Winter climate change in plant–soil systems: summary of recent findings and future perspectives

Response on root regrowth potential to soil moisture in Sedum species during winter in Særheim, Norway

Purpose

The impact of winter moisture on root metabolism and root integrity has potential consequences for the geographical distribution of drought-adapted succulent species and for their long-term performance on green roofs. The interacting effects of soil characteristics and precipitation frequency on root mortality under winter conditions and the potential to grow new roots in spring were evaluated for six Sedum species under controlled conditions.

Methods

To test for the impact of soil moisture during winter on root regrowth potential in six Sedum species, we used a combination of two substrates with differing water-holding capacity and four contrasting watering regimes. Specially, for the fine and coarse substrates, total pore volume was 42 and 46 %, respectively, and maximum water-holding capacity (i.e. field capacity) was 0.50 and 0.33 kg water per L, respectively. The four watering treatments involved overhead watering to runoff (approx. 10 mm): once every second week, once a week, three times per week and three times per week with 1 cm standing water in trays from January to March 2019.

Results

It was found that winter soil moisture had no major impact on root mortality or root regrowth potential in spring. Root mortality was not affected by watering frequency and regrowth potential showed no directional response to increased watering frequency, although species-specific responses were involved. Root diameter did not differ between the substrates, but there were some differences between the species. Sedum rupestre had on average the thickest roots (0.17 mm), followed by S. acre, S. anglicum and S. sexangulare (0.15-0.16 mm), while S. album and S. hispanicum had the thinnest roots (0.12-0.13 mm). Moreover, effects of watering frequency on root mortality and regrowth potential were not influenced by soil water-holding capacity across species. We concluded that winter soil moisture had no negative effects on root performance within the range of treatments tested here.

Conclusions

Root response to transient waterlogging or moist but unsaturated soil may not be an important mechanism for determining the survival and distribution of temperate Sedum species during winter.

Ecosystem Processes Show Uniform Sensitivity to Winter Soil Temperature Change Across a Gradient from Central to Cold Marginal Stands of a Major Temperate Forest Tree

The magnitude and frequency of soil frost events might increase in northern temperate regions in response to climate warming due to reduced insulation caused by declining snow cover. In temperate deciduous forests, increased soil frost severity can hamper tree growth and increase the mortality of fine roots, soil fauna and microorganisms, thus altering carbon and nutrient cycling. From single-site studies, however, it is unclear how the sensitivities of these responses change along continental gradients from regions with low to high snowfall. We conducted a gradient design snow cover and soil temperature manipulation experiment across a range of lowland beech forest sites to assess the site-specific sensitivity of tree growth and biogeochemical cycling to soil cooling. Even mild and inconsistent soil frost affected tree increment, germination, litter decomposition and the retention of added 15N. However, the sensitivity of response (treatment effect size per degree of warming or cooling) was not related to prevailing winter climate and snow cover conditions. Our results support that it may be valid to scale these responses to simulated winter climate change up from local studies to regional scales. This upscaling, however, needs to account for the fact that cold regions with historically high snowfall may experience increasingly harsh soil frost conditions, whereas in warmer regions with historically low snowfall, soil frost may diminish. Thus, despite the uniform biotic sensitivity of response, there may be opposing directions of winter climate change effects on temperate forests along continental temperature gradients due to different trends of winter soil temperature.

Schrenk spruce leaf litter decomposition varies with snow depth in the Tianshan Mountains

Seasonal snowfall, a sensitive climate factor and the main form of precipitation in arid areas, is important for forest material circulation and surface processes and profoundly impacts litter decomposition and element turnover. However, how the thickness and duration of snow cover affect litter decomposition and element release remain unclear. Thus, to understand the effects of snow on litter decomposition, fiber degradation and their relationships with soil properties, a field litterbag experiment was conducted under no, thin, medium, and thick snow cover in a Schrenk spruce (Picea schrenkiana) forest gap in the Tianshan Mountains. The snow cover period exhibited markedly lower rates of decomposition than the snow-free period. The litter lignin, cellulose and N concentrations in the pregrowing season and middle growing season were significantly higher than those in the deep-freeze period, and the litter C and P concentrations were significantly higher during the onset of the freeze-thaw period, deep-freeze period and thaw period than in the late growing season. The litter cellulose, C and N concentrations were significantly higher under thick snow cover than under no snow cover in most stages. Moreover, the correlations among litter mass, cellulose, lignin/cellulose and soil bulk density varied with snow cover depth. The temporal variations and snow cover depth affected the decomposition process significantly. The former affected lignin, cellulose and P, and the latter affected cellulose, C and N and changed the litter-soil properties relationship. These differences provide references for understanding how winter conditions affect material cycling and other ecological processes under climate change.

Understanding the Fertilizer Management Impacts on Water and Nitrogen Dynamics for a Corn Silage Tile-Drained System in Canada

Effective management of dairy manure is important to minimize N losses from cropping systems, maximize profitability, and enhance environmental sustainability. The objectives of this study were (i) to calibrate and validate the DeNitrification-DeComposition (DNDC) model using measurements of silage corn ( L.) biomass, N uptake, soil temperature, tile drain flow, NO leaching, NO emissions, and soil mineral N in eastern Canada, and (ii) to investigate the long-term impacts of manure management under climate variability. The treatments investigated included a zero-fertilizer control, inorganic fertilizer, and dairy manure amendments (raw and digested). The DNDC model overall demonstrated statistically "good" performance when simulating silage corn yield and N uptake based on normalized RMSE (nRMSE) < 10%, index of agreement () > 0.9, and Nash-Sutcliffe efficiency (NSE) > 0.5. In addition, DNDC, with its inclusion of a tile drainage mechanism, demonstrated "good" predictions for cumulative drainage (nRMSE < 20%, > 0.8, and NSE > 0.5). The model did, however, underestimate daily drainage flux during spring thaw for both organic and inorganic amendments. This was attributed to an underestimation of soil temperature and soil water under frequent soil freezing and thawing during the 2013-2014 overwinter period. Long-term simulations under climate variability indicated that spring applied manure resulted in less NO leaching and NO emissions than fall application when manure rates were managed based on crop N requirements. Overall, this study helped highlight the challenges in discerning the short-term climate interactions on fertilizer-induced N losses compared with the long-term dynamics under climate variability.

Black spruce assimilates nitrate in boreal winter

Winter has long been considered a dormant season in boreal forests regarding plant physiological activity such as nutrient acquisition. However, biogeochemical data clearly show that soil can remain unfrozen with substantial rates of nutrient transformation for several weeks following autumn snowfall. Here we examined nitrate (NO3--N) assimilation by black spruce (Picea mariana (Mill.) Britton, Sterns and Poggenb.) during summer and winter in Interior Alaska to test our hypothesis that this boreal species is able to assimilate NO3--N, even at the very low temperatures typical of early winter. Nitrate reductase activity (NRA) was measured in current year needles and fine roots of black spruce as an indicator of NO3--N assimilation in the summer and winter at two boreal forest sites. Nitrate concentration in the needles and roots were also measured to determine whether NO3--N was available in plant tissue for the enzyme. Nitrate reductase activity and NO3--N were detected in needles and roots in the winter as well as the summer. The results of a generalized linear mixed model showed that season had minimal effects on NRA and NO3--N concentration in this species. Additionally, the effect of incubation temperature for the NRA assays was tested at 30 °C and -3 °C for samples collected in the winter. Substantial enzyme activity was detected in winter-collected samples, even in incubations conducted at -3 °C. These results indicate that this dominant tree species in the boreal forests of Interior Alaska, black spruce, has the capacity to assimilate NO3--N below freezing temperatures, suggesting that the physiological activity required for nitrogen (N) resource acquisition may extend beyond the typical growing season. Our findings coupled to biogeochemical evidence for high microbial activity under the snow also indicate that winter N acquisition should be taken into account when estimating the annual N budgets of boreal forest ecosystems.

Declines in northern forest tree growth following snowpack decline and soil freezing

Changes in growing season climate are often the foci of research exploring forest response to climate change. By contrast, little is known about tree growth response to projected declines in winter snowpack and increases in soil freezing in seasonally snow-covered forest ecosystems, despite extensive documentation of the importance of winter climate in mediating ecological processes. We conducted a 5-year snow-removal experiment whereby snow was removed for the first 4-5 weeks of winter in a northern hardwood forest at the Hubbard Brook Experimental Forest in New Hampshire, USA. Our results indicate that adverse impacts of reduced snowpack and increased soil freezing on the physiology of Acer saccharum (sugar maple), a dominant species across northern temperate forests, are accompanied by a 40 ± 3% reduction in aboveground woody biomass increment, averaged across the 6 years following the start of the experiment. Further, we find no indication of growth recovery 1 year after cessation of the experiment. Based on these findings, we integrate spatial modeling of snowpack depth with forest inventory data to develop a spatially explicit, regional-scale assessment of the vulnerability of forest aboveground growth to projected declines in snowpack depth and increased soil frost. These analyses indicate that nearly 65% of sugar maple basal area in the northeastern United States resides in areas that typically experience insulating snowpack. However, under the RCP 4.5 and 8.5 emissions scenarios, we project a 49%-95% reduction in forest area experiencing insulating snowpack by the year 2099 in the northeastern United States, leaving large areas of northern forest vulnerable to these changes in winter climate, particularly along the northern edge of the region. Our study demonstrates that research focusing on growing season climate alone overestimates the stimulatory effect of warming temperatures on tree and forest growth in seasonally snow-covered forests.

Warmer winters increase the rhizosphere carbon flow to mycorrhizal fungi more than to other microorganisms in a temperate grassland

A decisive set of steps in the terrestrial carbon (C) cycle is the fixation of atmospheric C by plants and the subsequent C-transfer to rhizosphere microorganisms. With climate change winters are expected to become milder in temperate ecosystems. Although the rate and pathways of rhizosphere C input to soil could be impacted by milder winters, the responses remain unknown. To address this knowledge-gap, a winter-warming experiment was established in a seminatural temperate grassland to follow the C flow from atmosphere, via the plants, to different groups of soil microorganisms. In situ ¹³ CO₂ pulse labelling was used to track C into signature fatty acids of microorganisms. The winter warming did not result in any changes in biomass of any of the groups of microorganisms. However, the C flow from plants to arbuscular mycorrhizal (AM) fungi, increased substantially by winter warming. Saprotrophic fungi also received large amounts of plant-derived C-indicating a higher importance for the turnover of rhizosphere C than biomass estimates would suggest-still, this C flow was unaffected by winter warming. AM fungi was the only microbial group positively affected by winter warming-the group with the closest connection to plants. Winter warming resulted in higher plant productivity earlier in the season, and this aboveground change likely induced plant nutrient limitation in warmed plots, thus stimulating the plant dependence on, and C allocation to, belowground nutrient acquisition. The preferential C allocation to AM fungi was at the expense of C flow to other microbial groups, which were unaffected by warming. Our findings imply that warmer winters may shift rhizosphere C-fluxes to become more AM fungal-dominated. Surprisingly, the stimulated rhizosphere C flow was matched by increased microbial turnover, leading to no accumulation of soil microbial biomass.

Loss of testate amoeba functional diversity with increasing frost intensity across a continental gradient reduces microbial activity in peatlands

Soil microbial communities significantly contribute to global fluxes of nutrients and carbon. Their response to climate change, including winter warming, is expected to modify these processes through direct effects on microbial functions due to osmotic stress, and changing temperature regimes. Using four European peatlands reflecting different frequencies of frost events, we show that peatland testate amoeba communities diverge among sites with different winter climates, and that this is reflected through contrasting functions. We found that exposure to harder soil frost promoted species β-diversity (species turnover) thus shifting the community composition of testate amoebae. In particular, we found that harder soil frost, and lower water-soluble phenolic compounds, induced functional turnover through the decrease of large species (-68%, >80μm) and the increase of small-bodied mixotrophic species (i.e. Archerella flavum; +79%). These results suggest that increased exposure to soil frost could be highly limiting for large species while smaller species are more resistant. Furthermore, we found that β-glucosidase enzymatic activity, in addition to soil temperature, strongly depended of the functional diversity of testate amoebae (R²=0.95, ANOVA). Changing winter conditions can therefore strongly impact peatland decomposition process, though it remains unclear if these changes are carried-over to the growing season.

Consequence of altered nitrogen cycles in the coupled human and ecological system under changing climate: The need for long-term and site-based research

Anthropogenically derived nitrogen (N) has a central role in global environmental changes, including climate change, biodiversity loss, air pollution, greenhouse gas emission, water pollution, as well as food production and human health. Current understanding of the biogeochemical processes that govern the N cycle in coupled human-ecological systems around the globe is drawn largely from the long-term ecological monitoring and experimental studies. Here, we review spatial and temporal patterns and trends in reactive N emissions, and the interactions between N and other important elements that dictate their delivery from terrestrial to aquatic ecosystems, and the impacts of N on biodiversity and human society. Integrated international and long-term collaborative studies covering research gaps will reduce uncertainties and promote further understanding of the nitrogen cycle in various ecosystems.