Nitrogen dynamics under elevated carbon dioxide

How will high carbon dioxide concentrations affect the cycling of nitrogen in crop production systems.


29 Jan 2010

2009 Annual Interpretive Summary

The Australian Grains Free Air Carbon Dioxide Enrichment (AGFACE) facility was established to compare wheat growth, yield, and development under ambient atmospheric carbon dioxide (a[CO2] ~380 ppm) and elevated atmospheric carbon dioxide (e[CO2] ~550 ppm). Experiments on fertilizer N recovery, straw decomposition, and greenhouse gas production have been undertaken to estimate how e[CO2] and a changing climate could affect crop production systems.

Elevated CO2increased crop biomass at the end of tillering, anthesis, and maturity. Although plant and grain N contents declined, crop N uptake was 24% higher with e[CO2]. Stubble C:N ratio was not affected by e[CO2].

Wheat was grown with 15N enriched urea in PVC microplots at 50 kg N/ha in the AGFACE facility. Harvest biomass increased by 23% and N uptake increased by 17% under e[CO2]. Like the main experiment, C:N ratio of the stubble was not affected by e[CO2] and it had no significant effect on the proportion of N derived from fertilizer (%Ndff) for grain, stem, and root. There were no significant effects of e[CO2] on 15N recoveries in soil and total fertilizer N losses. The effects of e[CO2] and irrigation on straw decomposition and soil respiration was also undertaken within the AGFACE experiment. Pure cotton cloth, wheat straw, and pea straw were decomposed using litter-bag method for 140 days. The mass remaining was the highest for cotton cloth (90%), then wheat (73%), and pea (50%). Total C content of wheat and pea straw and total N content of pea straw were reduced only under e[CO2] and irrigated conditions. Soil CO2 emissions were increased by e[CO2] only under irrigation.

In these experiments, the C:N ratio and degradation of organic residues in the wheat crop is not affected by CO2levels, although large amounts of residue would enter soil nutrient cycles. These data indicate that e[CO2] increases plant N demand, but does not increase the efficiency with which fertilizers are used nor the likely supply of N from residues. Further research is planned to investigate mineralization and N fixation under e[CO2] and these data will be used to develop N strategies for future cropping systems. ANZ-04