Effects of organic materials and inorganic fertilizer on soil microbial biomass at Kabete, Kenya
Baaru, Mary Wamuyu
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Recently, the issue of sustainable agro ecosystem production has been of great concern. This has led to the need of understanding and developing management strategies, which conserve and protect the soil resources. Micro biota is one of the critical agents in relation to soil fertility and plant growth, due to among other reasons; its participation in nutrients cycling and in the formation and stabilization of soil aggregates. In order to develop sustainable and productive agroecosystems, there is therefore need to understand the dynamics of soil microbial biomass. This study was designed to evaluate the influence of organic inputs, inorganic fertilizers and their combinations on soil microbial biomass. The experiment was set-up at Kabete, Kenya, which is an on going trial established in 1999 (Kabete Nitrogen Management NI). It involved use of different combinations of Tithonia diversifolia, Senna spectabilis, Calliandra calothyrsus as the organic nutrient resources and urea as the nitrogen source. The experimental design was a complete randomized block design with 10 treatments each replicated 4 times. Soil Samples were collected to a depth 0-10 cm for two maize cropping seasons and this was done before incorporating the inputs and every two months thereafter within the season and also at harvesting time. The soils were analyzed for microbial carbon and nitrogen content using the chloroform fumigation extraction and incubation methods at ICRAF laboratory. Plant samples (maize ears and stovers) were also collected for yield data. Analysis of variance (ANOVA) was conducted on the data and means separated using LSD at 5% significance difference using Genstat for Windows Version 6. All treatments gave higher soil microbial biomass N (Nmic) than the control treatment throughout the two maize cropping seasons. Further, organic+urea treatments resulted in higher values of Nmic than sole organic treatments. Microbial biomass carbon (Cmic) in soils treated with organic and organic+urea inputs was higher than either control or urea treatments in both seasons. Treatments receiving organic+urea inputs gave higher values of Cmic than their corresponding sole organic treatments. Plant growth seemed to stimulate Cmic growth, as there was an increase in Cmic at the peak of plant growth. Moreover, Cmic was found to be positively correlated with total organic carbon and maize yield. Soil respiration in the control and urea treatments was lower than all the other treatments in both seasons and was found to be significantly lower than sole tithonia and tithonia+urea in the month of October 2003. Further, organic+urea treatments gave higher soil respiration than their corresponding sole organic treatments though the difference was not significant. A decrease in basal respiration was noted after addition of organic materials indicating a less stressed microbial community. The results of this study indicated that application of sole organic materials increased soil microbial biomass. However, organic+urea treatments did increase microbial biomass above sole organic treatments, suggesting that combining organic materials and urea is a better option for optimum soil production. Therefore, the advocacy should be to add organic resources where finances are limiting for the purchase of urea, otherwise combining of organic and inorganic inputs would be ideal for a productive and sustainable ecosystem.