About

Randy Kolka is an Adjunct Assistant Professor in the Department of Soil, Water, and Climate at the University of Minnesota. His educational background includes a BS from the University of Wisconsin-Stevens Point, an MS from the University of Minnesota, and a PhD from the University of Minnesota. His research involves watershed or landscape level soil and hydrological studies focusing on upland, riparian, and wetland soil carbon, mercury, and nutrient dynamics. He investigates how upland land use relates to pollution transport to surface waters and explores ecosystem restoration and the connections between upland and riparian or wetland communities. Additionally, he studies the functions and implications of global climate change.

Research topics

• Environmental science
• Ecology
• Geology
• Biology
• Geography
• Chemistry
• Environmental chemistry
• Physical geography
• Atmospheric sciences
• Soil science
• Botany
• Meteorology

Selected publications

• Priority research questions in global peatland science

  Communications Earth & Environment · 2026-04-28

  articleOpen access

  Abstract Peatlands are among Earth’s largest terrestrial carbon stores and are crucial for climate regulation, biodiversity conservation, and water security. Yet peatlands worldwide are deteriorating under pressures from climate change and human disturbance. Strategic, globally coordinated research is urgently needed to protect, restore and manage peatlands so they can continue to deliver essential ecosystem services. To meet this challenge, here we present a global research prioritisation for peatland science, based on a two-stage online survey and expert voting exercise involving 467 participants from 54 countries. We identify 50 priority research questions spanning carbon dynamics, climate impacts, restoration and management, technological innovation, and community and policy engagement. These questions provide a community-informed agenda to guide peatland research over the next decade. Addressing them will help close critical knowledge gaps, strengthen evidence-based decision making, and support the role of peatlands in achieving global climate and biodiversity goals.

  PublisherDOI

• Soil and tree stem greenhouse gas fluxes from nutrient-rich and nutrient-poor tropical peatland forests

  2026-03-14

  articleOpen access

  Tropical peatland forests are significant sources and sinks of greenhouse gases (GHG), yet the relative contributions of soil and tree stem fluxes have remained poorly quantified, particularly CH4 and N2O fluxes across gradients of nutrient availability. We conducted simultaneous measurements of CO2, CH4 and N2O fluxes from both soil and tree stems using soil and stem chamber in two contrasting tropical peat swamp forests: a nutrient-rich in Quistococha, Peru and a nutrient-poor in Maludam, Sarawak, Malaysia. Our results showed higher soil CO2, CH4 and N2O fluxes from Quistococha nutrient-rich forest. Tree stem respiration was consistently higher in the nutrient-poor forest across all dominant species in both forests. Tree stem CH4 fluxes exhibited distinct patterns, with significantly higher emissions from the nutrient-rich forest, while displaying species-specific behaviour among dominant tree species. Mauritia flexuosa palm stems in Quistococha showed high emission of CH4 from stems with potential CH4 sinks from specific species from both forests. N2O emissions were also species-specific and higher from the nutrient-rich forest, with negligible fluxes observed from the species in the nutrient-poor forest. From stem fluxes to tree fluxes upscaling, we found that the majority of total ecosystem GHG flux originated from soil with minimal contribution from the dominant tree species. In conclusion, these findings highlighted tree stems from tropical peatland can act as sources and sinks and that nutrient availability influence on the magnitude of greenhouse gas emissions.

  PublisherDOI

• Influences on growth and survival of seedlings planted in black ash stands of Northern Michigan and Wisconsin

  Forest Ecology and Management · 2026-02-28

  articleOpen access

  The invasive emerald ash borer ( Agrilus planipennis ) is a threat to all North American ash (genus Fraxinus ) trees, but wetland ecosystems dominated by black ash ( F. nigra ) are of particular concern to land managers. The rapid and extensive death of black ash on the landscape causes unprecedented ecological and hydrological impacts to cascade through the ecosystem. A strategy to combat such impacts and maintain these wetlands as forested is to underplant these stands with seedlings of alternative species that are capable of maturing into canopy trees. This study planted seedlings of seven non-ash tree species in black ash wetlands across northern Michigan and Wisconsin and assessed the factors that have the greatest effect on seedling survival and growth. First year survival across all species was 54 %, and by two years after planting total survival was 47 % with a mean relative growth rate of 5.47 cm/yr. The best-performing species in terms of both survival and growth rates were river birch ( Betula nigra ), silver maple ( Acer saccharinum ), and swamp white oak ( Quercus bicolor ). Bur oak ( Q. macrocarpa ) and red maple ( A. rubrum ) also exhibited high rates of survival despite having below average growth rates. The main factors influencing seedling performance were hydrology and microtopography, with greatly increased survival for seedlings planted on higher microsites further from the water table. Insufficient natural regeneration of canopy species observed in the understory of all stands warrants planting within these systems to avoid the vegetative community shift to dominance of herbaceous or shrub species. • Two years after planting, black ash stands had insufficient natural regeneration. • Black spruce had the lowest survival and growth rates of the planted species. • Hydrology and microtopography had the most influence on seedling performance. • It is important to consider multiple goals when designing planting initiatives.

  PublisherDOI

• Mapping the distribution and condition of mountain peatlands in Colombia for sustainable ecosystem management

  Journal of Environmental Management · 2025-03-12 · 2 citations

  article
  PublisherDOI

• A Large Amazonian Peatland Carbon Sink Was Eliminated by Photoinhibition of Photosynthesis and Amplified Ecosystem Respiration

  Geophysical Research Letters · 2025-06-30 · 1 citations

  articleOpen access

  Abstract The fate of tropical peatland carbon cycling under environmental change is highly uncertain. We found that a palm swamp peatland in the Peruvian Amazon that was a strong annual sink for 2 years switched to carbon neutral in the absence of a major anthropogenic disturbance. We attributed the change in carbon sink strength to (a) photoinhibition of canopy photosynthesis when skies were clearer and thus solar irradiance higher and (b) increased ecosystem respiration when the water table position was below the peat surface, and heterotrophic respiration was amplified. These mechanisms were not, however, synchronous in time. The importance of photoinhibition as a driver of changes in peatland carbon budgets is a novel finding, and an understudied mechanism of canopy photosynthetic impairment. Shifts in climate that increase periods with sustained high solar irradiance and/or low water table are thus likely to amplify interannual variability in the carbon sink strength.

  PublisherOA PDFDOI

• The Effects of Forest Harvesting on Total and Methylmercury Concentrations in Surface Waters Depend on Harvest Practices and Physical Site Characteristics

  Environmental Science & Technology · 2025-07-22 · 2 citations

  articleOpen access

  Forest harvesting can lead to mercury (Hg) mobilization from soils to aquatic habitats and promote the transformation of inorganic Hg to highly neurotoxic and bioaccumulative methyl-Hg (MeHg). Multiple past studies reveal broad variation of stream water MeHg and total Hg (THg) concentration responses to forest harvesting, which has confounded messaging to forest and resource managers. To advance beyond divergent and sometimes contradictory findings, we synthesized information for 23 previously studied catchments in North America and Fennoscandia and compiled a uniform set of soil, landscape, and harvesting properties to identify forest management, riparian, and hillslope factors that influence responses of stream water MeHg and THg concentrations. From this synthesis, we found catchments with high soil moisture and organic soil layers >100 cm to be at highest risk for disturbance-induced increases in MeHg formation after harvest but not necessarily affecting concentrations of MeHg in stream waters. Instead, the combination of MeHg formation in soils along with factors that affect mobilization with runoff to streams most influenced how forest harvest affects MeHg concentrations in stream waters.

  PublisherDOI

• Hydrologic and Landscape Drivers of Seepage Lake ANC Status in Northern Wisconsin, USA

  Lakes & Reservoirs Science Policy and Management for Sustainable Use · 2025-01-01

  articleOpen access

  ABSTRACT In some seepage lakes, acid neutralising capacity (ANC) is regulated by precipitation chemistry and in‐lake biogeochemical processes with little influence from groundwater and catchment runoff. However, additional environmental contexts, for example, landscape position and changing precipitation dynamics, may also contribute to variability in ANC across lakes over time. We used a mixed effects model to assess the influence of changing precipitation patterns and the chemistry of hydrologic inputs on ANC in seven northern Wisconsin, USA seepage lakes from 1984 to 2018. We observed differential ANC responses across lakes to the concomitant changes in precipitation acidity and post‐drought oxidative acidity pulses from adjacent wetland soils. Although all study lakes lie within a 4 km 2 area, mean lake ANC values ranged from 32.8 ± 25.2 μeq L −1 to 71.9 ± 26.8 μeq L −1 and were inversely related to lake landscape position. Lakes with higher mean ANC also showed lower relative variability over time, suggesting that these lakes were better buffered than those with lower ANC. Mixed effects modelling explained 50% of the ANC variability across all lakes, while model explanatory power ranged from 39% to 71% when ANC was assessed within individual lakes. The range of modelled ANC accuracy highlights the complex nature of biogeochemical regulation in individual seepage lakes, despite their shared geographic setting. Our study demonstrates the utility of mixed effects modelling for repeated measures analysis, emphasising the necessity of long‐term studies to evaluate seepage lake ANC responses to changing hydrologic and biogeochemical inputs.

  PublisherOA PDFDOI

• Influence of barrens restoration treatments on soil carbon, nitrogen, and mercury pools and emissions

  Soil Science Society of America Journal · 2024-04-05 · 1 citations

  article1st authorCorresponding

  Abstract Currently barrens communities only represent about 1% of their original area in the Great Lakes region. To maintain or restore barrens vegetation, prescribed fire is often applied to limit the regeneration of undesirable species and shrubs. Vegetation community response is a combination of direct fire effects on the vegetation vitality and the indirect effect of soil nitrogen (N) loss that favors nutrient‐poor adapted barren communities. In this study, we assessed forest floor and upper mineral soil (0–5 cm) pools of carbon (C), N, and mercury (Hg) before and after prescribed fire of the Moquah Barrens in northwest Wisconsin. Although we took measurements in four distinct cover types, we found no relationship between cover type and soil pools. Across all cover types, prescribed fire led to considerable emissions of C, N, and Hg in the forest floor but only Hg in the upper mineral soils (0–5 cm), presumably because maximum fire temperatures were met for Hg volatilization. We classified fire severity and soil surface temperatures at the quadrat scale, but no discernable relationships with emissions were observed. The lack of detectable relationships is likely the result of a mismatch between the scales of response variables and predictors. As a result, we calculated ecosystem‐scale fire emissions based on the total area burned because we could not discern other smaller scale predictors. Overall emissions from dormant, spring season prescribed fires at the Moquah Barrens were approximately 11,000 Mg (5.5 Mg ha −1 ) for C, 350 Mg for N (0.17 Mg ha −1 ), and 4,500 g for Hg (2.3 g ha −1 ).

  PublisherDOI

• Predictive soil health indicators across a boreal forest to agricultural conversion gradient

  Agricultural & Environmental Letters · 2024-04-24 · 1 citations

  articleOpen access

  Abstract A changing climate offers new opportunities to expand agriculture in northern latitudes, and understanding forest‐to‐agriculture land conversion impacts is critical to ensure soil sustainability. Using the Comprehensive Assessment of Soil Health (CASH) framework, we identified a minimum suite of indicators with little collinearity to reliably predict soil impacts during the conversion of boreal forest to agriculture and a time since conversion gradient (forest, <10 years, >10 and <50 years, and >50 years since conversion). We sampled paired forest and agricultural sites and used multiple linear regression to assess 16 indicators and found four‐ and six‐indicator models predicted the CASH score with varying but reasonable accuracy depending on conversion class. Organic matter, water aggregate stability, and pH were consistent predictors across all classes, as well as one or more micronutrients. The CASH framework appears to be more suitable for agricultural soils and as time since conversion proceeds.

  PublisherOA PDFDOI

• Infiltration within native prairie vegetative strips embedded in row crop fields across Iowa

  Journal of Soil and Water Conservation · 2024-01-01 · 3 citations

  article

  The integration of native prairie vegetative strips into row crop agriculture is a promising conservation strategy that has gained momentum in adoption rates throughout the US Midwest. Previous studies have shown that prairie strip establishment can lead to several positive soil and water quality outcomes, such as reductions in surface runoff and nutrient and sediment exports. However, the impacts of prairie strips on soil infiltration are not well known. In this study, the Cornell Sprinkle Infiltrometer system was used to measure differences in field-saturated infiltration rate between prairie strip and row crop treatments at six sites across Iowa after five to seven years since prairie strip establishment. Additionally, approximate sorptivity was calculated to compare trends in early infiltration between the two treatments at each site. Measurements were taken over a two-year span during summer and fall testing periods. Further, at two additional prairie strips sites, a separate approach using the tension infiltrometer generated hydraulic conductivity data for prairie strip and row crop treatments at 3, 4, and 14 years since prairie strip establishment. Differences between prairie strip and row crop were mostly undetected across nearly all sites in field-saturated infiltration rate and saturated hydraulic conductivity at 5 to 7 and 14 years after prairie strip establishment, respectively. However, at one site, saturated hydraulic conductivity was significantly greater within prairie strip than row crop, and at another, field-saturated infiltration rate was 3.6 times greater in prairie strip than row crop. Therefore, considering trends from both prairie strip age and infiltration testing method groups, differences in saturated infiltration capacity between prairie strip and row crop appear to be related to site-specific characteristics like soil texture, row crop tillage, and soil organic matter, especially at earlier stages of prairie strip establishment. Comparing trends in sorptivity approximations between the two treatments determined that prairie strips had 26% and 38% greater early infiltration than row crops during fall testing periods, but no treatment difference was found in the summer testing period. Since significant results were mostly limited to the fall, a combination of initial soil moisture and surface roughness disparities between treatments likely explain the observed treatment differences in approximate sorptivity. Within prairie strips, greater early infiltration relative to row crops delays and limits surface runoff generation. Therefore, this study suggests that a row crop field containing prairie strips will generate less surface runoff than a comparable 100% row crop field during a given rainfall event at the end and potentially beginning of the annual corn (<i>Zea mays</i> L.) and soybean (<i>Glycine max</i> [L.] Merr.) growing season in Iowa. By improving early infiltration and subsequently limiting runoff generation and sediment transport, prairie strips can be a valuable soil and water conservation tool.

  PublisherDOI

Frequent coauthors

• Stephen D. Sebestyen

  Northern Research Station

  132 shared

• Evan S. Kane

  Northern Research Station

  72 shared

• John B. Bradford

  United States Geological Survey

  61 shared

• Carl P. J. Mitchell

  The Scarborough Hospital

  57 shared

• Edward A. Nater

  University of Minnesota

  55 shared

• Erik A. Lilleskov

  45 shared

• D. Tyler Roman

  US Forest Service

  43 shared

• Brian J. Palik

  Northern Research Station

  42 shared