It is widely recognized that nitrogen and phosphorus are often nutritional limiting factors of plant growth in most tropical soils [1] [2] [3] [4] [5]. To raise the level of these elements in such poor acid soils, the use of nitrogen and phosphate fertilizers is a real bargain [4]. Because the increased production recorded the recent decades could be attributed to more half to the increased use of inorganic fertilizers [6]. Among all inputs applied in order to maximize agricultural output, nitrogenous fertilizers rank first. They make up 50% of all nutrient inputs. In addition, among the nutrients brought by these fertilizers, the most important is nitrogen [7], which can be attributed in some measure, 75% of the increase in crop yields recorded [6].

Thus, to meet future food needs, it will obviously continue to make more use of the inorganic nitrogen fertilizer. The efficiency of these fertilizers, however, is one of the lowest among plant nutrients. Therefore, high rates of N fertilization especially when using ammonium sulphate have also been shown to acidify soils. Yet these are severely limited by prohibitive purchasing costs and general lack of availability by the smallholder farmers [8] [9] [10] [11] and may eventually pollute the environment [6], degraded ecosystems and disrupt human health [12]. Nitrogenous fertilizers are subject to losses due to denitrification leading to release of nitrogen N₂ and NO₂ gases, volatilization of ammonia, run-off and leaching which can contaminate surface and underground water. To remedy this situation, technologies that will reduce N fertilizer input by resource-poor farmers in the Nutrient Management Systems (NMS) are urgently needed. Nitrogen input through biological nitrogen fixation (BNF) by grain legumes can help to maintain soil N reserves as well as substitute for N fertilizer requirement for large crop yields. So, researchers in the world exploited the legume Rhizobium symbiosis as a substitute for expensive N fertilizers [13] [14] [15] [16] [17]. To enrich soil-plant system in nitrogen and compensate for N potential losses, the phenomenon of BNF is an alternative present in virtually all ecosystems. Through a series of biological systems, nitrogen fixation is considered to have a major role in maintaining the productivity of tropical soils [18]. According to Chris et al. [19], Nitrogen (N) contributing by legumes to other crops in the system depends on the species, biological N₂ fixation and growth of legumes as determined by climate and soil, and management of residues.

So, in this practice, the choice of legumes varieties that nodules easily with indigenous rhizobia, is of capital importance [13]. Among these species, pigeon pea (Cajanus cajan L. (J. Millop)) has the capacity of growing in low soils in nitrogen without the addition of nitrogen fertilizer while improving the economy in fixed nitrogen for subsequent cereal crops [20], and provided a quality of produced food for humans [21] [22] [23] and a forage appreciated the cattle (Otero cited by Skerman) [22]. Although, the use of legumes seems to solve the problem due to lack of N in the soil; therefore, the amount of the nitrogen fixed by legumes is dependent not only to generic features of bacteria and the host plant, but also to environmental and agricultural practices [24]. Thus, the fertilizer phosphorus and nitrogen plays an important role [6] [25]. Mont Amba soils, however, have characteristically low available P making application of P through fertilizers necessary. Apart from P deficiency limiting plant growth, it can also limit symbiotic N₂ fixation as the latter has been noted to have a higher P requirement for optimal functioning than either plant growth or nitrate assimilation. If this P constraint is overcome, grain legumes in the cereal based cropping system of NMS should be able to fix a greater amount of N₂. For a mineral concentration of P is required for Nod factors (having an important role in the process of recognizing bacteria-plant) can occur (Mckay and Djodievic, 1993 quoted by Dommergues et al. [14] ). Thus, in the case of deficient soils, it is certain that the application of P improved nodulation (nodule number and weight) and increases the amount of N₂ fixed [24]. This confirmation is supported by the results of Sanginga et al. [26] on Leucaena leucocephala and Sanginga et al. [27] on cowpeas. Thus the use of organic manure instead of mineral fertilizers seems advantageous because of the disadvantages of mineral fertilizers recorded. Because these appear to be balanced organic fertilizer in essential elements (NPK) and to a lesser extent, S, Ca, Mg and other trace elements for plant growth [28]. This study then attempts to assess the impact of the form and nature of the different sources of nitrogen on growth and modulation of pigeon pea.

In general, all soil treatments had each a different effect on growth and nodulation of pigeon pea. Indeed, the mineral nitrogen (urea) had an effect on the border between the feces of pigs upstream and downstream chicken droppings both on nodulation, biomass production and on total height growth of plants of pigeon pea. Thus, the quantity of mineral nitrogen (0.5 g) applied to the soil could be treated as starter N for pigeon pea, which should stimulate nodulation as observed elsewhere [24].

Note also that the irregularity of the growth in both diameter and height in plants receiving 0.5 g of urea as a fertilizer, could be due to the split application of this fertilizer, because every time it has been applied, growth was stimulated immediately and then gradually slowed down as N becomes deficient by leaching and other losses (Table 1 and Table 2). These results corroborate the observation by Danso and Eskew [6] and Joly [18]. This is the observation made also for the treatment of chicken manure, the effect on growth was rapid during the first week and gradually slowed down as the age of plants evolved. In fact, the dung being a light organic fertilizer, N would have been quickly released, used in part by the seedlings, and then lost resulting in depth by leaching and through other channels as observed by Danso and Eskew [6] and Joly [18].

This depletion of N would have resulted in the decline in growth of seedlings that received the treatments equivalent to 0.5 kg and 1 kg of chicken manure to the end of the test. By cons, poor nodulation observed in the seedlings have undergone treatments with chicken manure is due to the concentration of N that would have inhibited the nodulation compared to the control and treatment to 0.5 g of urea, given that the excess of available nitrogen inhibits nodulation [24], on one hand. On the other hand, it is thought that the chicken manure has a high concentration of metals (Pb, Cu and Zn) and soluble salts, which thus have inhibited the growth and nodulation by rhizobia by acting on toxicity [31] [32] [33] [34].

We can also note the alternation of several organic substances contained in the fertilizer that are suspected to be toxic to rhizobia as to cause inhibition of nodulation [35] [36]. However, we found that pigeon pea grows on the substrate significantly improved by the application of pig manure (soil application of 1 kg and 0.5 kg) with good symbiotic characteristics (Table 4), among others. The conclusive evidence of the pronounced effect compared to other treatments is the rise in number of nodules per plant (Table 4). When closely related to the acceleration of the activity of nitrogenase, this parameter determines the concentration of N in plants and therefore the increase in biomass production as reported Abd-Alla et al. [37]. As such, we agree with Abd-Alla et al. [37] that there was in the feces of pigs a number of beneficial elements including a high concentration of P, with N, supported the good performance of both fertilizer on growth than on the nodulation of pigeon pea in our trial. In addition to P, one can also refer to metals such as Co, Cu, etc., which have a potential to improve the nodulation of legumes (Chatel et al. 1978; Bollard, 1983). Indeed, Abd-Alla and Abdel [38] had by the application of Co and Cu (100 mg/kg) significantly increased the number of nodules and the production of dry biomass in faba bean (Vicia faba). In this case, the feces of pigs are a good source of major elements (NPK) and other related items.

They are supposed to stimulate nodulation and increase the survival and multiplication of rhizobia in the soil and improve their natural habitat as found by Heckman et al. [39] of treated human waste, and therefore they enhance the growth and biomass production of crops. The presence of nodules on the control treatment shows that the legume under study have found a strain of rhizobia indigenous to the same nodules in the soil and pigeon pea nodulation can start even in very poor soils in N such that our test.

At the end of our observations, it appears that all the treatments applied to the soil produced, each, different effect on growth and nodulation of pigeon pea. However, treatment of 1 kg of pig manure ensured a smooth behavior of plants in terms of pigeon pea nodulation, growth and biomass production. This result indicates the power of pig feces to serve as fertilizer and organic amendment in improving fertility and soil physical properties of Mont-Amba although its optimum amount to be determined. So pig feces are richer in nutrients that stimulate growth and nodulation of pigeon pea, in contrast to the dung of hens on the contrary, reduced the performance against the observations reported by Zuang [28] on vegetable crops.

The authors declare no conflicts of interest regarding the publication of this paper.

Muyayabantu, G.M., Kabwe, N.M.M., Mutombo, J.M.T. and Dikand, K.B. (2019) Effect of the Form and Nature of Nitrogen on Growth and Nodulation of Pigeon Pea (Cajanus cajan (L.) Millsp) on Sandy Soil of Mont-Amba, in Democratic Republic of Congo. American Journal of Plant Sciences, 10, 1457-1467. https://doi.org/10.4236/ajps.2019.108103