MAIZE-SOYBEAN PRODUCTION AND MICROBIAL DIVERSITY AS AFFECTED BY RHIZOBIA INOCULATION AND CROPPING SYSTEMS IN A FERRALSOL IN WESTERN KENYA

Abstract

The use of promiscuous soybean varieties and commercial inoculants has been shown to increase production and improve soil fertility. However, most cropping systems are characterized by indigenous bradyrhizobia whose populations are often low or ineffective and can be increased in the presence of the host legume for several seasons. The contribution of a commercial inoculant (Legumefix®) in soils with low levels of indigenous rhizobia to soybean and maize production in different cropping systems was evaluated in field experiments in three sites in Siaya County using a randomized complete block design. Maize and soybean were planted as sole crops or in rotations to give a total of seven treatments for four seasons (short rain (SR) 2014, long rain (LR) 2015, SR 2015 and LR 2016). To assess the nodule occupancy, profiles were assayed using Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP). Denaturing gradient gel electrophoresis (DGGE) was used for genetic finger printing of the amplified 16S-23S rDNA intergenic spacer region to assess the bacterial communities in the study soils and were clustered using a dendrogram. Shannon Weaver diversity index (Hʹ) was used to calculate Hʹ values. Data was subjected to analysis of variance (ANOVA) to generate standard error of difference between means (p<0.05). During the first season, the inoculated and noninoculated treatments recorded a 91.6% and 21.6% difference in nodule weight and soybean grain yields, respectively. Soybean grain yields increased significantly under rotation when maize and soybean were planted in the LR and SR, respectively. A strong, positive and significant correlation between soybean grain yield and nodule weight was recorded. Maize grain yield ranged from 1195 kg ha-1 to 5450 kg ha-1 with inoculated and non-inoculated soybean-maize system recording a rotational yield gain of 15.7% and 7.3% in LR 2015, 24.9% and 24% in LR 2016, respectively. Soybeanmaize system outperformed mono maize system in cumulative net benefits (218 and 128 KES, 000 ha-1, respectively). The nodule occupancy for intergenic spacer profiles groups (IGS) profile I ranged from 71% to 100% while IGS profile II recorded higher nodule occupancy than IGS profile III in non-inoculated treatments. The rotational systems had higher Hʹ values than monocropping systems. Based on these results, inoculation is necessary and in cases where the inoculum is absent, building up of the indigenous rhizobia could boost soil bacterial diversity for agricultural sustainability. Sequencing and commercialization of IGS group II in the study area is recommended. Key words: native rhizobia; net returns; nodule occupancy; rotational yield gain