SOIL ORGANIC-MATTER POOLS IN A VOLCANIC-ASH SOIL UNDER FALLOW OR CULTIVATION WITH APPLIED CHICKEN MANURE

Frequent applications of chicken manure to a volcanic-ash soil resulte d in an increased crop yield several years after the applications ceas ed. Improved nutrient cycling through the soil organic matter (SOM) wa s thought be the cause of this. An area that for several years had bee n under either unimproved fallow or continuous bean-maize intercroppin g (with chicken manure application) was planted with maize intercroppe d with the legume Arachis pintoi. Soil samples were fractionated into several sizes of aggregates and incubated for 6 weeks in intact form a nd after crushing the aggregates. Between 1.2 and 3.1% of the C in any aggregate size class existed in easily decomposable (i.e. not physica lly or chemically protected) form. Less than 0.8% of the macroaggregat e C was physically protected by the aggregate structure. More than 97% of the C in any aggregate size class can be considered resistant. The large- or small-macroaggregate-protected SOM (i.e. in aggregates > 20 00 mu m and 250-2000 mu m, respectively), once made available, was mor e easily decomposed than the unprotected SOM. The large SOM content (4 5.8-57.3 g C kg(-1)) but yet limited mineralizability indicates that m echanisms other than soil macroaggregation are important for protectin g SOM in this soil. Binding of organic molecules onto allophane minera ls is likely to be such a mechanism. The larger yield and nutrient upt ake by the maize in the former bean-maize plots compared with the form er fallow plots could not be explained by differences in SOM decomposi tion. We think that the frequent applications of chicken manure to the former bean-maize plots increased the available phosphorus in these s trongly P-sorbing soils by increasing the cycling of organic-P or by b locking some of the P-reactive sites. This, however, needs to be furth er investigated.