Soil organic matter and nutrient dynamics in a Kenyan nitisol under long-term fertilizer and organic input management

Three soil fertility management practices relevant to smallhold farming sys tems in the East African Highlands were tested in an 18 yr-old experiment o n a humic nitisol (Kikuyu Red Clay) under a maize-bean rotation. These prac tices were the addition of mineral fertilizers (120 kg N and 52 kg P ha(-1) yr(-1)), application of cattle manure (10 t ha(-1) yr(-1)) and retention o f maize stover. Eight treatments arranged as a 2x2 x2 factorial were examin ed for their effects on crop yield, soil organic matter (SOM) fractions and soil chemical properties. Total crop yields of maize and beans ranged betw een 1.4 t ha(-1) yr(-1) when maize stover was retained without external inp uts to 6.0 t ha(-1) yr(-1) when stover was retained and fertilizers and man ure applied. Soil organic C contents to a depth of 15 cm ranged between 23. 6 t ha(-1) (14.3 mg g(-1)) with combined addition of mineral fertilizers an d stover removal to 28.7 t ha(-1) (17.4 mg g(-1)) with chemical fertilizati on, manuring and stover retention. Differences in particulate organic matte r and microbial biomass among treatments were proportionately larger than c hanges in total soil organic carbon. All land managements resulted in an ov erall decline in SOM over time and the greatest average rate of loss, 557 k g C ha(-1) yr(-1), was observed with fertilization and crop residue removal . Addition of manure and retention of maize stover reduced this loss by 49% . Carbon balances suggest that particulate organic matter is more efficient ly restocked by manure than maize stover. Particulate organic matter is a k ey fraction for understanding soil fertility changes in humic nitisols of t he Kenyan Highlands and has potential for use as an indicator of soil quali ty. (C) 1999 Elsevier Science Ltd. All rights reserved.