from the results that the native PGPR isolates have been found effective in soil microcosm and jhum fields at Mokokchung, Nagaland. The PGPR consortium was found much more promising than the individual isolates in jhum paddy yield improvement. These observations opened the avenues of crop productivity improvement in jhum farming system using potential PGPR consortium and low-cost biofertilizer formulations in the suitable carrier materials. The study suggests that the application of versatile PGPR consortium in jhum farming system will be helpful for soil health and crop productivity improvement, restoration of degraded jhum lands, and achieving agricultural sustainability and social prosperity in the rural areas of North East India. Therefore, the tested PGPR consortium is suggested for large-scale application to enhance paddy yield in jhum agriculture system of Nagaland.

Author contribution statement

Krishna Giri, Rupjyoti C. Baruah, Rajarshi Bhattacharyya, Navajyoti Bora, Bhanushree Doley: Conceived and designed the experiments, Performed the experiments.

Deep Chandra Suyal, Narendra Kumar, Niren Das: Analyzed and interpreted the data.

Krishna Giri, Gaurav Mishra, Deep Chandra Suyal, Niren Das: Contributed reagents, materials, analysis tools or data.

Bhanushree Doley,krishna Giri, Deep Chandra Suyal: Wrote the paper.

Funding statement

Dr. Krishna Giri was supported by Indian Council of Forestry Research and Education, Dehradun [Grant No. RFRI/January-2017/SCD-1].

Data availability statement

Data used for the analysis is available from the corresponding authors upon request.

Declaration of interest's statement

The authors declare no competing interests.

Additional information

No additional information is available for this paper.