Assessment of Some Leguminous Weeds as Potential Green Manure Crops under Mizoram, North East India

Green manuring is a renewable resource and can be an essential substitute for improving soil quality and plant nutrition (White and Brown, 2010; Baba et al., 2019). Due to its increased sustainability in cropping systems through reducing soil erosion, improving soil physical properties and increasing soil organic matter and fertility level green manuring is regarded as a good management practice in all agricultural production systems (Tajeda et al., 2008). Leguminous green manure crops because of their symbiotic relationships with N₂-fixing bacteria result in the delivery of a significant amount of nutrients to the soil-plant system and make it potentially accessible in successive cultivations (Perin et al., 2003).
       
Leguminous species exhibit great diversity in their biomass yield and nutrient uptake, which must be related to the inherent traits of each species, the handling of crop leftovers and the current soil and climate conditions. Dry mass production stands out among the ideal features for species selection for green manure because it is associated with the ability to enhance nutrients through symbiosis with microbes, soil cover and nutrient recycling (Teodora et al., 2015; de Oliveiria Miranda et al., 2020). The choice of an appropriate species of legume has a great impact on the amount of biomass, N accumulation and rate of nutrient release into the available forms (Hirpa et al., 2009). Many species of leguminous plants exhibit an exceptional capacity to produce large amounts of biomass and accumulate high nutrient concentrations (Matos et al., 2008). The most productive green manure crops produced about 4-5 t ha^-1 of dry biomass in 50-60 days (Irin and Biswas, 2021). Baba et al., (2019) stated that Crotolaria spp are a potential source of green manure as they produce large amounts of biomass, containing higher nitrogen content.
       
Understanding green manure legumes’ biomass and nutrient content is important for optimizing their benefits and integrating them effectively into crop rotations. Moreover, knowledge of the nutrient content of green manure legumes can help farmers reduce their reliance on synthetic fertilizers and minimize the environmental impacts associated with their use. We can identify the best species for a given location by assessing the potential biomass of local leguminous species. Although many leguminous weeds are abundantly growing in the wild, their growth behavior, biomass and nutrient accumulation are not known. Therefore, the study was conducted to evaluate the growth, biomass production and nutrient accumulation of the three locally available leguminous weeds of Mizoram, India to assess their suitability as potential green manure crops. 

The experiment was conducted under field conditions at the Horticulture Research farm of Mizoram University, Aizawl, India during the month of April- June of 2021 and 2022. The soil of the experimental site was acidic in nature with a pH of 4.82 with soil organic carbon content of 1.52%. The average monthly rainfall received during the cropping seasons in the years 2021 and 2022 was 195.3 mm and 252.1 mm respectively.
       
We used three leguminous weed species, i.e., Crotolaria micans Link., Aeschynomene indica L. and Calopogonium mucunoides Desv. (Fig 1 a, b and c respectively) for the present study. The experiment was laid out in plots in Randomized Block design with four replications for each species. Raised beds of size 1.5 m ´ 1.5 m were prepared for each species. Pre-treated seeds of the selected legumes were sown in line spaced 30 × 10 cm during the first week of April. After 90 days of sowing, fresh biomass of shoot and root was recorded separately. Sub-samples from each plot were collected and kept in a hot air oven at 60 p C until a constant weight was obtained to determine the dry weight for the shoot and root biomass. To evaluate the growth performance, 10 individual plants were selected randomly from each plot and data were recorded.
 

       
The oven-dried samples were milled and passed through a 1 mm sieve for nutrient analysis. Nitrogen was determined after digesting the samples with H₂SO₄ by the micro Kjeldahl method (AOAC, 1995). Phosphorous contents were measured colorimetrically by digesting the plant samples with Diacid (3:1: Nitric acid: Perchloric acid) mixture (Koenig and Johnson, 1942). Potassium, Calcium, Magnesium and other micronutrients (Zn, Cu and Fe) contents were determined by atomic absorption spectrophotometer.   
       
Averaged pooled data were subjected to analysis of variance and means were compared by least significant difference at 5% (Gomez and Gomez, 1984).

The study indicated that there were significant differences among the growth parameters of the species, except for the root length (Table 1). Crotolaria micans presented the highest shoot length (87.54 cm) followed by Calopogonium mucunoides (58.50 cm) and the least by Aeschynomene indica (46.25 cm). Duarte et al., (2013) studied different green manuring crops and found that Crotolaria juncea had a longer height (129.3 cm) than the other at its flowering stage. Similarly, Irin and Biswas, (2021); Romulo et al., (2013) reported that C. juncea had the longest height among the other crops.
 

       
There was no significant difference in the root length which ranged from 24.29 cm to 28.41 cm which was lower than the range recorded by Fageria et al., (2016) where the maximum root length varied from 27.40-35.47 cm in five tropical legume cover crops. Many environmental factors, including soil moisture content, temperature and the physical, chemical and biological characteristics of the soil, have also an impact on root growth (Fageria, 2009). Basal diameter and no. of primary branches ranged from 2.56-7.10 mm and 8.00-26.51 respectively (Table 1).
 
Fresh and dry biomass of the shoot and root
 
There was a significant difference in the fresh shoot and root biomass. The fresh shoot biomass ranged from 2.10-14.20 t ha^-1 with C. micans producing significantly higher fresh shoot biomass followed by C. mucunoides and A. indica (Fig 2). The fresh root biomass produced was in the order of C. micans (1.61 t ha^-1) > A. indica (0.35 t ha^-1) > C. mucunoides (0.28 t ha^-1). C. micans represented higher biomass accumulation, which may be attributed to its fast and determinate growth habit which will increase the soil fertility and also its morphological characteristics as shown in Table 1.  However, Vimala et al., (1999) showed that C. mucunoides after 3 months produced the highest fresh biomass (39 t ha^-1). The dry biomass for shoot and root ranged from 0.58 - 2.72 t ha-¹ and 0.14-0.56 t ha^-1 respectively (Fig 3). Irin and Biswas, 2021; Pereira et al., (2016) reported that Crotolaria juncea produced significantly higher fresh biomass (35 tons/ha and 45.76 Mg ha^-1 respectively). Several other studies have supported the potentiality of Crotolaria spp for dry matter production (Torres et al., 2005; Teodora et al., 2011; Mattar et al., 2015)). The plant populations in normal conditions without any competition will increase the plant biomass (Feichtinger et al., 2004; Daudu et al., 2006). However, Pramanik et al., (2009); Chand et al., (2015) reported that Sesbania recorded significantly higher fresh shoot biomass compared to Crotolaria. Sesbania aculeata and Crotaria juncea produced 5.3-6.3 and 6.2-8.2 t ha^-1 of dry biomass respectively (Hiermath and Patel, 1996). Similarly, Miah et al., (2015) reported fresh shoot and root biomass of 5 green manure crops ranged from 6.24 to 52.62 t ha^-1 and 1.32-11.3 t ha^-1 respectively; dry shoot and root biomass ranged from 0.86-6.3 t ha^-1 and 0.20-1.59 t ha^-1 respectively which was higher than our findings. However, a study by Chilagane et al., (2018) reported the highest dry biomass (15.13 t ha^-1) in velvet beans which was more than that of C. juncea (11.75 t ha^-1).
 

 

       
The differences in biomass among the studied species may also be attributed to the region’s availability of light, temperature and climatic factors. Thomas and Palaniappan, (2012) commented on some leguminous green manure crops being sensitive to photoperiod including Crotolaria juncea and further observed that the vegetative growth period was restricted due to shorter growth period and low temperature.
 
Nutrient content and accumulation
 
Nutrient contents in the shoot and root biomass was significantly different among the species (Table 2). C. micans exhibited the highest nitrogen content in the shoot (29.40 g kg^-1) and root (21.91 g kg^-1), followed by A. indica and C. mucunoides. Significantly higher P content for shoot and root was observed in C.micans, 5.24 g kg^-1 and 4.38  g kg^-1 respectively. The highest K content in the shoot was observed in C. micans (21.47 g kg ^-1), while C. mucunoides exhibited the highest in the root (17.60 g kg^-1). Ca content in shoot biomass ranged from 0.52-0.69 g kg^-1 with A. indica having the maximum whereas, Ca content in the root ranged from 0.03-0.22 g kg^-1 with C. mucunoides having the highest. The highest N values found in C. micans can be attributed to the plant’s high biological nitrogen fixation efficiency by the plant’s root nodules. The P contents in the studied legume were higher than those studied by Duarte et al., (2013). Contents of micronutrients among the species were in order Fe > Zn > Cu (Table 2). Fageria et al., (2002); Fageria et al., (2016) also reported an order (Fe > Mn > Zn > Cu) of micronutrient concentration in crop plants. Duarte et al., (2013) also reported that in the aerial part of the studied green manure plants, the content of Fe was higher and Cu content was the lowest.  
 

       
The leguminous species accumulated significantly different amounts of nutrients in their shoot and root biomass (Table 3). C. micans was statistically superior for the accumulation of N, P and K in both the shoot and root biomass. Furthermore, the highest dry biomass production of C. micans is an added plus in contributing the highest amount of nutrient accumulation among the studied legumes. C. juncea (126 kg ha^-1) recorded the highest total N values before the cultivation of eggplant (Castro et al., 2004). Pereira et al., (2016) accumulated about 377 kg ha^-1 N at 92 days and Duarte et al., (2013) accumulated about 175.8 kg ha^-1 N during the full flowering phase. In 55 days of growth, C. juncea, S. aculeata and V. unguiculata respectively accumulated 103, 84 and 67 kg of N (Datt and Bhardwaj, 1995). Calcium accumulation in the shoot biomass ranged from 3.53 to 18.33 kg ha^-1 with, C. micans having the highest and the least by C. mucunoides. However, A. indica accumulates higher amount of Ca in the root biomass (0.27 kg ha^-1). C. micans recorded higher accumulation of Zn, Fe and Cu (1445.02, 72.65 and 42.23 g ha^-1 respectively) in the shoot biomass and (900.06, 10.49 and 29.15 g ha^-1 respectively) in the root biomass (Table 3).
 

       
The varying nutrient accumulation in the shoot and root biomass of the leguminous weed species in this study and other studies may be attributed to genetic variations in the species, ages and cultivation practices of the crops.

It is concluded that Crotolaria micans was the most promising species in terms of growth performance, biomass production and nutrient accumulation after 90 days of sowing.
       
The high nutrient accumulation capacity, in a short period, makes C. micans potential species for use as a green manuring crop. However, a detailed investigation is required in terms of its decomposability, nutrient release, crop response and allelopathic impact through systematic on-station and off-farm trials in certain to its suitability to be recommended as a green manure crop.

We are also thankful to the Head, Department of Horticulture Aromatic and Medicinal plants, Mizoram University, Aizawl for allowing us to conduct the field experiment in their Research farm.

All authors declare that they have no conflicts of interest.