Soil Organic Matter Status under Different Agroforestry Management Practices in Three Selected Sites in Kenya
Soil organic matter (SOM) plays an important role in ensuring a healthy soil status. This study was designed to evaluate the influence of organic resource management on SOM-related soil properties in the Kabetc, Maseno and the Embu experiments in Kenya. The choice of these experiments was based on the different organic resources applied, their lifespan as wel1 as their unique ecological locations that characterize most smallholder fanning areas in Kenya. Soil samples were collected from these experiments before the long rains season of 2002 and prepared for analyses. Soils were analyzed for inorganic nitrogen, total carbon, nitrogen, carbon mineralization, bulk density and soil moisture retention. In addition, SOM aggregate and size fractionation, potassium permanganate oxidation and the carbon isotope labeling techniques were used to determine the quality of the various SOM fractions formed. All the data collected was subjected to analyses of variance (ANOV A) and the means separated at P :s 0.05. Mineral N was significantly different (P :s 0.05) across the treatments in HI Ernbu and PM I Maseno experiments and tended to be higher in organic treatments as compared to the control and the fertilizer treatments. N I Kabete experiment had the lowest C~ N and DC values pointing to the young age of this experiment as well as the low quantity of the organic residues applied. On the other hand, HI Embu experiment had high soil C values of over 2.0% indicating a positive effect of continued application of organic residues. Potassium pennanganate oxidizable carbon was significantly different for N I Kabete and PM I Maseno experiments and tended to vary according to the differences in organic resource management regimes in these experiments. The bulk density was not significantly affected by organic residue management regimes at any of the sites. Soil moisture retention trends were more defined in the older PM I Maseno as compared to the younger N I Kabete and HI Ernbu experiments. Aggregate mineral fraction (MF) size distribution were dominated by macroaggregates (250-500 urn and >500 urn) 11l the three experiments. HI Embu experiment had higher aggregate light fraction (LF) proportions as compared to N I Kabete and PM I Maseno experiments indicating the beneficial effects of continued organic residue application. Similarly, higher proportions of aggregate LF C and N were observed in macroaggregate fractions for the three experiments with organic treatments having higher proportions of both aggregate MF and LF C and N. The Ouc signatures of the macroaggregates (>250 urn) LF were more negative as compared to the 0 DC values in the microaggregate (53-250 ~1I11) LF. This pointed to C contribution to the most recently incorporated organic matter from C3 organic resources being applied. The results thus indicated that studies on soil physical properties require prolonged experimentation for the effects among the treatments to be isolated. Application of organic residues tended to improve SOM and related soil chemical properties thus justifying the need for continued application of organics to improve soil status.