Soil Organic Matter (SOM) and nutrient cycling

Abstract

Recognized for long as the kingpin of agricultural soil management, soil organic matter (SOM) has attained added importance lately because of its significant role as a carbon sink. This chapter will focus on the issues related to SOM in soil productivity enhancement, which is of special relevance to the nutrient-depleted tropical soils; issues related to carbon storage for climate change mitigation and its environmental impacts that are of global significance will be discussed in separate chapters. One of the acclaimed roles of agroforestry is the potential benefit of soil improvement through the addition of nutrient-rich tree-and-shrub foliage (litter) to crop fields, which upon decomposition releases nutrients to the associated crops at critical stages of their growth. Numerous studies have been conducted since the 1980s to assess litter quality and nutrient-supplying ability of the foliage of various shrubs and trees used in tropical agroforestry systems (AFS), and the results have led to management recommendations under varying ecological conditions. Nutrient cycling, a related ecological process of major significance in AFS, refers to the continuous, dynamic transfer of nutrients in the soil-plant system. The principles of nutrient cycling and the role of woody perennials in enhancing it under AFS are briefly reviewed. Other issues included in the chapter are uptake of nutrients from the deeper soil horizons (the so-called deep capture of nutrients), and the role of tree biomass including roots in enhancing nutrient use efficiency. Also included in the chapter are some estimates and their underlying premises of the amounts of biomass (both aboveground and belowground) that will need to be added to the soil to maintain soil organic matter under AFS in the three major climatic zones of the tropics.