Changes in soil organic matter as influenced by organic residue management regimes in selected experiments in Kenya

The failure to understand the dynamics of soil organic matter (SOM) is a major limitation to the sustainability of smallholder production systems that predominantly relied on organic resources for the maintenance of soil fertility. This study evaluated the influence of organic resource management on SOM in three selected experiments in central and western highlands of Kenya. Results showed that soil carbon (C), nitrogen (N) and carbon-13 (C-13) values in the three experiments were depending on the amounts of the organic residues applied as well as the duration of application indicating that organic residue management practices have a profound impact on the final contribution to the SOM pools. Kabete experiment had the narrowest C, N and 13C values pointing to its young age as well as the low quantity of the organic residues applied. On the other hand, Embu experiment had soil C values above the critical level of 2.0% indicating a positive effect of continued application of organic residues. In all the three sites, aggregate mineral fraction (MF) size distribution were dominated by macroaggregates (250-500 mu m and > 500 mu m) which on average accounted for about 72%, 65% and 69% of the dry soil weight for Maseno, Kabete and Embu experiments, respectively. Similarly higher proportions of aggregate light fractions (LF) C and N were observed in macroaggregate fractions for the three experiments with organic treatments having higher proportions. The C-13 signatures of the LF in the macroaggregates (> 250 mu m) were more negative as compared to the C-13 values in the microaggregate (53-250 mu m) LF suggesting a more C contribution from C3 vegetation to the most recently incorporated SOM pool.