Response of Transplanted Rice to Seedling Root-dip in Phosphorus and Biofertilizer Slurry in Acid Soils of North East India- A Participatory Assessment in Farmers’ Field

Phosphorus deficiency is one of the major limiting factors for rice production in many parts of the world (Saito et al., 2019; Tanaka et al., 2015; Vandamme et al., 2016). Farmers sometimes apply excess amounts of P to achieve higher production. The world resources of P are finite and therefore, P should be used as efficiently as possible in order to conserve the base resources (FAO, 2017). Efficient P fertilizer management is also key to improving rice yield for smallholder farmers who use little amount of fertilizers. Therefore, it is necessary to develop appropriate strategies for the effective use of P fertilizers in rice production systems.

Rice production is mainly constrained by low P-use efficiency (PUE) and P-recovery efficiency (PRE) in acid soils (Fageria et al., 2015), because a considerable part of P added to acid soils is fixed as insoluble phosphates of Al⁺³ and Fe⁺² (Bhattacharyya et al., 2015; Redel et al., 2016). In the anticipated phosphate crisis by 2050 (Gilbert, 2009), the strategies for P management in agriculture must focus on minimization of quantity of applied-P (Richardson et al., 2009), possibly recycling (Ashley et al., 2011) and rhizosphere-based P management (Martínez et al., 2015).
       
Several studies have demonstrated significant effects of nursery P and P-dipping to increase rice yields and agronomic P use efficiency (AEP) as the yield gain per unit of P applied. Excess yield gains with a micro-dosing nursery P may have a risk of P mining from soils. The P-dipping retains the P input-output balance and achieves a consistently high AEP at 74-152 kg kg^-1 (Rakotoson et al., 2022). Dipping seedling roots into P-enriched slurry just before transplanting (P-dipping) is reported to improve P fertilizer use efficiency in transplanted lowland rice production systems. The P-dipping technique with the slurry attached to seedling roots, assures the P supply after transplanting and avoids the P-mining risk (Oo et al., 2020a). Various studies on P dipping technology reported significant yield increases by 10-50% with 40-60% reductions in P application rates relative to broadcasting or incorporating P at transplanting (Raju et al., 1980; Ramanathan and Kothandaraman, 1984; Balasubramanian et al., 1995; De Datta et al., 1990).
       
Application method of P fertilizer is of paramount importance as it can minimize P fixation by soils and thereby increase P efficiency. So, there is a need to develop P application method, which can support better root development and architecture at the early stage of crop growth for exploration of more soil volume and thereby enhancing more nutrient uptake (Kalidas and Thakuria, 2018). Dipping of rice roots in single superphosphate (SSP)-soil slurry just before transplanting can save P-fertilizer up to 40-60% of the recommended P dose applied in conventional practice and increases PUE and PRE (Ru-kun et al., 1982; Hooper, 1991; Balasubramanian et al., 1995; Talukdar et al., 2001). In addition to the P slurry, application of Phosphate Solublizing Bacterial (PSB) biofertilizer may further enhance the PUE in acid soils.
       
P deficiency in the north east hill region of India is related to the highly weathered soils which are low in bioavailable P contents and high in P-sorption capacity because of soil acidity and high content of Fe and Al oxides (Batjes, 2011; Bekunda et al., 2010; Nishigaki et al., 2019). Available P content in surface soils of the hill region of Assam is reported to be very low to medium (0.2 to 1760 ppm), but in sub-surface soils it is very low i.e. 0.2 to 15.6 ppm (Chakravarty and Baruah, 1987). Inadequate P application due to limited purchasing capacity of the small and marginal farmers of the region is another key issue associated with P deficiency in rice. Under such soil and capital constraints, it is imperative to find effective P fertilizer management practices for rice production that will achieve higher yield per unit of P fertilizer applied. For this goal, field experiments in actual farmers field condition is essential to assess the efficacy, acceptability, yield advantage as well as economic benefits of the applied P fertilizers.
       
The objective of the experiment was to study the effect of the P-dipping technique in rice in farmers’ field condition in comparison to P incorporation and INM and to identify a suitable cost effective method of application of phosphatic fertilizer in rice for the acid soils of the hill region of North East India.

A participatory field trial was conducted by Krishi Vigyan Kendra, Karbi Anglong during the year 2020 and 2021 in farmers’ field of Karbi Anglong district (25°32'N to 26°36'N latitudes and 92°10'E to 93°50'E longitude) under hill zone of Assam which falls in the Eastern Himalayan region of India. The experiments were conducted at representative fields in the region where farmers continuously cultivated rice once a year without crop rotation and with little or no mineral fertilizer inputs prior to the start of the experiments.
       
The composite soil samples from experimental rice fields were analyzed before and after the experiment for physico-chemical properties (Table 1).
 

       
Three different management options of phosphorus were tested in comparison to the farmers existing practice as control in four villages as replications. The management options were application of recommended dose of fertilizers (RDF) for the region (N:P₂O₅: K₂O @ 60:20:40 kg ha^-1) (T₁), root dipping in P slurry and microbial consortia (T₂), INM package recommended by Assam Agricultural University (T₃) and farmers existing practice as control (T₄).
       
For the treatment T₁, entire P (as SSP) and K fertilizer was applied as basal and half of N was applied as basal and remaining half was applied in two splits at maximum tillering and panicle initiation stage. For the root dipping method (T₂), mud slurry bed was prepared in one corner of the main field and SSP (@ 7.0 kg/ha was mixed thoroughly with mud. Roots of uprooted rice seedlings after washing were dipped in the SSP amended mud slurry for over-night (10 hours). The SSP slurry treated roots of rice seedlings were again dipped in biofertilizer amended mud slurry and incubated for 2 hours. After this treatment, seedlings were transplanted with 50 % RDF. INM treatment (T₃) consisted of application of organic manure @ 1tha^-1 (on dry weight basis) along with mixed inoculum of Azospirillum amazonense A-10 andBacillus megaterium P-5 @ 4 kgha_-1, rock phosphate @ 10 kg P₂O₅ per ha and muriate of potash (MOP) @ 40 kg K₂O per ha. Rock phosphate component was used along with the biofertilizer as slurry to treat the seedling roots.
       
Ten hills per plot were marked for recording plant height, number of tillers and root weight. At maturity, grain and straw yields (t ha^–1) were determined by harvesting plants from an area of 4 m² in the center of each plot. The number of hills per harvested area was recorded for conversion of grain and straw yield per hectare basis.  Grain yield is expressed based on filled grain weight, corrected to 14% moisture content by using a grain moisture sensor. The straw yield was expressed on dry matter basis by oven-drying at 70°C for more than 72 h. Grain and straw were separated for each of 10 marked plants at harvest and subjected to nutrient content analysis. For nutrient content analysis, plant samples were sun-dried and washed with 0.01N HCl followed 4 rinses with distilled water. Finally, the cleaned straw and grain were oven-dried at 65°C to a constant weight. The oven dried grain and straw samples were ground in a Willey Mill for analysis of tissue P concentration and uptake.
       
Harvest index (HI) was calculated as the ratio of grain yield and biological yield multiplied by 100. Uptake of P in biomass (Straw and grain) was determined by multiplying P content (%) with their corresponding yield data. P use efficiency (PUE) and P recovery efficiency (PRE) were calculated by the following equations: All statistical analyses were performed using SPSS v. 12.0 (SPSS Inc. Chicago, IL, USA). Data were subjected to analysis of variance (ANOVA) and means significantly different were separated using Duncan’s Multiple Range Test (DMRT) (Duncan, 1955).

Growth and yield parameters
 
Different methods of P application in transplanted rice crop significantly influenced the growth and yield parameters (Table 2). The highest plant height, maximum number of effective tillers and root biomass were recorded in SSP root dip+PSB followed by INM practices. P root dip had significantly greater root biomass (4.36 g plant^-1) which was significantly higher than RDF (3.82 g plant^-1).
 

       
SSP root dip+PSB facilitated higher P uptake during root-dipping just before transplantation, which helped in development of more vigorous root system during early establishment of plant in the main field (Kalidas and Thakuria, 2018). Higher P uptake at the initial stage of crop growth provides an early advantage in root development (Chatterjee and Khan, 2005). Similar to the present result He et al., (2003) reported changes in root morphology and architecture indicated by increased total root length, root fineness and relative root allocation in the high-phosphorus layers, suggesting altered root morphology and preferential root proliferation in the high-phosphorus regions.
       
The rice seedlings when incubated overnight in SSP amended soil-water slurry before transplanting results maximum possible P uptake which help them in development of robust root system at the early stage of crop growth (Talukdar et al., 2001). Thus, robust root system can explore more soil volume and can uptake more P which is immobile by nature in soil (Arruda et al., 2016). This result is consistent with previous observations which observed that P-dipping produced greater shoot biomass (Rakotoson et al., 2022) and root biomass (Kalidas and Thakuria, 2018) from a very early growth stage after transplanting and eventually resulted in greater rice yields than those in conventional P application via broadcasting. Higher availability of soil P had direct positive influence on effective tiller numbers (Alam et al., 2009).
       
Grain and straw yields were significantly higher in all P treated plots compared to that in no P input control plots (P<0.05, Table 2). The highest grain yield achieved was in SSP-root-dip + PSB plot which was significantly higher than other options of P applications. Highest grain and straw yield of 5.80 and 7.82 t ha^-1 was recorded with root dip. However, straw yield was comparable with the INM. Similar to our result, Talukdar et al., (2001) reported that rice seedlings dipped overnight in SSP amended soil-water slurry just before transplantation recorded grain yield of 2.84 t ha^-1, while basal recommended dose of SSP recorded 2.46 tha^-1 in an acidic alluvium soil of Brahmaputra valley. Balasubramanian et al., (1995) reported 13% higher grain yield of rice in SSP-root dip method over that in SSP basal application as broadcast @ 26 kg P₂O₅ ha^-1.
       
Dipping seedling roots in P-enriched slurry transfer a considerable amount of P to the main field in the form of slurry attached to seedling roots at transplanting. The P thus transferred along with the root creates a P hotspot near the root zone which enhances root development and growth and facilitates the P uptake by rice plants even under highly P fixing soils (Oo et al., 2020b). Thus, the significantly higher root weight in both the P dipping treatments (T₂ and T₃) may be attributed to the better P availability in the root zone. The better root growth might have helped the plants to explore more moisture and nutrients resulting enhanced growth and yield of the crop. The relationship between crop yield and root biomass is often demonstrated to be significant and almost invariably linear (Li et al., 2009).
       
Grain yield was significantly increased by the root dip method (Table 2). Root dip application showed significantly higher yields (5.80 t ha^-1), compared with INM (5.53 t ha^-1) and RDF (5.46 t ha^-1). The average increase in the grain yield by Root dip, INM and RDF was 36.47%, 30.11% and 28.47%, respectively, compared to the control treatment. Similar to the present result, significant increase in shoot biomass, tiller number and photosynthetic efficiency due to P application, resulting in improved grain yield has been reported in many previous studies (Fageria et al., 2013;  Andrianary et al.,  2021). The highest benefit: cost ratio (Table 3) was observed in root dip method (2.44) followed by RDF (2.23), INM (2.20) and lowest was recorded in farmers practice (1.86). Although, higher yield was obtained in INM over RDF but high cost of organic inputs in INM package increased the cost of cultivation which resulted lower B:C ratio. The highest B:C ratio in root dip method was attributed to higher yield as well as reduction of cost of cultivation due to reduced use of chemical fertilizers.
 

 
P uptake and P efficiency indices
 
The P uptake was significantly higher in the root dip treatment (Table 4) and it was increased by 27.08% and 34.19% over RDF and INM, respectively. Highest P uptake in root dip method (23.98 kg ha^-1) was significantly higher as revealed from DMRT than RDF (18.57 kg ha^-1) and INM (17.87 kg ha^-1). However, no significant difference in uptake was observed between RDF and INM. A localized supply of phosphorus affects root morphology and root system architecture and thereby affect phosphorus uptake by rice plants (He et al., 2003). Larger root systems enable plants to access a greater volume of soil and to acquire more nutrients from various depths. Enhanced root growth could have enabled these plants to avail themselves of otherwise-unavailable subsoil P that could not be accessed by plants grown with conventional methods of P application (Barison and Uphof, 2011).  The result of the present study (Table 4) demonstrated that P-dipping can achieve high applied P use efficiency (1193.42) in transplanted rice compared to conventional incorporation of P (625.43). Thus, P-dipping is a potential strategy to overcome low applied P use efficiency in high P-fixing soils and hence reduce the need for excess P application. PRE is linked with the crop P uptake efficiency and it was higher in root dip (2.480) compared to INM (1.366) and conventional incorporation of P (0.794) probably as a result of increased P uptake by the crop mediated through better root growth and availability of applied as well as native soil P.
 

P-root dipping method of P application was found to be resource-efficient fertilizer management practices compared to P broadcasting in improving growth, grain yields and applied P use efficiencies on P-deficient lowlands in the hill zone of Assam with high P fixing soils.  This new method of P application is a practical approach which will overcome the low P use efficiency in lowland rice production and help farmers for maximizing the return with higher B:C ratio. Besides, this method is affordable for smallholder farmers in the hill region of Assam, as they have little financial capacity to purchase commercial fertilizers.

The authors are thankful to Dr. P.K. Pathak, Director of Extension Education, Assam Agricultural University, Jorhat, Assam for support to carry out this study. The authors would like to thank all the participant farmers for their excellent cooperation and participation in this study.

The experiment was conducted under the budget of Krishi Vigyan Kendra, Karbi Anglong, funded by Indian Council of Agricultural Research (ICAR), New Delhi.

The author declares that they have no conflicts of interest.