Increasing SOC has been proposed as a potential way to slow the rising atmospheric

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1 Published January, 2010 Soil & Water Management & Conservation Tillage and Residue Removal Effects on Soil Carbon and Nitrogen Storage in the North China Plain Zhangliu Du Tusheng Ren* Dep. of Soil and Water Sciences China Agricultural University Beijing, China Chunsheng Hu Center for Agricultural Resources Research Institute of Genetic and Developmental Biology Chinese Academy of Science Shijiazhuang, China Little information is available about their influences of conservation tillage on the distribution and storage of soil organic C (SOC) and total N in soil profiles in the North China Plain. We investigated the changes in SOC and total N as related to the shift from conventional to conservation tillage using a long-term field experiment with a winter wheat (Triticum aestivum L.) corn (Zea mays L.) double cropping system. The experiment included four tillage treatments for winter wheat: moldboard plow without corn residue return (MP R), moldboard plow with corn residue return (MP+R), rotary tillage (RT), and no-till (NT). Compared with the MP R treatment, returning crop residue to the soil (MP+R, RT, and NT) increased SOC and total N in the 0- to 30-cm soil layer, but no distinct changes in SOC and total N concentration were observed among the four treatments at soil depths >30 cm. Compared with the MP+R treatment, the RT and NT treatments increased SOC and total N concentration significantly in the 0- to 10- cm layer but decreased SOC and total N concentration in the 10- to 20-cm layers. As a consequence, soil profile SOC and total N storage did not vary among the MP+R, RT, and NT treatments. Thus under the experimental conditions, conservation tillage (RT and NT) increased SOC and total N contents in the upper soil layers, but did not increase SOC and total N storage over conventional tillage (MP+R) in the soil profile. Abbreviations: MP R, moldboard plow without corn residue return; MP+R, moldboard plow with corn residue return; NT, no-till with corn residue; RT, rotary tillage with corn residue; SOC, soil organic carbon; SOM, soil organic matter; SR, stratification ratio. Increasing SOC has been proposed as a potential way to slow the rising atmospheric CO 2 concentrations caused by burning fossil fuels (Paustian et al., 1997; Lal, 2004). Some researchers have recommended conservation tillage as an effective practice for improving SOC in agricultural ecosystems (Lal and Kimble, 1997; Lal, 2004). For example, using a global database of 67 long-term experiments, West and Post (2002) estimated that a change from conventional tillage to NT could sequester an average of 57 ± 14 g C m 2 yr 1. Ussiri and Lal (2009) reported that in comparison to moldboard plowing and chiseling, NT significantly increased the SOC pool in the top 0- to 30-cm layer after 43 yr of continuous corn. The increase was attributed largely to the addition of corn residue. Therefore, using crop residue for biomass energy production could have detrimental effects on SOC storage (Wilhelm et al., 2004; Lal, 2009). Other studies indicated that SOC and total N stocks did not increase under NT compared with conventional tillage, but they were more stratified under NT (Angers et al., 1997; Wander et al., 1998; Dolan et al., 2006; Gál et al., 2007; Yang et al., 2008). Considering the entire soil profile, the net effect of NT on SOC and total N storage may be negative (e.g., Blanco-Canqui and Lal, 2008) or neutral (Angers et al., 1997; Yang and Wander, 1999; Machado et al., 2003; Dolan et al., 2006; Blanco- Canqui and Lal, 2008). Angers and Eriksen-Hamel (2008) compiled the available literature (23 studies with 47 site comparisons) in which SOC ( 30 cm) had been measured under paired NT and full-inversion tillage (FIT, the plowing depth was about 23 cm) for >5 yr. They showed that in most cases NT had significantly greater SOC storage than FIT in the surface layer, whereas at the 21- to 25-cm layer and This research was funded by the Basic Research Development Program of China (973 Program, no. 2009CB118607) and the National Science and Technology Supporting Programs of China (no. 2006BAD02A15 and 2006BAD15B02). Soil Sci. Soc. Am. J. 74: Published online 13 Nov doi: /sssaj Received 4 Feb *Corresponding author (tsren@cau.edu.cn). Soil Science Society of America, 677 S. Segoe Rd., Madison WI 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. 196 SSSAJ: Volume 74: Number 1 January February 2010

2 the layer below the plowing depth (about cm), the average SOC stock was significantly greater under FIT than NT. These studies clearly indicate that continuous NT practice leads to profile stratification of soil organic matter (SOM), with greater accumulation in the upper soil layer. The degree of SOC and N pool stratification with depth, expressed as a ratio, could indicate soil ecosystem function (Franzluebbers, 2002). Furthermore, the influences of tillage system on SOC and total N storage varied with the soil depth considered. According to West and Post (2002), global data sets of tillage system effects on SOC concentration and storage have focused more on the plow layer (0 30-cm depth), providing an overemphasis on C changes in the near-surface zones. Several studies (e.g., Baker et al., 2007; Gál et al., 2007) questioned the sampling methodology that had been used in nearly all of these tillage studies, in which only the surface soil ( 30 cm) was taken. This sampling protocol has restricted the available information of tillage consequences on the SOM status of the deeper soil profile. Because tillage systems can greatly alter the mass and distribution of soil properties under the tilled layers, the studies that do not consider these lower layers might have biased the results. To clarify the effects of conservation tillage systems on SOC and total N storage, the soil layers deeper than 30 cm should be considered. The North China Plain, with an area about 35 million ha and a population of 130 million, is one of the most important agricultural regions in China. The predominant cropping system in the region is winter wheat summer corn double cropping. Traditional tillage in this region involves moldboard plowing to a depth of about 18 cm, followed by a sequence of harrowing, smoothing, and rolling. These operations are done after all crop residues have been removed for use as fodder for animals or as fuel for cooking and heating. Removal of the aboveground crop residue and tillage may lead to SOC depletion and subsequent soil quality degradation (Wang et al., 2007a). Recently, conservation tillage systems have gradually become more accepted by the farmers due to the economic benefits (less labor, time, energy, and machinery costs) and soil and water conservation. It is widely believed that the alternative tillage systems are beneficial to SOC sequestration. For example, Li et al. (2007) measured CO 2 fluxes during a winter wheat growing season under different tillage systems. They observed that NT and rotary tillage treatments reduced CO 2 emissions relative to conventional tillage, and concluded that these conservation tillage systems improved soil C sequestration, although they did not directly measure SOC storage in the soil. A comparative 2-yr field study by Wang et al. (2007b) indicated that NT resulted in an accumulation of SOC in the 0- to 10-cm surface soil layer. Unfortunately, these conclusions were made from indirect estimations or short-term tillage experiments. Since the changes of SOC in response to changes in tillage systems can take several years to develop, the conclusion with respect to C sequestration based on shorter term tillage experiments might be biased. The objective of this study was to determine the long-term (7-yr) influences of transforming conventional tillage systems to conservation tillage systems on the SOC and total N distribution and storage in the soil profile under a wheat corn double cropping system. MATERIALS AND METHODS Site Description The study was conducted at the Luancheng Agroecosystem Experimental Station (37 53 N, E, elevation 50 m) of the Chinese Academy of Sciences, located on the piedmont of the Taihang Mountains. The average annual air temperature is about 12.2 C, and the annual mean precipitation is about 536 mm, with 70% of it occurring from July to September. The soil texture is silt loam (13.8% sand, 66.3% silt, and 19.9% clay) (an Argic Rusty Ustic Cambisol, Gong, 1999). Winter wheat (early October early June) and corn (mid-june later September) double cropping is the prevalent cropping system in the region. Experimental Design and Management The tillage experiment was established during the winter wheat season in Initially, there were three tillage and crop residue management treatments: the conventional practice of moldboard plowing (MP+R) and two conservation systems, rotary tillage and no-till. For the MP+R treatment, after corn harvest, all crop residue was chopped into small pieces (5 10-cm long) and then incorporated into the soil using a moldboard plow, with a tillage depth of about 18 cm. Chopping crop residue into small pieces is a common practice because the small size tractors (normally <15 horsepower) are not able to complete tillage operations with large-size corn stalks in the field. Winter wheat seeding and fertilizer application were accomplished using a conventional seeder (15-cm row spacing, 3 5-cm seeding depth). For the RT treatment, after corn harvest, all crop residue was chopped into small pieces (5 10-cm long), and a rotary tiller was used to mix the residue with the soil, resulting in a shallow (approximately 10-cm) disturbed zone. More than 95% of the residue was incorporated into the soil. Winter wheat seeding and fertilizer placement were completed using the same seeder as the MP+R. For the NT treatment, winter wheat was seeded into standing corn stubble using a paired-row no-till wheat seeder (20 cm between pairs, 10 cm between rows, 3 5-cm seeding depth), and fertilizers were applied at the same time. The no-till seeder had a rotary coulter that was able to make a loose strip for the two crop rows. There were three replications per treatment, and the plot size was 1120 m 2 (16 by 70 m). In 2004, the MP+R plot was split into two subplots with different residue management strategies: half of the plot had chopped corn residue incorporated (the MP+R treatment) and the other half had the crop residue removed (designated as the MP R treatment). Based on the mass and organic C and total N concentrations of corn residue, it was estimated that the annual organic C input was about 3.7, 3.7, and 3.4 Mg ha 1 for MP+R, RT, and NT, respectively, and the annual total N input was about 0.10, 0.10, and 0.09 Mg ha 1 for MP+R, RT, and NT, respectively. No additional tillage operations were used before planting the summer corn crops. Corn was planted into wheat stubble (15 20-cm in height) after winter wheat harvest. Chemical fertilizers (including urea and diammonium hydrogen phosphate) and irrigation were identical for all the treatments. At winter wheat seeding, fertilizer rates were 130 kg ha 1 for N and 121 kg ha 1 for P. In addition, 138 kg N ha 1 was surface broadcast shortly after initiation of the elongation stages of winter wheat and corn. Depending on rainfall, three or four irrigations (40 50 mm each time) were applied for wheat and corn using a sprinkler system. SSSAJ: Volume 74: Number 1 January February

3 Soil Sampling and Analysis Soil samples were collected after winter wheat harvest in June 2007 and June 2008, 7 yr after establishment of the tillage treatments. In 2007, the samples were obtained from the top 30 cm at four increments: 0 to 5, 5 to 10, 10 to 20, and 20 to 30 cm. Triplicate soil cores were taken from the four layers using a stainless steel ring (5 cm high and 5 cm in diameter) for bulk density (ρ b ) determination (Grossman and Reinsch, 2002). Three representative soil samples were collected with a hand auger (4.1-cm diameter) from each plot and pooled together to make a composite sample for each soil layer and replication. A portion of the samples was air dried and passed through a 2-mm sieve for analysis of the particle size distribution. A portion of the sieved soil was further ground and passed through a 0.25-mm sieve for total SOC and total N concentration determination. Soil particle size distribution was determined by the pipette method (Gee and Or, 2002). In June 2008, soil samples were also collected in a similar way down to a 50-cm depth. These samples were separated into six intervals: 0 to 5, 5 to 10, 10 to 20, 20 to 30, 30 to 40, and 40 to 50 cm. Again, SOC and total N concentrations and ρ b were determined for each sampled layer. Soil C and N concentrations were determined by dry combustion of duplicate subsamples using a CNS analyzer (Vario Max CNS, Elementar, Hanau, Germany; Nelson and Sommers, 1996). Since the parent material was calcareous, the samples for total SOC and N concentration analyses were treated to remove any inorganic C. The samples were pretreated with 0.5 mol L 1 HCl until fizzing stopped, washed with deionized water until neutralized, and then air dried and ground again. Total N and SOC storage (Mg ha 1 ) were estimated from the product of the elemental concentrations, soil bulk density, and soil depth. Soil profile SOC and total N storage were the sum of the individual soil layer values. Following Franzluebbers (2002), we calculated the stratification ratio (SR), the ratio of SOC or total N concentration of the surface soil layers (0 5 and 5 10 cm) and the corresponding values of the deeper layers (10 20 and cm). Statistical Analyses To assess the treatment effects on soil ρ b, SOC and total N concentration and storage, and SR, we performed a one-way ANOVA using the SPSS 11.0 software (SPSS, 2001). The means were compared using Fisher s protected LSD test. Unless otherwise stated, all differences discussed are significant at the P < 0.05 probability level. RESULTS AND DISCUSSION Soil Bulk Density Soil bulk density influences the estimates of SOC and total N storage expressed on a unit area basis. Some researchers reported that in the upper 30-cm soil layer, the ρ b of NT was greater than the ρ b of tilled soil (e.g., Yang and Wander, 1999; Gál et al., 2007), but other researchers indicated that soil ρ b was equal to or smaller under NT relative to tilled systems (e.g., Angers et al., 1997; Ussiri and Lal, 2009). Therefore, it is generally considered that the impacts of tillage systems on soil ρ b are site specific, probably depending on soil type, climatic regime, residue management, and the duration of experiment. In our study, after winter wheat harvest, the soil ρ b ranged from 1.22 to 1.51 g cm 3 in 2007 and from 1.32 to 1.62 g cm 3 in 2008 (Fig. 1). The NT treatment had significantly Fig. 1. Soil bulk density profile influenced by tillage management (MP R, moldboard plow without corn residue; MP+R, moldboard plow with corn residue; RT, rotary tillage with corn residue; NT, no-till with corn residue). Error bars represent LSD (P < 0.05) for comparison among tillage treatments at the same depth. higher ρ b values than the other treatments in the 0- to 20-cm layer, but no significant differences among tillage treatments were observed at soil depths >20 cm. For example, in 2007, the ρ b of the NT plots was 7, 8, and 11% higher than that of the MP R treatment in the 0- to 5-, 5- to 10-, and 10- to 20-cm layers, respectively. These data indicated that tillage disturbance only affected ρ b in the 0- to 20-cm layer, which agreed with Puget and Lal (2005), who showed that there were no differences in ρ b between plow, chisel, and NT below 30 cm. In addition, there was no statistical difference between ρ b values of the MP+R and MP R treatments, indicating that residue removal alone did not significantly change the soil ρ b. Therefore, under the experimental conditions, soil mechanical disturbance by tillage equipment was the key factor impacting soil ρ b. For all of the treatments, soil ρ b was greater in 2008 than in 2007, possibly caused by the temporal variability of ρ b. Total Nitrogen and Soil Organic Carbon Concentrations and Storage The results of SOC and total N concentrations across the soil profiles are presented in Fig. 2. For the conventional tillage systems (MP R and MP+R), SOC and total N concentrations were relatively uniform within the 0- to 20-cm layers but decreased considerably in the 20- to 30-cm layers. For the conservation tillage systems (RT and NT), SOC and total N concentrations showed a continuously declining trend in the 0- to 30-cm layer. At soil depth >30 cm, SOC and total N concentrations were relatively low and uniform for all of the tillage treatments. Comparison of the four tillage treatments indicated that the RT and NT treatments had higher SOC and total N concentrations than the MP R and MP+R treatments in the 0- to 10-cm layer, but the reverse trend was observed in the 10- to 20-cm layer. In June 2007, for example, SOC and total N concentrations of the NT treatment were 26 and 22% higher, respectively, than those of the MP+R treatment in the 0- to 5-cm layer, but the MP+R treatment had 19 and 9% 198 SSSAJ: Volume 74: Number 1 January February 2010

4 Fig. 2. Vertical distribution of soil organic C and total N concentrations influenced by tillage management (MP R, moldboard plow without corn residue; MP+R, moldboard plow with corn residue; RT, rotary tillage with corn residue; NT, no-till with corn residue). Error bars represent LSD (P < 0.05) for comparison among tillage treatments at the same depth. Table 1. Soil organic C storage in the 0- to 30- and 0- to 50-cm depths in 2007 and 2008, respectively, as affected by tillage and residue management (MP R, moldboard plow without corn residue; MP+R, moldboard plow with corn residue; RT, rotary tillage with corn residue; NT, no-till with corn residue). Soil organic C storage Soil layer MP R MP+R RT NT MP R MP+R RT NT cm Mg ha c 6.93 c 8.26 b 9.56 a 7.10 c 7.72 c 9.07 b 9.83 a b 7.16 b 8.79 a 8.54 a 7.48 c 8.21 b 9.51 a 9.07 a ab a ab b c a ab b a a a a a a a a a 6.16 a 6.23 a 5.61 a a 5.73 a 5.98 a 5.80 a b ab a a b a a a b a a a Values followed by a different lowercase letter in the same row among tillage treatments are significantly difference at P < higher SOC and total N, respectively, than the NT treatment in the 10- to 20-cm layer. Similar trends were also observed in soil samples obtained in June At soil depths >30 cm, the four tillage treatments had similar SOC and total N concentrations. Estimated SOC storage values on an area basis are presented in Table 1. The results demonstrate several characteristics. First, the MP+R, RT, and NT treatments had similar SOC storage, either in the 0- to 30-cm layer in 2007 (ranging Mg ha 1 ) or in the 0- to 50-cm layer in 2008 (ranging Mg ha 1 ). These treatments also showed significantly higher SOC storage values than the MP R treatment, where crop residue was moved out of the field. For example, in June 2007, the treatments with residue return (MP+R, RT, and NT) on average increased SOC storage by 8% relative to the residue removal treatment. Second, treatment influences on SOC storage mainly appeared in the 0- to 20-cm layer, and no treatment difference was observed at soil depth >20 cm. While the NT and RT treatments had larger SOC storage than the MP+R treatment in the 0- to 10-cm soil layer, SOC storage in the 10- to 20-cm soil layer was in the order of MP+R > RT > NT. Third, the differences between NT and RT, the two conservation tillage systems, were significant in the 0- to 5-cm layer only, where NT had 15.7 and 8.4% more SOC storage than RT in 2007 and 2008, respectively. For the conventional tillage systems, the MP+R treatment had higher SOC storage than the MP R treatment at all depth intervals, but the difference was significant only in the 5- to 10- and 10- to 20-cm layers in Table 2 presents the total N storage in the soil profiles. In both 2007 and 2008, treatment influences on total N storage had a trend similar to the SOC storage, with the exception that the difference between NT and RT treatments in the 0- to 5-cm layer was not significant in Tillage practices influence the eventual location of aboveground crop residues in the soil, and subsequently the SOC and total N distributions within soil profiles. Allmaras et al. (1996) demonstrated that moldboard plowing to a depth of 25 cm buried 70% of aboveground residue at a depth of 12 to 24 cm, while chisel plowing to a depth of 15 cm left nearly 60% of the residue at a depth of 0 to 6 cm. With NT there is a minimum soil disturbance, and crop residue is maintained at the soil surface, which contributes to a SOC storage increase at or near the soil surface (Franzluebbers, 2002; Dolan et al., 2006; Angers and Eriksen-Hamel, 2008; Blanco-Canqui and Lal, 2008; Yang et al., 2008). In our study, crop residue under the MP+R treatment was incorporated into the soil and distributed relatively uniformly in the plow layer (0 18 cm). For the RT treatment, soil disturbance from the rotary tiller and subsequent crop residue distribution was limited to the 0- to 10-cm layer. The crop residue of the NT plots was retained mostly near the soil surface where soil dis- SSSAJ: Volume 74: Number 1 January February

5 turbance occurred only at crop seeding. As a result, the largest SOC storage values in the 0- to 5-, 5- to 10-, and 10- to 20-cm layer appeared in the NT, RT, and MP+R plots, respectively. The differences in the degree of soil disturbance, bulk density, and soil physical conditions may also affect root growth and distribution, and consequently SOC and total N concentrations in the soil profile (Ball-Coelho et al., 1998; Baker et al., 2007). Allmaras et al. (2004) demonstrated that more SOC storage might occur in subsurface soils with conventional tillage than with NT due to decomposition of buried crop residues and roots. Using a 5-yr field trial, Qin et al. (2006) found that NT resulted in higher corn root length density in the 0- to 5-cm layer but lower root length density at depths below 10 cm compared with conventional tillage. There was no difference in root length density between the tillage systems below 50 cm. At our experiment site, an earlier study on winter wheat root distribution demonstrated that the crop under the NT system generally had a relatively smaller root system than that of the RT and MP+R treatments, but the difference between the RT and MP+R treatments was not significant (Dong et al., 2007). Since the MP+R, RT, and NT treatments showed similar SOC storage in the soil profiles, we concluded that the influence of root distribution on SOC storage was relatively small compared with the contribution from crop residue input. Total Nitrogen and Soil Organic Carbon Stratification Ratio To further quantify the SOC and total N distribution as modified by tillage treatments, the SR of the SOC and total N concentrations were evaluated. Since there were no differences between treatments at soil depths >30 cm and the general trends were similar between 2007 and 2008, only the data for the 0- to 30-cm layer in 2007 are presented (Fig. 3). In the 0- to 20-cm layer, the SR value of SOC was about 1.03 for the moldboard plow treatments (MP R and MP+R) and was 1.32 to 1.59 for the conservation tillage treatments (RT and NT) (Fig. 3a). When the 0- to 30-cm layer was considered, the SR value of the SOC was Table 2. Total N storage in the 0- to 30- and 0- to 50-cm depths in 2007 and 2008, respectively, as affected by tillage and residue management (MP R, moldboard plow without corn residue; MP+R, moldboard plow with corn residue; RT, rotary tillage with corn residue; NT, no-till with corn residue. Total soil N storage Soil layer MP R MP+R RT NT MP R MP+R RT NT cm Mg ha c 0.80 c 0.97 b 1.07 a 0.99 b 0.99 b 1.18 a 1.20 a b 0.85 b 1.01 a 1.01 a 1.04 c 1.06 bc 1.23 a 1.16 ab a 1.80 a 1.68 a 1.66 a 1.97 b 2.20 a 2.05 b 1.95 b a 1.48 a 1.36 a 1.35 a 1.59 a 1.72 a 1.79 a 1.76 a a 1.45 a 1.34 a 1.41 a b 1.25 ab 1.27 ab 1.35 a b 4.93 a 5.03 a 5.09 a 5.59 b 5.97 a 6.25 a 6.07 a b 8.66 a 8.87 a 8.84 a Values followed by a different lowercase letter in the same row among tillage treatments are significantly difference at P < Fig. 3. (a) Soil organic C (SOC) and (b) total N stratification ratios influenced by tillage management (MP R, moldboard plow without corn residue; MP+R, moldboard plow with corn residue; RT, rotary tillage with corn residue; NT, no-till with corn residue). Values followed by a different lowercase letter within a specific depth ratio and among treatments are significantly different. Bars are standard errors. 200 SSSAJ: Volume 74: Number 1 January February 2010

6 increased considerably 1.37 to 1.45 for MP R and MP+R and 1.74 to 2.04 for RT and NT but there were no significant differences between MP R and MP+R or between RT and NT. The NT treatment had an SR value >2, the threshold value proposed by Franzluebbers (2002). The SR results for total N concentration showed trends similar to SOC, but the SR values for the N concentrations were smaller than those for the SOC (Fig. 3b). Therefore, 7 yr after conversion from conventional tillage to conservation tillage, the RT and NT treatments indicated the presence of profile stratification of SOC and total N. The conventional tillage system indicated relatively uniform SOC and total N concentrations in the tilled layer. These results were consistent with other reports that the SR of SOC for a moldboard plow system ( ) was generally less than for a NT system ( ) (Díaz-Zorita and Grove, 2002; Franzluebbers, 2002; Sá and Lal, 2009). Sampling Depth and Soil Organic Carbon and Total Nitrogen Storage Some researchers have argued that shallow sampling schemes (<30 cm) might lead to confounding conclusions concerning the influence of tillage systems on SOC storage (Dolan et al., 2006; Baker et al., 2007; Gál et al., 2007). This is reasonable because SOC storage under NT is concentrated near the surface, while in plowed soil it is more uniformly distributed in the soil profile. Consequently, the apparent SOC gains from NT that are based only on shallow sampling depth would vanish when deeper soil samples are included. For example, based on a 28-yr field experiment, Gál et al. (2007) observed that NT had 23 Mg ha 1 more C than moldboard plowing in the top 30-cm layer, while for a 1-m profile NT resulted in an overall gain of only 10 Mg ha 1. Our measurements from the 30- to 50-cm layer in 2008 showed that SOC and total N concentration were relatively small and uniform, and there were no apparent differences among the different tillage systems (Fig. 2). This was caused by the fact that soil disturbance by tillage and crop residue distribution was limited to the 0- to 18-cm layer for the MP R and MP+R treatments and an even shallower soil layer for the RT and NT treatments. Therefore, under the experimental conditions, a sampling depth of 30 cm was probably appropriate for studying SOC and total N concentration and storage. Nevertheless, we do not rule out a deeper sampling strategy in the future, as the influences of tillage system on crop root distributions and earthworm activities may be extended to below 30 cm (Riley et al., 2005; Qin et al., 2006). Furthermore, it is necessary to investigate whether SOC and total N distributions in soil depend on soil texture. A judgment about sampling depth must be made on a case-by-case basis depending on site-specific conditions. CONCLUSIONS This study clearly indicated that tillage systems strongly affected the soil bulk density and the distributions of SOC and total N in soil profiles located in the North China Plain. Compared with other treatments, the NT practice significantly increased soil bulk density by 5 to 8% in the top 20-cm layer. Crop residue and mechanical disturbance from tillage interactively affected SOC and total N storage distribution and storage in the soil profile. There was SOC and total N accumulation in the top layer of conservation tillage plots (RT and NT), but SOC and total N were more evenly distributed in the plowed layer under conventional tillage systems (MP R and MP+R). As a result, more SOC and total N stratification was observed with NT and RT treatments than with MP+R and MP R treatments. When the deeper soil profile (0 30 or 0 50 cm) was considered, conversion of conventional to conservation tillage systems did not result in significant increases in SOC and total N storage as long as a similar amount of crop residue was returned to the soil. Thus, returning crop residue to the soil or leaving crop residue on the soil surface is crucial to maintaining the SOM level. On the other hand, moldboard plowing coupled with residue removal had significantly less SOC and total N storage than the treatments where the residue was returned to the soil. The results of this study indicated that, under the experimental conditions, a sampling depth of 30 cm was appropriate to assess the changes in SOC and total N in soil profiles under different tillage systems. REFERENCES Allmaras, R.R., S.M. Copeland, P.J. Copeland, and M. Oussible Spatial relations between oat residue and ceramic spheres when incorporated sequentially by tillage. Soil Sci. Soc. Am. J. 60: Allmaras, R.R., D.R. Linden, and C.E. Clapp Corn-residue transformations into root and soil carbon as related to nitrogen, tillage, and stover management. Soil Sci. Soc. Am. J. 68: Angers, D.A., M.A. Bolinder, M.R. Carter, E.G. Gregorich, C.F. Drury, B.C. Liang, R.P. Voroney, R.R. Simard, R.G. Donald, R.P. Beyaert, and J. Martel Impact of tillage practices on organic carbon and nitrogen storage in cool, humid soils of eastern Canada. Soil Tillage Res. 41: Angers, D.A., and N.S. Eriksen-Hamel Full-inversion tillage and organic carbon distribution in soil profiles: A meta-analysis. Soil Sci. Soc. Am. J. 72: Baker, J.M., T.E. Ochsner, R.T. Venterea, and T.J. Griffis Tillage and soil carbon sequestration: What do we really know? Agric. Ecosyst. Environ. 118:1 5. Ball-Coelho, B.R., R.C. Roy, and C.J. Swanton Tillage alters corn root distribution in coarse-textured soil. Soil Tillage Res. 45: Blanco-Canqui, H., and R. Lal No-tillage and soil-profile carbon sequestration: An on-farm assessment. Soil Sci. Soc. Am. J. 72: Díaz-Zorita, M., and J.H. Grove Duration of tillage management affects carbon and phosphorus stratification in phosphatic Paleudalfs. Soil Tillage Res. 66: Dolan, M.S., C.E. Clapp, R.R. Allmaras, J.M. Baker, and J.A.E. Molina Soil organic carbon and nitrogen in a Minnesota soil as related to tillage, residue and nitrogen management. Soil Tillage Res. 89: Dong, W.X., S.Y. Chen, C.S. Hu, and C.M. Yi The effect of minimum tillage and no-tillage on growth and yield of winter wheat. (In Chinese with English abstract.) Acta Agric. Boreali-Sin. 22: Franzluebbers, A.J Soil organic matter stratification ratio as an indicator of soil quality. Soil Tillage Res. 66: Gál, A., T.J. Vyn, E. Michéli, E.J. Kladivko, and W.W. McFee Soil carbon and nitrogen accumulation with long-term no-till versus moldboard plowing overestimated with tilled-zone sampling depths. Soil Tillage Res. 96: Gee, G.W., and D. Or Particle-size analysis. p In J.H. Dane and G.C. Topp (ed.) Methods of soil analysis. Part. 4. Physical methods. SSSA Book Ser. 5. SSSA, Madison, WI. Gong, Z.T Chinese soil taxonomy. Sciences Press, Beijing. Grossman, R.B., and T.G. Reinsch Bulk density and linear extensibility. p In J.H. Dane and G.C. Topp (ed.) Methods of soil analysis. Part. 4. Physical methods. SSSA Book Ser. 5. SSSA, Madison, WI. Lal, R Soil carbon sequestration impacts on global climate change and food security. Science 304: Lal, R Soil quality impacts of residue removal for bioethanol production. SSSAJ: Volume 74: Number 1 January February

7 Soil Tillage Res. 102: Lal, R., and J.M. Kimble Conservation tillage for carbon sequestration. Nutr. Cycling Agroecosyst. 49: Li, L., H.L. Zhang, F. Chen, and S.J. Li CO 2 flux and its correlation with soil temperature in winter wheat growth season under different tillage measures. (In Chinese with English abstract.) Chin. J. Appl. Ecol. 18: Machado, P.L., O.A. Sohi, S.P. Gaunt, and J.L. Gaunt Effect of no-tillage on turnover of organic matter in a Rhodic Ferralsol. Soil Use Manage. 19: Nelson, D.W., and L.E. Sommers Total carbon, organic carbon, and organic matter: Laboratory methods. p In D.L. Sparks (ed.) Methods of soil analysis. Part 3. Chemical methods. SSSA Book Ser. 5. SSSA, Madison, WI. Paustian, K., O. Andren, H. Janzen, R. Lal, P. Smith, G. Tian, H. Tiessen, M. van Noordwijk, and P. Woomer Agricultural soil as a C sink to offset CO 2 emissions. Soil Use Manage. 13: Puget, P., and R. Lal Soil organic carbon and nitrogen in a Mollisol in central Ohio as affected by tillage and land use. Soil Tillage Res. 80: Qin, R., P. Stamp, and W. Richner Impact of tillage on maize rooting in a Cambisol and Luvisol in Switzerland. Soil Tillage Res. 85: Riley, H., M. Bleken, S. Abrahamsen, A.K. Bergjord, and A.K. Bakken Effects of alternative tillage systems on soil quality and yield of spring cereals on silty clay loam and sandy loam soils in the cool, wet climate of central Norway. Soil Tillage Res. 83: Sá, J.C.M., and R. Lal Stratification ratio of soil organic matter pools as an indicator of carbon sequestration in a tillage chronosequence on a Brazilian Oxisol. Soil Tillage Res. 103: SPSS SPSS for Windows, Version SPSS, Inc., Chicago, IL. Ussiri, D.A.N., and R. Lal Long-term tillage effects on soil carbon storage and carbon dioxide emissions in continuous corn cropping system from an Alfisol in Ohio. Soil Tillage Res. 104: Wander, M.M., M.G. Bidart, and S. Aref Tillage impacts on depth distribution of total and particulate organic matter in three Illinois soils. Soil Sci. Soc. Am. J. 62: Wang, X.B., D.X. Cai, W.B. Hoogmoed, O. Oenema, and U.D. Perdok. 2007a. Developments in conservation tillage in rainfed regions of North China. Soil Tillage Res. 93: Wang, Y., Z.J. Li, B. Han, Z.Q. Shi, T.Y. Ning, X.D. Jiang, Y.H. Zheng, M. Bai, and J.B. Zhao. 2007b. Effects of conservation tillage on soil microbial biomass and activity. (In Chinese with English abstract.) Acta Ecol. Sin. 27: West, T.O., and W.M. Post Soil organic carbon sequestration rates by tillage and crop rotation: A global data analysis. Soil Sci. Soc. Am. J. 66: Wilhelm, W.W., M.F. Johnson, J. Hatfield, J.W.B. Voorhees, and D.R. Linden Crop and soil productivity response to corn residue removal: A literature review. Agron. J. 96:1 17. Yang, X.M., C.F. Drury, W.D. Reynolds, and C.S. Tan Impacts of longterm and recently imposed tillage practices on the vertical distribution of soil organic carbon. Soil Tillage Res. 100: Yang, X.M., and M.M. Wander Tillage effects on soil organic carbon distribution and storage in a silt loam soil in Illinois. Soil Tillage Res. 52: SSSAJ: Volume 74: Number 1 January February 2010