Soil form, carbon storage and stabilisation
Soil C can change under different land uses. For example, soil C has been shown to decline with intensification of dairy farming on flat land, and with irrigation in Canterbury.
Soil is a physical, chemical and biological self-organising system where chemical and biological interactions on physical surfaces, such as clays, occur within soil pores. As a consequence, the physical structure and architecture of soil can not only be influenced by the chemical and biological activity in soil, but can also play a significant role in affecting chemical and biological activity.
Various models have been developed to predict soil C dynamics. Most, however, do not take account of the soil structure and architecture. We are learning how soil structure and architecture may influence soil C and N storage and stabilisation and if the research can be used to improve modelling of C dynamics.
X-ray computed tomography
With soil X-ray computed tomography (CT) we can quantify in 3D a soil’s architecture. Figure 1 shows an example of a 3D CT image under two orchard systems on the same soil with different soil pore networks. Quantifying the pore size distribution, topology and the pore surface density can help us understand the biogeochemical interactions that affect the gains and losses of soil C. We are currently analysing micro-CT data generated using a synchrotron to quantify microbial habitat within soil aggregates under low and high fertility pasture.
Aggregation
We have recently shown that land use affects soil aggregation and C dynamics within organic matter pools associated with soil aggregates.
With the intensification of some of our hill country sheep farms, application of N can increase production. In collaboration with AgResearch and the University of Waikato we found soil particulate organic matter (both outside and inside microaggregates), and microaggregate- associated silt and clay C, increased with increased N and stocking rate within the first 3 years.
Previous studies have shown that increased N inputs may cause a decline in soil C; however, we found no decline. There was no relationship between the natural abundance of the isotopes δ15N and δ13C in any soil fraction and the amount of N leached, suggesting organic matter decomposition was not a contributing factor to leached N. Rather, there was an increase in the particulate organic matter inside microaggregates (the physically protected particulate organic matter) with increasing N fertiliser rate. Not surprisingly, a slight change in the isotope abundance within the particulate organic matter outside microaggregates indicated this fraction was the most susceptible fraction to start decomposing at this early stage of increasing soil N nutrition.
By measuring the soil architecture and soil C fractions we are able to understand “how the soil works”, which may make it possible to develop models for improved management strategies.
John Scott
Markus Deurer
E: Markus.Deurer@plantandfood.co.nz
