April 5th, 2017
Sometimes moisturizing can be a bad thing: the effect of soil moisture on nitrate transport and transformations in soil
Nitrogen is one of the most important nutrients for the growth of agricultural crops, and because of this, nitrate (a form of nitrogen that plants use) is commonly applied to crops in the form of fertilizer. However, nitrate easily enters into groundwater through the soil, or into surface waters by run-off. It is also subject to transform into various forms of nitrogen from microbial activity in the soil. Currently, there is no accurate method for estimating a field’s soil nitrogen supply to plants in Atlantic Canada. Because of this, too much fertilizer is often applied to fields. With Atlantic Canada’s wet climate, nitrate often migrates out of fields with soil water or is lost to the air as a gas. When large amounts of nitrate enter aquatic environments, it can have devastating impacts to aquatic life. When large amounts of nitrate are converted into gas that is lost to the atmosphere, it can contribute to global warming.
As a student attending Dalhousie’s Agricultural Campus, I have been studying the effects of soil moisture content on nitrate transport and transformations in soil. The way I am conducting my research is quite easy. Using plastic cups to hold soil samples, I applied different amounts of water that contained nitrate and bromide in order to achieve a variety of soil moisture contents. Bromide was added to act as a conservative tracer for nitrate. Bromide is often used a tracer to study nitrate movements because it has similar chemical properties but it is not subject to microbial transformations like nitrate. An anion exchange membrane was placed in the middle of each soil column. An anion exchange membrane is simply a little polymer sheet that is designed to attract negatively charged nitrate ions. These membranes were replaced after 7, 14, and 21 days. Based on the amount of nitrate and bromide that was removed using the membranes, I was able to determine how much and how fast the chemicals moved through the soil to the membrane. Using the amount of bromide that was removed with the membranes, I was also able to estimate how much nitrate did not make it to the membrane, offering an indication of the transformations that nitrate may have undergone. It was found that the greatest movement of nitrate occurred in soils with 30% soil moisture. However, at higher soil moisture contents of 40% and 50%, nitrate was lost from the soil, likely by a process known as denitrification. Denitrification is a process that occurs in high moisture soils, where nitrate is converted to various gaseous forms of nitrogen, some of which are greenhouse gases.
This study has revealed that using ion exchange membranes may be a practical way to predict nitrate availability for plant uptake in response to soil moisture content based on its ability to measure net nitrate consumption and production from microbial activity. This could minimize the potential for environmental impact due to over fertilization, on both waterways and the atmosphere.