Microbial communities mediate every step of the soil nitrogen cycle, yet the structure and associated nitrogen cycle functions of soil microbial communities remain poorly studied in tropical forests. ...Moreover, tropical forest soils are often many meters deep, but most studies of microbial nitrogen cycling have focused exclusively on surface soils. The objective of our study was to evaluate changes in bacterial community structure and nitrogen functional genes with depth in soils developed on two contrasting geological parent materials and two forest types that occur at different elevations at the Luquillo Critical Zone Observatory in northeast Puerto Rico. We excavated three soil pits to 140 cm at four different sites representing the four soil × forest combinations (n = 12), and collected samples at ten-centimeter increments from the surface to 140 cm. We used bacterial 16S rRNA gene-DGGE (denaturant gradient gel electrophoresis) to fingerprint microbial community structures, and quantitative PCR to measure the abundance of five functional genes involved in various soil nitrogen transformations: nifH (nitrogen fixation), chiA (organic nitrogen decomposition), amoA (ammonia oxidation), nirS (nitrite reduction) and nosZ (nitrous oxide reduction). Multivariate analyses of DGGE fingerprinting patterns revealed differences in bacterial community structure across the four soil × forest types that were strongly correlated with soil pH (r = 0.69, P < 0.01) and nutrient stoichiometry (r2 ≥ 0.36, P < 0.05). Across all soil and forest types, nitrogen functional genes declined significantly with soil depth (P < 0.001). Denitrification genes (nirS and nosZ) accounted for the largest proportion of measured nitrogen functional genes. Measured nitrogen functional genes were positively correlated with soil carbon, nitrogen and phosphorus concentrations (P < 0.001) and all genes except amoA were significantly more abundant in the Inceptisol soil type compared with the Oxisol soil type (P < 0.03). Greater abundances and a stronger vertical zonation of nitrogen functional genes in Inceptisols suggest more dynamic nitrogen transformation processes in this soil type. As the first study to examine bacterial nitrogen functional gene abundances below the surface 20 cm in tropical forest soils, our work provides insight into how pedogenically-driven vertical gradients control the nitrogen-cycling capacity of soil microbial communities. While previous studies have shown evidence for redox-driven hotspots in tropical nitrogen cycling on a watershed scale, our study corroborates this finding on a molecular scale.
•Studied bacterial community structure and N-cycle genes in tropical forest soils.•Community structure is distinct across two soil types and two forest types.•N-cycle genes are more abundant in younger, sandier soil type, especially at depth.•Redox conditions may mediate subsoil N-cycling capacity of bacterial community.
Decomposition of soil organic matter (SOM) is mediated by microbial extracellular hydrolytic enzymes (EHEs). Thus, given the large amount of carbon (C) stored as SOM, it is imperative to understand ...how microbial EHEs will respond to global change (and warming in particular) to better predict the links between SOM and the global C cycle. Here, we measured the Michaelis–Menten kinetics maximal rate of velocity (Vmax) and half‐saturation constant (Km) of five hydrolytic enzymes involved in SOM degradation (cellobiohydrolase, β‐glucosidase, β‐xylosidase, α‐glucosidase, and N‐acetyl‐β‐d‐glucosaminidase) in five sites spanning a boreal forest to a tropical rainforest. We tested the specific hypothesis that enzymes from higher latitudes would show greater temperature sensitivities than those from lower latitudes. We then used our data to parameterize a mathematical model to test the relative roles of Vmax and Km temperature sensitivities in SOM decomposition. We found that both Vmax and Km were temperature sensitive, with Q10 values ranging from 1.53 to 2.27 for Vmax and 0.90 to 1.57 for Km. The Q10 values for the Km of the cellulose‐degrading enzyme β‐glucosidase showed a significant (P = 0.004) negative relationship with mean annual temperature, indicating that enzymes from cooler climates can indeed be more sensitive to temperature. Our model showed that Km temperature sensitivity can offset SOM losses due to Vmax temperature sensitivity, but the offset depends on the size of the SOM pool and the magnitude of Vmax. Overall, our results suggest that there is a local adaptation of microbial EHE kinetics to temperature and that this should be taken into account when making predictions about the responses of C cycling to global change.
Phosphatase enzymes play a key role cycling phosphorus from organic to plant-available pools, particularly in tropical soils where inorganic phosphorus is often limited. However, most studies of ...phosphatase activity have focused only on surface soils, despite the large quantities of carbon and nutrients stored in tropical subsoils. The goal of this study was to determine how acid phosphatase kinetic parameters change with depth across two parent materials (represented by Oxisols and an Inceptisols) and two distinct forests (lower and upper montane) at the Luquillo Critical Zone Observatory in northeast Puerto Rico. We collected samples from five soil pits at each of four soil × forest types, and measured apparent phosphatase kinetic parameters (AppVmax and AppKm) and soil nutrients at 0, 20, 50, 80, 110 and 140 cm depths. Across all sites, AppVmax declined 97% and AppKm declined 85% from the surface to 140 cm depth. The ratio of AppVmax to AppKm (i.e., Ka) did not change through the first meter of soil profiles but was significantly reduced by 50% 140 cm. Total carbon, nitrogen and extractable phosphorus all declined exponentially with depth. Carbon concentrations and AppVmax were both significantly greater in Oxisols compared with Inceptisols, and in the higher elevation montane forest compared to the lower elevation forest. The scaling relationship we observe between AppVmax and AppKm is common for environmental systems, although the degree of correlation in our study (R2 = 0.48) is unusually high, suggesting these parameters are both driven by changes in energy and nutrient availability along depth profiles. However, the consistency of Ka with depth indicates that overall catalytic capacity of phosphatase is maintained across a range of substrate concentrations. The larger variability in AppVmax compared with AppKm suggests microorganisms exert more control over phosphatase production than substrate availability. Our findings indicate that subsoil microbial communities are not metabolically dormant, but rather contribute to P-cycling at rates comparable to their surface counterparts. Further research on ecology of microorganisms in resource-limited tropical subsoils is warranted to better understand microbial contributions to biogeochemical cycles throughout tropical soil profiles.
•Measured changes in phosphatase kinetics with depth in two tropical forest soils.•AppVmax, AppKm, carbon and phosphorus concentrations declined with depth.•Catalytic capacity of phosphatase (Ka) is constant within the first meter of soils.•Vmax is more plastic than Km in response to changing soil resource availability.
Soil microbes produce extracellular enzymes that degrade carbon (C)‐containing polymers in soil organic matter. Because extracellular enzyme activities may be sensitive to both increased nitrogen (N) ...and temperature change, we measured the effect of long‐term N addition and short‐term temperature variation on enzyme kinetics in soils from hardwood forests at Bear Brook, Maine, and Fernow Forest, West Virginia. We determined the Vmax and Km parameters for five hydrolytic enzymes: α‐glucosidase, β‐glucosidase, β‐xylosidase, cellobiohydrolase, and N‐acetyl‐glucosaminidase. Temperature sensitivities of Vmax and Km were assessed within soil samples subjected to a range of temperatures. We hypothesized that (1) N additions would cause microbial C limitation, leading to higher enzyme Vmax values and lower Km values; and (2) both Vmax and Km would increase at higher temperatures. Finally, we tested whether or not temperature sensitivity of enzyme kinetics is mediated by N addition. Nitrogen addition significantly or marginally significantly increased Vmax values for all enzymes, particularly at Fernow. Nitrogen fertilization led to significantly lower Km values for all enzymes at Bear Brook, but variable Km responses at Fernow Forest. Both Vmax and Km were temperature sensitive, with Q10 values ranging from 1.64–2.27 for enzyme Vmax and 1.04–1.93 for enzyme Km. No enzyme showed a significant interaction between N and temperature sensitivity for Vmax, and only β‐xylosidase showed a significant interaction between N and temperature sensitivity for Km. Our study is the first to experimentally demonstrate a positive relationship between Km and temperature for soil enzymes. Higher temperature sensitivities for Vmax relative to Km imply that substrate degradation will increase with temperature. In addition, the Vmax and Km responses to N indicate greater substrate degradation under N addition. Our results suggest that increasing temperatures and N availability in forests of the northeastern US will lead to increased hydrolytic enzyme activity, despite the positive temperature sensitivity of Km.
Blockage of vasoactive intestinal peptide (VIP) receptors during early embryogenesis in the mouse has been shown to result in developmental delays in neonates, and social behavior deficits ...selectively in adult male offspring. Offspring of VIP deficient mothers (VIP +/−) also exhibited developmental delays, and reductions in maternal affiliation and play behavior. In the current study, comparisons among the offspring of VIP deficient mothers (VIP +/−) mated to VIP +/− males with the offspring of wild type (WT) mothers mated to VIP +/− males allowed assessment of the contributions of both maternal and offspring VIP genotype to general health measures, social behavior, fear conditioning, and spatial learning and memory in the water maze. These comparisons revealed few differences in general health among offspring of WT and VIP deficient mothers, and all offspring exhibited normal responses in fear conditioning and in the acquisition phase of spatial discrimination in the water maze. WT mothers produced offspring that were normal in all tests; the reduced VIP in their VIP +/− offspring apparently did not contribute to any defects in the measures under study. However, regardless of their own VIP genotype, all male offspring of VIP deficient mothers exhibited severe deficits in social approach behavior and reversal learning. The deficits in these behaviors in the female offspring of VIP deficient mothers were less severe than in their male littermates, and the extent of their impairment was related to their own VIP genotype. This study has shown that intrauterine conditions had a greater influence on behavioral outcome than did genetic inheritance. In addition, the greater prevalence of deficits in social behavior and the resistance to change seen in reversal learning in the male offspring of VIP deficient mothers indicate a potential usefulness of the VIP knockout mouse in furthering the understanding of neurodevelopmental disorders such as autism.
Tropical subsoils contain large reservoirs of carbon (C), most of which is stored in soil organic matter (SOM). Subsoil OM is thought to be particularly stable against microbial decomposition due to ...various mechanisms and its position in the soil profile, potentially representing a long-term C sink. However, few experiments have explicitly investigated SOM stability and microbial activity across several orders of magnitude of soil C concentrations as a function of soil depth. The objective of this study was to evaluate the biological stability of SOM in the upper 1.4 m of tropical forest soil profiles. We did so by measuring CO2 evolution during a 90-day laboratory incubation experiment on a sample set that was previously characterized for C and nutrient concentrations and microbial biomass. We concurrently measured the energy content of SOM using differential scanning calorimetry (DSC) as an index of the energy available for microbial metabolism, with the hypothesis that the biological stability of SOM would be inversely related to the energy contained within it. Cumulative CO2 evolution, mean respiration rates, and the energy density of SOM (energy released during combustion normalized to soil C) all declined with soil depth (P < 0.01). Biological indices of C stability were well correlated with measures of SOM energy. There was no change in the mean respiration rate as a function of depth when normalized to soil C, and a trend toward increased respiration per-unit microbial biomass (P = 0.07). While reduced microbial respiration in subsoils suggests an increase in the biological stability of SOM, we suggest this is driven principally by concurrent declines in energy availability as measured by DSC and the size of the microbial biomass pool. On a per-unit biomass basis, subsoil OM may be as prone to decomposition and destabilization as surface SOM.
•Examined C stability in 1.4 m deep profiles of tropical soils from Puerto Rico.•Measured biological stability by incubation and energy content by thermal analysis.•Cumulative respiration and energy availability declined exponentially with depth.•Respiration rates normalized to microbial biomass trended positively with depth.•Low energy availability responsible for increased C stability with depth.
Pharmacological studies indicate that vasoactive intestinal peptide (VIP) may be necessary for normal embryonic development in the mouse. For example, VIP antagonist treatment before embryonic day 11 ...resulted in developmental delays, growth restriction, modified adult brain chemistry and reduced social behavior. Here, developmental milestones, growth, and social behaviors of neonates of VIP‐deficient mothers (VIP +/−) mated to VIP +/− males were compared with the offspring of wild type mothers (VIP +/+) mated to VIP +/+ and +/− males, to assess the contributions of both maternal and offspring VIP genotype. Regardless of their own genotype, all offsprings of VIP‐deficient mothers exhibited developmental delays. No delays were seen in the offspring of wild type mothers, regardless of their own genotype. Body weights were significantly reduced in offspring of VIP‐deficient mothers, with VIP null (−/−) the most affected. Regardless of genotype, all offspring of VIP‐deficient mothers expressed reduced maternal affiliation compared with wild type offspring of wild type mothers; +/− offspring of wild type mothers did not differ in maternal affiliation from their wild type littermates. Play behavior was significantly reduced in all offsprings of VIP‐deficient mothers. Maternal behavior did not differ between wild type and VIP‐deficient mothers, and cross‐fostering of litters did not change offspring development, indicating that offspring deficits were induced prenatally. This study illustrated that the VIP status of a pregnant mouse had a greater influence on the growth, development and behavior of her offspring than the VIP genotype of the offspring themselves. Deficiencies were apparent in +/+, +/− and −/− offspring born to VIP‐deficient mothers; no deficiencies were apparent in +/− offspring born to normal mothers. These results underscore the significant contribution of the uterine environment to normal development and indicate a potential usefulness of the VIP knockout mouse in furthering the understanding of neurodevelopmental disorders with social behavior deficits such as autism.
Abstract Decomposition of soil organic matter ( SOM ) is mediated by microbial extracellular hydrolytic enzymes ( EHE s). Thus, given the large amount of carbon ( C ) stored as SOM , it is imperative ...to understand how microbial EHE s will respond to global change (and warming in particular) to better predict the links between SOM and the global C cycle. Here, we measured the M ichaelis– M enten kinetics maximal rate of velocity ( V max ) and half‐saturation constant ( K m ) of five hydrolytic enzymes involved in SOM degradation (cellobiohydrolase, β‐glucosidase, β‐xylosidase, α‐glucosidase, and N ‐acetyl‐β‐ d ‐glucosaminidase) in five sites spanning a boreal forest to a tropical rainforest. We tested the specific hypothesis that enzymes from higher latitudes would show greater temperature sensitivities than those from lower latitudes. We then used our data to parameterize a mathematical model to test the relative roles of V max and K m temperature sensitivities in SOM decomposition. We found that both V max and K m were temperature sensitive, with Q 10 values ranging from 1.53 to 2.27 for V max and 0.90 to 1.57 for K m . The Q 10 values for the K m of the cellulose‐degrading enzyme β‐glucosidase showed a significant ( P = 0.004) negative relationship with mean annual temperature, indicating that enzymes from cooler climates can indeed be more sensitive to temperature. Our model showed that K m temperature sensitivity can offset SOM losses due to V max temperature sensitivity, but the offset depends on the size of the SOM pool and the magnitude of V max . Overall, our results suggest that there is a local adaptation of microbial EHE kinetics to temperature and that this should be taken into account when making predictions about the responses of C cycling to global change.
Tropical forest soils contain large pools of carbon, most of which is stored as soil organic matter. In spite of its significant role in the global carbon cycle, the dynamics of tropical soil organic ...matter, including the soil microbial communities that produce, maintain and decompose it, are poorly understood. This dissertation investigates controls on the structure and biogeochemical functions of soil microbial communities and soil organic matter, using a combination of laboratory experiments and natural gradients present at the Luquillo Critical Zone Observatory. First, we investigate linkages between soil enzyme activities, nutrient availability and plant roots in surface mineral soils through a greenhouse pot experiment. Three subsequent studies "dig deeper", by investigating microbial community structures and functions (carbon, nitrogen and phosphorus cycling capacity) along the upper 1.4 meters of soil profiles. Finally, we use NMR spectroscopy to profile the chemical composition of soil organic matter across various depths, soil and forest types, coupled with additional thermal and chemical analyses to evaluate acid-treatment effects on soil organic matter composition. We find that dynamic microbial communities exist along the upper 1.4 meters of tropical soil profiles and that, on a per biomass basis, subsoil microbial communities have similar capacity to participate in carbon and nutrient mineralization as their surface counterparts. While microbial activity is strongly correlated to soil carbon concentrations and hence energy availability, soil organic matter chemistry appears to be driven by landscape scale factors as well as pit-scale factors. Because even small amounts of active soil carbon below the surface few centimeters of the soil profile could produce significant carbon fluxes over large spatial and temporal scales, models that aim to predict the future changes to the global carbon cycle should begin to consider the capacity for carbon cycling to occur throughout the deep critical zone.
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