Project Abstract

<p></p><p>                <b>ABSTARCT&nbsp;</b><br></p><p>Earthworms may have an influence on the production of N2O, a greenhouse gas, as a result of the ideal environment contained in their gut and casts for denitrifier bacteria. The objective of this study was to determine the relationship between earthworm (<i>Lumbricus terrestris</i>&nbsp;L.) population density, soil water content and N2O&nbsp; emissions in a controlled greenhouse experiment based on population densities (90 to 270 individuals ) m-2 found at the Guelph Agroforestry Research Station (GARS) from 1997 to 1998. An experiment conducted at considerably higher than normal densities of earthworms revealed a significant relationship between earthworm density, soil water content and N2O&nbsp; emissions, with mean emissions increasing to 43.5 g&nbsp; ha -1 day -1 at 30 earthworms 0.0333 m-2 &nbsp;at 35% soil water content. However, a second experiment, based on the density of earthworms at GARS, found no significant difference in N2O emissions (5.49 to 6.99 g ha-1 day -1 ) aa a result of density and 31% soil water content.</p> <br><p></p>

Project Overview

<p> </p><p><b>INTRODUCTION&nbsp;</b><br></p><p>The presence of earthworms can be seen as an added benefit to many agricultural systems since earthworms contribute greatly to the overall physical properties of agricultural soils [<a target="_blank" rel="nofollow">1</a>]. Previous studies in sole cropping systems have focused on the ability of earthworms to facilitate soil mixing and the decomposition of organic matter, which is especially important in agricultural systems [<a target="_blank" rel="nofollow">2</a>–<a target="_blank" rel="nofollow">4</a>]. Earthworms also affect soil properties, by increasing soil porosity and decreasing bulk density and through bioturbation and cast deposition on the soil surface [<a target="_blank" rel="nofollow">1</a>]. Earthworm activity stimulates mineralization of N in residues, which promotes the availability for plants and microorganisms of inorganic forms of N from plant material [<a target="_blank" rel="nofollow">1</a>, <a target="_blank" rel="nofollow">5</a>].</p> However, increased earthworm population might increase the production of nitrous oxide (N2O) emissions from agricultural soils. Over 50% of <i>in situ </i>N2O emissions, in some soils, could be a result of earthworm activity [<a target="_blank" rel="nofollow">6</a>]. Recent research suggests that, globally, earthworms could be producing up to  kg of N2O annually [<a target="_blank" rel="nofollow">6</a>]. Conventional agricultural practices, which aim to encourage earthworm populations due to their positive influence on soil properties are the highest anthropogenic sources of N2O emissions. On a global scale, annual emissions of N2O were 16.2 Tg in 2004 [<a target="_blank" rel="nofollow">7</a>], and as a result, earthworms could be responsible for nearly 2% of global emissions. <br><p></p><p> </p><p>One reason for this is that the earthworm gut is an ideal environment for denitrification [<a target="_blank" rel="nofollow">8</a>–<a target="_blank" rel="nofollow">10</a>]. Using microsensors, Horn et al. [<a target="_blank" rel="nofollow">9</a>] determined that the earthworm gut is anoxic and contains copious carbon substrates for microorganisms and is therefore ideal for N2O production. Denitrification is enhanced when the earthworm ingests denitrifier bacteria with organic matter [<a target="_blank" rel="nofollow">1</a>, <a target="_blank" rel="nofollow">8</a>–<a target="_blank" rel="nofollow">10</a>]. When gaseous N2O is produced, it is able to escape the permeable epidermis of the earthworm and diffuses from the soil surface [<a target="_blank" rel="nofollow">9</a>].</p><p>At the Guelph Agroforestry Research Station (GARS) in Guelph, Ontario, Canada, Price and Gordon [<a target="_blank" rel="nofollow">11</a>] found that earthworm density was greater in a Tree-Based Intercropping (TBI) system than in a conventional agricultural monoculture. A TBI system is defined as “an approach to land use that incorporates trees into farming systems and allows for the production of trees and crops or livestock from the same piece of land in order to obtain economic, ecological, environmental and cultural benefits” [<a target="_blank" rel="nofollow">12</a>]. These systems incorporate leaf litter and increase soil water content, which could encourage higher earthworm populations compared to sole cropping systems. In turn, this could increase the overall volume of the earthworm gut, thereby facilitating denitrification and higher N2O emissions from a TBI system. Price and Gordon [<a target="_blank" rel="nofollow">11</a>] also speculated that the reason earthworm densities were higher in the intercropped system compared to the conventional monoculture was because earthworms move to an area with a lower soil temperature, which in turn are areas that also have higher soil water content.</p><p>Currently, very little information exists on the influence that earthworm density has on N2O emissions from agricultural soils, and specifically those potentially associated with a TBI system. The objective of this study was to investigate the relationship, if any, between N2O flux, earthworm density, and gravimetric soil water content, taking into account the earthworm densities calculated by Price [<a target="_blank" rel="nofollow">13</a>] in the TBI and monoculture systems located at GARS and using the most common earthworm species found in GARS, the common nightcrawler (<i>Lumbricus terrestris </i>L). It was hypothesized that N2O flux would be higher as earthworm density and soil water content increased.</p><p><b>MATERIALS AND METHOD</b></p><h5>2.1. Study Design</h5><p>The first experiment was conducted in the Science Complex Phytotron at the University of Guelph, Guelph, Ontario, Canada. The purpose of the first experiment was to determine the optimal soil water content for earthworm activity resulting in the highest N2O emissions. The experiment was a two factorial, completely random design with four replications for a total of 64 experimental units. The first factor was earthworm density (see below) and the second factor was gravimetric soil water content (15%, 25%, 35%, and 45%).</p><p>Soil was collected from GARS and homogenized using a 2 mm sieve. The soil is sandy loam in texture with an average pH of 7.2 [<a target="_blank" rel="nofollow">14</a>]. A leaf litter mixture composed of silver maple (<i>Acer saccharinum</i>&nbsp;L.) and poplar (<i>Populus spp.</i>) leaves was also collected from GARS, dried at C for one week, and mixed into the homogenized soil to achieve a soil organic matter content of approximately 3%. Four kilograms of the soil and leaf litter mixture was then put into each of the 5 L polypropylene mesocosms, equipped with an airtight lid and rubber septum for sampling. The lids were only placed on the mesocosms at the time of N2O sampling. The surface area of each mesocosm was 0.033 m2.</p><p>Earthworm density was calculated based on data collected in the spring of 1997 from GARS by Price [<a target="_blank" rel="nofollow">13</a>]. The three earthworm densities included high, medium, and low earthworm densities, representing populations found 0 m, 3 m, and 6 m from the tree row in a TBI system, respectively. However, these values were tripled in order to ensure the detection of N2O for the purpose of finding optimal soil water content and also to represent an earthworm invasion where populations could initially be very high and decline over time [<a target="_blank" rel="nofollow">15</a>]. These values were 30, 20, and 10 earthworms per 4 kg of soil or 0.033 m, for the high, medium, and low treatments, respectively, and a control with no earthworms. <i>L. terrestris </i>were purchased from Kingsway Sports (Guelph, Ontario, Canada). Earthworms were counted and weighed prior to being added to the mesocosms.</p><p>Prior to adding the earthworms, each mesocosm was fertilized with urea (46-0-0, N-P-K), which represented the N fertilizer requirement for corn planted at GARS (215 kg ha−1). Deionized water was applied to each mesocosm for one week prior to adding the earthworms in order to achieve the desired gravimetric soil water content for each treatment. A small hole in the bottom of each mesocosm allowed for proper drainage. During the course of the experiment, soil water content was maintained by weight. The mesocosms were weighed every day for the entire course of the experiment and deionized water was added to bring each mesocosm to the desired water content.</p><p>The mesocosms were placed in a greenhouse with a constant air temperature of C and monitored light conditions of 16/8 hr cycles. Soil temperature was monitored using Priva soil temperature sensors (Priva North America Inc., Vineland Station, Ontario, Canada) to ensure a constant soil temperature of approximately C. N2O sampling technique and calculations will be explained in the following section.</p><p>A second experiment was conducted from February 2009 to March 2009 in the Science Complex Phytotron at the University of Guelph. Experiment &nbsp;was a completely random design with four replications for a total of 16 experimental units. A control with no earthworms and earthworm densities of 9 (high), 6 (medium), and 3 (low) earthworms per mesocosm were used for a total of four treatments. The high, medium, and low density treatments were calculated based on actual densities found by Price [<a target="_blank" rel="nofollow">13</a>] at GARS representing an earthworm density adjacent to the tree row, 3 m from the tree row, and 6 m from the tree row in a TBI system, respectively; a control with no earthworms was also included.</p><p>Optimal gravimetric soil water content was determined in Experiment &nbsp;and was found to be 31%. This soil water content treatment was held constant for all four earthworm density treatments over the duration of the experiment. Methods for soil preparation, maintaining gravimetric soil water content, and monitoring temperature were the same as in Experiment .</p><h5>2.2. Sampling Procedure</h5><p>At the time of N2O sampling, the airtight lid was placed onto each mesocosm and a 30 mL air sample using a 26-gauge needle and syringe was taken at , 30, and 60 min to calculate N2O flux over an hour. Air samples were deposited into 12 mL Exetainers (Labco Limited, United Kingdom) and analyzed using a SRI Model 8610C Gas Chromatograph (Torrance, California, USA) at Environment Canada (Burlington, Ontario, Canada). N2O samples were taken once a week for four weeks beginning at 10:00 AM.</p><p>A soil sample was taken from each mesocosm, both before the addition of earthworms and after the last week of sampling. This was done to measure the initial and the final nitrate (), ammonium (), and total inorganic N (TIN) concentrations to determine if there was a change over the course of the experiment. Soil samples were stored in the freezer until analysis. N content was measured following a 2N KCl extraction [<a target="_blank" rel="nofollow">16</a>], and samples were run through an Astoria 2-311 Analyzer (Astoria-Pacific Inc., Oregon, USA). Measurements of soil inorganic and organic carbon (C) were also done for initial and final C content using a Leco C determinator (Leco Corporation, St Joseph, MI, U.S.A.). However, results for soil N and C are not reported here and are part of a larger study.</p><h5>2.3. N2O Flux Calculation</h5><p>N2O flux was calculated using the ideal gas law; the molar volume of N2O at C and 1 atm is 44.0128 L/mol. The N2O flux was adjusted for air temperature and pressure using the following formula:where <i>T</i>&nbsp;is the air temperature and <i>P</i>&nbsp;is the air pressure on the day of sampling. These values were taken into consideration because a temperature greater than C increases molar volume and, air pressure that is greater than atmospheric decreases molar volume.</p><p><br></p><p></p>