Soil Science Society of America Journal

Soil Sci. Soc. Am. J. 78: 609–623 doi:10.2136/sssaj2013.10.0426 Received 2 Oct. 2013. *Corresponding author (malkaisi@iastate.edu). © 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. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Residue Removal and Management Practices Effects on Soil Environment and Carbon Budget

Soil & Water Management & Conservation

In agricultural systems, like any other ecosystem, the alteration of one part of the system may have large effects on the system as a whole. Under current agri-cultural practices, crop residue is left on the soil surface after harvest with NT

systems, or is incorporated into the soil with CT, where the residue decomposi-tion environment is more suitable (Franzluebbers et al., 1995; Al-Kaisi and Yin, 2005). However, crop residues left after harvest is being considered as a potential feedstock source for bioethanol production which can contribute to the reduction of fossil fuel use and net greenhouse gas (GHG) emissions (Wilhelm et al., 2004; Graham et al., 2007). It is most likely that lignocellulosic ethanol production will become a viable option and could create an annual market for crop residue from approximately 143 million tons to potential 583 to 805 million tons (U.S. tons; U.S. Department of Energy, 2011).

Jose G. Guzman Carbon Management and Sequestration Center The Ohio State Univ. Columbus, OH 43210

Mahdi M. Al-Kaisi*Agronomy Dep.Iowa State Univ.Ames, IA 50011

Agriculture management practices can significantly affect soil C storage through changes in C inputs and losses. This study investigated the short-term effects of tillage (no-tillage [NT] and conventional tillage [CT]), residue removal (0, 50, and 100%), and N rates of 0, 170, and 280 kg N ha-1 on soil C storage. Studies were established in 2008 to 2011 on a Nicollet clay loam (fine-loamy, mixed superactive, mesic Aquic Hapludolls) and Canisteo clay loam (Fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquolls) soil association at Ames, central Iowa site (AC) and a Marshall silty clay loam (Fine-silty, mixed, superactive, mesic Typic Hapludolls) soil association at Armstrong, southwest Iowa site (ASW) in continuous corn (Zea Mays L.). Findings from the C budget show that under CT and N rate of 170 kg N ha-1 in continuous corn, there was no significant change in net soil C with no residue removal. Increasing N rate from 170 to 280 kg N ha-1 resulted in greater potential C inputs from above and belowground biomass, although C losses were not significantly different, especially with NT. Thus, a portion of soil surface residue could be removed without causing a net loss of soil C. Converting from CT to NT led to lower soil C losses, but C inputs varied due to soil temperature and water content differences and seasonal variability in a given year. Consequently, averaged across both tillage systems and at 280 kg ha-1 N rate for continuous corn approximately 5.10 and 4.18 Mg ha-1 of the residue should remain on the field to sustain soil C in 2010 and 5.23 and 5.18 Mg ha–1 in 2011 for AC and ASW sites, respectively. These finding suggest that residue removal needs to be approached on yearly basis with particular consideration to site’s yield potential and weather condition as the residue biomass production can be variable.

Abbreviations: AC, Ames Central Iowa; ASW, Armstrong Southwest Iowa; CT, conventional tillage; DAP, days after planting; NT, no tillage; Rm, microbial respiration; RR, root respiration; SOC, soil organic carbon; UAN, urea-ammonium nitrate 32% nitrogen.

Published April 8, 2014

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Some studies have suggested that to meet the demands of the bioethanol industry, farmers will need to increasingly adapt to NT practices to offset potential soil organic carbon (SOC) losses due to crop residue removal (Kim and Dale, 2003). These losses are due to tillage effects, by increasing oxidation of SOC shortly after tillage operations (Reicosky et al., 1997; Al-Kaisi and Yin, 2005), and the destruction of aggregates that physi-cally protects SOC from microbial activities (Six et al., 2000). It has also been suggested that increases in N fertilization rates may further aid in soil C sequestration (Wilts et al., 2004; Van Vleck and King, 2011), due to increases in aboveground biomass and especially root biomass, which can contribute to more stable SOC derived C than does aboveground residue (Rasse et al., 2005). However, potential increases in C input from changes in N fertilization rate could be counter balanced by N fertilization effects on C mineralization and carbon dioxide (CO2) emissions in the field. Some studies have shown a suppressive effect of N on C mineralization and others showing a stimulatory positive effect (Kowalenko et al., 1978; Al-Kaisi et al., 2008). Although it is clear that agriculture management practices can significantly affect soil C storage through C inputs and losses, the detection of SOC changes due to management practices are often not ob-served until several years later in long-term studies with direct SOC measurements (Conen et al., 2003; Smith, 2004). The cal-culation of a C budget from C inputs (i.e., aboveground residue and root biomass) minus C outputs (C loss via microbial respira-tion, Rm), provides valuable insights into the processes contrib-uting to changes in SOC due to changes in management prac-tices on a finer temporal scale (Duiker and Lal, 2000).

In C budget studies, the primary mechanism for quantify-ing C loss from soils is by measuring soil surface CO2 emissions. Most of the soil surface CO2 emissions are a product of decom-position of plant litter and soil organic matter via Rm, and from root respiration (Rr). These two components of soil surface CO2 can have different responses to temperature (Fang and Moncrieff, 2001; Lloyd and Taylor, 1994) and soil water content which can restrict Rm and Rr, and can also slow down oxygen diffusion and emissions of CO2 (Bouma and Bryla, 2000). Thus the contribu-tions of these components need to be accounted for to quantify C losses in the soil (Raich and Mora, 2005).

Currently, there are few studies that have evaluated the ef-fects of residue removal rates under different management prac-tices on potential changes in SOC (Linden et al., 2000; Wilhelm et al., 2004; Dolan et al., 2006), for a conclusive estimate on how much crop residue can sustainably (i.e., increase or no net loss of C) be removed in a system. Additionally, sustainability of crop residue removal will also depend heavily on cropping sys-tem (Doran et al., 1984), climate, and soil type (Mu et al., 2008) which need to be specified regionally. We hypothesized that po-tential SOC losses due to residue removal, could be counter bal-anced by altering management practices that have lower intensity of tillage and greater N fertilization input. The objectives of this study were to: (i) examine the effects of tillage, N fertilization rates, residue removal, and their interactions on soil temperature

and soil water content and soil environment, (ii) effects of these factors that affect Rm and Rr, and (iii) evaluate a soil C budget to determine how much crop residue can sustainably be removed in Central and Southwest Iowa.

MATErIALS ANd METHodSExperimental Sites and Treatments

The study was established in fall of 2008 on a Nicollet clay loam and Canisteo clay loam soil associations at Iowa State University, Agronomy Research Farm (AC) in Central, Iowa (42.4°N; 95.5°W) and a Marshall silty clay loam soil associa-tion at Armstrong Research and Demonstration Farm (ASW) Southwest, Iowa (41.3°N; 95.1°W) in continuous corn. The mean air temperature and annual precipitation at the AC site are 8.7°C and 975 mm, respectively (data from 1982 to 2011). In the ASW site, mean air temperature and annual precipitation are 9.5°C and 909 mm, respectively (data from 1982 to 2011). Both sites were previously in corn-soybean [Glycine max (L.) Merr.] rotation under CT(chisel plow in fall and chisel plus disk in the spring to a depth of 15 cm). The chisel plow treat-ment was implemented using a commercially available model with straight shanks and twisted chisel plow sweeps at the bot-tom mounted on a tool bar. The shanks were mounted on four tool bars in a staggering order to ensure an effective spacing of 30 cm between shanks (Al-Kaisi and Yin, 2005). Source of N was liquid urea-ammonium nitrate 32% N (UAN) which was side-dressed injected in May after planting using agronomic rates of 170 kg N ha-1 during the corn phase (Blackmer et al., 1997). Phosphorus and potassium fertilization was done periodically to maintain optimum soil concentrations so as not to restrict corn growth during the length of this study.

Treatments were established in 2008 to 2011 for both sites in a randomized, complete-block design with split-split arrange-ment and three replications. Each individual plot dimensions were 6-m wide by 15-m long. The main treatment was tillage practice (NT and CT), which was split into three different residue removal rates (0, 50, and 100%) which was further split into three N fertilization rates of 0, 170, and 280 kg N ha-1. As done previously from past management, source of N was 32% liquid UAN which was injected side-dressed in May after plant-ing. Shortly after corn harvest, desired rates of residue removal were accomplished by adjusting down-pressure on raking equip-ment before baling of residue. For 100% removal, corn stalks and leaves were first mowed then raked clear down (very high down-pressure) for residue to be collected by baler. After baling, plots were hand raked to achieve nearly 100% removal. For 50% removal, residue was not mowed and a decreased down-pressure of the rake equipment was set so as to leave approximately 50% of the soil surface covered after baling. Actual removal of residue by mass varied by N rate, tillage, site, and year ranging from 46 to 64 and 90 to 99% for 50 and 100% residue removal treatments, respectively. Both sites were planted using a 111-d maturity corn variety (P33W84) at a seeding density of 79,000 seeds ha-1.

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Soil organic Carbon and Bulk density Measurements

Soil samples were collected after harvest in mid-October, before establishment of treatments in 2008 and 3 yr afterward in 2011, to measure changes in SOC. Twelve 1.7-cm diam. soil cores were collected from depths of 0 to 15, 15 to 30, 30 to 45, and 45 to 60 cm in each treatment plot. Soil cores for each depth were combined into a single homogeneous sample. Soil samples at field moisture were 2-mm sieved and then air dried before be-ing analyzed. The total carbon (TC) was determined by dry com-bustion using a CN analyzer (LECO Corporation, St. Joseph, MI). A separate carbonate analysis using a modified pressure cal-cimeter method was used to determine inorganic C concentra-tion, which was subtracted from TC concentration to calculate SOC concentration (Sherrod et al., 2002). Concurrently with TC soil samples were being taken, three individual bulk density (rb) samples were collected for each depth from each treatment plot, using a 1.7-cm diam. soil probe. Soil cores were taken at the same soil depths as TC samples and were then oven dried at 105°C for 24 h and weighed. Bulk density (Mg m–3) was cal-culated as the dried soil mass divided by the soil core volume. The SOC concentrations (mg g-1 dry soil) that were measured in 2008 through 2011 were multiplied by mean rb values and soil depth thickness to convert SOC concentrations to mass per area basis (Mg ha-1) per plot basis by soil depth. For soil samples col-lected after baseline year in 2008, SOC content was calculated similar to initial samples, although concentrations of SOC for each soil profile depth were adjusted for gains or losses in soil mass due to changes in rb, for the original or equivalent soil mass (ESM) in 2008 to determine changes in SOC stocks (Lee et al., 2009). For the purpose of comparing changes in SOC with C budget approach rates, data from 2009 through 2011 were used since the C budget measurements started in 2009 season.

Soil Surface Carbon dioxide EmissionsDuring the growing season from April to October, soil

surface CO2 measurements were taken in 6- to 10-d inter-vals coupled with soil moisture (TRIME-FM Time Domain Reflectometry, Mesa Corp., Medfield, MA) and soil tempera-ture (thermometer attached to LI-COR 6400, LI-COR, Inc., Lincoln, NE) at the 5-cm soil depth in each plot using a por-table infrared CO2 gas analyzer (LI-COR 6400) with a soil res-piration chamber. Measurements were taken between 800 and 1100 h to approximate the 24-h mean soil surface CO2. Soil sur-face CO2 measurements were conducted by placing a soil respira-tion chamber over a 10-cm diam. polyvinyl chloride (PVC) ring placed into the ground for the entire year at the 3-cm soil depth and leaving approximately 2 cm of the ring above the soil surface. Two PVC rings were placed in each plot, one within the plant row and one in between plant rows. During the non-growing season, biweekly or monthly readings were taken due to lower soil surface CO2 emissions from reduced soil temperatures.

Cumulative soil surface CO2 emissions for the growing sea-son were calculated as follows (Grote and Al-Kaisi, 2007):

( ) ( )1 12 1

Cumulative CO kg ha *( – )

2

ni i

i ii

X Xt t− +

+

+=∑ [1]

where, Xi is the first soil surface CO2 emission (kg ha-1 d-1) read-ing, and Xi+1 is the following reading at times ti and ti+1, respec-tively; n is the last CO2 emission reading during the growing sea-son and i is the first CO2 emission reading in the growing season. Since soil surface CO2 emissions were only measured weekly dur-ing the growing season, a linear regression model using soil sur-face temperature as the predictor variable was used to estimate missing weekly soil surface CO2 measurements. This resulted in an additional cumulative 1500 kg CO2 ha-1 on average from early November to late March for 2010 through 2011 in both sites. Soil surface CO2 emissions across treatments were not significantly different and remained relatively low compared with growing sea-son measurements due to soil being mostly near or below freezing temperatures (Fang and Moncrieff, 2001; Dornbush and Raich, 2006). During winter months, soil surface CO2 emissions rep-resented approximately 9% of the total annual soil surface CO2 emissions, which is within the range reported in studies done in the North-central USA (Kessavalou et al., 1998; Dornbush and Raich, 2006).

Separation of Microbial and root respirationTo quantify the percentage of Rr contribution to soil surface

CO2 emissions, a clipping and root exclusion experiment was con-ducted in the AC site (Hanson et al., 2000). In each treatment plot, soil surface CO2 respiration was measured between corn rows with no roots (BRn), where a metal cylinder was installed as a physical barrier at the 15-cm soil depth, and with roots present (BRr) to esti-mate Rr between corn rows using following calculation:

r n2 r

r

BR BRCO in-betweenrows( )% 100BR

R ×−= [2]

In the end of the study, root biomass samples were collected in the BRn treatments to confirm that there were no roots present.

To estimate Rr contribution to soil surface CO2 emissions within corn rows, a clipping of a corn plant in each treatment plot was done every 2 to 3 wk to account for increases in root biomass throughout the growing season. Measurements of soil surface CO2 were taken in rings by clipping aboveground bio-mass with roots (Cl) and un-clipped aboveground biomass (UC), to estimate Rr within rows using following calculation:

2 r(UC-Cl)CO within rows ( ) % 1 00

UCR = × [3]

Subsequently, by subtracting Rr contribution (%) from 100%, contribution of microbial respiration (Rm) to soil surface CO2 emissions was estimated.

Potential Total Carbon Inputs from Above and Belowground Plant Biomass

Above and belowground biomasses were measured to quan-tify potential C inputs from plant biomass. Corn residue left after

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harvest was collected in mid-October. Plant samples were col-lected within a 0.10-m2 area frame that was placed in-between and within rows in each treatment plot. Plant biomass was dried at 65°C for 7 d, and weighed to determine dry matter weight (Mg ha-

1), and ground to fine powder for analysis. Total C concentrations from plant biomass were determined by dry combustion using a CN analyzer (LECO, St. Joseph, MI) and concentration values were multiplied by the plant biomass (dry matter m-2) to deter-mine aboveground potential total C inputs in Mg ha-1.

Using a soil core sampling method, belowground plant root biomass was taken right after harvest in mid-October. A 10.8-cm diam. golf course hole-cutter was used to collect cores to a 15-cm depth. A core was collected directly on the corn crown and two additional cores at each side; this was done twice for each treat-ment plot for a total of six cores. All samples were frozen until analysis could be performed. Cores were processed by sieving and flotation of roots using a hydropneumatic elutriation system (Gillison’s Variety Fabrication, Inc., Benzonia, MI) equipped with 530-mm screens (Smucker et al., 1982). Any non-root de-bris was removed during this time. The roots were then dried at 65°C until all water was evaporated, and roots were dry and brit-tle. Root samples were then processed and analyzed by dry com-bustion using a CN analyzer (LECO) as done in aboveground plant samples for potential belowground total C inputs. Total root biomass for the entire soil depth was estimated by collecting and rinsing three soil samples for root biomass that were dug to a depth of 1.2 m by 0.20 m-2 in bordering corn that was planted next to treatment plots. Samples were also collected using 10.8-cm diam. golf course hole-cutter to the 15-cm depth to estimate contribution of root biomass in the top 15-cm soil depth to total soil profile in 2010 and 2011. On average, approximately 60% of the total root biomass occurred in the top 15-cm soil depth.

Estimation of Soil Carbon BudgetA soil carbon budget for each site was estimated by mea-

suring net ecosystem productivity using a similar approach by Duiker and Lal (2000) and Kucharik et al. (2006). The soil car-bon budget was calculated as the difference between above and belowground biomass C input, and the C loss through organic matter decomposition (i.e., Rm) for the entire year:

-1 -1mNet carbon change (Mg C ha yr ) (ANPP + BNPP ) R= − [4]

where, ANPP is the aboveground plant biomass net primary production C, BNPP is the belowground plant biomass net primary production C, and Rm is C loss as CO2 due to micro-bial respiration.

Statistical AnalysesData for TC, soil surface CO2 emissions, above and be-

lowground biomass, soil temperature, soil water content and C budget results were analyzed using the procedure of Repeated Measures in the Proc Mixed model of SAS. A compound sym-metry covariance structure was used for repeated measures. Type

of tillage was considered as the main plot, residue removal as the split-plot, N fertilization as split-split-plot, and date of measure-ment as the repeated measure variable. Mean soil surface CO2 in-between plant rows and within plant row were used in this statistical analysis. Mean separation was determined using the PDIFF procedure and significance was evaluated at p £ 0.10, unless otherwise stated. To determine the relationship between Rm with soil surface CO2, soil temperature, soil water content, days after planting (DAP), and their interactions, a multiple backward elimination regression analysis using JMP 10.0 (SAS Institute, 2002) was conducted to test the effects of these predi-cator variables for Rm. Additionally, correlations between SOC changes and C budget values were done using PROC CORR procedure in SAS.

rESuLTS ANd dISCuSSIoNManagement Effects on Soil Temperature and Soil Water Content

The effects of residue removal on soil temperature and water content varied by site, year, tillage, and N fertilization rate (Fig. 1–4). Typically, soil was cooler and wetter in the AC site com-pared with the ASW site. Soil temperatures were significantly cooler and wetter during the growing season in 2010 compared with 2011. From May through October, soil temperatures were on average 1.5°C cooler with 0.03 m3 m-3 greater water con-tent in 2010 than in 2011 in the AC site. Similarly, differences of 1.2°C cooler soil temperature and 0.03 m3 m-3 greater water content were observed in 2010 than in 2011 in the ASW site.

Soil temperatures typically followed a parabolic trend during the growing season, peaking in late July (Fig. 1–4). The exception was at the AC site in 2010, where soil temperatures peaked just before corn canopy completely covered the soil sur-face late in June. During the early stages of corn growth from May through early July, soil temperatures in general were ap-proximately 1°C cooler with no residue removal than with 50 and 100% residue removal treatments under NT for both sites and years. Under CT, there were no significant differences in soil temperature across the different residue removal rates, although soil temperatures were on average 1°C warmer than NT when 50 and 100% of residue was removed and 1.5°C warmer when no residue was removed. Warmer soil temperatures under CT compared with NT can be attributed to field cultivation done to prepare for planting (Licht and Al-Kaisi, 2005; Al-Kaisi and Kwaw-Mensah, 2007). Once the corn canopy had completely covered the soil surface starting in mid-July, there was no signifi-cant difference in tillage and residue removal effects on soil tem-perature until after grain harvest and fall tillage (mid-October) was completed. During this time, CT typically had higher soil temperatures than NT across residue removal and N fertilization treatments. However, the exception was with the 100% residue removal under NT treatment, which generally was similar to that or was higher than CT across all residue removal and N fertil-ization treatments. Nitrogen fertilization rate also indirectly af-fected soil temperatures primarily by altering corn growth and

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canopy shading. Beginning in early July, soil temperatures were on average 0.5 to 1°C warmer in treatments when no N was ap-plied compared to those that did receive N in both sites and years until late October.

Soil water content fluctuated with rainfall events through-out the growing season in the AC and ASW site (Fig. 1–4). No-till on average had a soil water content of 0.34 m3 m-3 during the growing seasons of 2010 and 2011, 0.03 m3 m-3 higher than

CT in the AC site. Similarly, in the ASW site, NT on average had a soil water content of 0.31 m3 m-3 during the growing sea-sons in 2010 and 2011, 0.03 m3 m-3 higher than that for CT. In general, there were no significant differences between residue removal rates or N fertilization rates effects on soil water content observed in these sites during 2010 and 2011. These were unex-pected results, for instance, the removal of residue we hypoth-esized would decrease soil water content due to greater evapo-

Fig. 1. Tillage and residue removal effects on daily soil temperature, water content, microbial respiration, and root respiration by N fertilization rate during the growing season in the Ames Central (AC) site in 2010. Where, T is tillage, P is planting, N is nitrogen fertilization, H is harvest, r is residue removal, and FT is fall tillage.

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ration losses. However, precipitation events occurred frequent enough in these sites during 2010 and 2011 (Fig. 1–4), so as to minimize differences in soil water content by residue and N fer-tilization treatments. In studies done in drier conditions, surface cover by crop residue have shown to increase soil water content and vary by N fertilization rates due to differences in shading and water uptake (Sainju et al., 2012).

Management Effects on Microbial and root respiration

Using stepwise regression analysis from results in the clip-ping experiment to separate Rr and Rm, DAP, soil surface CO2 emissions, and soil water content (qv), were significant predictors for the percentage of Rm contribution to soil surface CO2 emis-sions (Table 1). Whether these predictors were positive or nega-

Fig. 2. Tillage and residue removal effects on daily soil temperature, water content, microbial respiration, and root respiration by N fertilization rate during the growing season in the Ames Central (AC) site in 2011. Where, T is tillage, P is planting, N is nitrogen fertilization, H is harvest, r is residue removal, and FT is fall tillage.

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tive drivers for the percentage of Rm contribution to soil surface CO2 emissions were dependent on management practice and whether soil surface CO2 emissions were from within the row or in-between rows. For example, under treatments with no residue removal and N was applied, DAP, and soil surface CO2 emis-sions were negative drivers, while qv was positive for Rm. This in-dicates that the contribution of Rm relative to Rr, was negatively

affected by increasing root growth as associated with DAP, and that high soil surface CO2 emissions values were due to greater contribution from Rr, especially in dry days. This can be attrib-uted to how CO2 is produced and transported in the soil under different environmental conditions (Moncrieff and Fang, 1999). In wet soil conditions, most of the soil surface CO2 emissions originates from upper portion of the soil profile (approximately

Fig. 3. Tillage and residue removal effects on daily soil temperature, water content, microbial respiration, and root respiration by N fertilization rate during the growing season in the Armstrong Southwest (ASW) site in 2010. Where, T is tillage, P is planting, N is nitrogen fertilization, H is harvest, r is residue removal, and FT is fall tillage.

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15 cm) where the majority of Rm occurs due to higher tempera-ture, soil organic matter, and slower diffusions rates (Gaudinski et al., 2000). In contrast, under dry soil conditions, most of the upper portion of the soil profile CO2 that is produced is instan-taneously released to the surface, while the slow transport of CO2 from lower depths is increased, resulting in higher soil sur-face CO2 emissions. At lower soil depths (below 10 cm), most of

the CO2 that is produced is primarily from Rr (Moncrieff and Fang, 1999).

Microbial respiration during the growing season varied sig-nificantly across year, site, tillage, residue removal, and N fertil-ization rates (Fig. 1–4). In general, Rm was 22 and 29% greater in CT than NT in the AC and ASW sites, respectively. In the ASW site, Rm increased with increased residue removal in both years. This could be due to increased soil temperatures increas-

Fig. 4. Tillage and residue removal effects on daily soil temperature, water content, microbial respiration, and root respiration by N fertilization rate during the growing season in the Armstrong Southwest (ASW) site in 2011. Where, T is tillage, P is planting, N is nitrogen fertilization, H is harvest, r is residue removal, and FT is fall tillage.

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ing decomposition rates (Fig. 3 and 4). However, in the AC site, residue removal effects on Rm varied by year. During the wetter, cooler growing season in 2010 (Fig. 1), significantly lower Rm was observed with 50% residue removal treatments compared with 0%, due to reduced C substrate, (Davidson et al., 1998). In 2011, significantly lower Rm was observed with 0% residue re-moval treatments compared with 50 and 100%, which could be due to cooler soil conditions limiting decomposition and not C substrate. There were no general trends observed on N fertiliza-tion rates effects on Rm, although, Rm was typically lower under 280 kg N ha-1 fertilization rate compared with 170 kg N ha-1. This lack of general trend between Rm and N fertilization rates are in part due to N fertilization effects on corn growth which indirectly influences soil temperature and water content due to shading and water uptake. Additionally, N fertilization also

influences the quantity and quality of residue left on the soil surface (Halvorson et al., 1999). There is no consensus on N fertilization effects C mineralization and CO2 emissions in the field, with some studies showing a suppressive effect of N on C mineralization and others showing a stimulatory positive effect (Kowalenko et al., 1978; Al-Kaisi et al., 2008). During the grow-ing season, Rm typically coincided with soil temperature in both sites. Many studies have shown a positive association with CO2 emissions via Rm with soil temperature when C substrate and wa-ter content are not limiting (Davidson et al., 1998).

Estimates of Rr increased from zero, 10 d after corn planting to peaking at 165 kg CO2 ha-1 d-1 in mid-July to early August, and then declined with time until corn was harvested (Fig. 1–4). Contribution of Rr to soil surface CO2 emissions ranged from 0 to 88%, varying by DAP, qv, and management treatments.

Table 1. Contribution of microbial respiration (Rm) percentage to total soil surface Co2 emissions (TCo2) within and in-between planted rows as predicted by days after planting (dAP), TCo2, and soil water content (qv) as affected by tillage (no-till, NT; con-ventional till, CT), residue removal, and N fertilization in 2010 and 2011 at the Ames Central site (AC).

Tillage Residue removal, % N fertilization, kg N ha-1 Regression model F(P > F) R2

Within rowCT 0 0 91.49- 0.31DAP- 2.05TCO2 + 0.99qv 7(0.019) 0.49

CT 0 170 72.85- 0.24DAP- 2.49TCO2 + 1.02qv 4(0.078) 0.40

CT 0 280 131.72- 0.24DAP- 2.67TCO2- 0.78qv 15(0.001) 0.68

NT 0 0 91.24 + 0.16DAP- 5.89TCO2 + 0.06qv 9(0.005) 0.52

NT 0 170 102.64 + 0.01DAP- 2.07TCO2- 0.28qv 1(0.348) 0.17

NT 0 280 97.83- 0.11DAP- 1.75TCO2 + 0.10qv 2(0.240) 0.14

CT 50 0 64.74 + 0.20DAP + 0.17TCO2- 0.13qv 2(0.231) 0.23

CT 50 170 80.37- 0.07DAP- 2.97TCO2 + 0.60qv 2(0.168) 0.31

CT 50 280 83.74- 0.25DAP- 0.01TCO2 + 0.51qv 2(0.163) 0.27

NT 50 0 75.30 + 0.19DAP- 4.81TCO2 + 0.29qv 8(0.008) 0.52

NT 50 170 47.99- 0.27DAP + 0.01TCO2 + 1.34qv 10(0.003) 0.52

NT 50 280 93.32 + 0.25DAP- 2.32TCO2- 0.94qv 2(0.257) 0.21

CT 100 0 91.29 + 0.21DAP- 3.50TCO2- 0.58qv 6(0.027) 0.43

CT 100 170 40.62 + 0.34DAP + 1.29TCO2 + 60qv 13(0.001) 0.57

CT 100 280 73.43 + 0.01DAP + 0.94TCO2 + 0.68qv 5(0.038) 0.36

NT 100 0 85.09- 0.06DAP- 0.20TCO2 + 0.36qv 1(0.348) 0.17

NT 100 170 97.95- 0.09DAP- 5.14TCO2 + 0.53qv 8(0.128) 0.48

NT 100 280 124.00- 0.07DAP- 6.29TCO2- 0.64qv 7(0.015) 0.46

In-between rows

CT 0 0 113.18- 0.30DAP- 4.24TCO2 + 0.31qv 8(0.036) 0.76

CT 0 170 84.34- 0.02DAP- 3.58TCO2 + 0.66qv 4(0.088) 0.50

CT 0 280 91.44- 0.02DAP- 5.53TCO2 + 0.37qv 0.7(0.384) 0.24

NT 0 0 115.17- 0.46DAP + 9.46TCO2- 0.30qv 22(0.001) 0.91

NT 0 170 135.02- 0.45DAP + 4.19TCO2- 1.10qv 6(0.062) 0.69

NT 0 280 90.81- 0.05DAP + 0.17TCO2 + 0.13qv 0.2(0.475) 0.13

CT 50 0 108.53- 0.03DAP- 2.02TCO2- 0.38qv 0.5(0.420) 0.20

CT 50 170 107.59- 0.12DAP- 3.88TCO2 + 0.21qv 1(0.316) 0.33

CT 50 280 29.86 + 0.52DAP- 3.56TCO2 + 0.54qv 8(0.038) 0.71

NT 50 0 97.67- 0.16DAP + 5.80TCO2- 0.03qv 14(0.007) 0.85

NT 50 170 25.08 + 0.34DAP + 18.48TCO2 + 0.28qv 2(0.306) 0.24

NT 50 280 115.66- 0.70DAP- 4.29TCO2 + 0.97qv 31(0.001) 0.95

CT 100 0 109.95- 0.01DAP- 1.43TCO2- 0.43qv 1(0.251) 0.34

CT 100 170 64.31 + 0.17DAP + 0.74TCO2 + 0.76qv 8(0.028) 0.58

CT 100 280 63.37 + 0.05DAP + 3.72TCO2 + 0.40qv 0.1(0.491) 0.11

NT 100 0 84.28 + 0.19DAP + 2.82TCO2- 0.49qv 3(0.343) 0.49

NT 100 170 39.03 + 0.37DAP- 0.04TCO2 + 0.51qv 3(0.331) 0.51

NT 100 280 72.95 + 0.15DAP- 0.71TCO2 + 0.07qv 0.9(0.355) 0.26

618 Soil Science Society of America Journal

Average cumulative Rr contribution to soil surface CO2 emis-sions during the growing seasons in both sites was approximately 25% when no residue was removed and N fertilization rate of 170 kg N ha-1. Currently, there are no conclusive references to evaluate the accuracy of estimated Rr in this study, although studies done in agroecosystems have reported cumulative Rr to soil surface CO2 emissions ranging from 10 to 45% (Rochette et al., 1999; Raich and Mora, 2005). Root respiration typically coincided with Rm, with some exceptions. This was not surpris-ing since Rr has been coupled with photosynthesis rates, which is influenced by environmental conditions such as soil temper-ature and water content similar to Rm (Davidson et al., 1998; Kuzyakov and Gavrichkova, 2010). In general, Rr was greater in CT compared with NT until mid-August, and Rr was greater in NT until corn was harvested, when Rr ceased. In 2010 at the AC site, under NT with 0% residue removal and when N was ap-plied, Rr peaked 2 wk later than the peak of Rm with same treat-ments. This can be attributed to slower corn growth compared with treatments with 50 and 100% residue removal under NT, and CT across all residue removal rates, due to slower accumu-lating growing degree days (Al-Darby and Lowery, 1987). In 2011, Rr under NT treatments across all residue removal rates and when N was applied, peaked approximately 3 wk later than the peaks of Rm with the same treatments. Additionally, Rr in general increased with greater N fertilization rates, due in large part to increase in root biomass (Table 2 and 3).

Management Effects on Cumulative Carbon Losses via Microbial respiration

Cumulative CO2 loss via Rm was greater with 0% residue re-moval treatments than with 50% residue removal (Fig. 1 and 4). Previous studies have also shown a decrease in soil surface CO2 emissions with residue removal, due to less available C substrate for mineralization (Dao, 1998). However, the highest cumula-tive CO2 loss via Rm occurred with 100% residue removal in AC

site in 2010 (Fig. 1) and in both years for the ASW site (Fig. 3 and 4). This may be attributed to significantly warmer soil tem-peratures with 100% residue removal during the growing season (Fig. 1–4).

Cumulative CO2 loss via Rm in CT was significantly higher compared with NT in both sites and years except at the AC site in 2011. In 2010, Rm rates were more pronounced and higher than in 2011 due to more extreme soil moisture cycling (Mosier et al., 2006; Hernandez-Ramirez et al., 2009; Fig. 1–4). At AC site, CT treatments had 12 and 2% (not significantly different) greater cumulative CO2 loss via Rm than NT in 2010 and 2011, respectively. At ASW site, CT treatments had 15% greater cu-mulative CO2 loss via Rm than NT in both years. Warmer soil temperatures under CT compared with NT, during field cultiva-tion to prepare for planting early in the growing season (Al-Kaisi and Yin, 2005), is the primary reason for CT having greater cu-mulative CO2 emissions than NT, even though NT typically had similar or slightly higher soil temperature from July to October (Licht and Al-Kaisi, 2005; Al-Kaisi and Kwaw-Mensah, 2007).

residue and Management Practices Effects on Aboveground and root Biomass

Potential C inputs from aboveground biomass varied by site, year, tillage, residue removal rate, and N fertilization rate (Table 2 and 3). In general, residue remaining after baling de-creased by 58 and 96% with 50 and 100% residue removal treat-ments, respectively. Averaged across tillage and residue removal levels, the lowest aboveground biomass production occurred with 0 kg N ha-1 fertilization rates at 1.4 Mg ha-1, and increased by 47 and 58% with 170 and 280 kg N ha-1 fertilization rates, respectively. Averaged across N fertilization and residue removal rates, residue remaining after baling was 10% greater in CT com-pared with NT. Potential C inputs from residue remaining after harvest when 170 and 280 kg ha-1N was applied in this study, fall within the range of 2.0 to 5.7 Mg C ha-1 as reported by other

Table 2. Above and belowground biomass left after harvest as affected by tillage (no-till, NT; conventional till, CT), residue removal, and N fertilization by year in the Ames Central site (AC).

residue removal

Nitrogen rates, kg N ha–1

Tillage 0 170 280 0 170 280

2010 2011

% Aboveground biomass, Mg ha–1

CT 0 3.4 ± 0.7d† 6.0 ± 1.7b 6.1 ± 0.7ab 2.4 ± 0.5ef 4.8 ± 1.1b 5.7 ± 0.5a50 2.4 ± 0.3ef 2.0 ± 0.7ef 2.4 ± 0.3ef 2.0 ± 0.3fg 1.8 ± 0.5g 1.6 ± 0.2g

100 0.1 ± 0.1h 0.4 ± 0.2gh 0.3 ± 0.1gh 0.1 ± 0.1h 0.2 ± 0.1h 0.2 ± 0.1hNT 0 2.7 ± 1.1de 4.9 ± 0.3c 7.0 ± 0.6a 3.1 ± 0.6cd 3.5 ± 0.2c 5.0 ± 0.4b

50 1.7 ± 0.1fg 2.1 ± 1.1ef 2.8 ± 0.5de 2.7 ± 0.2de 1.7 ± 0.2g 2.0 ± 0.3fg100 0.1 ± 0.1h 0.1 ± 0.1h 0.2 ± 0.1h 0.1 ± 0.1h 0.1 ± 0.1h 0.1 ± 0.1h

Belowground biomass, Mg ha–1‡CT 0 1.5 ± 0.1g 2.6 ± 0.1cde 3.8 ± 0.5ab 1.3 ± 0.2fg 1.8 ± 0.2cde 2.2 ± 0.3ab

50 2.0 ± 0.4efg 3.2 ± 0.4bcd 3.7 ± 1.2ab 1.5 ± 0.2f 2.1 ± 0.2abcd 1.9 ± 0.2bcde100 1.9 ± 0.2fg 3.4 ± 0.3ab 3.5 ± 0.6ab 1.6 ± 0.1ef 2.0 ± 0.2abcd 1.9 ± 0.3bcde

NT 0 2.5 ± 0.4def 3.6 ± 0.7ab 4.0 ± 0.4a 1.0 ± 0.1g 2.2 ± 0.3a 2.0 ± 0.1abcd50 1.8 ± 0.1g 3.2 ± 0.6bc 3.5 ± 0.4ab 1.4 ± 0.1f 1.8 ± 0.2de 2.2 ± 0.3a

100 1.9 ± 0.4fg 3.8 ± 0.2ab 3.4 ± 0.6ab 1.4 ± 0.1f 2.2 ± 0.2a 2.1 ± 0.2abc†Means with the same lower-case letter within each year are not significantly different at p £ 0.1.‡Belowground biomass for the top 15-cm soil depth.

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studies under the same soil associations in Iowa (Al-Kaisi et al., 2005; Grote and Al-Kaisi, 2007).

Potential C inputs from root biomass in the top 15-cm soil depth, varied by site, year, and N fertilization rate (Table 2 and 3). Mean root biomass across treatments and years in the AC site was 2.4 Mg ha-1, 21% lower compared with ASW site. Additionally, mean root biomass across treatments and sites in 2010 was 3.0 Mg ha-1, 23% greater than that in 2011. In general, mean root biomass was 1.9 Mg ha-1 when no N was applied and increases of 35 and 41% were observed with 170 and 280 kg N ha-1 fer-tilization rates, respectively. Potential C inputs from root biomass when N was applied from this study, fall within range of 0.5 to 2 Mg C ha-1 as reported by other studies in corn cropping systems (Grote and Al-Kaisi, 2007; Mu et al., 2008).

Soil Carbon Budget under different residue removal and Management Practices

Calculations of net C changes in a system from estimates of potential C inputs from aboveground and root biomass minus C losses from Rm, resulted in differences among sites, years, till-age, residue removal levels, and N fertilization rates (Fig. 5 and 6). Differences in residue removal levels were the largest deter-minate factor in net C changes. As residue removal increased, greater net C losses were observed. In 2010, NT treatments typi-cally had greater net C gains compared with CT treatments in both sites. In 2011 when soil conditions were drier, there were no significant differences in net C change when comparing CT and NT treatments in the AC site and in the ASW site with 170 kg N ha-1 fertilization rate. However, in the ASW site when no residue was removed and under relatively high N fertilization rate of 280 kg N ha-1, CT had a greater net gain in C than NT. This was primarily due to CT treatments having a greater sum of potential C inputs from aboveground and root biomass com-pared with NT treatments in this study (Table 3), and lower C

losses via Rm in 2011 compared with 2010 due to drier soil con-ditions (Fig. 3 and 4). When no N was applied, significant net C losses were estimated regardless of tillage, and this increased with increased residue removal levels. Significant net potential C increases were only observed under treatments that received a relatively high N fertilization rate of 280 kg N ha-1, and less than 50% residue removal at the ASW site in 2010 and 2011. At the AC site, significant net C gains were only observed in 2010 under NT with <50%residue removal and relatively high N fer-tilization rate of 280 kg N ha-1. These findings are in agreement with longer continuous corn studies such as in the Morrow plots, which concluded the greater input of C from corn residue due to increased N fertilization, resulted in greater SOC (Khan et al., 2007). However, under different cropping rotations, many studies have shown N fertilization stimulated mineralization of SOC, especially in the subsurface layers (Soon and Arshad, 2002; Mack et al., 2004; Khan et al., 2007).

Soil organic Carbon ChangeThe negative effects of residue removal on SOC were most

profound after the first year (Guzman, 2012), although only the average changes in SOC after the second and third years are shown in this paper to coincide with when soil C budgets were done (Table 4). In both sites in general, under soil management systems in Iowa with CT and 170 kg N ha-1 fertilization rate and no residue removal, changes in SOC were not significantly dif-ferent from zero for the 0- to 60-cm soil profile. In the poorly drained soils at the AC site, converting from CT system to NT, aided in offsetting potential SOC losses from residue removal. When averaged across residue removal rates, changes in SOC were -0.05 Mg C ha-1 yr-1, compared with CT which was on av-erage -0.79 Mg C ha-1 yr-1. This was expected due to the strati-fication of organic C in the soil profile and decreases in soil or-ganic matter mineralization in wet and cold soil conditions, even

Table 3. Above and belowground biomass left after harvest as affected by tillage (no-till, NT; conventional till, CT), residue removal, and N fertilization by year in the Armstrong Southwest site (ASW).

residue removal

Nitrogen rates, kg N ha–1

Tillage 0 170 280 0 170 280

2010 2011

% Aboveground biomass, Mg ha–1

CT 0 3.4 ± 0.4d† 5.8 ± 0.9bc 7.2 ± 0.6a 4.2 ± 0.5d 7.1 ± 1.1b 8.8 ± 0.7a

50 0.5 ± 0.1g 1.4 ± 0.7f 2.9 ± 0.7de 0.5 ± 0.1h 2.8 ± 0.3f 3.2 ± 0.2ef

100 0.2 ± 0.1g 0.2 ± 0.1g 0.3 ± 0.1g 0.2 ± 0.1h 0.2 ± 0.2h 0.3 ± 0.2gh

NT 0 1.1 ± 0.6gf 5.2 ± 0.2c 6.4 ± 0.3b 1.3 ± 0.7g 6.3 ± 0.3c 7.7 ± 0.4b

50 0.6 ± 0.1g 2.5 ± 1.0e 2.9 ± 0.6de 0.6 ± 0.3gh 3.6 ± 0.6de 3.4 ± 0.4ef

100 0.2 ± 0.1g 0.2 ± 0.1g 0.3 ± 0.1g 0.1 ± 0.1gh 0.1 ± 0.1gh 0.1 ± 0.1gh

Belowground biomass, Mg ha–1‡

CT 0 2.0 ± 0.3e 2.7 ± 0.2cd 3.9 ± 1.3a 2.2 ± 0.1e 3.2 ± 0.2abcd 3.4 ± 0.1abc

50 2.1 ± 0.2de 3.5 ± 0.1ab 4.0 ± 0.5a 2.0 ± 0.1e 3.1 ± 0.3bcd 2.9 ± 0.5cd

100 2.7 ± 0.2cd 3.2 ± 0.6bc 4.0 ± 0.2a 2.1 ± 0.2e 3.4 ± 0.3abc 3.1 ± 0.1abcd

NT 0 2.3 ± 0.1de 3.6 ± 0.4ab 4.1 ± 0.3a 2.2 ± 0.1e 3.3 ± 0.3abc 3.6 ± 0.7a

50 1.9 ± 0.2e 3.5 ± 0.3ab 3.6 ± 0.2ab 2.2 ± 0.2e 3.2 ± 0.2abcd 3.2 ± 0.4abcd100 2.4 ± 0.1de 3.2 ± 0.7bc 4.0 ± 0.1a 2.1 ± 0.3e 2.8 ± 0.2d 3.4 ± 0.5ab

†Means with the same lower-case letter within each year are not significantly different at p £ 0.1.‡Belowground biomass for the top 15 cm soil depth.

620 Soil Science Society of America Journal

when 50 and 100% of the residue was removed (Al-Kaisi and Yin, 2005). Gains in SOC were highest with 0% residue removal when averaged across tillage systems at 0.70 Mg C ha-1 yr-1, com-pared with 50 and 100% residue removal treatments, which was on average -0.70 and -1.26 Mg C ha-1 yr-1.

In the ASW site, the magnitude of changes in SOC was smaller than the AC site. Residue removal was the only man-agement practice in which significant differences between treat-ments were observed. The highest SOC gains occurred with 0% residue removal when averaged across tillage systems at

Fig. 5. Tillage and residue removal effects on net soil C change by N fertilization rate in the Ames Central (AC) site in 2010 and 2011.

Fig. 6. Tillage and residue removal effects on net soil C change by N fertilization rate in the Armstrong Southwest (ASW) site in 2010 and 2011.

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0.21 Mg C ha-1 yr-1, compared with 50 and 100% residue removal treatments, which was on average -0.06 and -0.37 Mg C ha-1 yr-1, respectively in the ASW site from 2009 through 2011. In both sites, Pairwise Correlations test shows that changes in SOC cor-related well with soil C budget results (Table 4). Although, dif-ferences between management treatments were more evident with soil C budget results.

Management Practices to offset Potential Carbon Losses due to residue removal

By fitting a linear model to determine the net C change as a function of residue remaining on the soil surface, the amount of resi-due that must remain to maintain soil C in a system can be estimated (Fig. 5 and 6). In the AC site, approximately 6.6 and 5.2 Mg ha-1 of residue after harvest with 170 and 280 kg N ha-1 fertilization rates, respectively, must remain to have a net C change of zero on average. In the ASW site, approximately 6.6 and 4.5 Mg ha-1 of residue after harvest with 170 and 280 kg N ha-1 fertilization rates, respectively must remain to maintain soil C in a system.

Determining how much residue can be sustainably removed depends mainly on above and belowground biomass production and the minimum residue requirement for either maintaining SOC level or preventing soil erosion ( Johnson et al., 2006). In this study, under management practices of CT and N fertiliza-tion rates of 170 kg N ha-1 in continuous corn in Iowa, net C change was approximately zero when no residue was removed for both sites and years. With no other alteration of management practices, any removal of soil surface residue resulted in net C

losses for both sites. However, increasing N fertilization rates from 170 to 280 kg N ha-1, made it possible for removal of soil surface residue of approximately 2.1 Mg ha-1 (31%), and yet still have a net C change of zero in 2010 for both sites. This was pri-marily due to increases in potential C inputs from aboveground and root biomass, and C losses via Rm not being significantly different when compared with 170 kg N ha-1fertilization rate. However, increasing N fertilization rate does not necessarily result in greater potential for residue removal. For instance, in 2011, where drier soil conditions lowered potential C inputs from aboveground and root biomass in the AC site, but in-creased in the ASW (Table 2 and 3). These results suggest that the quantity of aboveground residue that can be removed at least in the short term is primarily dependent on how much more po-tential C input is increased from previous management C inputs, assuming C losses remained unchanged as observed in this study when N fertilization was increased.

Converting from CT to NT as done in this study, did result in lower C losses via Rm (Fig. 1–4), but typically also reduced potential C inputs in these sites (Table 2 and 3). As a result, till-age effects on potential C changes in these systems varied by soil temperature and water content in a given year. Corn growth in both years in these sites was negativity impacted by cooler and wetter soils in the spring and was amplified under NT, as report-ed in other studies in Iowa (Kasper et al., 1990). In systems where soil water content is insufficient for crop growth, studies have re-ported lower C losses and equal or greater aboveground and root biomass when tillage systems were converted into NT (Licht and

Table 4. Correlations between changes in soil organic carbon (SoC) and average soil C budget under different management practices at the 0- to 60-cm soil profile depth in the Ames Central site (AC) and Armstrong Southwest site (ASW) from 2009 through 2011.

Ames site (AC) Armstrong site (ASW)

Till residue removal N rate SoC change Soil C budget SoC change Soil C budget

% kg ha-1 ——————————–Mg ha-1 yr-1—————————–CT 0 0 0.10 bcde† -0.93 cde -0.15 abc -1.25 ef

CT 0 170 0.18 bcd -0.43 bc 0.48 a -0.54 de

CT 0 280 0.70 b 0.07 ab 0.34 a 1.32 ab

CT 50 0 -0.62 cdefg -1.63 fg -0.86 cd -2.26 gh

CT 50 170 -1.25 fg -1.55 ef 0.28 ab -1.21 ef

CT 50 280 -0.96 defg -1.55 ef 0.39 a -0.60 de

CT 100 0 -0.91 defg -1.82 fgh -0.09 abc -2.33 gh

CT 100 170 -2.77 h -1.87 fgh -0.59 bcd -2.35 gh

CT 100 280 -1.55 g -2.29 h -1.07 d -2.79 h

NT 0 0 0.43 bc -0.78 cd 0.38 a -1.62 fg

NT 0 170 0.24 bcd -0.12 b -0.26 abcd 0.46 bc

NT 0 280 2.57 a 0.60 a 0.46 a 1.56 a

NT 50 0 -0.13 bcdef -1.42 def -0.17 abc -1.67 fg

NT 50 170 -0.19 bcdef -0.93 cde 0.37 a -0.35 cde

NT 50 280 -1.03 efg -0.95 cde -0.34 abcd -0.12 cd

NT 100 0 -0.90 defg -1.98 fgh -0.25 abcd -2.15 fgh

NT 100 170 -0.30 bcdef -1.74 fgh -0.15 abc -2.03 fgh

NT 100 280 -1.11 efg -2.25 gh -0.11 abc -1.73 fg

Pairwise correlations Pairwise correlations

r = 0.78 r = 0.53

p £ 0.0001 p £ 0.0001

†Means with the same lower-case letter within each column are not significantly different at p £ 0.10.

622 Soil Science Society of America Journal

Al-Kaisi, 2005). Under these systems, a greater potential for resi-due removal is expected under NT than in CT.

It is important to keep in mind, that these findings are for the first 3 yr since establishment of treatments, and that the long-term removal of residue is expected to reduce SOC input, when crop production and soil quality is reduced (Karlen et al., 1994). Significant decreases in soil quality parameters such as SOC, bulk density, water infiltrations, aggregate stability, and compac-tion were observed after only 3 yr of residue removal in these sites (Guzman, 2012) and in other studies (Blanco-Canqui and Lal, 2009). In addition to reduced tillage and N fertilization, further C budget studies need to be conducted in systems which incor-porate cover crops, manure applications, and alternative biomass crops such as switch grass or miscanthus to determine how much crop residue can sustainably be removed and meet the demands of biomass for ethanol production and animal feed (Wilhelm et al., 2004; Lal, 2009).

CoNCLuSIoNSThe findings of this study suggest that a portion of the corn

residue that is left on the soil surface after harvest can be removed sustainably (i.e., net increase or no net loss of soil C) in Central and Southwest Iowa with changes in management practices. Under management practices of CT and N fertilization rates of 170 kg N ha-1 in continuous corn in Iowa, net soil C change was approximately zero when no residue was removed for both sites and years. With no other changes in the fore mention manage-ment practices, any removal of soil surface residue resulted in net soil C losses in AC and ASW sites. Increasing N fertilization rates from 170 to 280 kg N ha-1, resulted in greater potential C inputs from aboveground and root biomass, and C losses via Rm did not significantly differing when compared with 170 kg N ha-

1fertilization rate. How much residue could be removed depend-ed on how much more potential C input is increased from pre-vious residue C inputs, and varied by year and site. Converting from CT to NT did result in lower C losses via Rm, but typically also reduced potential C inputs from above and belowground in these sites. As a result, tillage effects on potential soil C changes in these systems varied by soil temperature and water content in a given year. In 2010, averaged across both tillage systems and at 280 kg ha-1 for continuous corn approximately 5.10 and 4.18 Mg ha-1 of the residue should remain on the field to sustain soil C in the AC and ASW sites, respectively. In 2011, drier soil conditions resulted in approximately 5.23 and 5.18 Mg ha-1of the residue should remain on the field averaged across both till-age systems with 280 kg ha-1 N rate to sustain soil C in the AC and ASW sites, respectively. These findings suggest that residue removal needs to be approached on yearly basis with particular consideration to site’s yield potential and weather condition for each location as the residue biomass production can be variable.

ACKNoWLEdGMENTSSupport for this research project provided by the Iowa State University, Agronomy Department Endowment.

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