Root length and nitrate under Sesbania sesban: Vertical and horizontal distribution and variability

Measurements of root and nutrient distributions in agroforestry systems hav e often been obtained from unreplicated plots. We, therefore, measured the vertical and horizontal distribution of root length and soil nitrate in rep licated plots of sesbania [Sesbania sesban (L.) Men.] to assess plot-to-plo t variation and identify appropriate sampling schemes. Sampling was on prof ile walls exposed from pits at two on-farm sites (Ochinga and Muange) in Ke nya. At Ochinga, soil was sampled at 27 0.15-m depth intervals and eight 0. 15-m-wide locations perpendicular to 2.25-m-wide rows of 19-month-old sesba nia. At Muange, sampling was 18 months after establishment of sesbania from seedlings with a 1 m by 1 m spacing. Soil was sampled at 17 0.15-m depth i ntervals and nine 0.15-m-wide sampling locations - one exactly below the tr ee and four each on opposite sides of the tree. Root length density general ly decreased with distance from sesbania and with soil depth. Soil nitrate, however, followed different trends in the four replications at Ochinga and the three replications at Muange. At Ochinga, nitrate above 2.5-m depth cl early increased with distance from the tree in two replications but followe d no distinct trend in another replication. At Muange, nitrate above 1.2-m depth decreased with distance from the tree in one replication, increased w ith distance from the tree in another replication and followed no clear tre nd in another replication. The suitability of various sampling schemes for measuring root length and nitrate was accessed from percent bias between me asured values at sampling locations in the scheme and actual values for an entire transect of sampling locations. Bias associated with selection of sa mpling locations tended to be higher within the 2.25-m-wide sesbania rows t han the 1 m by 1 m sesbania spacing. Soil sampling in systems with rows of young trees should be at several locations along a transect perpendicular t o tree rows in order to minimize bias.