Soil warming during winter period enhanced soil N and P availability and leaching in alpine grasslands: A transplant study

Abstract

Alpine meadows are strongly affected by climate change. Increasing air temperature prolongs the growing season and together with changing precipitation patterns alters soil temperature during winter. To estimate the effect of climate change on soil nutrient cycling, we conducted a field experiment. We transferred undisturbed plant-soil mesocosms from two wind-exposed alpine meadows at ~2100 m a.s.l. to more sheltered plots, situated ~300-400 m lower in the same valleys. The annual mean air temperature was 2°C higher at the lower plots and soils that were normally frozen at the original plots throughout winters were warmed to ~0°C due to the insulation provided by continuous snow cover. After two years of exposure, we analyzed the nutrient content in plants, and changes in soil bacterial community, decomposition, mineralization, and nutrient availability. Leaching of N and P from the soils was continuously measured using ion-exchange resin traps. Warming of soils to ~0°C during the winter allowed the microorganisms to remain active, their metabolic processes were not restricted by soil freezing. This change accelerated nutrient cycling, as evidenced by increased soil N and P availability, their higher levels in plants, and elevated leaching. In addition, root exudation and preferential enzymatic mining of P over C increased. However, any significant changes in microbial biomass, bacterial community composition, decomposition rates, and mineralization during the growing season were not observed, suggesting considerable structural and functional resilience of the microbial community. In summary, our data suggest that changes in soil temperature and snow cover duration during winter periods are critical for altering microbially-mediated processes (even at unchanged soil microbial community and biomass) and may enhance nutrient availability in alpine meadows. Consequently, ongoing climate change, which leads to soil warming and decreasing snow insulation, has a potential to significantly alter nutrient cycling in alpine and subalpine meadows compared to the current situation and increase the year-on-year variability in nutrient availability and leaching.

Fig 1. Design of mesocosms.

(a) The undisturbed plant-soil core was placed in the plastic box with perforated bottom. Four ion-exchange resin traps–parallels with two different ion-exchange resins that trapped N and P, were placed beneath the core, covering the entire bottom, and secured by another plastic box of the same size but without bottom. (b) Photo of mesocosm in the field together with a thermometer in its vicinity, marked with colored string.

Fig 2. Course of soil temperatures at sampling sites.

Daily mean temperature measured at the 2-cm soil depth at the high (H) and low (L) transplant sites in (a) Velka Studena (VS) and (b) Furkotska (FU) valleys during the mesocosms exposure from Sept 2013 to Sept 2015.

Fig 3. Effect of downward transfer of mesocosms on soil microbial biomass (MB).

Concentrations of C (a), N (b), and P (c) in soil microbial biomass (MB) in the high (H) and transferred (H→L) mesocosms in Furkotska (FU) and Velka Studena (VS) valleys (mean as line, standard error as box and standard deviation as whiskers are shown, n = 3). Asterisks mark significant effect of downward transfer on microbial biomass (p < 0.05).

Fig 4. Effect of downward transfer on activity of hydrolytic enzymes.

(a) Total potential activity of hydrolytic exoenzymes (mean as line, standard error as box and standard deviation as whiskers are given, n = 3) and (b) proportional investments in C, N and P mining (mean standard deviations are given, n = 3) in the soils of in situ H and downward transferred H→L mesocosms. Asterisks mark the significant effect of downward transfer on depicted characteristics (p < 0.05). Abbreviations: FU, Furkotska valley; VS, Velka Studena valley.

Fig 5. Effect of downward transfer on microbial processes.

(a) Rates of potential C mineralization, (b) potential net N mineralization and (c) net nitrification in the in situ H and transferred H→L mesocosms (mean as line, standard error as box and standard deviation as whiskers are shown, n = 3). Abbreviations: FU, Furkotska valley; VS, Velka Studena valley.

Fig 6. Effect of downward transfer on nutrient leaching.

In situ (a) P and (b) N leaching from original and downward transferred soils in Furkotska (FU) and Velka Studena (VS) valleys in 2014 (Sept 2013–Sept 2014) and 2015 (Sept 2014–Sept 2015). N leaching composes of ammonium- and nitrate-N leaching (means and standard deviations are given, n = 3). Asterisks mark significant changes in element leaching between the in situ H and transferred H→L mesocosms in particular year (p < 0.05).

Fig 7

The plant tissue C/N (a) and C/P (b) ratios in the in situ H and transferred H→L mesocosms in Furkotska (FU) and Velka Studena (VS) valleys in 2015 (mean as line, standard error as box and standard deviation as whiskers are given, n = 3), asterisks mark significant effect of downward transfer (p < 0.05). Correlations between N (c) and P (d) concentration in the above- and belowground plant biomass with the amount of mineral N and P, respectively, contained in ion-exchange resin traps in 2015.