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Corn residue is one of the sources of SOC in most Midwest row crops systems. On average, Iowa produces 0.1 billion metric tonnes (1.0 × 108 Mg) of corn and 67

million metric tonnes (6.7 × 107 Mg) of corn residue (Ertl, 2013). The benefits of leaving corn residue in the field include: prevent-ing soil erosion, adding plant nutrients and C to the soil, and as an energy source for soil microorganisms (Malhi et al., 2006; Ertl, 2013; Hossain and Puteh, 2013). However, excessive amounts of corn residue at the soil surface can negatively affect the soil pro-cesses by reducing soil temperature (Licht and Al-Kaisi, 2005) and increasing soil moisture beyond optimal levels for decomposition (Schomberg et al., 1994). Plant residue contains carbohydrates, lig-nin, tannins, fats and oils, waxes, proteins, and minerals (Ruther et al., 2003; Kramer and Gleixner, 2006) in varying concentrations, which influence the rate of decomposition by microorganisms. Materials high in lignin with low N require additional N, which soil microorganisms immobilize from the soil (Schomberg et al., 1994). However, the efficient decomposition of plant residue by soil microorganisms requires optimum soil temperature, moisture, and pH (Kirschbaum, 1995; Recous et al., 1995; Scott et. al., 1996; Bauer et. al., 2008; Han et al., 2012). These factors simultaneously affect SOC mineralization as a pivotal soil function in relation to nutrient supply to plants, soil structural stability and supporting soil biodiversity, greenhouse gas emissions, and CO2 concentration in the atmosphere (Paterson and Sim, 2013).

Among the factors that influence plant residue decomposition, temperature is a major factor that controls SOC mineralization (Conant et al., 2011). Studies have well established a significant positive correlation between soil temperature and the rate of SOC mineralization (Kirschbaum, 1995; Gudasz et al., 2010). However, the nature of the plant material under decomposition further determines the rate of decomposition. Plant residue higher in lignin is more resistant to microbial decomposition (Stotzky 2000; Ruther et al.,2003; Flores et al., 2005; Fang et al., 2007; Xue et al., 2011) than those higher in cellulose, hemi-cellulose, and starch compounds (Heal et al., 1997; Yanni et al., 2010). Therefore, the decomposition of lignin-rich materials with low N will require additional N to increase decomposition, which soil microorgan-isms utilize from the soil (Schomberg et al., 1994).

Effect of Nitrogen Fertilizer Application on Corn Residue Decomposition in Iowa

Mahdi M. Al-Kaisi,* David Kwaw-Mensah, and En Ci

Published in Agron. J. 109:1–13 (2017) doi:10.2134/agronj2016.11.0633

Copyright © 2017 by the American Society of Agronomy5585 Guilford Road, Madison, WI 53711 USAAll rights reserved

AbstrActCorn (Zea mays L.) residue is one of the sources of soil organic carbon (SOC) in row cropping systems in the Midwest. Farmers in Iowa apply liquid N to corn residue after harvest, assuming it will increase corn residue decomposition. The objective of this study was to investigate the effectiveness of N application for increasing corn residue decomposition. The study included two fields with three N rates (0, 34, or 67 kg N ha–1) of liquid 32% urea ammonium nitrate (UAN) applied after harvest, and two laboratory incubation experiments with three temperatures (0, 25, and 35°C) in 2012 and 2013. The experiment design was a randomized complete block in four replications in a no-tillage system (NT). The average mass of residue organic carbon (OC) after harvest was in the range of 1.6 to 1.7 Mg ha–1. Residue OC in the field declined sharply in both years, particularly dur-ing the first 3 mo, with no significant difference between N treatments. The only difference in field residue decomposition occurred after 6 mo (P = 0.0241) at the Olson location in 2012, where a greater (>21%) amount of remaining OC was associated with 0 kg N ha–1 than with other N treatments. The incuba-tion study showed an increasing rate of residue decomposition with increasing soil temperature with no N application effects on residue decomposition. These findings show that air and soil temperatures are the driving force for residue decomposition, especially at 25°C, rather than with N addition.

M.M. Al-Kaisi and D. Kwaw-Mensah, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011; E. Ci, College of Resources and Environment, Southwest University, Beibei District, Chongqing, China. Received 2 Nov. 2016. Accepted 4 June 2017. *Corresponding author (malkaisi@iastate.edu).

Abbreviations: NIACC, North Iowa Area Community College; NT, no-tillage; OC, organic carbon; SOC, soil organic carbon; UAN, urea ammonium nitrate, corn residue.

core Ideas• Application of 32% urea ammonium nitrate after harvest has no

effect on residue decomposition.• Three months after N addition, 54 to 69% of residue remained

with no N rate differences.• The remaining amount of residue after 12 mo was 35 to 49%

across all N treatments.• Incubation study shows soil temperature as a major factor in

residue decomposition.• Nitrogen addition suppressed CO2–C evolution during residue

incubation.

soIl tIllAge, conservAtIon & MAnAgeMent

Published online July 27, 2017

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Soil fauna and microorganisms draw available N from mineral soils proportionally to decompose the available C in plant residue (Bengtsson et al., 2003; Cabrera et al., 2005). Therefore, the rate of SOC assimilation by soil microorganisms depends on the source of microbial biomass N, the rate of decomposition of the plant mate-rials, the residual or any recently mineralized soil N and recycled microbial biomass, the C flow, and the C/N ratio (Barak et al., 1990; Hadas et al.,1992). The C/N ratio of plant residue is used as an index for assessment of N release or immobilization. Vigil and Kissel (1991) showed the break-even point between net N immo-bilization and mineralization to be a C/N ratio of 41, and research summarized by Whitmore (1996) showed the break-even point between C/N ratios of 20 and 40. Corn plant residue generally has a wider C/N ratio (70:1), and contributes little plant-available N to the soil during decomposition. Therefore, when C in the soil is low, soil microorganisms fully utilize the available C that is in the residue to release excess mineral N to the soil. However, a higher amount of soil C relative to N will result in a slower rate of residue decomposition, as N is not immediately available to soil microorganisms (Vigil and Kissel, 1991; Quemada and Cabrera, 1995). Thus, the role of N in microbial decomposition of plant residue emphasizes the importance of the C/N ratio. Fog (1988) attempted to reconcile the different relationships in different ranges of C/N ratios and decomposition rates (Scheffer, 1984; Berg and McClacgherty, 1987). Studies dealing with N additions to crop residue have documented that as long as the C/N ratio is >30, any added N will be immobilized. Therefore, the success-ful integration of crop residue management in cropping systems requires the understanding of the impact of crop residue on the physical, chemical, and biological properties of the soil.

The fall application of N after corn harvest by Iowa farmers varies widely through the state in different forms and method-ologies. In 2011, a survey of Iowa corn farmers regarding the N application showed more than 50% of corn farmers agreed, “that farmers apply excess fertilizer as yield insurance” (Arbuckle, 2014). The excess application of N as yield insurance includes a certain amount of N applied after harvest as a belief that it will help with residue breakdown, even though there is no actual data to support such an assertion. Promoting best management practices for N use by discouraging fall application can be useful environmen-tally and economically. Therefore, the application of N fertilizer such as 32% UAN solution [CO (NH2)2NH4NO3] to corn residue after harvest to increase the rate of residue decomposition should be considered carefully. Studies have shown opposite or inconsistent effects of N application on corn residue decomposi-tion (Fog, 1988; Al-Kaisi and Guzman, 2013; Grandy et al., 2013). In a study where the effects of mineral N on corn residue decomposition were investigated by applying mineral N to the corn residue and tilling the N-treated residue with the soil, the decomposition of 14C labeled corn residue varied depending on the stage of decomposition and the land use type (Chen et al., 2007). These authors further reported that N only accelerated the rate of residue decomposition in the first 5 d in the reduced tillage and grassland soils but not in tilled soils. However, the rate of corn residue decomposition decreased in the different soil management systems after 11 d. Al-Kaisi and Guzman (2013) also showed similar results in a corn residue decomposition study, where the rate of decomposition of corn residue treated with N was not significantly different. One reason why N additions suppress C

mineralization and CO2 evolution is that mineral N inputs may lead to increased decomposer efficiencies, more rapid formation of recalcitrant material, and decreased growth rate of decomposers (Ågren et al., 2001). The increase in decomposing efficiencies is due to changes in decomposing communities that have a greater N requirement (Ågren et al., 2001). Riggs et al. (2015) reported decreased microbial respiration with N additions. Although decomposition of labile C was stimulated by added N, decomposi-tion of recalcitrant C was suppressed. Moran et al. (2005) showed greater residue-C transformation into humin C with mineral-N input. Fog (1988) in a review, also concluded that N addition to residue negatively affects residue decomposition for the following reasons: (i) N addition to organic matter changes the composition of decomposers through competition, (ii) ammonia suppresses the production of enzymes required to degrade lignin and other recalcitrant compounds, and (iii) ammonia and amino compounds react with organic matter to form recalcitrant material. Other studies have confirmed the role of the enzymes in residue decom-position (Turner et al., 2002; Awad et al., 2012), where the enzyme β-cellobiosidade breaks down the cellulose in plant residue into smaller oligosaccharides, which are subsequently degraded by the enzyme β-glucosidase into glucose. To address the perception that applying N fertilizer solution to corn residue after harvest increases the rate of residue decomposition, we designed this study to investigate the effect of N fertilization on the rate of corn residue decomposition in the field and in the laboratory. Choice of organic matter decomposition models depends on types of C pools being measured. Generally, SOC mineralization is evaluated by using a two-component exponential model (Ct = C0 (1 – exp–kt) + (TOC – C0) × (1 – exp–ht), where labile and recalcitrant C pools are involved (Ci et al., 2015). Alternatively, crop residue decomposition, as in this study, is often evaluated using the first-order kinetic model Ct = C0 (1 – exp–kt) (Murwira et al., 1990). Previous research has shown that the first-order kinetic model is best fit to describe OC mineralization of field and laboratory crop residue decomposition similar to those in this study (Al-Kaisi and Guzman, 2013). The first-order kinetic model is widely used to determine plant material decomposition, and it is more appropri-ate, given the nature of C source in plant residue that is readily decomposed by different microbial species (Hadas et al., 2004). In a decomposition study using a range of organic materials, Saviozzi et al. (1993) demonstrated that the first-order model was versatile and proved a good fit for C-mineralization data. Based on the perception that applying N fertilizer solution to corn residue after harvest increases the rate of residue decomposition, the objective was to investigate the effectiveness of N application for increasing corn residue decomposition. Therefore, our hypothesis was that N application to corn residue after harvest has no effect on corn residue decomposition particularly when soil temperature, which is dependent on air temperature, is continually declining after the time of N application.

MAterIAls AnD MetHoDssite Description and Field experiment Design

The field experiments were conducted at two locations in Iowa (Table 1) in 2012 and 2013. One location in northern Iowa was at the North Iowa Area Community College (NIACC) research farm near Mason City (43°09¢12² N, 93°12¢03² W) 348 m above sea level in Cerro Gordo County, Iowa. The

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second location was at the Olson farm located 8 km north of Van Horne (42°0¢32² N, 92°5¢20² W) 288 m above sea level in Benton County, central Iowa. The soil at the NIACC location is poorly drained silty loam (fine-loamy, mixed, superactive Typic Endoaquou) and the Olson location has well-drained silty clay loam (fine-silty, mixed, superactive Typic Argiudoll) soil. The experimental design for the study was a randomized complete block in NT with N rate (0, 34, and 67 kg N ha–1) of 32% UAN in four replications. For each site, liquid N was applied to both experiments after corn grain harvest in early November, 2011 and 2012, using a Case-IH Floater (Heartland Ag Inc., Grand Island, NE) N applicator.

Field Decomposition experiment

For each site, immediately after applying the liquid N of 0, 34, or 67 kg N ha–1, corn residue samples were collected from each N treatment plot by randomly placing a 1 by 1 m white polyvinyl chloride frame in each plot to collect residue samples. A 120-g sub-sample of the corn residue from each N treatment was placed in a 78 by 32 cm (2496 cm2) nylon mesh bag, which was left in the field for 360 d. There were 48 nylon mesh residue bags for the experi-ment (three N treatments × four time periods × four replications). Initially, corn residue samples for time 0 analysis were collected as baseline samples for comparing weight loss from decomposition of corn residue samples that were subsequently retrieved from the field at four different time periods (i.e., 3, 6, 9, or 12 mo) from the beginning of the experiment. Residue bags in the field were fastened to the ground with thin horse-shoe iron pins to avoid being blown away by wind. After every 3 mo (90 d period), 12 corn residue bags were retrieved from the field and brought to the laboratory for processing and cleaning from soil. Residue bags were washed and hanged to drain any excess water for 24 h and oven-dried at 60°C for 3 d using an air-forced oven (S124838 Sheldon VWR 1685 1600 Hafo Series Horizontal Air Flow, Gilroy, CA). Oven-dried corn residue samples were weighed using a ULINE scale (H-1651, Pleasant Prairie, WI), and residue weight loss for each sample was determined by subtracting the oven-dry sample weight from the initial oven-dry weight of the sample collected from the field at time zero (0 d). Oven-dried samples were finely ground using a Wiley Mill Model 2 C steel pulverizer (Arthur H. Thomas Co., Philadelphia, PA) and stored in paper bags prior to the laboratory analyses of OC and N using a CN Analyzer (Leco Corporation, St. Joseph, MI).

laboratory Incubation experiment DesignThe laboratory experiment was a sequential incubation study

conducted at three different temperatures (0, 25, and 35°C) for 90 consecutive days (90 d) to simulate residue decomposition in the field during winter, spring, and summer seasons. Residue samples were transferred after 30 d at 0 to 25°C for another 30 d and lastly to 35°C for the last 30 d of the experiment. This was done to simu-late the field conditions, where residue samples were exposed to the gradual transitioning of colder seasonal temperature in winter to warmer spring and summer temperatures as the season progresses in the field. Residue samples used for the laboratory incubation experiment were taken from the same corn residue samples with the same N treatments initially collected at time 0 for the field experi-ments. Soil samples used for the incubation experiment with corn residue samples were collected for the top 15 cm from plots at each field experiment. The experiment at each incubation temperature was conducted in a dark incubation room for 1 mo (30 d). Four grams each of ground corn residue samples (2-mm sieve) were evenly spread on 0.1 kg of soil at 60% water-filled pore space placed in a 0.9 L capacity wide-mouth Mason glass jar. The soil in each Mason jar was occasionally sprayed with small amounts of deion-ized water to maintain the 60% water-filled pore space. Mason jars with soil without residue samples were used as control treatments, and completely empty Mason jars were used as blanks to trap atmo-spheric CO2 during the experiment. Forty Mason jars were used for the laboratory experiment. Treatments included three jars with soil and corn residue samples treated with different N rates (0, 34, or 67 kg N ha–1), one control jar (soil without corn residue), and one completely empty jar, arranged as a factorial experiment in four replications for both experiments’ at each incubation temperature (0, 25, and 35°C). A 20-mL scintillation vial containing 5 mL of 1 M NaOH solution was placed in each Mason jar to capture CO2 evolved from the soil and corn residue during incubation. The amount of CO2 evolved from residue decomposition trapped in the base (1 M NaOH) solution at specific time periods in days at each temperature was determined by titration. Base traps were changed on the following days of incubation: 1, 4, 7, 14, 18, 21, 25, 29, and 33, and titration was performed at the same time by adding 5 mL of 2 M BaCl2 solution and two to three drops of phenolphthalein indicator to each base trap in the 20 mL vial and titrating with 1 M HCl solution using digital micro burette until an endpoint was reached. The amount of CO2 retained in each base trap after the endpoint was determined by using the following formula from (Stotzky, 1965).

CO2 = (B – V) × (NE) [1]

Table 1. Major soil information by location (USDA-NRCS, 2013).

LocationSoil

associationSoil

series ClassificationSoil

textureDrainage

class SOM† pH†Latitude, longitude g kg–1

NIACC 43°09′12″ N, 93°12′03″ W Clyde–Readlyn

loamClyde fine-loamy, mixed,

superactive, mesic Typic Endoaquolls

Silty clay loam

Poorly and very poorly drained

54 6.1

Olson

42°0′32″N, 92°5′20″ W Tama Tamafine-silty, mixed, superactive, mesic Typic Argiudolls

Silty clay loam Well-drained 48 6.2

† Soil organic matter (SOM) and pH values represent the top 0 to15 cm of soil depth.

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where B = volume (mL) of the acid needed to titrate the trap solution from the empty Mason jars (blanks) to the endpoint, V = volume (mL) of acid needed to titrate the trap solution from the residue sample Mason jars to the end point, N = normal-ity of the acid, (mL–1), E = the equivalent weight of C in CO2; E = 6 if data is to be expressed in terms of C (mg CO2–C). The data was expressed on a unit mass basis of dry residue and soil (CO2–C mg kg–1 residue d–1). The amount of residue C mineral-ized was calculated by subtracting amounts of CO2–C evolved from treatments without residue (soil) from those with residue and soil for each N treatment. The number of days at each incubation tem-perature was 30 d. After 30 d in the room at 0°C, all the Mason jars were moved to a 25°C incubation temperature, and subsequently to a 35°C incubation temperature for the final 30 d of the experiment.

First-order rate constants (k) describing initial rates of residue decomposition (Table 2) were calculated by using the following one-pool kinetic model from Murwira et al. (1990). The first-order model assumes that plant C was readily decomposable and residue CO2–C emission was proportional to the C decay rate where Cresidue(t) is the available labile C at any time. Plant substrate in this model was considered as a single C pool. Therefore, this model is suitable for calculating plant residue decomposition in many studies (Saviozzi et al., 1993; Hadas et al., 2004).

Ct = C0 (1 – exp–kt) [2]

where Ct is carbon content at time t (day), C0 is initial carbon con-tent, k is first-order rate constant, and t is time (day). Using decay coefficient k values, times required for 50, 75, 90, and 99% residue C mineralization was estimated.

t(0.50) = ln 2 (k–1)

t(0.75) = ln 4 (k–1)

t(0.90) = ln10 (k–1)

t(0.99) = ln 100(k–1)

statistical Analysis

Field experimentThe field experiment was a completely randomized block design

with three N rates, five sampling times of residue bags, and four replications. Data were analyzed by site at P = 0.05 using the MIXED procedure of SAS (SAS Institute, 2011). Year and sam-pling time were considered as fixed effects and sampling time was also treated as a repeated measure. Several covariance structures were tested and the heterogeneous compound symmetry covari-ance structure had the best fit with12.4% coefficient of variance. Residue decomposition rate (change in weight) was the dependent variable and year, and sampling time, N rate, and their interactions as independent variables. The GLM procedure of SAS was used to determine the least square means of weight loss in the field and percentage of C mineralized.

Table 2. First-order decay model Ct = C0 (1– exp–kt)† parameters for sequential corn residue decomposition in the laboratory at 0, 25, and 35°C.

Location Temperature N2012 2013

C0 k R2 C0 k R2

°C kg ha–1 g kg–1 d–1 g kg–1 d–1

NIACC‡ 0 0 2.82 0.002 0.99 1.88 0.021 0.9934 3.16 0.003 0.99 2.76 0.017 0.9967 3.16 0.005 0.99 2.99 0.017 0.99

LSD (0.05) 0.008 0.001 0.009 0.00125 0 3.21 0.043 1.00 4.28 0.040 1.00

34 2.81 0.053 1.00 4.20 0.048 0.9967 2.52 0.051 0.99 3.98 0.051 1.00

LSD (0.05) 0.017 0.001 0.012 0.00535 0 3.51 0.021 0.99 3.67 0.024 0.99

34 4.39 0.013 0.99 2.39 0.032 0.9967 2.62 0.011 0.99 2.33 0.025 0.99

LSD (0.05) 0.011 0.001 0.015 0.002Olson 0 0 2.43 0.020 0.99 1.79 0.034 0.99

34 3.51 0.013 0.99 2.05 0.031 0.9967 3.23 0.006 0.99 1.80 0.034 0.99

LSD (0.05) 0.013 0.001 0.012 0.00225 0 2.99 0.054 0.99 3.39 0.040 0.99

34 2.60 0.057 0.99 3.43 0.044 0.9967 2.22 0.060 0.99 3.65 0.027 1.000

LSD (0.05) 0.012 0.001 0.018 0.00235 0 2.78 0.021 0.99 2.02 0.029 0.99

34 3.06 0.009 1.00 1.99 0.032 0.9967 3.06 0.025 0.99 1.83 0.032 0.99

LSD (0.05) 0.019 0.001 0.010 0.002† Ct = C0 (1– exp–kt), Ct = carbon content at time t, C0 = initial carbon content, k = first-order rate constant and t = time (day).‡ NIACC, North Iowa Area Community College.

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Fig. 1. Average monthly precipitation and air temperature North Iowa Area Community College (NIACC) and Olson locations for the growing seasons of November 2011 to November 2012 and November 2012 to November 2013.

Fig. 2. Remaining corn residue organic carbon (OC) in the field at North Iowa Area Community College (NIACC) and Olson locations. Different uppercase letters at specific times indicate significant difference at P = 0.05.

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laboratory experiment

The laboratory experiment was a two × five factorial within each year, with two locations, five levels of treatments (three glass jars of N-treated residue, control containing only soil, and blank or empty jar), and four replications. The data for each year were com-bined and analyzed by using PROC mixed procedure, where year and location were treated as fixed variables with repeated measure-ments for sampling times. The statistical model used treated resi-due decomposition rate (CO2 evolution) as dependent variable and year, time of retrieving base traps, N rate, and their interactions as independent variables. First-order constant (k) was estimated using

a nonlinear model (nls) parameter estimation in R software (ver-sion 2.13.2). Data were analyzed by site for each year.

resUltsThe weather conditions at both locations of the field experi-

ments are summarized in Fig. 1. The NIACC and Olson loca-tions in northern and east-central, Iowa, respectively, have a 30-yr average precipitation and temperature of 868 mm and 16.1°C per annum, and 743 mm and 16.9°C per annum, respectively. During the experiment years of 2012 and 2013, the average annual pre-cipitation and temperature at the NIACC and Olson locations

Table 3. Effect of N rate on remaining organic carbon (OC) mass percent of corn residue in field at different periods (months) at two locations in Iowa.

Location Year N rateMonths

0 3 6 9 12 kg ha–1 ——————————— residue organic C, % ——————————–

NIACC† 2012 0 100 50.3 45.3 41.1 40.434 100 55.6 50.9 46.2 40.967 100 54.8 51.0 49.0 43.3

Avg. 100 53.5 49.1 46.4 41.5Olson 2012 0 100 72.6 67.3 56.0 48.8

34 100 66.7 53.8 52.6 48.567 100 67.3 56.0 53.5 49.1

Avg. 100 68.9 59.0 54.0 48.8NIACC 2013 0 100 77.0 65.8 56.2 46.4

34 100 71.2 59.0 47.9 42.567 100 67.2 58.5 50.0 40.1

Avg. 100 71.8 61.1 51.4 43.0Olson 2013 0 100 74.0 63.0 55.0 42.0

34 100 72.1 65.4 55.8 36.567 100 75.5 65.3 51.0 26.5

Avg. 100 73.9 64.6 53.9 35.0† NIACC, North Iowa Area Community College.

Fig. 3. Rate of CO2–C release in 2012 at three temperatures (0, 25, and 35°C) from corn residue treated with three different N rates at two sites (North Iowa Area Community College [NIACC] and Olson). There were no significant differences at P = 0.05 in CO2–C release rate at each incubation temperature for both sites.

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were 585 mm and 17°C and 1140 mm and 15.6°C, and 448 mm and 17.7°C, and 804 mm and 16.6°C, respectively. Generally, the average annual temperature and precipitation at the NIACC are 0.8°C colder and 125 mm higher than the annual averages at the Olson site.

nitrogen Application effects on Field residue Decomposition

Corn residue at two separate locations treated with three dif-ferent N rates (Fig. 2) showed a gradual decline in residue OC mass over time for a period of 12 mo after UAN application. The total OC mass of the initial corn residue samples (0 time) for both locations after harvest was 1.6 to1.7 Mg ha–1. In 2012, the trend of corn residue OC mass decline was similar for all N treatments with no significant differences among treatments. At one location (Olson-2012) the control treatment showed a greater amount of remaining OC mass compared to treatments with higher N rates only at time periods after 6 mo, at P = 0.05 (Fig. 2). In 2013, the remaining mass of residue OC was not significantly different for all N treatments at the NIACC location. The remaining residue OC mass and its percentages during each period of the 2 yr of field experiments are presented in Table 3. The percentage of OC mass that remained at the NIACC location in 2012 for the periods of 6, 9, and 12 mo were 49.1, 46.4 and 41.5%, respectively, compared to 61.1, 51.4, and 43.0%, respectively in 2013. In the meantime, the percentage of OC mass that remained at the Olson location for the periods of 6, 9, and 12 mo in 2012 were 59.0, 54.0, and 48.8% respectively, compared to 64.6, 53.9, and 35.0, respec-tively in 2013. During the drought season of 2012, the percent-ages of remaining OC mass after 3 mo (December to February) averaged across all N rates at the NIACC and Olson locations were 53.5 and 68.9%, respectively.

nitrogen and temperature effects on residue Decomposition in a laboratory Incubation studyResults of the rate and cumulative CO2–C releases during

laboratory incubation of corn residue with different N rates at different incubation temperatures are presented in Fig. 3 and 4 and Fig. 5 and 6, respectively. The results of CO2–C emission rates at each incubation temperature for 2012 and 2013 were not significantly different for the different N rates (Fig. 3 and 4). Similarly, the cumulative CO2–C emission from three N-treated residue at 0°C and later at 35°C were not significantly different for both years, except at 25°C in 2012, where cumulative CO2–C evolutions associated with three N rates were significantly dif-ferent (P < 0.0001), after 27 d to the last day of the experiment for the NIACC location and from 18 d to the last day of incuba-tion for the Olson location (Fig. 5), where the control treatment (0 kg N ha–1) had the greatest CO2–C emission. Although there were no significant differences in the CO2–C emission at 0 and 35°C among N treatments, the rate of residue decomposition at the 0°C was much lower than that at 35°C (Fig. 5).

In 2013, cumulative CO2–C emissions from residue treated with different N rates were not significantly different at 0 and 25°C temperatures for NIACC, and at 0 and 35°C for Olson (Fig. 6). However, CO2–C emission at 35°C for NIACC and 25°C for Olson showed a significant difference (P < 0.0001) from 21 to 30 d (Fig. 6), where greater CO2–C evolution was associated with control and low N rate. Generally, CO2–C emission was much greater at 25°C incubation temperature than the other two temperatures (0 and 35°C) for both locations.

Kinetic Approach of residue Decomposition

The decomposition of corn residue in the laboratory (Table 4) was examined using the simple first-order kinetic model, Ct = C0 (1 – exp–kt) (Murwira et al., 1990). The first-order kinetic model describes the residue decomposition process very well, where

Fig. 4. Rate of CO2–C release in 2013 at three temperatures (0, 25, and 35°C) from corn residue treated with three different N rates at two sites (North Iowa Area Community College [NIACC] and Olson). There were no significant differences at P = 0.05 in CO2–C release rate at each incubation temperature for both sites.

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significant variance is associated with all treatments. In both years of the study, the effect of temperature on the k (d–1) value was incon-sistent, but showed an increasing trend with increasing temperature in most cases without any effect of increasing N rate. However, corn residue had different k values in both years and a tendency of higher k values at 25°C than the 0 and 35°C temperatures. In addition, the k values associated with different temperatures showed lower values with the 0°C temperature in both years. The initial C content (C0) differed among N rates in most cases (Table 4).

Assuming constant decay rates for specific treatments, time peri-ods t0.5, t0.75, t0.90, and t0.99 required for 50, 75, 90, and 99% of residue C mineralization were calculated (Table 4). The incubation was conducted sequentially with increasing temperature from 0 to 35°C, but concurrently for samples from both locations at each incubation temperature. For residue samples collected in 2012, the average first half-life, t(0.5) of corn residue treated with 0, 34, or 67 kg N ha–1 from both locations was 6 d. However, as the amount of corn residue for each N rate decreased, the number of days for

Fig. 6. Cumulative CO2–C release in 2013 at three temperatures (0, 25, and 35°C) from corn residue treated with three different N rates at two sites (North Iowa Area Community College [NIACC] and Olson). At 25°C and 35°C for Olson and NIACC, CO2–C release for the three N rates from Day 18 to 31 were significantly different at P = 0.05 as indicated by different uppercase letters.

Fig. 5. Cumulative CO2–C release in 2012 at three temperatures (0, 25, and 35°C) from corn residue treated with three different N rates at two sites (North Iowa Area Community College [NIACC] and Olson). At 25°C CO2–C release for the three N rates from Day 27 to 33 for NIACC and Day 18 to 33 for Olson were significantly different at P = 0.05 as indicated by different uppercase letters.

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subsequent time periods, t(0.75), t(0.90), and t(0.99) increased regard-less of N rate (Table 5). At the 0°C temperature, the differences in the number of days between periods from t(0.5) to t(0.99) were 7, 15, and 27 d, respectively. Similar trends of period percentages for corn residue decomposition were observed at 25 and 35°C. At the 25°C temperature, where the average first half-life, t(0.5) of corn residue treated with 0, 34, or 67 kg N ha–1 from both loca-tions was 4 d, and the differences in the number of days between time periods percentages from t(0.5) to t(0.99) were 9, 13, and 16 d, respectively. At the 35°C incubation temperature, the average first half-life, t(0.5) of corn residue treated with 0, 34, or67 kg N ha–1 from both locations was 5 d, and the differences in the number of

days between time periods percentages from t(0.5) to t(0.99) were 6, 13, and 25 d, respectively (Table 5).

The laboratory results in 2013 followed the same trend as in 2012 (Table 5). At 0°C temperature, the average first half-life, t(0.5) of corn residue treated with 0, 34, or 67 kg N ha–1 from both locations was 5 d. Also, as the amount of corn residue for each N rate decreased, the number of days for subsequent time periods percentages were greater than that for the half-life [t(0.5)] for the different N rates (Table 5). At the 0°C incubation temperature, the differences in the number of days between time periods per-centages from t(0.5) to t(0.99) were 8, 13, and 20 d, respectively. At the 25°C incubation temperature, the average first half-life

Table 4. First-order decay Ct = C0 (1– exp–kt) model used for corn residue decomposition treated with different N rate at different temperature in the laboratory. Assuming constant decay rate, days required for 50, 75, 90, and 99% C mineralization was estimated using k values.

Location Temperature N rate k R2 t(0.5)† t(0.75) t(0.90) t(0.99)°C kg ha–1 d–1 ————————————— d—————————————

2012NIACC‡ 0 0 0.002 0.99 7 15 32 63

34 0.003 0.99 7 14 30 6067 0.005 0.99 6 13 28 57

25 0 0.043 1.00 4 8 19 4134 0.053 1.00 4 8 19 4067 0.051 0.99 4 8 19 40

35 0 0.021 0.99 5 10 23 4634 0.013 0.99 5 11 24 5067 0.011 0.99 5 11 25 51

Olson 0 0 0.020 0.99 5 10 23 4734 0.013 0.99 5 11 24 5067 0.006 0.99 6 12 27 55

25 0 0.054 0.99 4 8 19 4034 0.057 0.99 4 8 18 3967 0.060 0.99 4 8 19 40

35 0 0.021 0.99 5 10 22 4634 0.009 1.00 5 12 26 5367 0.025 0.99 4 10 22 45

2013NIACC 0 0 0.021 0.99 5 10 22 46

34 0.017 0.99 5 10 23 4867 0.017 0.99 5 10 23 48

25 0 0.040 1.00 4 9 20 4234 0.048 0.99 4 8 19 4167 0.051 1.00 4 8 19 40

35 0 0.024 0.99 4 10 22 4634 0.032 0.99 4 9 21 4367 0.025 0.99 4 10 22 45

Olson0 0 0.034 0.99 4 9 21 43

34 0.031 0.99 4 9 21 4467 0.034 0.99 4 9 21 43

25 0 0.040 1.00 4 9 20 4234 0.048 0.99 4 8 19 4167 0.051 1.00 4 9 21 45

35 0 0.029 0.99 4 9 21 4434 0.032 0.99 4 9 21 4367 0.032 0.99 4 9 21 43

† t(0.5) = half-life (50%), t(0.75) = 75%, t(0.90) = 90%, t(0.99) = 99%.‡ NIACC, North Iowa Area Community College.

10 Agronomy Journa l • Volume 109, Issue 5 • 2017

[t(0.5)] of corn residue treated with 0, 34, or 67 kg N ha–1from both locations was 4 d, and the differences in the number of days between time periods percentages from t(0.5) to t(0.99) were 6, 13, and 19 d, respectively. At the 35°C incubation tempera-ture, the average first half-life [t(0.5)] of corn residue treated with 0, 34, or 67 kg N ha–1 from both locations was 4 d, and the differences in the number of days between time periods percentages from t(0.5) to t(0.99) were 9, 13, and 18 d, respec-tively (Table 5). Overall, the average number of days for 99% of corn residue to decompose at the NIACC and Olson locations were 22 and 20 d, respectively.

DIscUssIonnitrogen Application effects on residue

Decomposition in the FieldCorn residue generally decomposes at a slower rate than other

crops such as soybean [Glycine max (L.) Merr.], due to the high C/N ratio of corn residue (Reis et al., 2011). Contrary to farm-ers’ perceptions, avoiding post-harvest UAN application to corn residue can reduce input cost per hectare or acre from labor and fertilizer costs. We found, in this study, that the control treat-ment, with no UAN, had 1.7% greater decomposition, especially at the NIACC location in 2012 than corn residue treated with UAN (Fig. 2). In 2012, which was a drought year, the average loss of corn residue OC mass across all N rates during the first 3 mo after harvest at both locations was 31.8%. In 2013, which had a wet spring, the average loss of corn residue mass across all N rates at both locations during the first 3 mo after harvest was 27.2%. The low percentage of residue decomposition at the NIACC site was largely due to cooler temperatures at that location. Other studies also found that residue decomposition is highly influenced by temperature and moisture availability (Kirschbaum, 1994; Paul, 2001; Reis et al., 2011) rather than N application, consistent with findings of Al-Kaisi and Guzman (2013). The rate of decomposition in corn residue over a 9-mo period following the first 3 mo was slower. At the end of the 12-mo period, the average mass of organic C residue remaining across all N rates at the NIACC and Olson locations was 41.5 and 48.8%, respectively, in 2012. The drought conditions and high temperatures led to the lower percentage of residue decom-position at the Olson site. In 2013, the average mass remaining

after the 12-mo period at NIACC was 43.0% and at Olson, 35%. The normal weather conditions at both sites in 2013 dem-onstrated the temperature effect, especially at the NIACC, site where a greater percentage of residue remained as compared to the Olson site. An average of 45.1% residue organic C remained after 12 mo across NIACC and Olson in 2012 compared to 39.0% in 2013 across both locations (Table 3).

The lack of UAN effect on corn residue decomposition in the field may be attributed to several factors including: (i) the time of application of N after corn harvest when air temperature had dropped below the optimum level (Fig. 1) for microbial activity (Ci et al., 2015), and (ii) limited availability of soil moisture from lack of precipitation after corn senescence dur-ing the months of November and December (Fig. 1). There is no consensus on how N application to plant residue affects C mineralization, with some studies showing a suppressive effect on C mineralization (Al-Kaisi and Guzman, 2013; Grandy et al., 2013), and others showing a stimulatory effect (Fog, 1988; Green et al., 1995). One reason why N additions sup-press C mineralization and CO2 evolution is that mineral N inputs may lead to increased decomposer efficiencies, more rapid formation of recalcitrant material, and decreased growth rate of decomposers when N is continuously supplied (Ågren et al., 2001). The increase in decomposing efficiencies is due to changes in decomposing communities that have a greater N requirement (Ågren et al., 2001). The implication for this might be a greater proportion of residue C being stabilized as soil organic matter instead of CO2 emission. Riggs et al. (2015) reported decreased microbial respiration with N addition. Although decomposition of labile C was positively stimulated by added N, decomposition of recalcitrant C was suppressed. Moran et al. (2005) showed greater residue-C transformation into humin-C with mineral-N input. In this study, laboratory incubation of corn residue treated with N decomposed less using CO2 evolution as an indicator of decomposition (Fig. 4). This might be attributed to the applied N effect in acidifying the soil to further lower the soil pH, which was initially at pH 4.6 to 5.8. The additions of N fertilizers such as NH4NO3–N reduce soil pH (Schwab et al., 1989), and microbial activity (Rousk et al., 2009). Green et al. (1995) also reported that N additions can suppress C mineralization from SOM.

Table 5. Summary of number of days for determining corn residue decomposition with different N rates under different incubation tem-peratures at different percentages of life-times (50, 75, 90, and 99%).

N rate

Temperature, °C0 25 35

t(0.05)d t(0.75) d t(0.90) d t(0.99) d t(0.05) d t(0.75) d t(0.90) d t(0.99) d t(0.05) d t(0.75) d t(0.90) d t(0.99) dkg ha–1

20120 6 13 28 55 4 13 28 41 5 10 23 46

34 6 13 27 55 5 13 22 45 5 12 25 5267 6 13 28 56 4 13 28 40 5 11 24 48

Avg. 6 13 28 55 4 13 26 42 5 11 24 492013

0 5 13 28 45 4 9 20 42 4 13 28 4534 5 13 22 46 4 8 19 41 4 13 21 4367 5 13 28 46 4 13 28 42 4 13 28 44

Avg. 5 13 26 46 4 10 23 42 4 13 26 44

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nitrogen and temperature effects on residue Decomposition in laboratory Incubation

The N application in the incubation study did not show significant effects of adding UAN on the cumulative CO2–C evolution from corn residue decomposition from both locations at any incubation temperature. In both years (2012 and 2013), the driving force for increasing residue decomposition was tem-perature, with the greatest rate of CO2–C evolution (Fig. 3 and 4) and cumulative CO2–C release (Fig. 5 and 6) occurring at the incubation temperature of 25°C. This incubation experiment was conducted sequentially. Therefore, the amount of CO2–C released at 35°C was lower compared to 25°C because of the greater amount of mineralizable (labile) C that decomposed at 25°C (Fig. 5 and 6). The depletion of most readily decompos-able C components during the 30 d at 25°C may cause such differences. However, studies have confirmed the increase in CO2–C evolution with increasing temperature (Gudasz et al., 2010; Wang et al., 2010; Lavoie et al., 2011) was expected when the laboratory experiments were conducted simultaneously. In an incubation study with northern Alaskan soils at two tem-peratures (5 and 15°C) and two levels of N addition (with and without N) to test for N limitation of SOM decomposition, temperature always had a strong positive effect on SOM decom-position across soil layers and ecosystem types, whereas the effect of N addition was inconsistent (Lavoie et al., 2011). Although Green et al. (1995) reported that N addition to corn residue greater than 2 g residue kg–1 of soil stimulated CO2 evolution and C mineralization, our study showed temperature and not N as the stimulating factor of corn residue decomposition. Our findings have environmental and economic benefits. Avoiding post-harvest N application will reduce production cost and any potential environmental concerns associated with N application.

Kinetic Approach of residue Decomposition

The initial C content of corn residue from both study locations was similar (1.6–1.7 Mg ha–1 and the effect of N application on the rate of residue C decomposition at the different temperatures was not significantly different. The findings of this study reveal greater k constant and Co values for the incubation temperature of 25°C with corn residue from both locations and years (Table 3). The k value provides insight about the rate of decomposition of organic C (Ouyang et al., 2008). It appears that the maximum k value was at the incubation temperature of 25°C, which confirms that temperature has a positive effect on residue and SOC decom-position (Lavoie et al., 2011). The increase in mineralization rate with increasing temperature from 0 to 25°C is a reflection of the increase in microbial activity in mineralizing the residue OC. Ci et al. (2015) reported that SOC mineralization, especially with label C, is highly influenced by temperature fluctuation during the mineralization process that affects the pool size (C0) and mineral-ization rate (k).

Using the kinetic model to determine decomposition rate con-stant (k) and time (days) for the various percentages of residue life time at different incubation temperatures and N rate showed that half-life of residue [t(0.5)] was approximately 4 d. There were no sig-nificant differences with different N rates at different incubation temperatures, but the greatest k values were achieved at the incuba-tion temperature of 25°C. It has been documented in other studies that temperature sensitivity of organic C decomposition decreases

with increasing temperature (Kirschbaum, 1994). Although the temperature sensitivity is not constant across the range of tempera-tures of 4 to 28°C (Winkler et al., 1996), 25°C appears to be the optimum temperature for C mineralization. Kirschbaum (1994) reported 20 to 30°C as the moderate to high range of temperature for C mineralization. Vigil and Sparks (1995) reported maximum microbial activity for crop residue decomposition at 32°C, which significantly drops at 41°C.

In this study, C mineralization increased over time at the incubation temperature of 25°C with no significant differences between N treatments, yet with fewer number of days (almost 1–2 d less) at t(0.75) compared to the other two incubation tem-peratures (0 and 35°C). Although it took longer (approximately 7–15 d) to achieve a greater percentage of C mineralization at 0 and 35°C than at 25°C (4–8 d) (Table 4), the residue with lower OC content did not show any significant difference in the number of days (1–3 d) to achieve 99% [t(0.99)] of residue decomposition between N rate treatments. The differences in residue decom-position reveal that N application has no significant effect on residue OC mineralization. Corn residue decomposition is highly driven by temperature (P < 0.0001), with a significant interaction between N and temperature (P < 0.0001), where optimum OC mineralization occurred at 25°C. Optimum microbial activity during residue decomposition is highly affected by temperature and moisture as the significant drivers in OC mineralization (Vigil and Sparks, 1995; Winkler et al., 1996; Kirschbaum, 2000; Paul, 2001). Although one would have expected greater CO2–C release at 35°C, this was not the case in this study because residue samples were sequentially incubated by moving incubation jars after 30 d from one incubation temperature to the next starting at 0°C (Fig. 4). Such processes led to the mineralization of the majority of easily decomposable components of residue. Therefore, the effect of temperature on residue decomposition was considerable at 25°C with greater CO2–C release from corn residue in both years, and higher k values at 25°C than the other two temperatures (0 and 35°C). The higher k value and rate of decomposition of residue at 25°C than that at 35°C may be due to less availability of labile OC after the samples moved to the 35°C incubation temperature.

conclUsIonsThe findings from this study show that N application had no

effect on residue decomposition in the field or laboratory. The effect of N application on plant residue decomposition under dif-ferent incubation temperatures shows that residue decomposition is highly sensitive to temperature changes rather than N addition. The role of temperature and moisture in residue OC mineraliza-tion was evident in the number of days it took for corn residue to decompose (i.e., half-life) at the lowest temperature of 0°C. Most strikingly, N application did not have an effect on residue decom-position even at highest temperature (35°C). In most cases, the addition of N caused a slowdown in residue OC mineralization. The relevance and application of these findings are critical to the environmental and economic implications of corn residue manage-ment by farmers in the Midwest. This study demonstrates that applying N to corn residue does not improve residue decomposi-tion. Therefore, withholding N will improve farmers’ profits and prevent any potential N loss to the environment.

12 Agronomy Journa l • Volume 109, Issue 5 • 2017

AcKnowleDgMents

This research was partially funded by the Iowa Soybean Association. The authors express their thanks to Kevin Muhlenbruch, a faculty at NIACC and his staff, for allowing us to conduct this research at NIACC Research Farm in Cerro Gordo County and also to John and Ben Olson, farmers in East central Iowa (Benton County) who provided the second site for this research on their farm. Our thanks extend to graduate and undergraduate students who were part of our research program and to Extension staff from both counties where research sites were located, for their contributions during field work and outreach activities.

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