Nitrogen dynamics under elevated carbon dioxide

Within less than 50 years, atmospheric carbon dioxide concentration [CO2] will likely be double that observed in 1950. In this higher [CO2] world the sustainability of global crop production may be in jeopardy unless nitrogen (N) management is changed. The effects of elevated atmospheric [CO2] on soil N dynamics and greenhouse gas emissions in semi-arid cropping systems are poorly understood. A meta-analysis of current literature was conducted to quantitatively estimate the effects of elevated [CO2] on soil N dynamics in grain crop and legume pasture systems. The research reported in this thesis investigated the effects of elevated [CO2] on crop N demand, fertilizer N recovery, symbiotic N2 fixation, residual N (fertilizer and legume) availability to a subsequent crop, and greenhouse gas emissions from cropping systems in southern Australia (Horsham) and northern China (Changping) using free-air CO2 enrichment (FACE) facilities and glasshouse chambers.

15N-labelled (10.22 atom%) granular urea was applied (at a rate according to local practice) to field (FACE) microplots and glasshouse pots to determine the recovery of fertilizer N in the plant-soil system. Symbiotic N2 fixation in legumes was assessed using the 15N natural abundance method. A foliar labelling technique with 15N-labelled urea solution (98.26 atom%) was used to quantify below-ground legume N. The total C, total N and δ15N of plant and soil samples were analyzed by isotope ratio mass spectrometry. The fluxes of nitrous oxide (N2O), methane (CH4) and CO2 were measured by closed static chambers at various key growth stages of a wheat crop. The gas samples were analyzed by gas chromatography.

Elevated [CO2] generally increased crop biomass (11–84%) and grain yield (10–70%) across a range of crops, except when the wheat crop was grown under a hot and dry period, or when legumes experienced phosphorus (P) deficiency. Results of the meta-analysis indicated that grain N removal worldwide is stimulated by an average of 17% in crops grown under elevated [CO2].

The recovery of fertilizer N by the wheat crop or in the soil was not affected by elevated [CO2] in FACE experiments in Australia and China. The [CO2]-induced increase in plant N uptake (18–44%) was satisfied mostly by increased uptake of indigenous N (19–50%) at both sites. Irrespective of [CO2], fertilizer N recovery by wheat grown under FACE was stimulated (13–609%) by supplementary irrigation (higher rainfall scenario) in Horsham, but reduced (47%) by high N application in Changping. A glasshouse experiment showed that in the absence of residue, elevated [CO2] increased fertilizer N recovery in the plant by 7%, but when residue was incorporated elevated [CO2] decreased the recovery in the plant by 8%.

Under FACE conditions in Changping, elevated [CO2] increased both the proportion (from 59 to 79%) and the amount (from 166 to 275 kg N ha–1) of shoot N derived from the atmosphere (Ndfa) by soybean cultivar Zhonghuang 13, but had no significant effect on either parameter for the other cultivar Zhonghuang 35. Under controlled environments (glasshouse chambers in Horsham), the proportions of Ndfa for chickpea, field pea and barrel medic were not affected by elevated [CO2] regardless of P availability. However, elevated [CO2] increased the amount of N fixed by chickpea (20–86%), field pea (44–51%) and barrel medic (114–250%) when P fertilizer was applied, but the effect was not apparent when these crops were phosphate limited.

The contribution of N derived from field pea to a subsequent wheat crop wasreduced from 20% under ambient [CO2] to 11% under elevated [CO2], but was higher than that from fertilizer (4–5%) regardless of [CO2]. The grain yield and total N uptake by the wheat grown after field pea were 24% and 11%, respectively, higher than that when the wheat followed N-fertilized barley irrespective of [CO2].
Elevated [CO2] increased the emission of N2O (108%), CO2 (29%) and CH4 (from –0.14 to 3.45 μg C m–2 h–1) from soil in Horsham, with changes being greater during the wheat vegetative stage than later in the growing season. Supplementary irrigation (higher rainfall scenario) reduced N2O emission by 36% when averaged across [CO2] treatments. Supplementary irrigation increased CO2 flux by 26% at ambient [CO2], but not at elevated [CO2], and had no impact on CH4 flux. At the Changping site, elevated [CO2] increased N2O (60%) and CO2 (15%) emission, but had no significant effect on CH4 flux. Elevated [CO2] increased overall N2O emission by 27% and 75% under low N and high N inputs, respectively.

Results of the present research suggest that under future elevated CO2 atmospheres (i) there will be an increase in crop demand for N; (ii) higher fertilizer N application rates and greater use of legume intercropping using locally appropriate agricultural management practices will be required so that crops are able meet the additional N demand due to higher [CO2]; and (iii) increases in the terrestrial C sink may be less than expected as [CO2]-induced increase in greenhouse gas emissions (CO2-equivalent) will be significant as atmospheric [CO2] rises.

The following refereed journal and conference articles were published from the PhD of Dr Lam.
Journal Articles
Lam, S.K., Chen, D., Norton, R. & Armstrong, R. (2012). The effect of elevated atmospheric carbon dioxide concentration on the contribution of residual legume and fertilizer nitrogen to a subsequent wheat crop. Plant & Soil (published online).
Lam, S.K., Chen, D., Norton, R. & Armstrong, R. Crop residue incorporation negates the positive effect of elevated atmospheric CO₂ concentration on wheat productivity and fertilizer nitrogen recovery under controlled environments. Plant & Soil (published online).
Lam SK, Chen D, Norton R, Armstrong R, and Mosier AR. 2013. Influence of elevated [CO2] and supplementary irrigation on greenhouse gas emissions from a spring wheat crop in southern Australia. Journal of Agricultural Science, Cambridge. (published online).
Lam SK, Chen D, Norton R, Armstrong R, and Mosier AR. 2012. Nitrogen dynamics in grain crop and legume pasture systems under elevated CO₂: A meta-analysis. Global Change Biology 18, 2853-2859.
Lam SK, Hao X, Lin E, Han X, Norton R, Mosier AR, and Seneweera S. 2012. Effect of elevated carbon dioxide on growth and nitrogen fixation of two soybean cultivars in northern China. Biology & Fertility of Soils,48, 603-606
Lam SK, Han X, Erda L, Norton R, and Chen D. 2012. Does elevated atmospheric carbon dioxide concentration increase wheat nitrogen demand and recovery of nitrogen applied at stem elongation? Agriculture, Ecosystems and Environment, 155, 142-146.
Lam, S.K., Chen, D., Norton, R. and Armstrong, R. 2012. Does phosphorus stimulate the effect of elevated [CO₂] on growth and symbiotic nitrogen fixation of grain and pasture legumes? Crop & Pasture Science, 63, 53-62.
Lam, S.K., Chen, D., Norton, R. and Armstrong, R. 2012. Nitrogen demand and the recovery of ¹⁵N-labelled fertilizer in wheat grown under elevated carbon dioxide in southern Australia. Nutrient Cycling in Agroecosystems, 92, 133-144.
Lam SK, Lin E, Norton R, and Chen D. 2011. Soil gas fluxes of N₂O, CO₂ and CH₄ under mini-FACE wheat field in semi-arid environment in northern China. Soil Biology and Biochemistry 43, 458-461.

Refereed Conference Articles
Lam SK, D Chen, R Norton, R Armstrong, A Mosier, 2012. More nitrogen may be required for future grain production systems: A meta-analysis on the effect of elevated [CO₂] on nitrogen dynamics. In: Proceedings of the 5th Joint Australian and New Zealand Soil Science Conference: Soil solutions for diverse landscapes. Hobart. (Eds LL Burkitt and LA Sparrow). p 442-445. (Australian Society of Soil Science Inc.)
Lam SK, D Chen, R Norton, R Armstrong, A Mosier, 2012. The availability of residual legume and fertilizer nitrogen to a subsequent wheat crop under elevated CO₂ concentration. In: Proceedings of the 5th Joint Australian and New Zealand Soil Science Conference: Soil solutions for diverse landscapes. Hobart. (Eds LL Burkitt and LA Sparrow). p 471-474. (Australian Society of Soil Science Inc.)
Lam SK, R Norton, R Armstrong, D Chen 2010. Effect of elevated carbon dioxide on 15N- fertilizer recovery under wheat in Australia. Proceedings of International Soil Science Congress, Brisbane, 2010. http://www.iuss.org/19th%20WCSS/Symposium/pdf/1399.pdf
Lam SK, R Norton, E Lin, R Armstrong, D Chen 2010. Soil gas fluxes of N₂O, CO₂ and CH₄ under elevated carbon dioxide under wheat in northern China. Proceedings of International Soil Science Congress, Brisbane, 2010. http://www.iuss.org/19th%20WCSS/Symposium/pdf/2142.pdf
Norton, R.M., Lam, S.K., Chen, D., Fitzgerald, G. & Armstrong, R 2010. N dynamics under elevated carbon dioxide in the Australia FACE experiment. In: Farming's Future: Minimising footprints and maximising margins (eds L.D. Currie and C.L. Christiansen). Occasional Report No 23. Fertilizer and Lime Research Centre, Massey University, Palmerston North, New Zealand, 344-351.
Lam SK, R Norton, D Chen 2010. Effect of elevated carbon dioxide on straw decomposition and soil respiration under wheat in Australia. Proceedings of International Soil Science Congress, Brisbane, 2010.