TECHNICAL REPORTS: PLANT AND ENVIRONMENT INTERACTIONS
Tillage, Cropping Sequence, and Nitrogen Fertilization Effects on Dryland Soil Carbon Dioxide Emission and Carbon Content Upendra M. Sainju,* Jalal D. Jabro, and Thecan Caesar-TonThat USDA–ARS Management practices are needed to reduce dryland soil CO2 emissions and to increase C sequestration. We evaluated the effects of tillage and cropping sequence combinations and N fertilization on dryland crop biomass (stems + leaves) and soil surface CO2 flux and C content (0- to 120-cm depth) in a Williams loam from May to October, 2006 to 2008, in eastern Montana. Treatments were no-tilled continuous malt barley (Hordeum vulgaris L.) (NTCB), no-tilled malt barley– pea (Pisum sativum L.) (NTB-P), no-tilled malt barley–fallow (NTB-F), and conventional-tilled malt barley–fallow (CTB-F), each with 0 and 80 kg N ha−1. Measurements were made both in Phase I (malt barley in NTCB, pea in NTB-P, and fallow in NTB-F and CTB-F) and Phase II (malt barley in all sequences) of each cropping sequence in every year. Crop biomass varied among years, was greater in the barley than in the pea phase of the NTB-P treatment, and greater in NTCB and NTB-P than in NTB-F and CTB-F in 2 out of 3 yr. Similarly, biomass was greater with 80 than with 0 kg N ha−1 in 1 out of 3 yr. Soil CO2 flux increased from 8 mg C m−2 h−1 in early May to 239 mg C m−2 h−1 in mid-June as temperature increased and then declined to 3 mg C m−2 h−1 in September–October. Fluxes peaked immediately following substantial precipitation (>10 mm), especially in NTCB and NTB-P. Cumulative CO2 flux from May to October was greater in 2006 and 2007 than in 2008, greater in cropping than in fallow phases, and greater in NTCB than in NTB-F. Tillage did not influence crop biomass and CO2 flux but N fertilization had a variable effect on the flux in 2008. Similarly, soil total C content was not influenced by treatments. Annual cropping increased CO2 flux compared with crop–fallow probably by increasing crop residue returns to soils and root and rhizosphere respiration. Inclusion of peas in the rotation with malt barley in the no-till system, which have been known to reduce N fertilization rates and sustain malt barley yields, resulted in a CO2 flux similar to that in the CTB-F sequence.
Copyright © 2010 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 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. J. Environ. Qual. 39:935–945 (2010) doi:10.2134/jeq2009.0223 Published online 22 Feb. 2010. Received 13 June 2009. *Corresponding author (
[email protected]). © ASA, CSSA, SSSA 5585 Guilford Rd., Madison, WI 53711 USA
C
arbon dioxide emission from agricultural practices has been considered a significant anthropogenic source (25%) of greenhouse gas responsible for global warming (Post et al., 1990; Duxbury, 1994). The emission occurs primarily from oxidation of soil organic matter and root and microbial respiration due to cropping, return of nonharvestable crop residue in the soil, tillage, and other management practices (Curtin et al., 2000; Sainju et al., 2008). In contrast, soil is also an important sink for atmospheric CO2 which is absorbed by plant biomass through photosynthesis and converted into soil organic matter after crop residues are returned to the soil (Lal et al., 1995; Paustian et al., 1995). Soils are an important reservoir of C in the terrestrial ecosystem, contributing about 1500 Pg C, three times greater than that stored in the vegetation (Schlesinger, 1997). Out of this, agricultural soils contain about 170 Pg C to a depth of 1 m (Cole et al., 1996), 54 Pg C of which has been estimated to be lost through CO2 emission in the last two centuries alone (Paustian et al., 1995). Since 47% of the earth’s surface has been classified as drylands (United Nations Environmental Program, 1992), C sequestration in plants and soils and loss as CO2 emissions from the soil surface constitute important components of the global C cycle in dryland soils (FAO, 2004). Traditional farming systems, such as conventional tillage with crop–fallow, have resulted in a decline of dryland soil organic C by 30 to 50% from their original levels in the last 50 to 100 yr in the northern Great Plains (Haas et al., 1974; Peterson et al., 1998). Intensive tillage increases the mineralization of organic C (Bowman et al., 1999; Schomberg and Jones, 1999), and fallowing increases its loss by reducing the amount of plant residue returned to the soil (Black and Tanaka, 1997; Campbell et al., 2000), and by increasing soil water and temperature (Haas et al., 1974). Therefore, novel management practices are needed to increase C sequestration and reduce CO2 emission in dryland soils. Some of the management practices that influence dryland soil CO2 emission and C storage are tillage, cropping sequence and intensity, and N fertilization (Curtin et al., 2000; Sainju et al., 2008). Decreased tillage intensity reduces soil disturbance and microbial activities, which in turn lowers CO2 emission (Curtin et al., 2000). In contrast, increased tillage intensity increases CO2 emission by increasing aeration due to greater soil disturbance (Roberts and Chan, 1990) and by physical degassing of dissolved USDA–ARS, Northern Plains Agricultural Research Lab., 1500 North Central Ave., Sidney, MT 59270. Assigned to Associate Editor Elizabeth Baggs. Abbreviations: CTB-F, conventional-tilled malt barley–fallow; DOY, day of year; NTB-F, no-tilled malt barley–fallow; NTB-P, no-tilled malt barley–pea; NTCB, no-tilled continuous malt barley; STC, soil total C.
935
CO2 from the soil solution (Jackson et al., 2003). Cropping can increase CO2 emission and C storage compared with fallow by increasing root respiration and the amount of crop residue returned to the soil (Curtin et al., 2000; Amos et al., 2005; Sainju et al., 2007, 2008). Similarly, residue quality, such as C/N ratio, can alter the decomposition rate of residue (Kuo et al., 1997), thereby influencing CO2 emission (Al-Kaisi and Yin, 2005). Increased cropping intensity can also increase CO2 emission and C storage by increasing residue C input (Curtin et al., 2000; Al-Kaisi and Yin, 2005). Nitrogen fertilization, however, has variable effects on CO2 emission and C storage (Mosier et al., 2006; Al-Kaisi et al., 2008). Management practices can also indirectly influence CO2 emission by altering soil temperature and water content, as CO2 flux is related to these parameters (Parkin and Kaspar, 2003; Amos et al., 2005). Tillage can enhance soil water loss, while 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 both influence soil temperature and water content compared with fallow by affecting shade intensity and evapotranspiration (Curtin et al., 2000; Amos et al., 2005). Rewetting of dry soil due to irrigation or rainfall can also increase CO2 flux by increasing microbial activity, C mineralization, and respiration (Calderon and Jackson, 2002). Information on soil surface CO2 emission and C storage due to soil and crop management practices under dryland cropping system is limited. We hypothesized that no-tilled annual cropping with the recommended rate of N fertilization would reduce soil CO2 flux and increase C storage when compared with conventional-tilled crop–fallow with no N fertilization. Our objectives were (i) to determine the influence of tillage, cropping sequence, and N fertilization on crop biomass (stems + leaves) residue returned to the soil, (ii) to quantify the individual and combined effects of tillage, cropping sequence and phase, and N fertilization on soil surface CO2 flux from 2006 to 2008, and (iii) to relate CO2 flux with crop C input and soil total (organic + inorganic) C (STC) content at the 0- to 120-cm depth after 3 yr under dryland cropping systems in the northern Great Plains.
Materials and Methods Experimental Site and Treatments The experiment was conducted from 2006 to 2008 on a dryland farm, 8 km west of Sidney (48°33′ N, 104°50′ W) in eastern Montana. The site is characterized by wide variations in mean monthly air temperature from −8°C in January to 23°C in July and August. The mean annual precipitation (105yr average) is 350 mm, 70% of which occurs during the crop growing season (April–August). The soil is a Williams loam (fine-loamy, mixed, superactive, frigid, Typic Argiustolls) with 350 g kg−1 sand, 325 g kg−1 silt, 325 g kg−1 clay, and pH 7.2 at the 0- to 20-cm depth. The STC concentrations at 0- to 5- and 5- to 20-cm depths at the initiation of the experiment in April 2006 were 13.3 and 10.6 g kg−1, respectively. The previous cropping system was conventional-tilled spring wheat (Triticum aestivum L.)–fallow–safflower (Carthamus tinctorius L.) rotation conducted for the past 6 yr. 936
Main-plot treatments were no-tilled continuous malt barley (NTCB), no-tilled malt barley–pea (NTB-P), no-tilled malt barley–fallow (NTB-F), and conventional-tilled malt barley– fallow (CTB-F), each with two subplot N fertilization rates of 0 and 80 kg N ha−1. While NTCB had only one phase (malt barley), other cropping sequences had two phases in the rotation. For example, NTB-P had malt barley and pea phases, NTB-F had malt barley and no-tilled fallow phases, and CTB-F had malt barley and conventional-tilled fallow phases. Malt barley was planted annually in NTCB, in rotation with pea in NTB-P, and in rotation with fallow in NTB-F and CTB-F. The first phase (Phase I) of the cropping sequence in NTCB, NTB-P, NTB-F, and CTB-F contained malt barley, pea, no-tilled fallow, and conventional-tilled fallow, respectively, whereas the second phase (Phase II) contained malt barley in all treatments. Because of the variable effect of climate on crop biomass and soil CO2 emission from year to year, both phases of the cropping sequence occurred in every year. As a result, values for crop biomass, soil CO2 emission, and STC content for a rotation were calculated as the average values of phases in a year. The 80 kg N ha−1 is the recommended rate of N fertilization for malt barley in the dryland cropping system in the experimental site. In NTCB, NTB-P, and NTB-F, plots were left undisturbed, except for fertilizer application and the planting of crops in rows. The CTB-F is the conventional-tilled farming system where plots were tilled with field cultivator equipped with C-shanks and 45-cm-wide sweeps and coiledtoothed spring harrows with 60-cm rods. Plots were tilled to a depth of 10 cm during planting and fallow periods three to four times a year for seedbed preparation and to control weeds. Nitrogen fertilizer was applied to malt barley at 80 kg N ha−1 in the first phase of the cropping sequence in NTCB but was not applied to fallow in other sequences. For pea crops, N fertilizer was applied at 5 kg N ha−1 while applying P as monoammonium phosphate. In the second phase of the cropping sequence, N fertilizer was applied to malt barley at 0 or 80 kg N ha−1 in all treatments. Weeds in no-tilled treatments were controlled by applying preplant and postharvest herbicides and in conventional-tilled treatment by a combination of herbicides and conventional tillage to a depth of 10 cm as needed. Glyphosate [N-(phosphonomethyl) glycine] was applied at 1.8 kg a.i. ha−1 to all plots as preplant and postharvest herbicide. For malt barley, growing season weeds were controlled by applying a mixture of bromoxynil (3, 5-dibromo-4-hydroxybenzonitrile) and fanoxaprop-P {(R)-2-[4(6-chloro-1, 3-benoxazol-2-yloxy) phenoxy] propionic acid} at 0.5 kg a.i ha−1. For peas, sonalan [N ethyl-N-2(2-methyl-2 propenyl)-2, 6 dinitro4-trifluromethyl benzeneamine] was applied at 1.0 L ha−1 in the fall followed by postemergence herbicide of a mixture of bentazon [3-isopropyl-1-H-2,1,3-benzothiadiazin-(3H)-one 2,2–dioxide] and sethoxydim {2-[1-(ethoxyimino) butyl]-5–2[2-(ethylthio)propyl]-3-hydroxy-2- cyclohexene-1-one}at 0.12 kg a.i ha−1. Treatments were laid out in split-plot arrangement in a randomized complete block with three replications. The split plot size was 12.0 by 6.0 m.
Crop Management In April, 2006 to 2008, six-row malt barley (cv. Certified Tradition [Busch Agricultural Resources, Fargo, ND]) was Journal of Environmental Quality • Volume 39 • May–June 2010
planted to a depth of 3.8 cm at 45 kg ha−1, while peas (cv. Majoret [Macintosh Seed, Havre, MT]) were sown at 101 kg ha−1 with a no-till drill equipped with double-shoot Barton (Flexi-Coil, http://www.flexicoil.com/barton.asp) disk openers. Pea seeds were inoculated with Rhizobium sp. At the same time, N fertilizer as urea (46% N) and monoammonium phosphate (18% N, 46% P) at 80 kg N ha−1, P fertilizer as mono-ammonium phosphate at 29 kg P ha−1, and K fertilizer as muriate of potash (60% K) at 27 kg K ha−1 were banded to malt barley in the first phase of the cropping sequence. For peas, P and K fertilizers from sources and at rates similar to malt barley as above were banded, which also supplied 5 kg N ha−1. No fertilizers were applied to the fallow phase. In the second phase, P as triple superphosphate (45% P) and K as muriate of potash were banded to malt barley at the same rates as in the first phase. Nitrogen as urea was broadcast at 0 or 80 kg N ha−1 to malt barley in all treatments a week after planting. No irrigation was applied. In August, 2006 to 2008, malt barley and pea grain yields were determined from a swath of 12.0 by 1.5 m using a combine harvester and biomass (stems + leaves) yields from an area of 1.0 by 1.0 m outside yield rows. After grain harvest, crop biomass residues were returned to the soil.
Carbon Dioxide Flux Measurements and Soil Sampling and Analysis Immediately after planting, soil surface CO2 flux was measured weekly in all treatments from May to October, 2006 to 2008, at the experimental site until the ground froze. All measurements were taken between 0900 h and 1200 h of the day to reduce variability in CO2 flux due to diurnal changes in temperature (Parkin and Kaspar, 2003). The CO2 flux was measured with an Environmental Gas Monitor chamber containing infrared CO2 analyzer attached to a data logger (model EGM-4, PP System, Haverhill, MA). The chamber was 15 cm tall and 10 cm in diameter, with sharp edge at the bottom, and was able to measure CO2 flux over the range of 0 to 9.99 g CO2–C m−2 h−1. The chamber was placed at the soil surface by firmly pushing the edge to a depth of 5 mm for 2 min in each plot until CO2 flux measurement was recorded in the data logger. Measurements were made randomly from two places in the plot, and the average value was used for a treatment. At the time of CO2 measurement, soil temperature near the chamber was measured from a depth of 0 to 15 cm using a probe attached to the data logger. The probe was pushed by hand into the soil while measuring temperature. Similarly, gravimetric soil water content was measured near the chamber by collecting a soil sample from 0 to 15 cm with a hand probe (2.5 cm diam.) firmly pushed into the soil every time CO2 flux was measured. The moist soil was oven-dried at 110°C and water content was determined. The gravimetric water content was converted into volumetric water content by multiplying it by bulk density of the soil core (determined by the weight of ovendried soil divided by the volume of the core). In October 2008, additional soil samples were collected from the 0- to 120-cm depth with a hydraulic probe to determine STC concentration. These were collected from five places within a plot, divided into 0- to 5-, 5- to 10-, 10- to 30-, 30- to 60-, 60- to 90-, and 90- to 120-cm depths, composited within a depth, air-dried, Sainju et al.: Management Effects on Dryland Soil Carbon Dioxide Emission
and sieved to 2 mm. A subsample from each treatment and depth was ground and sieved to 0.5 mm to determine STC concentration (g kg−1) by using the high induction furnace C and N analyzer (LECO, St. Joseph, MI). An additional undisturbed soil core was taken from each treatment and depth to determine bulk density by dividing the weight of oven-dried soil at 110°C by the volume of the core. The STC content (Mg ha−1) at a depth was determined by multiplying STC concentration by soil depth and bulk density.
Data Analysis Data for CO2 flux and soil temperature, water, and STC content were analyzed using the Analysis of Repeated Measures procedure in the MIXED model of SAS (Littell et al., 1996). Tillage and cropping sequence combination was considered as the main-plot factor and fixed variable, N fertilization as the subplot factor and another fixed variable, date of measurement as the repeated measure variable, and replication as the random variable. Similarly, data for crop biomass were analyzed by using the MIXED procedure after considering tillage and cropping sequence combination as the main-plot factor and fixed variable, N fertilization as the subplot factor and another fixed variable, year as the repeated measure variable, and replication as the random variable. Because crops were absent during the fallow phase of the cropping sequence, data for biomass yields were considered as missing in the fallow treatments in Phase I for analysis. To calculate annualized mean biomass yield, biomass was averaged across phases within a cropping sequence and average value was used for a cropping sequence treatment. Similarly, to calculate annualized mean yield in crop– fallow treatments, biomass was divided by 2 due to absence of crops during fallow. Means were separated by using the least square means test when treatments and interactions were significant (Littell et al., 1996). When main-plot treatments were significant, orthogonal contrasts were used to determine the individual effects of tillage and cropping sequence on these parameters. Statistical significance was evaluated at P ≤ 0.05, unless otherwise mentioned.
Results and Discussion Crop Biomass Yield Because of the similar effect of treatments on crop grain and biomass (stems + leaves) yields of crops, only biomass yield was considered. Since biomass was returned to the soil after grain harvest, it served as a C input to the soil, which could potentially influence soil CO2 emission due to increased C substrate availability. In contrast, C was removed during grain harvest and therefore was not related to soil CO2 emission. Crop biomass yield was significantly influenced by tillage and cropping sequence combination and year (Table 1). Nitrogen fertilization influenced biomass only for Phase II of the cropping sequence. Interactions were significant for tillage and cropping sequence × year for both phases and N fertilization × year for Phase II of the cropping sequence. Biomass varied with treatments, cropping phases, and years (Table 2). Biomass was greater in malt barley than in peas in Phase I of the cropping sequence in 2006 and 2007. In Phase II, biomass was greater in barley following fallow than following 937
Table 1. Analysis of variance for crop biomass (stems + leaves) residue returned to the soil. Cropping sequence phase Annualized Phase I Phase II mean
Source Tillage and cropping sequence (CS) N fertilization rate (N) CS × N Year (Y) CS × Y N×Y CS × N × Y
**
**
**
NS† NS *** *** NS NS
*** NS *** *** ** NS
** NS *** *** * NS
* Significance at P ≤ 0.05. ** Significance at P ≤ 0.01. *** Significance at P ≤ 0.001. † NS, not significant.
barley or peas in 2008. Annualized biomass was greater in NTCB and NTB-P than in NTB-F and CTB-F in 2006 and 2007. Compared with no N fertilization, N fertilization increased barley biomass in Phase II of the cropping sequence or annualized biomass in 2007. Because data for crop biomass yields during fallow phases in NTB-F and CTB-F were missing, values for annualized biomass for N fertilization treatments were different from the calculated average values of Phase I and Phase II of a cropping sequence in a year. Tillage did not influence biomass. Biomass among years followed the order: 2007 > 2006 > 2008. Averaged
across years, biomass was greater in NTCB and NTB-P than in NTB-F and CTB-F, and greater with 80 than with 0 kg N ha−1. It is not surprising to observe greater biomass with malt barley than with peas in the first phase of the cropping sequence during periods of adequate precipitation in 2006 and 2007 (Table 2). Growing season precipitation (April–August) was 154 mm in 2006, 210 mm in 2007, and 111 mm in 2008. Peas produce about half of the biomass compared with barley in the dryland cropping system when growing season precipitation is adequate (Lenssen et al., 2007). During lower growing season precipitation in 2008 than in 2006 and 2007 (Fig. 1D, 2D, and 3D), biomass was similar in malt barley and pea. During a lower precipitation season, peas even outperform barley due to their lower water requirement (Miller et al., 2002; Lenssen et al., 2007). While barley biomass was similar among tillage and cropping sequence treatments in Phase II of the cropping sequence in 2006 and 2007, greater biomass following fallow than following barley or pea in 2008 was probably due to greater water conservation during fallow in Phase I of the cropping sequence in 2007 that increased succeeding barley yield. Fallowing increases soil water storage and succeeding crop and biomass yields (Lenssen et al., 2007; Sainju et al., 2007). Because of the absence of crops during fallow, annualized biomass remained lower in NTB-F and CTB-F than in NTCB and NTB-P in 2006 and 2007. In contrast, greater barley biomass following fallow in Phase II resulted in similar annualized biomass among tillage and cropping sequence treatments in 2008. The effect of N fertilization in increasing biomass was observed only during adequate soil
Table 2. Effects of tillage, cropping sequence, and N fertilization on crop biomass (stems + leaves) residue returned to the soil from 2006 to 2008. 2006
Treatment
2007 2008 Year Annualized Annualized Annualized annualized Phase I† Phase II† Phase I Phase II Phase I Phase II mean mean mean mean −1 ——————————————————————————— Mg ha ———————————————————————————
Tillage and cropping sequence‡ NTCB NTB-P NTB-F CTB-F Contrast Till vs. no-till¶ AC vs. C-F in NT# CB vs. B–P in NT††
2.50a§ 1.86b — —
2.28a 2.53a 2.77a 2.46a
2.39a 2.20a 1.39b 1.23b
4.50a 3.32b — —
3.13a 3.36a 2.80a 3.52a
3.82a 3.35a 1.40b 1.76b
1.55a 1.78a — —
1.36b 1.67b 3.00a 2.74a
— —
0.72 0.44
0.36 2.18**
— —
−0.26 −1.48***
−0.13 0.09
−0.02 1.06**
−0.31
−0.27
0.13
— —
−0.31 −0.36
−0.16 0.91**
0.64*
−0.25
0.19
1.18***
2.51a 2.65a
3.92a 3.92a
N fertilization rate (kg N ha−1) 0 80
— —
2.38a 2.64a
−0.23
2.34b 4.04a
0.47
0.23
2.75b 3.99a
1.61a 1.71a
2.18a 2.21a
1.45a 1.72a 1.50a 1.37a
2.21a 2.25a
2.55a 2.42a 1.43b 1.45b
2.49b 2.96a
* Significant at P ≤ 0.05. ** Significant at P ≤ 0.01. *** Significant at P ≤ 0.001. † Phase I contained malt barley in NTCB (no-tilled continuous malt barley), pea in NTB-P (no-tilled malt barley–pea), no-tilled fallow in NTB-F (no-tilled malt barley–fallow), and conventional-tilled fallow in CTB-F (conventional-tilled malt barley–fallow). Phase II contained malt barley in all treatments where N fertilizer was applied at 0 and 80 kg N ha−1. ‡ Tillage and cropping sequences are CTB-F, NTB-F, NTB-P, and NTCB. § Numbers followed by different letters within a column in a set are significantly different between treatments at P ≤ 0.05 by the least square means test. ¶ CTBF − NTBF. # Annual cropping vs. crop-fallow in no-till [(NTCB + NTB-P)/2 − NTBF]. †† Continuous malt barley vs. malt barley–pea in no-till (NTCB − NTB-P). 938
Journal of Environmental Quality • Volume 39 • May–June 2010
water content, such as in 2007, probably a result of increased N availability. Similarly, the sequential order of increasing biomass among years was related to crop growing season precipitation, indicating that growing season precipitation has a major influence on dryland crop production (Halvorson et al., 2002; Sainju et al., 2009). Differences in crop biomass residue returned to the soil among treatments have been expected to influence soil CO2 fluxes due to differences in C inputs, as discussed below.
Soil Temperature and Water Content Soil temperature increased from 7°C in April to as much as 37°C in August and then declined to 1°C in October in 2006, 2007, and 2008 (Fig. 1B, 2B, and 3B). Soil temperature normally did not vary among treatments, except in some measurement dates in 2007. In contrast, soil water content varied with treatments, date of measurements, and years (Fig. 1C, 2C, and 3C, Table 3) and responded largely to precipitation (Fig. 1D, 2D, and 3D). During lower precipitation in July and August, water content was greater in NTB-F and CTB-F than in other treatments, especially in 2007 and 2008. Both mean soil temperature and water content among years followed the trend: 2007 > 2006 > 2008, a case similar to that obtained for crop biomass yield.
Soil Surface Carbon Dioxide Flux Soil surface CO2 flux was significantly influenced by tillage and cropping sequence combination and time of measurement in all years and phases of the cropping sequence, except for tillage and cropping sequence effect in 2008 Phase II (Tables 2 and 3). Nitrogen fertilization influenced CO2 flux in 2006 Phase II. Interactions were Fig. 1. Effect of tillage and cropping sequence combination on (A) soil surface CO2 flux, (B) soil significant for tillage and cropping sequence temperature, (C) soil volumetric water content at the 0- to 15-cm depth, and (D) precipitation from May to October in 2006. CTB-F, conventional-tilled malt barley–fallow; NTB-F, no-tilled malt × time of measurement in all years and crop- barley–fallow; NTB-P, no-tilled malt barley–pea; NTCB, no-tilled continuous malt barley. LSD ping sequence phases and for N fertilization (0.05) is the least significant difference between treatments in a measurement date at P ≤ 0.05. × time of measurement in 2008. nounced during these peak periods. For example, CO2 flux The CO2 flux increased from 8 mg C m−2 h−1 in early May was greater in NTCB or NTB-P than in NTB-F or CTB-F in −2 −1 to as much as 238 mg C m h in mid-June and then declined DOY 151, 221, and 237 in 2006, from DOY 165 to 178 in −2 −1 to 4 mg C m h at the end of October (Fig. 1A, 2A, and 2007, and from DOY 160 to 167 or from DOY 211 to 218 3A). While the trend in CO2 flux was similar to trend in soil in 2008. Similarly, CO2 flux varied with N fertilization during temperature, peak fluxes were usually obtained after substanpeak periods in 2008 (Fig. 4). tial precipitation (>10 mm) events. For example, greater CO2 Cumulative CO2 flux from May to October was greater in fluxes from day of year (DOY) 221 to 237 in 2006 were probNTCB and NTB-P than in NTB-F in 2006, greater in NTCB ably related to the precipitations from DOY 190 to 221, fluxes than in NTB-F and CTB-F in 2007, and greater in NTCB than from DOY 158 to 178 in 2007 related to the precipitations in NTB-P and NTB-F in 2008 (Table 3). In the no-tilled system, from DOY 151 to 165, and fluxes from DOY 161 to 167 or annual cropping increased CO2 flux compared with crop-fallow 211 to 216 in 2008 related to precipitations in DOY 154 or in 2006 and 2007 and continuous malt barley increased CO2 flux 190 to 211, all during periods of higher temperature. Most compared with malt barley–pea in 2008. As with crop biomass of the significant differences among treatments were also proSainju et al.: Management Effects on Dryland Soil Carbon Dioxide Emission
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2000; Calderon and Jackson, 2002; Sainju et al., 2008). The greater CO2 flux during these periods or greater cumulative flux in NTCB and NTB-P than in NTB-F probably resulted from greater root respiration from annual crops and greater amounts of crop residue returned to the soil than in crop–fallow treatment (Table 2), where these factors were virtually absent during the fallow phase. Root respiration constitutes about 30% of the total CO2 flux from the soil surface (Rochette et al., 1999). Although tillage did not influence overall CO2 flux, it appeared to have some impact in the flux in CTB-F, where tillage resulted in the flux similar to that in NTB-P. Probably, lower CO2 flux by peas than by malt barley (Table 4) in NTB-P resulted in the flux similar to that produced by CTB-F. Similar trends in CO2 flux, crop biomass yield, precipitation or soil water content, and soil temperature among years (Tables 2, 3, and 4) further suggest that the flux is largely controlled by cropping system, substrate availability, and soil and climatic conditions. The greater or lower levels of CO2 flux with 80 than with 0 kg N ha−1 during peak periods in 2008 (Fig. 4) suggests that N fertilization has a variable effect on CO2 emission. Greater CO2 flux with 80 kg N ha−1 occurred during higher precipitation events or soil water contents, whereas greater flux with 0 kg N ha−1 occurred during lower soil water contents (Fig. 3C, 3D, and 4). It is possible that N fertilization reduced the soil C/N ratio or increased N availability, thereby increasing microbial activity and CO2 flux, especially during higher soil water contents (Sainju et al., 2008). The nonsignificant effect of N fertilization on CO2 flux (Table 3), however, suggests that N fertilization overall did not impact CO2 flux. This is similar to results reported by several researchers Fig. 2. Effect of tillage and cropping sequence combination on (A) soil surface CO2 flux, (B) soil (Rochette and Angers, 1999; Amos et al., temperature, (C) soil volumetric water content at the 0- to 15-cm depth, and (D) precipitation 2005), who observed variable effects of N ferfrom May to October in 2007. CTB-F, conventional-tilled malt barley–fallow; NTB-F, no-tilled malt tilization on CO2 flux, while others (Ding et barley–fallow; NTB-P, no-tilled malt barley–pea; NTCB, no-tilled continuous malt barley. LSD (0.05) is the least significant difference between treatments in a measurement date at P ≤ 0.05. al., 2007; Al-Kaisi et al., 2008) found that N fertilization reduced CO2 flux. The observed yield and total precipitation, cumulative CO2 flux among years cumulative CO2 flux values of 1.44 to 3.25 followed the order: 2007 > 2006 > 2008. Tillage and N fertilizaMg C ha−1 from May to October in a year under dryland croption overall did not influence CO2 flux. ping systems in this experiment were lower than the reported The relationships between CO2 flux and soil temperature and annual values of 2.88 to 4.86 Mg C ha−1 for irrigated cropping water content are well known (Follett, 1997; Sainju et al., 2008). systems in the central Great Plains (Mosier et al., 2006), a fact A CO2 flux as high as 341 mg C m−2 h−1 following tillage and that appeared reasonable because irrigation increases CO2 flux heavy rain in dry soils in the northern Great Plains has been compared with no irrigation (Sainju et al., 2008). reported (Curtin et al., 2000). Significant differences in CO2 The effects of tillage, crop species, fallow, and N fertilization flux among treatments during peak periods in warmer temperaon CO2 flux can be better revealed by describing the fluxes by ture following substantial precipitation were probably due to difphases of the cropping sequence. In Phase I, 2006–2008, CO2 ferences in microbial activity, root respiration, and C substrate flux was greater in no-tilled malt barley or peas or conventionalavailability when soil water content was adequate (Curtin et al., tilled fallow than in no-tilled fallow during peak periods (Fig. 5). 940
Journal of Environmental Quality • Volume 39 • May–June 2010
Similarly, in Phase II, CO2 flux was greater in barley following barley or pea in the no-tilled system or barley following fallow in the conventional-tilled system than in barley following fallow in the no-tilled system during peak periods. Cumulative CO2 flux from May to October in Phase I was greater in barley than in fallow in 2006, greater in barley than in pea and fallow in 2007, and greater in barley than in pea and fallow in the no-tilled system in 2008 (Table 4). In Phase II, cumulative CO2 flux was greater in barley following barley or pea than following fallow in the no-tilled system in 2006 and 2007 (Table 4). Tillage increased the flux compared with no-tillage in 2008 Phase I and N fertilization increased the flux compared with no N fertilization in 2006 Phase II. Since the experiment was continued in the same place from 2006 to 2008, differences in CO2 flux among treatments within a phase of the cropping sequence could also be influenced by treatments in the previous phase in addition to climatic and crop biomass variations among years. For example, greater CO2 flux in cropping than in fallow phase in 2006 Phase I could be solely due to increased root respiration by crops (Table 4). Similarly, in 2007 Phase I, greater flux in barley than in pea or fallow could be solely due to differences in root respiration rates among treatments because residual substrate availability from previous crop, i.e., barley biomass in 2006 Phase II was not different among treatments (Table 2). In contrast, greater CO2 flux in barley following pea than following barley in 2007 Phase II could be a result of difference in residue quality (N concentration or C/N ratio) of the previous crop, i.e., barley vs. pea, in 2006 Phase I. Although barley produced greater biomass than pea in 2006 Phase I, rapid decomposition of pea compared with barley residue Fig. 3. Effect of tillage and cropping sequence combination on (A) soil surface CO flux, (B) soil 2 due to its higher N concentration or lower temperature, (C) soil volumetric water content at the 0- to 15-cm depth, and (D) precipitation C/N ratio during adequate soil water con- from May to October in 2008. CTB-F, conventional-tilled malt barley–fallow; NTB-F, no-tilled malt barley–fallow; NTB-P, no-tilled malt barley–pea; NTCB, no-tilled continuous malt barley. LSD tent may have increased CO2 flux in barley (0.05) is the least significant difference between treatments in a measurement date at P ≤ 0.05. following pea than following barley in 2007 could be due to variations in root respiration rates and residual Phase II. Carbon and N concentrations in effects of residue from 2006 Phase I, since biomass was not crop residues were not measured in this experiment; however, different among treatments in 2007 Phase II and between pea average N concentration or C/N ratio from 2004 to 2007 −1 and barley in 2008 Phase I. Increases in CO2 flux due to tillage from a nearby experiment was 18.5 g kg or 21.6, respectively, −1 may have resulted in similar flux levels between tilled–fallow for pea and 11.9 g kg or 33.6 for barley. Crop residue with and barley in 2008 Phase I. Nondifference in crop biomass higher N concentration or lower C/N ratio decomposes more among treatments in 2006 Phase II or increased soil water conrapidly than with lower N concentration or higher C/N ratio tent during fallow in 2007 Phase I probably resulted in similar (Kuo et al., 1997). Increased soil water content during fallow CO2 fluxes among treatments in 2008 Phase II. As stated above in 2006 Phase I probably increased microbial activity and CO2 regarding the cropping sequence, increased N availability probflux that could have resulted in flux levels in barley following ably increased CO2 flux with 80 than with 0 kg N ha−1 in 2006 fallow similar to that in barley following barley in 2007 Phase Phase II. II. In 2008 Phase I, differences in CO2 flux among treatments Sainju et al.: Management Effects on Dryland Soil Carbon Dioxide Emission
941
Table 3. Effects of tillage, cropping sequence, and N fertilization on dryland cumulative soil surface CO2 flux from May to October and mean soil temperature and water content (0- to 15-cm depth) across measurement dates and cropping phases from 2006 to 2008. Treatment
2006
Cumulative CO2 flux 2007 2008
2006
————— Mg C ha−1 ————— Tillage and cropping sequence (CS)† NTCB NTB-P NTB-F CTB-F Contrast Till vs. no-till§ AC vs. C-F in NT¶ CB vs. B-P in NT# N fertilization rate (kg N ha−1) 0 80 Significance CS† N rate (N) CS × N Time of measurement (T) CS × T N×T CS × N × T
Mean soil temp. 2007
2008
—————— °C ——————
2.49a‡ 2.45a 2.00b 2.24ab
3.25a 3.05ab 2.47c 2.74bc
1.70a 1.48b 1.44b 1.55ab
19.4a 19.4a 19.5a 19.2a
21.3a 21.1a 21.2a 20.4b
17.2a 17.2a 17.0a 16.7a
0.24 0.47*** 0.04
0.27 0.68*** 0.20
0.11 0.15 0.22*
−0.26 −0.11 0.04
−0.78*** 0.01 0.21
−0.26 0.23 0.04
2.26a 2.33a
2.86a 2.89a
1.54a 1.54a
19.4a 19.3a
21.0a 20.9a
** NS NS
** NS NS
* NS NS
NS†† NS NS
***
***
***
*** NS NS
*** NS NS
*** ** NS
Mean soil water content 2006 2007 2008 —————— m3 m−3 ——————
0.171b 0.177ab 0.177ab 0.184a
0.171d 0.179c 0.201b 0.209a
0.137c 0.144b 0.160a 0.160a
0.009* 0.003 −0.006
0.008** −0.026** −0.008**
0.001 −0.019* −0.007*
17.0a 17.0a
0.179a 0.175a
0.192a 0.188b
0.150a 0.150a
* NS NS
NS NS NS
* NS NS
** ** NS
*** NS NS
***
***
***
***
***
***
NS NS NS
*** NS NS
NS NS NS
* NS NS
*** NS NS
*** NS NS
* Significant at P ≤ 0.05. ** Significant at P ≤ 0.01. *** Significant at P ≤ 0.001. † Tillage and cropping sequences are CTB-F, conventional-tilled malt barley–fallow; NTB-F, no-tilled malt barley–fallow; NTB-P, no-tilled malt barley–pea; and NTCB, no-tilled continuous malt barley. ‡ Numbers followed by different letters within a column in a set are significantly different between treatments at P ≤ 0.05 by the least square means test. § CTBF − NTBF. ¶ Annual cropping vs. crop-fallow in no-till [(NTCB + NTB-P)/2 − NTBF]. # Continuous malt barley vs. malt barley-pea in no-till (NTCB − NTB-P). †† NS, not significant.
Despite using no-tillage practices, NTCB may be a lessattractive management option to reduce CO2 emission because of greater flux resulting from the increased amount of crop residue returned to the soil and possibly root respiration. The nonsignificant difference in CO2 flux between NTB-P and CTB-F
in all years (Table 3), however, suggests that NTB-P may be an appropriate management practice to sustain CO2 emission in the dryland cropping system. Such a management system has also been known to sustain dryland crop yields, reduce N fertilization rate, soil erosion, and risk of reduced farm income, and improve soil quality and productivity compared with the conventional system (Miller et al., 2002; Lenssen et al., 2007; Sainju et al., 2006, 2009). However, further studies are needed to quantify CO2 fluxes derived from soil, crop residue, and root respiration, and those fixed in the plant biomass from the atmosphere through photosynthesis and sequestered in the soil for various management practices over a number of years before a practice can be recommended to reduce CO2 emission.
Soil Carbon Content
Fig. 4. Effect of N fertilization on soil surface CO2 flux from May to October in 2008. LSD (0.05) is the least significant difference between treatments in a measurement date at P ≤ 0.05. 942
Both soil bulk density and STC content in October 2008, 3 yr after the initiation of the experiment, were not influenced by treatments but increased with depth (Table 5). The increase in STC below 10 cm was due to increases in inorganic C concentration. Since both soil organic and inorganic C contents Journal of Environmental Quality • Volume 39 • May–June 2010
Table 4. Effects of tillage, cropping sequence, and N fertilization on dryland cumulative soil surface CO2 flux from May to October, 2006 to 2008, at various cropping phases. Treatment
2006 Phase I†
Phase II†
Cumulative CO2 flux 2007 Phase I Phase II
2008 Phase I
Phase II
———————————————————————— Mg C ha−1 ———————————————————————— Tillage and cropping sequence (CS)‡ NTCB NTB-P NTB-F CTB-F Contrast Till vs. no-till¶ AC vs. C-F in NT# CB vs. B-P in NT†† N fertilization rate (kg N ha−1) 0 80 Significance CS‡ N rate (N) CS × N Time of measurement (T) CS × T N×T CS × N × T
3.10a§ 2.98ab 2.34c 2.52bc
2.24a 2.16ab 1.94b 2.19ab
3.70a 2.72b 2.17c 2.26bc
0.18 0.70** 0.12
0.25 0.26* 0.08
0.09 1.04*** 0.98**
2.73a 2.73a
2.05b 2.23a * * NS *** * NS NS
* NS‡‡ NS *** *** NS NS
2.79b 3.36a 2.77b 3.12ab
1.77a 1.34bc 1.15c 1.55ab
1.65a 1.62a 1.74a 1.55a −0.19 −0.11 0.03
0.35 0.31 −0.57*
0.40* 0.41* 0.43*
2.66a 2.77a
3.07a 3.00a
1.45a 1.45a
1.64a 1.64a
*** NS NS *** *** NS NS
* NS NS *** * NS NS
** NS NS *** *** NS NS
NS‡‡ NS NS *** * *** NS
* Significant at P ≤ 0.05. ** Significant at P ≤ 0.01. *** Significant at P ≤ 0.001. † Phase I contained malt barley in NTCB (no-tilled continuous malt barley), pea in NTB-P (no-tilled malt barley–pea), no-tilled fallow in NTB-F (no-tilled malt barley–fallow), and conventional-tilled fallow in CTB-F (conventional-tilled malt barley–fallow). Phase II contained malt barley in all treatments where N fertilizer was applied at 0 and 80 kg N ha−1. ‡ Tillage and cropping sequences are CTB-F, NTB-F, NTB-P, and NTCB. § Numbers followed by different letters within a column in a set are significantly different between treatments at P test.
0.05 by the least square means
¶ CTBF − NTBF. # Annual cropping vs. crop-fallow in no-till [(NTCB + NTB-P)/2 − NTBF]. †† Continuous malt barley vs. malt barley-pea in no-till (NTCB − NTB-P). ‡‡ NS, not significant.
were influenced by tillage and cropping sequence (Cihacek and Ulmer, 2002; Mikahilova and Post, 2006; Sainju et al., 2007), we decided to examine the effect of management practices on STC instead of soil organic C, as has been used normally for evaluation of C storage in acidic soils where the presence of inorganic C is negligible. Because the depths of soil samples taken 3 yr after the start of the experiment were different from those at the initial sampling in April 2006, except at 0 to 5 cm, only changes in STC content at the 0- to 5-cm depth from 2006 to 2008 were considered. Assuming that soil bulk density at 0 to 5 cm in April 2006 was similar to that in October 2008 (1.39 Mg m−3, Table 5), changes in STC at 0 to 5 cm from 2006 to 2008 were not significantly influenced by treatments, although STC increased by 0.5 Mg C ha−1 (9.2 Mg C ha−1 in April 2006 to 9.7 Mg C ha−1 in October 2008). Since most of the significant effects of tillage, cropping sequence, and N fertilization on soil C occur at 0 to 5 cm (Franzluebbers et al., 1995; Schomberg and Jones, 1999; Sainju et al.: Management Effects on Dryland Soil Carbon Dioxide Emission
Halvorson et al., 2002), the nonsignificant effect of treatments on changes in STC at 0 to 5 cm from 2006 to 2008 also indicates that it is unlikely that treatments will have any major impact on changes in STC below the 5-cm depth within a 3-yr period. The nonsignificant effect of treatments on STC shows that C levels do not change after 3 yr of management under dryland cropping systems. This is because STC is highly variable in the soil profile due its large background and spatial heterogeneity (Franzluebbers et al., 1995). Crop biomass yields and C inputs are often lower in drylands than in irrigated regions due to limited precipitation, shorter growing season, and longer cold winters in the northern Great Plains, and this ultimately leads to slower C turnover rates (Halvorson et al., 2002; Sherrod et al., 2003; Sainju et al., 2006, 2007). Also, shallow tillage conducted to a depth of 10 cm in the conventional tillage system under dryland cropping systems does not have much impact on soil organic and inorganic C contents compared with no-tillage even after 6 to 21 yr of management (Sainju et al., 2006, 2007). 943
Fig. 5. Effect of tillage and cropping sequence phases (Phase I and II) on soil surface CO2 flux from May to October, 2006 to 2008. CTB-F, conventional-tilled malt barley–fallow; NTB-F, no-tilled malt barley–fallow; NTB-P, no-tilled malt barley–pea; NTCB, no-tilled continuous malt barley. Letter in the parenthesis denotes corresponding phases in the cropping sequence : B, malt barley; F, fallow; P, pea. LSD (0.05) is the least significant difference between treatments in a measurement date at P ≤ 0.05.
Soil C storage is normally determined by the balance between C inputs from crop residues and rhizodeposition and C loss through CO2 emission as a result of root and microbial respiration and/or organic C mineralization, soil erosion, and leaching and surface runoff of water soluble C. Assuming that C loss through erosion, leaching, and runoff is negligible, C loss as CO2 emission from the soil from May to October in a year (1.97–2.48 Mg C ha−1, average from 2006 to 2008, Table 3) could be higher than C added from aboveground biomass (0.58–1.02 Mg C ha−1, average from 2006 to 2008, Table 2, assuming that crop residue contains 40% C). Considering that root and rhizodeposition C is 1.2 times greater than aboveground biomass (stems + leaves) C (Johnson et al., 2006), the total amount of C supplied by above- and belowground biomass and rhizodeposition in a year as influenced by the management practices in this experiment ranges from 1.28 to 2.42 Mg C ha−1. These values are lower than the values of C loss as CO2 emission from May to October in a year. Furthermore, C loss as CO2 emission from November to April has not been taken into Table 5. Soil bulk density and total C (STC) content (mean ± SD) from 0- to 120-cm depth averaged across treatments. Soil depth
Soil bulk density
STC content
cm
Mg m−3 1.39 ± 0.20 1.45 ± 0.15 1.61 ± 0.14 1.62 ± 0.17 1.63 ± 0.19 1.65 ± 0.20
Mg C ha−1 9.7 ± 2.1 9.4 ± 1.9 37.2 ± 4.0 142.1 ± 15.0 142.4 ± 16.4 114.4 ± 17.8
0–5 5–10 10–30 30–60 60–90 90–120 944
account because data were not available. The results show that C loss as CO2 emission could be higher than C inputs from crop residue and rhizodeposition, but the overall C loss did not alter soil C storage as influenced by 3 yr of management practices under dryland cropping systems in the northern Great Plains.
Conclusions Soil surface CO2 flux varied with treatments and measurement dates and increased from spring to summer as soil temperature increased, followed by a decline in the autumn in 2006 to 2008. Increased fluxes were noted following substantial precipitation when most of the differences among treatments in fluxes were pronounced. Annual cropping increased crop biomass returned to the soil and CO2 flux compared with crop–fallow, but the flux varied with crop species and fallow treatments. While cropping increased CO2 flux compared with fallow, and malt barley increased the flux compared with pea, the presence of the previous crop in the cropping phase also influenced CO2 flux due to differences in residue quality (C/N ratio) and quantity and soil water content. Tillage did not influence crop biomass and CO2 flux. Nitrogen fertilization increased crop biomass compared with no N fertilization in 2007 but had a varied effect on CO2 flux in 2008. Soil total C content was not influenced by treatment but increased with depth. The CO2 flux increased with precipitation and crop biomass among years. Although no-tilled crop–fallow reduced CO2 flux, annual cropping increased the flux probably by increasing root and microbial respiration and availability of C substrate. Using malt barley–pea instead of continuous malt barley in the no-tilled system, however, resulted in a CO2 flux that was Journal of Environmental Quality • Volume 39 • May–June 2010
similar to that in conventional-tilled malt barley–fallow. Since conventional tillage and fallow have been known to reduce soil C storage, no-tilled malt barley–pea rotation with reduced N fertilization rate to malt barley may be a management option to sustain CO2 emission in the dryland cropping systems. Evaluation of C balance with C added from crop residue and estimates of rhizodeposition vs. C loss as CO2 emission from the soil suggests that net C loss as CO2 emission did not alter soil C storage as influenced by 3 yr of management practices under dryland cropping systems in the northern Great Plains.
Acknowledgments We sincerely acknowledge the help provided by Johnny Rieger, Joy Lyn Barsotti, and Christopher Russell for collecting gas, soil, and plant samples in the field and analyzing them in the laboratory and Andy Lenssen, Mark Gaffri, Michael Johnson, and Randall Obergfell for managing the experimental plots. We also acknowledge USDA– ARS GRACEnet Project for providing part of the funds to conduct this experiment.
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