Does clay stabilise organic matter in New Zealand soils?

Carbon accumulation (16.8%) as aluminium-humus in a black layer (1 m thick) near Mt Aso, Japan.

Soil organic matter is stabilised in soils, and up to half of it can be 1000 years old or more. Scientists think that clay stabilises soil organic matter by various chemical (e.g. as inert molecules) and physical (e.g. in very small pores) processes.

Clays have reactive surfaces and their surface area can be measured from water adsorption measurements. We have looked for a relationship between soil organic matter (carbon %) and clay for soils held in the National Soils Database. There is almost no relationship for topsoils under pasture (Figure 1). However, when we plot carbon % against soil surface area there is a good relationship (R2=0.61) (Figure 2). Soils with the greatest surface area are the Allophanic soils, derived from volcanic materials, and these soils are often not considered in overseas literature.

As many factors affect soil organic matter stabilisation, the relationship in Figure 2 is not perfect. Landcare Research is studying these other factors, which in addition to those mentioned above include stabilisation by iron oxides and aluminium, and by different types of clay minerals. Landcare Research is developing a thermal decomposition index to characterise soil organic matter stability in soils of varying mineralogies.

Fresh soil organic matter (e.g. from fresh roots and litter) makes up the labile pool, and with time becomes partially decomposed and partially stabilised in soil. A portion of the soil organic matter tends to be stabilised for 10–20 years, and we call this the “decadal pool”. This pool typically accounts for over half the organic matter in soils. These pools have recently been measured (using 14C from nuclear bomb testing) in work in association with GNS. The residence time of organic matter in the decadal pool is 17 years for an Allophanic soil compared with 9 years for a non-allophanic soil.

Other work on soil organic matter stabilisation is currently being funded by the Sustainable Land Management and Climate Change (SLMACC) initiative of MPI (Ministry for Primary Industries) and from the New Zealand Agricultural Greenhouse Gas Research Centre, in conjunction with Plant & Food, AgResearch, and Massey University (biochar). Soil surface area has been used to help assess the maximum amount of soil organic matter stored in New Zealand soils in these projects.

Roger Parfitt & Guodong Yuan