Prof Brian Chambers, Head of Soils & Nutrients, ADAS UK Ltd(free)
The extent to which the recycling of organic materials to agricultural land can benefit soil carbon and potentially mitigate climate change is reviewed in this paper. Increases in soil organic carbon (SOC) following the addition of a range of organic materials were estimated from a number of long-term experiments in the UK. Carbon retention was greatest from biosolids, green compost and paper crumble additions, and least from farm manures, with an estimated 630-1800 kg/ha C retained in the topsoil at typical application rates. It is debatable whether these increases in SOC can be counted as genuine additional carbon storage (against a present/recent day baseline), as with the probable exceptions of compost and paper crumble applications, almost all other organic materials considered were already being applied to land. However, the existence of such storage following biosolids and farmyard manure additions should be formally recognised in the baseline. Changes in greenhouse gas (GHG) emissions as a result of organic material additions were also considered, particularly nitrous oxide (N2O), with the recent UK GHG Emissions Inventory indicating that c.1%, 8% and 23% of total N2O emissions from agriculture result from biosolids, farm manure and manufactured fertiliser N additions to agricultural land, respectively. The predominant justification for returning organic materials to soil should be for maintaining and enhancing existing SOC levels and reducing/replacing manufactured fertiliser use, rather than carbon storage for climate change mitigation per se.
Keywords Soil carbon storage, organic materials, nitrous oxide, climate change
The loss of soil organic carbon (SOC) is seen as one of the most important threats facing UK soils (Defra, 2009; Dobbie et al., 2011). SOC is fundamental to the maintenance of soil fertility and function; it not only provides a food source and habitat for the soil biological community and drives the recycling of nutrients, but is also a central component of soil aggregation and the maintenance of structure and water relations. Indeed, a soil with a higher SOC content than the same soil at a lower SOC content has been shown to have more stable structure, be less prone to runoff and erosion, have greater water retention and increased biological activity (Bhogal et al., 2009; Johnston et al., 2009). Moreover, soils are also an important carbon (C) store and changes in soil C stocks can have implications for climate change (Powlson et al., 2011). Consequently, there has been much discussion on the possibility of mitigating climate change by increasing SOC stocks through land-use and management changes (e.g. Smith et al., 2008).
Around 90 million tonnes of farm manures (Williams et al., 2000), 3-4 million tonnes of biosolids (Water UK, 2010) and 1.9 million tonnes of compost (WRAP, 2012) are applied (on a fresh weight basis) annually to agricultural land in the UK. These materials provide a valuable source of both nutrients and organic matter, and could potentially increase SOC stocks. However, changes in SOC are generally slow to occur and difficult to measure against the large background C content in arable soils which are most likely to benefit from organic material additions in the UK. There are approximately 1 million ha of tillage land in the UK receiving organic material additions (out of a total tillage area of c. 5 million ha). Notably, even small increases in SOC storage per hectare could overall lead to important increases in C storage at a national level. This paper reviews the extent to which the recycling of organic materials to agricultural land can benefit soil carbon and potentially mitigate climate change.