Relationships between SOM and nitrogen availability

Mr Thomas Carter1, Dr Diane Allen2, Dr Ben Macdonald3, Dr Mark Farrell1

1CSIRO, Agriculture And Food, Urrbrae, Australia, 2DES, Brisbane, Australia, 3CSIRO, Agriculture And Food, Canberra, Australia

Soil organic matter (SOM) is intrinsically linked to nitrogen (N) availability, not only harbouring the majority of soil N as solid organic matter, but also mediating its release and processing by the soil microbial community. Given the recent advances in our understanding of both SOM and N processing over the past 20 years, gaps still exist in our understanding of the role of SOM in N release. Our objective was to build a better understanding of the diversity of N pools and processes in relation to SOM across a broad range of Australian soils of different types and land uses, and to relate this to plant N uptake. A total of 358 topsoil (0-10 cm) samples were collected from 89 sites in Australia, consisting of 13 different land uses. These were measured for pools of N (bulk DON, free amino acid N (FAA-N), nitrate and ammonium) and rates of mineral N and plant available DON production quantified. A subset of 100 soils were selected for a pot experiment to observe plant N uptake. Investigating the different variables measured from the pot experiment showed that the strongest relationship to plant N concentrations was microbial biomass N (r=0.634, P<0.001, n=97). Other factors that played an important role were found to be pH, C:N ratio, FAA-N and DON. When examining relationships between soil C content and the various N pools and rates, surprisingly little direct correspondence was observed (r=0.1771) between total soil C and proteolysis, the main rate limiting step of N production. The relationship between total C and plant N uptake was not much stronger (r=0.2560). These findings indicate that across this diverse range of soils, SOM content has only a small direct influence on plant N availability, and it is likely that other multi-step processes are involved.


Tom Carter is a Research Support Officer at CSIRO with over 10 years experience in a number of methods to quantify soil organic matter and nutrient pools and fluxes. Most recently, he has focussed his expertise in the use of radio-labelled tracers to understand fluxes of low molecular weight compounds such as amino acids and sugars.

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