Surprising Relationships Between Soil pH and Microbial Biomass and Activity in a Northern Hardwood Forest

doi:10.21203/rs.3.rs-1620018/v1

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Surprising Relationships Between Soil pH and Microbial Biomass and Activity in a Northern Hardwood Forest

https://doi.org/10.21203/rs.3.rs-1620018/v1

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published 14 Mar, 2023

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Soil microbes mediate major biogeochemical processes in forest ecosystems. Soil pH is considered a “master variable” controlling these processes and many biogeochemical processes in turn influence pH. Long-term measurements of soil pH, microbial biomass carbon (C) and nitrogen (N) content, microbial respiration, potential net N mineralization and nitrification rates, denitrification potential, and soil nitrate (NO₃^-) and ammonium (NH⁺ ₄) concentrations have occurred since 1994 at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA. Naturally acidic soils at the HBEF have been further acidified by anthropogenic acid deposition. Soils in a HBEF watershed have been experimentally treated with calcium silicate to ameliorate acid deposition. While we expected to observe strong positive relationships between soil pH and microbial biomass/activity, weak and/or curvilinear relationships between microbial biomass and activity and soil pH were evident, with peaks at unexpectedly low values (~ 4.5) and values decreased at higher pH values, especially in the calcium-treated soils. It is likely that complexities in plant: microbial interactions inhibit and/or mask microbial response to changes in pH in these acidic soils. These results raise fundamental questions about pH as a controller of microbial processes and how ecosystems recover in response to decreases in acid deposition.

microbial respiration

nitrogen cycle

mineralization

nitrification

denitrification

forest soil

northern hardwood forest

Soil microorganisms play a major role in nutrient cycling, plant production, soil formation, and other critical ecosystem processes. However, much remains unknown about how microbes mediate these processes, and how they respond to environmental change and disturbances such as atmospheric deposition and climate change which in turn impact abiotic factors such as soil pH (Paul 2014). Soil has considerable capacity to buffer acid deposition; however, when hydrogen ion inputs exceed the soil buffering capacity, soil acidification occurs (van Breemen et al. 1983). Acidification alters mineral solubility and nutrient availability, and thus impacts the availability and toxicity of nutrients and elements such as free Fe ³⁺ and Al ³⁺ in the soil (Driscoll et al. 2001). In forests, acid addition has been found to negatively impact Mg ²⁺, Ca ²⁺, soil respiration, fine root biomass, microbial biomass C, microbial biomass N, and bacterial biomass, while positively affecting soil iron, NH₄ ⁺-N, available phosphorus (P), and fungal biomass (Meng et al. 2019).

The HBEF Long Term Ecological Research (LTER) site in the White Mountains of New Hampshire (Fig. 1) presents unique opportunities for study of patterns in soil microbial biomass and activity. The HBEF is dominated by northern hardwood forest and has been impacted by acid deposition for over 50 years (Likens et al. 2021). In October 1999, a watershed at HBEF (Watershed 1) was experimentally treated with wollastonite (CaSiO₃) to replace calcium estimated to have been leached from the ecosystem by decades of acid deposition (Cho et al. 2010; Cho et al. 2012). This treatment allows for evaluation of pre- and post-treatment effects of the calcium addition as well as comparisons to an untreated, reference watershed (Bear Brook) at HBEF.

Long-term measurements of microbial biomass and activity were initiated in a reference area (Bear Brook watershed) at the HBEF in 1994 and in Watershed 1 in 1998 (Groffman 2019). Early analysis (Bohlen et al. 2001) focused on relationships among microbial biomass, N transformation rates (net N mineralization, net nitrification, denitrification potential), and soil, plant, and water parameters. This analysis established that microbial biomass is a useful indicator of broad-scale variations in N cycling processes within ecosystems and that plant-soil-microbial relationships regulate spatial and temporal patterns of N cycling processes.

The extensive body of long-term monitoring and research on Watershed 1 and Bear Brook watershed provided an excellent opportunity for analysis of the importance of pH as a controller of microbial and ecosystem processes. We hypothesized that there would be strong positive relationships between pH and C and N cycle processes in soils at Hubbard Brook and that these relationships would emerge in comparisons between the calcium-treated and reference watersheds and in long-term temporal patterns where pH was increasing (reference watershed) or decreasing (calcium-treated watershed) over time. The analysis provides fundamental basic, scientific information on an important controller of microbial biomass and activity variables that are central to forest ecosystem nutrient cycling processes. More practically, the results are relevant to a widespread environmental problem, acid deposition, which is ongoing in many parts of the world while in other areas, such as the northeast U.S., recovery or deacidification is underway (Jandl et al. 2012; Likens et al. 1996; Oulehle et al. 2017; Oulehle et al. 2011)

The HBEF was established in 1955 in the White Mountains of New Hampshire (43 °56’N, 71°45’ W) with 3,160 ha (~ 7,800 acres) of forest. The climate is continental with short cool summers and long cold winters featuring constant snowpack averaging 1.5 m. The average air temperature is 19°C In July and − 9°C in January and annual precipitation averages 140 cm (Campbell et al. 2021). Soils are dominated by Spodosols (Haplorthods) of various drainage classes formed from glacial till, with pH values ranging from 3.4 to 3.8 in the surface organic horizons (Johnson et al. 2000). The forest floor organic horizons are typically 5–9 cm in thickness and are comprised of the undecomposed litter layer (Oi), a partially decomposed fibrous layer with a noticeable root mat (Oe), and a highly decomposed humic layer (Oa/A). The mineral soils are fairly shallow with high amounts of coarse fragments and loamy sand to sandy loam texture (Johnson et al. 2000).

Sample collection has taken place every July since 1994 at the HBEF for analysis of soil NO₃^- and NH₄⁺ concentrations, microbial biomass C and N content, microbial respiration, potential net N mineralization and nitrification rates, and denitrification potential in the Bear Brook Watershed (a reference area located just west of the biogeochemical reference watershed (Watershed 6) (west of Watershed 6). Soil samples were collected at four different elevations (525, 585, 685, and 775 m above sea level). Sampling was conducted at least a week following rainfall events to avoid the effects of re-wetting dry soils on microbial activity. In 1994 and 1995, only the first two forest floor soil horizons (Oie and Oa/A) were sampled. Beginning in 1996 the Oie, Oa/A horizons and the upper 10 cm of mineral soil were sampled. Samples were split into three layers in the field and each horizon was placed into a sample bag. Soil samples were placed in a cooler and transported to the Cary Institute of Ecosystem Studies in Millbrook, New York, USA and stored at 4°C until analyses, which were completed within 10 days of sampling. Analysis of soil pH of these samples began in 2000.

In mid-October 1999, 55 tons of pelletized Ca in the form of wollastonite (CaSiO₃) were added to the 11.8 ha watershed 1 to replenish calcium estimated to have been leached from the ecosystem by several decades of acid deposition. In 1998, the sampling routine for microbial biomass and activity was extended to four matching elevations on this watershed.

Composite soil samples were mixed by hand, and roots and stones over 2 mm were removed. Soil moisture content was determined by drying at 60^o C for 48 h (McInnes et al. 1994). Amounts of inorganic N (NO₃^- and NH₄⁺) in soil were determined by extraction with 2 M KCl followed by colorimetric analysis using a Flow Injection Analyzer. Soil pH was measured with a glass electrode in a 1:2 (Oa/A, mineral soil) or 1:4 (Oie) soil:water slurry.

Field-moist soils were used for microbial biomass, respiration, N mineralization, and denitrification potential analyses. The chloroform-fumigation-incubation method (Jenkinson & Powlson 1976) was used to determine soil microbial biomass. A 20-g soil sample was fumigated for 24 hours with distilled chloroform in a humid vacuum desiccator, and then it was inoculated with 0.5 g of fresh soil and placed in 1-L air-tight glass jars and incubated for 10 days at room temperature. In separate jars, controls composed of 20.5 g of soil were incubated simultaneously alongside the fumigated samples. Gas samples collected at the end of the incubations were analyzed for CO₂ content by gas chromatography. Inorganic N (NO₃^- and NH₄⁺) was extracted from fresh samples before the incubations and from jars at the end of the incubation as described above.

Microbial biomass C was calculated as B_c=F_c/0.41, where B_c is microbial biomass C, F_c represents the C CO₂-C produced over the 10-day incubation of flush from fumigated soils, and 0.41 is a correction factor that accounts for the extraction efficiency of the method for microbial biomass C. Estimates of microbial biomass N were not corrected and are determined as the inorganic N produced over the 10-day incubation of fumigated soils.

The production of CO₂ in the unfumigated incubation serves as an estimate of microbial respiration, the accumulation of inorganic N in these incubations is a measure of potential net N mineralization. The accumulation of NO₃^- in the unfumigated incubation is a measure of potential net nitrification.

Denitrification potential was measured using a denitrification enzyme activity (DEA) assay that involves quantifying the production of nitrous oxide (N₂O) from soils treated with NO₃^- (100 mg N/kg), glucose (500 mg/kg), chloramphenicol (125 mg/kg) and acetylene (C₂H₂, 10 kPa) and incubated under anaerobic conditions for 90 minutes (Groffman et al. 1999; Smith & Tiedje 1979). Gas samples taken at 30 and 90 minutes were analyzed for nitrous oxide by gas chromatography. The denitrification enzyme activity was then calculated as the accumulation of nitrous oxide between 30 and 90 minutes.

Data were analyzed using R v. 1.1.3.1093 (R Core Team 2013). Linear and polynomial regression models were run using pH or time as the independent variable and pH and microbial biomass and activity parameters as the dependent variable. Models were considered to be statistically significant at p < 0.05. Specific p-values form both linear and polynomial regressions are presented in Table 1.

Table 1

Regression statistics for linear and polynomial relationships between microbial biomass and activity variables and soil pH over all samples as well as stratified by reference versus calcium treated watershed and by soil horizon. Significant relationships are shown in bold, *p < 0.05, **p < 0.01, ***p < 0.001.
	Linear	Polynomial	Linear	Polynomial	Linear	Polynomial	Linear	Polynomial
	p-value
	Microbial biomass C		Microbial biomass N		Microbial Respiration		Nitrate
All data	0.33	0.32	0.25	0.39	0.61	0.61	< 0.001***	< 0.001***
Reference	0.78	0.77	< 0.05*	< 0.01**	0.81	0.74	< 0.001***	< 0.001***
Treated	0.30	0.29	0.64	0.56	0.36	0.38	0.06	0.14
Oi/Oe	< 0.05*	< 0.05 *	< 0.001***	< 0.001***	0.06	< 0.05*	0.01**	0.01**
Reference	0.34	0.26	0.12	0.17	0.11	0.08	< 0.01**	< 0.01**
Treated	0.36	0.28	< 0.05*	< 0.01**	0.37	0.31	0.05	0.09
Oa/A	0.69	0.73	0.37	0.54	0.53	0.60	< 0.001***	< 0.001***
Reference	< 0.001***	< 0.001***	0.47	0.68	< 0.001***	< 0.001***	< 0.01**	< 0.01 **
Treated	0.358	0.330	0.0838	0.0844	0.727	0.702	0.433	0.319
Mineral	0.209	0.2063	< 0.05*	< 0.05*	0.36	0.33	< 0.001***	< 0.001***
Reference	0.76	0.72	0.15	0.15	0.06	0.07	0.22	0.26
Treated	0.29	0.26	0.52	0.47	0.38	0.34	< 0.05 *	< 0.05 *

Table 1

continued
	Linear	Polynomial	Linear	Polynomial	Linear	Polynomial	Linear	Polynomial
	p-value
	Denitrification potential		Ammonium		Mineralization		Nitrification
All data	0.85	0.74	0.07	0.12	0.27	0.22	0.96	0.76
Reference	0.18	0.09	0.06	< 0.05*	< 0.001***	< 0.001***	< 0.01**	< 0.001***
Treated	0.62	0.63	0.48	0.38	0.61	0.51	< 0.01**	< 0.01**
Oi/Oe	< 0.05*	< 0.05*	0.55	0.30	< 0.01**	< 0.001***	0.88	0.71
Reference	0.07	0.06	0.23	0.20	< 0.001***	< 0.001***	0.09	0.08
Treated	0.20	0.18	0.99	0.81	0.17	0.14	< 0.05*	0.05
Oa/A	0.44	0.44	< 0.01**	< 0.01**	0.15	0.08	< 0.01**	< 0.01**
Reference	0.83	0.68	< 0.01**	< 0.05*	0.05	< 0.05*	< 0.05*	< 0.05*
Treated	0.40	0.46	0.75	0.74	0.51	0.45	0.97	0.86
Mineral soil	0.09	0.09	< 0.05*	0.06	0.17	0.12	< 0.05*	< 0.05*
Reference	0.16	0.17	< 0.01**	< 0.01**	0.22	0.16	0.16	0.14
Treated	< 0.05*	< 0.05*	0.41	0.52	0.80	0.73	0.71	0.64

Regression statistics for both linear and polynomial relationships between microbial biomass and activity variables and soil pH are presented in Table 1. The largest number of significant relationships were observed in the Oi/Oe horizon (7 out of 8 relationships examined). Of all the response variables, nitrate had the highest number of significant relationships (27 out of 48 relationships examined). The most significant correlations were with microbial biomass N in the Oi/Oe horizon, and nitrate in the Oa/A horizon and mineral horizons. Peak values for the majority of curvilinear relationships occurred in the 2008–2011 time frame.

Soil pH significantly increased over time in the Oa/A and mineral soil horizons in the Bear Brook Reference Watershed (Fig. 2a) and significantly decreased over time in the Oi/Oe and Oa/A horizons in the calcium-treated watershed 1 (Fig. 2b). There was a positive, but non-significant trend in the mineral soil in watershed 1.

Microbial biomass C showed a significant curvilinear relationship with pH in the Oie horizon with peak values at pH ~ 4.25 and lower values at higher and lower pH (Fig. 3a). In the reference watershed the peak values occurred in 2005–2006. For the calcium-treated watershed, the peak of the curvilinear relationship for microbial biomass C occurred in 2008–2009. There were no clear relationships between microbial biomass C and pH in the Oa/A or mineral soil horizons.

Microbial respiration also showed a significant curvilinear relationship with pH in the Oie horizon with peak values at pH ~ 4.5, and lower values at higher and lower pH (Fig. 3b). The curvilinear peak occurred in 2005–2006 in the reference watershed and in 2009–2010 for the calcium-treated watershed. There were no significant relationships between microbial respiration and pH in the Oa/A or mineral soil horizons.

Microbial biomass N exhibited significant curvilinear relationships in both the Oie and mineral soil horizons. The mineral horizon showed peak values at pH ~ 4.25 and the Oie horizon showed peak values at pH ~ 4 (Fig. 4a). No relationship between microbial biomass N and pH was found in the Oa/A horizon. In the reference watershed, peak values occurred in 2009–2010 in the Oi/Oe horizon and in 2008–2009 in the Oa/A soil horizon. In the calcium-treated watershed, peak values occurred in 2009–2010 in the Oi/Oe horizon and in 2010 in the mineral soil horizon.

Significant curvilinear relationships were observed for potential net N mineralization with pH in the Oie horizon with peak values at pH ~ 4.25 (Fig. 4b). There was no relationship between potential net N mineralization and pH in the Oa/A or mineral soil horizons. The curvilinear peak for potential net mineralization occurred in 2009–2010 for the reference watershed and in 2010–2011 in the calcium-treated watershed.

Potential net nitrification exhibited significant curvilinear relationships with pH in the mineral soil and Oa/A horizons. The mineral soil horizon showed peak values at pH ~ 4.25 and the Oa/A horizon showed peak values at pH ~ 3.8 (Fig. 4c). No relationship was evident between potential net nitrification and pH in the Oie horizon. In the reference watershed, peak values occurred in 2009, while in the calcium-treated watershed peak values occurred in 2009–2010.

Denitrification potential demonstrated significant curvilinear relationships with pH in the Oie horizon, with peak values at pH ~ 4.25 (Fig. 4d). The peak values occurred in 2009–2010.

Ammonium showed significant curvilinear relationships with pH in the Oa/A horizon with peak values at pH ~ 4. No relationship was found between ammonium and pH in the Oie horizon (Fig. 5a). Peak values occurred in 2009–2010.

Nitrate was the only variable to show significant curvilinear relationships in each soil horizon – mineral, Oa/A, and Oie. The mineral horizon had peak values at pH ~ 4 .25, the Oa/A horizon had peak values at pH ~ 3.8, and the Oie horizon had peak values at pH ~ 4.25 (Fig. 5b). In the reference watershed, peak values occurred in 2009–2010. In the calcium-treated watershed, peak values occurred in 2006–2007 in the Oi/Oe horizon, 2008–2009 in the Oa/A horizon, and 2009 in the mineral soil horizon.

Our results are surprising and raise questions about fundamental relationships between soil pH and biological processes. While we expected strong positive relationships between soil pH and microbial biomass and activity, we instead observed significant curvilinear relationships with peaks at unexpectedly low values, between pH ~ 3.5 and ~ 4.5. These relationships are based on 20 years of data that span reference and Ca-treated watersheds and organic and mineral soil horizons.

Microbial biomass C is a key functional pool for C and N cycling in organic detritus since it represents the active biological component of soil organic matter. Microbial respiration is a more dynamic, short-term indicator of soil biological activity. The fact that there were many significant relationships between these variables and pH in the Oie horizon, which is the most biologically active zone of the soil profile, suggests that soil pH is a key controller of biological activity at this northern forest site, but the nature of the controls is unexpected. The curvilinear relationships observed may suggest that the soil communities are adapted to the inherently low pH at the site. Addition of calcium to Watershed 1 experimentally increased the pH but clearly did not shift this adaptation to a higher pH. The curvilinear relationships contrast with our expectations and other results from Watershed 1 (Sridevi et al. 2012) showing that decreased soil calcium was correlated with reduced microbial biomass and C sequestration of forest trees.

Our results also show potential adaptation of N cycling processes to low pH in Hubbard Brook soils. We observed significant curvilinear relationships for potential net N mineralization in the Oa/A horizon and for potential net nitrification in the mineral and Oa/A horizons. The most consistent variable was the soil nitrate pool, which was the sole variable that was significantly correlated with pH in all three horizons.

An alternative explanation for the curvilinear relationships observed centers on plant: microbial interactions. Increases in pH could stimulate microbial biomass and activity to a certain level at which plants begin to respond and limit the ability of microbes to respond further. Similar dynamics have been observed in other studies (discussed below) at HBEF. Thus, the complexities of plant: microbial interactions may give the appearance that microbes are adapted to/optimized for the low values while in reality, their ability to respond to increases in pH is being inhibited by plants.

Complex effects of soil pH at a naturally acidic watershed

Forest soils at HBEF are naturally acidic, with low base cation supply due to low rates of mineral weathering and low exchangeable base cation concentrations in the O horizon (Lawrence et al. 1995; Likens et al. 1996). The low levels of exchangeable Ca in mineral soils are due to low rates of mineral weathering and are associated with elevated concentrations of Al in the O horizon and streams at the site (Likens et al. 1998; Palmer et al. 2004).

The addition of calcium silicate to watershed 1 in 1999 rapidly (by 2000) increased exchangeable Ca, soil pH, cation exchange capacity, and base saturation and decreased exchangeable acidity in organic soil horizons (Cho et al. 2010; Cho et al. 2012; Johnson et al. 2014). In the mineral soil layer, short-term increases of exchangeable Ca, soil pH, cation exchange capacity, and base saturation were much smaller and exchangeable acidity exhibited small decreases to no change.

Over the last 20 years, pH has declined in the Oa/A and Oie horizons of the treated watershed, consistent with processing and loss of the added calcium (Johnson et al. 2014). Soil pH has increased in the mineral and Oa/A horizons of the reference watershed, consistent with deacidification and reduction of acid deposition (Likens et al. 2021). Our sampling sites, which range across elevation gradients in both reference and treated watersheds, thus encompass a wide range of pH conditions for the northern hardwood forests represented by the HBEF.

Several studies at HBEF have found that soil pH controls on soil biological processes are more complex that might be expected from consideration of the chemical dynamics of base cations, acidity and aluminum. Fiorentino et al. (Fiorentino et al. 2003) investigated the effects of the calcium addition to Watershed 1 on P pools and cycling and N cycle processes. While there were no significant differences in soil solution or fine root P concentration, microbial biomass P, microbial C to P ratios, available organic and inorganic P fractions were lower in the Oe horizon of the calcium treated watershed than the reference watershed. They concluded that the soil pH changes associated with the Ca addition had increased the rates of P cycling in the forest floor horizons, with enhanced flow of P to plants at the expense of microbes (Fiorentino et al. 2003).

Several studies of the watershed 1 experiment have reported complex effects of calcium additions on N cycling processes (Battles et al. 2014; Juice et al. 2006; Rosi-Marshall et al. 2016). Groffman et al. (Groffman et al. 2006) observed declines in the N content of the microbial biomass, potential net and gross N mineralization rates, and soil inorganic N pools in the Oie horizon of the treated watershed. These results clearly suggested that the calcium addition did not stimulate microbial N cycling, and the authors suggested that plants had responded more dynamically to the calcium addition than microbes, inhibiting the ability of the microbes to respond to the increases in calcium and soil pH. This idea was supported by soil mesocosm studies without plants where calcium additions resulted in marked increases in soil microbial N cycling process rates (Groffman & Fisk 2011a; Groffman & Fisk 2011b).

These results for P and N response to the calcium addition to Watershed 1 lead to the idea that plant:microbial interactions underlie the surprising curvilinear relationships between microbial biomass and activity and soil pH observed in this study. It is logical that increases in pH caused either by natural variation or calcium addition should stimulate microbial biomass and activity. However, if the increases in pH reach a certain level where plants begin to respond, the flow of C, N, and P to microbes could be reduced by plant uptake, resulting in apparent declines in microbial biomass and activity at higher pH. Thus, the complexities of plant: microbial interactions may make it appear that the microbes are adapted to/optimized for low pH while in reality their ability to respond to increases in pH is inhibited by plants.

The previous research at the HBEF highlight that there are important temporal trends in microbial biomass and activity that are not addressed in our analysis of pH controls here. Key trends include marked declines in acid and N deposition, which combined with a lengthening growing season and increases in atmosphere CO₂ concentrations have led to a marked reduction in N availability (oligotrophication) at the site (Groffman et al. 2018). These temporal trends are being monitored and analyzed in other studies.

The importance of microbial community composition

Much of the effects of soil pH and acid deposition on ecosystem processes are mediated via effects on microbial community composition. Despite advances in technology and understanding, soil biota have proven to be very difficult to study and characterize. A meta-analysis of over 1300 published datapoints, found that variation in microbial biomass is predictable across biomes with microbial biomass C representing 0.6–1.1% of soil organic C and 1–20% of total plant biomass C (Fierer et al. 2009). This analysis also found that bacterial community composition and fungal: bacterial gene ratios were strongly driven by soil pH and soil C:N ratios (Fierer et al. 2009). A continental scale analysis also found that soil pH was a strong predictor of bacterial community structure (Lauber et al. 2009). This analysis found significant correlations between soil pH and the abundance of acidobacteria, actinobacteria, and bacterioidetes. A study that utilized marker gene sequencing to characterize soil bacterial communities found that 53% of microbes had predictable habitat preferences and strong links between microbial community composition and plant productivity (Delgado-Baquerizo et al. 2018). These studies suggest that there are predictable patterns in soil microbial communities that should be useful for assessing and predicting the effects of environmental changes on these communities.

Previous studies (Sridevi et al. 2012) have found significant differences in bacterial populations in the reference and calcium-treated watersheds at HBEF. Acidobacteriaceae, Comamonadaceae, and Pseudomonadaceae were lower in the Ca-amended soils, while Flavobacteriaceae and Geobacteraceae showed an opposite pattern. Ammonia-oxidizing Nitrosomonadaceae were lower in organic horizon soils of the Ca-amended watershed and an opposite pattern was observed in the mineral soil. The decreases in Acidobacteriacease and Nitrosomonadaceae are consistent with the concept that microbes respond to increases in pH as expected, but that plant domination of the N cycle constrains this response. The presence of Acidobacteria in HBEF soils is important because these bacterial taxa play a major role in the function of the forest soil ecosystem and are viable in acidic, nutrient-limited environments such as forests at HBEF. The presence of specific bacteria can be indicative of how pH impacts the bio-availability of soil nutrients and can potentially be a limiting agent in nutrient pathways.

A metagenomics survey in the reference watershed at the Hubbard Brook focused on the genes involved in denitrification, a process that responds to N enrichment, found evidence that acidity affected the communities that carry out this process (Roco et al. 2019). It has long been recognized that low soil pH inhibits expression of the genes responsible for the last step of the denitrification process (Payne 1981; Zumft 1997). The more recent study showed a series of adaptations to acidity, including removal of toxic intermediates in the denitrification process (nitric oxide) that suggest significant capacity for adaptation to highly acid conditions in HBEF soils (Roco et al. 2019). Further evidence for adaptive capacity was found under experimental soil warming and increased soil freeze/thaw cycles which suppressed bacterial taxa with the genetic potential for the final steps of denitrification (Garcia et al. 2020). This analysis suggested that a key to adaptation to new conditions in HBEF soils is the emergence of taxa that trade-off growth for stress tolerance traits. Another study found that winter snowpack decline associated with global warming increased bacterial richness and phylogenetic diversity (2016). However, any effects of changing soil climate conditions may have been mediated by effects on root growth, which is impaired by soil freezing associated with the snowpack decline (Sorensen et al. 2019).

Much of the microbial community response to changing environmental conditions may be focused in “microzones” or “microbial hotspots” of higher pH in the generally acidic soil matrix at the HBEF (Kuzyakov & Blagodatskaya 2015). These microzones could be associated with Ca-containing mineral particles, or plant roots and could serve as refugia for pH sensitive microbes. Detailed analysis of hotspot characteristics would be required to evaluate this idea.

Regulation of microbial biomass and activity in forest soils

It is undeniable that pH has a strong effect on soil microbial biomass, activity and community composition. However, our results suggest that these effects are strongly mediated by plants and that evaluations of responses of the soil microbial community to environmental change must consider plant: microbial interactions. These interactions greatly complicate assessment and prediction of how ecosystem C and N cycling will respond to environmental change, which will depend on the varying resistance or resilience of taxa within groups or “functional guilds” of microbes that cycle C and N. At the same time, plant responses to environmental change can be more dynamic than environmental responses, as they have access to both aboveground and belowground resources and the ability to shift allocation of these resources in response to environmental stress. Thus, our initial hypothesis that it is useful to view soil pH as a “master variable” controlling belowground biogeochemical processes may be only partially correct. Rather, the effects of pH, and other environmental variables must be evaluated in the context of whole-ecosystem, multi-component interactions and dynamics.

Acknowledgment

This research was supported by a grant from the U.S. National Science Foundation Long-Term Ecological Research Program (Award # 1637685). We thank Lisa Martel for help with field, laboratory and data analysis and Clare Kohler for help with statistical analysis and graphing. Data are available at: https://portal.edirepository.org/nis/mapbrowse?scope=knb-lter-hbr&identifier=252.

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Surprising Relationships Between Soil pH and Microbial Biomass and Activity in a Northern Hardwood Forest

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Abstract

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Introduction

Materials And Methods

Results

Discussion

Complex effects of soil pH at a naturally acidic watershed

The importance of microbial community composition

Regulation of microbial biomass and activity in forest soils

Declarations

References

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