Assessing the vulnerability of organic matter to C mineralisation in pasture and cropping soils of New Zealand
Sam McNally A D , Mike Beare A , Denis Curtin A , Craig Tregurtha A , Weiwen Qiu A , Francis Kelliher B and Jeff Baldock C
+ Author Affiliations
- Author Affiliations
A The New Zealand Institute for Plant & Food Research Limited, Private Bag 4704, Christchurch, 8140, New Zealand.
B AgResearch, Lincoln Research Centre, Private Bag 4749, Christchurch 8140, New Zealand.
C CSIRO Land and Water, PMB2, Glen Osmond, SA 5064, Australia.
D Corresponding author. Email: Sam.McNally@plantandfood.co.nz
Soil Research 56(5) 481-490 https://doi.org/10.1071/SR17148
Submitted: 2 June 2017 Accepted: 28 March 2018 Published: 2 July 2018
Abstract
In New Zealand, pastoral soils have substantial organic carbon (OC) stocks, which may be vulnerable to loss from disturbance and environmental perturbations. We assessed OC vulnerability using two approaches. For the first approach, we postulated that the OC deficit of continuously cropped soils relative to nearby pastoral soils would provide a measure of the quantity of potentially vulnerable OC in pastures. As a test, soils were sampled to a depth of 15 cm at 149 sites and the total organic carbon (TOC) and particulate organic carbon (POC) contents were measured. The second approach involved measurement of OC mineralisation in a laboratory assay (98 day aerobic incubation at 25°C). For the pastoral soils, the mean TOC and POC was about twice that of the cropped soils. On average, 89% more OC was mineralised from the pastoral soils compared with the cropped counterparts. However, the quantity of OC mineralised in pasture soils was small relative to the potential for OC loss inferred from the difference in TOC between pastoral and cropped soils. Carbon mineralisation was explained using a two-pool exponential model with rate constants of the ‘fast’ and ‘slow’ pools equating to 0.36 ± 0.155 and 0.007 ± 0.003 day–1 respectively. The larger, slow OC pool correlated strongly with hot water extractable OC whereas the fast pool was related to OC extracted using cold water. Our results suggest that water extraction (using cold and hot water) can provide a rapid estimate of the quantity of mineralisable OC across a wide range of New Zealand soils.
Additional keywords: C mineralisation, cropping, pasture SOC, vulnerability.
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