Are soil carbon stocks controlled by a soil’s capacity to protect carbon from decomposition?

Dr Miko Kirschbaum1, Dr Donna Glitrap1, Dr Sam McNally2, Dr Liyin Liang1, Dr Carolyn Hedley1, Dr Gabriel Moinet3, Dr Michael Blaschek1, Dr Michael Beare2, Dr Benny Theng1

1Landcare Research – Manaaki Whenua, Palmerston North, New Zealand, 2Plant and Food Research, Lincoln, New Zealand, 3Landcare Research – Manaaki Whenua, Lincoln, New Zealand

Greenhouse gas emissions can be off-set by increasing soil organic carbon (SOC), but the factors controlling changes in SOC storage must be understood to identify suitable management practices. Soils differ in their ability to stabilise SOC, but do soils have a maximum capacity to stabilise carbon, or does stabilisation simply act to reduce turn-over rates without limits? Here, we use observations from two specific NZ sites, and from national soils data to gain insights into the controls of SOC stabilisation. They showed:

  1. When other factors such as climate, soil fertility, and pasture management were the same, SOC was linearly correlated with soil specific mineral surface area (Sm).
  2. At each soil depth, SOC and Sm were linearly related, with the slopes and intercepts of the relationships decreasing with depth.
  3. Small intercept values at zero Sm implied that SOC was mostly protected by the soil matrix rather than biochemically, that mineral surface area was the functionally relevant measure of stabilisation capacity, and that its effectiveness was independent of carbon input rates.

We analysed New Zealand’s national soils data based for evidence of a maximum stabilisation capacity. We reasoned that if SOC was limited by maximum stabilisation capacity it should result in a skewed distribution of SOC around mean values. Some points could be much lower than the maximum stabilisation capacity, but points could not exceed that maximum capacity. SOC in the national data set, however, was normally distributed, thus being inconsistent with maximum stabilisation capacity as a SOC limitation.

Instead, our analysis suggested that protected SOC, Cp, could be described as:

Cp, = Cin Sm / f(T, W, …), where Cin is the carbon input rate, and f(T, W, …) is a SOC turn-over rate depending on temperature, soil moisture or any other factors able to affect decomposition rates.


Miko Kirschbaum is an ecophysiologist and modeller. He works for Landcare Research and is based in Palmerston North, New Zealand. He has had a long-standing interest in the effects of climate change on ecosystems and has extensively studied the effects of increasing temperature and CO2 concentration on biological productivity and soil carbon storage. His recent work has aimed to identify ways to modify farm management in order to mitigate climate change by increasing soil carbon storage.

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