Photosynthetic Pigments, Symbiotic Traits, Yield Attributes and Nutrient Uptake of Soybean Crop as Influenced by Soil Amendments on Degraded Vertisol of Chambal Tablelands

Soybean [Glycine max (L.) Merrill] is a major kharif oilseed crop of central and western India is commonly known as golden bean, containing about 40-42 per cent protein and 18-22 per cent oil occupying almost 11.6 million ha registering productivity of 10.45 q ha^-1 in vertisol under rainfed ecosystem of Madhya Pradesh, Rajasthan, Maharashtra, Karnataka, Chhattisgarh and Telagana (Dupare et al., 2020). When compared to other soybean cultivating countries, crop productivity is low in India. Major constraints in soybean productivity is mainly attributed to improper land treatment, imbalance nutrient supply, abiotic biotic stress erratic weather patterns and moisture stress (Ramesh et al., 2017). Soybean-mustard and soybean-wheat is the common cropping system of the central and western India. With negligible application of sulfur and continuous cropping of oilseed crops, large scale sulfur deficiency is commonly observed in vertisol (Chahal et al., 2020). Being oilseed legume crop, sulfur requirement of soybean is comparatively higher than cereal crops (Rashmi et al., 2018). In India, 41 percent (Singh 2001) and in Rajasthan, 34% of soils are deficient in S (Shukla et al., 2021). Soil salinity, is another major challenge in sustaining oilseed crop production in semi-arid tropics (Ramesh et al., 2017). In this regard, gypsum is known to sustain Ca/ Na and K/Na, in soil solution, reducing soil pH and providing sulfur nutrient to crops (Bello et al., 2021). Maintenance of soil organic matter is pre-requisite for sustainable crop production, however, governed by other factors such as temperature, rainfall, cropping practices etc. Farm yard manure (FYM), crop residues (CR), green manure, root exudates significantly contribute to soil organic matter build  up in soils. Gypsum supply S, that enhances crops tolerance and resistance to biotic and biotic stress, by increasing chlorophyll components, amino acids and other metabolites thereby improving nutrient uptake (Cannon 2017). Crop residues (CR) application offers innumerable benefits including organic matter addition, moisture conservation, reducing soil erosion, soil fertility improvement and their recycling onto arable lands thus are inevitable strategy for improving soil health and sustainability of agroecosystem on long term basis (Turmel et al., 2015). Crop residue Furthermore, conjoint use of gypsum with organic amendments have great potential in improving crop production and improving crop health. Previous studies on the use of gypsum and FYM as sole amendment has been performed with lower doses on soybean crop (Raza et al., 2018; Bhairwa et al., 2021). Moreover, earlier studies either evaluated saline soil reclamation with gypsum mostly on rice cropping system (Murtaza et al., 2016; Sharma et al., 2016). Imbalanced use of NPK fertilizers, no use of organic sources and sulphur fertilizer by the farmers in the region results in poor yield of soybean crop as well as deterioration of soil health. Limited information of high dose of gypsum and organic amendments application on soybean crop has been reported for saline soils. The study would provide new insights on the impact of soil amendments on crop phenological and yield attributes such as chlorophyll a/b, carotenoids, nodulation traits, carbohydrate equivalent yield and nutrient harvest index in soybean crop grown under saline-sodic soils. Keeping this in view, the present investigation was planned to assess the effect of soil amendments on chlorophyll pigments, symbiotic traits, crop growth and yield attributes, profitability, nutrient uptake and nutrient harvest index (NHI) of soybean crop in degraded vertisols of western India.

The experiment was conducted for conducted for two consecutive years (2017-18 and 2018-19) at IISWC (Indian Institute of Soil and Water Conservation), Research farm, Kota (Latitude- 25°13' 29" to 25°14' 18" N; Longitude-75°52'18"  to 75°52' 44" E) which is a table land adjacent to Chambal ravine landform in semi arid region of south eastern Rajasthan state, India. Initial soil properties before experiment were recorded having soil pH 8.2, low soil organic carbon 3.5 g kg_-1, high exchangeable sodium percentage (ESP) value of 16.7%, low available sulfur (S) content of 4.4 mg kg^-1. Gypsum was broadcasted manually at the rate of 2.5 t ha^-1 before crop sowing. During the experiment 10 t ha^-1 of farm yard manure (FYM) applied one month before sowing and 3 t ha^-1 of mustard crop residue (CR) were applied to cover soil suface after sowimg. Soybean as the test crop sown at 30 × 10 cm spacing during kharif season of two years (2017- 2018). Total rainfall received during soybean crop growth period of 2017 and 2018 was 495 and 798.0 mm, respectively. Crop was sown with the commencement of south west monsoon during last week of June to first week of July every year. The experiment was laid out in random block design (RBD) with three replications and eight treatments viz.; T1: Control (absolute control); T2:  Recommended Dose of Fertilizer (RNPK) for Soybean (N:P:K- 20:40:40 kg ha^-1); T3:  RNPK+Gypsum (2.5t ha^-1); T4: RNPK+FYM (10 t ha^-1); T5: RNPK+CR (mustard stover @ 3 t ha^-1); T6: RNPK+Gypsum+CR; T7:  RNPK+Gypsum + FYM; T8: RNPK+Gypsum+CR+FYM. Nitrogen and phosphorus applied through diammonium phosphate DAP (18% N, 46% P), urea (46% N) and muriate of potash MoP (49.8% K) respectively. Almost 2/3^rd of N and full dose of P and K was applied as basal dose during sowing and remaining 1/3rd N was applied after 35 DAS. Soybean biometric observations like nodules per plant and nodule dry weight per plant was recorded from randomly selected five plants at 45 DAS. Dried nodules were used for total N analysis by micro-kjeldahl method. Dry matter accumulation (DMA) at 30, 60 DAS and harvest stage were recorded from all the treatments. Other attributes like chlorophyll a, b, total chlorophyll and carotenoid content assessed during 45 and 60 DAS were calculated using equations based on the specific absorption coefficients (Henry and Grime, 1993). Yield attributing parameters like plant height, pods per plant, 100 seed weight, grain and stover yield was recorded during maturity and harvest. For plant analysis, seed and straw was digested with acid mixtures and was analysed for N by micro kjeldahl method, P by vanadomolybdate yellow color method and K content by flame photometer. Nutrient harvest index for N, P, K, S was calculated by dividing nutrient uptake in seed with total nutrient uptake by crop and multiplied by hundred. The economic yield of soybean crop converted into equivalent value of carbohydrate (kg ha^-1) (Gopalan et al., 2004) and carbon output was calculated based on the biomass of the plant which contains on an average 44% carbon (Lal, 2004). Data was statistically analysed following standard procedure based on analysis of variance (ANOVA) using SPSS software (version 23). Means were compared with Turkeys’ test at 5% level of significance.

Impact of amendments on chlorophyll pigments, symbiotic traits and biomass of soybean
 
Chlorophyll a, b and carotenoid content in soybean leaf at 30 and 60 DAS significantly (p<0.05) increased in T6, T7 and T8 treatments against control and T2 (Table 1). Gypsum addition supplied sulfur, owing to its role in N metabolism and chlorophyll content in leaves and photosynthetic pigments. In addition, FYM amended treatments showed higher total chlorophyll content in T8 (1.561 mg g^-1) and T7 (1.593 mg g^-1) at 30 DAS because integration of organic amendments helps in maintenance of essential nutrients, improves nitrogen uptake and with increased N, chlorophyll (‘a’ and ‘b’) also increases (Cannon 2017). Moreover, sulfur applied through gypsum plays crucial for Rubisco and chlorophyll, which are two crucial components for photosynthesis. Raza et al., (2018) reported that addition of sulfur improves Chl a and b content and the ratio between Rubisco and plant soluble protein thereby improving net photosynthetic rate of plants. Dry matter accumulation (DMA) recorded at 30, 60 DAS and at harvest illustrated that it increased gradually with advancement of crop age (Table 2). The combination of gypsum with FYM and CR showed higher values of DMA at all stages over control treatments. Our results are in line with those of Gupta et al., (2018) and Raghuwanshi et al., (2017) who reported similar growth response in soybean and other pulse crops with addition of sulfur and organic manures. Reduced photosynthesis under S deficient conditions of T1(control) and T2 cause decreases in CO₂ assimilation rate and reduce dry matter accumulation in plants. The number of nodules per plant and its dry weight was significantly influenced by different combinations of amendment (Table 2). The number of nodules per plant varied from 11.2 to 33.6 and maximum number of nodules per plant (31.6-33.6) was observed in T5, T6, T7 and T8 treatments which were statistically at par with each other and was two times higher than T2. Addition of gypsum with organic amendments improved the environment for better nodulation and N fixation in soybean. Similarly, maximum dry weight of nodules was also higher in sole and combined amendment treatments over control plots without any amendments (Table 2). Nodule N content followed the similar trend of nodulation and was highest for T8 (2.36%) followed by T7 (2.26%), T6 (2.05) and T5 (1.95%) which was significantly higher than control. Sulfur is an important component of ferredoxin, a Fe protein found in chloroplast. Gypsum added S which is acid producing amendment, regulating soil reaction by improving rhizobium activity showing positive impact on soybean yield. This protein plays metabolic role in N absorption by rhizobacteria in root nodules (Cannon, 2017; Raghuwanshi et al., 2017) and sulfur being component of amino acids, its addition promotes root nodulation in legumes. Improvement in nodulation of soybean crop with addition of FYM with 100% NPK showed maximum and higher nodulation over only NPK (Bairwa et al., 2021).
 

 

 
Impact of amendments on yield attributes, crop yield and profitability analysis
 
The data on soybean yield attributes such as plant height, branches per plant, 100 seed weight (Table 3) were significantly higher for T8> T7> T6> T5 treatments followed by T3>T5. Maximum number of soybean pods per plant and 100 seed weight was observed in T8 (36.9 pods plant^-1 and 12.8) followed by T7 (36 pods plant^-1 and 12.4) and T6 (35.8 pods plant^-1 and 12.5) respectively. Application of gypsum, crop residue, FYM with chemical fertilizer (T8) in soybean produced highest mean grain yield (1304 kg ha^-1) followed by T7 (1264 kg ha^-1) > T6 (1251 kg ha^-1) and least yield (550 kg ha-1) was observed in control plots. Soybean grain and biological yield varied significantly (p<0.05) across the treatments with amendment application. Compared to T2, soybean grain yield increased by 49-53% combined amendment (T6, T7, T8) and 18-38% (T4, T3, T5) in sole amendment treatments. Similarly, these treatments exhibited better performance in soybean biological yield (Table 3). Positive influence of amendments on root nodulation, total chlorophyll content and dry matter accumulation attributed to higher yield in these treatments. Further gypsum application enhanced available S content, bio resource cycling through FYM, CR enhanced soil organic matter, nutrient mobility and soil water storage capacity enhancing crop yield (Rashmi et al., 2021; Gupta et al., 2016). Yadav et al., (2019) reported addition of sulfur through gypsum or other sources attributes to improved vigour and development of legume crops as reflected in dry matter accumulation and crop yield. Further, carbohydrate equivalent (266 and 264 kg ha^-1) was recorded highest in combined application of organic amendments and gypsum. Similarly, carbon output (1712 kg CO₂ eq. ha^-1) was recorded highest in T8 followed by T6 (1696.9 kg CO₂ eq.  ha^-1) and T7 (1609 kg CO₂ eq. ha^-1).  This is because of the positive response of combination of soil amendments with RNPK which improved soil environment for optimum crop growth which resulted in higher yield and total harvested crop biomass. Upadhyay et al., (2022) reported higher carbohydrate equivalent yield and carbon output with maximum crop and stover yield in wheat and rice crops. Gypsum amendment addition fetched higher net return especially in T3 (₹ 14499/-) and T6 (₹ 15247/-) treatments as compared to other gypsum treatments (Table 3). Low net return in T7 and T8 treatments was because of high cost of FYM transportation and application in field. Our results are in line with Singh et al., (2016) who reported higher cost of FYM and higher energy required for handling, transport, mixing of FYM during agricultural operations. Likewise, highest profitability was observed in RNPK+Gypsum (1.59) and RNPK+Gypsum+CR (1.48) treatments. However, lower values were observed in T7 (1.33) and T8 (1.13) because of high cost incurred on FYM and proportionate return from these amendments were less compared to the cost incurred. Therefore, it is important to note that addition of gypsum with other organic amendments like CR, biofertilizers, green manures and other wastes can be cost effective strategy to reduce not onliy runoff, soil loss, but improve soil quality indices thereby improving crop yield in such degraded ecosystem (Ali et al., 2021).
 

 
Impact of amendments on soybean nutrient uptake and nutrient harvest index
 
Uptake of nutrients by soybean crop (Table 4) increased significantly (p<0.05) with different amendment treatment over control. Among nutrients, highest uptake by soybean crop was observed in N followed by K, S and P uptake. Results showed N, P, K and S uptake increased by 90%, 83%, 93.5% and 118% respectively across T6, T7, T8 treatments over T2 plots. Higher nutrient uptake is because of application of gypsum and organic amendment which improved soil properties, enhanced nutrient cycling which might have increase bioavailability of nutrients (Rashmi et al., 2021). Nutrient harvest index (NHI) signifies nutrients transport from soil and fertilizer into economic parts of the crop. Nitrogen, phosphorus, potassium and sulfur index varied from 55.3 to 60.7, 51.7 to 60, 17.3 to 21.8 and 41.2 to 49.4 per cent, respectively, under different soil amendment combinations (Table 4). Application of organic amendments and gypsum with recommended fertilizer dose, illustrated lower NHI, PHI, KHI, SHI values over the T1 and T2 treatment. Thus, NHI decreased with combined amendment application as compared to application of gypsum, organic resources and fertilizers alone. This reduction in N, P, K and S indices would increase nutrient use efficiency and reducing nutrients removal from agricultural soil leading to sustainable land use (Sheoran et al., 2017).
 

The present study evaluated use of soil amendments like gypsum (2.5 t ha^-1) with CR (3 t ha^-1) in conjunction with recommended fertilizer dose (T6) improved photosynthetic pigments and other yield attributes increasing soybean yield by 53% and produced maximum return of Rs 15247 kg ha^-1 over RNPK alone in degraded vertisol. This study suggests a strong basis of incorporating amendments in soil management practices to improve crop productivity and nutrient uptake under similar conditions eleswhere.

None.