Soil nitrogen cycle unresponsive to decadal long climate change in a Tasmanian grassland

Abstract

Increases in atmospheric carbon dioxide (CO 2) and global air temperature affect all terrestrial ecosystems and often lead to enhanced ecosystem productivity, which in turn dampens the rise in atmospheric CO 2 by removing CO 2 from the atmosphere. As most terrestrial ecosystems are limited in their productivity by the availability of nitrogen (N), there is concern about the persistence of this terrestrial carbon sink, as these ecosystems might develop a progressive N limitation (PNL). An increase in the gross soil N turnover may alleviate PNL, as more mineral N is made available for plant uptake. So far, climate change experiments have mainly manipulated one climatic factor only, but there is evidence that single-factor experiments usually overestimate the effects of climate change on terrestrial ecosystems. In this study, we investigated how simultaneous, decadal-long increases in CO 2 and temperature affect the soil gross N dynamics in a native Tasmanian grassland under C3 and C4 vegetation. Our laboratory 15 N labeling experiment showed that average gross N mineralization ranged from 4.9 to 11.3 lg N g-1 day-1 across the treatment combinations, while gross nitrification was about ten-times lower. Considering all treatment combinations, no significant effect of climatic treatments or vegetation type (C3 versus C4 grasses) on soil N cycling was observed.

Composition and activity of nitrifier communities in soil are unresponsive to elevated temperature and CO2, but strongly affected by drought

“…Also, our results are in line with meta-analyses that observed that the effect of temperature on N cycling processes is less pronounced in grasslands, in comparison with other ecosystem types [ 36 , 73 ]. In addition, other studies in grassland systems also reported no significant effects of elevated temperature on nitrification [ 72 , 74 ]. Our results indicate that increased temperature might have altered abiotic and biotic soil parameters that may be masking individual temperature effects.…”

“…Likewise, eCO 2 did not affect gross N mineralization and NH 4 + levels, highlighting that substrate availability for nitrifiers did not change in response to eCO 2 . In addition, other studies also reported very few effects of elevated atmospheric CO 2 concentration on belowground N processes in heathlands and temperate grasslands [ 70 – 72 ] showing that soil parameters such as pH and inorganic N concentrations likely play a more significant role than increased atmospheric CO 2 on overall nitrification.…”

“…Global reviews have shown that the interaction of different climate change factors can lead to nonadditive effects [ 6 ], although a more recent meta-analysis showed that antagonistic and synergistic effects are quite rare [ 7 ]. A recent study in a grassland system assessed the effect of elevated temperature and elevated CO 2 and found that while combinatory effects were antagonistic, they were mostly nonsignificant [ 72 ]. Regarding nitrifier community composition, only active AOB were significantly affected by the interactive effect of future climate conditions and drought [eTeCO 2 × [D]).…”

Nitrogen dynamics after two years of elevated CO2 in phosphorus limited Eucalyptus woodland

“…The weak or missing N cycle response under FACE treatment in some ecosystems -grassland, heathland or forest observed here and elsewhere (Wild et al 2018;Schleppi 2019;Rütting 2020), could be due to limitation of another nutrient, such as P (Dijkstra et al 2013;. At EucFACE, evidence derived from Eucalyptus leaf C:N:P stoichiometry revealed strong P re-translocation compared to N in both ambient and CO 2 treated trees (Crous et al 2019), emphasising that P is highly limiting for the trees in this ecosystem.…”

“…Using a 14 C approach, Farrell et al (2013) determined amino acid and peptide half-life (alanine, dialanine and trialanine) across grassland, bush and forest in Western Australia. With 15 N techniques, Rütting & Hovenden (2020) determined gross N mineralization rate in a Tasmanian grassland (TasFACE). Our study is however, the first conducted in Australian woodland within an eCO 2 field facility (EucFACE).…”