The impacts of warming and snow exclusion varied by soil type. Compared to the fine soil, the coarse soil generally experienced higher losses of C and most nutrients, except \({\text{P}\text{O}}_{4}^{3-}\). However, it was common for the impact of climate treatments to differ considerably by soil type. For example, on fine soils, snow exclusion elevated N losses, but on coarse soils, snow exclusion had no effect. Furthermore, while high plant biomass was able to mitigate N losses from coarse soils, increasing biomass had little impact on N losses from fine soils. Overall, our results provide experimental evidence that interactions among climate treatments and soil properties are an important factor in determining the magnitude of climate change effects on ecosystem biogeochemistry.
Soil and climate treatment impacts on abiotic factors
Climate treatments altered abiotic factors that influence processes related to our hypotheses. Overall, climate treatments had the expected effects: on average, warming raised surface temperatures by 2°C and increased soil temperatures to a depth of 60 cm; snow exclusion decreased soil temperatures to 60 cm, induced deep soil freezing, and reduced snowpack depth. However, between-year variation in winter air temperatures and onset of snowpack resulted in different impacts of the warming treatment between winters. In 2014, warming reduced early winter snowpack, causing soils to freeze deeper than in controls. Conversely, in 2015 warmed mesocosms experienced less soil freezing overall than controls due to faster soil thaw in the spring (Fig. 2). Importantly, late snowpack development in 2015 allowed deep soil freezing in all treatments that persisted under the snowpack (Hardy et al. 2001).
In addition to soil temperature, soil type and climate treatments impacted soil moisture throughout the experiment. Fine soils consistently held twice as much moisture as coarse soils. However, the leachate drained from coarse soils was only 10% higher than from fine soils, likely due to fine soils having higher plant biomass (Fig. S3) and consequently greater evapotranspiration potential (Kosiba 2017). Finally, both climate treatments resulted in drier soils than controls in the second year.
Coarse textured soils had greater C and nutrient leaching, except for\({PO}_{4}^{3-}\)
Cation exchange capacity, clay content, and organic matter content generally correlate with reduced leachate losses. In our study, the coarse soil exceeded the fine soil in each of these metrics (Table S1). Notwithstanding, in all cases with a significant soil effect, except \({\text{P}\text{O}}_{4}^{3-}\), the coarse soil experienced higher leachate losses. The high fine gravel content of the coarse soil likely reduced its water holding capacity and increased its hydraulic conductivity compared to the fine soil, thereby diminishing its storage capacity for cations and nutrients (Dudley et al. 2008). The nutrients that experienced significant losses across years by soil type were DOC and Ca (higher losses from coarse soils), and \({\text{P}\text{O}}_{4}^{3-}\) (higher losses from fine soils). The effect of soil on losses of all other nutrients varied between years, but in all significant cases, coarse textured soils experienced higher losses (2014: Al; 2015: TDN, mineral N, and Mg).
Contrary to our expectations, the loss of \({\text{P}\text{O}}_{4}^{3-}\) was greater from fine- vs. coarse-textured soils. However, this could be associated with between-soil differences in pH and cation contents. Namely, the fine soil had a lower pH (6.2) than the coarse soil (7.6; Table S1). In soils with pH < 7, \({\text{P}\text{O}}_{4}^{3-}\) tends to be available but can be fixed, to some degree, by aluminum (Penn and Camberato 2019). Because aluminum availability in the fine soil was relatively low (Table S1), and aluminum tends to be non-soluble at neutral pH (pH 6-8; Lindsay and Walthall 1996), \({\text{P}\text{O}}_{4}^{3-}\) may have been more easily lost from the fine soil than expected based on its hydraulic conductivity. Conversely, in soils with pH > 7, like the coarse soil, P is fixed by Ca (Penn and Camberato 2019), which was very abundant in the coarse soil (Table S1). Thus, the Ca content of the coarse soil may have reduced the susceptibility of \({\text{P}\text{O}}_{4}^{3-}\) to leaching despite that soil’s reduced capacity for nutrient storage relative to the fine soil.
Throughout the experiment, one of the largest differences we observed was the change in the magnitude of N loss (mineral N and TDN) across years. Coarse soils lost a similar amount of N in both years, but N loss from fine soils dropped to extremely low levels in 2015, despite no associated decrease in leachate volume, causing a significant soil effect (Fig. 4). Greater N uptake due to the higher plant biomass (24% higher) supported by the fine soil provides one possible explanation for the large reduction in leachate N loss in 2015. However, while increasing plant biomass did reduce mineral N losses in coarse soils in some cases, it had no impact on N losses from fine soils, suggesting that higher plant uptake might not be responsible for this difference. Alternatively, greater nitrification rates were measured on the coarse soil during the 2015 growing season, which could have created a pool of nutrients vulnerable to leaching given the low water holding capacity and larger volume of water leached from that soil.
Warming and snow exclusion did not have strong impacts on C and nutrient loss independent of soil type
Despite consistently altering abiotic conditions in the mesocosms, climate treatment impacts on nutrient losses were inconsistent, and varied interannually (e.g., Ca and Mg). Warming increased springtime net nitrification regardless of soil type, in accordance with prior observations (but see also Barnard et al. 2005; MacDonald et al. 1995). But warming effects varied with soil type in the winter and had no impact on summer nitrification rates. The lack of a summertime response to warming could be explained by the lower soil moisture in that treatment. As we observed, warming treatments tend to dry soils (Xu et al. 2013), which can reduce microbial activity such as nitrification (Liu et al. 2009). Additionally, plant activity dominates terrestrial water movement during the summer (Jasechko et al. 2013). Thus, early spring water availability during plant dormancy combined with elevated temperatures in the warming treatment could explain the ephemeral nature of the nitrification response to warming. Finally, net nitrification did not respond to snow exclusion, unlike previous findings that linked decreased net nitrification to soil freezing (Shibata et al. 2013).
Climate treatments consistently affected C and nutrient responses differently depending on soil type, as observed in wintertime nitrification rates and losses of DOC, TDN, total mineral N, \({\text{P}\text{O}}_{4}^{3-}\), Mg, and Al during one or both years. This occurred even though climate treatments did not affect the abiotic conditions of the soils differently (Table S5). In all cases, either soil or treatment, but not their interaction, altered soil temperature and moisture, snow, or soil freezing conditions. Additionally, differences in plant biomass across mesocosms did not account for the varying effects of climate treatment by soil type, and accounting for plant biomass at times revealed treatment and biomass interactions with soils (i.e., \({\text{P}\text{O}}_{4}^{3-}\)).
Throughout the experiment, patterns of N loss (TDN and total mineral N) consistently responded to climate treatments differently according to soil type. Across years, snow exclusion elevated N losses from only fine soils. This finding coincides with well-documented increases in \({\text{N}\text{O}}_{3}^{-}\) loss following soil freezing (Campbell et al. 2014; Mitchell et al. 1996), although with variability (Groffman et al. 2011; Judd et al. 2011). Elevated \({\text{N}\text{O}}_{3}^{-}\) losses after soil freezing have been attributed to root mortality (Tierney et al. 2001) and decreased root nutrient uptake (Campbell et al. 2014). In both our soil types, differences in plant biomass in the snow exclusion treatment had no impact on mineral N losses (the majority of which was \({\text{N}\text{O}}_{3}^{-}\), Fig. 4b and d), suggesting that root uptake in these plots was unable to reduce mineral N losses. Our results therefore indicate that soil differences could additionally account for variability in the \({\text{N}\text{O}}_{3}^{-}\) leaching response to soil freezing, perhaps due to water content during freezing or thawing events or the type of soil frost (e.g., concrete or granular, Fuss et al. 2016).
Warming also increased mineral N and TDN losses from fine soils (except for total mineral N in 2015), but not coarse soils. Previous soil warming in temperate forests found no increase in N leaching (Melillo et al. 2011) or soil solution \({\text{N}\text{O}}_{3}^{-}\) (Sanders-DeMott et al. 2018) under warming conditions despite increased N mineralization (Melillo et al. 2011), a well-documented response to warming (Rustad et al. 2001; Salazar et al. 2020). In those cases, the tight cycling of N between plants and soil accounted for the lack of increased leachate losses despite accelerated N processing under warming (Melillo et al. 2011). Although increasing plant biomass reduced total mineral N losses from coarse soils in 2015 (Fig. 4d), we found no other instances where increasing plant biomass reduced N losses from the warming treatment. These results of increased N loss due to warming from only one soil type show the tightness of nutrient recycling can vary between soils under climate change.
Although elevating temperature increased N losses from our fine soil type, N cycling processes often correlate more with moisture conditions than temperature alone (Beier et al. 2008; Groffman et al. 2009). Nevertheless, overwinter nitrification rates in our study increased under warming on only the drier coarse soils, and over the summer the coarse soils experienced higher nitrification rates than the moister fine soils regardless of climate treatment. In sum, the moister fine soil which supported greater plant biomass had lower rates of summer nitrification and higher leachate N losses under warming, while the drier coarse soil which supported less plant biomass had higher rates of summer nitrification and no response of N loss to warming. These results run counter to expectations based on abiotic moisture and temperature conditions alone, highlighting the role of soil characteristics in mediating biogeochemical losses under climate change.