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

Abstract

It is uncertain how the predicted further rise of atmospheric carbon dioxide (CO2) concentration will affect plant nutrient availability in the future through indirect effects on the gross rates of nitrogen (N) mineralization (production of ammonium) and depolymerization (production of free amino acids) in soil. The response of soil nutrient availability to increasing atmospheric CO2 is particularly important for nutrient poor ecosystems. Within a FACE (Free-Air Carbon dioxide Enrichment) experiment in a native, nutrient poor Eucalyptus woodland (EucFACE) with low soil organic matter (≤ 3%), our results suggested there was no shortage of N. Despite this, microbial N use efficiency was high (c. 90%). The free amino acid (FAA) pool had a fast turnover time (4 h) compared to that of ammonium (NH4+) which was 11 h. Both NH4-N and FAA-N were important N pools; however, protein depolymerization rate was three times faster than gross N mineralization rates, indicating that organic N is directly important in the internal ecosystem N cycle. Hence, the depolymerization was the major provider of plant available N, while the gross N mineralization rate was the constraining factor for inorganic N. After two years of elevated CO2, no major effects on the pools and rates of the soil N cycle were found in spring (November) or at the end of summer (March). The limited response of N pools or N transformation rates to elevated CO2 suggest that N availability was not the limiting factor behind the lack of plant growth response to elevated CO2, previously observed at the site.

Rainfall frequency and soil water availability regulate soil methane and nitrous oxide fluxes from a native forest exposed to elevated carbon dioxide

“…For example, the anaerobic dependency of N 2 O emissions can be observed by the increase of N 2 O fluxes with the reduction of soil air‐filled pore space, that is, higher pulses of N 2 O production under higher soil water availability in this study, which constrains oxygen availability throughout the soil profile (Groffman et al., 2009). However, it is important to mention that, despite the anaerobic dependency of N 2 O emissions observed in our study, nitrification pathways are expected to play an important role under aerobic conditions (Hu et al., 2015) but probably constitute most of the low fluxes detected, linked to low substrate availability (Andresen et al., 2020) and no impact on net nitrification rates (Hasegawa et al., 2016; Hu et al., 2016).…”

“…Andresen et al. (2020) showed a lack of strong response in N transformation rates to eCO 2 , supporting the observed lack of plant growth and litter inputs (Ellsworth et al., 2017; Jiang et al., 2020). However, it also suggested that P limitation could be leading soil microbes to target P rather than N (Dijkstra et al., 2013) and thus preventing the stimulation of N mineralization (Rütting & Andresen, 2015).…”

The influence of elevated CO2 and soil depth on rhizosphere activity and nutrient availability in a mature Eucalyptus woodland

“…Furthermore, the gross N mineralization rate increased in the rhizosphere compared to bulk soil. Given the positive links found between gross N mineralization and SOM decomposition (Bengtson et al, 2012;Zhu et al, 2014) these findings suggest that root-microbe interactions are facilitating decomposition and increasing nutrient availability (Andresen et al, 2020).…”