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a Pontificia Universidad Catolica de Chile, Santiago, Chile
b Dep. of Soil and Crop Sci., Colorado State Univ., Fort Collins, CO 80523
* Corresponding author (gary.peterson{at}colostate.edu)
Received for publication September 14, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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0.16) for the more intense WCMF cropping system to have higher SOC and SON contents than the traditional WF system (C = 6.6 g kg-1 for WF compared with 7.5 g kg-1 for WCMF and N = 0.70 g kg-1 for WF compared with 0.74 g kg-1 for WCMF). From 1985 to 1993, gains in SOC (967 kg ha-1) and SON (74 kg ha-1) occurred in the surface 0- to 2.5- and 2.5- to 5-cm depths while losses were observed in the 5- to 10-cm depth (SOC = -694 kg ha-1; SON = -44 kg ha-1). Climate strongly modified these effects but did not reflect a clear ET gradient effect. The results suggest that higher levels of surface SOC and SON can be attained by increasing cropping intensity under no-till management.
Abbreviations: ET, evapotranspiration • SOC, soil organic carbon • SON, soil organic nitrogen • WCMF, wheat–corn (or sorghum)–millet–fallow • WF, wheat–fallow
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Several research projects have focused on the long-term effects of soil management practices on soil characteristics, particularly tillage management (Dick, 1983; Follett and Peterson, 1988; Ismail et al., 1994) and concluded that reduced tillage usually results in improvements in both chemical and physical soil properties. Reeves (1997), in a review of long-term experiments, reported that conservation tillage maintained or even increased soil organic C (SOC) when coupled with more intensive cropping. Summer fallow with intensive tillage, a common practice in more arid environments, results in larger soil organic matter losses than continuous cropping in these same environments (Haas et al., 1957; Ridley and Hedlin, 1968). However, little research has been done to address the effect of cropping intensity on residue accumulation and on soil characteristics under no-till conditions in soils previously managed under conventional tillage practices. Wood et al. (1991), working in the Great Plains of eastern Colorado, found that imposing no-till management on soils previously managed under tilled wheat–fallow (WF) systems altered the depth distribution and contents of SOC and soil organic N (SON) after only 4 yr. They found that surface soil (0–10 cm) SOC and SON contents were maintained at higher levels by more intense cropping systems (less summer fallow time) and were decreased by the less intense WF system. They concluded that higher equilibrium levels of organic C and N can be established in no-till soils through the return of greater amounts of plant residues that accompany greater cropping intensity. Work by Campbell et al. (1990)( 2000) in Saskatchewan, Canada, supports the concept that frequent fallow depletes SOC, and they report that increased crop residue returned to the soil with continuous cropping is the major means of replenishing soil organic matter.
Crop residue production (nonharvested plant parts) and decomposition in the central Great Plains are controlled by water availability. Extrapolation of results to climatic regions beyond local experiments can best be accomplished if linked to climatic factors that control water availability, specifically, climatic precipitation and potential evapotranspiration (ET). Climate in the central Great Plains can be characterized as a grid with two variables: annual precipitation, which ranges from 250 to 450 mm, and open pan evaporation (index of potential ET), which ranges from 1600 to 2000 mm for a growing season (Farnsworth and Thompson, 1982; Peterson and Westfall, 1999). One expects residue accumulation and decomposition rates to vary with these two parameters; therefore, they are a key for interpreting results related to crop residues found on and in the soil.
Time itself can be a major variable when assessing the effects of management changes on soil organic matter levels. Switching from tilled to no-till management practices represents a dramatic change in how C is added to the soil. Intensifying cropping also changes C inputs relative to a WF system. Soil organic matter contents reported by Wood et al. (1991) are a benchmark to which we can compare C and N contents at any following date.
The objectives of this study were to (i) determine the effect of cropping intensity, climate gradient, and soil depth on levels and changes in soil C, soil N, and residue parameters after 8 yr of no-till management in dryland cropping systems and (ii) relate soil and residue parameters to soil C and N levels.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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All three sites chosen had been cultivated, mainly in dryland WF or sorghum for more than 50 yr until 1985 when no-till management was established. Management systems (rotations) were placed across the soil sequences at each site and represent the third variable: increased cropping intensity with decreasing summer fallow time. No-till cropping systems are WF, wheat–corn (or sorghum)–fallow, wheat–corn (or sorghum)–proso millet–fallow (WCMF), opportunity crop, and perennial grass, all managed with no-till techniques to maximize water storage potential. Grain sorghum replaces corn at the high-ET site, Walsh. All phases of each cropping system are present every year. For this residue study, only the WF and WCMF systems were sampled. If the reader desires information on the other cropping systems, details are provided by Peterson et al. (1993). The fourth driving variable, time, is present in the sense that the experiment was planned with a minimum time horizon of 20 yr (five cycles of the 4-yr rotation) (Peterson et al., 1988, 1993).
An experimental unit is an individual soil area within a site and within a cropping system. All units are 6.1 m wide but vary in length, depending on the length of the soil catena (ranging from 185–305 m). The experimental design is a split block that includes location (Sterling, Stratton, and Walsh), slope position (summit, sideslope, and toeslope), and cropping system (WF, wheat–corn–fallow, WCMF, opportunity, and grass) variables in two blocks (Peterson et al., 1988, 1993).
Fertilizer N is applied to each experimental unit according to soil tests performed for each soil, within each rotation and specific for the crop present in a given year (Peterson et al., 1988, 1993).
Selected Plots and Sampling Procedure
We selected two of the no-till cropping systems, WF and WCMF on one slope position (summit) across the three locations (ET gradient), for the residue reported herein. Table 1 contains a summary of initial soil properties measured in 1986 and the average amount of N fertilizer applied. We chose the fallow phase preceding wheat planting, in both the WF and WCMF systems, as the experimental unit for sampling. Thus, by our sampling time in June 1993, WF had produced four wheat crops and had approximately 56 mo of fallow during the 8-yr period. The WCMF system had produced two wheat crops, two corn (sorghum) crops, two millet crops, and had approximately 24 mo of fallow during the same period.
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An area of 30 by 75 cm was excavated in each plot to a depth of 10 cm, in 2.5-cm increments, to determine soil residue distribution. Surface plant residues and residues from soil layers were collected separately. Plant residues were separated from each soil layer using water under high pressure and a 0.6-mm sieve. Remaining soil particles were then separated from residues by flotation of the residue.
Sample Preparation and Laboratory Analysis
Soil cores were air-dried, finely ground (0.2 mm), and analyzed for organic C and N. We measured SOC by wet oxidation (Nelson and Sommers, 1982). Total soil N was determined colorimetrically, following a micro-Kjeldahl procedure (Bremmer and Mulvaney, 1982), with a Lachat autoanalyzer (Lachat QuickChem Syst., Mequon, WI). The total soil N procedure did not include NO3–N, and extractable NH4–N was subtracted from total soil N to obtain SON.
Plant residues were oven-dried at 70°C, finely ground (0.2 mm), and analyzed for total C and N. Carbon was determined by the diffusion method (Snyder and Trofymow, 1984).
Total N in the residues was determined colorimetrically, following a micro-Kjeldahl procedure (Bremmer and Mulvaney, 1982). Residues from the surface and residues from the 0- to 2.5-cm soil depth also were analyzed for acid detergent fiber (ADF), cellulose, and lignin (Goering and Van Soest, 1970).
Experimental Design and Data Analysis
The treatment arrangement was a 3 x 2 x 4 factorial in a split-split block experimental design, with two replications, that included location [Sterling (low ET), Stratton (medium ET), and Walsh (high ET)] as main plots, cropping system (WF and WCMF) as split-blocks, and soil depth (0–2.5, 2.5–5, 5–7.5, and 7.5–10 cm) as the split-split block. At each field location, cropping systems were randomly assigned to strips within each block. Depths in our analysis were treated as parallel to cropping systems within each block. The assumption is that the correlation between all depths is the same for all pairs of depths, and although this may result in some inaccuracy, it is still a commonly used procedure (P. Chapman, personal communication, 1999). An example of the basic analysis of variance (ANOVA) is given in Table 2.
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Analyses of variance were performed using the procedure GLM of Statistical Analysis System (SAS) (SAS Inst., 1991), testing for all main effects and interactions. A paired t-test procedure (Ott, 1993) was performed to compare SOC and SON gains from 1985 to 1989 and from 1989 to 1993. Correlation and regression analyses among soil and residue parameters were performed using procedures CORR and REG in SAS (SAS Inst., 1991).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Soil Organic Carbon, Soil Organic Nitrogen, and Carbon/Nitrogen Ratio
Carbon and Nitrogen Contents
Surface soils (0–2.5 cm) had the highest organic C and N contents, ranging from 4.5 to 13.3 g C kg-1 and from 0.4 to 1.2 g N kg-1. The C/N ratio also was larger for this surface soil layer. Soil organic C, SON, and soil C/N ratio all decreased with depth across locations and cropping systems. For SOC and SON, however, the degree to which they were affected depended on the location because the location x depth interaction was significant (Table 3). The interaction occurred because the stratification of SOC and SON was more pronounced at the low- and medium-ET locations (Sterling and Stratton, respectively) than at the high-ET location (Walsh).
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0.16) for the more intense WCMF cropping system to have higher SOC and SON contents than the traditional WF system (Table 3). Franzluebbers et al. (1994) reported that SOC and total Kjeldahl N increased with increasing C input associated with more intensive cropping systems over the long term. Even in conventionally tilled systems, cropping intensification can increase SOC, as reported by Varvel (1994).
Changes in Soil Carbon and Nitrogen (1985–1993)
Changes in SOC, SON, and soil C/N ratio in the surface soil (0–10 cm) were calculated for two periods, 1985 to 1993 and 1989 to 1993. The changes that occurred during the first 4 yr after conversion to no-till (1985–1989) were reported by Wood et al. (1991). Since conversion to no-till management in 1985, gains in SOC, SON, and C/N ratio generally occurred in the surface 0- to 2.5- and 2.5- to 5-cm soil depths (Table 4). Losses were observed in the 5- to 10-cm soil depth, irrespective of cropping intensity. However, the magnitude of these effects depended on location because the location x depth interaction was significant for SOC and SON (Table 4). There were gains in SOC and SON down to the 5-cm soil depth at the medium- and high-ET sites (Stratton and Walsh, respectively) while at the low-ET site (Sterling), gains occurred just in the surface 0- to 2.5-cm soil. Moreover, SOC and SON losses in the 5- to 10-cm soil depth were larger at the low-ET site (Sterling) compared with the medium- and high-ET sites (Stratton and Walsh, respectively).
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After 8 yr of no-till, the more intense cropping system (WCMF) tended (P 0.25) to gain SOC and SON and have a wider C/N ratio while the less intensive WF appeared to lose SOC and SON and have a narrower C/N ratio. Because of its larger biomass production, we expected that WCMF would gain more SOC and SON and would have a wider gain in soil C/N ratio than WF in the first 0- to 2.5-cm layer and that these differences would be evident, even in deeper layers in the WCMF system. Wood et al. (1991) found that more intense cropping increased SOC more in the 0- to 2.5-cm layer than the less intensive WF on summit positions.
Changes in Soil Carbon and Nitrogen (1989–1993)
There were gains in SOC and SON in the 0- to 2.5- and 2.5- to 5-cm depths and losses in the 5- to 10-cm layer during the period of 1989 through 1993. The magnitudes of these changes were 0 to 2.5–>2.5 to 5–>5- to 10-cm soil depths (Table 5). For SON, these effects depended on location and cropping system because the corresponding three-way interaction was significant. The interaction is explained by differences in the magnitude of the changes in SON among soil depths, across locations and cropping systems, although we observed no definite pattern.
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Changes in Soil Carbon and Nitrogen by Rotational Period
When comparing the gains in SOC and SON, at three soil depths and for the rotational periods 1985 to 1989 and 1989 to 1993, we found that equal gains in SOC occurred in the 0- to 2.5-cm depth during both rotational periods. A larger gain in SON occurred in this soil layer during the second rotational period (1989–1993) compared with the first 4 yr after conversion to no-till (Fig. 2) . A net gain in SOC and SON was observed in the 2.5- to 5-cm layer during the second rotational period compared with a net loss during the first period. In the 5- to 10-cm layer, similar net losses in SOC and SON were observed in both periods.
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Residue Variables
The amount of residue found on the soil surface and within the surface soil is directly affected by the factors controlling biomass production and microbial decomposition. In our case, these factors are represented by location (ET gradient) and cropping system. From this point on, we will define surface residue as that on the soil surface, and residue in the soil layers will be identified as soil residue. Residue quality also is a function of the crop types in a particular cropping system and of the degree of decomposition undergone by the residue before the sampling time. Differences between residue located on the soil surface and the residue present within soil layers are expected because of their possible different nature (shoots vs. roots) and opportunity for decomposition.
Soil and Surface Residue Quantity and Carbon and Nitrogen in Residue
Quantity of soil residue decreased dramatically with depth and interacted with cropping system (Fig. 3)
. We found maximum residue levels in the 0- to 2.5-cm depth in the WCMF system, but only in the surface soil did WCMF have more residues than WF. Differences in residue quantity between the first soil layer and following depths were larger for WCMF than for WF, which was mirrored by the amount of residue C (Fig. 4)
. Residue N decreased with depth as one would expect, but there was no interaction between soil depth and cropping system for N (Fig. 4).
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Neither cropping system nor ET gradient affected the soil residue composition in this study. Only soil depth had a significant effect on residue C and N content and C/N ratio (Table 7). Residue C and N content increased with depth as did the residue C/N ratio. The lowest contents for C and N were generally observed in residues found nearest the surface (0–2.5 cm), which occurs because of greater C and N mineralization in this layer.
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Residue Carbon and Nitrogen Contents and Carbon/Nitrogen Ratio
Residue C and N contents and C/N ratio were highly dependent on residue position (Fig. 5)
regardless of location (ET gradient). Surface residue had a higher C content but smaller N content than the residue found in the first 0 to 2.5 cm of soil. Consequently, the C/N ratio for surface residue was wider than for residue in the 0- to 2.5-cm soil depth. Differences in residue as related to position were probably due to differences in age. Surface residue is younger than that found in the 0- to 2.5-cm soil depth, which had been continually accumulating in this layer since the experiment started. Age determines the degree of microbial decomposition. Rate of decomposition would be faster for residue that was in the soil (soil residue) compared with surface residue. The different composition of surface residue compared with soil residue is another reason for residues being different. At the soil surface, a straw-like residue was found while in the 0- to 2.5-cm soil depth, we identified a mixture of partially decomposed residue coming from the surface and crop roots.
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0.07) to affect the lignin/N ratio but was not significant at the 5% probability level (Fig. 8). These results reflect the degree of residue decomposition as lignin content was greatest for residue found in the 0- to 2.5-cm soil depth.
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Cropping system did not affect cellulose content of residues but did affect their lignin and ADF content (P 0.1). According to these findings, WCMF would potentially mineralize more N than WF because of its lower ADF and lignin content. However, these results also could mean that WF had undergone more decomposition than WCMF, which is reflected by its higher lignin content.
Relationship between Some Selected Soil and Residue Variables
Considering All Soil Depths
Correlation analysis showed that SOC, SON, and soil C/N ratio were all related (P 0.01) to the amount of residue present in all soil layers (Table 8). When using a linear model, the amount of residue in the soil explained 35, 22, and 30% of the variability in SOC, SON, and soil C/N ratio, respectively. These results are in general agreement with those of many authors (Larson et al., 1972; Black, 1973; Rasmussen et al., 1980; Rasmussen and Parton, 1994). Generally, however, surface residue has been used instead of soil residue to study crop residue influence on SOC and SON.
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0.05) with soil residue in the 0- to 10-cm depth (r = 0.53) and the amounts of C (r = 0.69) and N (r = 0.59) in the residue. Soil residue increased linearly as surface residue increased. Theoretically, when no residue was present on the surface, there was almost 2000 kg ha-1 of residue remaining in the 0- to 10-cm soil layer. Every 100 kg ha-1 increase in surface residue increased the amount of residue in the 0- to 10-cm soil layer by 40 kg ha-1.
No significant relationship (P 
0.05) existed between surface residue and SOC or SON in any soil depth. These results do not agree with those of Wood et al. (1990), who reported that after 4 yr of no-till, a quadratic relationship existed between added residues and surface (0–5 cm depth) SOC concentration. The different conclusion probably occurred because we correlated measured soil surface residue with SOC while they correlated SOC with the total amounts of residue produced or added by the crops over time.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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0.16) for the more intense WCMF cropping system to have higher SOC and SON contents than the traditional WF system. From 1985 to 1993 and from 1989 to 1993, gains in SOC, SON, and C/N ratio occurred in the surface 0 to 2.5 cm and in the 2.5 to 5 cm of soil while losses were observed in the 5- to 10-cm soil depth. However these effects depended on ET gradient (location) in the former case and on ET gradient (location) and cropping system in the latter period. Also, the direction of the ET gradient effect was not consistent in either case.
Soil organic C and SON buildup was directly related to the amount of residue present in each soil layer, but no relationship was observed between the amount of surface residue and SOC and SON.
There were differences in amount and quality between surface residue and 0- to 2.5-cm soil residue. In some cases, cropping system modified this effect. Potentially, WCMF would mineralize more N than WF because of its larger amounts of C and N and average better residue quality in terms of ADF and lignin content compared with WF. However it is possible that by the sampling time, WF could have undergone more decomposition than WCMF, as evidenced by its narrower C/N ratio and larger ADF and lignin contents.
The results of this study suggest that the production of greater amounts of above- and belowground plant residues promoted by greater cropping intensity under no-till management can create higher levels of organic C and N in the surface soil. Results also demonstrate the need for a permanent monitoring of changes in soil properties when studying the effects of cropping intensity under no-till conditions and indicate that the residue found within surface soil layers is an important factor to consider when interpreting these effects.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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