Starter N and P Fertilizers Have Dissimilar Effects on Native Mycorrhizal and Bradyrhizobial Symbiosis of Four Promiscuous Soybean Varieties in Acid Soils of Cameroon

Influence of starter nitrogen (N) and phosphorus (P) applications on mycorrhizal colonization and nodulation of soybean (Glycine max (L.) Merill) was investigated at two sites of humid forest margins of Cameroon. Four soybean varieties, TGX 1838-5E (var1), TGX 1879-7E (var2), TGX 1828-4E (var5) and TGX 1805-13F (var6), were grown for four months in fields from different fallow ages, in a factorial combination of 30 kg N.ha-1 and 30 kg P.ha-1, with five replicates, in a randomized complete block design. Soil acidity varied strongly with site, being 5.9 and 4.5, at Nkometou and Mengomo, respectively. Fractional mycorrhizal colonization (FMC) was not affected by soil pH. FMC significantly varied among fields between 20% and 40%, was significantly reduced by P fertilization while effect of N amendment was contrasted. Nodulation was strongly influenced by soil pH: high nodulation in Nkometou but extremely low in Mengomo. In Nkometou, early maturing soybean varieties (var2 and var5) yielded higher nodule number and mass than late-maturing ones (var1 and var6). Var2 exported the highest biological N and var6 the lowest. Plant P uptake only differed among fields. Nodulation and grain yield did not respond to the fertilizer rates. Negative, highly significant correlations were established between nodule number and mass (r= - 0.726; p< 0.0001; n= 68), between nodule number and FMC (r= - 0.682; p< 0.0001; n= 68). However, a positive and highly significant correlation was obtained between FMC and nodule mass (r= 0.976; p< 0.0001; n= 68). Such biological reactions to fertilization could be attributed to effective indigenous arbuscular mycorrhizal fungi and bradyrhizobia

Soil structure of an Oxisol as influenced by land use systems in the forest margin zone of Southern Cameroon

Soil structure is the key for controlling soil quality. To assess changes in soil structure and its related indices under different land-use systems, an on-farm investigation was carried out on an Oxisol of Southern Province of Cameroon. Six land-use systems (LUS) consisting of a primary forest, a 30-year old secondary forest, a 25-year old cocoa field, a 15-year old natural fallow, a 3- year old Chromolaena odorata fallow and a 2-month old groundnut (Arachis hypogea) field were tested. Soil samples were collected at 0-5 and 5-10 cm depths and were used to determine particle size distribution (hydrometer method), bulk density and aggregatestability. Soil resistance to penetrometer was measured at the same depths in the field using a hand penetrometer. It was found that the primary forest and the cocoa field were associated with highest claycontents (74.6 and 52.0%, respectively) compared to other LUS. However, bulk density was significantly higher under cocoa field (1.09-1.26 g.cm-3) as compared to primary forest (0.72-0.89 g.cm-3). Soil resistance topenetrometer was the lowest under forests (1.2 - 5.2 bars) as compared to cropped fields (9 -12.5 bars) at 0-5 cm depth. Similar trend was observed at 5-10 cm depth. The proportion of aggregates less than 2 mm indiameter was the least under primary forest (27%) as compared to groundnut field (52%). In contrast, soil aggregates under primary forest and cocoa field were more stable, with the highest mean weight diameter(MWD) of 3.37 and 3.00 mm, respectively.Keywords: Cameroon, humid forest, land use systems, Oxisol, soil structure, structural stability