Abstract/Summary

Application of soil amendments, such as composts, to soil to improve fertility and crop yield is commonplace in horticultural systems. A potential additional beneficial outcome of application of these materials is the augmentation of the soil carbon pool, a vital component of the global C cycle. This research utilises a pre-established long term field experiment in Wisley (UK). Since 2008, amendments that are commonly used by and/or available for use in the wider horticultural community (peat, garden compost, composted bark, bracken and horse manure, and spent mushroom compost) were applied annually (and plants grown) to 3 m x 3 m plots. All of the amendments were successful in elevating soil quality above the bare control plots. Individual soil physico-chemical properties sue as pH, bulk density and nutrient contents were influenced by the application of amendments. Therefore selection of amendment should be driven by individual needs and they can do so with detriment to the overall soil quality. A combination of physical fractionation techniques and nuclear magnetic resonance analysis indicated the difference in total C concentration between treatments results from an increase in unprotected particulate organic matter, rather than an increase in organic matter being occluded in aggregates or in organo-mineral complexes, and persist as a result of accumulation of alkyl C components. A method known as the Tea Bag Index was used to determine decomposition rate constants in each of the plots. The differences seen in decomposition rate in the plots is largely determined by the carbon:nitrogen ratio of the amendment, and the presence of plants on the plots that compete with microorganisms for nitrogen. In fact, nitrogen, and competition for it, was found to be the limiting factor for plant yield, decomposition of amendments and decomposition of the tea bags.