Influence of soil organic matter on forest productivity

Soil organic matter (SOM) is generally assumed to be important to forest productivity, but its direct influence has been difficult to clearly demonstrate. SOM has a myriad of interactions with other soil properties, and levels of SOM depend on plant factors such as productivity and litter chemistry, and on environmental factors such as temperature and water. SOM is thus both cause and effect with respect to productivity. Additionally, SOM is inversely related to productivity where conditions such as low temperatures or reduced aeration are adverse for both plant growth and for microbial activity, and SOM accumulates. Conventional experimental methods are unlikely to provide a wholly-inclusive general demonstration of the effects of SOM on forest productivity because the relationship is complex and site-specific. In spite of that caveat, circumstantial evidence indicates that SOM positively affects long-term forest productivity, with its specific role and contribution depending on the limiting site factors. In coarse-textured soils, SOM is important for retaining water and for supplying and retaining nutrients. As soils become finer, those roles become less important but its role in promoting favourable soil physical properties increases. Forest management practices can alter the amount and type of SOM, but because inherent soil or site characteristics sometimes compensate for or mitigate the effects of SOM change, the direct impacts on productivity may be equivocal. Nonetheless, because of the strong ties of SOM to a wide range of soil properties and functions across soil textures, most prudent forest management regimes should maintain or enhance SOM levels.