About

Martin C. Rabenhorst is a Professor in the Department of Environmental Science & Technology at the University of Maryland. His research focuses on the genesis, morphology, and classification of hydromorphic soils, as well as pedogenesis and soil-landscape relations. He specializes in the hydropedology of non-tidal wetlands and coastal marshes, and the pedogenesis and resource inventory of subaqueous soils. His work also involves developing technology for documenting reducing soil conditions. Professor Rabenhorst has contributed to the understanding of soil and geologic mapping over mafic and ultramafic parent materials, the genesis of Maryland soils formed from serpentinite, and the morphology and classification of wetland and tidal marsh soils. He teaches courses related to soil morphology, wetland soils, and pedological investigations, equipping students with both theoretical knowledge and practical skills in soil description, classification, and interpretation for land use and environmental management.

Research topics

• Geology
• Environmental science
• Soil science
• Geography
• Ecology
• Geomorphology
• Mathematics
• Remote sensing
• Oceanography
• Earth science
• Climatology
• Mineralogy
• Botany
• Biology

Selected publications

• On-farm soil test data for the Lower Chesapeake Legacy Phosphorus project, Caroline County, Maryland, USA, 2022, with corresponding topographic metrics derived from LIDAR DEM

  Open MIND · 2026-05-21

  datasetOpen access

  The USDA Legacy Phosphorus Project seeks to leverage watershed research with USDA NRCS (Natural Resource Conservation Service) and USDA ARS (Agricultural Research Service) to advance the science, innovation, and forecasting of legacy phosphorus (P) mitigation strategies. This project utilizes Conservation Effects Assessment Project (CEAP) watersheds in Lower Chesapeake Bay area.This dataset supports spatial analysis of Legacy Phosphorus dynamics in the Lower Chesapeake Bay watershed. It includes tabular data, shapefiles, and raster maps derived from field measurements and Random Forest modeling.The main data file contains multiple sheets to guide the user, see 'readme' and 'column descriptions' for details. The 'Data Summary' sheet has the primary data in tidy format.This project is funding by USDA-NRCS Conservation Effects Assessment Project (Agreement NRC21IRA0010879) including USDA Legacy Phosphorus Study, with additional funding provided by USDA-ARS intramural projects under Natural Resources and Sustainable Agricultural Systems National Programs, as well as the USDA Long-Term Agroecosystem Research (LTAR) Network.

  PublisherOA PDFDOI

• The Evolving Classification of Acid Sulfate Soils

  European Journal of Soil Science · 2025-01-01 · 3 citations

  articleOpen access1st authorCorresponding

  ABSTRACT The process of classification helps us organize knowledge and it helps us to better appreciate relationships and connections. Classification also facilitates communication, and good classification systems will be (to some degree) practical and utilitarian. While the problematic nature of acid sulfate (AS) soils has been recognized (by some) at least since the 18th century, much of what we understand about AS soils only began to be recognized around the time of the 1st International AS Soils Conference in 1972. As our modern soil classification systems emerged during the last half century, many of their architects knew to incorporate these challenging and unique soils. Thus, as our experience and understanding of these soils has expanded, we have also seen corresponding changes or adaptations in the classification systems to accommodate the new knowledge. In this paper we examine revised principles, perspectives and structures for the classification of AS soils through three systems used broadly around the world: US Soil Taxonomy, the World Reference Base for Soil Resources and the Australian Soil Classification. All of these classification systems have accommodated AS soils throughout their history and each has demonstrated distinctive changes during particular periods. As our concepts and understanding of AS soils have developed, these have become encoded (quickly or slowly) in our classifications. This paper will explore how various AS soil concepts have been addressed within these three classification systems, how these concepts have changed through time, and how changes in each system have preceded, followed or paralleled the other systems. We highlight differences between systems that have existed and that may remain, and offer perspectives on the rationale for these distinctives. We will also demonstrate the ways in which collaborations and shared knowledge have drawn parts of these systems more closely together.

  PublisherOA PDFDOI

• Impact of Water Halinity on the Presence of Hypersulfidic Materials in Estuarine Tidal Marsh Soils, Chesapeake Bay ( <scp>USA</scp> )

  European Journal of Soil Science · 2025-03-01 · 1 citations

  articleOpen access1st authorCorresponding

  ABSTRACT In brackish tidal marsh soils, sulfate reduction processes commonly lead to the formation of Fe sulfide minerals, and if the accumulated potential acidity exceeds the ability of other components for neutralisation, can lead to the occurrence of hypersulfidic soil materials, which if disturbed and oxidised, can become extremely acid (sulfate) soils. In estuarine/riverine marshes that are fed by fresh water flowing into an estuary, a pronounced halinity gradient exists along the course of the stream, with upstream portions being fresher and downstream sections being more strongly influenced by salts. Thus, it is expected that hypersulfidic materials will be less prevalent in upstream sections, and this is reflected in the concepts used in soil mapping of the marshes in the Chesapeake Bay estuary (hypersulfidic materials not being recognised when stream halinity is lower than about 2 ppt). This study was designed to examine tidal marsh soils that span a halinity gradient in estuarine/riverine marshes. Soils at eight sites were identified for study that had stream halinity ranging between 0.10 and 8.8 ppt. Soil morphology was described and samples collected from each horizon, which were examined by documenting pH change during moist aerobic incubation (MAI). Surprisingly, all soils, even those with halinity between 0.10 and 1.0 ppt, contained horizons that became extremely acid (pH &lt; 4.0) within 14 weeks during MAI. Examination of salts that developed in the samples during MAI were demonstrated by X‐ray diffraction to be mainly sulfate salts, confirming that the acidity was derived from oxidation of sulfide minerals. We expect that occasional pulses of sulfate enriched water, such as occurs during storm events, may provide sufficient stream water sulfate to lead to formation and accumulation of Fe sulfide minerals sufficient to form hypersulfidic materials. Continued rising sea levels under the current warming climate scenario might also exacerbate this worldwide. These observations suggest that a review of the mapping paradigm used in Chesapeake Bay may be in order. Potential modifications to existing soil maps of marshes around Chesapeake Bay should perhaps recognise soils with hypersulfidic materials extending further up the tidal estuary than previously recorded. This work may also have implications for mapping of similar estuarine tidal marsh soils in other parts of the country or the world.

  PublisherOA PDFDOI

• Assessing the topographic distribution of legacy soil phosphorus in agricultural fields of the Delmarva Peninsula, Mid‐Atlantic Coastal Plain, USA

  Journal of Environmental Quality · 2025-11-29

  articleOpen access

  Abstract Phosphorus (P) management remains a challenge in agricultural watersheds. The Choptank River Conservation Effects Assessment Project watershed, located in Maryland and Delaware and draining to the Chesapeake Bay, contains legacy soil P from historical dairy and poultry manure applications. These practices elevated soil P beyond crop needs, contributing to persistent P export to aquatic ecosystems. We assessed spatial P distribution and analyzed GIS (Geographic Information Systems)‐derived landscape features driving legacy P movement on a farm (47 ha). We hypothesized that P accumulates in drained lowlands and depressional areas due to gravity‐driven processes that accelerate P‐enriched water to receiving waters via overland flow. In collaboration with the US Department of Agriculture Legacy P project, we collected 105 soil samples (0‐ to 5‐cm and 5‐ to 15‐cm depths) and 14 ditch sediment samples across five topographic openness classes from a farm with &gt;100 years of dairy manure application. Average Mehlich‐III P concentrations were 218 and 179 mg kg −1 at 0‐ to 5‐cm and 5‐ to 15‐cm depths, respectively, with legacy areas defined by P content &gt; 100 mg kg −1 . Soil P and clay particle size were positively correlated ( r = 0.42, p &lt; 0.05), increased as landscape openness decreased, and were negatively correlated with topographic openness (ranging from −0.2 to −0.4, p &lt; 0.05), indicating accumulation of P and clay in low‐lying areas. These patterns suggest that historical field‐level managements have primarily shaped P distribution, while hydrologic and landscape properties further influence its redistribution via transport pathways and drainage. These findings support the development of landscape models to map critical source areas in low‐relief watersheds and guide targeted mitigation in high‐risk P export zones.

  PublisherOA PDFDOI

• Advances in technology for using Indicator of Reduction in Soils (IRIS) to quantify porewater sulfide levels in the coastal zone

  Soil Science Society of America Journal · 2025-03-01

  articleOpen access1st authorCorresponding

  Abstract Soluble sulfide is toxic to many plants and animals and is especially problematic in brackish environments of the coastal zone (e.g., marshes and benthic environments). In addition to traditional techniques for measuring porewater sulfide in marsh and subaqueous systems (peepers, sippers, and centrifugal extraction), over the last decade or so, Indicator of Reduction in Soils (IRIS) has been added to the arsenal of available methods. Soluble sulfide reacts with the Fe oxide coatings on IRIS devices to form gray to black iron monosulfide (FeS) stains and coatings, the color of which is a function of both the concentration of the sulfide and the time of exposure. Challenges in using IRIS for sulfide analysis stem from the fact that the dark FeS colors fade quickly over a period of minutes to hours. During the last few years, significant advances in IRIS technology, as well as recent advances in digital image acquisition and image analysis, have allowed us to develop an IRIS approach for quickly and effectively collecting and quantifying porewater sulfide levels in coastal environments (e.g., subaqueous areas and marshes). This article will introduce new tools for deploying IRIS in subaqueous settings and will also demonstrate the utility of the new digital technology for image acquisition and analysis, as sulfide data from two marsh sites and four subaqueous soil sites are presented and discussed.

  PublisherDOI

• On-farm soil test data for the Lower Chesapeake Legacy Phosphorus project, Caroline County, Maryland, USA, 2022, with corresponding topographic metrics derived from LIDAR DEM.

  Ag Data Commons · 2025-01-01

  datasetOpen access

  The USDA Legacy Phosphorus Project seeks to leverage watershed research with USDA NRCS (Natural Resource Conservation Service) and USDA ARS (Agricultural Research Service) to advance the science, innovation, and forecasting of legacy phosphorus (P) mitigation strategies. This project utilizes Conservation Effects Assessment Project (CEAP) watersheds in Lower Chesapeake Bay area.This dataset supports the articleForoughi, M., Du, L., Scott, I.S.P.C., Hively, W.D., Simpson, Z.P., Smith, Z.J., Hapeman, C.J., Rabenhorst, M.C., Weil, R.R., and McCarty, G.W. Assessing the Topographic Distribution of Legacy Soil Phosphorus in Agricultural Fields of the Delmarva Peninsula, Mid-Atlantic Coastal Plain, USA., <i>Journal of Environmental Quality</i>, in progress.The main data file contains multiple sheets to guide the user, see 'readme' and 'column descriptions' for details. The 'Data Summary' sheet has the primary data in tidy format.for sake of space, only the selected fits are included here ("best performing models by site") -- unzip that folder to use.

  PublisherDOI

• Carbon stocks in Mid‐Atlantic tidal marshes

  Soil Science Society of America Journal · 2025-07-01 · 1 citations

  articleOpen accessSenior authorCorresponding

  Abstract Sound estimations of blue carbon (C) stocks have important implications for global carbon accounting. This is especially true in tidal marshes because of their capacity to accumulate and store large quantities of C. Reliable field data, however, have historically been limited. More recent research has focused on general estimates of C stocks in the conterminous United States but without regard to the differences in soil characteristics, which vary based on the particular geomorphic setting where a marsh has formed. This may lead to inaccuracies in C stock estimates at a regional scale. In this study, we set out to measure tidal marsh C stocks in the Mid‐Atlantic region and to understand the impact of geomorphology on the variability of C storage. We collected and analyzed 455 samples from 72 pedons distributed across five pedogeomorphic units (PGUs) representative of the Mid‐Atlantic region: (1) submerged upland, (2) estuarine fresh, (3) estuarine non‐fresh, (4) coastal barrier, and (5) coastal mainland. Carbon stocks were measured for each pedon, and significant differences in mean C stocks were found among the five PGUs. Differences became more pronounced with increased sampling depth (up to 200 cm). Additionally, we found that C stocks change spatially and systematically within certain types of marshes. These results suggest that geomorphic setting, which influences the pedogenesis of tidal marsh soils, has a meaningful impact on C storage and must be considered for accurate C accounting.

  PublisherOA PDFDOI

• Advances in making Mn oxide‐coated sands

  Soil Science Society of America Journal · 2024-02-28

  articleOpen access1st authorCorresponding

  Abstract Manganese (Mn) oxide‐coated sand has been suggested as an amendment for scrubbing metals in water filtration beds and also as a less concentrated medium for uniformly amending soils with Mn oxides in mesocosm scale studies. Earlier work at the lab bench scale, using potassium permanganate (KMnO 4 ) solutions that were reduced with sodium (Na) lactate, resulted in sands coated with about 0.13% Mn. The goal of this project was to increase the amount of Mn oxide that could be coated on sand to make it a more useful amendment and also to attempt to scale up the procedure to produce larger (kg) quantities of coated sand. Titration experiments examined the effects of (1) varying the molar ratio of Na lactate to KMnO 4 , (2) varying the rate at which the titration was accomplished, and (3) varying the concentration (molarity) of the original KMnO 4 solution. The results of this work led to an optimal approach utilizing 0.32 M KMnO 4 solution that was titrated to a final lactate:permanganate ratio of ∼1.1 with 10% of the lactate being added every 10 min while the suspension was being stirred. The proportion of sand to an initial solution was also increased 5–20 fold to between 50 and 200 g per 100 mL of solution. Applying this method and using a large 20‐ to 30‐L reaction vessel yields sands coated with up to 0.7% Mn in batches 5–10 kg is size, which could be useful as an amendment in mesocosm scale studies, or as a component of treatment filter beds. The examination of various size fractions of the coated sands demonstrated that more Mn was coated on finer sand fractions, which appears to be a function of the particle surface area available for the coating of Mn oxides, and at a rate of 0.3–0.5 µg Mn mm −2 of the particle surface.

  PublisherOA PDFDOI

• Redox conditions and Indicator of Reduction in Soils (IRIS) films in soils of a hypersaline wetland

  CATENA · 2024-11-14 · 2 citations

  articleOpen access

  • Strongly reducing conditions develop in hypersaline wetland gypsum-rich soils. • A conspicuous black sulfidic soil layer prevails during the annual cycle. • Mn and Fe IRIS films quantified the current redox processes and their variability. • The oxide-coating removal rates are very low compared to non-carbonate environments. Information about the reducing conditions in hypersaline wetland soils is scarce though redox traits are of ecological and agronomical interest. We studied the soils and water in Salineta playa-lake (NE Spain) plus the soil redox conditions using IRIS (Indicators of Reduction in Soils) films during 17 months. Soils had a pH varying from 6.8 to 7.8, a mean gypsum content of 38 %, a mean organic carbon content of 0.6 %, and soil salinity ranged from 219 to 66 dS m −1 in the saturated paste extract. Soil horizons showed distinct morphological features consistent with a redox potential (Eh) ranging from +434 to −108 mV. Minimum Eh occurred in the upper soil horizon indicating intensified reducing conditions with a transition to aerobic conditions with depth. Sulfidic and Anoxic conditions prevailed during the annual cycle despite temporary drying. The IRIS films revealed an irregular removal of manganese and iron oxide coatings through time and through the film depth as triggered by pulses of the water level. The monthly removal rate was 10.6 % for manganese and 5 % for iron films. Iron removal showed a delayed and irregular response due to the alkaline conditions. The persistent accumulation of sulfides in the soil resulted in a distinctive black soil layer which can be a key feature for future monitoring of the impacts of the agricultural flows from surrounding irrigated lands. Further research on pedogenesis of arid wetlands will help the soil resource inventories and the understanding of the various life adaptations to these conditions.

  PublisherDOI

• Potential interference of organic acids and ferrous iron in the interpretation of Fe and Mn indicators of reduction in soil

  Soil Science Society of America Journal · 2023-06-09 · 5 citations

  article

  Abstract Removal of manganese (Mn) and iron (Fe) oxide coatings from indicators of reduction in soil (IRIS) is considered a function of microbial reduction of these elements under sufficiently reducing conditions, but there may be situations where removal is abiotic. We examined whether abiotic removal of Mn and Fe oxide coatings may be facilitated by organic acids and whether loss of Mn can take place via chemical reduction by ferrous iron (Fe 2+ ) in vitro. Loss of color from Mn‐IRIS in citric acid solution was highest at 1 and 10 mM regardless of pH, with no loss observed at lower concentrations. Effects of citric acid on Fe‐IRIS were also limited to 1 and 10 mM, but color loss only occurred at pH 4.5 and the rate and extent of loss were lower than for Mn‐IRIS. Color loss from Mn‐IRIS in oxalic acid was highest at 1 and 10 mM and increased with decreasing pH. Effects of oxalic acid on loss of color from Fe‐IRIS were limited to 1 and 10 mM. Loss of color from Mn‐IRIS was limited to 10 mM salicylic acid and pH 4.5, with no effect on Fe‐IRIS regardless of concentration and pH. Only the two highest concentrations of Fe 2+ (50 and 100 mM) resulted in reduction of the Mn‐IRIS coating and subsequent deposition of an Fe oxide coating. Organic acids and pH have a differential effect on color loss from Mn‐ and Fe‐IRIS, with only high concentrations of organic acids at low pH possibly interfering with interpretation.

  PublisherDOI

Frequent coauthors

• Mark H. Stolt

  Louisiana Department of Natural Resources

  16 shared

• D. S. Fanning

  University of Maryland, College Park

  14 shared

• Gregory W. McCarty

  Beltsville Agricultural Research Center

  13 shared

• Megan Lang

  United States Fish and Wildlife Service

  13 shared

• Brian A. Needelman

  University of Maryland, College Park

  13 shared

• L. P. Wilding

  12 shared

• Amr H. Hussein

  Tanta University

  8 shared

• Barret M. Wessel

  Michigan State University

  8 shared

Education

• Ph.D.

  Texas A&M University

  1983

• M.S.

  University of Maryland

  1978

• B.S.

  University of Maryland

  1975

Awards & honors

• AGNR Awards
• Excellence in Extension Award
• Excellence in Instruction Award
• Excellence in Research Award