Michelle L. Haddix

Experimental warming shows that decomposition temperature sensitivity increases with soil organic matter recalcitrance

Soil C decomposition is sensitive to changes in temperature, and even small increases in temperature may prompt large releases of C from soils. But much of what we know about soil C responses to global change is based on short-term incubation data and model output that implicitly assumes soil C pools are composed of organic matter fractions with uniform temperature sensitivities. In contrast, kinetic theory based on chemical reactions suggests that older, more-resistant C fractions may be more temperature sensitive. Recent research on the subject is inconclusive, indicating that the temperature sensitivity of labile soil organic matter (OM) decomposition could either be greater than, less than, or equivalent to that of resistant soil OM. We incubated soils at constant temperature to deplete them of labile soil OM and then successively assessed the CO2-C efflux in response to warming. We found that the decomposition response to experimental warming early during soil incubation (when more labile C remained) was less than that later when labile C was depleted. These results suggest that the temperature sensitivity of resistant soil OM pools is greater than that for labile soil OM and that global change-driven soil C losses may be greater than previously estimated.

Redistribution of pyrogenic carbon from hillslopes to stream corridors following a large montane wildfire

Pyrogenic carbon (PyC) constitutes a significant fraction of organic carbon in most soils. However PyC soil stocks are generally smaller than what is expected from estimates of PyC produced from fire and decomposition losses, implying that other processes cause PyC loss from soils. Surface erosion has been previously suggested as one such process. To address this, following a large wildfire in the Rocky Mountains (CO, USA), we tracked PyC from the litter layer and soil, through eroded, suspended, and dissolved solids to alluvial deposits along river sides. We separated deposited sediment into high- and low-density fractions to identify preferential forms of PyC transport, and quantified PyC in all samples and density fractions using benzene polycarboxylic acid markers. A few months after the fire, PyC had yet to move vertically into the mineral soil and remained in the organic layer or had been transported off site by rainfall driven overland flow. During major storm events PyC was associated with suspended sediments in river water, and later identified in low-density riverbank deposits. Flows from an unusually long-duration and high magnitude rain storm either removed or buried the riverbank sediments approximately one year after their deposition. We conclude that PyC redistributes after wildfire in patterns that are consistent with erosion and deposition of low-density sediments. A more complete understanding of PyC dynamics requires attention to the interaction of post-fire precipitation patterns and geomorphological features that control surface erosion and deposition throughout the watershed. Index Terms: Carbon Cycling, Soils, Biogeochemistry.

Design and Operation of a Continuous 13C and 15N Labeling Chamber for Uniform or Differential, Metabolic and Structural, Plant Isotope Labeling

Tracing rare stable isotopes from plant material through the ecosystem provides the most sensitive information about ecosystem processes; from CO2 fluxes and soil organic matter formation to small-scale stable-isotope biomarker probing. Coupling multiple stable isotopes such as 13C with 15N, 18O or 2H has the potential to reveal even more information about complex stoichiometric relationships during biogeochemical transformations. Isotope labeled plant material has been used in various studies of litter decomposition and soil organic matter formation1-4. From these and other studies, however, it has become apparent that structural components of plant material behave differently than metabolic components (i.e. leachable low molecular weight compounds) in terms of microbial utilization and long-term carbon storage5-7. The ability to study structural and metabolic components separately provides a powerful new tool for advancing the forefront of ecosystem biogeochemical studies. Here we describe a method for producing 13C and 15N labeled plant material that is either uniformly labeled throughout the plant or differentially labeled in structural and metabolic plant components. Here, we present the construction and operation of a continuous 13C and 15N labeling chamber that can be modified to meet various research needs. Uniformly labeled plant material is produced by continuous labeling from seedling to harvest, while differential labeling is achieved by removing the growing plants from the chamber weeks prior to harvest. Representative results from growing Andropogon gerardii Kaw demonstrate the systems ability to efficiently label plant material at the targeted levels. Through this method we have produced plant material with a 4.4 atom%13C and 6.7 atom%15N uniform plant label, or material that is differentially labeled by up to 1.29 atom%13C and 0.56 atom%15N in its metabolic and structural components (hot water extractable and hot water residual components, respectively). Challenges lie in maintaining proper temperature, humidity, CO2 concentration, and light levels in an airtight 13C-CO2 atmosphere for successful plant production. This chamber description represents a useful research tool to effectively produce uniformly or differentially multi-isotope labeled plant material for use in experiments on ecosystem biogeochemical cycling.

Molecular composition of soil organic matter with land-use change along a bi-continental mean annual temperature gradient

Soil organic matter (SOM) is critical for maintaining soil fertility and long-term agricultural sustainability. The molecular composition of SOM is likely altered due to global climate and land-use change; but rarely are these two aspects studied in tandem. Here we used molecular-level techniques to examine SOM composition along a bi-continental (from North to South America) mean annual temperature (MAT) gradient from seven native grassland/forest and cultivated/pasture sites. Biomarker methods included solvent extraction, base hydrolysis and cupric (II) oxide oxidation for the analysis of free lipids of plant and microbial origin, ester-bound lipids from cutin and suberin, and lignin-derived phenols, respectively. Solid-state 13C nuclear magnetic resonance (NMR) was used to examine the overall composition of SOM. Soil cultivation was found to increase the amount of microbial-derived compounds at warmer temperatures (up to 17% increase). The cultivated soils were characterized by much lower contributions of plant-derived SOM components compared to the native soils (up to 64% lower at the coldest site). In addition, cultivation caused an increase in lignin and cutin degradation (up to 68 and 15% increase, respectively), and an increase in the amount of suberin-derived inputs (up to 54% increase). Clear differences in the molecular composition of SOM due to soil cultivation were observed in soils of varying mineral composition and were attributed to disturbance, different vegetation inputs, soil aggregate destruction and MAT. A high organic allophanic tropical soil was characterized by its protection of carbohydrates and nitrogen-containing compounds. The conversion of native to cultivated land shows significant shifts in the degradation stage of SOM. In particular, cutin-derived compounds which are believed to be part of the stable SOM pool may undergo enhanced degradation with long-term cultivation and disruption of soil aggregates. On a per year basis, the total amount of cutin decreased only at the two forest sites that were converted to pasture, likely due to cutin degradation or to changes in vegetation and litter quality associated with land-use change. Overall, our study highlights that the implementation of different agricultural management practices enhances the degradation of recalcitrant SOM compounds that may become a source of atmospheric CO2 with increasing land-use and climate change.

Soil N Dynamics Related to Soil C and Microbial Changes During Long-term Incubation

Knowledge of the pools and fluxes of C and N soil components is required to interpret ecosystem functioning and improve biogeochemical models. Two former grassland soils, where wheat or corn are currently growing, were studied by kinetic analysis of microbial biomass C and N changes, C and N mineralization rates, acid hydrolysis, and pyrolysis. Nearly twice as much C as N was mineralized during incubation. Modeling of changes during incubation demonstrated that two-pool first-order kinetics effectively described losses of microbial biomass C and N and concurrent N mineralization. Loss of microbial biomass N during incubation accounted for a significant portion of the N mineralized. Microbial biomass N content and soil N mineralization rates were strongly affected by soil type and soil management. Nitrification, but not N mineralization, was inhibited during the latter stages of incubation in one of the soils. We believe nitrifier populations had dropped below effective levels. Nonacid hydrolysable C was increased in both amount and mean residence time by cultivation and incubation. Hydrolysis removed a larger amount of N than incubation. Data after pyrolysis of soils, in argon at 550°C, closely reflected results for both C and N found after cultivation and incubation. This technique should be further investigated to identify the recalcitrant forms of C and N in soils. The dynamics of soil C and soil N, although related, are not identical; thus, management can be targeted to soil C or N cycling in ecosystem functioning or to soil organic matter dynamics in global change.

Soil bacterial community responses to altered precipitation and temperature regimes in an old field grassland are mediated by plants

The structure and function of soil microbiomes often change in response to experimental climate manipulations, suggesting an important role in ecosystem feedbacks. However, it is difficult to know if microbes are responding directly to environmental changes or are more strongly impacted by plant responses. We investigated soil microbial responses to precipitation and temperature manipulations at the Boston-Area Climate Experiment in Massachusetts, USA, in both vegetated and bare plots to parse direct vs. plant-mediated responses to multi-factor climate change. We assessed the bacterial community in vegetated soils in 2009, two years after the experiment was initiated, and bacterial and fungal community in vegetated and bare soils in 2011. The bacterial community structure was significantly changed by the treatments in vegetated soils. However, such changes in the bacterial community across the treatments were absent in the 2011 bare soils. These results suggest that the bacterial communities in vegetated soils were structured via plant community shifts in response to the abiotic manipulations. Co-variation between bacterial community structure and temperature sensitivities and stoichiometry of potential enzyme activities in the 2011 vegetated soils suggested a link between bacterial community structure and ecosystem function. This study emphasizes the importance of plant-soil-microbial interactions in mediating responses to future climate change.