Carbon dioxide, a major greenhouse gas responsible for global warming, is

Soil Biology & Biochemistry Land Use and Management Practices Impact on Plant Biomass Carbon and Soil Carbon Dioxide Emission Upendra M. Sainju* William B. Stevens Thecan Caesar-TonThat Jalal D. Jabro USDA-ARS Northern Plains Agricultural Research Lab. Sidney, MT 59270 Land use and management practices may influence plant C inputs and soil CO 2 emission. We evaluated the effect of a combination of irrigation, tillage, cropping system, and N fertilization on plant biomass C, soil temperature and water content at the 0- to 15-cm depth, and CO 2 emission in a sandy loam soil from April to October, 2006 to 2008, in western North Dakota. Treatments were two irrigation practices (irrigated and unirrigated) and six cropping systems (conventional-tilled malt barley [Hordeum vulgare L.] with N fertilizer [CTBFN], conventional-tilled malt barley with no N fertilizer [CTBON], no-tilled malt barley pea [Pisum sativum L.] with N fertilizer [NTB- PN], no-tilled malt barley with N fertilizer [NTBFN], no-tilled malt barley with no N fertilizer [NTBON], and no-tilled Conservation Reserve Program [NTCRP]). Plant biomass C was greater in NTBFN than in NTBON in 2006 and 2007 but was greater in NTB-PN than in CTBON, NTBON, or NTCRP in 2008. Soil temperature was greater but water content was lower in NTCRP than in CTBFN and NTBFN. Soil CO 2 flux peaked immediately following heavy rain or irrigation (>15 mm). Total CO 2 flux from April to October was greater in the irrigated than in the unirrigated practice and greater in NTCRP than in annual cropping systems. Soil CO 2 emission was probably related more to soil temperature and water content or tillage than to aboveground plant C input. Because of reduced CO 2 flux compared with CTBON and NTCRP but similar biomass yield as NTBFN and CTBFN, NTB-PN may be used to reduce CO 2 emission from croplands in the northern Great Plains. Abbreviations: CTBFN, conventional-tilled malt barley with nitrogen fertilizer; CTBON, conventional-tilled malt barley with no nitrogen fertilizer; NTB-PN, no-tilled malt barley pea with nitrogen fertilizer; NTBFN, no-tilled malt barley with nitrogen fertilizer; NTBON, no-tilled barley with no nitrogen fertilizer; NTCRP, no-tilled Conservation Reserve Program. Carbon dioxide, a major greenhouse gas responsible for global warming, is emitted from crop- and grasslands due to oxidation of soil organic matter, root and microbial respiration, and return of unharvested plant residues to the soil (Curtin et al., 2000; Frank et al., 2006; Sainju et al., 2008). In contrast, soil is also an important sink of atmospheric CO 2, which is absorbed by plant biomass through photosynthesis and converted into soil organic matter after plant residue is returned to the soil (Lal et al., 1995; Paustian et al., 1995). The balance between the amounts of plant residue C (fixed through photosynthesis) added to the soil and the rate of C mineralized as CO 2 emission in unmanured soil determines the level of soil C storage (Rasmussen et al., 1980; Peterson et al., 1998). The CO 2 emission from the soil to the atmosphere is the primary mechanism of soil C loss (Fortin et al., 1996; Parkin and Kaspar, 2003). Land use and management practices can influence soil CO 2 emission by influencing plant growth, soil disturbance, and nutrient levels, such as N (Frank et al., 2006, Sainju et al., 2008). Grasslands emit more CO 2 than croplands due to extensive root systems and greater soil organic C levels (Dugas et al., 1999; Frank et al., 2006). Conversion of grasslands to croplands can reduce the soil organic C level by emitting CO 2 (Davidson and Ackerman, 1993). Although initial tillage of grassland resulted in more CO 2 emission than from an undisturbed perennial Soil Sci. Soc. Am. J. 74:1613 1622 Published online 12 Aug. 2010 doi:10.2136/sssaj2009.0447 Received 4 Dec. 2009. *Corresponding author (upendra.sainju@ars.usda.gov). 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. SSSAJ: Volume 74: Number 5 September October 2010 1613

grassland (Sainju et al., 2008), adoption of no-tilled or reduced tilled management systems can reverse the trend of CO 2 loss (Fortin et al., 1996; Curtin et al., 2000). Some of the management practices that influence soil CO 2 emission are irrigation, tillage, cropping system, and N fertilization (Curtin et al., 2000; Sainju et al., 2008). Irrigation can increase CO 2 emission compared with no irrigation by increasing soil water availability (Sainju et al., 2008), microbial activity, C mineralization, and respiration (Calderon and Jackson, 2002). Decreased tillage intensity reduces soil disturbance and microbial activity, which in turn, lowers CO 2 emissions (Curtin et al., 2000). In contrast, increased tillage intensity increases CO 2 emissions by increasing soil aeration and disrupting soil aggregates (Roberts and Chan, 1990) and by physical degassing of dissolved CO 2 from the soil solution ( Jackson et al., 2003). The cropping system can influence CO 2 emissions by affecting the crop residue quality and quantity returned to the soil (Curtin et al., 2000; Amos et al., 2005; Sainju et al., 2008). Nitrogen fertilization, however, has variable effects on CO 2 emissions (Mosier et al., 2006; Al-Kaisi et al., 2008). Land use and management practices can also indirectly influence CO 2 emissions by altering soil temperature and water content, because CO 2 flux is related to these parameters (Parkin and Kaspar, 2003; Amos et al., 2005). Perennial grasses can lower soil temperature and water content compared with annual crops by providing shade and absorbing more water during periods when annual crops do not absorb much water, such as before emergence and after harvest (Frank et al., 2006). Tillage can dry up the soil but no-tillage can conserve soil water and reduce temperature because of decreased soil disturbance and increased residue accumulation at the soil surface (Curtin et al., 2000; Al- Kaisi and Yin, 2005). Similarly, cropping system and crop type can influence soil temperature and water content compared with fallow by affecting the shade intensity and evapotranspiration (Curtin et al., 2000; Amos et al., 2005). Information on the effects of management practices on soil surface CO 2 emissions in croplands and grasslands in the northern Great Plains is limited. We hypothesized that no-till management with N fertilization would reduce soil CO 2 flux compared with conventional tillage without N fertilization in an annual cropping system and that CO 2 flux would be lower in annual than in perennial cropping systems. Our objectives were to: (i) determine the influence of irrigation, tillage, cropping system, and N fertilization on plant biomass (stems and leaves) C returned to the soil from 2006 to 2008; (ii) quantify the effect of management practices on soil surface CO 2 flux; and (iii) compare CO 2 fluxes in annual and perennial cropping systems in the northern Great Plains. MATERIALS AND METHODS Experimental Site and Treatments Soil surface CO 2 flux was measured from 2006 to 2008 in an experiment established in 2005 when land in a no-tilled Conservation Reserve Program (NTCRP) planting (perennial system) was converted to annual cropping systems at Nesson Valley (48.1 N, 103.1 W) in western North Dakota. The average air temperature at the experimental site ranged from 5 C in January to 32 C in July and August, with annual precipitation of 373 mm. The soil was a Lihen sandy loam (sandy, mixed, frigid, Entic Haplustoll) with 720 g kg 1 sand, 120 g kg 1 silt, 160 g kg 1 clay, and a ph of 7.7 in the 0- to 20-cm depth before the initiation of the experiment in April 2005. At the same time, soil organic C concentrations in the 0- to 5- and 5- to 20-cm depths were 13.7 and 9.9 g kg 1, respectively. The resident vegetation in NTCRP for the last 24 yr contained a mixture of alfalfa (Medicago sativa L.), crested wheatgrass [Agropyron cristatum (L.) Gaertn], and western wheatgrass [Pascopyrum smithii (Rydb.) A. Love]. The resident vegetation in all treatments, except NTCRP, was killed by applying glyphosate [N-(phosphonomethyl)glycine)] at 3.5 kg a.i.ha 1 in April 2005. Treatments consisted of two irrigation practices (irrigated vs. unirrigated) and six cropping systems (conventional-tilled malt barley with 67 to 134 kg N ha 1 [CTBFN], conventional-tilled malt barley with 0 kg N ha 1 [CTBON], no-tilled malt barley pea rotation with 67 to 134 kg N ha 1 [NTB-PN], no-tilled malt barley with 67 to 134 kg N ha 1 [NTBFN], no-tilled malt barley with 0 kg N ha 1 [NTBON], and NTCRP). In NTB-PN, both malt barley and pea phases were present in every year. The recommended N fertilization rates for irrigated and unirrigated malt barley were 134 and 67 kg N ha 1, respectively. The variation in N rate between irrigated and unirrigated malt barley was due to the differences in grain yield and N uptake between irrigated and unirrigated conditions. Soil samples to a depth of 60 cm were tested for NO 3 N content before applying N fertilizer. No N fertilizer was applied to pea and the grasses. In 2008, N rate for malt barley following pea was 11 kg N ha 1 less than for continuous malt barley due to N contribution from pea residue. Conventional-tilled malt barley plots were tilled initially (April 2005) using a rototiller to a depth of 10 cm. In subsequent years, these plots were tilled to the 10-cm depth with a singlepass field cultivator. No-tilled malt barley and pea were planted with a no-till drill that also banded fertilizers to a depth of 5 cm, 2.5 cm away from the seed row. The same drill was also used to plant conventionaltilled malt barley. The NTCRP treatment consisted of alfalfa and grasses that were maintained from the previous 24 yr. Both annual and perennial cropping systems were subjected to irrigated treatments. Treatments were arranged in a randomized complete block, with irrigation as the main plot and cropping system as the split-plot factor. Each treatment had three replications. The size of each experimental unit was 10.6 by 3.0 m. Crop Management In late April, 2006 to 2008, malt barley (cv. Certified Tradition, Busch Agricultural Resources, Fargo, ND) was planted at the 3.8-cm depth at 90 kg ha 1 in the irrigated treatment and at 67 kg ha 1 in the unirrigated treatment with a no-till drill. Similarly, pea (cv. Majorete, Macintosh Seed, Havre, MT) was planted at 200 kg ha 1 in irrigated and unirrigated treatments. In irrigated malt barley, half of the N fertilizer as urea (or 67 kg N ha 1 ) was banded at planting and the other half was broadcast at 4 wk after planting. In unirrigated malt barley, all N fertilizer was banded at planting. Phosphorus fertilizer (as triple super phosphate at 25 kg P ha 1 ) and K fertilizer (as muriate of potash at 21 kg K ha 1 ) were banded for both malt barley and pea at planting but not for grasses. Appropriate types and amounts of herbicides and pesticides were applied to control weeds and pests during growth 1614 SSSAJ: Volume 74: Number 5 September October 2010

and after the harvest of the malt barley and pea. In irrigated plots, water was applied from 10 to 34 mm per application for a total of 236 mm in 2006, from 13 to 31 mm for a total of 56 mm in 2007, and from 6 to 25 mm for a total of 47 mm in 2008. Each treatment received water as needed, using a self-propelled irrigation system, based on soil water content and crop demand. In late July and early August, 2006 to 2008, malt barley and pea were harvested from an area of 10.6 by 1.5 m using a plot combine after determining biomass (leaves and stems) from two 0.5-m 2 areas per plot outside yield rows. Similarly, the biomass of plants in NTCRP was determined from two 0.5-m 2 areas per plot. After grain harvest, biomass residue of malt barley and pea was returned to the soil. The aboveground biomass of plants in NTCRP was also returned to the soil without harvest. The C concentration (g C kg 1 ) in plant biomass was determined by a high-induction furnace C and N analyzer (LECO Corp., St. Joseph, MI) after oven drying the sample at 60 C and grinding to 1 mm. The C content (Mg C ha 1 ) was determined by multiplying plant biomass yield by C concentration. Carbon Dioxide Flux Measurements Immediately after planting, soil surface CO 2 flux was measured weekly in all treatments from April to October, 2006 to 2008. All measurements were taken between 0900 and 1200 h to reduce variability in CO 2 flux due to diurnal changes in temperature (Parkin and Kaspar, 2003). The CO 2 flux was measured with an Environmental Gas Monitor chamber attached to a datalogger (Model EGM-4, PP Systems, Haverhill, MA). The chamber was 15 cm tall and 10 cm in diameter with a sharp edge at the bottom, and had the capacity to measure CO 2 flux from 0 to 2398 kg CO 2 C ha 2 d 1. The chamber was placed at the soil surface with the sharp edge gently pushed into the soil to a depth of 5 mm for good soil contact for 2 min in each plot until CO 2 flux measurement was recorded by the datalogger. A flag was placed as a marker in the plot so that the CO 2 flux could be measured in the same place throughout the study. Although a small incision on roots was made, especially in NTCRP when the chamber was pushed in the ground for the first time for CO 2 measurement, this did not significantly alter CO 2 flux because subsequent measurements were made in the same place throughout the study. At the time of CO 2 measurement, soil temperature near the chamber was measured from a depth of 0 to 15 cm using a probe attached to the datalogger. Similarly, gravimetric soil water content was measured near the chamber by collecting a soil sample from the 0- to 15-cm depth with a hand probe (2.5-cm diameter) every time CO 2 flux was measured. The moist soil was oven dried at 110 C and the water content was determined. Because soils were frozen to >1-m depth and insignificant fluxes occur during November to March (Frank et al., 2006), CO 2 flux and soil temperature and water content were not measured during this period. Data Analysis Data for plant biomass and C content were analyzed using the Analysis of Repeated Measures procedure in the MIXED model of SAS (Littell et al., 1996). Irrigation was considered as the main plot, cropping system as the split plot, and year as the repeated-measure variable for data analysis. Similarly, data for CO 2 flux, soil temperature, and water content in each year were analyzed as above after considering irrigation as the main plot, cropping system as the split plot, and date of Table 1. Analysis of variance for plant biomass (stems and leaves) yield and C content. Source Biomass yield Biomass C content Irrigation (I) * * Cropping system (CS) *** *** I CS * * Year (Y) ** ** I Y NS NS CS Y ** *** I CS Y NS NS * Significant at P 0.05. ** Significant at P 0.01. *** Significant at P 0.001. NS, not significant. measurement as the repeated-measure variable. Irrigation and cropping system were considered as fixed effects and replication and irrigation replication interaction as random effects. In the NTB-PN treatment, data were averaged across the barley and pea phases and the average value was used for the crop rotation in the analysis. Means were separated by using the least square means test when treatments and interactions were significant (Littell et al., 1996). When treatments were significant, orthogonal contrasts were used to determine the effect of an individual management practice on soil and plant parameters. Statistical significance was evaluated at P 0.05 unless otherwise stated. RESULTS AND DISCUSSION Plant Biomass Yield and Carbon Content Plant biomass yield and C content were significantly influenced by irrigation, cropping system, and year (Table 1). Interactions were significant for irrigation cropping system and cropping system year. Plant biomass yield and C content, averaged across years, were greater in NTBFN, CTBFN, and NTB-PN than in other cropping systems in the irrigated practice (Table 2). In the unirrigated practice, biomass yield and C content were greater in NTBFN and CTBFN than in NTBON and NTCRP. Biomass yield and C content, averaged across irrigation practices, were greater in NTBFN than in NTBON in 2006, greater in NTBFN and CTBFN than in other cropping systems in 2007, and greater in NTBP-N, NTBFN, and CTBFN than in other cropping systems in 2008 (Table 3). Averaged across cropping systems and years, biomass yield and C content were greater in the irrigated than in the unirrigated practice (Table 2). Averaged across irrigation and years, biomass yield and C content were greater in CTBFN, NTB-PN, and NTBFN than in other cropping systems. Averaged across irrigation and cropping systems, biomass yield and C content were greater in 2007 and 2008 than in 2006 (Table 3). Nitrogen fertilization increased biomass yield and C content compared with no N fertilization, and annual cropping increased biomass yield and C content compared with the perennial system (Table 2). Continuous malt barley decreased biomass yield and C content compared with malt barley pea in the no-till system. Tillage did not influence biomass yield and C content. The greater plant biomass yield and C content in NTBFN, CTBFN, and NTB-PN than in other cropping systems in the SSSAJ: Volume 74: Number 5 September October 2010 1615

Table 2. Effects of irrigation and cropping system (CS) on plant biomass (stems and leaves) yield and C content averaged across years. Irrigation Cropping system Biomass yield Mg ha 1 Biomass C content Mg C ha 1 Irrigated CTBFN 4.84 2.07 CTBON 2.90 1.21 NTB-PN 4.68 1.99 NTBFN 5.04 2.17 NTBON 2.54 1.06 NTCRP 2.65 1.14 Unirrigated CTBFN 3.87 1.68 CTBON 3.19 1.38 NTB-PN 3.47 1.48 NTBFN 3.89 1.70 NTBON 2.43 1.04 NTCRP 1.45 0.63 LSD (0.05) 0.98 0.35 Means Irrigated 3.78 a 1.61 a Unirrigated 3.05 b 1.32 b CTBFN (CS1) 4.35 a 1.87 a CTBON (CS2) 3.05 b 1.30 b NTB-PN (CS3) 4.08 a 1.74 a NTBFN (CS4) 4.47 a 1.94 a NTBON (CS5) 2.49 bc 1.05 bc NTCRP (CS6) 2.05 c 0.88 c Contrast Till vs. no-till (CS1, CS2 vs. CS4, CS5) 0.22 0.09 N fertilization vs. no N (CS1, CS4 vs. CS2, CS5) 1.64*** 0.73*** Continuous barley vs. barley pea (CS4, CS5 vs. CS3) 0.60* 0.24* Annual vs. perennial system (CS3, CS4, CS5 vs. CS6) 1.63*** 0.69*** * Significant at P 0.05. *** Significant at P 0.001. CTBFN, conventional-tilled malt barley with 67 to 134 kg N ha 1 ; CTBON, conventional-tilled malt barley with 0 kg N ha 1 ; NTB-PN, no-tilled malt barley pea rotation with 67 to 134 kg N ha 1 applied to barley; NTBFN, no-tilled malt barley with 67 to 134 kg N ha 1 ; NTBON, no-tilled malt barley with 0 kg N ha 1 ; NTCRP, no-tilled Conservation Reserve Program (perennial system). Numbers followed by different letters in a column in either irrigation practice or cropping system are significantly different at P = 0.05 by the least square means test. irrigated practice (Table 2) was probably due primarily to N fertilization to malt barley, followed by N input from pea residue that probably increased barley biomass yield. In the unirrigated practice, N fertilization also increased biomass yield and C content in NTBFN and CTBFN. The lack of irrigation, however, probably reduced N mineralization from pea residue, resulting in biomass yield and C content in NTB-PN similar to that in CTBON. Lower biomass yield and C content in continuous malt barley with and without N fertilization than in the malt barley pea rotation in the no-till system suggests that pea plays an important role in increasing biomass yield and C content of succeeding malt barley due to N supplied by its residue. This is because pea, being a legume, has a higher N concentration than barley (Miller et al., 2002; Lenssen et al., 2007). Biomass yield Table 3. Effect of cropping system on plant biomass (stems and leaves) yield and C content from 2006 to 2008 averaged across irrigation practices. Year Cropping system Biomass yield Biomass C content Mg ha 1 Mg C ha 1 2006 CTBFN 3.31 1.41 CTBON 2.79 1.19 NTB-PN 3.06 1.30 NTBFN 3.37 1.44 NTBON 2.20 0.94 NTCRP 2.38 1.04 2007 CTBFN 5.37 2.36 CTBON 3.26 1.42 NTB-PN 3.93 1.67 NTBFN 5.41 2.39 NTBON 2.37 1.03 NTCRP 1.44 0.62 2008 CTBFN 4.38 1.85 CTBON 3.07 1.29 NTB-PN 5.24 2.24 NTBFN 4.61 1.97 NTBON 2.89 1.20 NTCRP 2.33 0.99 LSD (0.05) 1.07 0.48 Mean 2006 2.85 b 1.22 b 2007 3.63 a 1.58 a 2008 3.75 a 1.59 a CTBFN, conventional-tilled malt barley with 67 to 134 kg N ha 1 ; CTBON, conventional-tilled malt barley with 0 kg N ha 1 ; NTB-PN, no-tilled malt barley pea rotation with 67 to 134 kg N ha 1 applied to barley; NTBFN, no-tilled malt barley with 67 to 134 kg N ha 1 ; NTBON, no-tilled malt barley with 0 kg N ha 1 ; NTCRP, no-tilled Conservation Reserve Program (perennial system). Numbers followed by different letters in a column are significantly different at P = 0.05 by the least square means test. and C content were lower in NTCRP than in other treatments, probably because N, P, and K fertilizers were not applied to perennial plants, although root biomass can be higher in perennial grasses than in annual crops (Frank et al., 2006). Biomass yield and C content varied among years, probably due to differences in growing season precipitation (April October). Greater biomass yield and C content in 2007 and 2008 than in 2006 (Table 3) was probably a result of greater precipitation. The April to October precipitation was higher in 2007 and 2008 than in 2006 (Fig. 1), although less than the normal precipitation of 283 mm. During the dry period (April August) in 2006, biomass yield and C content were greater in NTBFN and NTB-PN than in other cropping systems, except in CTBFN, probably because better conservation of soil water under no-till than under conventional tillage (Sainju et al., 2008), along with N fertilization, increased plant biomass yield. When precipitation was adequate (close to normal) in 2007 and 2008, greater N availability due to N fertilization and N supplied by pea residue may have increased biomass yield and C content in NTBFN, CTBFN, and NTB-PN compared with other cropping systems. The nonsignificant effect of tillage on biomass yield and C content was similar 1616 SSSAJ: Volume 74: Number 5 September October 2010

to the results reported by several researchers in the northern Great Plains (Halvorson et al., 2002; Sainju et al., 2009). Soil Temperature and Water Content Soil temperature increased from April to August and then declined (Fig. 2 5). In contrast, soil water content varied with date of measurement, generally peaking immediately following irrigation and heavy precipitation. Soil temperature was greater but water content was lower in NTCRP than in other cropping systems at some measurement dates. Soil temperature, averaged across measurement dates, was normally greater in NTCRP than in other cropping systems in the unirrigated practice in 2006 and 2008 (Table 4). In the irrigated practice, soil temperature was greater in CTBON than in other cropping systems in 2008. Averaged across irrigation practices and measurement dates, soil temperature was greater in NTCRP than in CTBFN and NTBFN in all years (Table 5). In contrast, soil water content was lower in NTCRP than in CTBFN, CTBON, and NTBON. Irrigation decreased soil temperature but increased water content compared with no irrigation. Tillage reduced soil water content compared with no-tillage in 2006 and 2007. Similarly, N fertilization reduced soil temperature compared with no N fertilization in 2007 and 2008 and reduced water content in 2007. The greater soil temperature in NTCRP than in other cropping systems was probably due to lower aboveground biomass in perennial than in annual cropping systems (Table 2) as a result of reduced shading intensity (Curtin et al., 2000; Amos et al., Fig. 1. Daily rainfall during CO 2 measurement dates from April to October, 2006 to 2008, at the experimental site. Fig. 2. Effect of irrigation on soil surface CO 2 flux and soil temperature and water content at the 0- to 15-cm depth, averaged across cropping systems, from April to October, 2006 to 2008. Arrows indicate timing of irrigation in irrigated treatments. The LSD (0.05) bar shows the least significant difference at P = 0.05 among treatments within a measurement date. SSSAJ: Volume 74: Number 5 September October 2010 1617

Fig. 3. Effect of cropping system on soil surface CO 2 flux and soil temperature and water content at the 0- to 15-cm depth, averaged across irrigation practices, from April to October 2006: CTBFN, conventional-tilled malt barley with N fertilizer; CTBON, conventional-tilled malt barley with no N fertilizer; NTB-PN, notilled malt barley pea with N fertilizer; NTBFN, no-tilled malt barley with N fertilizer; NTBON, no-tilled barley with no N fertilizer; and NTCRP, no-tilled Conservation Reserve Program (perennial system). The LSD (0.05) bar shows the least significant difference at P = 0.05 among treatments within a measurement date. 2005). In contrast, lower water content in NTCRP was probably due to greater soil water uptake by perennial plants due to greater root biomass (Frank et al., 2006), followed by increased evapotranspiration due to increased temperature. The greater water content in NTBON than in other cropping systems was probably a result of lower water uptake due to lower biomass yield (Table 2), followed by increased soil water conservation in the no-till system (Sainju et al., 2008). Reduced soil temperature and water content with N fertilization than without was probably related to increased biomass growth that increased the shading intensity and water uptake (Sainju et al., 2008). Soil Surface Carbon Dioxide Flux Differences in plant C input and soil temperature and water content among treatments and measurement dates resulted in significant effects of irrigation, cropping system, and date of sampling on soil surface CO 2 flux from 2006 to 2008 (Table 4). Interactions were significant for irrigation cropping system in Fig. 4. Effect of cropping system on soil surface CO 2 flux and soil temperature and water content at the 0- to 15-cm depth, averaged across irrigation practices, from April to October 2007: CTBFN, conventional-tilled malt barley with N fertilizer; CTBON, conventional-tilled malt barley with no N fertilizer; NTB-PN, notilled malt barley pea with N fertilizer; NTBFN, no-tilled malt barley with N fertilizer; NTBON, no-tilled barley with no N fertilizer; and NTCRP, and no-tilled Conservation Reserve Program (perennial system). The LSD (0.05) bar shows the least significant difference at P = 0.05 among treatments within a measurement date. 2008 and irrigation date of sampling and cropping system date of sampling from 2006 to 2008. The CO 2 flux ranged from 10 kg CO 2 C ha 1 d 1 in April 2007 and 2008 to 400 kg CO 2 C ha 1 d 1 in August 2008 as soil temperature and water content changed (Fig. 2). The flux declined substantially after July 2007 and August 2008. Fluxes were pronounced immediately following substantial precipitation and irrigation (>15 mm), which slightly decreased soil temperature but increased water content. Irrigation increased CO 2 flux compared with no irrigation during dry periods in July and August in 2006 and 2007, during active plant growing periods in May and June in all years, or even after August in 2008. Total CO 2 flux from April to October, averaged across cropping systems, was 29 to 50% greater in the irrigated than in the unirrigated practice (Table 5). The flux was similar in 2006 and 2007, both of which were lower than in 2008. Carbon dioxide fluxes of 300 to 600 kg CO 2 C ha 1 d 1 following heavy rain and irrigation in cropland and grassland soils in the northern Great Plains have been reported (Curtin et al., 2000; 1618 SSSAJ: Volume 74: Number 5 September October 2010

Sainju et al., 2008). Considering these values, it is not surprising to obtain a CO 2 flux of as much as 400 kg CO 2 C ha 1 d 1 following substantial rainfall and irrigation in this experiment. Greater flux in the irrigated than in the unirrigated practice or following substantial rainfall suggests that CO 2 flux increased with enhanced microbial activity and root respiration due to increased soil water content (Van Gestel et al., 1993; Curtin et al., 2000). Root respiration accounts for 30 to 50% of the total soil CO 2 flux (Rochette and Flanagan, 1997; Rochette et al., 1999; Curtin et al., 2000). Although total precipitation from April to October was lower in 2006 than in 2007 and 2008 (Fig. 1), greater CO 2 flux in 2008 than in 2006 (Table 5) could be a result of a more uniform distribution of precipitation that resulted in higher microbial and root respiration. Among land use systems, CO 2 flux was greater but soil water content was lower during many measurement dates in NTCRP than in the annual cropping systems from 2006 to 2008 (Fig. 3 5). Total CO 2 flux from April to October was greater in NTCRP than in annual cropping systems in the irrigated practice (Table 4). Averaged across irrigation practices, total CO 2 flux was greater in NTCRP than in annual cropping systems in all years (Table 5). In annual cropping systems, CO 2 flux was greater in CTBON than in other treatments at some measurement dates following irrigation or substantial rainfall (Fig. 3 5). Total CO 2 flux was greater in CTBON than in NTB-PN and NTBON in the unirrigated practice in 2008 (Table 4). Similarly, total CO 2 flux, averaged across irrigation practices, was greater in CTBON than in CTBFN, NTBFN, and NTB-PN in 2006 and 2007 and greater than in NTB-PN, NTBFN, and NTBON in 2008. Tillage increased CO 2 flux compared with no-tillage in 2007 and 2008 but N fertilization decreased the flux compared with no N fertilization in 2007. The greater CO 2 flux in NTCRP than in annual cropping systems was probably due to higher soil temperature at certain measurement dates (Fig. 3 5; Table 5), followed by increased root respiration due to greater root biomass. Soil temperature was 0.57 to 0.81 C greater in the perennial system (NTCRP) than in annual cropping systems (Table 5). Increased soil temperature can increase microbial activity and root respiration (Parkin and Kaspar, 2003; Amos et al., 2005). Although aboveground plant biomass and C content were lower in NTCRP than in annual cropping systems (Tables 2 and 3), it has been reported that root biomass can be 10 to 15 times greater in grassland than in cropland soils in the northern Great Plains (Frank et al., 2006). Most of the increased fluxes in NTCRP occurred before emergence (April and May) and after harvest (August October) of cereal crops in other cropping systems (Fig. 3 5). This indicates that plants in the perennial system continue to grow using soil water and respiring more than cereal crops during these periods, thereby resulting in greater CO 2 flux and lower soil water content in NTCRP than in annual cropping systems. Although soil C content was not measured in this experiment, several researchers (Dugas et al., 1999; Frank et al., 2006) have reported that greater CO 2 flux in grasslands than in croplands also results from greater Fig. 5. Effect of cropping system on soil surface CO 2 flux and soil temperature and water content at the 0- to 15-cm depth, averaged across irrigation practices, from April to October 2008: CTBFN, conventional-tilled malt barley with N fertilizer; CTBON, conventional-tilled malt barley with no N fertilizer; NTB-PN, no-tilled malt barley pea with N fertilizer; NTBFN, no-tilled malt barley with N fertilizer; NTBON, no-tilled barley with no N fertilizer; and NTCRP, no-tilled Conservation Reserve Program (perennial system). The LSD (0.05) bar shows the least significant difference at P = 0.05 among treatments within a measurement date. substrate C availability because soil organic C and microbial biomass C are higher in grasslands than in croplands (Frank et al., 2006). Such increases in CO 2 fluxes were pronounced more in the irrigated than in the unirrigated practice due to greater soil water availability (Table 4). In croplands, greater CO 2 flux in CTBON than in other cropping systems was probably due to tillage, followed by the absence of N fertilization (Table 5). This explanation was supported by the comparison of individual treatments that showed increased CO 2 flux with tillage but decreased flux with N fertilization. Tillage can increase CO 2 emissions compared with no-tillage by increasing aeration due to greater soil disturbance and disruption of soil aggregates (Roberts and Chan, 1990) and through physical degassing of dissolved CO 2 from the soil solution ( Jackson et al., 2003). In contrast, N fertilization can reduce CO 2 flux compared with no N fertilization (Ma et al., 1999; Al-Kaisi et al., 2008), probably due to reduced soil ph (Smolander et al., 1994; Ladd et al., 1994). Other mechanisms by which N fertilization could have reduced CO 2 flux compared SSSAJ: Volume 74: Number 5 September October 2010 1619

Table 4. Interaction effect of irrigation and cropping system on total soil surface CO 2 flux from April to October and average soil temperature and water content at the 0- to 15-cm depth across measurement dates from 2006 to 2008. Irrigation Cropping system Total soil surface CO 2 flux Soil temperature Soil water content 2006 2007 2008 2006 2007 2008 2006 2007 2008 Mg CO 2 C ha 1 C g kg 1 Irrigated CTBFN 19.2 20.2 27.8 18.4 16.7 16.5 79 69 71 CTBON 21.8 24.6 30.2 18.8 17.9 18.3 81 71 74 NTB-PN 19.8 19.0 24.0 18.5 17.2 16.6 82 72 71 NTBFN 19.2 17.1 23.6 18.2 16.8 16.6 83 74 68 NTBON 20.8 22.5 21.6 18.6 17.8 17.2 88 79 72 NTCRP 32.8 34.0 37.1 18.5 17.9 16.9 71 71 64 Unirrigated CTBFN 12.9 15.7 19.9 19.6 17.4 17.7 64 65 65 CTBON 15.6 19.0 23.6 19.6 17.7 18.0 64 74 64 NTB-PN 12.6 14.6 18.7 19.8 17.5 17.4 67 68 65 NTBFN 13.3 14.6 19.8 19.8 17.7 17.6 66 67 63 NTBON 12.8 16.5 18.8 19.9 17.9 18.0 67 73 63 NTCRP 21.3 25.2 21.6 21.4 18.7 18.8 53 62 57 LSD (0.05) NS NS 4.0 1.1 NS 0.9 NS 5 NS Significance Irrigation (I) *** *** *** *** * *** *** *** *** Cropping system (CS) *** *** *** * * * *** *** * I CS NS NS ** * NS * NS * NS Date of sampling (D) *** *** *** *** *** *** *** *** *** I D *** *** *** *** NS *** *** *** *** CS D *** *** *** NS NS * *** *** * I CS D NS NS NS NS NS NS NS NS NS * Significant at P 0.05. ** Significant at P 0.01. *** Significant at P 0.001. CTBFN, conventional-tilled malt barley with 67 to 134 kg N ha 1 ; CTBON, conventional tilled malt barley with 0 kg N ha 1 ; NTB-PN, no-tilled malt barley pea rotation with 67 to 134 kg N ha 1 applied to barley; NTBFN, no-tilled malt barley with 67 to 134 kg N ha 1 ; NTBON, no-tilled malt barley with 0 kg N ha 1 ; NTCRP, no-tilled Conservation Reserve Program (perennial system). NS, not significant. Table 5. Effects of irrigation and cropping system (CS) on total soil surface CO 2 flux from April to October and average soil temperature and water content at the 0- to 15-cm depth across measurement dates from 2006 to 2008. Cropping Total soil surface CO 2 flux Soil temperature Soil water content Irrigation system 2006 2007 2008 2006 2007 2008 2006 2007 2008 Mg CO 2 C ha 1 C g kg 1 Irrigated 22.2 a 22.9 a 27.1 a 18.5 b 17.4 b 17.0 b 80 a 73 a 70 a Unirrigated 14.8 b 17.7 b 20.3 b 20.0 a 17.8 a 17.9 a 63 b 68 b 63 b CTBFN (CS1) 16.1 c 18.0 cd 24.0 bc 19.0 b 17.0 b 17.1 b 71 b 67 c 68 a CTBON (CS2) 18.8 b 21.7 b 26.9 b 19.2 ab 17.8 ab 18.1 a 73 b 72 b 69 a NTB-PN (CS3) 16.1 c 16.9 d 21.2 cd 19.2 ab 17.3 b 17.0 d 74 ab 70 bc 68 a NTBFN (CS4) 16.1 c 15.9 d 21.6 cd 19.0 b 17.3 b 17.1 b 74 ab 70 bc 66 ab NTBON (CS5) 16.8 bc 19.6 bc 20.1 d 19.3 ab 17.9 ab 17.6 ab 77 a 76 a 68 a NTCRP (CS6) 27.0 a 29.7 a 29.5 a 19.9 a 18.3 a 17.8 a 62 c 67 c 60 b Contrasts Till vs. no-till (CS1, CS2 vs. CS4, CS5) 1.0 2.2* 4.5*** 0.01 0.15 0.27 3.5* 3.5* 1.5 N fertilization vs. no N (CS1, CS4 vs. CS2, CS5) 1.7 3.7** 0.7 0.25 0.69** 0.80** 2.5 5.5** 1.5 Continuous barley vs. barley pea (CS4, CS5 vs. CS3) 0.4 1.0 0.4 0.03 0.25 0.36 1.5 3.0* 1.0 Annual vs. perennial system (CS3, CS4, CS5 vs. CS6) 10.8*** 12.2*** 8.4** 0.78* 0.81** 0.57* 13*** 5.0** 7.3** * Significant at P 0.05. ** Significant at P 0.01. *** Significant at P 0.001. CTBFN, conventional-tilled malt barley with 67 to 134 kg N ha 1 ; CTBON, conventional-tilled malt barley with 0 kg N ha 1 ; NTB-PN, no-tilled malt barley pea rotation with 67 to 134 kg N ha 1 applied to barley; NTBFN, no-tilled malt barley with 67 to 134 kg N ha 1 ; NTBON, no-tilled malt barley with 0 kg N ha 1 ; NTCRP, no-tilled Conservation Reserve Program (perennial system). Numbers followed by different letters within a column in either irrigation practice or cropping system are significantly different at P 0.05 by the least significant difference test. 1620 SSSAJ: Volume 74: Number 5 September October 2010

with no fertilization are reductions in soil temperature and water content (Table 5) because CO 2 emission is linearly related to soil temperature and water content (Parkin and Kaspar, 2003; Amos et al., 2005; Sainju et al., 2008). The CO 2 flux was not significantly different between continuous barley and barley pea in the no-till system, although continuous barley produced lower biomass yield and C content (Table 2). These results suggest that CO 2 emissions are related more to irrigation, tillage, and N fertilization than to differences in aboveground plant C inputs as influenced by cropping system in sandy loam soils in the northern Great Plains. Because NTB-PN has been known to reduce the N fertilization rate to malt barley following pea (in 2008 as described above; Lenssen et al., 2007) but produced a biomass yield similar to NTBFN and CTBFN (Table 2), lower CO 2 flux in this system than in CTBON from 2006 to 2008 (Table 5) suggests that no-tilled annual cropping of a barley pea rotation may be used as a management option to reduce CO 2 emissions and sustain malt barley yields compared with conventional-tilled continuous barely in the northern Great Plains. Other benefits of using a barley pea rotation compared with continuous barley are reduced incidences of weeds, pests, and diseases that impact barley yields (Miller et al., 2002; Lenssen et al., 2007). It should be noted that CO 2 emissions from different land use and management practices measure only one part of the C cycle. Because plants can absorb CO 2 from the atmosphere through photosynthesis, which is converted into soil organic matter after the residue is returned to the soil, changes in soil organic C levels due to management practices should also be taken into account when estimating C cycling in annual and perennial cropping systems. Grasslands contain higher soil organic C and belowground C inputs than croplands (Frank et al., 2006); therefore, greater CO 2 flux from perennial than from annual cropping systems does not necessarily mean that perennial systems would have a greater net emission of CO 2 than annual cropping systems. Similarly, in annual cropping systems, a reduction in CO 2 flux from one management practice does not mean that adopting this practice will help to reduce the global warming potential. A more complete analysis of the C cycle including belowground C inputs and changes in soil C storage would be needed to make that assessment. Total CO 2 fluxes of 14.8 to 20.3 Mg CO 2 C ha 1 during the crop growing season (April October) in the unirrigated practice in annual and perennial cropping systems obtained in this experiment were greater than other reported values of 2.8 to 10.2 Mg CO 2 C ha 1 under dryland cropping systems in the northern Great Plains (Curtin et al., 2000; Frank et al., 2006; Sainju et al., 2008). This could be due to differences in soil and climatic conditions among locations. The soil in this experiment was a sandy loam compared with silt loam and loam at the other sites. It is possible that plant residue and soil organic C could have mineralized more rapidly in the coarse-textured soil in our experiment compared with other sites, thereby resulting in increased CO 2 emissions. Another factor resulting in increased CO 2 emissions from our site could also be due to the continued mineralization of soil organic C as a result of conversion of perennial to annual cropping systems because organic C levels are higher in grasslands (Dugas et al., 1999; Frank et al., 2006). SUMMARY AND CONCLUSIONS Land use and management practices influenced plant biomass C and soil temperature, water content, and CO 2 emissions. Although aboveground plant C input was greater in annual cropping systems, CO 2 flux was greater in the perennial system, probably due to increased soil temperature and root respiration, especially before the emergence of annual crops in the spring and after harvest in the fall. In croplands, aboveground plant C input was lower but CO 2 flux was greater in CTBON than in other cropping systems. Nitrogen fertilization increased plant C input but reduced CO 2 flux compared with no N fertilization. Tillage did not influence plant C input but increased CO 2 flux compared with no-tillage. In contrast, irrigation increased both plant C input and CO 2 flux compared with no irrigation. With adequate N fertilization, crop rotation had little effect on plant C input and CO 2 flux. Emissions of CO 2 usually peaked immediately following irrigation or substantial rainfall. Differences in CO 2 emissions among land use and management practices were related more to soil temperature, water content, and N levels than to aboveground plant C input. Because of reduced CO 2 flux compared with conventional-tilled malt barley with no N fertilization but similar biomass yields as no-tilled or conventional-tilled malt barley with N fertilization, a no-tilled malt barley pea rotation may be used as a management option to reduce CO 2 emissions from croplands and sustain crop yields in the northern Great Plains. To evaluate the importance of CO 2 flux to the overall global warming potential of a cropping system, additional data including changes in soil C levels with time and underground C inputs should be measured. ACKNOWLEDGMENTS We greatly appreciate the help provided by Bryan Gebhard, Bill Iversen, Dale Spracklin, and Randall Obergfell for field work and plot maintenance, and Chris Russell and Joy Barsotti for plant and gas sample collection and analysis. REFERENCES Al-Kaisi, M.M., M.L. Kruse, and J.E. Sawyer. 2008. Effect of nitrogen fertilizer application on growing season carbon dioxide emission in a corn soybean rotation. J. Environ. Qual. 37:325 332. Al-Kaisi, M.M., and X. Yin. 2005. Tillage and crop residue effects on soil carbon and carbon dioxide emission in corn soybean rotations. J. Environ. Qual. 34:437 445. Amos, B., T.J. Arkebauer, and J.W. Doran. 2005. Soil surface fluxes of greenhouse gases in an irrigated maize-based agroecosystem. Soil Sci. Soc. Am. J. 69:387 395. 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