Carbon, nitrogen and phosphorus sequestration within soil aggregates of maize and tree-based agroforestry systems

Direct contributions by tree crops to the nutrient capital and nutrient cycling in Sub Saharan Africa have been demonstrated for several soil types. Soil aggregate sequestration of carbon (C), nitrogen (N) and phosphorus (P) was investigated on a P deficient Oxisol in Western Kenya, during four crop cycles of maize and tree rotations (1997--1999) in an agroforestry management system. Concentric layers of soil aggregrates (4--8 mm across) were removed using stainless steel meso-SAE chambers, developed for separating concentric layers from interior regions by gentle erosion of aggregate surfaces. Plant available phosphorus supplied by concentric layers was evaluated by resin exchange. Maize removed plant available P from the exterior surface layers of soil aggregates when applied fertilizer P was insufficient to meet plant demand. Poor plant growth in the continuous maize cropping system resulted in 48% more accumulations of plant available P to surface layers of soil aggregates. The contemporary maize crop contributed as much as 50% of the current carbon onto surface layers in July 1998 (field study) and August 1999 (pot experiment). However, this influx was not apparent when whole aggregate measurements were taken. During the same time period, C contents in surface layers of soil aggregates, 4--8 mm across, declined nearly 23% when fertilizer P was applied to planted tree crops. These results suggest that uniform separation and analyses of soil aggregate layers should be integrated into current research methods to further enhance our understanding of the rapid and short term changes in C and P associated with management modifications in low input crop production systems of sustainable tropical agriculture.