The molecular microbial perspective of organic matter turnover and nutrient cycling in tropical agroecosystems - What do we know?

A primary goal of low-input small-holder farming systems in the tropics is the appropriate management of organic matter (OM) turnover and nutrient cycling via adapted agricultural practices. These emphasize the promotion of soil organic matter (SOM) turnover and carbon (C) sequestration, nutrient use efficiency and soil microbial activity. Soil microbial communities are acknowledged as key players in the terrestrial C and nutrient (e.g., nitrogen (N) and phosphorus (P)) cycles. They respond sensitively to agricultural management with shifts in their community structure as well as functional properties (i.e., decomposition and mineralization). This may be in particular evident for tropical, agriculturally managed soils which show an accelerated microbial decomposition activity induced by favorable climatic and unique physicochemical soil conditions. Molecular techniques advanced the understanding about the composition of soil microbial communities and partially their functions standing in close interaction with SOM dynamics. So far, such methods have rarely been used for elucidating microbial community dynamics including composition and functioning in tropical soils under agricultural use. The primary objective of this article is thus to summarize the existing literature on tropical soil microbial ecology as drivers of OM turnover and crop nutrient supply in soils under agricultural use. This included the highlighting of the latest efforts in deploying particularly nucleic acid-based, cultivation-independent techniques to study the compositional status of soil microbial decomposer communities and, to a smaller extent, their functional attributes in response to land use change and OM management in tropical agroecosystems. The majority of available studies on tropical microbial ecology so far concentrated primarily on the description of compositional microbial community dynamics. It was, however, hardly questioned if detected structural microbial community changes substantially influenced microbial key processes which actually maintain ecosystem functioning and soil productivity. This merit remains substantially unexplored in tropical soils under agricultural use as altered microbial community compositions may be only transient with time with potentially negligible consequences on relevant microbial functioning. There are, however, a few specialized key functional microbial groups whose presence or absence may actually affect the performance, speed and recovery of important ecosystem processes including the transformation of OM and supply of crop nutrients (e.g., N and P). These may finally regulate and determine the productivity of tropical, low-input small-holder farming systems which rely essentially on indigenous soil fertility. Consequently, research recommendations are discussed with emphasis on unique characteristics of tropical environments and tropical agroecosystems to improve the current understanding about the link between microbial key players and productivity of tropical, agriculturally managed soils.