Do enzyme activities during decomposition follow predicted patterns? A test of the conceptual model of litter decay.

Surprisingly, there remains a paucity of research examining specific interactions between the relationship between microbial community behavior and plant litter chemistry during decomposition. A more mechanistic understanding of the relationship between these drivers will ultimately help determine the trajectory of litter decomposition and the conditions in which soils serve as either a source or sink for atmospheric C. In order to examine these relationships, a laboratory incubation was established using _Acer saccharum_ litter and a sandy soil (< 1.5% organic matter). Extracellular enzyme activities ([BETA]-glucosidase, N-acetyl glucosaminidase, leucine-amino peptidase, acid phosphatase, phenol oxidase, and peroxidase) were monitored on a consistent basis along with instantaneous rates of carbon dioxide production, microbial biomass (carbon and nitrogen) and phospholipid fatty acid biomarkers (PLFA), and nitrogen and phosphorus availability. Microbial biomass and microbial respiration peaked within the first week of the experiment. This was likely due to the high availability of water soluble substrates early in decay that can be obtained without the production of extracellular enzymes. [BETA]-glucosidase (BG), N-acetyl glucosaminadase (NAG), and acid phosphatase activities increased quickly following the first week and peaked within the first month (at approximately 15% mass loss). Leucine amino peptidase was not detected during the incubation, which may be due to its strong positive correlation with soil pH, while other hydrolytic enzymes tend to track concentrations of soil organic matter. Phenol oxidase and peroxidase activities were not measurable until the second month of the experiment (> 25% mass loss), likely following the depletion of more labile substrates. A second increase in BG activity was observed between Days 83-111, which may be due to an increase in the availability of cellulose that was previously shielded by lignin, since oxidative enzyme activity was first detected on Day 68. We also observed some shifts in microbial PLFAs along with enzyme activities during decomposition. Prior to the increases in enzyme activity we observed a high proportion of PLFA 18:1[omega]7c, which is a bacterial biomarker. As enzyme activities increased, we observed a decrease in this biomarker and an increase in 18:2[omega]6,9c, a fungal biomarker that was correlated with BG and NAG activity. We did not observe any clear relationships between PLFAs and lignolytic enzyme activity, however. Overall, we observed a distinct functional shift in microbial substrate use that may be associated with either changes in composition of the microbial community or community shifts in enzyme production

Continental-scale patterns of extracellular enzyme activity in the subsoil: an overlooked reservoir of microbial activity

Chemical stabilization of microbial-derived products such as extracellular enzymes (EE) onto mineral surfaces has gained attention as a possibly important mechanism leading to the persistence of soil organic carbon (SOC). While the controls on EE activities and their stabilization in the surface soil are reasonably well-understood, how these activities change with soil depth and possibly diverge from those at the soil surface due to distinct physical, chemical, and biotic conditions remains unclear. We assessed EE activity to a depth of 1 m (10 cm increments) in 19 soil profiles across the Critical Zone Observatory Network, which represents a wide range of climates, soil orders, and vegetation types. For all EEs, activities per mass of soil correlated positively with microbial biomass (MB) and SOC, and all three of these variables decreased logarithmically with depth (p &lt; 0.05). Across all sites, over half of the potential EE activities per mass soil consistently occurred below 20 cm for all measured EEs. Activities per unit MB or SOC were substantially higher at depth (soils below 20 cm accounted for 80% of whole-profile EE activity), suggesting an accumulation of stabilized (i.e. mineral sorbed) EEs in subsoil horizons. The pronounced enzyme stabilization in subsurface horizons was corroborated by mixed-effects models that showed a significant, positive relationship between clay concentration and MB-normalized EE activities in the subsoil. Furthermore, the negative relationships between soil C, N, and P and C-, N-, and P-acquiring EEs found in the surface soil decoupled below 20 cm, which could have also been caused by EE stabilization. This finding suggests that EEs may not reflect soil nutrient availabilities deeper in the soil profile. Taken together, our results suggest that deeper soil horizons hold a significant reservoir of EEs, and that the controls of subsoil EEs differ from their surface soil counterparts. </div

Soil Microbial Responses to Elevated CO2 and O3 in a Nitrogen-Aggrading Agroecosystem

Climate change factors such as elevated atmospheric carbon dioxide (CO2) and ozone (O3) can exert significant impacts on soil microbes and the ecosystem level processes they mediate. However, the underlying mechanisms by which soil microbes respond to these environmental changes remain poorly understood. The prevailing hypothesis, which states that CO2- or O3-induced changes in carbon (C) availability dominate microbial responses, is primarily based on results from nitrogen (N)-limiting forests and grasslands. It remains largely unexplored how soil microbes respond to elevated CO2 and O3 in N-rich or N-aggrading systems, which severely hinders our ability to predict the long-term soil C dynamics in agroecosystems. Using a long-term field study conducted in a no-till wheat-soybean rotation system with open-top chambers, we showed that elevated CO2 but not O3 had a potent influence on soil microbes. Elevated CO2 (1.5×ambient) significantly increased, while O3 (1.4×ambient) reduced, aboveground (and presumably belowground) plant residue C and N inputs to soil. However, only elevated CO2 significantly affected soil microbial biomass, activities (namely heterotrophic respiration) and community composition. The enhancement of microbial biomass and activities by elevated CO2 largely occurred in the third and fourth years of the experiment and coincided with increased soil N availability, likely due to CO2-stimulation of symbiotic N2 fixation in soybean. Fungal biomass and the fungi∶bacteria ratio decreased under both ambient and elevated CO2 by the third year and also coincided with increased soil N availability; but they were significantly higher under elevated than ambient CO2. These results suggest that more attention should be directed towards assessing the impact of N availability on microbial activities and decomposition in projections of soil organic C balance in N-rich systems under future CO2 scenarios

Elevated atmospheric nitrate deposition in northern hardwood forests: Impacts on the microbial mechanisms of plant litter decomposition.

The burning of fossil fuels and subsequent atmospheric deposition of nitrate (NO3-) has increased the global input of nitrogen (N) to many terrestrial ecosystems. This has the potential to alter the cycling of carbon (C) in these ecosystems by reducing microbial-mediated decomposition. After assimilating anthropogenic N, the soil microbial community can release that N as ammonium (NH4+), which can inhibit the activity of lignin-degrading soil fungi. My primary objectives were to determine if increases in NO3- deposition have altered microbial community composition and function in upland temperate forests and if these changes have altered decomposition. I hypothesized that anthropogenic N deposition will fundamentally altered the flow of C in the microbial foodweb, which, in turn, will alter ecosystem-level patterns of C cycling. This idea was tested in four sugar maple-dominated northern hardwood ecosystems in Michigan which have received experimental N additions (30 kg NO3--N ha-1 y -1) since 1994. I determined microbial community composition by phospholipid fatty acid analysis and microbial function by measuring the activities of extracellular enzymes that decompose plant tissue. Measuring the flow of C through the soil microbial foodweb was achieved using 13C labeled compounds (cellobiose and vanillin) that are common products of lignocellulose decomposition. Increases in NO3- deposition significantly (p 3- deposition can potential reduce rates of decomposition. This may increase the capacity for terrestrial ecosystems to accumulate C through soil organic matter formation. Therefore, anthropogenic N deposition, by slowing the flow of C through the microbial foodweb, can be a potent modifier of ecosystem-level patterns of C cycling.Ph.D.Biological SciencesEcologyMicrobiologySoil sciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124044/2/3121916.pd

Data from: Chronic phosphorus enrichment and elevated pH suppresses Quercus spp. leaf litter decomposition in a temperate forest

The potential of excessive nitrogen (N) suppressing litter decay in is widely recognized. The role of phosphorus (P) in plant litter decay is less understood and is generally assumed to have a weak influence in decomposition outside tropical or wetland ecosystems. Based on the “microbial mining” hypothesis that suggests the availability of limiting nutrients is the driving mechanism of decay, P could have a strong influence on carbon (C) cycling in low-P ecosystems. The objective of this study was to determine if increasing P availability will influence the decomposition of leaf litter in a low-P temperate forested ecosystem. If the availability of a limiting nutrient is a driving mechanism mediating decay, then increasing the availability of P in a low-P environment should inhibit decay. The experiment was held in a low-P mixed mesophytic forest in the unglaciated Alleghany Plateau (USA). Since 2009, P availability has been increased either directly by phosphate fertilizer and/or indirectly by raising soil pH. Starting in 2011 and for three consecutive years, Quercus spp. leaf litter bags were deployed and harvested at least every year for three years for temporal replication. After three years, elevating pH and/or P litter mass loss was ~52% and significantly (P < 0.001) lower than the control at 73% mass loss. The mean residence time (MRT; k yr-1) for the control was 2.1 years, but elevating P with pH increased MRT to 5.3 years. Results support the “microbial phosphorus mining” hypothesis that P has the potential to inhibit decay for Quercus spp. leaf litter in temperate forests and potentially increase soil C

Soil, litter, and mass loss measurments

Contains raw data for soil mineral properties & processes (Table 1), Quercus spp. litter chemistry (Table 2), and from the litter bag experiment (Figures 2 & 3)

Atmospheric nitrate deposition and the microbial degradation of cellobiose and vanillin in a northern hardwood forest

Human activity has increased the amount of N entering terrestrial ecosystems from atmospheric NO3- deposition. High levels of inorganic N are known to suppress the expression of phenol oxidase, an important lignin-degrading enzyme produced by white-rot fungi. We hypothesized that chronic NO3- additions would decrease the flow of C through the heterotrophic soil food web by inhibiting phenol oxidase and the depolymerization of lignocellulose. This would likely reduce the availability of C from lignocellulose for metabolism by the microbial community. We tested this hypothesis in a mature northern hardwood forest in northern Michigan, which has received experimental atmospheric N deposition (30kgNO3--Nha-1y-1) for nine years. In a laboratory study, we amended soils with 13C-labeled vanillin, a monophenolic product of lignin depolymerization, and 13C-labeled cellobiose, a disaccharide product of cellulose degradation. We then traced the flow of 13C through the microbial community and into soil organic carbon (SOC), dissolved organic carbon (DOC), and microbial respiration. We simultaneously measured the activity of enzymes responsible for lignin (phenol oxidase and peroxidase) and cellobiose (β-glucosidase) degradation. Nitrogen deposition reduced phenol oxidase activity by 83% and peroxidase activity by 74% when compared to control soils. In addition, soil C increased by 76%, whereas microbial biomass decreased by 68% in NO3- amended soils. 13C cellobiose in bacterial or fungal PLFAs was unaffected by NO3- deposition; however, the incorporation of 13C vanillin in fungal PLFAs extracted from NO3- amended soil was 82% higher than in the control treatment. The recovery of 13C vanillin and 13C cellobiose in SOC, DOC, microbial biomass, and respiration was not different between control and NO 3- amended treatments. Chronic NO3- deposition has stemmed the flow of C through the heterotrophic soil food web by inhibiting the activity of ligninolytic enzymes, but it increased the assimilation of vanillin into fungal PLFAs. © 2004 Elsevier Ltd. All rights reserved

Atmospheric Nitrate Deposition, Microbial Community Composition, and Enzyme Activity in Northern Hardwood Forests

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153176/1/saj2sssaj20041320.pd

Interactions between leaf litter quality, particle size, and microbial community during the earliest stage of decay

With global change expected to alter aspects of the carbon (C) cycle, empirical data describing how microorganisms function in different environmental conditions are needed to increase predictive capabilities of microbially-driven decomposition models. Given the importance of accelerated C fluxes during early decay in C cycling, we characterized how varying litter qualities (maple vs. oak) and sizes (ground vs. 0.25 cm2 vs. 1 cm2), and contrasting soils (sandy vs. loamy), altered microbial biomass-carbon and community structure, respiration, enzyme activities, and inorganic nutrients over the initial 2 weeks of decomposition. Our hypotheses were (1) mixing ground maple with loam should result in a quicker, more prolonged respiration response than other treatments; and (2) “priming”, or substrate-stimulated soil organic matter turnover, should be minimal over the first few days due to soluble C substrate uptake. Respiration peaks, biomass increases, nutrient immobilization, low enzyme activities, and minimal priming occurred in all treatments over the first 72 h. These general features suggest soluble C compounds are degraded before polymeric substrates regardless of litter size or type, or soil. Ground litter addition to the high C and microbial biomass loam resulted in a more prolonged respiration peak than the poorly aggregated sand. Priming was greater in loam than the C limited sandy soil after the first 72 h, likely due to co-metabolism of labile and recalcitrant substrates. We conclude that the general features of early decay are widespread and predictable, yet differences in litter and soil characteristics influence the temporal pattern and magnitude of C flux