Carbon, Nitrogen, and Phosphorus Stocks from Fallow of Forage Legumes on Alfisols of Guinea Savanna Nigeria

The aim of this study was to assess soil carbon (C), nitrogen (N), and phosphorus (P) stocks from fallow of two forage legumes: Centrosema pascuorum (Cp) and Macrotyloma uniflorum (Mu) on Alfisols of Guinea Savanna, Nigeria. The study was conducted at the Institute for Agricultural Research (IAR) experimental field Samaru Zaria, Nigeria (2008 to 2009). Treatment consisted of 3 plots (Cp planted, Mu planted, and control - natural vegetation regrowth). Plot size was 5 m * 3 m = 15 m2 replicated three times for each treatment. Pre-experimental composite soil samples were taken with an auger at 0-15 depth. The soil samples collected were air-dried, grounded, sieved with a 2 mm sieve, and the less than 2 mm fraction was analysed for C, N, and P. The results of the analysis of soil organic C, total N, and available P were 6.1 gkg-1, 0.53 gkg-1, and 8.75 mgkg-1 respectively. Soil pH was 5.9. The plots were left fallow for one year and again soil samples were collected at 0-15 cm depth and analysed. The results showed that Cp significantly improved and had higher soil available P (13.74 mgkg-1) after one year followed by Mu (7.68 mgkg-1) and control (4.6 mgkg-1). On the other hand, the control plots significantly had highest soil organic C (5.9 gkg-1) compared to 5.2 and 3.7 gkg-1 from Cp and Mu. Similarly, higher total N (2.9 gkg-1) was recorded from control plots compared to 1.4 and 0.5 gkg-1 from Mu and Cp respectively. Results from this study indicate that one year fallow of cultivated Cp has potential to improve soil available P compared to Mu and natural vegetation regrowth. In terms of plant nutrient uptake, the N and P content of Mu was the highest (4.28 and 2.65 % respectively). The natural vegetation from the control plots had the lowest (1.95 %) N concentration. There was no significant difference in the P content of the natural vegetation from the control plots and Cp (1.92 and 1.84 % respectively). Fallow periods of more than a year of cultivated Centrosema pascuorum and Macrotyloma uniflorum or their incorporation into soils are suggested for further studies.