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Biogeochemistry

I plead the soil amendment – Improving the function of restored wetlands

Ballantine, K. a, Groffman, P. M., Lehmann, J., & Schneider, R. L. (2014). Stimulating Nitrate Removal Processes of Restored Wetlands. Environmental Science & Technology. doi:10.1021/es500799v

 

The input of excessive nutrients, such as nitrogen (i.e. in the form of nitrate, NO3), to coastal waters often causes many issues, particularly regarding water quality.  For example, excess nutrients (from fertilizers, sewage treatment plants, etc…) can stimulate algal ‘blooms’ which eventually begin to die-off and decompose, depleting the water of oxygen leaving it in a hypoxic (low-oxygen) or anoxic (no-oxygen) state.  One mechanism of mitigation to this problem is to restore and/or create wetlands, which are estimated to reduce as much as 30-85% of the nitrate contained in runoff each year.  Wetlands are known providers of “ecosystem services,” valuable services, such as buffering storm surges and improving water quality in a region.  Wetlands, with the right conditions, can remove excess nitrate (NO3) from a system through the process of denitrification, where the NO3is ultimately converted to nitrogen gas (N2) through a series of processes carried out by certain denitrifying bacteria.

Simple Nitrogen cycle schematic courtesy of ecosystems.mbl.edu

Simple Nitrogen cycle schematic courtesy of ecosystems.mbl.edu

Motivation

This denitrifying bacteria largely gets its energy by feeding on “labile” organic carbon (scientific-speak for what’s biologically available to an organism), but unfortunately, the soil present in restored wetlands does not typically mimic the same characteristics as the soil that’s been present in existing wetlands for centuries and they do not always have enough labile carbon to allow for denitrification to take place.  This study investigates the efficacy of amending soil in restored wetlands to help them better function as water quality improvers. The motivation for this research is the surprisingly little amount of research that’s been done on this topic in the past, and the fact that what is out there is a matter of conflicting opinions.

What they did

A group of researchers in western New York looked at four different newly restored wetlands with the hopes of elucidating the effects of carbon additions on denitrification rates and other processes within the nitrogen cycle.  Each of the wetlands had different hydrology patterns (i.e. duration inundated with water) and different background soil characteristics, which were all measured and characterized throughout the study.  Four different ‘treatments’ of 8kg of organic carbon were randomly applied to plots in the form of straw, topsoil, a 50:50 mix of straw and biochar (what’s formed when biomass is burned under low oxygen conditions), and biochar.  Control plots were also employed to test for background levels.

A suite of soil samples were taken throughout the experiment to test for denitrification potential (the potential capability a system may have to undergo denitrification), properties that affect denitrification, (such as labile carbon content, soil carbon content, pH, moisture, and hyrology), along with other processes involving the nitrogen cycle.  Statistics were then run to see which, if any, treatments were producing significant effects on the wetlands.

What they found

Overall, the researchers determined that the various soil amendments studied in these restored wetlands dramatically affected processes occurring in the nitrogen cycle, which are essential for denitrification.

In terms of ‘background’ conditions that affect denitrification (with or without soil amendments), sites that were intermittently flooded were found to have higher denitrification potential over sites that were constantly inundated.  In addition, sites with higher microbial N biomass and higher nitrate levels showed higher denitrification potential.

Ballantine et al - nitrate removal 2

When assessing the effects of soil amendments on denitrification potential and nitrogen cycling processes, the researchers found that the nitrate content, the carbon to nitrogen ratio (C:N), and the microbial biomass of N were the three most important factors in determining denitrification potential of the wetlands, which are different than the previously reported carbon availability predictor.   Plots that were amended with topsoil were found to have the highest denitrification potential, which the authors attribute to a proper mix of labile carbon and nitrogen.  The effects of the straw and biochar plots to improve denitrification potential were less obvious and merit further research.  These restored wetlands acted similarly to other restored wetlands, but their capacity for nutrient cycling and sustained plant production falls short of what natural systems can do.  Conversely, the plots amended with both the straw and the mix of straw and biochar had the highest labile carbon pool measured.

This study addresses an important point that restored wetlands are not always designed in the most efficient ways and often fall short of providing their desired function.  The numerous soil characteristics measured here are all important considerations when choosing a proper site for wetland restoration. Future research will help lead to observations about long term trends and differences that may occur across different treatments.

 

To read about a different method of nutrient mitigation, Click here!

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