Determining Trace Gas Efflux from Container Production of Woody Nursery Crops

Significance to the Nursery Industry

Agriculture production is a large contributor to greenhouse gas (GHG) emissions. Due to climate change concerns, scientists have investigated ways to reduce GHG emissions through altering agricultural practices. Past work has concentrated on row crops, forests, and animal production systems, with little attention given to specialty sectors such as container nurseries. In order to reduce emissions from the container nursery industry, baseline emission levels from common production practices must first be established. This study investigated efflux of carbon dioxide (CO2), methane (CH₄) and nitrous oxide (N₂O) from a woody nursery crop (dwarf yaupon holly) grown in four different container sizes for one year. Results showed that CO₂ and N2O emissions generally increased as container size increased, while CH₄ emissions were minimal throughout the study in all container sizes evaluated. These results begin to provide baseline data needed to determine the environmental impact of the container nursery industry on climate change.

Introduction

High concentrations of GHG are thought to be a main factor causing global warming (6, 9); atmospheric concentrations of CO₂, CH₄, and N₂O have increased by approximately 35, 155, and 18%, respectively, since 1750 (6, 13, 16, 30). Agriculture in the United States is a large contributor to GHG emissions, second only to energy production (15). Emissions of CO₂, CH₄, and N₂O from agriculture collectively account for an estimated one-fifth of the annual increase in GHG emissions. When land use changes involving clearing of land, biomass burning, and soil degradation are included, the overall radiative forcing from agriculture production is one third of the man-made greenhouse effect (4). Therefore, concerns for global climate change necessitate development of mitigation strategies to reduce trace gas emissions from agriculture.

Mitigation of trace gas emissions by altering agriculture production practices has been widely researched (4, 17, 19, 21, 29, 33). Adoption of no-till agriculture (34), feed supplementation in ruminant animals (4, 21), and increased efficiency of N fertilization (17, 18) have been shown to successfully reduce emissions of CO₂, CH₄, and N₂O, respectively.

Much of the work on reducing GHG emissions from agriculture has been conducted in row crops, forests, and animal production systems; however, virtually no research has focused on contributions from specialty crop industries such as horticulture. Horticulture is a multi-billion dollar industry which impacts the landscape of rural, suburban, and urban environments. In 2006, there were 7,300 nursery crop producers in the top 17 states, occupying approximately one-half million acres (35). Although horticulture production occupies much less acreage than most agronomic crops, horticulture is one of the fastest growing sectors in agriculture (10), and the impact of this industry on climate change (either positively or negatively) has not been thoroughly investigated.

Reduction of GHG from horticultural production not only provides environmental benefits, but could provide new sources of revenue for producers. Farmers in other agricultural sectors are now earning financial incentives in the emerging carbon (C) trading market and through government incentives to reduce GHG emissions (3, 7, 25, 32).

Changing production practices to mitigate GHG emissions might not only provide new financial opportunities for agricultural producers, but may become required by law. Congress has been slow or hesitant to pass any major climate change bills. As a result, the U.S. Environmental Protection Agency is now beginning to regulate CO₂ and other GHG emissions, and in some cases even those from agriculture (24) which could dramatically impact production (2).

Greenhouse Gas Reduction through Agricultural Carbon Enhancement network (GRACEnet) is a program initiated by the Agricultural Research Service of the USDA to identify and develop strategies that will enhance soil C sequestration, reduce GHG emissions, and provide a scientific basis for possible C credit and trading programs (14). One of the goals of GRACEnet is to begin to establish baseline estimates of net GHG emissions from existing agricultural systems in order to explore ways to reduce these emissions. GRACEnet's primary objectives focus on determining emissions from row crop and animal production systems. For horticulture producers to benefit from the same C trading or offset programs, baseline trace gas emissions (CO₂, N₂O, and CH₄) from current production practices must be established in order to develop strategies to reduce these emissions.

Determining GHG flux from differing container sizes will establish both a baseline for common nursery container production practices and the relative importance of container size on these emissions. The objective of this research was to determine efflux patterns of CO₂, CH₄, and N₂O associated with different nursery container sizes under common production practices. If a direct relationship between potting media volume and trace gas efflux can be established, scaling up to industry-wide emissions levels can be accomplished using estimates of the number and size of plants produced in container nurseries (22).

Materials and Methods

This experiment was conducted at the Paterson Greenhouse Complex, Auburn University, AL. On April 1, 2010, Ilex vomitoria ‘Nana’ (dwarf yaupon holly) liners [2.5 cm (1 in)] were transplanted into four different nursery container sizes: 3 liter (trade gal; TG), 3.8 liter (#1; 1 gal), 7.6 liter (#2; 2 gal), and 11.4 liter (#3; 3 gal). Containers were filled with a pinebark:sand (6:1 v:v) media (TG, #1, #2, and #3 were filled with media to a volume of 2.05, 3.15, 5.15, and 10.1 liters, respectively) which had been previously amended with 8.3 kg·m⁻³ (14 lbs·yd⁻³) of 17N-2.2P-4.2K (17-5-11) Polyon® control-release fertilizer (10–12 month), 3.0 kg·m⁻³ (5 lb·yd⁻³) of ground dolomitic limestone, and 0.9 kg·m⁻³ (1.5 lb·yd⁻³) of Micromax® micronutrient. The study used seven replicates for each container size which contained plants; there were no differences in plant size at study initiation. Three additional replications per container size contained only media and served as controls. After potting, all containers with plants were placed in full sun on a nursery container pad in a randomized complete block design and received daily overhead irrigation [1.3 cm (0.5 in)] via impact sprinklers. Media only containers were placed directly adjacent to containers with plants on the full sun nursery container pad in a similar manner and received irrigation as described above. At the time of study initiation, an additional ten dwarf yaupon holly plants, similar in size to those used during the gas sampling portion of the study, were selected and used to determine approximate initial plant biomass. Plant growth index [(plant height + width1 + width2) / 3] was measured, shoots were cut at the soil line, soil was removed from roots, and shoots and roots were dried for approximately 72 hours at 55C (130F) in a forced-air oven and weighed. Roots and shoots were then ground separately to pass through a 0.2 mm (0.08 in) mesh sieve. Concentrations of C and N were determined using a LECO 600-CHN analyzer (St. Joseph, MI).

Trace gases emitted from the containers were sampled in situ weekly for 1 year (April 1, 2010, to March 31, 2011) using the static closed chamber method (11, 12). Custom-made gas flux chambers were designed and constructed based upon criteria described in the GRACEnet protocol (1, 28) to accommodate nursery containers. A structural base consisting of polyvinyl chloride (PVC) cylinders [25.4 cm (10 in) inside diameter by 38.4 cm (15.1 in) tall] was sealed at the bottom. During gas measurements, the entire plant-pot system was placed inside the base cylinder and a vented flux chamber [25.4 cm (10 in) diameter × 11.4 cm (4.5 in) height] was placed on top of the base cylinder. The top flux chambers were constructed of PVC, covered with reflective tape, and contained a center sampling port. Gas samples for CO₂, CH₄, and N₂O were taken at 0, 15, 30, and 45 min intervals following chamber closure. At each time interval, gas samples [10 mL (0.6 in⁻³)] were collected with polypropylene syringes and injected into evacuated glass vials [6 mL (0.4 in⁻³)] fitted with butyl rubber stoppers as described by Parkin and Kaspar (28). Corresponding air temperature data were collected for each sampling period using Hobo Portable Temperature Data Loggers (Model H08-032-08 with Solar Shield, Onset Computer Corp., Bourne, MA; Fig. 1).

View Figure

• Download as Powerpoint
• Download Figure

Gas samples were analyzed using a gas chromatograph (Shimadzu GC-2014, Columbia, MD) equipped with three detectors: thermal conductivity detector for CO₂, electrical conductivity detector for N₂O, and flame ionization detector for CH₄. Gas concentrations were determined by comparison with standard curves developed using gas standards obtained from Air Liquide America Specialty Gases LLC (Plum-steadville, PA). Gas fluxes were calculated from the rate of change in concentration of trace gas (CO₂, N₂O, or CH₄) in the chamber headspace during the time intervals while chambers were closed (0, 15, 30, and 45 min) as described by Parkin and Venterea (27). Calculations in this study were used to express data as mg CO₂-C, μg CH₄-C, and μg N₂O-N trace gas per day for each container size. Estimates of cumulative efflux were calculated from gas efflux at each sampling date integrated over time using a basic numerical integration technique (i.e., trapezoidal rule).

Upon study completion (March 31, 2011), all plants used during the gas sampling portion of the study were also measured (growth index), weighed, dried, ground, and analyzed as described above to determine C accumulation in plant biomass grown in each container size over the course of the study. Trace gas data was analyzed on each sampling date (data not shown), across all dates, and cumulatively. All trace gas and growth data were analyzed using the Proc Mixed procedure in SAS (SAS® Institute version 9.1, Cary, NC). Means were separated using Fisher's Least Significance Difference Test in the Proc Mixed procedure. In all cases, differences were considered significant at p ≤ 0.05. Linear correlations between temperature and CO₂ efflux were calculated using the Proc Corr procedure in SAS and were considered significant at p ≤ 0.05.

Results and Discussion

Weekly trace gas emissions indicated a significant positive relationship between container size (with plants) and CO₂ efflux, with efflux increasing as container size increased (Fig. 2). On 30 of the 50 sampling dates, #3 containers had higher efflux than any other container size (data not shown). This pattern was also observed when cumulative CO₂ efflux was calculated over the course of one year (Table 1). Additionally, on 13 sampling dates (with plants), #2 containers had higher flux than #1 or TG containers (data not shown). Heterotrophic respiration from decomposition of larger quantities of growth media likely resulted in greater CO₂ loss and thus higher flux rates from these containers. Efflux from media only containers showed that the pinebark media accounted for an estimated 30, 34, 41 and 47% of yearly cumulative efflux from the TG, #1, #2, and #3 containers, respectively. Similarly to patterns observed in containers with plants (Fig. 2; Table 1), emissions from media only containers indicated a significant positive relationship between container size and CO₂ efflux, with efflux increasing as container size increased (Fig. 3; Table 1). Higher levels of plant respiration from the larger plants in the #3 containers (Table 2) resulted in greater CO₂ loss, especially during the growing season (Fig. 2). In addition to effects of container size, there was a positive linear correlation between CO₂ efflux and temperature (p = < 0.0001, R² = 0.29). Carbon dioxide efflux was consistently highest during late spring and summer months when larger differences in efflux among container sizes were observed (Figs. 2 and 3). Carbon dioxide efflux has been shown to be highly dependent upon temperature and water content (8); while water content was not monitored in this study, container moisture levels were uniform due to daily controlled irrigation.

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

Table 1. Cumulative trace gas (CO₂ CH4, and N₂O) efflux from container production of woody nursery crops.

View Fullscreen

Table 2. Biomass, carbon, and nitrogen content of dwarf yaupon holly shoots and roots^z.

View Fullscreen

Mean N₂O efflux (with plants), averaged over the course of the study, was highest in #3 containers, followed by #2 containers, with no difference among the other two container sizes (Fig. 4). Yearly cumulative N₂O efflux also showed that most N₂O was lost in #3 containers (Table 1). Over the course of the study, #3 containers had higher N2O efflux than all other containers on 32 of the 50 sampling dates (data not shown). Because fertilizer was incorporated into the media prior to planting on a volume basis, larger containers had more fertilizer than smaller containers, likely causing a higher N₂O efflux. Further, all plants were similar in size at the beginning of the study and less fertilizer could be utilized by the plant in the larger containers, resulting in higher losses via N₂O efflux. This is further illustrated by observing N₂O efflux from media only containers (Fig. 5). As expected, N₂O efflux was higher in media only containers (Fig. 5; Table 1) than efflux observed in containers with plants (Fig. 4; Table 1), but followed the same general trends. Wagner-Riddle et al. (36) showed that N₂O emissions will be reduced in agricultural soils when farmers avoid fallowing, and a new crop is planted as soon as possible after plowing to increase plant N use; this concept seems to be applicable to container plant production.

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

Nitrous oxide emissions increased dramatically in May, 2010, and remained high through July of the same year before leveling off in late summer (Figs. 4 and 5). This is likely because the release rate of the controlled-release fertilizer used in this study is highly dependent upon soil temperature, which may have caused higher N₂O fluxes during warmer months. However, no increases in N₂O flux were observed in 2011 (Figs. 4 and 5) as most of the fertilizer (10–12 month formulation) was likely utilized or leached as soluble nitrate.

Methane efflux was consistently low in all containers for the duration of study (data not shown). Yearly cumulative CH₄ efflux showed no differences regardless of container size, with or without plants (Table 1). It is likely these values were close to or below the detection limits of the gas chromatograph. Previous work has shown that CH₄ fluxes in non-saturated soils are generally small (31) and so it is not surprising, given the media was well drained, the anaerobic conditions needed for CH₄ are not common in well-managed container production systems. Based on results from this study, CH4 efflux does not appear to have a significant effect on total trace gas emissions from container-grown nursery crops.

Our results showed that loss of both CO₂ and N2O were greatest in the largest containers, while CH₄ efflux was low regardless of container size. While CO₂ and N₂O losses were higher in larger containers, smaller containers would likely have higher total trace gas emissions on a per acre basis. For example on a 0.4 ha (1 A) production bed, #3 gallon containers spaced 15 cm (6 in) apart (about 26,000 plants) would have approximately half [50 kg (110 lbs)] of the cumulative CO₂-C efflux (Table 1) of TG containers [96 kg (212 lbs) of CO₂-C efflux] spaced 5 cm (2 in) apart (about 98,000 plants). Therefore, while trace gas emissions increased with increasing container size, a larger number of smaller containers will likely have higher efflux than a lower number of larger containers in a given area of production space. Further, data indicate that trace gas emissions from container nursery production may be higher (for a given area of production) than from soils in row crop production systems (5). However, nursery production acreage is much smaller than that used for agronomic crops. For example, approximately 90 million acres of corn were harvested in the United States in 2007, compared with approximately 0.5 million acres of nursery stock (26). Thus, the nursery industry is likely producing only a fraction of total GHG emissions produced from agronomic production.

It is important to note that our container trace gas flux data do not necessarily reflect net emissions as they do not account for C sequestered in growing biomass. Foliar analysis (Table 2) showed that 18.6, 32.3, 35.3, and 42.4 g C were contained in holly biomass (roots and shoots) grown in TG, #1, #2, and #3, respectively. Further, container nursery systems may contribute to C sequestration by placing large amounts of C-rich growing media belowground when plants are transplanted into the landscape (23). Average dry weight of pinebark in media only containers was 769.5, 1160.1, 1810.2, and 3315.7 g in the TG, #1, #2, and #3, respectively. Using a C percentage of 49.2% (previously determined using analysis methods described above) for the pinebark media used in this study, estimated C stored underground following landscape transplanting would be approximately 378.6, 570.8, 890.6, and 1631.3 g for the TG, #1, #2, and #3 containers, respectively. Subtracting cumulative CO₂-C efflux (Table 1) from the total C stored in biomass and media (i.e., following landscape outplanting) would result in a net C gain (in biomass and media) of 396.3, 601.6, 924.3, and 1671.8 g from the TG, #1, #2, and #3 containers, respectively. However, the longevity of this C storage is still unknown. The life span and growth rate of nursery crops will vary greatly depending on species and environment, and no long term studies have investigated the longevity of the pinebark media after being placed underground in the landscape. While our data suggest a net C gain from nursery container production, this storage may only be realized in the short term as longevity of this storage potential requires further investigation.

While high N₂O levels were observed at times, it is likely only a fraction of total N was lost via N₂O. Cumulative N₂O efflux from containers with plants (Table 1) indicates that approximately 0.6, 0.9, 2.0, and 5.4 mg of N were lost via N₂O over the course of the study in the TG, #1, #2, and #3 containers, respectfully. Considering the amount of N applied at potting (approximately 3, 5, 7, and 14 g N in the TG, #1, #2, and #3 containers, respectfully) most N was either used by the plant or more likely lost via leaching. Although not measured in this study, it appears that N leaching is likely more of an environmental concern in container production than N₂O emissions.

Data presented here indicate that container production of a typical woody nursery crop using common production practices would likely be a net C sink while in production and after being planted into the landscape. The benefits of this sink will depend on the longevity of the media and the rate of plant biomass accumulation over time. Further investigation is needed to determine the impact of different production variables (e.g., growing media, fertilization and irrigation practices, and other plant species) on trace gas emissions. While uncertainty remains regarding the overall impact of the nursery industry on climate change, results from this study begin to determine the overall environmental impact of the container nursery industry and provide baseline data of trace gas emissions from container-nursery production systems needed to evaluate future mitigation strategies.