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16.2D: Wetland Soils - Biology

16.2D: Wetland Soils - Biology


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Wetlands are considered one of the most biologically diverse of all ecosystems.

Learning Objectives

  • Assess the composition of wetland soils

Key Points

  • Nutrient cycling in lakes and freshwater wetlands depends heavily on redox conditions.
  • Some anaerobic microbial processes include denitrification, sulfate reduction and methanogenesis and are responsible for the release of N2 (nitrogen), H2S (hydrogen sulfide) and CH4 (methane).
  • Other anaerobic microbial processes are linked to changes in the oxidation state of iron and manganese and as a result of anaerobic decomposition, the soil stores large amounts of organic carbon because decomposition is incomplete.

Key Terms

  • denitrification: The process by which a nitrate becomes molecular nitrogen, especially by the action of bacteria.
  • methanogenesis: The generation of methane by anaerobic bacteria.
  • heterotroph: An organism that requires an external supply of energy in the form of food as it cannot synthesize its own.

A wetland is a land area that is saturated with water, either permanently or seasonally, such that it takes on the characteristics of a distinct ecosystem. Primarily, the factor that distinguishes wetlands from other land forms or water bodies is the characteristic vegetation that is adapted to its unique soil conditions: Wetlands consist primarily of hydric soil, which supports aquatic plants. The water found in wetlands can be saltwater, freshwater, or brackish. Main wetland types include swamps, marshes, bogs and fens. Sub-types include mangrove, carr, pocosin, and varzea.

Wetlands play a number of roles in the environment, principally water purification, flood control, and shoreline stability. Wetlands are also considered the most biologically diverse of all ecosystems, serving as home to a wide range of plant and animal life.

In balanced soil, plants grow in an active and steady environment. The mineral content of the soil and its heartiful structure are important for their well-being, but it is the life in the earth that powers its cycles and provides its fertility. Without the activities of soil organisms, organic materials would accumulate and litter the soil surface, and there would be no food for plants.

The soil biota includes:

Megafauna: size range – 20 mm upward, e.g. moles, rabbits, and rodents.

Mesofauna: size range – 100 micrometres to 2 mm, e.g. tardigrades, mites, and springtails.

Microfauna and Microflora: size range – 1 to 100 micrometres, e.g. yeasts, bacteria (commonly actinobacteria), fungi, protozoa, roundworms, and rotifers.

Of these, bacteria and fungi play key roles in maintaining a healthy soil. They act as decomposers that break down organic materials to produce detritus and other breakdown products. Soil detritivores, like earthworms, ingest detritus and decompose it. Saprotrophs, well represented by fungi and bacteria, extract soluble nutrients from delitro. The ants (macrofaunas) help by breaking down in the same way but they also provide the motion part as they move in their armies. Also the rodents, wood-eaters help the soil to be more absorbent.

Nutrient cycling in lakes and freshwater wetlands depends heavily on redox conditions. Under a few millimeters of water heterotrophic bacteria metabolize and consume oxygen. They therefore deplete the soil of oxygen and create the need for anaerobic respiration. Some anaerobic microbial processes include denitrification, sulfate reduction and methanogenesis and are responsible for the release of N2 (nitrogen), H2S (hydrogen sulfide) and CH4 (methane). Other anaerobic microbial processes are linked to changes in the oxidation state of iron and manganese. As a result of anaerobic decomposition, the soil stores large amounts of organic carbon because decomposition is incomplete.

The redox potential describes which way chemical reactions will proceed in oxygen deficient soils and controls the nutrient cycling in flooded systems. Redox potential, or reduction potential, is used to express the likelihood of an environment to receive electrons and therefore become reduced. For example, if a system already has plenty of electrons (anoxic, organic-rich shale) it is reduced and will likely donate electrons to a part of the system that has a low concentration of electrons, or an oxidized environment, to equilibrate to the chemical gradient. The oxidized environment has high redox potential, whereas the reduced environment has a low redox potential.

The redox potential is controlled by the oxidation state of the chemical species, pH and the amount of oxygen (O2) there is in the system. The oxidizing environment accepts electrons because of the presence of O2, which acts as electron acceptors:

O2 + 4e + 4H+ → H2O

This equation will tend to move to the right in acidic conditions which causes higher redox potentials to be found at lower pH levels. Bacteria, heterotrophic organisms, consume oxygen while decomposing organic material which depletes the soils of oxygen, thus increasing the redox potential. In low redox conditions the deposition of ferrous iron (Fe2+) will increase with decreasing decomposition rates, thus preserving organic remains and depositing humus.


Structural and Functional Loss in Restored Wetland Ecosystems

Affiliations Integrative Biology Department, University of California at Berkeley, Berkeley, California, United States of America, Jasper Ridge Biological Preserve, Stanford University, Woodside, California, United States of America

Affiliation Integrative Biology Department, University of California at Berkeley, Berkeley, California, United States of America

Affiliation Department of Conservation of Biodiversity and Ecosystem Restoration, Pyrenean Institute of Ecology – CSIC, Zaragoza, Spain

Affiliation UMR CNRS 7205, Muséum National d'Histoire Naturelle, Paris, France


Mineralogical associations with soil carbon in managed wetland soils

Tyler L. Anthony, Department of Environmental Science, Policy, and Management, University of California Berkeley, 130 Mulford Hall, Berkeley, CA 94720, USA.

Ecosystem Science Division, Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, USA

Ecosystem Science Division, Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, USA

Tyler L. Anthony, Department of Environmental Science, Policy, and Management, University of California Berkeley, 130 Mulford Hall, Berkeley, CA 94720, USA.

Ecosystem Science Division, Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, USA

Abstract

Carbon (C)-rich wetland soils are often drained for agriculture due to their capacity to support high net primary productivity. Increased drainage is expected this century to meet the agricultural demands of a growing population. Wetland drainage can result in large soil C losses and the concentration of residual soil minerals such as iron (Fe) and aluminum (Al). In upland soils, reactive Fe and Al minerals can contribute to soil C accumulation through sorption to poorly crystalline minerals and coprecipitation of organo-metal complexes, as well as C loss via anaerobic respiration by Fe-reducing bacteria. The role of these minerals in soil C dynamics is often overlooked in managed wetland soils and may be particularly important in both drained and reflooded systems with elevated mineral concentrations. Reflooding drained soils have been proposed as a means to sequester C for climate change mitigation, yet little is known about how reactive Fe and Al minerals affect C cycling in restored wetlands. We explored the interactions among soil C and reactive Fe and Al minerals in drained and reflooded wetland soils. In reflooded soils, soil C was negatively associated with reactive Fe and reduced Fe(II), a proxy for anaerobic conditions (reactive Fe: R 2 = .54–.79 Fe(II): R 2 = .59–.89). In drained soils, organo-Al complexes were positively associated with soil C and Fe(II) (Al R 2 = .91 Fe(II): R 2 = .54–.60). Soil moisture, organo-Al, and reactive Fe explained most of the variation observed in soil C concentrations across all sites (p < .01). Reactive Fe was negatively correlated to soil C concentrations across sites, suggesting these Fe pools may drive additional C losses in drained soils and limit C sequestration with reflooding. In contrast, reactive organo-Al in drained soils facilitates C storage via aggregation and/or formation of anaerobic (micro)sites that protect residual soil C from oxidation and may at least partially offset C losses.


Connecting carbon and nitrogen storage in rural wetland soil to groundwater abstraction for urban water supply

We investigated whether groundwater abstraction for urban water supply diminishes the storage of carbon (C), nitrogen (N), and organic matter in the soil of rural wetlands. Wetland soil organic matter (SOM) benefits air and water quality by sequestering large masses of C and N. Yet, the accumulation of wetland SOM depends on soil inundation, so we hypothesized that groundwater abstraction would diminish stocks of SOM, C, and N in wetland soils. Predictions of this hypothesis were tested in two types of subtropical, depressional-basin wetland: forested swamps and herbaceous-vegetation marshes. In west-central Florida, >650 ML groundwater day −1 are abstracted for use primarily in the Tampa Bay metropolis. At higher abstraction volumes, water tables were lower and wetlands had shorter hydroperiods (less time inundated). In turn, wetlands with shorter hydroperiods had 50–60% less SOM, C, and N per kg soil. In swamps, SOM loss caused soil bulk density to double, so areal soil C and N storage per m 2 through 30.5 cm depth was diminished by 25–30% in short-hydroperiod swamps. In herbaceous-vegetation marshes, short hydroperiods caused a sharper decline in N than in C. Soil organic matter, C, and N pools were not correlated with soil texture or with wetland draining-reflooding frequency. Many years of shortened hydroperiod were probably required to diminish soil organic matter, C, and N pools by the magnitudes we observed. This diminution might have occurred decades ago, but could be maintained contemporarily by the failure each year of chronically drained soils to retain new organic matter inputs. In sum, our study attributes the contraction of hydroperiod and loss of soil organic matter, C, and N from rural wetlands to groundwater abstraction performed largely for urban water supply, revealing teleconnections between rural ecosystem change and urban resource demand.

Figure S1. Plan and elevation projections of sampling scheme.

Figure S2. Frequency of recording water table does not affect calculations of hydroperiod.

Table S1. Descriptive site information.

Table S2. Wetland location does not confound soil variable-hydroperiod correlations.

Table S3. Correlations among predictor variables.

Table S4. Soil variables as functions of soil texture and hydrology: model comparisons.

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.


Electronic supplementary material is available online at http://dx.doi.org/10.6084/m9.figshare.c.4341131.

Published by the Royal Society. All rights reserved.

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Abstract

Any change in the intensity and sign of CO2 flux between soil and atmosphere is expected to have a significant impact on climate. The net emission of CO2 by soils depends on antagonistic processes: the persistence of dead plant matter and the mineralization of soil organic matter. These two processes are partly interdependent: their interaction is known as the “priming effect” (PE), i.e. the stimulation of the mineralization of stable soil organic matter by more labile fresh organic matter.

Documenting the response of PE to global change is needed for predicting long term dynamics of ecosystems and climate change. We have tested the effects on PE of temperature, nutrient availability, biodegradibility of added organic matter (fresh vs. decomposed), soil cover (agricultural vs. forest soil) and interactions.

Our results suggest that the biodegradability of plant debris (wheat straw, fresh or pre-decomposed) is the first determinant of the intensity of PE, far ahead of temperature and nutrients: fresh wheat straw addition induced up to 800% more CO2 emission than pre-decomposed one. The raise of temperature from 15 to 20 °C, increased basal soil organic matter mineralization by 38%, but had little effect on PE. Interactions between biodegradability of straw and the other factors showed that the agricultural soil was more responsive to all factors than the forest soil.

We have shown in our study that the intensity of PE could be more dependent on soil cover and plant residue management than on other drivers of global change, particularly temperature and nutrients. There is an urgent need to assess the genericity of our results by testing other soil types and plant debris for a better integration of PE in models, and for identifying alternative land carbon management strategies for climate change mitigation.


Wetland Ecology


Photo courtesy Jennifer Key,
TPWD 2004, Angelina County

Wetlands, which are fluctuating ecosystems inherently difficult to categorize, are often found at the intersection of terrestrial habitat and aquatic habitat and usually include elements of both systems. Many wetlands are unique to a certain degree, as their individual characteristics are determined by a combination of factors such as climate, soils, hydrology, and vegetation.

One of the most important factors that determines the overall nature of a wetland however, is hydrology, since the timing, quantity, and duration of water flow strongly influences both abiotic and biotic factors within a wetland. Abiotic factors that are determined by hydrology in a wetland could include soil texture, water quality, or topography, whereas biotic factors influenced by hydrology in a wetland would be plant and animal types, diversity, or quantity. Of course, hydrology doesn't always affect biology, as animals such as beavers can change the nature of a stream by constructing a dam, or vegetation can build up over time in an area and reduce available surface water through increasing evapotranspiration.

Most wetlands experience a fluctuating water level on a seasonal or even yearly basis, so some areas that are difficult to identify as wetlands during the summer may be completely inundated during the winter. Texas contains several different types of wetlands, each of which offers a varying degree of wetland functions and values, including providing habitat for wildlife.

Abiotic factors:
Nonliving elements of the environment, such as soil or climate.

Biotic factors:
Living elements of the environment, such as plants, animals, or bacteria.


INTRODUCTION

For many millennia, humans have been cultivating land for food production. Initially, human settlements primarily occurred in fertile areas along rivers. In the floodplains of Mesopotamia, such settlements were the very cradle of human civilization 6000 years ago. From the early beginning of agricultural activities, such riverine wetlands have been recognized as valuable land areas for food and fodder production, because they have fertile soils as a result of regular sediment deposition during flood events. Access to waterways for transport was a major additional advantage. In the course of history, wetlands have been reclaimed for agriculture in many parts of the world with ever more effective drainage and land amelioration measures. The natural wetland ecosystems reclaimed in this way have lost much of their original character, leading to reduced biodiversity and reduced performance of functions other than crop productivity (Hassan et al., 2005). For the global resource of freshwater wetlands, it is certain that substantial wetland areas have been lost because of drainage and development, although quantitative estimates are only available for a number of regions more than 50 % of the area of peatlands, depressional wetlands, riparian zones, lake littoral zones and floodplains has been lost, mostly through conversion to intense agricultural use, in North America, Europe and Australia (Millennium Ecosystem Assessment, 2005).

Although wetland protection is officially a priority for the 159 nations (as of 2009) that have ratified the Ramsar Convention (www.ramsar.org), wetlands continue to be under threat of being drained and reclaimed. Based on the expected growth of the world population in the next 25 years, the need for food products will increase 50 % by 2030 (Hassan et al., 2005). In addition, there is a growing trend to grow energy crops for use in biofuel production (Smeets et al., 2007). At the same time, measures to enhance 𠆌limate-neutral’ economic activities will result in initiatives to plant forests in open areas, including non-forested wetlands. All these developments will lead to a greater pressure to reclaim still remaining natural areas for agricultural purposes. This could mean that wetlands run an increasingly higher risk of being drained and destroyed. Another consequence may be the active search for more flood-tolerant and salt-tolerant crop varieties that may grow successfully under limited periods of waterlogging or drought-associated salt stress. This may lead to agricultural activities in wetlands that leave the water regime of the wetland intact but still disturb the wetland ecosystem by adding fertilizer or pesticides. Important aspects of a wetland's character will therefore be harmed and functions other than productivity may still be diminished or destroyed.

The aim of this review is to evaluate the impacts of the agricultural use of wetlands from different perspectives, with special attention to the consequences of past and current developments of land-use dynamics and new agricultural approaches for wetland functions and their benefits worldwide. We first briefly review the history of wetland use, and address to what extent agricultural productivity went together with high biodiversity and other valuable functions. Subsequently, we illustrate these developments with case studies where wetland reclamation has (had) dramatic consequences and give an overview of the latest developments in crop science with respect to flood tolerance. Finally, we summarize the information and give an opinion on the degree of sustainability of various types of agricultural use in wetlands. Sustainability has ecological, economic and social dimensions (Falvey, 2004). We refer to sustainable agriculture in wetlands as the ability to produce food indefinitely, without causing severe or irreversible damage to the wetland ecosystem character.


Unit 1.3 Wetland Metaphors

CLICK HERE for pdf file (for grades 1-12). Common objects can be used a physical metaphor for natural wetland functions. Students will:

  • Describe characteristics of wetlands.
  • Appreciate the importance of wetlands to wildlife and humans.
  • Identify ecological functions of wetlands.

Additional extension activities:

  • Create graphic illustrations of metaphors. Write poems or stories using the metaphors, and publish them in the school newspaper.
  • Create riddles to have classmates guess the critter.
  • Make a Wetland Newsletter of creative writing for the school community.

Federal Agency Definitions of Wetland Tracking

The Federal Geographic Data Committee, Wetlands Subcommittee developed definitions for restoration and related activities designed to aid agencies in accurately reporting wetland increases due to their program activities. Many different definitions of these terms have been used by various agencies. The definitions, below, provide standard terminology for the more than 15 agencies involved in wetland restoration, related activities, and/or mitigation.

Restoration: the manipulation of the physical, chemical, or biological characteristics of a site with the goal of returning natural/historic functions to former or degraded wetland. For the purpose of tracking net gains in wetland acres, restoration is divided into:

  • Re-establishment: the manipulation of the physical, chemical, or biological characteristics of a site with the goal of returning natural/historic functions to a former wetland. Re-establishment results in rebuilding a former wetland and results in a gain in wetland acres.
  • Rehabilitation: the manipulation of the physical, chemical, or biological characteristics of a site with the goal of repairing natural/historic functions of degraded wetland. Rehabilitation results in a gain in wetland function, but does not result in a gain in wetland acres.

Establishment: the manipulation of the physical, chemical, or biological characteristics present to develop a wetland that did not previously exist on an upland or deepwater site. Establishment results in a gain in wetland acres.

Enhancement: the manipulation of the physical, chemical, or biological characteristics of a wetland (undisturbed or degraded) site heighten, intensify, or improve specific function(s) or for a purpose such as water quality improvement, flood water retention or wildlife habitat. Enhancement results in a change in wetland function(s) and can lead to a decline in other wetland function, but does not result in a gain in wetland acres. This term includes activities commonly associated with the terms enhancement, management, manipulation, directed alteration.

Protection/Maintenance: the removal of a threat to, or preventing decline of, wetland conditions by an action in or near a wetland. Includes purchase of land or easement, repairing water control structures or fences, or structural protection such as repairing a barrier island. This term also includes activities commonly associated with the term preservation. Protection/Maintenance does not result in a gain of wetland acres or function.