Influence of microclimate on crop growth, soil respiration and carbon sequestration under varied agronomic management in sali paddy in Assam

Introduction

Assam profoundly relies on rice (Oryza sativa L) as its primary crop, cultivated across 2.5 m ha, yielding 5.2 mt with a productivity of 2.1 t ha -1 (Economic Survey of India (2017-18). Despite being a rice surplus state presently, projections indicate a future demand of 13 mt by 2050 due to declining cultivable land and accelerated population growth (Anon., 2014) [1] . A targeted yield of 4.0 t ha -1 is essential, to maintain self-sufficiency of the state, that necessitating a significant fillip from the existing productivity level (2.1 t ha -1 ). Up-scaling of the productivity of kharif rice is decisive, as it holds the potential to play a crucial role in achieving this target. The effective use of land, water, nutrients, labour, capital, and other resources would result from maximum yield performance from a declining paddy field area, fostering sustainable agricultural methods. This move towards higher yield performance guarantees the preservation of essential resources for future generations while also promoting crop diversity (Kalita, 2020) [2] . When compared to traditional approaches, rice plants grown with the SRI methodology develop quickly, generate more tillers and leaves, and improve resource utilization, which results in better grain yields (Thakur et al., 2010 and Kalita et al., 2023) [3,4] . The synergistic combination of timely and meticulously applied agronomic techniques that generate a favourable microclimate and enable the crop to fully express its genetic potential is responsible for the higher yield of rice under SRI. Many research findings highlighted that lower plant density in the SRI facilitates better air circulation, reducing humidity within the plant canopy and resulting in decreased incidence of insect, pest and disease.

Consequently, this results in the growth of more robust and healthy rice plants.

Research conducted in North-east India opined modifications of plant spacing in SRI technique to optimize tiller production in this region of high rainfall. It stresses the urgent need for a more productive approach to kharif rice production that can address challenges related to food security, resource constraints, and changing climatic conditions (Das et al. 2014) [5] . This entail creating favorable microclimatic conditions to fully utilize the resources like light and water for better crop performance and yield sustainability (Kalita et al. 2023) [4] . The holistic manipulation in crop management practices in SRI techniques result in superior plant phenotypes and physiological processes, leading to increased resilience towards environmental stresses and ultimately greater crop yields (Visalakshmi et al., 2014) [6] . Worldwide researches indicate that through the adoption of SRI practices yields can be improved by 20-50%, and sometimes even more (Gopalakrishnan et al., 2013, Uphoff, 2015, Kalita, 2020 and Kalita et al., 2023) [7,8,2,4] . According to IPCC, 2013 [9] , the global emission of greenhouse gas (GHG) such as CO 2 , CH 4 and N 2 O is 76, 16 and 6 percent, respectively of which, CO 2 had increased from 280 ppm in the year 1850 to 395 ppm in the year 2014 that corresponding with global mean temperature rise of 0.85 °C in the time period of 1800 to 2012. The Paddy cultivation contributes 9 to 11% of the agricultural GHG emission, mainly CO 2 , CH 4 and N 2 O (IPCC, 2014) [10] . The rate of emissions of CO 2 can be mitigated by capturing it from the atmosphere, storing and maintaining in soil through "carbon sequestration". Indian soil possesses a large sink capacity for atmospheric CO 2 sequestration as it has inferior level of SOC. Improved agronomic practices could mitigate the impact of climate change through both carbon sequestration and minimized emissions of GHGs. The exchange of CO 2 between terrestrial ecosystems and the atmosphere is one of the key processes that affect atmospheric CO 2 concentration. The conventional rice production system has been pressurizing the natural resource base indiscriminately to feed the expanding population and thereby deteriorating soil health and rice ecosystem balance by escalating more emission of CO 2 , CH 4 , and N 2 O to the atmosphere (Ali et al., 2019) [11] . Among different rice ecosystem, the SRI is a promising carbon farming practices as it imparts low carbon footprints (Krishnan et al., 2017 and Kalita, 2020) [12,2] . In view of this, a field experiment was taken up to study the crop growth, yield and also to have an insight on status of SOC, CO 2 efflux and carbon sequestration potential (CSP) of kharif rice grown under SRI and conventional techniques at varied transplanting dates and hill densities.

Materials and Methods

The study was carried out at Nepalikhuti village of Golaghat district of Assam, India continuously for two years (2016 and 2017). The region has a subtropical humid climate with high annual precipitation (>2000 mm), high relative humidity (>80%), and temperatures ranging from 50 to 37 °C. There were 1242.60 mm and 1618.80 mm of rainfall throughout the experimental period of June to November of 2016 and 2017, respectively. The highest reported rainfall of 342.10 and 449.9 mm was received in the month of July of both years which coincided with the tillering stage of the crop. The weekly mean bright sunlight hour was 4.67 hours in the first year and 3.24 hours (0.5 to 6.8 hours) in the second. The soil of the experimental site is well drained sandy loam with acidic in reaction possessing medium level of soil organic carbon and available nitrogen, and lower in phoshorus pand potassium. The experiment was fitted in was factorial split plot design with two crop establishment methods viz. SRI (C 1 ) and conventional (C 2 ) transplanted in three different dates viz. 26 th June (D 1 ), 10 th July (D 2 ) and 25 th July (D 3 ) as the main plot treatment, and at four levels of hill densities viz 20cm x 15cm (S 1 ), 20cm x 20cm (S 2 ), 25cm x 20cm (S 3 ) and 25cm x 25cm (S 4 ) as sub plot treatment. The test variety was "Shraboni" which is a medium duration high yielding variety with average yield of 5 t ha -1 . The age of the transplanted seedlings was 12 and 21 days old for SRI and conventional technique, respectively. The crop was nurtured with 10 t ha -1 of FYM along with recommended integrated nutrient management package. At different stages of the crop such maximum tillering stage (MTS), flowering and physiological maturity (PM), growth parameters such as plant height, number of tillers m -2 etc were found out following standard procedures. The CO 2 flux per unit area per unit time was measured in-situ by alkali absorption method (Gupta and Singh, 1981) [13] . The CO 2 emission rate was calculated as follows:

Where, B: Volume of HCl needed to titrate NaOH solution from the control V: Volume of HCl needed to titrate NaOH solution in the vial N: 1.0 (Molarity of HCl) E: Equivalent weight (6 for C; 22 for CO 2 ). The emission was expressed as mg CO 2 -C m -2 d -1 .

Carbon sequestration potential was estimated in terms of increase in carbon stock in the soil. Here, CO 2 emission from the field was not taken in account. The SOC content was measured for both initial and final stage for all the treatments (Pathak et al. 2011 and Singh et al. 2014) [14,15] . The mass of SOC in the particular thickness of soil layer was then calculated as follows:

Where, MSOC: Mass of SOC (Mg ha -1 ), SOC: Organic C concentration in soil (%) BD: Bulk density (g m -3 ) and T: Thickness of surface layer (cm) Carbon sequestration potential (CSP) of the treatment was calculated as CSP treatment = MSOC treatment -MSOC initial Where, CSP treatment: Carbon sequestration potential of each treatment MSOC treatment: Final SOC (Mg ha -1 ) of each treatment MSOC initial: Initial SOC (Mg ha -1 ) In this study, the data pertaining to each parameter were subjected to statistical analysis using the "Analysis of Variance" method as outlined by Cochran and Cox (1957). The significance of treatment effects was evaluated through the Ftest, with the variance ratio (F) being considered significant. The critical difference (CD) was reported at a 5% significance level if the F-test gave significant results; otherwise, the standard error of the mean (S.Em) was mentioned.

Result and Discussion

Effect on crop growth Data on plant height and number of tillers m -2 as influenced by imposition of microclimatic variation in terms of method crop establishment, date of transplanting and various level of plant density are presented in Fig. 1 &2. With respect to varied methods of crop establishment, the mean plant height under SRI was significantly taller than that of the crops under conventional system at flowering and physiological maturity. However, crop establishment methods failed to show any significant variation in MTS. Throughout the crop growing stages, significantly higher number of tillers m -2 was recorded in SRI crops. Increased plant height and tiller numbers m -2 under SRI might be due to shallow planting of younger seedling in resulting in extensive root system that absorbing more nutrients from the soil consequently enhancing emergence of more tillers from the collar region compared to conventional method besides increasing plant height. The negligible disturbance of growing buds and the roots during transplanting of SRI seedling facilitated significant improvement in growth parameters of SRI crop (Thakur et al., 2015 and Hosain et al., 2018) [16,17] . Plant height was found to be not affected by date of transplanting at any stages of crop growth; however, significant variation was noted with respect to number of tillers m -2 . Highest tiller number was registered under early transplanting (26 th June) followed by intermediate (10 th July) and late transplanting (25 th July). In flowering and physiological maturity stage, 26 th June planted crop registered the maximum tiller numbers m -2 , which was statistically at par with 10 th July planted crop. Further, quantity of tillers recorded in 25 th July planted crop was found to be comparable as that of 10 th July. Such superiority of early transplanting dates with respect to number of tillers per plant was also reported by Sharma et al. (2011) [18] and Sharma et al. (2018) [19] . Interaction between crop establishment and date of planting was found to be non-significant. In respect of hill density, wider spacing i.e. hill density of 16 hills m -2 registered the tallest plant which was at par with hill density of 20 and 25 hills m -2 . Significantly lowest height was observed in 20cm x 15cm spacing (33 hills m -2 ). Similar trend was also noted with respect to tiller number m -2 , the lowest being recorded under hill density of 33 hills m -2 . The highest number of tillers was recorded in microclimatic variation imposed by 20 hills m -2 which was comparable with both 16 and 25 number of hill m -2 . The increased magnitude of plant height and tiller m -2 under lower hill density might be attributed to efficient utilization of growth resources in lower degree of intraspecific competition that triggered a substantial fillip in photosynthetic rate. Similar was reported by Singh et al. (2012) [20] and Chakrabortty et al. (2014) [21] .

Effect on Grain Yield

The two years of yield data along with the pooled data presented in Fig. 3 revealed that the superiority of SRI technique in terms of grain yield which was on an average 1.17 times higher than that achieved from conventional one. Significantly greater yield of 57.13 q ha -1 was obtained owing to SRI technique of crop establishment while the conventional crop registered 48.83 q/ha of grain yield. The improved yield under the SRI method can be attributed to the practice of planting young, single seedlings per hill in an environment free from hypoxic conditions. This approach likely promoted enhanced growth and physiological characteristics, leading to more efficient utilization of light, nutrients, and water. Consequently, this facilitated an accelerated photosynthetic rate and effective translocation of assimilates to the sink. These findings align with earlier studies by Mitra et al. (2013) [22] and Thakur et al. (2015) [16] and Kalita, et al. 2023 [4] . The Fig 3 also indicates the significant differences in crop performance due to various transplanting dates. Specifically, the early transplanting on 26 th June resulted in the highest grain yield of 54.86 and 58.16 q ha -1 in 2016 and 2017, respectively which was followed by the 15-day delayed transplanting on 10 th July and the 30-day delayed transplanting on 25 th July. The substantial improvement in grain yield associated with the early transplanting date (26 th June) was consistently observed in both years. Pooled analysis further confirmed the superiority of early planting which was 1.02 and 1.09 times significantly higher than that of the July plantings dates, viz. 10 th and 25 th July, respectively. The yield reduction under delayed transplanting conditions can be attributed to unfavorable weather parameters, leading to the development of a poorly developed canopy which eventually resulted in insufficient photosynthesis coupled with partial and ineffective translocation of assimilate from source to sink. Previous research studies, such as those conducted by Gautam et al. (2019) [23] and Kalita, (2020) [2] have also emphasized on the significant impact of meteorological conditions on growth, yield and overall production of paddy worldwide. Irrespective of crop establishment methods and transplanting dates, the grain yield, influenced by varying hill densities, revealed a significant drop off with increasing density levels.

The highest grain yield of 55.21 qha -1 and 58.29 qha -1 were recorded under the hill density of 25 cm x 25 cm in 2016 and 2017, respectively which were statistically comparable to hill densities of 20 and 25 hills m -2 in both years. Pooled analysis further confirmed a 1.12 times increase in grain yield when adopting a hill density of 16 hills m -2 compared to the closest density of 33 hills m -2 . The superior yield under wider hill densities can be attributed to enhanced yield attributing parameters, resulting in a larger sink size and efficient translocation of photosynthates to the sink, particularly when the crop was cultivated under optimal planting patterns. These findings align with previous research by Das and Chandra (2012) [24] , highlighting the importance of planting density in optimizing grain yield in rice cultivation.

Carbon sequestration study CO2 flux:

The CO 2 flux measured at maximum tillering stage are presented in Table 1 and Fig. 4 indicates significant influence of all the three factors viz. crop establishment methods, date of transplanting and various measures of hill density on emission of CO 2 from the soil. Significantly greater quantum of CO 2 efflux measuring to 5935.42 mg CO 2 m -2 d -1 and 6082.47 mg CO 2 m -2 d -1 was recorded under SRI establishment in 2016 and 2017; which were 454.11 and 456.44 unit higher than that of conventional one. The underlying reason of higher emission of CO 2 in SRI might be due to increased rate of respiration resulted from the luxuriant and vigorous vegetative growth specially the root system. This got more intensified as a result of the absence of the barrier i.e. standing water due to its unique water management practice in SRI resulting in accelerated decomposition of organic matter. These findings corroborate with those reported by Liu et al. (2013) [25] and Krishnan et al.

The CO 2 flux was found to be significantly influenced by the date of transplanting. In 2016, maximum efflux was recorded in 10 th July planting, which was at par with the late planting (25 th July) and significantly superior to the early one i.e. 26 th June planting. In the 2 nd year, maximum emission was registered in late planting which was followed by 10 th July and lastly by 26 th June planting. However, the quantum of emission in 25 th July planting was at par with its 15 days earlier planting. The tendency of more emission with delaying of transplanting date might attributable to increased mean daily temperature which led to increase in soil temperature and thereby, soil respiration. Increased emission of soil CO 2 owing to increased temperature was reported by many researchers (Sugihara et al., 2012 and Dilekoglu and Sakin, 2017) [26,27] . With the increase of hill density, emission of CO 2 was found to be increasing significantly in both the year. The maximum quantum of release was measured at highest density under study i.e. 33 hills m -2 which was comparable to its immediate lower one (25 hills m -2 ). This might attribute to increased root respiration per unit area per unit time under closer plant density.

Carbon sequestration at harvest

The appraisal of data presented in Table 2 reveals that the method of crop establishment could not bring any significant impact on organic-C and CSP of both 0-15 and 15-30 cm layer of soil at harvest though the SRI method, showed a marginal improvement in soil organic-C and CSP as compared to conventional one. However, a significant interaction effect of crop establishment methods and planting geometry on SOC was observed in the 2 nd year of investigation (Table 3). SRI crops raised at lower hill density showed significantly higher SOC build-up in comparison to conventional crop grown in 25cm x 20cm and 25cm x 25cm as well. The tendency of building-up of soil organic-C under SRI might attribute to the typical microclimate created specially as a result of conoweeding and water management practice that enhancing the performance of the rhizospheric microbes on decomposition of the large root system of the SRI crops. This is in agreement with the findings of Dumas-Johansen (2009) [28] and Rajkishore et al. (2015) [29] .

Conclusion

In nut-shell, it may be concluded that early planting of kharif paddy on 26 th June at 25cm x 25cm apart following SRI method could be a promising crop management option to access the complimentary crop microclimatic condition to perform the best. Besides, crop performance, the study draws attention to consideration of microclimatic factors to optimize carbon farming for better sequestration in kharif paddy field in long run.

Acknowledgement

AcknowledgementThe authors are thankful to Department of Agronomy, Assam Agricultural University (AAU), Jorhat and KVK-Golaghat, AAU-Khumtai for providing necessary facilities for field and lab works.

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