Climate Warming Alters Nutrient Storage in Seasonally Dry Forests: Insights From a 2,300 m Elevation Gradient

Abstract

Understanding potential response of forest carbon (C) and nutrient storage to warming is important for climate mitigation policies. Unfortunately, those responses are difficult to predict in seasonally dry forests, in part, because ecosystem processes are highly sensitive to both changes in temperature and precipitation. We investigated how warming might alter stocks of C, nitrogen (N), and phosphorus (P) in vegetation and the entire regolith (soil + weathered bedrock or “saprock”) using a space‐for‐time substitution along a bioclimatic gradient in the Sierra Nevada, California. The pine‐oak and mixed‐conifer forests between 1,160–2,015 m elevation have more optimal climates (not too dry or hot) for ecosystem productivity, soil weathering, and cycling of essential elements than the oak savannah (405 m) and subalpine forest (2,700 m). We found decreases in overstory vegetation nutrient stocks with decreasing elevation because of enhanced water limitation and greater occurrence of disturbances. Stocks of C, N, and P in the entire regolith peaked at the pine‐oak and mixed‐conifer forests across the bioclimatic gradient, driven by thicker regolith profiles and greater nutrient input rates. These observations suggest long‐term warming will decrease ecosystem nutrient storage at the warmer, transitional pine‐oak zone, but will increase nutrient storage at the colder, subalpine zone. Assuming steady‐state conditions, we found the mean residence time of ecosystem C decreased with projected rising air temperatures and increased following a major drought event across the bioclimatic gradient. Our study emphasizes potentially elevation‐dependent changes in nutrient storage and C persistence with warming in seasonally dry forests.

Climatic controls on soil and saprock nitrogen distribution and persistence in the Sierra Nevada

“…Given that TN concentrations were relatively low in saprock, the main factor influencing the magnitude of N pools was thickness, and the systematic trends in thickness are controlled by climate. This low concentration of N in saprock mirrors the trends seen in the relative concentrations of organic C in soil, compared to saprock in the same study sites, where saprock C represents up to 30% of the total profile C pool (Moreland et al., 2021; Yang et al., 2022), compared to the 37% profile N being accounted for by saprock in this study. Recent studies are highlighting that, in addition to biological fixation of atmospheric N 2 , the weathering of parent material can contribute to soil N pools and availability (Houlton & Morford, 2015).…”

“…We found important trends in the relative proportion of the light fractions (fLF and oLF) between sites that vary with climate, indicating that the accumulation of most N in the fLF and oLF is sensitive to changes in climate. As elevation increases, we observed a greater proportion of the soil TN contained within the fLF and oLF, mimicking C trends at these sites (Moreland et al., 2021; Rasmussen et al., 2005; Yang et al., 2022). This is due, in part, to the higher GPP at the higher elevation sites where there are greater inputs of N from aboveground plant and root sources (Goulden et al., 2012; Yang et al., 2022).…”

“…As elevation increases, we observed a greater proportion of the soil TN contained within the fLF and oLF, mimicking C trends at these sites (Moreland et al., 2021; Rasmussen et al., 2005; Yang et al., 2022). This is due, in part, to the higher GPP at the higher elevation sites where there are greater inputs of N from aboveground plant and root sources (Goulden et al., 2012; Yang et al., 2022). Furthermore, the mixed effect model results indicate that variability in the topsoil and subsoil fLF and oLF can be explained most by GPP, MAP, and DWP.…”

“…However, granite δ 15 N is reported to be between 1% and 10‰, indicating that the HF δ 15 N could also be representative of the parent rock and may not all be due to microbial processing (Holloway & Dahlgren 2002). Since the HF N pool increases with depth, this suggests that the subsoil N may be more persistent, compared to the topsoil, which is consistent with deep soil organic C at these sites (von Lützow et al., 2006; Moreland et al., 2021; Yang et al., 2022). HF N may be a large, overlooked persistent pool of N.…”

“…Subsoil OM concentration decreases with depth and typically has a longer MRT, compared to topsoil (Jobbágy & Jackson 2000; Moreland et al., 2021; Yang et al., 2022). Lower concentrations of OM, lower microbial abundance, and increased persistence of OM due to burial, aggregation, and organo‐mineral associations decrease the rate of subsoil OM decomposition (Chabbi et al., 2009; Rumpel & Kögel‐Knabner, 2011; Schrumpf et al., 2013).…”