Performance of Intercrops under Different Spacings of Jatropha curcas Plantations in Tamil Nadu, India

Demand for fossil fuels is increasing every day because of industrialization and population growth. Biofuels, obtained from renewable resources, are gaining importance as an alternative to fossil fuels due to its environmental benefits. Biofuels create new markets and hold enormous potential for farmers because they are produced from crops. Bio-diesel blending was started in India during 2015 and National Policy on Biofuels-2018 has the target of achieving 5% biodiesel-blending and 20% ethanol-blending by the year 2030 (MPNG, 2018). Among the non-edible oil-yielding plants for biodiesel production Jatropha curcas is the most promising oilseed plant (Maravi et al., 2015) due to oil-rich seeds, low cost of seeds, suitable fatty acid composition of the oil, adaptability to diverse agro-climatic conditions and short gestation period (Meher et al., 2013). Jatropha curcas L. is a plant native to North and Central America and it is now widespread all over the tropical and subtropical world (Kamel et al., 2018), can grow up to 3-4m height and the seeds contain about 30-40% of non-edible oil (Wani and Sreedevi 2007; Islam et al., 2016; Kamel et al., 2018).
       
The use of multiple tree species in agroforestry systems will improve soil fertility, maintain agricultural production (Rachel et al., 2012) and alleviate the conflicts between the increasing population and availability of limited arable land resources. Integrating trees in farmlands is considered as a promising approach in enhancing livelihood security (Jat et al., 2011; Glover et al., 2012; Subbulakshmi et al., 2017), because trees enhance and sustain soil health and therefore crop production (Barrios et al., 2012).
       
Economic yield of oil-bearing seeds can be obtained from the fifth year after planting and intercrops can be grown to get income and to utilize the available interspace between J. curcas by adoption of proper canopy management practices. Apart from income generation, intercrops act as smother crops and control erosion and weeds, conserves soil moisture and aids in the recycling of nutrients. Selection of suitable tree species, tree spacing and crop combination are important aspects of agroforestry practices. Hence this study was designed to find out the optimum spacing for J. curcas and to determine appropriate intercrop to get higher return in J. curcas based agroforestry system.

A field experiment was conducted at already established three and a half-year-old J. curcas plantation at Forest College and Research Institute, Mettupalayam, Tamil Nadu. The amount of rainfall received during the cropping period (July to November) was 380.4 mm distributed over 27 rainy days. The soil of the experimental site is red sandy loam, non-calcareous and low in available nitrogen, available phosphorus and organic carbon and medium in available potassium. The experiment was arranged in split-plot design and replicated thrice. The individual experimental plot size was 36 m².  Within each experimental plot, different spacings of J. curcas viz., 3m × 3m, 4m × 2m and 4m × 3m formed the main plots and different intercrops viz., blackgram, greengram, cowpea, groundnut, gingelly and sunflower were assigned to the subplot. Intercrops were raised in between the rows of J. curcas. The height of intercrops was measured at 60 days after sowing. The leaf area index was calculated by dividing the total leaf area of the plant by the land area occupied by the plant. The root volume of representative samples (two plants from each replication) of each intercrop selected at random was taken by the Xylometric method (Chaturvedi and Khanna, 1982). The fresh weight of the each intercrop sample was recorded at the time of harvest and dry weight was recorded after air drying the intercrop samples for 14 days to calculate plant dry matter production. Seed yields of intercrops were recorded for each plot and expressed as kg ha^-1.
       
The chemical properties of the soil from the plantation were analyzed both in the beginning and after intercropping. Soil samples were collected from different points within the replications using augur at 0-15cm depth and mixed to get final soil sample by quarter method. Standard methods were followed for analyzing soil pH, EC, available Nitrogen, Phosphorus and Potassium (Jackson 1973; Subbiah and Asija1956; Olsen et al., 1954; Stanford and English 1949). All the leaves of representative plant samples of all intercrops selected randomly were air dried and then dried in hot air oven at 65-70°C to a constant weight. The samples were pulverized into fine powder and analyzed for various nutrient content (N, P, K) (Humphries 1956; Jackson1973). Nutrient uptake (N, P, K) by intercrops were calculated by multiplying the dry matter production with the nutrient content and expressed as kg ha^-1.

Growth parameters of intercrops
 
Plant height
 
Significantly higher plant height in all the intercrops was recorded with the wider spacing of 4m × 3m at 60 days after sowing. Gingelly showed the highest plant height (140.3 cm) followed by sunflower (130.2 cm) at 4m × 3m spacing (Table 1). Higher plant height at wider spacing might be due to harvesting of more solar radiation which might have increased photosynthetic activity and vegetative growth. Rathee et al., (2017) also observed significant increase in plant height of coriander with increasing spacing of trees in poplar based agroforestry system at Hisar, Haryana. The spacings studied were 5 × 4 m, 10 × 2 m and 18 × 2 × 2 m.
 

 
Number of branches
 
In the present study, significantly more number of branches was recorded at the wider spacing of 4 m × 3 m in all the intercrops except sunflower (non-branching type). It was highest in gingelly (8.6) followed by blackgram (6.7) at 4m × 3m spacing (Table 1). Wider spacing favourably influenced the branching of all the five intercrops except sunflower. The number of branches increased with the increase in the spacing levels. Higher light intensity available at 4m × 3m and competition-free environment favoured the production of more branches. Gill and Ajit (1996) also reported that in the case of cowpea, the number of branches per plant was higher in association with mango grown at wider spacing.
 
Leaf area index (LAI)
 
LAI in all the intercrops was found significantly higher at wider spacing of 4 m × 3 m of J. curcas. Among intercrops, greengram exhibited the highest leaf area index (6.11) followed by blackgram (6.10) and cowpea (6.00). Gingelly recorded the least leaf area index of 1.08 at 3m × 3m spacing (Table 1). More photosynthetic efficiency in wider spacing due to ample availability of solar radiation and unlimited availability of nutrients, moisture and space might have produced more number of leaves and more leaf area of an individual leaf. This might have finally increased the LAI. Similar findings were reported by Basha (2006) in Jatropha based agroforestry system.
 
Root volume
 
Significantly higher root volume was observed at wider spacing of 4m × 3m in all the intercrops. Wider spacing might have enabled the intercrops to develop more roots because of less competition and spread freely to absorb more nutrients. Thakur and Dutt (2003) reported that in the closer spacing, root length, root number and root growth potential of wheat were affected to a greater extent in plants growing nearer to Morus alba. Sunflower (47.51 cm³) followed by gingelly (24.61 cm³) recorded the highest root volume, but due to unsuitable soil condition, they have not performed well. Among the pulses intercropped with J. curcas, cowpea recorded the highest root volume (10.02 cm³) due to higher biomass produced and drought hardiness of the crop (Table 1).
 
Dry matter production (DMP)
 
Dry matter production of all the intercrops was significantly influenced by different spacings of J. curcas. DMP was significantly higher at wider spacing of 4m × 3m spacing. This may be due to the production of more leaves per unit area, increased plant height, increased branches per plant and higher LAI under wider spacing, which intercepted more sunlight resulting in increased photosynthesis and accumulation of dry matter. The rate of dry matter production by a crop is mainly a function of the amount of solar radiation intercepted by its leaves where space, moisture, and nutrients are not the limiting factors (Monteith, 1977). Among the intercrops, groundnut (1757 kg ha^-1) and cowpea (1605 kg ha^-1) produced higher biomass and dry matter at 4 m × 3 m spacing (Table 1). As dry matter production is a function of growth and yield attributes better growth and yield of groundnut and cowpea increased the dry matter production.

All the intercrop growth parameters were in general significantly higher at 4 m × 3 m spacing of J. curcas except that differences between 4 m × 2 m and 4 m × 3 m spacing in respect of number of branches per plant in groundnut and LAI in black gram were non-significant.

Yield of intercrops
 
Different spacings of J. curcas significantly influenced the seed yield of intercrops. Among the spacings, significantly higher mean seed yield was recorded at wider spacing of 4m × 3m (463.45 kg ha^-1) followed by 4m × 2m spacing (400.42 kg ha^-1) (Table 2). Availability of sufficient solar radiation for physiological activity and photosynthesis of the crop and competition-free availability of resources for growth can be the major reason for increased growth and yield of intercrops in the wider spacing (Basha, 2006). Singh et al., (2004) also reported that wheat crop produced higher grain yield under kapok tree at wider tree spacing of 8m × 4m as compared to narrow tree spacing of 4m × 4m. In another study, lower yield of maize at closer spacing was observed in the J. curcas agroforestry system in the second year of cultivation (Abugre et al., 2015).

There was a significant difference in the performance of intercrops regarding seed yield. The maximum mean seed yield was recorded in the intercrop groundnut (787.33 kg ha^-1) followed by cowpea (554.00 kg ha^-1). Blackgram and greengram were found to be par with each other. The lowest mean seed yield was found in gingelly (129.33 kg ha^-1) and sunflower (136.00 kg ha^-1) (Table 2). The crops belonging to the leguminous group performed better in the J. curcas based agroforestry system. This may be due to their ability to fix the atmospheric nitrogen, good root system, growth and yields. The poor performance of gingelly and sunflower was attributed to the inability of the crops to adapt to the cropped soil and environmental conditions and they proved incompatible with J. curcas in the red soils of Mettupalayam. The interaction effect of spacing and intercrop for seed yield of intercrops were not significant.
 

 
Effect of spacing and intercropping on nutrient uptake of intercrops
 
Nutrient uptake is the function of DMP and nutrient contents of the crop and depends on soil and environmental factors. There were significant changes in the uptake of N, P and K due to varied spacings of J. curcas and also various intercrops grown in the interspaces. The mean uptake of nutrients (N-53.0 kg ha^-1; P-17.3 kg ha^-1; K-57.2 kg ha^-1) by the intercrops was higher in the wider spacing of J. curcas (4m × 3m) (Fig 1). Combined effect of higher dry matter accumulation and higher plant nutrient content might have contributed for a higher uptake of nutrients in the wider spacing. It is evident from the findings of Sharma (2005) who reported an increased nutrient uptake by clusterbean with a decrease in the density of trees in P. cineraria and A. albida agroforestry system. Among the intercrops, groundnut was superior in registering more uptakes of nutrients (N -67.3 kg ha^-1; P-19.9 kg ha^-1; K-65.3 kg ha^-1) followed by cowpea, blackgram and greengram (Fig 1). The more biomass produced by these leguminous crops combined with higher root volume and N fixation might have triggered higher nutrient uptake from the soil. Interaction effects were not observed between spacing and intercrops for nutrient uptake.
 

 
Effect of different spacing and intercrops on soil fertility
 
Adopting the agroforestry system can be a strategy for recovering the quality of soil and rejuvenating degraded lands into productive and sustainable production systems (Cherubin et al., 2018). In the present study, significant differences were not observed in soil pH and EC. The initial available N, P and K status of the experimental field were found to be 215 kg ha^-1, 9.5 kg ha^-1 and 235 kg ha^-1. There was a significant difference in post-harvest available soil N and P under different spacings. The highest available N (218 kg ha^-1) and P (10.82 kg ha^-1) was found in 4m × 3m spacing (Fig 2). Wider spacing of J. curcas would have reduced the crop competition for nutrition and hence resulted in higher available N and P. Uthappa et al., (2015) reported decrease in available P with increase in density from 500 trees ha^-1 to 1000 trees ha^-1in Poplar based agroforestry system due to more nutrient uptake by higher root density per unit area.
 

 
Different intercrops also significantly affected the soil available N and P (Fig 2). Among the intercrops, the highest available N was with cowpea (223.7 kg ha^-1) followed by blackgram (220.7 kg ha^-1). It might be due to leguminous intercrops; which through symbiotic association with soil microorganisms fix the normally unavailable atmospheric nitrogen in the soil. Similarly an increase in nitrogen with intercrops was reported under Leucaena leucocephala with maize and cowpea (Hulugalle and Kang, 1990).The highest soil available P was observed with cowpea (12.2 kg ha^-1) followed by greengram (10.5 kg ha^-1). This might be due to the fact that the release of P from the fixed P might have been encouraged by the rhizosphere activities of intercrops (Aggarwal et al., 1976). There was no significant difference in the available K status and interaction effects were not observed between spacing and intercrops. Ghimire and Bana (2015) also reported a non-significant effect of different tree densities on available K status in the Poplar and Indian mustard based agroforestry system.