Alley cropping in the moist savanna of West-Africa - III. Soil organic matter fractionation and soil productivity

In cropping systems with limited amounts of external inputs, the soil organ ic matter pool (SOM) may contribute significantly to plant nutrition. The i mpact of organic inputs on total SOM and particulate organic matter (POM) N contents as affected by soil type and the relationships between sources of N and maize N uptake were assessed for a set of alley cropping trials in t he West-African moist savanna. The trials were established in Niaouli (Beni n Republic), in Glidji, Amoutchou, and Sarakawa (Togo), and in Bouake and F erkessedougou (Cote d' Ivoire). The total soil N content, averaged over all treatments and years, varied between 324 and 1140 mg N kg(-1) soil. The PO M-N content varied between 50 and 160 mg N kg(-1) soil. The average proport ion of soil N belonging to the POM pool ranged between 9% and 29%. This was significantly related to the annual N inputs from maize stover and pruning s, when averaged over the different alley cropping treatments. The trial 'a ge' also appeared to be related to the impact of the different treatments o n the POM-N content. The Ferkessedougou soil contained a relatively higher proportion of total soil N in the POM pool because of its relatively high s ilt and clay content, compared to the other sites. The relative change in P OM-N content between 1996 and the initial sampling was about twice the rela tive change in total soil N content. This suggests that N incorporated in t he POM is relatively labile, compared to N incorporated in the other SOM fr actions. Maize N uptake was related to the amount of add pruning-N (partial r(2) of 27%), the rainfall during the growing season (partial r(2) of 17%) , the POM-N content (partial r(2) of 14%), and to a lesser degree to the PO M N concentration (partial r(2) of 5%), the fertilizer N addition rate (par tial r(2) of 3%), and the silt and clay content of the soil (partial r(2) o f 3%). The POM-N content was shown to be influenced by organic matter addit ions and soil characteristics and to contribute significantly to maize N su pply. This pool may be an important indicator for the soil fertility status of savanna soils.