Abstract:
Conversion of natural wetlands to cropped land can have significant impacts on the transformation of soil aggregates and associated nutrients, including soil phosphorus (P). However, little is known about the changes in P fractions in wetland systems, or the fate of aggregate fractions due to conversion of unused wetland areas to cropped systems.
We hypothesized that conversion of natural wetland systems will result to changes in the size of P pools and aggregate fractions associated with SOM and that these changes will be determined by the soil type and land use. Our objectives were to evaluate the effects of land use changes on: - (1) the proportions of soil P fractions; (2) the size and proportion of water stable aggregates in two different wetlands in East Africa. The wetlands comprise an inland valley in the highlands-Karatina (1700 masl) and a floodplain in the lowlands-Korogwe (320 masl). Land use treatments included the natural unused sites
which were dominated by Cyperus spp; the cropped fields under anaerobic and aerobic soil conditions and abandoned sites under fallow vegetation. Results indicated that conversion from natural systems to aerobic, anaerobic or abandoned fallow affected soil P fractions and the stability of soil aggregates. The floodplain soils responded sensitively to intensified land use with pronounced decreases in soil aggregate stability, the availability of labile P-fractions, and the supplying capacity of stable P-fractions. The inland valley showed higher contents of available P and stable P, while the stable P stocks in the
floodplain were apparently depleted quickly. This may imply that the conversion of wetland systems to cropped systems could have significant alterations of soil P fractions and the aggregate fractions associated with organic matter unless strategies to increase the organic matter level and hence improved aggregation and mineralizable nutrients are put into consideration.
