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Article

Carbon Accumulation, Soil Microbial and Enzyme Activities in Elephant Foot Yam-Based Intercropping System

by
Tamilselvan Ilakiya
1,*,
Ramakrishnan Swarnapriya
2,*,
Lakshmanan Pugalendhi
1,
Vellingiri Geethalakshmi
3,
Arunachalam Lakshmanan
4,
Manoj Kumar
5,* and
José M. Lorenzo
6,7
1
Department of Vegetable Science, Tamil Nadu Agricultural University, Coimbatore 641003, India
2
Floriculture Research Station, Kanyakumari 629302, India
3
Agro-Climate Research Centre, Tamil Nadu Agricultural University, Coimbatore 641003, India
4
School of Post Graduate Studies, Tamil Nadu Agricultural University, Coimbatore 641003, India
5
Chemical and Biochemical Processing Division, ICAR—Central Institute for Research on Cotton Technology, Mumbai 400019, India
6
Área de Tecnoloxía dos Alimentos, Facultade de Ciencias, Universidade de Vigo, 32004 Ourense, Spain
7
Centro Tecnológico de la Carne de Galicia, Parque Tecnológico de Galicia, Avd. Galicia n° 4, San Cibrao das Viñas, 32900 Ourense, Spain
*
Authors to whom correspondence should be addressed.
Agriculture 2023, 13(1), 187; https://doi.org/10.3390/agriculture13010187
Submission received: 29 November 2022 / Revised: 21 December 2022 / Accepted: 7 January 2023 / Published: 11 January 2023
(This article belongs to the Special Issue Soil Enzymatic Activity and Physicochemical Properties in Agriculture System)
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Abstract

:
Intercropping is a sustainable, eco-friendly, and economically beneficial cropping system. Elephant foot yam (EFY), a multifarious long-duration vegetable, takes 60 days or more to spread its canopy. Hence, this research assessed the impact of intercropping short duration vegetables, viz., cluster bean, radish, Amaranthus, and fenugreek, in elephant foot yam for two seasons (2021 and 2021/22). It included the analysis of parameters such as carbon accumulation, soil chemical properties, nutrient, enzyme, and microbial activities. The findings revealed that for both the seasons there was a significant (p < 0.01) rise in all the parameters examined in the intercropping patterns. Cluster bean (legume) outperformed the other intercrops utilised. Overall, carbon accumulation was improved by 54.40% when cluster beans were intercropped in EFY. Cluster bean intercropping increased the microbial and enzyme activities in the soil rhizosphere and improved soil organic carbon, microbial biomass carbon, nitrogen, phosphorus, and potassium by 31, 42, 28, 37, and 11%, respectively, compared to the sole crop. A positive correlation was observed between the soil microbes and enzyme activity with the soil chemical properties. As a result, the research concludes that intercropping cluster bean in EFY promotes carbon accumulation, soil nutrients, enzymes, and microbial community, which, in turn, favour the productivity of the elephant foot yam.

1. Introduction

The elephant foot yam of the family of Araceae is a notable tuber crop in the tropics. It is a staple food for tribes in most Asian countries [1,2]. The elephant foot yam is considered as the king of tubers for its multifaceted uses. The elephant foot yam has emerged as a candidate crop for food security as it can thrive on marginal soil in spite of low annual rainfall and drought [3]. Due to its production potential and appeal as a starchy vegetable with excellent nutritional and medicinal benefits, it is cultivated not only as a food security crop but also as a commercial crop [4]. It is a low-fat, healthy food rich in the essential fatty acids that increase the good cholesterol in the blood. It is used as a source for ayurvedic medicines that cure haemorrhoids, gastro-intestinal disorders, and rheumatism [5,6]. Being a wide-spaced and long-duration crop, elephant foot yam takes 60 days or even more to spread into full ground cover. As a result, enough space and sunlight are accessible in the early stages of growth to favour weed growth. Furthermore, the crop drains a significant amount of nutrients from the soil. To maintain soil quality and health, a higher quantity of organic matter is needed. The cultivation of short duration vegetables or legumes as an intercrop can raise the nitrogen and organic matter content in the soil [7,8].
Intercropping is the coinstantaneous growing of more than one species in the same place and is applied widely due to its environmental, economic, and ecological benefits [9]. The beneficial attributes of intercropping include complementation and increasing the efficiency of resources in spatial and temporal patterns, preventing the development of hostile plants, increasing insect and disease resistance, as well as improving soil enzyme activities [10,11]. The enzymes and microorganisms present in the soil play a major role in biochemical processes, including the nutrient element cycles. The diversity and composition of soil microbial communities are imperative for maintaining soil health, productivity, and plant biodiversity [12]. Soil enzymes, as an indicator of the soil quality, are needed for the synthesis and degradation of organic matter. Urease plays a crucial role in the effective use of urea fertiliser in soil, and differences in urease activity can be utilised as an indirect indicator of variations in the pool of potentially accessible N in a soil [13]. Dehydrogenases are an intracellular enzyme class called oxidoreductases that are present in all living microbial cells. Dehydrogenases transfer hydrogen between organic substrates to the inorganic acceptors, which is a crucial step in the oxidation process of soil organic matter [14]. Phosphatase activity is an excellent predictor of soil organic phosphorus mineralisation capability and biological activity. Phosphatases in the soils hydrolyse C-O-P ester bonds in the organic phosphorus molecules and release inorganic phosphorus [15]. In numerous reports, intercropping has been found to elevate the soil enzyme activities, which in turn play a dynamic role in maintaining organic matter, microbial activities, and the physical properties of the soil [16,17]. Intercropping also enhances the carbon accumulation potential as it produces higher biomass with more crop density. Capturing carbon in the agricultural cropping system has sparked widespread attention, not just to reduce the threat of global warming, but also to improve soil quality [18,19].
Thus, the present study was framed with the following objectives: to assess (i) the carbon accumulation; (ii) the soil chemical, enzyme, and microbial activities; and (iii) to explore the correlation between soil enzymes, chemical and microbial activities in an elephant foot yam-based intercropping system. Our hypotheses were that (1) intercropping favours carbon accumulation over monocropping due to higher plant density; (2) soil chemical, enzyme and microbial activity are high in an intercropping system due to the enhanced microbial composition that has as an interrelationship with the soil qualities and (3) there exists a considerable and positive relationship between the soil’s chemical, enzyme and microbial activities across all the cropping systems.

2. Materials and Methods

2.1. Site Description

The field experiment was taken up at the college orchard, Horticultural College and Research Institute (11°02′ North latitude, 77°03′ East longitude), Coimbatore, Tamil Nadu, India (Figure 1). The experiment was conducted for two seasons, i.e., April 2021–December 2021 and September 2021–May 2022. The soil under investigation was Typic Haplustalfs (World Reference Base soil classification system) with texture class of sandy clay loam containing 0.38 and 0.35% of organic carbon, 6.59 pH, 276 and 253 kg/ha of available nitrogen, 26 and 24 kg/ha of available phosphorus and 412 and 389 kg/ha of available potassium at season I and season II, respectively. Weather conditions varied over the growing seasons of 2021 and 2021/22 (Figure 2). On average, maximum temperatures in the first season ranged from 28.4 to 35.16 °C, whereas in the second season it varied from 28.4 to 34.64 °C. The daily minimum temperatures during the first season varied from 20.89 to 24.58 °C. In the second season, minimum temperatures varied from 20.67 to 24.58 °C. When comparing the amount of annual precipitation that was received, in the first season, it was 636.50 millimetres, while the second season it was 738.60 millimetres.

2.2. Experimental Treatments and Plot Management

The experiment encompassed five treatments, viz., T1—elephant foot yam (EFY)—sole crop; T2—EFY + cluster bean; T3—EFY + radish; T4—EFY + Amaranthus and T5—EFY + fenugreek, and was laid out with four replications in a randomised block design. The variety CO1 of elephant foot yam was planted at a spacing of 90 cm between rows and 60 cm between plants. Fertilizer application of 40:60:50 kg of NPK per hectare was made at 45 days after sowing. One month later, it was top-dressed with 40 kg of nitrogen and 50 kg of phosphorus. Irrigation was given once a week. The short duration vegetable crops, viz., cluster bean, radish, Amaranthus and fenugreek, were sown in the interspaces between elephant foot yam immediately after planting EFY.

2.3. Soil Carbon Accumulation

The aboveground biomass (AGB) was estimated by using a 1 m × 1 m quadrant. All plants that grew along the edges of the quadrant were cut at ground level and dried at 65–70 °C for one to two days, until they all weighed a constant weight. AGB was obtained by dividing the dry weight by the plot area. Belowground biomass (BGB) was also estimated by using a 1 m × 1 m quadrant. The living roots were collected from all the plants that were grown along the edges of the quadrant. The living roots were then dried at 65–70 °C for one or two days until they attained constant weight. BGB was calculated by dividing the dry weight obtained by plot area [20,21]. Total biomass (TB) was obtained by adding AGB and BGB. Aboveground carbon (AGC) and belowground carbon (BGC) were obtained by multiplying AGB and BGB by a factor of 0.45, respectively. Later, total carbon was obtained by adding AGC and BGC, as recommended by Woomer and Palm [22].

2.4. Soil Chemical Enzyme and Microbial Activities

The roots of each plant were pulled out and shaken manually to remove the loose soil; later, the soil adhering to the root surface was collected as rhizosphere soil [23]. In the monocropping system (T1), soil samples from the rhizosphere region of elephant foot yam roots were collected, whereas the five rhizosphere soil samples each from elephant foot yam roots near the intercrop side and intercrop roots near the elephant foot yam side were collected randomly and mixed together for the intercropping systems (T2, T3, T4 and T5 individually). Later, the collected soil samples for each treatment were sieved separately in a 2 mm sieve for further analysis [24].
The available nitrogen, phosphorus and potassium were estimated by standard procedures given by Subhiah and Asija, Olsen and Jackson, respectively [25,26,27].
Microbial biomass carbon was determined using fumigation–incubation technique (FIC) [28]. For this, 10 g of the soil sample was taken in a 100 mL beaker. Then, 5 mL of water was added to saturate the soil sample to 50% and incubated in BOD incubator for 5–7 days. After the incubation period, 2 mL of ethanol-free chloroform was added to fumigate the sample. A non-fumigated sample was also maintained simultaneously. The soil was then transferred to a conical flask with scintillation vial holding 5 mL of 0.5 N NaOH for fumigated sample and 0.2 N NaOH for non-fumigated sample. The flask was then sealed and kept for 5 days. During the incubation period, CO2 was evolved. The CO2 was estimated by titrating the alkaline traps with 0.5 N HCl for precipitation of CO32− with 50% BaCl2 using phenolphthalein indicator.
Soil organic carbon for the soil sample was analysed using chromic acid wet digestion method [29]. For this experiment, 0.5 g of sample was taken in a 500 mL conical flask. Then 10 mL of 1 N potassium dichromate and 20 mL of concentrated H2SO4 was added to it and mixed by gentle rotation. The contents were then allowed to stand for 30 min. After 30 min, 200 mL of distilled water, 10 mL of phosphoric acid and 1 mL of diphenylamine indicator were added and titrated against 0.5 N ferrous ammonium sulphate till a bright green colour appeared. A blank was also run simultaneously. From the titre value, organic carbon was calculated.
Urease activity was estimated by the standard procedure given by Tabatabai and Bremner [30]. Five grams of dry soil was taken in a 100 mL volumetric flask. To this 0.2 mL of toluene and 9 mL of Tris hydroxymethyl aminomethane (THAM) buffer (0.05 M, pH 9.0) was added and swirled in the flask for a few seconds, followed by the addition of 1 mL of 0.2 M urea solution and mixed thoroughly, stoppered, incubated for 2 h at 37 °C. Then 35 mL of KCl-Ag2SO4 was added followed by shaking immediately and allowed to cool to room temperature. Then the volume was made up to 50 mL by addition of KCl-Ag2SO4. After addition, the volumetric flask was stoppered and mixed thoroughly. From this solution, 20 mL of aliquot was pipetted out into a 100 mL distillation flask and steam distilled with 0.2 g of MgO for 4 min. A quantity of 25 ml of the distillate was collected in 50 mL Erlenmeyer flask containing 5 mL of 2% H3BO3− indicator solution. NH4+-N concentration of distillate was estimated by titrating with standard acid. The end point was the colour change from blue to pink. A control was also maintained and the same procedure was followed with the addition of 1 mL of 0.5 M urea solution after the addition of 35 mL of KCl-Ag2SO4 solution. The urease activity was expressed in μg of NH4 released/g of soil/hr.
According to the method suggested by Casida and her co-workers, the activity of dehydrogenases was determined [31]. A soil sample weighing six grams was collected from the experimental plot in a test tube and mixed with 0.1 g of calcium chloride. To this, 1 mL of 3 per cent 2, 3, 5-triphenyl tetrazolium chloride and 2.5 mL distilled water was added, mixed thoroughly, the tube was stoppered and incubated for 24 h at 37 °C. After incubation, 10 mL of methanol was added and the tube was stoppered followed by mixing for one minute and then the tube was unstoppered. The sample was then filtered using Whatman No. 40 filter paper. This process was repeated again and again till the volume reached 25 mL and the reading was taken at 485 nm. Simultaneously, from the stock solution of tri phenyl formazan, a series of the working standard, viz., 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 ppm, were prepared for standard graph. Methanol was run as a blank.
Acid phosphatase and alkali phosphatase were analysed by adopting the standard procedure of Tabatabai and Bremner [32]. One gram of the soil from the experimental site was placed in 100 mL Erlenmeyer flask. To that, 4 mL of modified universal buffer (pH 6.5 for acid phosphatase and pH 11.0 for alkali phosphatase) and 0.2 mL of toluene were added, followed by the addition of 1 ml of 0.05 M p-nitrophenyl phosphate (pH 6.5 for acid phosphatase and pH 11.0 for alkali phosphatase). Then, the flask was added and swirled for a few seconds and kept in an incubator. After 1 h of incubation, 1 mL of 0.5 M calcium chloride and 4 mL of 0.5 M sodium hydroxide were added. The soil suspension was filtered through Whatman No. 42 filter paper. The intensity of the yellow colour was measured immediately in a UV–Visible spectrophotometer at 420 nm. The p-nitrophenol content of the filtrate was computed from the standard curve. The enzyme activity was expressed in μg of p-nitrophenol released per gram of soil per hour.
The fresh soil samples were collected and preserved in refrigerated conditions (4 °C) and were used for estimating various microbes. The media, including nutrient agar, Martin’s rose Bengal agar, and the Kenknights agar media, were prepared to isolate the bacteria, fungi, and actinobacteria, respectively. A measure of 1 ml of the required dilution, i.e., 10−5 for bacteria and 10−4 for actinobacteria and fungi, was inoculated into their respective Petri plates after the serial dilution. The soil samples were spread over the media and incubated at 37 °C in dark conditions. The microbial colonies were visible readily at two to seven days for fungi and bacteria, and at seven to fourteen days for actinobacteria. In each plate, the numbers of colonies were counted. By taking into account the soil moisture and dilution factor, the number of colony forming units (cfu) per gram of dry soil was estimated as per the procedures suggested by Allen, Martin and Rangaswami [33,34,35].

2.5. Statistical Analysis

The data on carbon accumulation, soil chemical, enzyme, and microbial activities pertaining to two seasons were submitted to pooled analysis followed by Duncan’s multiple range test using STAR software. Differences with a p-value of less than 0.01 were considered statistically significant. The Pearson correlation coefficient of the soil chemicals, enzymes, and microbial activity was obtained. The figures and statistical analysis were computed using R and Origin softwares.

3. Results

3.1. Soil Carbon Accumulation

The biomass production was employed as dry matter to compute the carbon accumulation. The production of biomass was divided into three categories, viz., AGB, BGB, and TB. The results (Figure 3) indicated that in both seasons, the biomass production was influenced significantly (p < 0.01) as a result of the various intercrops cultivated with EFY. It was observed that the treatment T2 (EFY + cluster bean) reported higher AGB (11319 kg/ha), BGB (3131 kg/ha) and TB (14450 kg/ha) production in both the seasons. Data (Figure 3) depicted that all the biomass carbon traits were significantly (p < 0.01) different and the treatment T2 exhibited the highest AGC (5093 kg/ha), BGC (1409 kg/ha), and TC (6503 kg/ha). In contrast to the monocropping system (T1), intercropping with cluster bean (T2) increased the overall carbon accumulation by 54.40 percent, followed by T4 (Amaranthus) (20.56%) > T3 (radish) (17.70%) > T5 (fenugreek) (8.48%).

3.2. Soil Chemical Enzyme and Microbial Activities

In both seasons, the soil organic carbon (Figure 4A) was significantly (p < 0.01) higher in the intercropping treatments. An increase of 31, 28, 4 and 2% in SOC was observed in T2, T4, T5 and T3, respectively, compared to the sole crop of elephant foot yam. Values of SOC ranged between 0.31 and 0.40%. Intercropping significantly (p < 0.01) boosted the microbial biomass carbon (Figure 4A) in the soil as compared to monocropping. MBC ranged between 163–235 mg/kg in the different treatments. Here, a 42% increase in MBC was recorded in T2 followed by T4 > T5 > T3 compared to T1.
The soil nutrient status of the experiment site cultivated with elephant foot yam as a monocrop and intercropped with short duration vegetables is described in Figure 4B. A significant difference (p < 0.01) was obtained between the monocrop and intercrop treatments for nitrogen, phosphorus, and potassium content in both seasons. The nitrogen, phosphorus and potassium contents in T2 were found to be 28, 37 and 11% higher, respectively, over T1. The values ranged between 184 and 235 kg/ha for nitrogen; 15.88 and 21.70 kg/ha for phosphorus and 325 and 362 kg/ha for potassium.
The results (Figure 4C,D) showed that the soil enzyme urease, dehydrogenases, acid phosphatase, and alkali phosphatase were highly active in the intercropped systems compared to elephant foot yam grown on its own. The soil enzyme activities in both seasons were found to be highest in T2 and lowest in T1. Among them, the urease activity of T2 was 78% higher than T1, followed by T5 > T4 > T3 (63, 30, and 17%, respectively) (p < 0.01). The urease activity ranged between 21.29 and 37.82 NH3 µg/g/h. The treatment T2 (65% increase over T1) was found with maximum dehydrogenase activity and ranged between 1.58 and 2.61 µg TPF formed/g soil/h (p < 0.01). The enzyme acid and alkali phosphatase were observed to increase by 132 and 148%, respectively, over T1 (p < 0.05). The values ranged between 12.15 and 28.21 µg PNP/g/h for acid phosphatase and 7.35 and 18.22 µg PNP/g/h for alkali phosphatase.
The soil microbial count was found to be significantly influenced by the intercrops grown (Figure 4E). Here, in both the seasons, the bacteria, fungi, and actinobacteria count were found to be increased by 65, 43, and 84% in the cluster bean intercropped treatment T2, followed by the treatments T5 > T4 > T3 when compared to the sole crop. The ranges were 23–38 × 105 cfu/g for bacteria; 20–29 × 104 cfu/g for fungi and 8–14 × 104 cfu/g for actinobacteria.

3.3. Correlation between Soil Microbes and Soil Enzymes

As indicated in Figure 5, the soil microbial count (bacteria, fungi, actinobacteria) and the activities of enzymes (urease, dehydrogenases, acid phosphatase, alkali phosphatase) were all positively correlated, which indicates that each component mutually promotes the other. Urease and alkali phosphatase enzyme were significantly (p < 0.05) correlated with bacterial and fungal count, whereas, the acid phosphatase activity was significantly (p < 0.05) correlated with all three types of microbes, viz., bacteria, fungi and actinobacteria.

3.4. Correlation of Soil Microbes and Enzymes with Soil Chemical Properties

The relationship between the soil microbes and enzymes and the soil chemical properties is presented in Figure 6. All the traits were positively connected, indicating that each component is mutually related and encourages others. Fungal abundance was significantly (p < 0.05) correlated with nitrogen and microbial biomass carbon.

4. Discussion

The biomass production was significantly higher in the intercropping system when compared to the EFY-solo cropping. In this study, higher biomass was obtained in the legume intercropping. Numerous factors influence aboveground biomass production, including site quality, species growth habits, intercultural operations, management practices, and interaction in the rhizosphere region. In general, belowground biomass follows the same pattern as aboveground biomass [36]. The assessment of carbon accumulation was in accordance with the biomass production capacity in the present study. More biomass in any system can be ascribed to its higher carbon value. Differences in carbon might also be attributed to factors such as crown architecture, genetic makeup, and chemical and anatomical uniqueness of the species [37].
In both seasons, there was significant improvement in the soil organic carbon. The SOC was found higher in the elephant foot yam intercropped with cluster bean. Higher SOC could be attributable to the build-up of organic matter through the cluster bean biomass, root nodules, and massive leaf-fall breakdown in this system, which increased the microbial population and hastened the decomposition of crop leftovers [38]. Soil organic carbon improves soil quality by enhancing CEC, aggregate stability, biological activity, nutrient cycling, and by lowering bulk density. It was revealed that soil organic matter content on the surface of the soil accounts for 58% of carbon, corresponding to SOC [39]. Increased SOC storage not only aids in carbon accumulation but also improves the physio-biochemical aspects of the soil. The principal source of soil organic matter is mainly through carbon fixed by plants that acts as a substrate for microbial processes in SOM formation. The allocation of photosynthetic components below ground is equally critical for improving SOC concentration.
A marked build-up of microbial biomass carbon was observed in treatment T2 where EFY was grown with cluster bean. The soil microbial biomass accounts for a very small portion of the total soil carbon and is defined by its rate of turnover. The nutritional components N, P, S, and C are all labile in soil microbial biomass. MBC was found to be elevated near the root zone. It is a merit of the microbial population, providing a better insight into soil organic carbon cycling [40]. It encourages the development of new humus and boosts the total soil carbon content. Moreover, the current analysis found that the maximum enzyme activity detected in this treatment was indirectly linked to microorganisms present in the soil. Higher microbial activity was correlated with a higher MBC content. This might be attributed to the addition of greater residue and the inclusion of legume as the intercrop in this treatment [41,42].
The soil nutrient status, viz., nitrogen, phosphorus, and potassium, was found to be higher in elephant foot yam intercropped with cluster bean. Intercropping had a positive impact on soil nourishment. The combination of two species is typically thought to boost the efficiency of nutrients throughout the soil profile. Legumes (cluster bean) grown in an intercropping system boost phosphorus availability in general through nodules, root exudates, root turnover, and root cells sloughing off during the growth season. The root excretes more carbohydrates and protons (malate, malonate and citrate), making the root-borne phosphatase more potent in hydrolysing organic phosphorus. The hydrolysis of the organic phosphorus enhances the phosphate-solubilising bacteria such as Pseudomonas, which are found to be more common in intercropped soils and are linked with increased nitrogen and phosphorus content in soil [43,44]. This study has thrown a light that intercropping cluster bean with EFY is a way by which the soil nutrients can be improved and replenished. The primary nutrients necessary for plant development and production are nitrogen, phosphorus, and potassium.
Enzymes serve as catalysts in all biological reactions. Soil urease originates mainly from plants, plant litter on the soil surface, and microorganisms, both as intracellular and extracellular enzymes. The urease enzyme, which contains nickel, is a metalloenzyme that aids in the degradation of urea fertilizer in agricultural systems. Plant types or combinations of plant species have an impact on urease [45]. In this study, the treatment T2 (EFY + cluster bean) recorded increased urease activity because cluster bean, being a legume crop, has the capacity to fix nitrogen organically, which in turn stimulates urease activity. An increase in soil microbiota coupled with a rise in organic matter in the legume intercropped soil also increases the urease activity [46,47]. Various authors quote that intercropping, on the other hand, has been proven to boost urease activity because of N-cycling from soil [48,49].
In soil, dehydrogenase activity is often utilised as a biological indicator and assists vital biochemical processes. Dehydrogenases were found to be strongly linked to microbial biomass and facilitate the oxidation of organic matter through the transfer of electrons and protons from the substrate to the acceptors. Higher dehydrogenase activity was noted in the cluster bean-intercropped treatment. Legume-supplemented soil had a higher dehydrogenase level when compared to the monoculture elephant foot yam. This increased activity of the dehydrogenases might be attributed to the higher flux of root secretions, root exudation, and organic matter as a result of legumes being included in this system [50].
It was observed that the phosphatase activity of various crop combinations grown with EFY varied from one another. Phosphatase is a class of enzyme that can catalyse the hydrolysis of phosphoric acid esters and anhydrides. These enzymes are considered to be crucial in the P-cycle in soil ecosystems. Phosphatase is housed in soil microbes and plants [51]. These enzymes are classed as acid or alkaline phosphatases as they have optimum activity in their respective pH ranges. In addition, acid phosphatase is found in plant roots’ exudates and, in rare cases, in rhizospheric soil [52]. Alkaline phosphatase, on the other hand, is primarily produced by soil microbes. Here, both the activity of acid phosphatase and alkaline phosphatase was found to be higher in T2. The presence of enhanced substrate availability such as soil moisture, nitrogen, available phosphorus, organic carbon and potassium may explain greater phosphatase activity in this treatment [53]. The overall enzymatic activity was also improved and it was found that legume intercropping could benefit nonlegume crops by taking advantage of a leguminous crop’s strong phosphatase activity and consequent phosphorus liberation. These results corroborate the findings of Guenes and his co-workers, who reported the higher phosphatase activity of barley and peanut [54].
In ecological processes, microbial populations play an important role in maintaining soil functions. Soil microbes are crucial for soil biogeochemical processes such as the phosphorus, nitrogen, and other cycles. Some of the other functions of soil microbiota include mineral nutrient-availability regulation, SOM decomposition, atmospheric nitrogen fixation, mycorrhiza formation, and the production of biologically active substances that stimulate plant growth [55,56]. In this study, the combination of elephant foot yam with cluster bean increased the soil microbial communities, viz., bacteria (38 × 105 cfu/g), fungi (29 × 104 cfu/g), and actinobacteria (14 × 104 cfu/g), compared to the other treatment combinations. There was a substantial difference detected in the composition of the microbial community and its abundance in intercropped soil compared to that of the monoculture treatment. In the rhizosphere, one or both plant species can alter the microbial community architecture. The EFY and cluster bean roots interact directly with each other in an intercropping system, affecting root exudation and, as a result, enhanced microbial structure, diversity, and activity were observed. These findings are in accordance with those of Lian and Dang who reported that legume intercropping improves soil fertility by fostering a nutrient-tolerant soil microbial community in the rhizosphere of the elephant foot yam [57,58].
All the soil enzyme and soil microbial populations studied showed a positive correlation. Furthermore, the soil physiochemical parameters exhibited a positive correlation with soil enzyme and microbial activity. The activity of urease and alkali phosphatase enzymes were found to have a significant positive correlation with both the bacterial and the fungal count; whereas, the acid phosphatase activity was significantly correlated with all three types of microbes. The fungal count exhibited positive significant correlation with available nitrogen and microbial biomass carbon. These results indicated that soil microbes play a major role in the enzyme-mediated soil reactions. Increased enzyme activity could be attributed to the increased abundance of substrates that sustain these activities. Improvements in organic matter contribute to the build-up of microflora and the different classes of enzymes responsible for biochemical reactions in the soil [59,60].

5. Conclusions

The cropping system that is practiced has a significant impact on the chemical and natural qualities of the soil. Here, the outcome of the study highlighted that intercropping with elephant foot yam had a significant effect on the soil activities as well as the carbon accumulation. Cluster bean performed better than the other intercrops used. The total carbon accumulation of cluster bean intercropping reached 6.5 t/ha. The soil nutrient status was increased by over 28% N, 37% P and 11% K more than the control. The microbial activity was 38 × 105 cfu/g for bacteria; 29 × 104 cfu/g for fungi and 14 × 104 cfu/g for actinobacteria. Furthermore, the enzyme activities (urease, dehydrogenases, acid and alkali phosphatase) were found to be higher in the cluster bean intercropping treatment. Nitrogen fixation and the activity of beneficial root-associated microbes in the rhizosphere were all improved by intercropping with legumes. Through nodulation and root exudation, legume intercropping enriches soil nutrients, soil enzymes, and soil microorganisms, which in turn increase crop development and yield. Thus, farmers benefit when cluster bean is intercropped in elephant foot yam as it has dual production and profitability. Therefore, it contributes to cost-cutting strategies for communities of smallholder farmers who are constrained by resources.

Author Contributions

Conceptualisation, T.I., R.S. and L.P.; methodology, V.G. and A.L.; software, T.I. and J.M.L.; validation, R.S., L.P., V.G. and A.L.; formal analysis, T.I. and M.K.; investigation, T.I.; resources, R.S. and L.P.; data curation, T.I., M.K. and J.M.L.; writing—original draft preparation, T.I.; writing—review and editing, R.S., L.P., M.K. and J.M.L.; visualisation, V.G. and A.L.; supervision, R.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to Department of Vegetable Science, Tamil Nadu Agricultural University for their support.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. (a) Schematic map of the research area; (b) experimental field layout; (c) details on row arrangements of treatments studied.
Figure 1. (a) Schematic map of the research area; (b) experimental field layout; (c) details on row arrangements of treatments studied.
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Figure 2. Monthly metrological weather data for (a) season 1 (2021) and (b) season 2 (2021–2022) at Coimbatore, Tamil Nadu, India.
Figure 2. Monthly metrological weather data for (a) season 1 (2021) and (b) season 2 (2021–2022) at Coimbatore, Tamil Nadu, India.
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Figure 3. Soil carbon accumulation in elephant foot yam based intercropping system (AGB—aboveground biomass (kg/ha); BGB—belowground biomass; TB—total biomass; AGC—aboveground carbon (kg/ha); BGC—belowground carbon (kg/ha); TC—total carbon (kg/ha); T1—elephant foot yam (EFY) sole crop; T2—EFY + cluster bean; T3—EFY + radish; T4—EFY + Amaranthus and T5—EFY + fenugreek).
Figure 3. Soil carbon accumulation in elephant foot yam based intercropping system (AGB—aboveground biomass (kg/ha); BGB—belowground biomass; TB—total biomass; AGC—aboveground carbon (kg/ha); BGC—belowground carbon (kg/ha); TC—total carbon (kg/ha); T1—elephant foot yam (EFY) sole crop; T2—EFY + cluster bean; T3—EFY + radish; T4—EFY + Amaranthus and T5—EFY + fenugreek).
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Figure 4. (A) Carbon dynamics, (B) soil nutrient status, (C) urease and dehydrogenases, (D) acid phosphatase and alkali phosphatase and (E) soil microbial population in EFY-based intercropping system (T1—elephant foot yam (EFY)—sole crop; T2—EFY + cluster bean; T3—EFY + radish; T4—EFY + Amaranthus; T5—EFY + fenugreek. The above parameters were significant at p < 0.01 for all the treatments; SOC—soil organic carbon (%); MBC—microbial biomass carbon (mg/kg); N—available nitrogen (kg/ha); P—available phosphorus (kg/ha); K—available potassium (kg/ha). Bacteria (×105 cfu/g); fungi (×104 cfu/g); actinobacteria (×104 cfu/g); urease (NH3 µg/g/h); dehydrogenases (µg TPF formed/g soil/h); acid phosphatases (µg PNP/g/h) and alkali phosphatases (µg PNP/g/h).
Figure 4. (A) Carbon dynamics, (B) soil nutrient status, (C) urease and dehydrogenases, (D) acid phosphatase and alkali phosphatase and (E) soil microbial population in EFY-based intercropping system (T1—elephant foot yam (EFY)—sole crop; T2—EFY + cluster bean; T3—EFY + radish; T4—EFY + Amaranthus; T5—EFY + fenugreek. The above parameters were significant at p < 0.01 for all the treatments; SOC—soil organic carbon (%); MBC—microbial biomass carbon (mg/kg); N—available nitrogen (kg/ha); P—available phosphorus (kg/ha); K—available potassium (kg/ha). Bacteria (×105 cfu/g); fungi (×104 cfu/g); actinobacteria (×104 cfu/g); urease (NH3 µg/g/h); dehydrogenases (µg TPF formed/g soil/h); acid phosphatases (µg PNP/g/h) and alkali phosphatases (µg PNP/g/h).
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Figure 5. Correlation between soil microbes and soil enzymes; * and ** indicate a significant difference at p < 0.05 and p < 0.01, respectively; bacteria (×105 cfu/g); fungi (×104 cfu/g); actinobacteria (×104 cfu/g); urease (NH3 µg/g/h); dehydrogenases (µg TPF formed/g soil/h); acid phosphatases (µg PNP/g/h) and alkali phosphatases (µg PNP/g/h).
Figure 5. Correlation between soil microbes and soil enzymes; * and ** indicate a significant difference at p < 0.05 and p < 0.01, respectively; bacteria (×105 cfu/g); fungi (×104 cfu/g); actinobacteria (×104 cfu/g); urease (NH3 µg/g/h); dehydrogenases (µg TPF formed/g soil/h); acid phosphatases (µg PNP/g/h) and alkali phosphatases (µg PNP/g/h).
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Figure 6. Correlation of soil microbes and enzymes with soil chemical properties; *, ** and *** indicate a significant difference at p < 0.05, p < 0.01 and p < 0.001, respectively. OC—organic carbon (%); MBC—microbial biomass carbon (mg/kg); N—available nitrogen (kg/ha); P—available phosphorus (kg/ha); K—available potassium (kg/ha). Bacteria (×105 cfu/g); fungi (×104 cfu/g); actinobacteria (×104 cfu/g); urease (NH3 µg/g/h); dehydrogenases (µg TPF formed/g soil/h); acid phosphatases (µg PNP/g/h) and alkali phosphatases (µg PNP/g/h).
Figure 6. Correlation of soil microbes and enzymes with soil chemical properties; *, ** and *** indicate a significant difference at p < 0.05, p < 0.01 and p < 0.001, respectively. OC—organic carbon (%); MBC—microbial biomass carbon (mg/kg); N—available nitrogen (kg/ha); P—available phosphorus (kg/ha); K—available potassium (kg/ha). Bacteria (×105 cfu/g); fungi (×104 cfu/g); actinobacteria (×104 cfu/g); urease (NH3 µg/g/h); dehydrogenases (µg TPF formed/g soil/h); acid phosphatases (µg PNP/g/h) and alkali phosphatases (µg PNP/g/h).
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Ilakiya, T.; Swarnapriya, R.; Pugalendhi, L.; Geethalakshmi, V.; Lakshmanan, A.; Kumar, M.; Lorenzo, J.M. Carbon Accumulation, Soil Microbial and Enzyme Activities in Elephant Foot Yam-Based Intercropping System. Agriculture 2023, 13, 187. https://doi.org/10.3390/agriculture13010187

AMA Style

Ilakiya T, Swarnapriya R, Pugalendhi L, Geethalakshmi V, Lakshmanan A, Kumar M, Lorenzo JM. Carbon Accumulation, Soil Microbial and Enzyme Activities in Elephant Foot Yam-Based Intercropping System. Agriculture. 2023; 13(1):187. https://doi.org/10.3390/agriculture13010187

Chicago/Turabian Style

Ilakiya, Tamilselvan, Ramakrishnan Swarnapriya, Lakshmanan Pugalendhi, Vellingiri Geethalakshmi, Arunachalam Lakshmanan, Manoj Kumar, and José M. Lorenzo. 2023. "Carbon Accumulation, Soil Microbial and Enzyme Activities in Elephant Foot Yam-Based Intercropping System" Agriculture 13, no. 1: 187. https://doi.org/10.3390/agriculture13010187

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