Contrasting drivers of belowground nitrogen cycling in a montane grassland exposed to a multifactorial global change experiment with elevated CO2 , warming, and drought

Abstract

Depolymerization of high-molecular weight organic nitrogen (N) represents the major bottleneck of soil N cycling and yet is poorly understood compared to the subsequent inorganic N processes. Given the importance of organic N cycling and the rise of global change, we investigated the responses of soil protein depolymerization and microbial amino acid consumption to increased temperature, elevated atmospheric CO₂ , and drought. The study was conducted in a global change facility in a managed montane grassland in Austria, where elevated CO₂ (eCO₂ ) and elevated temperature (eT) were stimulated for 4 years, and were combined with a drought event. Gross protein depolymerization and microbial amino acid consumption rates (alongside with gross organic N mineralization and nitrification) were measured using ¹⁵ N isotope pool dilution techniques. Whereas eCO₂ showed no individual effect, eT had distinct effects which were modulated by season, with a negative effect of eT on soil organic N process rates in spring, neutral effects in summer, and positive effects in fall. We attribute this to a combination of changes in substrate availability and seasonal temperature changes. Drought led to a doubling of organic N process rates, which returned to rates found under ambient conditions within 3 months after rewetting. Notably, we observed a shift in the control of soil protein depolymerization, from plant substrate controls under continuous environmental change drivers (eT and eCO₂ ) to controls via microbial turnover and soil organic N availability under the pulse disturbance (drought). To the best of our knowledge, this is the first study which analyzed the individual versus combined effects of multiple global change factors and of seasonality on soil organic N processes and thereby strongly contributes to our understanding of terrestrial N cycling in a future world.

Keywords: T-FACE; amino acid consumption; climate warming; drought; elevated CO2; protein depolymerization; soil nitrogen cycling.

FIGURE 1

Graphic representation of the plots from which samples were collected and analyzed (detailed in Simon et al., 2020). The figure illustrates the different combinations of three temperature levels, three CO₂ levels, and drought (blue dashed squares). The gray numbers in brackets represent the number of sample replicates (plots) per treatment, and the number after the slash refers to the available replicates for the drought and rewetting treatment. The setup follows the response surface approach, which includes seven of nine possible treatment combinations (Piepho et al., 2017)

FIGURE 2

Response of soil organic nitrogen processes to elevated temperature and atmospheric CO₂ concentration. (a) Protein depolymerization (μg N g⁻¹ d⁻¹), (b) amino acid consumption (μg N g⁻¹ d⁻¹), and (c) mean residence times of free amino acids (h) in May, July, and October 2017 under various combinations of three temperatures and three CO₂ treatment levels. Data points correspond to ambient air temperature (ambient, blue), 1.5°C above ambient temperature (+1.5, orange), 3°C above ambient air temperature (+3, red) within levels of ambient atmospheric CO₂ concentration (ambient, white box), 150 ppm CO₂ above ambient level (+150, light gray box), and 300 ppm CO₂ above ambient (+300, dark gray box). Data are presented as mean ± 1 standard deviation (n = 2–8 per treatment, for details see Figure S2), along with raw data (semi‐transparent points). Statistical results of the corresponding generalized least squares models can be found in Table 1. Data for free amino acids, mineralization, and nitrification rates are presented in Figure S4

FIGURE 3

Drought and recovery effects on (a) protein depolymerization (μg N g⁻¹ d⁻¹), (b) amino acid consumption (μg N g⁻¹ d⁻¹), and (c) mean residence times of free amino acids (h) in ambient and future climate (+3°C, +300 ppm) plots. Drought effects were measured in July and recovery effects in October 2017 in the “+D” plots (red points). Data are presented as mean ± 1 standard deviation (n = 4–8 per treatment, for details see Figure S2), along with raw data (semi‐transparent points). Statistical results of two‐way ANOVAs for each variable can be found in Table S2. Points associated with no common letters (Piepho, 2018) are significantly different between groups (p < .05, Tukey's HSD test). Data for free amino acids, mineralization, and nitrification rates are presented in Figure S6

FIGURE 4

Relationship between protein depolymerization and other parameters, including plant and microbial descriptors, pool sizes, and enzyme activities. The correlation coefficients (ρ) and p‐values come from a repeated measures correlation, done for the entire dataset (n = 102), for the “eT eCO₂” subset (ambient, eT, and eCO₂ plots across seasons, n = 78), and for the “drought” experiment subset (ambient and “extreme” climate at the predrought, drought, and recovery dates, n = 60)