Carbon and nitrogen relationships in swards of Danthonia richardsonii in response to carbon dioxide enrichment and nitrogen supply

Abstract

Atmospheric C0₂ concentrations are increasing steadily, although not fast enough to account for all anthropogenic emissions. Uptake of extra carbon by the terrestrial biosphere in response to the CO₂ concentration increase may, in turn, be moderating the rate of increase. However, there are uncertainties pertaining to the ability of natural ecosystems to respond to this increase in terms of productivity and carbon sequestration, when productivity is restricted by nitrogen availability. This study provides experimental data on the response of microcosms of the wild, C₃ Australian grass Danthonia richardsonii Cashmore to C0₂ enrichment over 4 years of growth in the Canberra Phytotron, when productivity was restricted by low nitrogen availability. Complementary experiments with isolated plants elucidated the effects of C0₂ enrichment on carbon and nitrogen acquisition and allocation. The general growth response of D. richardsonii to C0₂ enrichment was compared with other C₃ grasses in further isolated plant experiments. Microcosms accumulated extra carbon under C0₂ enrichment. Microcosms were supplied continuously with 3 productivity limiting rates of N supply, 2.2, 6.7 or 19.8 g N m⁻² yr⁻¹, at atmospheric C0₂ concentrations of 359 or 718 μLL⁻¹. Controlled quantities of water were supplied and periodic drought imposed. The effect of C0₂ on total plant-soil system carbon was highly significant at all N supply rates, and did not diminish over time. Increased microcosm carbon was attained without a persistent increase in leaf area index, and above ground live carbon was not increased by C0₂ enrichment after the first year. Leaf turnover was higher at high C0₂, as was the amount of total senesced leaf. Root carbon was lower at high C0₂ at lowand mid-N, but higher at high-N. Rates of water-use were lower at high C0₂, resulting in a higher soil water content. At the higher N levels both soil microbial and non-microbial carbon were increased at high C0₂, while total soil carbon and non-microbial carbon were increased at low-N. Low- and mid-N microcosms gained significant amounts of nitrogen from the environment, attributed to a combination of nitrogen deposition and free-living nitrogen fixation. Nitrogen loss from the high-N microcosms was lower at high C0₂. Green leaf nitrogen concentration and total standing leaf nitrogen were reduced by C0₂ enrichment. Increased C:N ratios of senesced leaf at high C0₂ resulted in decreased decomposition (cumulated microbial respiration) in vitro. This was not reflected at the microcosm level, possibly owing to increased soil water content and microbial biomass. High C0₂ increased carbon accumulation by isolated plants when grown at several N levels (0.05, 0.2, 0.5 or 6 mg N plan⁻¹ day⁻¹ ) over 37 days. Net assimilation rate and leaf nitrogen productivity were increased at high C0₂• Whole plant (g N g⁻¹ C) and leaf nitrogen concentrations (g N g⁻¹ C or g N m⁻² ) were reduced by C0₂ enrichment when nitrogen supply restricted growth. Allometric relationships showed that carbon allocation and root nitrogen concentration (g N g⁻¹ C) were not affected by C0₂ enrichment. Nitrogen allocation to root, as a proportion of total plant nitrogen was increased at high C0₂, and leaf to root surface area ratio reduced, indicating a shift in investment from processes involved with carbon acquisition to those involved with nitrogen acquisition. The differences were small at the higher rates of N supply. As an isolated plant, D. richardsonii exhibited a similar response in dry matter or carbon accumulation to C0₂ enrichment ( -360 μL L⁻¹ and -720 μLL⁻¹ ) as other grasses when nitrogen - supply was abundant (Hoagland solution) or growth limiting (0.4 or 1.6 mg N planf1 day"1 ) over 71 days. Total transpiration (g H₂0 planf1 ) was reduced, and transpiration efficiency (g C g-1 H20) increased at high C0₂ in plants experiencing nitrogen limitation, but was not determined in the plants supplied with abundant N. D. richardsonii showed real increases in nitrogen use efficiency at high C0₂. This was expressed at the microcosm level as increases in total plant-soil system carbon gain a high C0₂ at all rates of nitrogen supply. The magnitude of this response is large enough to account for a significant proportion of global anthropogenic carbon emissions, if applicable to all ecosystems in the field.