Preparation and nutrient release kinetics of enriched biochar-based NPK fertilizers and agronomic effectiveness in direct seeded rice


 In present study, two enriched biochar-based fertilizers were prepared having fertilizer grade of 6-6-4 N-P2O5-K2O by intercalation of NPK fertilizers mixture solution as EB-1 and additional humic acid and seaweed extract as EB-2. In laboratory, batch experiment were done to compare nutrients (NH4+, NO3−, P and K+) release patterns of developed fertilizers along with conventional fertilizers. Enriched biochar fertilizers (EB) demonstrated much slower release pattern of NH4+, P and K+, however NO3− release was similar over conventional fertilizers. The cumulative release of N in EB fertilizers was similar to conventional fertilizer, however significantly less of P and K were released during the period of 72 hrs. The field response study of enriched fertilizers EB-2 revealed 29.5, 11.5 and 22.9% higher apparent use efficiency than conventional fertilizer. The slow nutrients release behaviour of EB fertilizers implies reduced losses and enhanced NUE as reflected by higher apparent recovery of N, P and K.


Introduction
Nutrient use e ciency (NUE) is a vital concept for assessing farming system which is greatly dependent on fertilizer management as well as soil-plant-water relationships. The nutrient use is mainly used to increase the crop performance by providing optimum nourishment while minimizing nutrient losses from the eld and supporting sustainability by contributing to soil quality components. Use e ciency of nitrogenous fertilizers is around 30-40 %, phosphate fertilizers are 15-20 %, potassic fertilizers is 50-60 % and micronutrient is 1-2 % (Singh et al. 2014). Among the different approaches initiated to increase the nutrient use e ciency; control release fertilizer may be a novel environment-friendly approach. Control-release fertilizers delays the availability of nutrients after application for plant uptake and use, or which extends the nutrient availability to the plant for signi cantly longer period compared to the conventional fertilizers and thereby enhance nutrient use e ciency. Biochar prepared from agricultural wastes viz., rice straw and husk may be utilized as a suitable matrix to prepare control release fertilizers (Singh et al. 2020;Chaturvedi et al. 2021). It will not only enhance NUE but also improve soil health and crop yield with most economic management of agro-wastes.
Biochar is a product of the pyrolysis of organic materials such as wood, rice husk, rice straw, leaves, grasses, crop residues and manure as a stable, recalcitrant organic carbon-rich amendment to improve soil bio-physiological and chemical properties (Lehmann et al., 2006). Biochar has a porous structure, huge surface area, a large number of functional groups, and plentiful mineral elements, that bene t loading of fertilizer nutrients and availability of heavy metals in the soil (Dong et al. 2019;Liu et al. 2019). Presence of surface functional groups on biochar strongly adsorbs various nutrient ions, including NH 4 + , NO 3 -, P and K + ions (Kimetu 2010;Mizuta et al. 2004). This promotes to load nutrients on biochar and reduce their losses (Singh et al. 2020;O'Connor et al. 2018). Sorption by the biochar is the primary reason for reduced mean cumulative leaching of nitrate and nitrite in biochar-amended soil (Mukherjee et al. 2014). Therefore, application of biochar as a soil amendment or slow-release fertilizer could make farming more productive by compensating for acidity, improving organic carbon, water retention and nutrient availability and bring more area under agriculture (Fryda and Visser 2015).
Despite the positive impacts of biochar in agricultural systems, it is usually very expensive to apply mainly due to the high cost of collecting feedstock as well as the high costs of pyrolysis plants. Biochar (Asai et al. 2009) due to the insu cient supply of nitrogen thus, augmenting biochar with fertilizer nutrients makes biochar composite suitable for plant growth and yield (Si et al. 2018) and heavy metal adsorption (Zhao et al. 2016). Due to the farming and ecological applications, enriched biochar fertilizers have received growing attention. Recent studies suggest that biochar-based fertilizers delay the release of nutrients in the soil displaying a slow-release effect (Chaturvedi et al. 2021). Enrichment of soil with biochar-based fertilizer adjust the soil pH, reduce the bulk density, improve soil aeration and water permeability and retention to increase the crops yield (Gao et al. 2012;O'Connor et al. 2018). Addition of enriched biochar in soil as a carrier of nutrients is enormous. Moreover, it could guide nutrient release in sustained manner and extended period due to its micro porous structure and extensive surface area (Ghezzehei et al. 2014;Singh et al. 2020). Collectively, these characteristics makes biochar-based fertilizer a novel approach that could reduce losses, increase nutrient bioavailability, mitigate GHG emission, improve soil health and consequently crop growth and biomass yield.

Ammonium (NH 4 + )
For determination of release pattern of NH 4 + from conventional fertilizers mix, EB-1 and EB-2 batch experiment was followed. These three fertilizers were taken in 10 conical asks for each sample on 1g N content basis. After that 10 g of soil sample from experimental eld was added to each ask and 30 mL of water also added to each ask. At predetermined time intervals selected asks samples were ltrated in 100 mL volumetric ask and after collecting ltrate lled the volumetric ask up to 100 mL. After collecting all the ltrates at all predetermined time intervals.
Modi ed kjeldhal method was followed to determine NH 4 + content in samples. Calculated NH 4 + content (ppm) in samples from recorded readings were plotted in graph against time intervals to get NH 4 + released pattern of these three fertilizers. Both the cumulative and instantaneous concentration graph at predetermined time intervals were drawn.
After determination of NH 4 + content of the samples, 0.2 g of Devarda's alloy was added to the samples and again Modi ed kjeldhal method was followed to determine NO 3 content in samples. Calculated NO 3 content (ppm) in samples from recorded readings were plotted in graph against time intervals to get NO 3 released pattern of these three fertilizers. Both the cumulative and instantaneous concentration graph at predetermined time intervals were drawn.

Phosphorus (P)
For determination of release pattern of P from conventional fertilizers mix, EB-1 and EB-2 batch experiment was followed. These three fertilizers were taken in 10 conical asks for each sample on 0.1 g P 2 O 5 content basis. After that 10 g of soil sample from experimental eld was added to each ask and 30ml of water also added to each ask. At predetermined time intervals selected asks samples were ltrated in 100 mL volumetric ask and after collecting ltrate lled the volumetric ask up to 100 mL. After collecting all the ltrates at all predetermined time intervals, ammonium molybdate and potassium antimony react method was followed to determine P content in samples.
Calculated P content (ppm) in samples from recorded readings were plotted in graph against time intervals to get P released pattern of these three fertilizers. Both the cumulative and instantaneous concentration graph at predetermined time intervals were drawn.

Potassium (K)
For determination of release pattern of K from conventional fertilizers mix, EB-1 and EB-2 batch experiment was followed. These three fertilizers were taken in 10 conical asks for each sample on 0.1 g K 2 O content basis. After that 10 g of soil sample from experimental eld was added to each ask and 30 mL of water also added to each ask. At predetermined time intervals selected asks samples were ltrated in 100 mL volumetric ask and after collecting ltrate lled the volumetric ask up to 100 mL. After collecting all the ltrates at all predetermined time intervals, Flame emission spectrophotometry method was followed to determine K + content in samples. Calculated K content (ppm) in samples from recorded readings were plotted in graph against time intervals to get K released pattern of these three fertilizers. Both the cumulative and instantaneous concentration graph at predetermined time intervals were drawn. conventional fertilizers mix as basal and rest half of N was applied in two equal splits at 30 and 60 DAS as top dressing through urea for treatment T1. Whereas for treatments T2, T3 and T4, half dose of N and full dose of P and K were applied through EB-1 as basal and rest half of N was applied in two equal splits at 30 and 60 DAS as top dressing through UEB-1. For treatments T5, T6 and T7, half dose of N and full dose of P and K were applied through EB-2 as basal and rest half of N was applied in two equal splits at 30 and 60 DAS through UEB-2 as top dressing. For evaluating nutrient-use e ciency of enriched biochar-based fertilizers apparent nutrient recovery (%) was determined using following formulae:

Results And Discussion
Characteristics of biochar and enriched biochar (EB-1 and EB-2) The surface morphology of biochar derived from rice husk is primarily performed by SEM and showed a highly porous structure with smooth and tight. The porous tubular morphology of rice husk biochar (RHB) favour it to store/reserve the nutrients and slow release to soil system (Fig. 1a&b) (Wen et al., 2017). Under pyrolysis temperature of 200 to 400 ℃, the feedstock was carbonized incompletely, so numerous oxygen functional groups were remaining in biochar, which could provide abundant negative potential charges for the adsorption of NH 4 + , owing to hydrogen bonding and electrostatic interactions (Cai et al., 2016). In the present study, NH 4 + released from the conventional fertilizers mix at the logarithmic rate i.e. released at faster rate with shorter time interval up to 1 h, after that release rate with increase in time interval were less and at the end of the experiment ( Gwenzi et al (2017) for biochar based slow-release fertilizer (BSRF).

Phosphorus (P) release pattern
In general, enriched biochar based fertilizers (EB-1 and EB-2) had consistently lower phosphorus release than the conventional fertilizers mix throughout the 36 hours monitoring period (Fig. 4a&b). Total phosphorus release was signi cantly lower for EB-1 and EB-2 than conventional fertilizers mix up to 9 h of starting of batch experiment and after that total releases were statistically at par. P release pattern for conventional fertilizers mix was characterized by general decline with time with its peak at 3 h after starting the experiment. The peak of P release pattern for both of the enriched biochar fertilizers were different. P release from EB-1 reached its peak around 6 h and from EB-2 reached its peak around 9 h after starting the experiment. Gwenzi et al. (2017) also found that in P release pattern the cumulative concentration of PO 4 3released from biochar based slow release fertilizer (BSRF) was approximately half of that of commercial fertilizer Compound D during a 72 days' period of sequential leaching. In the present study, there was no signi cant difference in P release pattern for both of EB-1 and EB-2. The P in the matrix of biochar is slowly released due to physical protection of the biochar pores network (Dias et al., 2018). The water action removes aliphatic groups (hydrophobicity) on biochar surface, increasing its a nity for water (Das and Sarmah, 2015) interacting with hydrophilic groups resulting a swelling of the biochar coating making a new arrangement which nally controls the water movement inside and outside of the fertilizer (Dias et al., 2018). Till date, limited studies have evaluated nutrient release patterns of slow release NPK fertilizer using biochar as a nutrient carrier. Yao et al. (2013) used biochar to remove P from aqueous solution and concluded that P-laden biochar could possibly act as a slow-release fertilizer. It did not include other nutrients such as cations and highly soluble and mobile nutrients in its study. Moreover, a conventional chemical fertilizer was also not taken for comparison. Therefore, the present study focused to develop a biochar-based slow-release NPK fertilizer and evaluate its nutrient release pattern.
Potassium (K + ) release pattern Peak K + releases from the conventional fertilizers mix as well as from the enriched biochar based fertilizers (EB-1 and EB-2) occurred at 1 h after starting the experiment but that for EB-1 was about 60.50 % and for EB-2 was about 44.22 % lower than that of conventional fertilizers mix (Fig. 5a&b). Overall cumulative concentration of K + released by both EB-1 and EB-2 were lower than that for conventional fertilizers mix up to 36 h of experiment. Similar result was reported by Gwenzi et al. (2017) that overall cumulative concentration of K + released by biochar based slow release fertilizer (BSRF) was 1.5 times lower than that of for commercial fertilizer Compound D and peak K + release was signi cantly lower for BSRF than that of Compound D. In the present study, there was no signi cant difference in K + release pattern for both of EB-1 and EB-2.
Evaluation of NH4 + , NO 3 -, PO 4 3-, and K release patterns con rmed the slow-release behavior of the biochar enriched fertilizers . Nutrient release patterns of biochar enriched slow-release fertilizers were characterized by an initial high release in the rst few observations mainly attributed to rapid diffusion and washout as also reported by Gwenzi et al. (2017). The release of PO 4 3and K by enriched fertilizers was characterized by distinct multiple peaks suggesting a number of release mechanisms which was not the case with N. Moreover, the cumulative PO 4 3and K releases of the enriched fertilizers (EB-1 &EB-2) were approximately half that of commercial fertilizer mix in period of study. The manifold mechanisms of nutrient release from biochar enriched fertilizers include pore diffusion, ion exchange on charged biochar surfaces of biochars, co-precipitation, complexation/ligand exchange, van der Waals/electrostatic forces, and p-cation interactions between solute ions and aromatic rings of biochar (Wang and Chen 2006;Berber-Mendoza et al., 2013). Moreover, the use of an adjuvants may enhance the slow-release behavior of the biochar-based fertilizers.
Apparent nutrient recovery (%) of N, P and K Apparent nitrogen recovery was higher under application of 100 % fertility dose through EB-2 + UEB-2 which was 29.5 % more than application of conventional fertilizers at that same fertility dose (Table-2). Even application of EB-1 + UEB-1 at 75, 100 and 125 % fertility levels had lower apparent N recovery as compared to EB-2 + UEB-2 at similar fertility levels, respectively. However, application of both the enriched biochar fertilizers at all the fertility levels had more apparent N recovery as compared to application of 100 % fertility level through conventional fertilizers. More N uptake from enriched biochar sources might be the principle reason for their higher apparent recovery. Apparent P and K recovery was higher under application of 100 % fertility dose through EB-2 + UEB-2 of 29.39 % and 83.06 % of P and K respectively. Further, it was 11.5 % and 22.9 % more than application of conventional fertilizers at that same fertility dose (Table-2). Huang et al. (2013) reported that due to addition of biochar, agronomic nitrogen use e ciency was increased by 43 % over no biochar addition at 100 % fertility level. Slow release of nitrogen, phosphorus and potassium from NPK enriched biochar-based fertilizers is mainly attributed to their slower release kinetics, reduced losses and indirectly by enhanced availability of soil nutrients thereby higher apparent use e ciency of N, P and K under enriched biochar fertilizers application. Several studies have shown that biochar application enhanced the microbial activity including soil enzyme activity and soil respiration (Steiner et al., 2008), promote root colonization by arbuscular mycorrhiza (Elmer and Pignatello, 2011). These root exudates and enhanced microbial activities could further promote release of nutrients from insoluble soil nutrient pools. Therefore, biochar based slow-release fertilizers have several potential niche applications in crop production, horticulture, olericulture including environmental remediation.

Conclusions
Two novel enriched biochar-based fertilizers (EB-1 and EB-2) were developed with much slower nutrient release patterns as compared to that of conventional chemical fertilizers. The slow nutrients (NH 4 + , P and K + ) release behavior Yao Y, Gao B, Chen J, Yang L (2013)   Cumulative (a) and Instantaneous (b) concentrations of P released Figure 5 Cumulative (a) and Instantaneous (b) concentrations of K+ released