Nutrient release from organic resources in Nitisols of the Central Highlands of Kenya

We carried out a litterbag experiment to assess the characteristics of animal manure and Tithonia diversifolia and their nutrient release dynamics in un-amended soils under different tillage systems. We conducted the study in Meru South and Kandara sub-Counties during short rains 2016 (SR16) and long rains 2017 (LR17) seasons. The treatments were; conventional tillage (ConvT) and minimum tillage (MinT). Litterbags were buried vertically in the soil to a depth of 8 cm at planting. T. diversifolia had high nitrogen concentration resulting in a significantly low C/N ratio. The recalcitrant pool (cellulose, lignin and polyphenols) was higher in animal manure compared with T. diversifolia. Nutrients (N and P) remaining from litterbags with animal manure were generally higher under MinT while in T. diversifolia they were lower under MinT in both sites. Nutrient content in both organic resources declined as the cropping season progressed. N and P release rate from animal manure was significantly higher under ConvT compared with MinT on the 8th week at Meru South site. Nutrients' release rates in T. diversifolia were significantly higher under ConvT by 57 and 48% for N and P, respectively, compared with MinT on the 6th week during SR16 season in Meru South. At Kandara site, N release rates from animal manure were significantly higher under ConvT by 60% on the 2nd week and 30% on the 8th week compared with MinT during SR16 season. The phosphorous release rate was significantly high under ConvT by 62% on the 2nd week, 43% on the 8th week and 25% on the 10th week during SR16 season. During LR17 season, N and P release rates in animal manure were significantly higher under ConvT by 54% compared with MinT on the 8th week at Kandara. Nitrogen release rates from T. diversifolia were significantly higher under ConvT by 40% on the 4th week, 30% on the 6th week and 42% on the 10th week during SR16 season. P release rate from T. diversifolia was significantly higher under ConvT by 41% on the 4th week and 32% on the 10th week compared with MinT. During LR17 season, N and P release rate from T. diversifolia was significantly higher under ConvT by 29 and 28% respectively, compared with MinT on the 8th week. Therefore, tillage, as a soil management option, influences nutrient availability from organic resources and should be considered in soil fertility management.