Performing Department
Environmental Science and Technology
Non Technical Summary
Soil carbon sequestration is commonly involved in greenhouse gas (GHG) mitigation strategies. While environmental variables may impact the balance/distribution of C among the pools of vegetation, soil and atmosphere, recent field and laboratory studies, as well as broad statistical analyses, have implicated manganese (Mn) in the plant/soil system as disproportionately driving decomposition of plant litter and soil organic matter toward greater CO2 released to the atmosphere (a non-desirable outcome). Nearly all work thus far has focused on plant litter and organic (O) soil horizons at high latitudes (e.g. boreal forests), but in temperate zones, most of the soil carbon is stored in mineral A horizons (not O or litter horizons), which is generally more decomposed/humified. We propose to examine mineral soil A horizons to determine whether soil Mn concentrations affect the decomposition rate or stability of soil organic matter. We will 1) measure CO2 respiration in soil A horizons, with and without Mn oxide amendments to evaluate/confirm preliminary observations and 2) determine if there is a soil Mn threshold, above which, there is no affect from Mn amendments. We will then 3) explore implications regarding whether soils differ in their carbon sequestration potential depending on their Mn content.
Animal Health Component
20%
Research Effort Categories
Basic
80%
Applied
20%
Developmental
(N/A)
Goals / Objectives
1. To examine a broad group of soil A horizons to determine whether they will confirm preliminary data suggesting that a modest addition of Mn will increase the rate and magnitude of SOM decomposition (CO2 respiration), and to evaluate whether low Mn soils behave differently from those with a high content of native extractable Mn.2. To determine if there is a threshold for native soil Mn concentration, above which, soils exhibit no increase in CO2 respiration with Mn additions.3. Based on the outcomes of efforts addressing Objectives 1 and 2, to develop a strategy for identifying which soils/lands (in Maryland and in the region) would be expected to function as "low Mn" soils (below the threshold) and which would be expected to function as "high Mn" soil (above the threshold).In addition to the above, with the results collected from this research, the PI hopes to submit an application to the USDA- AFRI - Bioenergy, Natural Resources, and Environment program RFA during the latter part of 2026.
Project Methods
Overall StrategyPrevious experimental work conducted by an UG researcher demonstrated the viability of using an innovative method to more uniformly collect gas samples and measure CO2 respiration from fairly small (30 to 60 g) soil samples. More importantly, his data suggested that microbial respiration of some (but not all) soil A horizons is significantly greater when amended with Mn. It also suggested the hypothesis that the distinguishing characteristic between soils that do or don't demonstrate this effect, might be the native level of Mn present in the soil. The fundamental approach for this project is to apply these recently developed methods to a broader group of soil A horizons, that have been strategically selected to permit testing of this hypothesis. Numerous soil A horizons, that vary in their level of native Mn, will be studied to compare the rate and magnitude of microbial CO2 respiration, with and without the addition of Mn.Site/Sample Selection, Collection and ProcessingApproximately 15 to 20 different soil A horizons will be collected from across the region that exhibit a range in the levels of extractable Mn. Guidance for locating samples will be obtained using soil and geological maps and from previously collected available data . It is anticipated that soils derived from mafic parent materials, or those in hydrological discharge areas, will be more likely to be higher in Mn. At each sampling location, an abbreviated soil morphological description will be made using standard methods. Approximately six to eight liters of each soil A horizon will be collected and transported to the lab. Each sample will be crushed to pass a 2 mm sieve, and will then be thoroughly homogenized. Each soil will be analyzed to determine the quantity of OC using high temperature dry combustion. Extractable iron (Fe) and manganese (Mn) levels will be measured using a modification of the dithionite-citrate extraction approach, and then measured by atomic absorption (AA) spectrophotometry.INITIAL EXPERIMENTATION (Objective 1)Microcosm Construction and Mn AmendmentTo test the initial hypothesis that additions of Mn to soils with low native Mn will demonstrate a significant increase in CO2 respiration while soils with high native Mn will not (objective 1), our initial experimentation will focus on samples at the two ends of the Mn concentration spectrum.Soils will be amended either with an inert quartz sand, or with a quartz sand that has been coated with birnessite that contains approximately 0.5% Mn. Our approach ensures thorough homogenization, resulting in an addition of approximately 0.25% Mn, representing a high, but realistic, Mn level. Soils without the Mn amendment will receive an equal amount of uncoated quartz sand.Thus, each experimental unit (microcosm) will be comprised of a 60 mL plastic syringe with a filter pad at the bottom (to retain the soil), to which will be added 30 g of soil and either 30 grams of Mn coated sand (MCS) or 30 grams of uncoated sand (S) (thoroughly homogenized). Each combination will be prepared in triplicate. Extractions will be run using a mechanical vacuum extractor (MVE). The MVE can accommodate 24 syringes, so 4 soils can be run simultaneously (4 soils X 3 replicates X 2 treatments).Collection and Analysis of Gas SamplesTo facilitate microbial respiration, soils must be moist. This will be accomplished by slowly submerging the microcosms from below, to minimize any trapping of air in the soil. Following established protocols, after 3 days, microcosms will be mounted on the MVE, and water will be extracted from the microcosms which thereafter will be maintained in a moist, aerobic condition. Using the MVE, approximately 60 mL of air will be drawn through the syringes each day, over an 8 hr period, and collected in a "bottom syringe" (also ≈60 mL) and the volume of gas in each syringe will be recorded. This procedure will be repeated daily for 21 days. As each syringe (containing a gas sample) is removed from the MVE, it will be sealed at the tip using a small piece of surgical tubing and clamp. Samples will be collected and analyzed daily for the first week. Daily extractions will continue during weeks two and three, but because after the first week, gas production rates generally become more stable, analyses will only be run on alternate days. Air will also be collected from the lab each day and analyzed in order to account for ambient CO2 contributions.Gas analyses will be performed using a Shimadzu GC-8A Gas Chromatograph which is available for use in the Stable Isotope Laboratory overseen by Prof. James Farquhar. Five mL subsamples of the gas will be collected into a small syringe with a needle using the surgical tubing seal on the collection syringe as a septum. Approximately three mL of this subsample will be injected into the GC column for analysis. Because gas standards can slowly diffuse through gas storage (Tedlar) bags, on each day that analyses are run, gas standards will be made fresh from a certified CO2 stock to ensure accuracy.Data and Statistical AnalysisBy joining the measured %CO2 concentration from each gas sample with the volume of the gas collected in each syringe, the mass of CO2 respired from each soil microcosm can be calculated. These data can be summed over the duration of the study and can also be normalized based upon the amount of OC in each soil. The effect of the treatment (Mn amendment) and differences among the soils will be tested using a multivariate analysis of variance that incorporates a repeated measures (time series) component (MANOVA).EXTENDED EXPERIMENTATION (Objectives 2 and 3)When the initial round of experimentation is complete, (which will be focused upon soils with Mn concentrations at the two ends of the spectrum described above), additional experimental effort will then be focused upon soils with intermediate native levels of Mn. The goal of this phase will be to assess whether there may be a particular Mn concentration threshold, above which, no significant effect is observed from amending the soil with Mn (Objective 2). For this phase, samples will be selected based upon the native concentration of extractable Mn as determined during the initial characterization efforts. An additional four to eight soils will be evaluated. The methods used will be the same as those during the earlier phase of the work.Provided that the results from work on objective 1 confirm preliminary data, and that the work on objective 2 allow us to identify some Mn concentration threshold, above which Mn additions have no significant impact on SOM decomposition, we will seek to develop a strategy for identifying and grouping those soils/lands that would be considered to be above or below this threshold (Objective 3). To accomplish this, we will use information gained from the initial sampling effort and analyses, as well as from other sources including the data from other thesis and dissertation projects, as well as information available in the USDA-NRCS Soil Characterization Database. Data from specific locations will be joined with soil series or soil map units or geological map units, in an attempt to create a guidance document describing which soils might be considered to be high in Mn (above the identified threshold) and which are not.