5. Discussion
Morphological characteristics are the physical or visually determinable attributes of a tree seedling. The major morphological criteria used to describe seedling quality are shoot height or shoot length, stem diameter or collar diameter, root length etc. Height increment and stem diameter enlargement are good traits for selecting fast-growing leguminous plants for agroforestry (Duguma and Tonye, 1994). Consequently, these morphological indicators of plant growth were correlated with the effect of fertilizers in this study.
The study clearly revealed that the fertilizers had the beneficial effect on the improvement of growth parameters of leguminous agroforestry tree seedlings. Application of fertilizers yielded better growth and seedling quality. The results of this study is also supported by Bhuiyan et al., 2000; Walker et al., 1993 and Sanginga et al., 1989 who reported the accleration of growth parameters on the application of fertilizers in Casuarina spp.; Sundralingam, 1983 in Tectona grandis; Paul and Emdad Hossain, 1996; Das, 1984 and RAPA, 1987 who found the beneficial use of cowdung for improving growth of leguminous tree seedlings; Ponder 1984; Schutt et al. 1985; Evan 1986; Stanton and Gerhold 1988; Stanturf et al. 1989; Gleason et al. 1990; Heilman and Fu-Guang 1993; Maze and Vyse 1993; Warren 1993 who reported the increment of growth on the application of N fertilizers; Israel, 1987; Vadez, et al., 1999; Vadez and Drevon, 2001 and Clarksonm, 1985; Verma et al., 1996 and Totey, 1992 who reported the effect of P fertilizers on plant heighr, collar diameter, root length were significant. This findings are also correlated with the findings of Sun et al., 1992 who found that Acacia mangium growth increased with P application and Verma et al., 1996 who reported the positive effect of phosphorus fertilizers on the growth of Dalbergia sissoo.
In some cases it was found that the higher doses of fertilizers reduced the seedling growth which might hamper the seedling growth by the initiation of toxic effects. This supported the findings of Van den Driessche (1980), who reviewed both the positive and negative effects of nursery fertilizers application on subsequent seedling growth and survival. Negative effects of fertilization were also reported for Sitka spruce (Benzian et al., 1974) and Lobolly pine (Pharis and Kramer, 1964; Schomaker, 1969). Similarly Hartley (1977) and Kadeba (1978) reported that addition of access fertilizer on Pinus caribaea depressed growth and increased mortality of the seedlings and higher doses reduced for growth of Gmelina arborea seedlings in a pot experiment (Kamis Awang and Ismail Katim, 1986).
Fertilizers have the important effect on Biological Nitrogen Fixation (BNF). Since nodulation (number of nodules/plant, nodule color, etc) is the indicator of BNF (Singleton, ?), so we studied the effect of fertilizers on nodulation. And the present study revealed that different fertilizers have different effects on nodulation. It was found that nitrogen fertilizers influenced the nodulation of leguminous agroforestry tree seedlings in two ways. First, in some cases the nodulation (nodule numbers/plant) was found to be promoted by the low doses/levels of N fertilizers (Urea). Zahran, 1999 and Afza et al., 1987, also reported these same findings. Second, in most of the cases, N fertilizers both higher and lower levels almost always depress or inhibit nodulation. These findings coincided with the findings of Hardarson, et al. 1991; MacDicken, 1994; Davidson and Robson, 1986; Eaglesham, 1989; Herdina and Silsbury, 1989; Zahran, 1999; Atkins et al., 1984; Arreseigor et al, 1997; Abdul-Waheb et al., 1996; Imsande, 1986; Purcell and Sinclair, 1990 and Sanginga et al., 1996. This results are also consistent with the findings of Fakir et al., 1988; Bhuiya et al, 1989; Solaiman, 1999 who found the suppression of nodulation in agroforestry crops due to nitrogen fertilization.
Another findings of this study were the positive effect of P fertilizers (TSP) on the nodulation of agroforestry tree seedlings. It was found that nodulation was significantly increased in both levels of phosphorous fertilizations in comparison to the control (without P fertilizations). Similar findings were reported by Gates and Wilson, 1974; Hicks and Loynachan, 1987; Paula and Siqueira, 1987; Adu-Gyamfi et al., 1989, Jasper et al., 1989; Munns, 1997; Israel, 1987; Vadez, et al., 1999; Vadez and Drevon, 2001; by Sun et al., 1992 who also found that Acacia mangium growth and N fixation increased with P application; by Sanginga et al., 1991 who reported that P application in Leucaena leucocephala improved the seedling biomass and nitrogen fixation.
The nodule shapes and color were also reported in this study. It was found that the nodule shapes in Mimosoideae (Acacia spp., Albizia spp., Leucaena leucocephala) were fan-shaped, finger-shaped, lobed and semi-globose. In Papilionoideae (Gliricidia sepium, Sesbania sesban) the nodule shapes were
bifurcate, globose and elongated. Nodules of Acacia spp. Leucaena leucocephala were mostly brown in colour and rough-surfaced. Most of the young nodules of rest of Albizia spp. (Used in experiment) were round, white and smooth-surfaced while older nodules were of various shapes. These findings were consistent with the findings of Anegbeh, 2002 who reported the characteristics of nodules of four agroforestry tree species in Niger Delta Nigeria and Corby, 1971.
However, present investigation clearly concluded that in the context of sustainable agriculture and agroforestry, the use of NFT and the application of fertilizers might be an effective biological strategy directed at sustainability. In an agroecosystem this means profitable production enhancing the BNF without damage to the environment. BNF by NFT is an appropriate alternative that will solve certain problems caused by disappearance of forest resources and the widespread loss and deterioration of soils.
Although, fertilizers shows the significant effects on BNF and growth of agroforestry trees, but still little is known about the effects, their quantification, mechanisms, measurement of nodule nitrogen fixation etc. due to the limited lab facilities. So, there is need for further facilities and studies will be broadened to evaluate the effect of fertilizers on BNF under agroforestry farming systems.
