Effect of Intercropping Mango with Lemongrass and Screwpine on the Soil Carbon Dioxide Flux and Soil Microbial Density

Mango (Mangifera indica L.) is widely cultivated in tropical and sub-tropical regions (Akin-Idowu et al., 2020) and extensively consumed globally. Approximately 50.65 million metric tonnes of mangoes were produced worldwide in 2027, spanning over 4.37 million acres (Akin-Idowu et al., 2020). In Malaysia, mango production in 2020 recorded an average yield of 4.0 metric tonnes per hectare, with a total output of 12,834.40 metric tonnes (Department of Agriculture, 2020).
       
In general, mango orchards are established as monocrop with wide spacing to accommodate the large size of the trees and also to facilitate agro-management. This practice of growing a single crop species simplifies management for farmers, allowing them to streamline cultivation practices effectively. However, this intensive monoculture farming method, depends largely on synthetic fertilizers and pesticides, can significantly degrade soil health and disrupt the surrounding ecosystem (Dias et al., 2020; Sharma et al., 2000). Moreover, mono-cropping practice was found to have negative effect on soil microbial density and diversity in long term (Lulie, 2017) and accelerate the soil CO₂ flux to atmosphere and in turn loss of essential nutrients into surrounding environment (Dyer et al., 2012). Indeed soil CO₂ flow is a good soil health indicator because it aids in determining the rate at which soil microorganisms degrade soil organic matter (Giacomo et al., 2014).
       
Intercropping can be an alternative to mono-cropping as it reduces the environmental impacts on agriculture. Intercropping is a multiple cropping technique that maximizes output from crops cultivated with little input use while utilizing existing resources more efficiently (Khomphet et al., 2021; Maitra et al., 2021). Kishore et al., (2021) reported that mango tree canopy provide shade to the pineapple and thereby increased the yield of pineapple. Zhong and Zeng (2019) reported that intercropping peanut with cowpea significantly enhanced microbial density compared to monocropping system. However, intercropping has drawbacks, including reduced availability of nutrients, water and light, but it nevertheless produces a lower yield (Murtaza et al., 2020). Abagandura et al. (2020) opined that intercropping of Prairie cordgrass with kura clover emitting higher cumulative CO₂ flux compared to Prairie cordgrass monocrop. Low temperature, carbon input and slow turnover time of carbon are the way to keep the soil CO₂ emission in check (Pries et al., 2017).
               
Aromatic plant intercropping is a novel strategy to be applied in an agro-ecosystem to keep the growing medium healthy. Both screwpine (Pandanus amaryllifolius) and lemongrass (Cymbopogon nardus) can thrive in the warm, humid climate of the tropics, making them suitable companion plants for mango trees (Skaria et al., 2006; Wongpornchai, 2006). This promising approach should be widely adopted to enhance the regenerative capacity of aromatic plants for subsequent harvests and to facilitate their rapid expansion into surrounding areas. As the literature is virtually silent on how intercropping mango with pandan and lemongrass influences soil microbial density and CO₂ flux, the present study was undertaken to comprehend the system in relation to the above-mentioned activities. 

A field experiment was conducted during 2021 to 2023 in a mango orchard (941.88 m²) of Chuping, Perlis, Malaysia (6^o31'21.9"N, 100^o17'08.4"E) in collaboration with Universiti Malaysia Perlis. Data were collected at six (vegetative), 12 (reproductive) and 18 months (post-harvesting) from seven-year-old mango trees planted in fine sandy loam soil (Ultisol) at 15 × 5 m spacing. The orchard was fertilized with 30 kg of organic fertilizer biannually. Annual rainfall was 1318.38 mm (2021–2022) and 1111.29 mm (2022–2023), with temperatures ranging from 24^oC to 33.86^oC. A randomized complete block design (RCBD) was employed for the experiment, consisting of five replications distributed in plots size of 1125 m². Treatments included mango monocropping (T1), mango-pandan intercropping (T2) and mango-lemongrass intercropping (T3). Aromatic plants (Pandanus amaryllifolius and Cymbopogon nardus) were vegetatively propagated and transplanted between mango trees at 1 × 1 m spacing in 15 m rows.
       
Soil samples (20 g) were randomly collected at 0-15 cm and 15-30 cm depths using a soil auger. Soil available C and N were measured using a CHNS analyser (PE 2400 SERIES II, Perkin Elmer, USA). Soil CO₂  flux was measured using the soda lime technique (Keith and Wong, 2006) with five PVC chambers per plot. The efflux of CO₂ was calculated as:
 

CO₂ efflux= (G-g × 1.69)/A × (24 h)/H × 12/44

Where,
G = Weight gain of sample.
g = Average blank weight gain.
A = Area of chamber and H is the exposure time.
       
Soil microbial analysis for Azotobacter and phosphate-solubilizing microorganisms (PSM) was carried out using the spread plate method with Beijerinkia’s and Pikovskaya mediums, respectively (Motsara and Roy, 2008). Comparison of treatments was done with the use of Tukey’s test at p<0.05 and the significant effects of the treatments were ascertained by analysis of variance (ANOVA). For the statistical study, version 9.2 of the Statistical study System (SAS) software was used.

Soil carbon and nitrogen analysis
 
Utilising aromatic plants as companion plants is a novel strategy for an agro-ecosystem. It maximizes land usage and generates additional income for the grower. After 18 months of intercropping mango and aromatic plants, there were no discernible changes between any plots for soil C and N at both depths (Fig 1). When comparing planting months in the T1, mono-cropping system across soil depths, soil C content was lowest at six months, peaked at 12 months, but declined thereafter (Fig 1). Similarly, a comparable trend was noted for the soil N levels in both T1 and T3 at a depth of 0-15 cm. Nevertheless, T3 had a relatively high soil C and N content with depth compared with T2 and T1 after 18 months planting. The highest organic matter input by the high leafy biomass of lemongrass in T3 had increased the soil C and N with depth after 18 months planting. This finding is consistent with the increasing soil organic matter can enhance the soil C and N level (Saha et al., 2024; Malone et al., 2023; Bhatt et al., 2019). Pankaj et al. (2017) has also reported that the lemongrass cultivation can enhance soil C and N levels as the plants form dense clumps over time.


 
Soil bulk density, soil moisture and soil temperature analysis
 
Bulk density (BD) is commonly used to assess the soil’s capacity for gas exchange, water retention and its suitability for supporting plant growth (Rabot et al., 2018). Between all plots, there were no variations in the soil’s BD and soil moisture (MC) (Table 1). Comparing planting months, treatment T1 initially exhibits the lowest MC for the first 6 months, but thereafter, it displays a rising trend at 18 months after planting. Lower soil MC in monocrop also observed by Mendis et al., (2022). This implies that soils subjected to intercropping demonstrate an enhanced capacity to retain moisture over prolonged durations compared to mono-cropping treatments.  High precipitation received during the post-harvest of mango tree could explain a relatively high soil MC across all the treatments. On average, there were no statistical differences in soil temperature across the treatments after 6 months of intercropping with aromatic plants (Table 1). However, treatment T2 had a lower soil temperature after 12 months of intercropping with screwpine. This suggests that the fast spreading and broad leaves of screwpine trees providing a shade and thereby resulting in a relatively low soil temperature in T2 after 12 months planting. After 18 months of planting, there were no variations in soil temperatures among the treatments. When comparing the planting months, the soil temperature was notably lower during the initial 6 months of planting across the treatments. This observation may be attributed to the reduced rainfall during the vegetative stage of the mango trees. Additionally, pruning activities, conducted for flushing induction, may have reduced shade coverage, consequently increasing soil temperature during this period.



Soil carbon dioxide (co₂) flux and microbial density
 
Regulating the soil CO₂ emission is essential measure to mitigating global climate change (Ibrahim et al., 2023). Plot T1 had the highest soil CO₂ flux across all the plots throughout 18 months of planting (Fig 2). Our findings suggest that intercropping mango tree with the aromatic plants can sequester more carbon in the soil. Intercrops increase the carbon allocation in promoting the root growth and thereby reduce the soil CO₂ emission (Machado et al., 2016). It appears that the T2 intercrop with screwpine exhibits a fluctuating trend in CO₂ flux over the planting months, as opposed to the more stable trend observed in T3 intercrop with lemongrass. This is also evidenced by the fluctuating pattern in microorganism activity observed in T2 over time (Table 2). Soil microbial community is a useful soil quality indicator and major driver of the elemental soil biogeochemical processes. No significant differences were observed in the density of Azotobacter spp. across the treatments regardless of planting stages at 0-15 cm depth but PSM shows the lowest density in T1 with increasing soil depth during the post-harvesting stage of mango trees (Table 2). At 15-30 cm depth, intercropping plots show more population of PSM over time compared with the mono-cropping which could be attributed to the increased biomass input in the intercrop plots improving the density of PSM. In general, intercropping improves the abundance of soil microorganisms (Zhao et al., 2023; Zhang et al., 2021). Between months, plot T2 initially recorded the lowest density of Azotobacter spp. at 6 months after planting, but increased gradually over time (Table 2). Similarly, plot T2 exhibited a fluctuating trend in PSM over time. In contrast, Plot T3 had the lowest density of PSM at the initial 6 and 12 months but increased after 18 months of planting. The lower density of microorganisms at early planting suggests that they may require time to establish and colonize the rhizosphere of the newly planted intercrops.

The present study indicated that intercropping mango with lemongrass increases the soil C and N with increasing soil depth after 18 months planting. Our findings also suggest that intercropping mango with lemongrass and screwpine can sequester more carbon in the soil. In addition, intercropping plots show more population of PSM than the mono-cropping which could be attributed to the increased biomass input in the intercrop plots improving the density of PSM. However, further long-term analysis is needed to reinforce the findings for selecting the most beneficial cropping patterns for farmers to improve the soil nutrients.

The authors acknowledge the support from the Fundamental Research Grant Scheme (FRGS) under a grant number of FRGS/1/2020/WAB04/UNIMAP/03/2 from the Ministry of Higher Education Malaysia. The authors would also like to thank En. Wan Mohamad Fishaal Bin Wan Daud from Koperasi Harumanis Y.A that provide the study site for the experiment. We are also thankful to UniMAP for providing the facilities in conducting the research.
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.

The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.