Effects of fertilizer-N and organic resource management on soil aggregates formation and carbon cycling in the central highlands of Kenya

Abstract

The maintenance of proper levels of soil organic matter (SOM) has been advocated as one of the main ways of combating soil fertility decline in sub Saharan Africa (SSA). SOM levels can be increased through increased aggregate formation as soil aggregates physically protect SOM, from its loss through decomposition. The objective of this study was to investigate how the amendment of soils of varying texture and fertility levels with fertilizer-N and organic resources affects aggregate formation and subsequent carbon (C) cycling in aggregates. The experiment was conducted in Embu and Machang' a areas of central Kenya and was based on a decomposition tube experiment that was set up in April 2005.

This experiment aimed at complimenting long-term field trials started in 2002 to establish the effect of application of various combinations of organic plus mineral resources on soil nutrient status. The main variable comprised of fertilizer-N and organic resources, with the sub-treatment being soil fertility levels. Maize stover and urea fertilizer were mixed with 3.2 kg of soil and set up in decomposition tubes. The application rate was 4 ton per hectare and 120 kg per hectare for the maize stover and fertilizer respectively. Four treatments were established, namely; control (no organic resources or fertilizer-N added), sole fertilizer-N, sole stover and combined stover and fertilizer-N, with each treatment having three replicates. To determine changes in soil aggregates, soil samples obtained from the decomposition tubes were fractionated through wet sieving. SOM fractionation was also conducted to obtain the various SOM fractions. All aggregates and SOM fractions obtained were then oven dried, ground, and analyzed for C. All the data collected was analyzed with the PROC MIXED procedure of SAS and the means separated at p <0.05.

Higher proportions of macroaggregates were observed for the Embu soils compared to the Machang'a ones. For both Embu and Machang'a soils, amendment of soils with sole or combined fertilizer-N and organic resources had a significant effect (p <0.05) on the proportions of all aggregate class sizes. Amendment of soils of varying fertility levels with sole or combined fertilizer-N and organic resources also had a significant effect on the distribution of SOM fractions for both Embu and Machang'a soils. Significant differences in whole soils, aggregates and SOM fractions percentage carbon levels were also observed. Overall, the silt and clay fraction had higher C levels compared to the other aggregate size classes indicating higher stabilization of C within this fraction.

From the results obtained from this study, the use of combined organic and mineral resources is recommended for the improvement and maintenance of soil fertility in high fertility soils. In low fertility coarse textured soils, the sole application of organic resources is recommended for the improvement and maintenance of soil fertility.