Dryland Crop Yields and Soil Organic Matter as Influenced by Long-Term Tillage and Cropping Sequence

Transcription

1 Iowa State University From the SelectedWorks of Andrew W. Lenssen March, 2009 Dryland Crop Yields and Soil Organic Matter as Influenced by Long-Term Tillage and Cropping Sequence Upendra M. Sainju, United States Department of Agriculture Andrew W. Lenssen, United States Department of Agriculture TheCan Caesar-TonThat, United States Department of Agriculture Robert G. Evans, United States Department of Agriculture Available at:

2 Dryland Crop Yields and Soil Organic Matter as Infl uenced by Long-Term Tillage and Cropping Sequence Upendra M. Sainju,* Andrew W. Lenssen, Thecan Caesar-TonThat, and Robert G. Evans ABSTRACT Novel management practices are needed to improve the declining dryland crop yields and soil organic matter contents using conventional farming practices in the northern Great Plains. We evaluated the 21-yr effect of tillage and cropping sequence on dryland grain and biomass (stems + leaves) yields of spring wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and pea (Pisum sativum L.) and soil organic matter at the 0- to 20-cm depth in eastern Montana, USA. Treatments were no-tilled continuous spring wheat (NTCW), spring-tilled continuous spring wheat (STCW), fall- and spring-tilled continuous spring wheat (FSTCW), fall- and spring-tilled spring wheat-barley ( ) followed by spring wheat-pea ( ) (FSTW-B/P), and the conventional spring-tilled spring wheat-fallow (STW-F). Spring wheat grain and biomass yields increased with crop growing season precipitation (GSP) and were greater in STW-F than in FSTCW and FSTW-B/P when GSP was <250 mm. Although mean grain and biomass yields were greater, annualized yields were lower in STW-F than in other treatments. In FSTW-B/P, barley and pea grain and biomass yields also increased with increased GSP. Soil organic C and total N were lower in STW-F than in other treatments and linearly related (R 2 = 0.64 to 0.78) with total annualized biomass residue returned to the soil from 1984 to Alternate-year summer fallowing increased spring wheat grain and biomass yields compared with annual cropping but reduced annualized yields and soil organic matter. For sustaining dryland crop yields and soil organic matter, no-tillage with annual cropping system can be adopted in the northern Great Plains. In the northern Great Plains, USA, the conventional spring-tilled spring wheat-fallow (STW-F) system is uneconomical due to the absence of crops during the fallow period (Aase and Schaefer, 1996; Dhuyvetter et al., 1996) and has contributed to the loss of soil organic matter (Aase and Pikul, 1995; Halvorson et al., 2002b). Fallowing is usually done to conserve soil water, control weeds, release plant nutrients, and increase succeeding crop yields (Aase and Pikul, 1995; Jones and Popham, 1997; Pikul et al., 1997), but extending the fallow period by reducing cropping intensity can reduce soil water storage efficiency, increase saline seeps development, and reduce soil organic matter contents (Tanaka and Aase, 1987; Black and Bauer, 1988). In the last 50 to 100 yr, STW-F has resulted in a decline of soil organic matter by 30 to 50% of their original levels (Mann, 1985; Peterson et al., 1998). Intensive tillage increases the oxidation of soil organic matter (Bowman et al., 1999; Schomberg and Jones, 1999) while fallowing reduces organic matter by reducing the amount of nonharvested plant residue returned to the soil (Black and Tanaka, 1997; Campbell et al., 2000). Therefore, improved soil and crop management practices are needed to sustain dryland crop yields, increase organic matter, and improve soil quality USDA-ARS, Northern Plains Agricultural Research Laboratory, 1500 North Central Avenue, Sidney, MT Received 3 Sept *Corresponding author (upendra.sainju@ars.usda.gov). Published in Agron. J. 101: (2009). doi: /agronj x Copyright 2009 by the American Society of Agronomy, 677 South Segoe Road, Madison, WI 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. and productivity to reduce the dependence of producers on federal aids transfer programs (Aase and Schaefer, 1996; Dhuyvetter et al., 1996). Great Plains dryland crop yields, when compared with rotations containing fallow, can be maintained or increased by using reduced tillage or increasing the cropping intensity (Aase and Pikul, 1995; Aase and Schaefer, 1996; Halvorson et al., 2002a). Aase and Pikul (1995) and Aase and Schaefer (1996) have found that no-tilled continuous spring wheat (NTCW) increased annualized spring wheat yields (mean spring wheat yield across crop rotation phases in a year) and soil organic matter and was the most profitable cropping system compared with STW-F. No-till increased spring and winter wheat yields compared with conventional till, especially during dry periods, due to increased water conservation as a result of increased residue accumulation at the soil surface (Halvorson et al., 1999; Campbell et al., 2004). Research conducted outside the United States also showed that no-till improved snow trap due to residues accumulated at the soil surface, and increased water and N-use efficiencies and crop yields compared with conventional till (Cantero-Martinez et al., 1999; Miller et al., 2002a; Huang et al., 2008). Improved water storage efficiency with reduced till allows producers to increase cropping intensity and crop yields compared with crop-fallow (Black et al., 1981; Halvorson and Reule, 1994). Crop yields in no-tilled annual cropping systems were generally equal to 80% of the yields in conventional-tilled crop-fallow systems during the crop year Abbreviations: FSTCW, fall- and spring-tilled continuous spring wheat; FSTW-B/P, fall- and spring-tilled spring wheat-barley ( ) followed by spring wheat-pea ( ); GSP, growing season precipitation; NTCW, no-tilled continuous spring wheat; STCW, spring-tilled continuous spring wheat; STW-F, spring-tilled spring wheat-fallow. Dryland Cropping Systems Agronomy Journal Volume 101, Issue

3 (Halvorson and Reule, 1994; Aase and Pikul, 1995; Lenssen et al., 2007a). Winter wheat yields in a winter wheat-corn (Zea mays L.)-fallow rotation was equal to or greater than those in winter wheat-fallow (Dhuyvetter et al., 1996). Crop diversification is one of the recommended practices as opposed to wheat monoculture in dryland cropping systems to reduce the risk of crop failure, farm inputs, and duration of fallow to improve economic and environmental sustainability (Matson et al., 1997; Struick and Bonciarelli, 1997; Gregory et al., 2002). Several pulse crops have been grown successfully in rotation with spring wheat to replace summer fallow in dryland cropping systems (Thomson et al., 1997; Miller et al., 2002a; Gan et al., 2003). Pulse crops not only increased succeeding crop yields but also their protein content (Miller et al., 2002a; Gan et al., 2003; Lenssen et al., 2007b). Among pulse crops, pea has been widely grown, especially in the northern Great Plains, due to its lower water requirement and increased benefit of increasing succeeding spring wheat yields and protein content (Lenssen et al., 2007a, 2007b). Information on the effects of tillage and cropping system on dryland crop yields and soil organic matter in the northern Great Plains is available (Aase and Pikul, 1995; Pikul et al., 1997; Halvorson et al., 2002b); however, little is known about their long-term (>20 yr) combined effects. Since the climate in the northern Great Plains is characterized by long cold winters, short but warm summers, large diurnal variations in temperature, and highly variable precipitation (Padbury et al., 2002), crop growth and yields vary from year to year (Stewart and Robinson, 1997). To reduce crop failure and assess the impact of climatic variability, long-term studies on the effects of tillage and cropping sequences on dryland crop yields and soil organic matter are needed. We hypothesized that, over the long term, reduced tillage with annual cropping system would sustain annualized dryland crop yields and soil organic matter contents compared with the conventional STW-F. Our objectives were Table 1. Monthly total precipitation from 1984 to 2004 near the study site 11 km north of Culbertson, MT. Total April August Total annual Year April May June July August mm yr avg The 105-yr average precipitation data were taken from Culbertson, MT, which is 11 km south of the study site. to: (i) evaluate the influence of tillage and cropping sequence combinations on dryland grain and biomass (stems + leaves) yields of spring wheat, barley, and pea from 1984 to 2004 and (ii) relate total annualized crop biomass residue returned to soil from 1984 to 2004 with soil organic C and total N contents at 0- to 20-cm depth in the northern Great Plains, USA. MATERIALS AND METHODS Site Description and Treatments The experiment was started by Aase and Pikul (1995) in The experimental site was located 11 km north of Culbertson (48 33 N, W) in eastern Montana, USA. 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 (105-yr average) is 340 mm, 70% of which occurs during the crop growing season (April August) (Table 1). The soil is a Dooley sandy loam (fine-loamy, mixed, frigid, Typic Argiboroll) with 0 to 2% slope. The soil sampled in 1983 before the initiation of the experiment had 645 g kg 1 sand, 185 g kg 1 silt, 170 g kg 1 clay, 1.50 Mg m 3 bulk density, 16.8 Mg ha 1 organic C, and 6.2 ph at the 0- to 8-cm depth (Aase and Pikul, 1995). Details of the experimental treatments and management were described by Aase and Pikul (1995) and Aase and Schaefer (1996). The treatments consisted of no-tilled continuous spring wheat (NTCW), spring-tilled continuous spring wheat (STCW), fall- and spring-tilled continuous spring wheat (FSTCW), fall- and spring-tilled spring wheat-barley ( ) followed by spring wheat-pea ( ) (FSTW-B/P), and spring-tilled spring wheat-fallow (STW-F). Spring wheat was planted annually in the spring in NTCW, STCW, and FSTCW. In FSTW-B/P, spring wheat-barley was the rotation from 1984 to 1999, after which barley was replaced by pea in 2000, forming spring wheat-pea rotation from 2000 to The STW-F represented the conventional farming system where spring wheat was planted in alternate year in the spring wheat-fallow sequence. Each phase of the crop rotation was present in every year. In STCW, plots were tilled with a sweep plow before spring wheat seeding to prepare a seedbed in the spring. In FSTCW and FSTW-B/P, plots were tilled with standard sweeps (0.45 m wide with medium crown) and rods in the fall, followed by tandem disk tillage in the spring to prepare the seedbed. In STW-F, plots were tilled with tandem disk before seeding in the spring, and sweep and rods during fallow periods three to four times to control weeds. All tilled plots were cultivated to a depth of 10 cm. In NTCW, plots were left undisturbed, except for applying fertilizers and seeding in rows. Weeds in NTCW were controlled by applying preplant and postharvest herbicides and in other treatments by a combination of herbicides and sweep tillage to a depth of 10 cm as needed. Treatments were arranged in a randomized complete block with four replications. Individual plot size was 12 by 30 m. Crop Management The rates of N, P, and K fertilizers to spring wheat, barley, and pea in all treatments were broadcast at the time of planting in the spring according to crop requirement under dryland conditions and soil NO 3 N test to a depth of Agronomy Journal Volume 101, Issue

4 cm. The fertilization rates were based on crop yield goals and protein content (spring wheat, 2350 kg ha 1 and 13%; barley, 2400 kg ha 1 and 12.5%; and pea, 1100 kg ha 1 and 20%) (Montana State University and North Dakota State University, 1997). As a result, from 1984 to 2004, N fertilization rates were adjusted every year according to soil NO 3 N levels at the 0- to 60-cm depth to provide total (soil + fertilizer) N to spring wheat and barley at 70 kg N ha 1. Nitrogen fertilizer was applied as urea (46% N) and monoammonium phosphate (18% N, 46% P). For pea in FSTW-B/P from 2000 to 2004, N fertilizer was applied at 5 kg N ha 1 when monoammonium phosphate was applied as P fertilizer. Long-term P requirements (from 1984 to 1996) were made by incorporating a single application of P fertilizer at 560 kg P ha 1 as monoammonium phosphate to a depth of 5 cm to spring wheat and barley in all treatments in 1983 (Aase and Pikul, 1995). From 1997 to 2004, P fertilizer was applied annually at 56 kg P ha 1 in all treatments. Potassium fertilizer was not applied from 1983 to 1996 because of abundant soil K levels to meet crop requirements (Aase and Pikul, 1995) but was applied at 48 kg K ha 1 as muriate of potash (60% K) from 1997 to 2004 in all treatments. Spring wheat [ Lew (unknown source) from 1984 to 1996 and McNeal (Foundation Seed, Montana State University, Bozeman, MT) from 1997 to 2004] was planted at 74 kg ha 1 ; barley [ Hector (unknown source) from 1984 to 1996 and Certified Tradition (Busch Agricultural Resources, Fargo, ND) from 1997 to 1999] at 84 kg ha 1 ; and pea [ Majoret (Macintosh Seed, Havre, MT) from 2000 to 2004] at 160 kg ha 1 in April of every year. These were planted using a doubledisk opener with a row spacing of 20 to 25 cm from 1984 to 1996 and a Versatile no-till drill from 1997 to Growing season weeds were controlled with selective postemergence herbicides appropriate for each crop. Contact herbicides were applied at postharvest and preplanting, and fallow plots were tilled with sweeps three to four times to control weeds. From 1984 to 1993 and in 1995, grain and biomass (stems + leaves) yields of spring wheat and barley were determined by cutting bundle samples from five 1-m long rows from six areas in July and August each year. The bundle samples were dried, weighed, threshed, and composited, from which grain and biomass yields were determined. In 1994 and from 1996 to 2004, grain yield was determined from a swath of 1.5 m width by 10 to 30 m long with a combine harvester in central rows. Biomass yield was measured by harvesting plants from an area of 0.5 by 1 m outside yield rows after separating grains from straw and ovendrying a subsample at 60 C for 3 d. After completing grain harvest from the rest of the plots, biomass residues containing stems and leaves were returned to the soil. Harvest index was calculated as grain yield/biomass yield. Soil Sample Collection and Analysis In October 2004, after 21 yr of tillage and cropping sequence treatments, soil samples were collected with a hand probe (5 cm i.d.) from the 0- to 20-cm depth from five places in the central rows of the plot and composited. Samples were collected after clearing crop residues at the soil surface. Samples were airdried, ground, and sieved to 2 mm for determining C and N concentrations. Two additional soil cores (5 cm i.d.) were taken from the 0- to 20-cm depth and composited to determine bulk density by dividing mass of the oven-dried soil at 105 o C by the volume of the probe. Soil organic C and total N concentrations in the samples were determined by using a dry combustion C and N analyzer (LECO Corp., St Joseph, MI) after grinding to <0.5 mm and pretreating the soil with 5% H 2 SO 3 to remove inorganic C (Nelson and Sommers, 1996). Soil organic C and total N contents (Mg C or N ha 1 ) were calculated by multiplying their concentrations (g C or N kg 1 ) by bulk density and thickness of the soil layer. Data Analysis Data for grain and biomass yields of spring wheat, barley, and pea from 1984 to 2004 as influenced by treatments were analyzed using the MIXED procedure of SAS with GSP considered as a split-plot and year as a repeated measure variable (Littell et al., 1996). Treatment was considered as the fixed effect and replication as the random effect. Similarly, data for soil organic C and total N contents in 2004 were analyzed using the MIXED procedure with treatment as the fixed effect and replication as the random effect. Means were separated by using the least square means test when treatments were significant (Littell et al., 1996). Statistical significance was evaluated at P 0.05, unless otherwise stated. Linear regression analysis was used to relate total annualized crop biomass residue returned to the soil from 1984 to 2004 and soil organic C and total N contents in For this, annualized biomass yield in FSTW-B/P was calculated by averaging spring wheat and barley or pea biomass yield in a year. Similarly, annualized biomass yield in STW-F was calculated by dividing spring wheat biomass by two, since biomass was returned to the soil once in two years in this treatment. Total annualized biomass yield in each treatment was calculated as the sum of annualized biomass yields from 1984 to RESULTS AND DISCUSSION Precipitation During the crop growing season (April August), monthly, annual, and crop growing season precipitations (GSP) varied among years (Table 1). The GSP was below the 105-yr average in 10 out of 21 yr (1984, 1985, 1987, 1990, 1995 to 1999, and 2002) and annual precipitation was below the average in 9 out of 21 yr (1984, 1985, 1988, 1990, 1994, 1995, 1997, 1999, and 2002). Similarly, monthly precipitation measured for 21 yr was below the average in April in 16 yr, in May in 12 yr, in June in 13 yr, in July in 6 yr, and in August in 12 yr. Since most dryland crops grow rapidly in May and June, precipitations during these months are critical for crop production, as soil water availability determine crop yields. Without adequate soil water availability, especially during these periods, crops are bound to fail, resulting in poor yields. Crop Grain Yields Spring wheat grain yields were strongly influenced by tillage and cropping sequence, year, and GSP (Table 2). Interactions were significant for tillage and cropping sequence year, tillage and cropping sequence GSP, year GSP, and tillage and cropping sequence year GSP. Because of the significant tillage and cropping sequence year GSP interaction, spring Agronomy Journal Volume 101, Issue

5 Table 2. Effects of tillage and cropping sequence combination, year, and growing season precipitation on mean spring wheat grain and biomass (stems + leaves) yields at the study site 11 km north of Culbertson, MT. Grain yield Biomass yield Harvest index Tillage and cropping sequence Mg ha 1 NTCW STCW FSTCW FSTW-B/P STW-F LSD (0.05) NS Contrast NT vs. ST + FST in CW Signifi cance P value Tillage and cropping sequence (TCS) <0.001 < Year (Y) <0.001 < TCS Y Growing season precipitation (GSP) <0.001 < TCS GSP Y GSP <0.001 < TCS Y GSP Tillage and cropping sequence are FSTCW, fall- and spring-tilled continuous spring wheat; FSTW-B/P, fall- and spring-tilled spring wheat-barley ( ) followed by spring wheat-pea ( ); NTCW, no-tilled continuous spring wheat; STCW, spring- tilled continuous spring wheat; and STW-F, spring-tilled spring wheat-fallow. NS, not signifi cant at P Contrast is shown as difference in mean values of the treatments as [NTCW- (STCW + FSTCW)/2]. Abbreviations used in contrast statement are CW, continuous spring wheat; FST, fall and spring till; NT, no-till; and ST, spring till. Table 3. Effects of year and growing season precipitation (GSP) on mean spring wheat grain and biomass (stems + leaves) yields at the study site 11 km north of Culbertson, MT. Parameter Grain yield Biomass yield Harvest index Mg ha 1 Year LSD (0.05) NS Average GSP <200 mm mm >250 mm LSD (0.05) NS NS, not signifi cant at P Fig. 1. Effect of tillage and cropping sequence combination on spring wheat grain yields from 1984 to 2004 at the experimental site, 11 km north of Culbertson, MT. Yields were divided according to (A) <200 mm, (B) mm, and (C) >250 mm GSP (crop growing season precipitation from April to August) in years. FSTCW represents fall- and spring-tilled continuous spring wheat; FSTW-B/P, fall- and spring-tilled spring wheat-barley ( ) followed by spring wheatpea ( ); NTCW, no-tilled continuous spring wheat; STCW, spring-tilled continuous spring wheat; and STW-F, spring-tilled spring wheat-fallow. The LSD (0.05) bar is the least significant difference between treatments at P = wheat grain yields among treatments and years were grouped according to GSP (Fig. 1). Grain yields normally increased with GSP (Table 3). With <200 mm GSP, grain yield averaged 0.8 Mg ha 1 for all treatments. Grain yield was usually greater in STW-F than in other treatments in most years, except in 1985, 1990, and 1996 when grain yield was not significantly different between STW-F and NTCW (Fig. 1A). A hailstorm damaged crops in 1995, resulting in lower grain yields in all treatments in that year. The greater yield with STW-F, however, declined as GSP increased above 200 mm. With 200 to 250 mm GSP, grain yield for all treatments averaged 2.1 Mg ha 1 (Table 3) and was greater in STW-F than in FSTW-B/P in 1989, 1994, and 1997 (Fig. 1B). With >250 mm GSP, grain yield for all treatments averaged 2.6 Mg ha 1 and was greater in STW-F than in other treatments in 1987, 1991, 1992, 1993, and 2000 (Fig. 1C). Similarly, grain yield was greater in FSTW-B/P and STW-F than in other treatments in 2001 and 2003 and greater than in FSTCW in Mean spring wheat grain yield across years was greater in STW-F than in other treatments (Table 2). Grain yield was 17% lower in NTCW than in STW-F in the crop year. Tillage did not influence grain yields in the continuous spring wheat 246 Agronomy Journal Volume 101, Issue

6 system. Mean grain yield across treatments was greater in 1986 than in other years (Table 3). Even with >200 mm GSP (Table 1), yields were lower in 1989 and In FSTW-B/P, barley grain yield was greater in 1986 than in other years (Table 4), a case similar to that observed for spring wheat yield. Similarly, pea grain yield was greater in 2004 than in 2000, 2002, and 2003, showing that even with GSP > 250 mm (except in 2002), factors other than GSP may influence pea yields. Averaged across treatments and years, mean crop yields were greater in spring wheat (1.91 Mg ha 1 ) and barley (1.78 Mg ha 1 ) than in pea (1.27 Mg ha 1 ) (Tables 3 and 4). The greater spring wheat grain yield in STW-F than in other treatments (Fig. 1, Table 2) was attributed to soil water conservation during fallow. Studies have shown that fallowing conserves soil water by reducing plant transpiration during the fallow year (Aase and Pikul, 1995; Farhani et al., 1998; Lenssen et al., 2007a). Although tillage overall did not influence grain yield in the continuous spring wheat system (Table 2), greater grain yield in NTCW than in STCW or FSTCW in 7 out of 21 yr (1985, 1990, 1991, 1996, 1999, 2001, and 2004) (Fig. 1) suggests that no-till increased spring wheat grain yields compared with tilled treatments probably by increasing wateruse efficiency and/or reduced evaporation losses (Farhani et al., 1998; Huang et al., 2008). This was noticeable especially during <200 mm GSP when grain yield was greater in NTCW than in STCW or FSTCW in 3 out of 6 yr (Fig. 1A). This suggests that no-till may play important roles in conserving soil water and maintaining grain yields especially during lower GSP (Lenssen et al., 2007a). Halvorson et al. (2000) reported similar results of greater spring wheat yield in no-till than in conventional till when GSP was <400 mm in North Dakota. Since barley was replaced by pea in rotation with spring wheat in FSTW-B/P in 2000, greater spring wheat grain yield in FSTW-B/P than in STCW and FSTCW in 2001 and 2003 or greater than in FSTCW in 2004 was probably due to (i) reduced incidences of diseases, pests, and weeds, (ii) soil N enrichment by pea residue because of its higher N concentration, and/or (iii) increased soil water content due to low water use by pea (Miller et al., 2002a). This was especially true when GSP was >250 mm (Fig. 1C). Pea uses less water than spring wheat or barley, thereby leaving more water available for succeeding crops and influencing their yields (Miller et al., 2002a; Lenssen et al., 2007a). Lenssen et al. (2007a) reported that spring wheat yield was 15% greater following three pulse crops than following spring wheat in Montana, USA. Similarly, Miller et al. (2002a, 2002b) reported that spring wheat yield was 21 37% greater following pulse crops than following spring wheat in Saskatchewan, Canada. Sainju et al. (2007) found that soil NO 3 N content at the 0- to 20-cm depth was greater in spring wheat-pea rotation than in continuous spring wheat in Montana. It could be possible that with >250 mm GSP, pea residue with higher N concentration than spring wheat, mineralized rapidly, thereby enriching soil N and increasing succeeding spring wheat yields. When GSP was <250 mm, such as in 2002 (Fig. 1B), pea residue may have mineralized slowly due to lack of adequate soil water, thereby having minimal effect on spring wheat yields. This is similar to those observed by Pikul et al. (1997) in eastern Montana, and Nielsen and Vigil (2005) in northeastern Colorado, who Table 4. Grain and biomass (stems + leaves) yields of barley and pea in rotation with spring wheat in the fall- and springtilled spring wheat-barley ( ) followed by spring wheat-pea ( ) (FSTW-B/P) treatment at the study site 11 km north of Culbertson, MT. Year Grain yield Biomass yield Harvest index Mg ha 1 Barley LSD (0.05) Average Pea LSD (0.05) NS Average NS, not signifi cant at P reported that spring wheat yields following legumes were similar to or lower than following spring wheat or fallow. Snow trap in the winter, however, was lower with pea residue than with spring wheat or barley residue because of its poor biomass yield and rapid decomposition in the soil (Miller et al., 2002a). Overall grain yield in FSTW-B/P was not different from yields in NTCW, STCW, and FSTCW (Table 2). More studies are needed to evaluate the complex effects of legumes and GSP on dryland spring wheat yields in the northern Great Plains. Although mean grain yield across years was greater in STW-F than in other treatments (Table 2), mean annualized spring wheat grain yield (grain yield/2) was lower in FSTW- B/P (0.98 Mg ha 1 ) and STW-F (1.26 Mg ha 1 ) than in other treatments. This is because spring wheat was grown once in 2 yr in FSTW-B/P and STW-F compared with that grown in every year in other treatments. While reduced return from spring wheat production can be compensated by increased returns from barley or pea production in FSTW-B/P, absence of crops in the fallow year reduces yields and returns in STW-F. Lower annualized spring wheat grain yields and net return in crop-fallow than in annual cropping, regardless of tillage, in drylands of the northern and central Great Plains, USA, were reported by various researchers (Aase and Pikul, 1995; Aase and Schaefer, 1996; Halvorson et al., 2002a). Aase and Schaefer (1996) reported that STW-F had the lowest and NTCW had the highest net return than STCW, FSTCW, and FSTW- B/P. Therefore, NTCW can not only sustain dryland spring wheat yields but also improve the potentials for increased net returns and reduced soil erosion due to increased residue cover Agronomy Journal Volume 101, Issue

7 Fig. 2. Effect of tillage and cropping sequence combination on spring wheat biomass (stems + leaves) yields from 1994 to 2004 at the experimental site, 11 km north of Culbertson, MT. Yields were divided according to (A) <200 mm, (B) mm, and (C) >250 mm GSP (crop growing season precipitation from April to August) in years. FSTCW represents falland spring-tilled continuous spring wheat; FSTW-B/P, fall- and spring-tilled spring wheat-barley ( ) followed by spring wheat-pea ( ); NTCW, no-tilled continuous spring wheat; STCW, spring-tilled continuous spring wheat; and STW-F, spring-tilled spring wheat-fallow. The LSD (0.05) bar is the least significant difference between treatments at P = at the soil surface. Our mean annualized spring wheat yields of 0.98 to 2.10 Mg ha 1 were slightly lower than the reported ranges of 1.12 to 2.23 Mg ha 1 for crop-fallow and continuous cropping systems, respectively, in eastern Montana and North Dakota, USA (Aase and Pikul, 1995; Aase and Schaefer, 1996; Halvorson et al., 2000) and in Victoria, Australia (O Connell et al., 2002). Variations in the amount and distribution of precipitation during the crop growing season (April to August) (Table 1) probably reflected differences in spring wheat, barley, and pea grain yields among years (Tables 3 and 4). The greater spring wheat grain yields in 1986 and 2004 than in other years (Table 3) were attributed to above-average GSP (Table 1). Similar results were obtained for barley and pea yields in FSTW- B/P, with greater barley yield in 1986 and pea yield in 2004 than in other years (Table 4). Although spring wheat grain yields generally increased with GSP (Table 3), factors other than GSP also may have influenced grain yields. For example, the causes for lower spring wheat grain yields in 1989, 1999, and 2003 (Fig. 1) were unknown. Crop Biomass Yields Spring wheat biomass yields also varied with treatments, years, and GSP, similar to grain yields (Figs. 1 and 2, Tables 2 and 3). Biomass yield was greater in FSTCW, FSTW-B/P, and STW-F than in STCW in 1999, greater in STW-F than in other treatments 2001, and greater in STW-F than in FSTCW and FSTW-B/P in 2002 and 2004 (Fig. 2). Mean biomass yield across years was greater in STW-F than in other treatments (Table 2). As with grain yield, biomass yield in NTCW was 85% of that in STW-F in the crop year. Mean biomass yield across treatments was greater in 2004 than in other years and increased with increased GSP (Table 3). The spring wheat harvest index was not influenced by treatment, year, GSP, and their interactions, and averaged 0.54 (Tables 2 and 3). Tillage did not influence biomass yield and harvest index in the continuous spring wheat system (Table 2). In FSTW-B/P, barley and pea biomass yields also varied with years, similar to their grain yields (Table 4). Barley biomass was greater in 1994 than in other years. The barley harvest index was greater in 1993 than in other years, except in 1987 and Pea biomass was greater in 2004 than in 2000, 2002, and Pea harvest index was not influenced by year and averaged Averaged biomass across treatments and years among crops followed trend similar to grain yield (spring wheat = barley > pea). Total biomass production was similar to grain yields (Fig. 1 and 2). As a result, using a constant harvest index, the yield of one component may be used to estimate the yield of the other in dryland spring wheat production. In some years with >200 mm GSP (1986, 1987, and 1992), however, greater grain yields than biomass yields resulted in higher harvest index (Tables 1 and 3). Harvest index in barley in FSTW-B/P, unlike in spring wheat, was greater in years with >250 mm GSP (1987, 1991, and 1993) (Tables 1 and 4), suggesting improved grain yield compared with biomass yield when soil water content is adequate. Overall harvest index in barley was, however, similar to that in spring wheat. Average harvest index in pea was also similar to those in spring wheat and barley. Pea harvest index was not significantly different among years, a case similar to that observed in spring wheat. Although spring wheat biomass yield was greater in STW-F than in other treatments (Table 2), annualized biomass yield, similar to grain yield, was significantly lower in this treatment (2.26 Mg ha 1 ) because of the absence of crop during the fallow year. In contrast, annualized biomass yield in FSTW- B/P (3.58 Mg ha 1 ) was not significantly different from those in NTCW, STCW, and FSTCW. This may have important implications on the effect of treatments on soil quality and productivity, as discussed below, since biomass residues were returned to the soil. Relationship between Crop Biomass Residue and Soil Organic Matter After 21 yr of treatments, soil organic C at the 0- to 20-cm depth was lower in STW-F than in other treatments (Table 5). In contrast, soil total N was greater in NTCW, STCW, and FSTCW than in STW-F and greater in NTCW and STCW than in FSTW-B/P. Similarly, C/N ratio was greater in FSTW-B/P and STW-F than in NTCW. While tillage did not 248 Agronomy Journal Volume 101, Issue

8 Table 5. Effect of tillage and cropping sequence combination on soil organic C and total N contents at the 0- to 20-cm depth in 2004 at the study site 11 km north of Culbertson, MT. Tillage and cropping sequence Soil organic C Soil total N C/N ratio Mg C ha 1 Mg N ha 1 NTCW 30.0a 2.56a 11.7b STCW 30.0a 2.43a 12.3ab FSTCW 28.3a 2.30ab 12.3ab FSTW-B/P 27.0a 1.99bc 13.6a STW-F 21.6b 1.60c 13.5a Contrast NT vs. ST + FST in CW * 0.6 Tillage and cropping sequence are FSTCW, fall- and spring-tilled continuous spring wheat; FSTW-B/P, fall- and spring-tilled spring wheat-barley ( ) followed by spring wheat-pea ( ); NTCW, no-tilled continuous spring wheat; STCW, spring-tilled continuous spring wheat; and STW-F, spring-tilled spring wheat-fallow. Numbers followed by different letters within a column are signifi cantly different at P 0.05 by the least square means test. Contrast is shown as difference in mean values of the treatments as [NTCW- (STCW + FSTCW)/2]. Abbreviations used in the contrast statement are CW, continuous spring wheat; FST, fall and spring till; NT, no-till; and ST, spring till. * Differences between treatments are signifi cant at P influence soil organic C in the continuous spring wheat system, soil total N was lower in tilled than in no-tilled treatments. Differences in the amount of annualized crop biomass residue among treatments resulted in a linear relationship between total annualized biomass residue returned to the soil from 1984 to 2004 and soil organic C and total N in 2004 (R 2 = 0.68 to 0.78, P 0.05, n = 20) (Fig. 3). An increase in biomass residue by 1 Mg ha 1 increased soil organic C by 0.28 Mg C ha 1 and total N by 0.04 Mg N ha 1. Biomass residue explained 68 to 78% of the variability in soil organic C and total N. The effect of crop residues in soil organic C and total N was clearly demonstrated by strong relationships between them. Crop residues serve as C and N inputs to the soil. As the amount of residue addition increased, soil organic C and total N also increased, provided that increased aboveground residue production also increased belowground (root) residue. Since biomass residue accounted for most of the variability in soil organic C and total N, the unexplained variability in their relationships could have resulted from some other factors, such as tillage. Although tillage did not influence soil organic C, it reduced soil total N (Table 5). Tillage reduces soil organic C and total N by incorporating crop residues, disrupting soil aggregation, and increasing aeration (Franzluebbers et al., 1999; Halvorson et al., 2002b). The lower soil organic C and total N in STW-F than in other treatments was probably due to lower amount of annualized biomass residue returned to the soil, followed by its increased decomposition due to tillage and fallow. This is consistent with the findings reported by several researchers in the northern (Aase and Pikul, 1995; Halvorson et al., 2002b; Sainju et al., 2006) and central Great Plains, USA (Peterson et al., 1998; Halvorson et al., 2002a). In contrast, greater soil total N in NTCW and STCW than in FSTW-B/P likely resulted from reduced mineralization due to decreased tillage frequency, followed by a difference in residue quality (C/N ratio), although annualized crop biomass residue returned to the soil in these treatments was similar. Legumes, such as pea, in FSTW-B/P, with lower C/N ratio than nonlegumes, such as wheat, mineralize rapidly in the soil, thereby Fig. 3. Relationship betwe en total annualized crop biomass (stems + leaves) residues returned to the soil from 1984 to 2004 and (A) soil organic C and (B) soil total N contents at the 0- to 20-cm depth in 2004 at the experimental site, 11 km north of Culbertson, MT. resulting in lower soil total N (Kuo et al., 1997). Increased soil organic C and total N contents will ultimately improve soil quality and productivity that are essential for sustaining crop productivity and improving environmental quality (Bauer and Black, 1994). The greater C/N ratio in FSTW-B/P and STW-F than in NTCW (Table 5) indicates that tillage and fallow probably mineralize soil total N more rapidly than soil organic C in the dryland cropping systems in the northern Great Plains. Since the depths at which initial (0- to 8-cm in 1983) and final (0- to 20-cm in 2004) soil organic C contents measured were different, it is difficult to measure C sequestration rates due to long-term tillage and cropping sequence. Although Aase and Pikul (1995) indicated that soil organic C was measured to a depth of 15 cm in 1983, they have reported data only for 0- to 8-cm. Estimating initial soil organic C concentration of 9.5 g kg 1 and bulk density of 1.51 Mg ha 1 at 8- to 20-cm (as found in the undisturbed grassland soil near the experiment), organic C content at 8- to 20-cm would be 17.2 Mg C ha 1. Summed up, initial organic C content at 0- to 20-cm would be 34.0 Mg C ha 1 (16.8 Mg C ha 1 at 0- to 8-cm Mg C ha 1 at 8- to 20-cm). This would result in C sequestration rates of 182, 182, 259, 318, and 563 kg C ha 1 yr 1 at the 0- to 20-cm depth for NTCW, STCW, FSTCW, FSTW-B/P, and STW-F, respectively. Considering the C/N ratio of the soil as 11.0, initial soil total N content at 0- to 20-cm would be 3.09 Mg N ha 1. This would result in N sequestration rates of 24, 30, 36, 50, and 68 kg N ha 1 yr 1 at the 0- to 20-cm Agronomy Journal Volume 101, Issue

9 depth for NTCW, STCW, FSTCW, FSTW-B/P, and STW-F, respectively. These values indicate that both soil organic C and total N decreased with time in the dryland cropping system of the northern Great Plains, regardless of tillage and cropping sequences. However, the conventional STW-F system resulted in 2.8 to 3.1-fold greater organic C and total N losses than NTCW. Aase and Pikul (1995) similarly noted that soil organic C content at 0- to 8-cm in this experiment decreased from 1983 to 1993 in all treatments, with negligible decline in NTCW and 480 kg C ha 1 yr 1 in STW-F. Although reduced amount of crop residue returned to the soil, followed by tillage and fallow could have resulted in greater losses of soil organic C and total N in STW-F, the reasons for losses of soil organic C and total N with time in other treatments were not known. CONCLUSIONS Dryland spring wheat grain and biomass yields varied with GSP and tillage and cropping sequence combinations in the semiarid northern Great Plains, USA. Although grain and biomass yields normally increased with GSP, they were greater in STW-F than in annual cropping systems, especially when GSP was <200 mm. However, annualized grain and biomass yields were lower in STW-F than in other treatments. Inclusion of pea in the crop rotation using tillage increased spring wheat yields when GSP was >250 mm. Total annualized crop biomass residue returned to soil from 1984 to 2004 was linearly related to soil organic C and total N after 21 yr, where lower amount of crop residue in the STW-F reduced these soil parameters. The no-tilled annual cropping system can be used to sustain dryland spring wheat yields and improve soil organic matter compared with the conventional STW-F, thereby reducing the potentials for soil erosion and energy and improving soil quality in the semiarid regions. Although spring wheat yields following barley were similar to following spring wheat, wheat yields can be sustained by including pea in the crop rotation when GSP was >250 mm. ACKNOWLEDGMENTS We sincerely acknowledge the help provided by Johnny Rieger and Lynn Solberg for collecting soil samples, Johnny Rieger for analyzing soil samples, and Rene France for maintaining crop data records. We also acknowledge J. Kristian Aase and Joseph Pikul, Jr. for conceiving, establishing, and maintaining experimental plots for the first ten years of this study. REFERENCES Aase, J.K., and J.L. Pikul, Jr Crop and soil response to long-term tillage practices in the northern Great Plains. Agron. J. 87: Aase, J.K., and G.M. 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10 Agron. J. 94: Miller, P.R., J. Waddington, C.L. McDonald, and D.A. Derksen. 2002b. Cropping sequence affects wheat productivity on the semiarid northern Great Plains. Can. J. Plant Sci. 82: Montana State University and North Dakota State University Agricultural Research Update. Regional Rep. No. 2. MSU, Eastern Agricultural Research Center, Sidney, MT. Nelson, D.W., and L.E. Sommers Total carbon, organic carbon, and organic matter. p In D.L. Sparks et al. (ed.) Methods of soil analysis. Part 3. Chemical methods. SSSA Book Ser. 5. SSSA, Madison, WI. Nielsen, D.C., and M.F. Vigil Legume green fallow effect on soil water content at wheat planting and wheat yield. Agron. J. 97: O Connell, M.G., D.J. Connor, and G.J. O Leary Crop growth, yield, and water use in long-term fallow and continuous cropping sequences in the Victorian milee. Aust. J. Exp. Agric. 42: Padbury, G., S. Waltman, J. Caprio, G. Coen, S. McGinn, D. Mortensen, G. Nielsen, and R. Sinclair Agroecosystems and land resources of the northern Great Plains. Agron. J. 94: Peterson, G.A., A.D. Halvorson, J.L. Havlin, O.R. Jones, D.G. Lyon, and D.L. Tanaka Reduced tillage and increasing cropping intensity in the Great Plains conserve soil carbon. Soil Tillage Res. 47: Pikul, J.L., Jr., J.K. Aase, and V.L. Cochran Lentil green manure as fallow replacement in the semiarid northern Great Plains. Agron. J. 89: Sainju, U.M., A. Lenssen, T. Caesar-Tonthat, and J. Waddell Tillage and crop rotation effects on dryland soil and residue carbon and nitrogen. Soil Sci. Soc. Am. J. 70: Sainju, U.M., A. Lenssen, T. Caesar-Tonthat, and J. Waddell Dryland plant biomass and soil carbon and nitrogen fractions on transient land as influenced by tillage and crop rotation. Soil Tillage Res. 93: Schomberg, H.H., and O.R. Jones Carbon and nitrogen conservation in dryland tillage and cropping systems. Soil Sci. Soc. Am. J. 63: Stewart, B.A., and C.A. Robinson Are agroecosystems sustainable in semiarid regions? Adv. Agron. 60: Struick, P.C., and F. Bonciarelli Resource use at the cropping system level. Eur. J. Agron. 7: Tanaka, D.L., and J.K. Aase Fallow method influences on soil water and precipitation storage efficiency. Soil Tillage Res. 9: Thomson, B.D., K.H.M. Siddique, M.D. Barr, and J.M. Wilson Grain legume species in low rainfall Mediterranean-type environments. Field Crops Res. 54: Agronomy Journal Volume 101, Issue