Soils from cold and snowy temperate deciduous forests release more nitrogen and phosphorus after soil freeze–thaw cycles than soils from warmer, snow-poor conditions
Authors: Kreyling Juergen , Schumann Rhena , Weigel Robert 
Published: August 2020

Abstract. The effects of global warming are most pronounced in winter. A
reduction in snow cover due to warmer atmospheric temperature in formerly
cold ecosystems, however, could counteract an increase in soil temperature
by reduction of insulation. Thus, soil freeze–thaw cycles (FTCs) might
increase in frequency and magnitude with warming, potentially leading to a
disturbance of the soil biota and release of nutrients. Here, we assessed how soil freeze–thaw magnitude and frequency affect
short-term release of nutrients in temperate deciduous forest soils by
conducting a three-factorial gradient experiment with ex situ soil samples
in climate chambers. The fully crossed experiment included soils from
forests dominated by Fagus sylvatica (European beech) that originate from different winter
climate (mean coldest month temperature range ΔT>4 K),
a range of FTC magnitudes from no (T=4.0 ∘C) to strong (T=-11.3 ∘C) soil frost, and a range of FTC frequencies (f=0–7). We hypothesized that higher FTC magnitude and frequency will increase the release of nutrients. Furthermore, soils
from cold climates with historically stable winter soil temperatures due to
deep snow cover will be more responsive to FTCs than soils from warmer, more
fluctuating winter soil climates. FTC magnitude and, to a lesser extent, also FTC frequency resulted in
increased nitrate, ammonium, and phosphate release almost exclusively in
soils from cold, snow-rich sites. The hierarchical regression analyses of
our three-factorial gradient experiment revealed that the effects of
climatic origin (mean minimum winter temperature) followed a sigmoidal curve
for all studied nutrients and was modulated either by FTC magnitude
(phosphate) or by FTC magnitude and frequency (nitrate, ammonium) in complex
twofold and, for all studied nutrients, in threefold interactions of the
environmental drivers. Compared to initial concentrations, soluble nutrients
were predicted to increase to 250 % for nitrate (up to 16 µg NO3-N kg−1DM), to 110 % for ammonium (up to 60 µg NH4-N kg−1DM), and to 400 % for phosphate (2.2 µg PO4-P kg−1DM) at the coldest site for the strongest magnitude and
highest frequency. Soils from warmer sites showed little nutrient release
and were largely unaffected by the FTC treatments except for above-average
nitrate release at the warmest sites in response to extremely cold FTC
magnitude. We suggest that currently warmer forest soils have historically already
passed the point of high responsiveness to winter climate change, displaying
some form of adaptation either in the soil biotic composition or in labile
nutrient sources. Our data suggest that previously cold sites, which will
lose their protective snow cover during climate change, are most vulnerable
to increasing FTC frequency and magnitude, resulting in strong shifts in
nitrogen and phosphorus release. In nutrient-poor European beech forests of
the studied Pleistocene lowlands, nutrients released over winter may be
leached out, inducing reduced plant growth rates in the following growing
season.