Subsoil root activity in tree-based cropping systems

An increasing number of studies indicate that (i) nutrient and water resources can be abundant in the subsoil and (ii) trees have deep root systems that can possibly reach these resources. It is less clear whether subsoil resources are actually improving water and nutrient status of tree-based cropping systems and whether they are significantly increasing crop production and yield. To answer such a question, the distribution of nutrient and water uptake by trees needs to be quantified. So-called `root activity distributions' give valuable information about actual subsoil use by trees whereas studies on root length or mass distributions do not often correlate with uptake distributions. Despite the usually lower relative root activity in the subsoil compared to the topsoil per unit soil, the large volume of subsoil in comparison to mostly shallow topsoil is an important resource for crop nutrient and water uptake. The present study compares published root activity distributions using the model Activity=A _max(1–k ^depth). The obtained regression constants k of 0.91–0.99 determined in this publication reflect the values computed by an earlier published survey for root biomass ranging from tundra to those of temperate forest biomes. Thus, tree crops can have shallow root activity and 75% of their total root activity in the first 0.1 m of soil, or very deep root activity with more than 90% below 0.1 m. Neither environmental factors (i.e., climate and soil properties available from these publications) nor plant species explain differences of root activity distributions with depth. The deepest root activity is found for fruit trees such as citrus, guava and mango. Shaded crops such as coffee and cacao tend to have shallower root activity than fruit trees. Monocots including oil palm, coconut or banana have root activity that can be both deep and shallow. Regional and temporal variations of subsoil root activity for the same tree species are significant and generally larger than differences between species. Root activity patterns of tree crops appear to be sufficiently flexible to allow for subsoil resource use. Consequently, management such as pruning, fertilization, liming and irrigation are shown to significantly affect subsoil root activity.