SEQUESTERING C IN STABLE SOIL ORGANIC MATTER FRACTIONS: HOW IMPORTANT IS FETILIZER-N IN SEQUESTERING C?

Principal Investigator:
Chris van Kessel
University of California - Davis
Department of Agronomy and Range Science
Davis, CA 95616

Collaborator:
J.W. van Groenigen, Dept. of Agronomy and Range Science, University of California, Davis.

Project Summary

Nitrogen plays a crucial role in the dynamics and sequestration of C in agro-ecosystems. Most C cycling models assume that the rate of residue decomposition is largely controlled by the C:N ratio and therefore the C and N decomposition are considered to be closely linked. However, recent findings from fertilized agro-ecosystems suggest that in those systems C and N sequestration in soil organic matter (SOM) occurs at different rates. Moreover, preliminary data suggest that in N-fertilized systems, residue-N contributed only marginally to the sequestration of N in the SOM pools.

Hence, we hypothesize that in N-fertilized agro-ecosystems the majority of N in stable SOM pools is from inorganic origin, i.e., fertilizer N, rather than residue-N. Although the C:N ratio in residue is an important factor controlling the rate of decomposition, residue-N plays only a limited role as a source of N in the sequestration process of C in stable SOM fractions. Our specific objectives are (i) to determine the relative contribution of residue-N and fertilizer-N in SOM pools, and (ii) to quantify differences in C and N sequestration pathways between a conventionally managed and an organic system.

The SAFS long-term experiment at UC-Davis will be used to test the above hypothesis. The relative contribution of residue-N and fertilizer-N in C sequestration will be quantified using labeled and unlabelled ¹⁵N fertilizer and single or double ¹³C¹⁵N-labeled residue. In addition, effectiveness of conventional vs. organic cropping systems in sequestering C and N will be tested using treatments receiving exclusively labeled fertilizer-N or labeled residue-N. Sequestration of C and N will be followed through the microbial biomass and the various SOM pools that are most sensitive to change (iPOM) will be separated by physical fractionations procedures. The proposed research is essential in devising management practices that will enhance C and N sequestration and the build-up of SOM in fertilized agro-ecosystems in California and beyond.