Previous studies have attempted to link foliar resorption of nitrogen and phosphorus to their respective availabilities in soil, with mixed results. Based on resource optimization theory, we ...hypothesized that the foliar resorption of one element could be driven by the availability of another element. We tested various measures of soil N and P as predictors of N and P resorption in six tree species in 18 plots across six stands at the Bartlett Experimental Forest, New Hampshire, USA. Phosphorus resorption efficiency (
P
< 0.01) and proficiency (
P
= 0.01) increased with soil N content to 30 cm depth, suggesting that trees conserve P based on the availability of soil N. Phosphorus resorption also increased with soil P content, which is difficult to explain based on single-element limitation, but follows from the correlation between soil N and soil P. The expected single-element relationships were evident only in the O horizon: P resorption was high where resin-available P was low in the Oe (
P
< 0.01 for efficiency,
P
< 0.001 for proficiency) and N resorption was high where potential N mineralization in the Oa was low (
P
< 0.01 for efficiency and 0.11 for proficiency). Since leaf litter is a principal source of N and P to the O horizon, low nutrient availability there could be a result rather than a cause of high resorption. The striking effect of soil N content on foliar P resorption is the first evidence of multiple-element control on nutrient resorption to be reported from an unmanipulated ecosystem.
Functional balance theory predicts that plants will allocate less carbon belowground when the availability of nutrients is elevated. We tested this prediction in two successional northern hardwood ...forest stands by quantifying fine root biomass and growth after 5–7 years of treatment in a nitrogen (N) x phosphorus (P) factorial addition experiment. We quantified root responses at two different levels of treatment: the whole-plot scale fertilization and small-patch scale fertilization of ingrowth cores. Fine root biomass was higher in plots receiving P, and fine root growth was highest in plots receiving both N and P. Thus, belowground productivity did not decrease in response to long-term addition of nutrients. We did not find conclusive evidence that elevated availability of one nutrient at the plot scale induced foraging for the other nutrient at the core scale, or that foraging for nutrients at the core scale responded to addition of limiting nutrients. Our observations suggest NP co-limitation of fine root growth and indicate complex interactions of N and P affecting aboveground and belowground production in early successional northern hardwood forest ecosystems.
Evaluating, and possibly ameliorating, the effects of base cation depletion in forest soils caused by acid deposition is an important topic in the northeastern United States. We added 850 kg Ca ha-¹ ...as wollastonite (CaSiO₃) to an 11.8-ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA, in fall 1999 to replace calcium (Ca) leached from the ecosystem by acid deposition over the past 6 decades. Soil microbial biomass carbon (C) and nitrogen (N) concentrations, gross and potential net N mineralization and nitrification rates, soil solution and stream chemistry, soil:atmosphere trace gas (CO₂, N₂O, CH₄) fluxes, and foliar N concentrations have been monitored in the treated watershed and in reference areas at the HBEF before and since the Ca addition. We expected that rates of microbial C and N cycle processes would increase in response to the treatment. By 2000, soil pH was increased by a full unit in the Oie soil horizon, and by 2002 it was increased by nearly 0.5 units in the Oa soil horizon. However, there were 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. Stream, soil solution, and foliar concentrations of N showed no response to treatment. The lack of stimulation of N cycling by Ca addition suggests that microbes may not be stimulated by increased pH and Ca levels in the naturally acidic soils at the HBEF, or that other factors (for example, phosphorus, or Ca binding of labile organic matter) may constrain the capacity of microbes to respond to increased pH in the treated watershed. Possible fates for the approximately 10 kg N ha-¹ decline in microbial and soil inorganic pools include components of the plant community that we did not measure (for example, seedlings, understory shrubs), increased fluxes of N₂ and/or N storage in soil organic matter. These results raise questions about the factors regulating microbial biomass and activity in northern hardwood forests that should be considered in the context of proposals to mitigate the depletion of nutrient cations in soil.
Exotic earthworms can profoundly alter soil carbon (C) and nitrogen (N) dynamics in northern temperate forests, but the mechanisms explaining these responses are not well understood. We compared the ...soil microbial community (SMC) composition (measured as PLFAs) and enzyme activity between paired earthworm-invaded and earthworm-free plots in northern hardwood forests of New York, USA. We hypothesized that differences in SMCs and enzyme activity between plots would correspond with differences in soil C content and C:N ratios. Relative abundance of several bacterial (mostly gram-positive) PLFAs was higher and that of two fungal PLFAs was lower in earthworm compared to reference plots, largely because of earthworm incorporation of the organic horizon into mineral soil. In surface mineral soil earthworms increased arbuscular mycorrhizal fungi (AMF) and gram-positive bacterial PLFAs, and decreased fungal (mostly saprotrophic) and several bacterial (gram-negative and non-specific) PLFAs. Earthworms also increased the activities of cellulolytic relative to lignolytic enzymes in surface mineral soil, and the relationships between enzyme activities and components of the SMC suggest a substrate-mediated effect on the SMC and its metabolism of C. A highly significant relationship between components of the SMC and soil C:N also suggests that earthworms reduce soil C:N through functional and compositional shifts in the SMC. Finally, changes in AMF abundances were linked to phosphatase activity, suggesting that earthworms do not necessarily inhibit P-acquisition by AMF-associated plants in our study system. We conclude that the combined influence of earthworm-related changes in physical structure, accessibility and chemistry of organic matter, and relative abundance of certain groups of fungi and bacteria promote C metabolism, in particular by increasing the activities of cellulolytic vs. lignolytic enzymes.
•Earthworms increased abundance of many bacterial relative to most fungal indicators.•Abundance of arbuscular mycorrhizal fungi increased by earthworms.•Earthworms increased activities of cellulolytic compared to lignolytic enzymes.•Changes in enzyme activities contribute to more complete carbon metabolism.•Soil microbial community composition correlated with enzyme activity and soil C:N.
Forest productivity on glacially derived soils with weatherable phosphorus (P) is expected to be limited by nitrogen (N), according to theories of long-term ecosystem development. However, recent ...studies and model simulations based on resource optimization theory indicate that productivity can be co-limited by N and P. We conducted a full factorial N × P fertilization experiment in 13 northern hardwood forest stands of three age classes in central New Hampshire, USA, to test the hypothesis that forest productivity is co-limited by N and P. We also asked whether the response of productivity to N and P addition differs among species and whether differential species responses contribute to community-level co-limitation. Plots in each stand were fertilized with 30 kg N·ha−1·yr−1, 10 kg P·ha−1·yr−1, N + P, or neither nutrient (control) for four growing seasons. The productivity response to treatments was assessed using per-tree annual relative basal area increment (RBAI) as an index of growth. RBAI responded significantly to P (P = 0.02) but not to N (P = 0.73). However, evidence for P limitation was not uniform among stands. RBAI responded to P fertilization in mid-age (P = 0.02) and mature (P = 0.07) stands, each taken as a group, but was greatest in N-fertilized plots of two stands in these age classes, and there was no significant effect of P in the young stands. Both white birch (Betula papyrifera Marsh.) and beech (Fagus grandifolia Ehrh.) responded significantly to P; no species responded significantly to N. We did not find evidence for N and P co-limitation of tree growth. The response to N + P did not differ from that to P alone, and there was no significant N × P interaction (P = 0.68). Our P limitation results support neither the N limitation prediction of ecosystem theory nor the N and P co-limitation prediction of resource optimization theory, but could be a consequence of long-term anthropogenic N deposition in these forests. Inconsistencies in response to P suggest that successional status and variation in site conditions influence patterns of nutrient limitation and recycling across the northern hardwood forest landscape.
While much research over the past 30 years has focused on the deleterious effects of excess N on forests and associated aquatic ecosystems, recent declines in atmospheric N deposition and unexplained ...declines in N export from these ecosystems have raised new concerns about N oligotrophication, limitations of forest productivity, and the capacity for forests to respond dynamically to disturbance and environmental change. Here we show multiple data streams from long-term ecological research at the Hubbard Brook Experimental Forest in New Hampshire, USA suggesting that N oligotrophication in forest soils is driven by increased carbon flow from the atmosphere through soils that stimulates microbial immobilization of N and decreases available N for plants. Decreased available N in soils can result in increased N resorption by trees, which reduces litterfall N input to soils, further limiting available N supply and leading to further declines in soil N availability. Moreover, N oligotrophication has been likely exacerbated by changes in climate that increase the length of the growing season and decrease production of available N by mineralization during both winter and spring. These results suggest a need to re-evaluate the nature and extent of N cycling in temperate forests and assess how changing conditions will influence forest ecosystem response to multiple, dynamic stresses of global environmental change.
Resource allocation theory posits that increased soil nutrient availability results in decreased plant investment in nutrient acquisition. We evaluated this theory by quantifying fine root biomass ...and growth in a long term, nitrogen (N) × phosphorus (P) fertilization study in three mature northern hardwood forest stands where aboveground growth increased primarily in response to P addition. We did not detect a decline in fine root biomass or growth in response to either N or P. Instead, fine root growth increased in response to N, by 40% for length (
P
= 0.04 for the main effect of N in ANOVA), and by 36% for mass, relative to controls. Fine root mass growth was lower in response to N + P addition than predicted from the main effects of N and P (
P
= 0.01 for the interaction of N × P). The response of root growth to N availability did not result in detectable responses in fine root biomass (
P
= 0.61), which is consistent with increased root turnover with N addition. We propose that the differential growth response to fertilization between above- and belowground components is a mechanism by which trees enhance P acquisition in response to increasing N availability, illustrating how both elements may co-limit northern hardwood forest production.
Understanding the responses of terrestrial ecosystems to global change remains a major challenge of ecological research. We exploited a natural elevation gradient in a northern hardwood forest to ...determine how reductions in snow accumulation, expected with climate change, directly affect dynamics of soil winter frost, and indirectly soil microbial biomass and activity during the growing season. Soils from lower elevation plots, which accumulated less snow and experienced more soil temperature variability during the winter (and likely more freeze/thaw events), had less extractable inorganic nitrogen (N), lower rates of microbial N production via potential net N mineralization and nitrification, and higher potential microbial respiration during the growing season. Potential nitrate production rates during the growing season were particularly sensitive to changes in winter snow pack accumulation and winter soil temperature variability, especially in spring. Effects of elevation and winter conditions on N transformation rates differed from those on potential microbial respiration, suggesting that N‐related processes might respond differently to winter climate change in northern hardwood forests than C‐related processes.
Snow cover is projected to decline during the next century in many ecosystems that currently experience a seasonal snowpack. Because snow insulates soils from frigid winter air temperatures, soils ...are expected to become colder and experience more winter soil freezethaw cycles as snow cover continues to decline. Tree roots are adversely affected by snowpack reduction, but whether loss of snow will affect root-microbe interactions remains largely unknown. The objective of this study was to distinguish and attribute direct (e.g., winter snowand/or soil frost-mediated) vs. indirect (e.g., root-mediated) effects of winter climate change on microbial biomass, the potential activity of microbial exoenzymes, and net N mineralization and nitrification rates. Soil cores were incubated in situ in nylon mesh that either allowed roots to grow into the soil core (2 mm pore size) or excluded root ingrowth (50 µm pore size) for up to 29 months along a natural winter climate gradient at Hubbard Brook Experimental Forest, NH (USA). Microbial biomass did not differ among ingrowth or exclusion cores. Across sampling dates, the potential activities of cellobiohydrolase, phenol oxidase, and peroxidase, and net N mineralization rates were more strongly related to soil volumetric water content (P < 0.05; R² = 0.25-0.46) than to root biomass, snow or soil frost, or winter soil temperature (R² < 0.10). Root ingrowth was positively related to soil frost (P < 0.01; R² = 0.28), suggesting that trees compensate for overwinter root mortality caused by soil freezing by re-allocating resources towards root production. At the sites with the deepest snow cover, root ingrowth reduced nitrification rates by 30% (P < 0.01), showing that tree roots exert significant influence over nitrification, which declines with reduced snow cover. If soil freezing intensifies over time, then greater compensatory root growth may reduce nitrification rates directly via plantmicrobe N competition and indirectly through a negative feedback on soil moisture, resulting in lower N availability to trees in northern hardwood forests.
Allocation processes are central to understanding patterns of productivity and nutrient retention in forest ecosystems. Nutrient addition experiments support ideas of resource allocation theory in ...soil with, for example, the addition of nitrogen promoting the activity of phosphatase enzymes in many different ecosystems. However, empirical evidence is needed to understand whether similar mechanisms operate spatially or temporally to balance availability of nutrients over the long-term. Therefore we examined the general question of whether N availability can influence phosphorus availability through effects on phosphatase enzyme activity, across hardwood forests in the northeastern United States that differ in age and soil properties. Using resin-available P and net N mineralization as our indices of plant-available nutrients, we found that the availability of P increased in relation to that of N across sites, with no effect of forest age. Net N mineralization was a significant predictor of soil phosphatase activity, suggesting that allocation to enzyme activity is a mechanism that could contribute to coupling of N and P availability in successional as well as mature northern hardwoods. However, resin-available P was not correlated with phosphatase activity. Bicarbonate-extractable P, an index of potentially available P, was not related to other soil variables across the stands. While N may influence the activity of phosphatase enzymes, this interaction does not appear to drive availability of P across these hardwood forest ecosystems. Instead we suggest that it operates at smaller scales and over shorter time scales than processes mediating the broader-scale coupling of N and P availability.
•We examined soil N and P cycling processes in forests in the northeastern US.•Soil processes varied widely across forest stands but did not differ by stand age.•Phosphatase activity and resin-available P increased with net N mineralization potential.•Phosphatase activity did not contribute to broad patterns of P availability.•Availability of N may limit soil phosphatase activity across these ecosystems.
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