Published November 10, 2014

Journal of Environmental Quality

TECHNICAL REPORTS Atmospheric Pollutants and Trace Gases

Nitrous Oxide Fluxes in Fertilized Pinus taeda L. Plantations across a Gradient of Soil Drainage Classes Raj K. Shrestha,* Brian D. Strahm, and Eric B. Sucre

F

orests have received much interest in the discussion of mitigating climate change. They cover one third of the global and U.S. land areas and can play an important role in regulating greenhouse gas (GHG) emissions and global climate (Smith et al., 2003; Pilegaard et al., 2006; World Bank, 2011). Nitrogen (N) fertilization is a common silvicultural practice in managed pine plantations (Albaugh et al., 2007) and is increasing in the United States and worldwide to enhance social, economic, and environmental services derived from forests. Application of N fertilizers to forest ecosystems have been shown to alter ecosystem properties and processes, including forest productivity, species composition and diversity, and losses of N to the atmosphere (primarily as nitrous oxide [N2O]) and to surface and groundwater (primarily as nitrate [NO3]) (Ussiri and Lal, 2013). Nitrous oxide is an important GHG. Its global warming potential, over a 100-yr time frame, is 310 times higher than that of CO2 (IPCC, 2007). Several studies have reported that significant amounts of N2O are emitted from forest ecosystems, with estimates ranging from 2.4 to 5.7 Tg N2O–N yr-1 (Brumme et al., 1999; IPCC, 2007). The application of synthetic N fertilizers to U.S. forest soils in 2010 resulted in direct N2O emissions of 0.4 Tg CO2 Eq. (USEPA, 2012). Therefore, understanding the impacts of forest fertilizer management on N2O fluxes is an essential first step to mitigating land management–induced GHG emissions. Forest ecosystems that are N limited tend to behave both as a source and sink for N2O (Bowden et al., 2000). Nitrogen fertilization has been reported to increase N2O production from N-limited forest soils via two principal mechanisms—microbial nitrification and denitrification (Billore et al., 1996; Hall and Matson, 1999; Venterea et al., 2003; Zhang et al., 2008; Jassal et al., 2010; Brown et al., 2012)—while simultaneously decreasing the CH4 sink strength (Sitaula and Bakken, 1993; Mochizuki et al., 2012). A review by Shrestha et al. (2014) reported that forest N fertilization increased N2O emissions from 20 to >500% compared with unfertilized forest. The IPCC has reported that 10 kg N2O–N is emitted for every 1000 kg of N

Abstract The effect of fertilizer management on nitrous oxide (N2O) fluxes in agricultural ecosystems is well documented; however, our knowledge of these effects in managed forests is minimal. We established a comprehensive research study to address this knowledge gap across a range of soil drainage classes (poorly, moderately, and well drained) common in southern pine plantation management. Fertilizer treatments in each drainage class comprised of control (no fertilizer), urea + phosphorus (P), and P-coated urea fertilizer (CUF). Fertilization (168 kg N ha-1) occurred independently during the spring, summer, and fall to assess the effects of application timing. Nitrous oxide sampling, using vented static chambers, started immediately after seasonal fertilizer application and was performed every 6 wk for more than 1 yr. Time-integrated net annual N2O emissions increased with urea (1.15 kg N2O–N ha-1) and CUF (0.88 kg N2O–N ha-1) application compared with unfertilized control (0.22 kg N2O–N ha-1). Mean annual N2O flux was significantly increased with fall fertilization (1.17 kg N2O–N ha-1) relative to spring (0.73 kg N2O–N ha-1) or summer (0.33 kg N2O–N ha-1). Similarly, average annual N2O flux was higher in poorly drained soils (1.40 kg N2O–N ha-1) than in moderately drained (0.46 kg N2O–N ha-1) and welldrained soils (0.39 kg N2O–N ha-1). This study suggests that N2O emissions after fertilization can be minimized by avoiding fall fertilization and poorly drained soils and by selecting enhancedefficiency N fertilizers over urea.

Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

R.K. Shrestha and B.D. Strahm, Dep. of Forest Resources and Environmental Conservation, 228 Cheatham Hall (0324), Virginia Tech, Blacksburg, VA 24061; E.B. Sucre, Southern Timberlands Technology, Weyerhaeuser Company, 1785 Weyerhaeuser Rd, Vanceboro, NC 28586. Assigned to Associate Editor Carlo Calfapietra.

J. Environ. Qual. 43:1823–1832 (2014) doi:10.2134/jeq2014.03.0109 Received 9 Mar. 2014. *Corresponding author ([email protected]).

Abbreviations: CUF, phosphorus-coated enhanced efficiency urea fertilizer; dbh, diameter at breast height; GHG, greenhouse gas; WFPS, water-filled pore space.

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fertilizer applied in a managed system (IPCC, 2007). However, N fertilization–induced N2O peaks last for a short period (2–3 wk) after summer fertilization and during soil thawing periods in early spring (Peng et al., 2011). Nitrogen fertilization effects on soil N2O emissions are controlled by soil N status, temperature, moisture, aeration, C availability, C/N ratio, pH, texture, land use, and forest composition and age (Shrestha et al., 2014). Under certain conditions, forest ecosystems can function as a sink for atmospheric N2O (Papen et al., 2001), as observed in managed forests in Canada (Kellman and Kavanaugh, 2008) and Europe (Goldberg and Gebauer, 2009; Inclán et al., 2012). These observations can be explained by the presence of a denitrifier population in soil, which, in the absence of soil NO3, can use atmospheric N2O as an electron acceptor (Papen et al., 2001). Forest N fertilization can alter these dynamics and can change soils from a net sink to a net source of atmospheric N2O (Papen et al., 2001). Synchronizing timing of N application with demand (Ussiri and Lal, 2013), managing water-filled pore space (WFPS) or soil water content (Kellman and Kavanaugh, 2008; Koehler et al., 2009), and/or applying controlled-release fertilizers (Akiyama et al., 2010; Halvorson et al., 2010) to increase N use efficiency are important factors to manage N2O emissions. Thus, it is important to understand these controlling factors and their interactions to manage atmospheric N2O fluxes in intensively managed southern pine forests. Extensive studies have been conducted on N fertilizer use and its impacts on growth in managed forest ecosystems (e.g., Elliot and Fox, 2006; Fox et al., 2007; Haase et al., 2007; Fujinuma et al., 2011; Ring et al., 2011; Vallack et al., 2012). However, the effects of N fertilization on N2O fluxes in managed forest ecosystems are poorly understood because of limited studies at the field scale (Ambus et al., 2006) despite an increase in acreage under forest fertilization (Albaugh et al., 2007). This scientific gap requires further investigation into the effects of N fertilizer management practices on N2O emissions to better manage N fertilization simultaneously for improved N use efficiency and reduced N2O emissions. Therefore, the objectives of this study are to identify the impact of N fertilizer type, season of N fertilizer application, and soil drainage class on N2O fluxes.

Materials and Methods

Treatments and Experimental Design The study was established as a replicated (n = 4) randomized complete block design with a 3 × 3 × 3 factorial of soil drainage class, season of fertilizer application, and fertilizer type. The three drainage classes investigated were poorly, moderately, and well drained. Within each soil drainage class, fertilizer was applied during three seasons (spring, summer, and fall) using three fertilizer treatments (control [no fertilizer], urea + triple superphosphate, and P-coated enhanced efficiency urea fertilizer [CUF] with volatility control) (Arborite, Weyerhaeuser Co.). To capture microtopographic variability imposed by site preparation, two subsample locations were identified in each plot: one on the bedded rows and another in the adjacent interbed area. Fertilization occurred manually at one point in time at the rate of 168 kg N ha-1. Nitrogen fertilizers (urea and CUF) were applied on 20 July 2011 for summer fertilization, on 3 Nov. 2011 for fall fertilization, and on 2 Apr. 2012 for spring fertilization treatment. The amount of P (1.1 kg P ha-1) added from coated urea was compensated for in urea treatments through a separate triple superphosphate addition.

Study Site Description Study sites were established at three different locations in the Lower Coastal Plain Physiographic Region in eastern North Carolina. All three sites are second-rotation loblolly pine (Pinus taeda L.) plantations with different establishment dates (Table 1). Additionally, each site is at a different stage of stand development and has incurred various silvicultural treatments. The collection of research sites creates a continuum of soil drainage classes of poorly, moderately, and well drained (Table 1), which is a main focal point of this research. These sites and other surrounding plantations occur on soils that were artificially drained in the late 1960s and early 1970s. In the absence of this drainage and bedding during site preparation, it would be difficult to establish viable plantations due to mortality and slow growth rates. The poorly drained site was located about 10 km east of Aurora, North Carolina in Beaufort County (76°39¢1.063¢¢ W, 35°16¢13.186¢¢ N). The elevation is approximately 1.8 m, and the site has an average annual precipitation of 1336 mm. Average

Table 1. Site characterization for poorly drained, moderately drained, and well-drained sites in North Carolina. Parameters County Town Elevation, m Landscape Tree species Establishment date Thinning date Pruning date Average height, m Average dbh,† cm Average plant density, plant ha-1 Soil type Fertilization history

Poorly drained Beaufort Aurora 1.79 terrace Pinus taeda L. 1995 2008 2008 17.4 23.1 59 To (Torhunta)–Tomotley fine sandy loam 2003: 83 kg N ha-1 and 9 kg P ha-1; 2007: 175 kg N ha-1 and 18 kg P ha-1

Moderately drained Pamlico Arapahoe 3.5 flat Pinus taeda L. 2001 none none 12.9 18.0 155 Ar–Argent loam none

Well-drained Craven Spring Hope 10.5 upland Pinus taeda L. 2003 2009 2011 12.5 18.3 163 La–leaf silt loam none

† Diameter at breast height. 1824

Journal of Environmental Quality

annual temperature is 22.3°C, ranging from 1°C in the winter to 30.8°C in the summer. This site occurs on the low marine surface of the Pamlico surface on a loamy terrace landform classified as a poorly drained Tomotley soil series (fine-loamy, mixed, semiactive, thermic Typic Endoaquults). One-year-old seedlings were planted in 1995. This is the oldest plantation of the three research sites that underwent a commercial thinning and pruning in 2008. At the time of study, stocking was 358 stems per hectare, with an average tree height of 17.4 m and diameter at breast height (dbh) of 23.1 cm. The moderately drained site was located about 8 km east of Arapahoe, North Carolina, in Pamlico County (76°46¢31.812¢¢ W, 34°59¢9.516¢¢ N). The elevation is approximately 3.5 m. This site has an average annual precipitation of 1352 mm and an average annual temperature of 22.6°C, ranging from 1.9°C in the winter to 31.2°C in the summer. One-year-old seedlings were planted in 2001. This site also occurs on the low marine surface of the Pamlico surface but on a mineral low terrace classified as a somewhat poorly drained Argent soil series (fine, mixed, active, thermic Typic Endoaqualfs). At the time of study, stocking was 946 stems per hectare, with an average tree height of 12.9 m and dbh of 18.0 cm. The well-drained site was located about 12 km north of Askins, North Carolina, in Craven County (76°57¢22.846¢¢ W, 35°14¢5.605¢¢ N). The elevation is approximately 10.5 m. This site has an average annual precipitation of 1385 mm. Average annual temperature is 23.5°C, ranging from 0.9°C in the winter to 32.2°C in the summer. One-year-old seedlings were planted in 2004. This site is situated on the uplands of the Talbot surface on upland clay flat classified as a moderately well drained Leaf soil series (fine, mixed, active, thermic Typic Albaquults). This plantation was the youngest and had a precommercial mechanical vegetation control treatment in 2011 using a drum chopper. At the time of study, stocking was 993 stems per hectare, with an average tree height of 12.5 m and dbh of 18.3 cm.

Soil Sample Collection and Analyses Soil samples were collected at 0- to 15-cm and 15- to 30-cm depths after removing plant debris from bed and interbed locations. Collected soil samples were sealed in plastic bags and transported to the laboratory in a cooler. Soil samples were then air-dried under shade. Large clods were gently crushed, stones were removed, and the samples were sieved (2 mm (Page-Dumroese et al., 1999).

Field Measurements of Nitrous Oxide Flux Soil–atmospheric exchange of N2O was measured using a vented static chamber technique (Hutchinson and Livingston,

2001). The static chamber design contains a sample port with a septum and a separate vent for equilibration of internal and external atmospheric pressures. Collars (inner diameter: 25 cm; height: 18 cm) were made of polyvinyl chloride. All collars were inserted about 9 cm into the soil surface with the help of soil knife in circumscribing collar location. Collars were left in the ground about 1 mo before sampling to minimize placement disturbance. Chambers were left open in the field for the entire sampling period to mimic the natural environment. A lid made of polyvinyl chloride material was used to close the chamber at the time of flux sampling. Measurements of the N2O fluxes began 1 d after seasonal fertilization and were performed every 6 wk in excess of 1 yr. On the day of measurement, chambers were closed with a lid and sealed with a Cando latex rubber band loop (15” silver; Best Priced Products, Inc.) wrapping around the joint of the lid and collar. Immediately after sealing, a 20-mL syringe was used to mix the headspace by pumping three times before taking flux samples. The flux samples were withdrawn from each chamber headspace with the syringe equipped with a one-way stopcock through a butyl rubber septum in the middle of the lid and labeled as a 0-min sample. Subsequent samples were collected 20, 40, and 60 min after sealing the chamber. For each headspace sample, 12 mL of gas was transferred to pre-evacuated (0.05 kPa), 10-mL, crimp-sealed vials with a butyl rubber septum after triple flushing the vials with N2 gas. Gas flux measurements were made between 10:00 and 16:00 h, when the diurnal temperature variation was expected to be minimal (Bajracharya et al., 2000). Air temperature and soil temperature at 10-cm depth were measured at each gas flux sampling date. Soil temperature was monitored using a digital Thermistor Thermometer (OAKTON Temp 5 Acorn Series, OAKTON Instruments), and percent volumetric soil water content across the 12-cm depth was monitored using HydroSense (Campbell Scientific, Inc.) (Fig. 1 and 2). Rainfall was monitored using an Onset Data Logging Rain Gauge with a HOBO Event Data Logger (Onset Headquarters). The gas samples were analyzed for N2O using a gas chromatograph (GC-2010, Shimadzu Corp.) equipped with an AOC-5000 Plus Autosampler (Shimadzu/CTC Analytics). The gas chromatograph was equipped with an electron capture detector at 325°C for N2O detection. The carrier gas was ultrahigh pure N2 at 26 mL min-1. The column temperature was isothermal at 80°C. The gas chromatograph was calibrated using standard gases obtained from Alltech. The rate of N2O fluxes (mg N2O-N m-2 d-1) was computed by fitting a linear regression of gas concentration against the time after the chamber was closed using Eq. [1]:

æ DGC öæ ÷÷çç V ö÷÷ k [1] F = çç èç Dt ÷øèç A ÷ø where F is gas flux (mass of gas m-2 d-1), ∆GC/∆t is the rate of change in gas (N2O) concentration inside chamber (mg N2O–N m-2 min-1), V is chamber volume (m3), A is soil surface area covered by chamber (m2), and k is the time conversion factor (1440 min d-1) Changes in headspace gas concentration with time were tested for nonlinearity as suggested by Rochette and EriksenHamel (2008) and Kroon et al. (2008). Annual fluxes were calculated by summing the product of daily N2O concentration

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and number of days for next sampling over a period of 1 yr. Negative fluxes of N2O indicate the uptake of N2O gas by soil, and positive fluxes indicate net emission from soil.

Statistical Analysis The daily and annual N2O fluxes and soil moisture and temperature regimes for each sampling date were analyzed using the GLM procedure in SAS 9.3 for Windows (SAS Institute Inc.) to detect the effects of the fertilizer type, season of fertilizer application, and drainage class on N2O fluxes. Means were separated using the least square significance test at P < 0.05. Soil temperature and moisture regimes in each treatment were correlated with N2O fluxes.

Result and Discussion Soil Measurements

Bulk density ranged from 1.03 to 1.37 Mg m-3 for the 0- to 15-cm depth and from 1.48 to 1.79 Mg m-3 for the 15- to 30-cm depth (Table 2), with the moderately drained site having the highest bulk density. Soil pH was acidic in all three sites. Carbon and N concentrations were higher in the poorly drained sites, followed by well-drained and moderately drained sites (Table 2). Phosphorus was higher in well-drained sites than in the other two sites. Soil temperature ranged from 6.8°C on 23 January to 26.4°C on 21 July (Fig. 1). Soil moisture ranged from 2.1 to 45.9% (Fig. 2). Annual rainfall amounts were 165, 200, and 137 cm at the poorly, moderately, and well-drained sites, respectively (Fig. 3).

Fig. 1. Soil temperature inside the experimental chamber at 12 cm depth for (A) summer, (B) fall, and (C) spring fertilization.

Effect of Fertilization on Temporal Dynamics and Net Annual Nitrous Oxide Fluxes Daily nitrous oxide fluxes ranged from -0.11 to 2.96 mg N2O–N m-2 d-1 (average, 0.09 mg N2O–N m-2 d-1) for summer fertilization, from -0.11 to 3.77 mg N2O–N m-2 d-1 (average, 0.27 mg m-2 d-1) for fall fertilization, and from -0.11 to 2.78 mg N2O–N m-2 d-1 (average, 0.16 mg N2O–N m-2 d-1) for spring fertilization, indicating that these pine plantations were at times both sources and sinks of N2O (Fig. 4–6). Nitrous oxide fluxes were not affected by microsite (bed or interbed). In general, both urea and CUF application increased N2O emissions irrespective of season of fertilization. These average values were well within the range of fluxes measured in montane (0.39 mg N2O–N m-2 d-1) (Martinson et al., 2013) and temperate (0.71 mg N2O–N m-2 d-1) (Eickenscheidt et al., 2010) forest ecosystems. Out of a total of 2376 individual N2O flux rate measurements for the entire observation period, 640 (27%) were negative, indicating that the soil functioned as a small sink for atmospheric N2O (-0.11 mg N2O–N m-2 d-1). The soil in the unfertilized control treatment did function as a sink for N2O during 36% of sampling events (mean uptake: 0.058 mg N2O–N m-2 d-1; range: -0.001 to -1.845 mg N2O–N m-2 d-1); however, the fertilized 1826

treatments functioned as a sink for N2O during only 22% of the sampling events (mean uptake: -0.052 mg N2O–N  m-2 d-1; range: -0.001 to -1.173 mg N2O–N m-2 d-1). Despite the fact that forest soils can act as a sink for atmospheric N2O, this consumption of N2O is often masked by net emissions from soil (Chapuis-Lardy et al., 2007). Understanding these competing processes can have important implications for global N budgets (Cicerone, 1989). In general, N2O fluxes from fertilized treatments were higher than from the unfertilized control treatment throughout the study period for all summer (Fig. 4), fall (Fig. 5), and spring (Fig. 6) fertilizations. Earlier studies have also reported an increase in soil N2O emissions of 20 to >500% with the application of N fertilizers compared with unfertilized forests (Brumme and Beese, 1992; Matson et al., 1992; Papen et al., 2001; Magill et al., 2000; Koehler et al., 2009; Jassal et al., 2008, 2010), whereas other studies show little to no effect (Bowden et al., 2000; Steudler et al., 2002; Maljanen et al., 2006). Ultimately, predicting the role of N fertilization on N2O fluxes depends on whether the added N is taken up by plants, incorporated into soil organic matter, or assimilated by soil microbes in the near term (Lohse and Matson, 2005). Journal of Environmental Quality

large peak of N2O flux (2.96 mg N2O–N m-2 d-1) was observed at the well-drained site especially with urea fertilization in the interbed (Fig. 4), which was five times higher than in the bedded microsite. This may be due to a combination of comparatively high moisture content and high temperature in the interbed (Fig. 1 and 2), contributing to denitrificationdriven N2O emissions (Kellman and Kavanaugh, 2008; Koehler et al., 2009). High fluxes from the interbed on the well-drained site may be due to high temperatures of 24°C (Fig. 1) and unusually heavy rainfall (30.6 cm in the 10 d before and 2.74 cm a few hours before the gas sampling event) (Fig. 3) (Smith et al., 1998; Linn and Doran, 1984). Fall fertilization significantly elevated N2O production for urea and CUF treatments compared with the unfertilized control. This was particularly evident during the summer sampling on the poorly drained soils both in the bed and interbed microsites (Fig. 5). The elevated N2O emissions were likely due to optimal soil temperature and moisture resulting in increased rates of nitrification (Avrahami et al., 2002). The most pronounced peak in N2O emissions in the interbed of poorly drained soils was observed with urea (3.2 mg N2O–N m-2 d-1) and CUF (2.5 mg N2O–N m-2 d-1) during the August sampling period (Fig. 5). Similarly, the most pronounced peak in N2O emissions in the bed of poorly drained Fig. 2. Soil moisture inside the experimental chamber at 10 cm depth for (A) summer, (B) fall, and soils was observed with urea application in (C) spring fertilization. September (2.7 mg N2O–N m-2 d-1) and Summer fertilization produced weak N2O fluxes throughout in August for CUF treatments (1.9 mg the 16-mo study period in all drainage classes with no clear N2O–N m-2 d-1). Nitrous oxide emissions from CUF were less fertilization effect except on the well-drained site (Fig. 4). Six in both the bed and interbed. Reduction in N2O emissions with weeks after summer fertilization (20 July 2011 sampling date), a CUF may be due to the slow release of available N, ultimately Table 2. Soil properties of all three sites in North Carolina by sampling depths. Drainage class Location Poorly drained (terrace)

bed interbed

Moderately drained (flat)

bed interbed

Well drained (upland)

bed interbed

Depth

Bulk density

cm 0–15 15–30 0–15 15–30 0–15 15–30 0–15 15–30 0–15 15–30 0–15 15–30

g cm-3 1.10 1.66 1.03 1.62 1.37 1.71 1.20 1.79 1.16 1.48 1.05 1.56

pH 4.69 4.52 4.67 4.51 4.25 4.51 4.25 4.54 4.24 4.39 4.03 4.25

C

N

——— g kg-1 ——— 81.6 3.67 34.6 1.34 84.7 3.65 26.7 1.02 25.9 1.03 16.0 0.60 32.8 1.25 10.8 0.40 41.2 1.51 19.7 0.68 69.8 2.91 43.2 1.52

P

K

——— mg kg-1 ——— 3.2 32.3 2.2 17.3 2.6 28.0 2.2 14.9 2.6 20.4 2.6 15.9 2.6 24.6 2.0 17.8 11.1 25.6 8.4 15.3 11.8 30.7 8.3 17.9

CEC†

Soluble salts

cmolc kg-1 9.02 4.22 8.90 4.18 7.29 5.63 8.41 5.80 7.29 5.63 8.41 5.80

mg kg-1 118 108 102 87 64 31 55 27 51 31 90 56

† Cation exchange capacity. www.agronomy.org • www.crops.org • www.soils.org 1827

Fig. 3. Precipitation received during the study period at road side and in the forest (through fall) for poorly, moderately, and well-drained sites, New Bern, North Carolina.

Fig. 4. Effects of summer fertilization on N2O fluxes in poorly, moderately, and well-drained soils of North Carolina. CUF, phosphorus-coated enhanced efficiency urea fertilizer.

making less substrate available for N2O production at any one time. Moderately and poorly drained soils had a similar trend for N2O production during the study period, except for a peak of N2O production that was observed in the bedded region on moderately drained soils. Fall fertilization encouraged strong N2O production in the spring especially in the poorly drained soil, followed by moderately and then well-drained soils. The 1828

effects of fertilization on N2O fluxes are likely to last from 2 to 3 wk (Peng et al., 2011) to 8 to 10 wk (Bouwman et al., 2002) after N application, but the magnitude of these effects depends on temperature, moisture, and nutrient availability. The unfertilized control treatment did not show any N2O peak during the entire observation period across all drainage classes. Journal of Environmental Quality

Fig. 5. Effects of fall fertilization on N2O fluxes in poorly, moderately, and well-drained soils of North Carolina pine ecosystems. CUF, phosphoruscoated enhanced efficiency urea fertilizer.

Fig. 6. Effects of spring fertilization on N2O fluxes in poorly, moderately, and well-drained soils of North Carolina pine ecosystems. CUF, phosphorus-coated enhanced efficiency urea fertilizer.

The N2O fluxes after spring fertilization treatment followed a similar pattern to the fall fertilization treatment, albeit slightly less in magnitude, with the highest flux rates observed in midsummer

through early fall (Fig. 6). Unlike other application periods, spring fertilization increased N2O production immediately after fertilization especially in the interbed of poorly drained

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Effects of Season of Fertilizer Application on Nitrous Oxide Fluxes

soils. Rainfall and associated changes in soil moisture are major drivers of N2O fluxes (Smith et al., 1998). Usually, stimulation of N2O emissions is triggered with rainfall events (Borken and Beese, 2005; Zona et al., 2013). Therefore, this increase may be due to increased WFPS associated with a 0.53-cm rainfall event that occurred shortly after spring fertilization but before the initial flux measurement. The combined effect of N fertilizer application and wet soils leads to optimum conditions for denitrification and N2O production (Linn and Doran, 1984; Brown et al., 2012). Approximately 4 mo after CUF application, another pronounced peak in N2O emissions in the interbed of moderately drained soil was observed in the August sampling. Again, this may be due to an increase in soil moisture content associated with a 1.56-cm rainfall event that occurred 1 d before flux measurement. This might have encouraged N release from CUF, making substrate available for nitrification and subsequent denitrification (Phillips et al., 2009). Few studies have reported no significant increases in N2O emissions after forest N fertilization (Bowden et al., 2000; Steudler et al., 2002; Maljanen et al., 2006). This could be due to trees competing for available N when WFPS is well below the optimum of 60 to 70% for denitrification (Steudler et al., 2002). Furthermore, the absence of fertilization responses on N2O emissions in these studies may be due to low rates of net nitrification (Bowden et al., 2000; Maljanen et al., 2006). In N-rich mature forest ecosystems, N addition increases soil N availability to nitrifying and denitrifying microorganisms (Hall and Matson, 1999). These results indicate that N fertilization alters the dynamics of N2O fluxes and changes ecosystem function from a weak sink to a moderate source of N2O. Annual time-integrated N2O fluxes were not affected by summer fertilization. However, fall and spring fertilization increased annual N2O emissions three to seven times more than the unfertilized control (Table 3). Annual N2O emissions from control, CUF, and urea treatments were 0.22, 0.88, and 1.15 kg N2O–N ha-1, respectively (Table 3). This value is much less than those reported in grassland (15–36 kg N2O–N ha-1) and agricultural (2–9 kg N2O–N ha-1) ecosystems (Pappa et al., 2011; Smith et al., 2012). It is also important to note that N-fertilizer is applied annually in agricultural ecosystems, whereas N-fertilizer is applied only two to four times over a period of a 25-yr rotation in an intensively managed southern pine forest. Furthermore, forest N fertilization increases leaf area and long-term C storage in biomass (Albaugh et al., 2012). Therefore, N fertilization in forest ecosystems can reduce overall GHG emissions if the CO2 sequestered in biomass and CH4 uptake in soil are valued.

Microbial emission and consumption of N2O after forest N fertilization is influenced by temperature, moisture, and nutrient availability. Therefore, time-integrated net annual N2O fluxes over a period of 1 yr were significantly influenced by the season of fertilizer application (Table 3). Reported time-integrated net flux magnitude of N2O represents a balance between soil sources and sinks activity instead of a gross estimate of N2O production. Irrespective of fertilizer treatments, annual N2O flux was significantly greater for fall fertilization (1.17 kg N2O–N ha-1; P < 0.05) than for soils fertilized in the spring (0.73 kg N2O–N ha-1; P < 0.05) and summer (0.33 kg N2O–N ha-1; P < 0.05). Regardless of season, N fertilization with urea consistently resulted in higher N2O emissions compared with the unfertilized control. Annual flux rates from both the urea and CUF treatments were significantly greater with fall application than with summer application but were not statistically different for sites fertilized in the spring. These results suggest that fall and spring N fertilization, either as urea or CUF, stimulated N2O emissions, but summer fertilization did not. If N is applied as urea, summer fertilization is better than fall fertilization at minimizing N2O emissions. However, if N is applied as CUF, both summer and spring fertilizations result in lower N2O emissions than fall fertilization.

Effect of Drainage Class, Soil Moisture, and Temperature on Nitrous Oxide Fluxes Drainage class significantly affected N2O flux (Table 4). Irrespective of fertilizer treatments and season of fertilization, the average time-integrated annual N2O flux was significantly higher in poorly drained soils (1.40 kg N2O–N ha-1) compared with moderately drained (0.46 kg N2O–N ha-1) and welldrained soils (0.39 kg N2O–N ha-1). Rates of N2O emissions followed along the expected gradient of drainage classes tested. The N2O emissions were less on the well-drained site where soil moisture content (Fig. 1) and rainfall (Fig. 3) were less than the poorly and moderately drained sites. Less rainfall from July to September of 2012, in combination with less moisture in welldrained sites, likely resulted in low observed fluxes (Fig. 1 and 3). Therefore, given adequate substrate availability, soil moisture, as a result of drainage class, might be the primary factor controlling N2O emissions (Koehler et al., 2009). Irrespective of season of application, fertilized treatments emitted significantly higher amounts of N2O in poorly drained soils than in moderately and well-drained soils (Table 4). Nitrous oxide emission from urea in poorly drained soils was about four

Table 3. Effects of fertilizer type and season of application on nitrous oxide emissions in pine ecosystems of North Carolina. Treatments

Control Urea CUF‡ Season average

Annual N2O flux Summer

Fall

Spring

Fertilizer average

———————————————————— kg N2O–N ha-1 yr-1 ———————————————————— 0.21aA† 0.23bA 0.22bA 0.22a 0.57aB 1.68aA 1.20aAB 1.15b 0.25aB 1.59aA 0.78abB 0.88b 0.34B 1.17A 0.73B

† Fertilizer treatments with same lowercase letters and season with same uppercase letters are not different at the 0.05% level. ‡ Phosphorus-coated urea fertilizer. 1830

Journal of Environmental Quality

Table 4. Effects of drainage class on nitrous oxide emissions in pine ecosystems of North Carolina. Treatments

Control Urea CUF‡ Drainage average

Poorly drained

Annual N2O flux Moderately drained

Well drained

——————————————— kg N2O–N ha-1 yr-1 ——————————————— 0.40bA† 0.17bB 0.08bB 2.28aA 0.51abB 0.66aB 1.52aA 0.69aB 0.42abB 1.40A 0.46B 0.39B

† Fertilizer treatments with same lowercase letters and drainage class with same uppercase letters are not different at the 0.05% level. ‡ Phosphorus-coated urea fertilizer.

times higher than in the moderately and well-drained soils. Similarly, N2O emissions from CUF in poorly drained soils were about two to three times higher than in the moderately and welldrained soils. The effects of urea application on N2O emission were similar to CUF application across all drainage classes. Soil temperature at the 0- to 10-cm depth showed positive but weak correlation with N2O emission for all three seasons (summer: r = 0.117, P = 0.01 [n = 864]; fall: r = 280, P = 0.01 [n = 792; and spring: r = 0.150, P = 0.01 [n = 720]) of fertilization, indicating that an increase in soil temperature encourages soil N2O emissions. A similar positive relationship between N2O fluxes and soil temperatures was observed in earlier studies (Smith et al., 1998; Kellman and Kavanaugh, 2008; Ullah and Moore, 2011). However, Maljanen et al. (2006) did not observe a relationship between soil or air temperature and N2O emission. Moisture at the 0- to 10-cm depth was positively correlated with N2O emissions only for summer fertilization but not for fall and spring fertilization. Similarly, Smith et al. (1998) observed an exponential relationship between N2O emission and WFPS.

Conclusions When considering the total C life-cycle emissions and associated production of forest-based feed stocks, this study provides a comprehensive assessment of direct N2O emissions associated with forest fertilization. Fertilizer application increased N2O emissions in managed pine forest ecosystems, ranging from four times higher for CUF to five times higher for urea compared with unfertilized control. However, these flux rates were much less than those reported in agricultural or grassland ecosystems. Many existing life-cycle analysis models do not have forest fertilizer emissions data; consequently, the expected emissions for forest fertilization are often purported under “agriculture” because they are assumed to be similar. These data will greatly improve existing life-cycle analysis models by providing precise data from managed forests across a range of soil conditions. Furthermore, N fertilization in forest ecosystem can reduce overall GHG emissions if the CO2 sequestered in biomass and CH4 uptake in soil is valued. As observed in this study, by applying fertilizer during periods when N demand and N uptake is highest, such as in spring and summer, soil N2O emissions can be reduced further during an average 25-yr rotation for loblolly pine. In addition to minimizing N2O loss in summer, enhanced-efficiency fertilizers, such as CUF, have lower N loss because losses via NH3 volatilization are reduced significantly compared with urea applied during same time of year. This provides landowners the opportunity to apply fertilizer in the summer when nutrient demand is highest. Lastly,

careful attention to soil drainage class should be considered, as well as a determinant of when to optimize uptake and minimize losses. Poorly drained soils had higher N2O emissions compared with moderately and well-drained soils in this study. Thus, applying during periods when soils are not saturated or when water the table is not high, such as in fall and winter, would be a good practice. Collectively, season of fertilizer application and inherent soil moisture should be carefully considered when attempting to mitigate N2O emissions after the application of N fertilizer in managed forest ecosystems.

Acknowledgments The authors thank Sam Fry and William Shackett for assistance with conducting this research and for collecting soil-air samples. Funding for this study was provided by Weyerhaeuser Company.

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