The rate of progression and stability of progressive nitrogen limitation at elevated atmospheric CO2 in a grazed grassland over 11 years of Free Air CO2 enrichment

Abstract

A decline in the availability of nitrogen (N) for plant growth (progressive nitrogen limitation or PNL) is a feedback that could constrain terrestrial ecosystem responses to elevated atmospheric CO 2 . Several long-term CO 2 enrichment experiments have measured changes in plant and soil pools and fluxes consistent with PNL but evidence for PNL in grasslands is limited. In an 11 year Free Air CO 2 Enrichment (FACE) experiment on grazed grassland we found the amount of N harvested in aboveground plant biomass was greater at elevated CO 2 but declined over time to be indistinguishable from ambient after 5 years. Re-wetting after a major drought resulted in a large input of N from mineralisation and a return to a higher N harvested under elevated CO 2 followed by a further decline. Over these two periods the amount of N in soil significantly increased at elevated CO 2 . Data from mesocosms introduced into the rings at intervals, and therefore having different lengths of exposure to CO 2 , showed plant N availability declined at elevated CO 2 reaching a new equilibrium after 6 years of exposure. We conclude that the availability of N for plants in this grassland is dynamic but the underlying trend at elevated CO 2 is for PNL.

Effects of long-term exposure to enriched CO2 on the nutrient-supplying capacity of a grassland soil

“…This suggests that the N availability of the soil was not influenced by enriched CO 2 during the pot experiment. Indeed, initial soil mineral N concentrations were equivalent for both CO 2 treatments and did not reflect a decrease in N availability under enriched CO 2 that has been previously observed in the field experiment (Newton et al 2010). It is possible that the soil disturbance when initiating the present pot experiment may have altered the soil nutrient pools from field conditions.…”

“…Evidence of nitrogen (N) limitation has been found in the New Zealand free air CO 2 enrichment (NZ FACE) experiment. Soil N availability, predominantly nitrate-N, measured with ion exchange membranes, has decreased under enriched CO 2 (Newton et al 2006(Newton et al , 2010. Over the 11 years of CO 2 enrichment, there has been a steady increase in soil total N and a decline in N in plant pools although there was a re-stimulation of soil N mineralization and plant N uptake following a severe drought (Newton et al 2010).…”

The Role of Plants in the Effects of Global Change on Nutrient Availability and Stoichiometry in the Plant-Soil System

“…Increased concentrations of atmospheric CO 2 also lead to reduced transpiration (Niklaus et al, 2001;Del Pozo et al, 2007), because the uptake of CO 2 is possible with higher stomatal closure (Samarakoon and Gifford, 1995), which can further limit the capacity to take up N (Berntson, 1994;Pritchard and Rogers, 2000;BassiriRad et al, 2001) and can lead to a progressive limitation of nutrients that can quickly limit the initial increase in plant production under elevated concentrations of atmospheric CO 2 (Murray et al, 2000;Zak et al, 2003;Luo et al, 2004;Newton et al, 2010;Norby et al, 2010).…”

Detection of historical changes in pasture growth and attribution to climate change

“…6). The availability of N for plant growth at elevated CO 2 has been found to both increase (Dijkstra et al 2008) and decrease (Schneider et al 2004, Hovenden et al 2008, Newton et al 2010 in experiments. Decreases are less likely where N inputs are maintained either through fertiliser application or by biological nitrogen fixation (Hu et al 2006), as was the case at the Winchmore site.…”

“…As well as being important, the CFE is often the response with the greatest uncertainty (Howden et al 2007, Lobell & Field 2008, McKeon et al 2008 Finger et al 2010, Rosenzweig et al 2014 because experimental data provide a wide range of values (Lobell & Field 2008) resulting from differences in experimental conditions, including potentially large differences between the results of experiments in controlled environments, such as growth chambers and glasshouses, and those of field experiments . To compare such an estimate with experimental data, we used the results of a free air carbon dioxide enrichment experiment on grazed pasture of the North Island of New Zealand -the NZFACE experiment (Newton et al 2006). Although separated geographically, the NZFACE and the Winchmore sites have common management features -the addition of P but not N fertiliser and the inclusion of grazing by sheep -and have similar patterns of pasture production (Fig.…”