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Agric. Res. Stn., Fort Valley State Univ., 1005 State University Drive, Fort Valley, GA 31030
* Corresponding author (sainjuu{at}mail.fvsu.edu)
Received for publication May 30, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: CP, chisel plowing • MP, moldboard plowing • NT, no-till • PCM, potential carbon mineralization • PNM, potential nitrogen mineralization
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Changes in soil organic C and N levels occur slowly because of their large pool size and inherent spatial variability (Franzluebbers et al., 1995a; Salinas-Garcia et al., 1997). In contrast, active fractions of organic C and N, such as potential C and N mineralization (PCM and PNM, respectively) and inorganic N, vary seasonally due to changes in the amount of plant residues brought by management practices (Franzluebbers et al., 1995a, 1995b; Salinas-Garcia et al., 1997), rhizodeposition of organic materials in soil from roots during crop growth (Buyanovsky et al., 1986), or seasonal changes in soil moisture and temperature (Kaiser and Heinemeyer, 1993). These fractions have been identified as early indicators of changes in soil organic C and N levels that alter nutrient dynamics in soil (Franzluebbers et al., 1995a, 1995b; Salinas-Garcia et al., 1997).
Conservation tillage, cover cropping, and N fertilization provide opportunity to improve soil quality and sustain crop productivity by conserving organic matter content without influencing crop yield (Doran, 1987). Although conservation tillage, cover cropping, and reduced rate of N fertilization can improve soil organic matter and sustain tomato yield (Sainju et al., 2000a, 2000b; Yaffa et al., 2000), their combined short- (as measured by PCM, PNM, and inorganic N contents) and long-term (as measured by organic C and N contents) effects on soil C and N and tomato yield, however, are not well known. While long-term changes in C and N contents may influence soil quality and productivity, short-term changes may affect nutrient availability and plant growth.
Our objectives were to (i) examine the effects of tillage, hairy vetch cover cropping, and N fertilization rate on soil organic C and N, PCM, PNM, inorganic N contents, and tomato yield and N uptake; (ii) observe relationships among cover crop C and N accumulations, soil C and N, and tomato yield and N uptake; and (iii) determine a best management practice that conserves soil C and N and sustains tomato growth.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The treatments consisted of three levels of tillage [NT, chisel plowing (CP), and MP], two cover crops (hairy vetch vs. winter weeds), and three rates of N fertilization (0, 90, and 180 kg N ha-1). The 180 kg N ha-1 rate is the recommended rate of N fertilization for tomato in central Georgia (Univ. of Georgia, 1995). The CP (or reduced tillage) treatment consisted of harrowing to a depth of 10 to 15 cm, followed by chiseling to a depth of 20 to 25 cm and leveling with a S-tine harrow. Similarly, the MP (or conventional tillage) treatment consisted of harrowing, followed by moldboard-plowing to a depth of 20 to 25 cm and leveling. Nitrogen fertilizer was applied to tomato in summer, not to cover crops. The experiment was arranged in a split split-plot design, with tillage as the main plot, cover crop as the split plot, and N fertilization as the split split-plot treatment in a randomized complete block. Each treatment had three replications. The size of the split-split plot was 7.2 by 7.2 m.
The description and dates of cultural practices used in the experiment are shown in Table 1. The CP and MP plots were tilled two times a year: in September or October for cover crop planting and in April of the following year for tomato transplanting. Plots were harrowed two to three times until plant residues were broken into small pieces and soil particles loosened before plowing. The NT plots were left undisturbed except for drilling cover crop seed, transplanting tomato, and hand-weeding at the soil surface. Every year, treatments were applied to the same plots.
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100 g) was collected for determination of dry matter yield and C and N concentrations. The rest of the sample was spread uniformly in the plot where it was collected before. In the plots without vetch, winter weeds [dominated by henbit (Lamium amplexicaule L.) and cut-leaf evening primrose (Oenolthea laciniate L.)] were collected as above. Plant samples were oven-dried at 60°C, weighed, and ground to pass a 1-mm screen for C and N analysis. After sampling, cover crop and weeds were mowed with a rotary mower in all plots, killed by spraying 3.36 kg a.i. ha-1 glyphosate [N-(phosphonomethyl) glycine] in NT plots, and incorporated into the soil by harrowing in CP and MP plots. Plant residues were allowed to decompose in the soil for 2 wk before tomato was transplanted. Cover crop data were not taken in 1995. At tomato transplanting in late April, P {from triple superphosphate [Ca(H2PO4)2]} and K [from muriate of potash (KCl)] fertilizers were broadcast at 56 kg ha-1 each, based on the soil test. At the same time, diazinon [O,O-diethyl O-(2-isopropyl-6-methyl-4-pyrimidinyl) phosphorothioate] (3.35 kg a.i. ha-1) was applied to control cutworms, and 0.57 kg ha-1 trifluralin [2,6-dinitro-N-dipropyl-4-(trifluoromethyl) benzeneamine] was applied to control weeds. Fertilizers, pesticide, and herbicide were incorporated into the soil by plowing in CP and MP plots and left at the soil surface in NT plots. Weeds in NT plots were controlled by hand-weeding with a spade at the soil surface and in CP and MP plots using cultivator, hand-weeding every week during tomato growth, or both. After land preparation, 5-wk-old tomato (cv. Sunbeam) seedlings were hand-transplanted at a spacing of 0.9 by 0.9 m. Each experimental unit contained eight 7.2-m-long rows of tomato, each row spaced 0.9 m apart. This spacing was used to produce large-size tomato fruits (Univ. of Georgia, 1995). Starter solution containing 3 g L-1 (equivalent to 0.4 kg ha-1) N, P, and K was applied to each tomato plant 1 wk after transplanting to improve early establishment. Nitrogen fertilizer [nitrate of soda (NaNO3)] was split into three doses, and each was broadcast at 3-wk intervals from the date of transplanting. Irrigation (equivalent to 25 mm of rain) was applied using reel rain gun immediately after fertilization and as needed to prevent water stress.
In July and August, tomato fruits were harvested every 3 to 4 d as the color turned from green to pink. Fruits were picked from five plants (4.05-m2 area) in the middle rows, weighed, cut into slices, oven-dried at 60°C, and ground to pass a 1-mm screen for N analysis. At the final fruit harvest, tomato plants were cut 2 cm above the ground, separated by leaves and stems, oven-dried, weighed, and ground to pass a 1-mm screen. Fruit and biomass data in 1995 were not taken although tomato was harvested.
Although the experiment was started in September 1994, soil samples were taken regularly only from September 1995 to August 1997 for C and N analysis. Soil samples were collected in September 1995 and 1996 (after tomato harvest and before cover crop planting), December 1995 and January 1997 (cover crop establishment), April 1996 and 1997 (after cover crop kill and at tomato transplanting), May 1996 and 1997 (tomato establishment), and August 1996 and 1997 (after tomato harvest). These were collected from 0- to 7.5- and 7.5- to 20.0-cm depths from five places in the middle rows with a push tube (5-cm diam.). After separating visible plant residues, samples were composited within a depth, air-dried, ground, and passed through a 2-mm sieve. No soil samples were collected from September 1994 to August 1995.
Laboratory Analysis
Nitrogen concentration in cover crop and tomato samples was determined by the H2SO4–H2O2 method as described by Kuo et al. (1997b). Carbon concentration in cover crop sample was determined by the Walkley–Black method (Nelson and Sommers, 1996), assuming that all plant C was oxidized during digestion. Carbon and N accumulated in cover crops and N uptake in tomato were determined by multiplying dry matter weight by C and N concentrations.
Because soil organic C and N change slowly over time (Franzluebbers et al., 1995a; Salinas-Garcia et al., 1997), organic C and N concentrations were determined only in August 1997 soil samples at the end of the experiment. In contrast, PCM, PNM, and inorganic N concentrations were determined in samples taken at all dates. Soil organic C concentration was determined by the Walkley–Black method. Total N was determined by the Kjeldahl method (Bremner, 1996). The NH4 and NO3 concentrations were determined by steam distillation (Mulvaney, 1996) after extraction with 2 M KCl. Inorganic N was determined as the sum of NH4 and NO3 concentrations and organic N as the difference between total and inorganic N. The PCM and PNM were determined by the method described by Salinas-Garcia et al. (1997), with following modifications: Ten grams of dry soil was moistened with water (0.20 kg kg-1 soil) and incubated in a 1-L jar at 21°C for 10 d. The jar contained beakers with 10 mL of 0.5 M NaOH to trap evolved CO2 and 20 mL of water to maintain high humidity. After 10 d, the CO2–C absorbed in NaOH was titrated with 0.05 M HCl from pH 8.3 to 3.7 to determine PCM (Horwath and Paul, 1994). The PNM was determined by extracting the incubated soil with 100 mL of 2 M KCl for 1 h and analyzing inorganic N by steam distillation. Soil C and N contents (Mg ha-1 or kg ha-1) at 0- to 7.5- and 7.5- to 20.0-cm depths were calculated by multiplying C and N concentrations (g kg-1 soil) by bulk density and depth. Because bulk density was not significantly affected by treatments and interactions at the beginning and end of the experiment, their average value was used to calculate C and N contents for a treatment. The bulk density at the end of the experiment ranged from 1.25 to 1.34 Mg m-3 at 0 to 7.5 cm and from 1.41 to 1.49 Mg m-3 at 7.5 to 20.0 cm.
Data Analysis
Data for cover crop C and N accumulations, tomato yield and N uptake, and soil organic C and N contents were analyzed using the MIXED procedure of SAS after testing for homogeneity of variance (Littell et al., 1996). For analyzing cover crop and tomato parameters, year was considered as the main plot, tillage as the split plot, cover crop as the split-split plot, and N fertilization as the split-split-split plot treatment. For analyzing soil organic C and N, tillage was considered as the main plot, cover crop as the split plot, and N fertilization as the split split-plot treatment. Because PCM, PNM, and inorganic N contents in soil were measured repeatedly over time, their data were analyzed using the procedure of Analysis of Repeated Measures (Littell et al., 1996). For this, date of sampling was considered as the main plot, tillage as the split plot, cover crop as the split-split plot, and N fertilization as the split-split-split plot treatment. Means were separated using the least square means test when treatments and interactions were significant. Correlation analysis was done to determine relationships among cover crop C and N accumulations, soil C and N contents, and tomato yield and N uptake on mean of three replicates. Statistical significance was evaluated at P 
0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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0.05). Hairy vetch, being a N-fixing legume, had two- to sixfold greater C and N accumulations, averaged across tillage and N fertilization rate, than winter weeds in the no-vetch plot in 1996 and 1997 (Table 2). Nitrogen accumulation in vetch and weeds was greater in 1996 than in 1997 due to higher N concentration (18.7–37.5 g kg-1 vs. 14.1–18.5 g kg-1). The lower N concentration in vetch in 1997 was probably due to greater proportion of weeds that were not controlled by cover crop. The C/N ratio was lower in vetch than in weeds.
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0.05). Except for the interaction of year with tillage, cover crop, and N fertilization, other interactions were not significant (P 0.05). In 1996, fruit yield and N uptake, averaged across cover crop and N fertilization rate, were greater in CP and MP than in NT, and dry matter weight was greater in MP than in NT (Table 3). In 1997, dry weight and N uptake, averaged across tillage and N fertilization, were greater with hairy vetch than with winter weeds. Fruit yield, averaged across tillage and cover crop, was also greater with 90 and 180 kg N ha-1 than with 0 kg N ha-1 in 1996 and 1997, and N uptake was greater with 180 than with 0 kg N ha-1 in 1997. Fruit yield, dry weight, and N uptake were similar in CP vs. MP and with 90 vs. 180 kg N ha-1. Averaged across years, fruit yield and N uptake were greater in CP and MP than in NT and greater with 90 and 180 kg N ha-1 than with 0 kg N ha-1, and dry weight was greater in MP than in NT.
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0.05) occurred between these treatments at the end of the experiment in August 1997. With or without cover crop, organic C and N, averaged across N fertilization rate, were greater in NT and CP than in MP at 0 to 7.5 cm but were greater in NT with vetch than in CP and MP with vetch or weeds at 7.5 to 20.0 cm (Table 4). At 0 to 20 cm, organic C and N were greater in NT with vetch than in MP with vetch or weeds. Averaged across cover crop and N rate, organic C was 20 to 23% greater and organic N 23 to 31% greater in NT and CP than in MP at 0 to 7.5 cm, but organic C was 16% greater and organic N 17% greater in NT than in CP at 7.5 to 20.0 cm after 3 yr. As a result, organic C and N decreased by 3 to 4% in NT, 7 to 13% in CP, and 26 to 33% in MP at 0 to 7.5 cm from 1994 to 1997. Similarly, at 7.5 to 20.0 cm, organic C and N decreased by 1 to 4% in NT, 18 to 19% in CP, and 6 to 14% in MP. At 0 to 20 cm, organic C and N decreased by 2 to 4% in NT, 14 to 16% in CP, and 18 to 19% in MP. While cover crop and N rate did not alter organic C, mean organic N was greater with vetch than with weeds across tillage and N rate and greater with 180 than with 90 kg N ha-1 across tillage and cover crop. The C/N ratio was lower in NT than in MP at 0 to 7.5 cm and lower with vetch than with weeds at both depths. Both soil organic C and N did not correlate with tomato yield and N uptake in 1997.
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0.001) on soil PCM and interacted significantly (P 0.01) with tillage. As a result, PCM—averaged across tillage, cover crop, and N rate—was greater in May and September 1996 and April 1997 than at other sampling dates and greater in April 1997 than in May 1996 (Fig. 2)
. While PCM, averaged across cover crop and N rate, was greater in MP than in NT and CP at 7.5 to 20.0 cm in May 1996, it was greater in NT and CP than in MP at 0 to 7.5 cm in April 1997. Averaged across cover crop, N rate, and date of sampling, PCM was 28 to 34% greater in NT and CP than in MP at 0 to 7.5 cm (Table 5). Similarly, mean PCM across tillage, N rate, and date of sampling was 7 to 10% greater with vetch than with weeds. Fertilization with N did not influence PCM.
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0.05, n = 18). Similarly, PCM was correlated with soil organic C and N in August 1997 (r = 0.71–0.76, P 0.01, n = 18). The proportion of organic C in PCM ranged from 1.1 to 1.8%, depending on tillage, N fertilization rate, and soil depth. The PCM in May 1996 and April 1997 was correlated with tomato yield and N uptake (r = 0.62–0.63, P 0.05, n = 18).
Potential Nitrogen Mineralization and Inorganic Nitrogen
As with PCM, date of soil sampling strongly influenced (P 0.001) soil PNM and inorganic N and interacted significantly (P 0.01) with tillage, cover crop, and N fertilization. Both PNM and inorganic N—averaged across tillage, cover crop, and N rate—were greater in May 1996 than in April 1997 (Fig. 3 and 4)
. Similarly, PNM and inorganic N were greater in May 1996 and April 1997 than at other sampling dates, except August and September 1996 and August 1997. Averaged across cover crop and N rate, PNM and inorganic N were greater in MP than in NT and CP at 7.5 to 20.0 cm in April and May 1996 but were greater in NT and CP than in MP at 0 to 7.5 cm in April 1997. Similarly, PNM and inorganic N, averaged across tillage and N rate, were greater with vetch than with weeds in April 1996 and 1997. Mean PNM and inorganic N across tillage and cover crop were greater with 180 than with 0 kg N ha-1 in May 1996 and August 1997. Averaged across cover crop, N rate, and date of sampling, PNM was 25% greater in NT and CP than in MP at 0 to 7.5 cm but was 17% greater in MP than in CP at 7.5 to 20.0 cm (Table 5). Similarly, mean PNM and inorganic N were 13 to 22% greater with vetch than with weeds across tillage and N rate and 10 to 31% greater with 180 kg N ha-1 than with 90 or 0 kg N ha-1 across tillage and cover crop.
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0.01, n = 18), PCM (r = 0.65–0.68, P 0.05, n = 18), and tomato yield and N uptake (r = 0.53–0.78, P 0.05, n = 18). The PNM and inorganic N were also correlated with soil organic N (r = 0.55–0.67, P 0.05, n = 18) in August 1997. The proportion of organic N in PNM ranged from 2.5 to 13.8%, depending on tillage, N rate, and soil depth. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The greater soil organic C and N contents in NT with vetch than in CP and MP with vetch or weeds at 0 to 7.5 cm and 7.5 to 20.0 cm (Table 4) may have resulted from increased C and N inputs from vetch compared with weeds (Table 2), followed by surface placement of its residue in NT. While plant residue addition and rate of residue decomposition determine organic C and N levels in soil (Kuo et al., 1997a, 1997b), placement of residue at the soil surface in NT increases organic C and N because of its reduced contact with microorganisms, thereby reducing the decomposition rate of residue compared with MP (Havlin et al., 1990; Franzluebbers et al., 1995a; Salinas-Garcia et al., 1997). Similarly, greater soil organic C and N levels with vetch than with weeds in NT may be attributed to increased C and N inputs from vetch. Furthermore, increased soil coverage with vetch compared with weeds in NT during winter also may have conserved organic C and N because coverage reduces soil erosion and fallowing, both of which reduce organic C and N levels (Grant, 1997). Because organic C and N were strongly correlated with one another (r = 0.90–0.92, P 0.001), both behaved similarly with tillage, cover crop, and N fertilization treatments. The results indicate that NT with legume cover crop can conserve both soil organic C and N better than CP and MP with or without cover crop.
We observed stronger influence of tillage on organic C and N than cover cropping or N fertilization (Table 4). We believe that the larger difference in organic C and N levels between NT and MP after 3 yr of the experiment was probably a result of twice-a-year cultivation in MP plots for cover crop and tomato transplanting as opposed to once-a-year cultivation performed in other studies (Havlin et al., 1990; Franzluebbers et al., 1995a; Salinas-Garcia et al., 1997). In these studies, the difference in organic C between NT and conventional tillage at 0 to 20 cm after 4 to 16 yr was 0.1 to 4.2 Mg ha-1 compared with 3.0 Mg ha-1 obtained in this study after 3 yr. Soil organic C decreases with increasing intensity and frequency of tillage (Franzluebbers et al., 1999). Although soil organic C was not influenced by cover crop or N rate, increased soil organic N with vetch compared weeds or increasing N rate probably resulted from increasing N input. Increased soil organic N concentration with cover cropping or N fertilization has been observed by several researchers (McVay et al., 1989; Liang and McKenzie, 1992; Kuo et al., 1997b; Omay et al., 1997).
The increased soil PCM, PNM, and inorganic N in May 1996 and April 1997—regardless of tillage, cover crop, and N fertilization (Fig. 2, 3, and 4)—probably resulted from increased C and N inputs from cover crop residue addition in April 1996 and 1997, followed by increased temperature from winter to spring (Fig. 1). While soil labile C and N pools following cover crop incorporation are proportional to the amount of C and N contributed by cover crop residues (Kuo et al., 1997a, 1997b), increasing temperature can increase microbial activities and N mineralization and availability (Alexander, 1961; Stanford et al., 1975). A significant correlation (r = 0.65–0.68, P 0.05, n = 18) observed among PCM, PNM, and inorganic N in April and May, 1996 and 1997, also indicates that increased microbial activities probably increased N mineralization and availability. However, greater PCM peak followed by lower PNM and inorganic N peaks in April 1997 (Fig. 2, 3, and 4) suggests that increased microbial activities also may immobilize N in soil microbial biomass. Similarly, increased PCM, PNM, and inorganic N in September 1996 and August 1997 probably resulted from tomato (leaf fall and root) residue addition and/or rhizodeposition to the soil during tomato growth (Franzluebbers et al., 1995a, 1995b; Salinas-Garcia et al., 1997). Addition of plant residue increases C availability in the soil, thereby stimulating microbial activities and C mineralization (Alexander, 1961; Hu et al., 1997). Labile C pools, such as PCM, microbial biomass C, or carbohydrate, increase from 7 to 28 d after incorporation of plant residues into soil (Franzluebbers et al., 1995b; Hu et al., 1997; Kuo et al., 1997a) because plant C decomposes rapidly in the soil, with half lives ranging from 2 to 9 wk (Buchanan and King, 1993; Kuo et al., 1997a). Therefore, crop residue addition can influence soil microbial activities and N mineralization and availability, which influence tomato yield and N uptake (Yaffa et al., 2000).
As with organic C and N, greater PCM, PNM, and inorganic N levels in NT and CP than in MP at 0 to 7.5 cm may have resulted from reduced degree of incorporation due to surface placement or reduced decomposition of plant residue in soil. In contrast, greater levels in MP than in NT and CP at 7.5 to 20.0 cm may have resulted from increased incorporation of residue at greater soil depth. Similarly, increased PCM, PNM, and inorganic N with vetch than with weeds or with increasing rate of N fertilization may have resulted from increased C and N inputs from plant biomass.
The difference in tomato yield and N uptake between 1996 and 1997 and in soil PCM, PNM, and inorganic N levels between May 1996 and April 1997 can be explained by the variation in the amount of cover crop C and N inputs and temperature and rainfall pattern in 1996 and 1997. Increased cover crop N accumulation (Table 2) followed by warmer temperature in May 1996 compared with 1997 (Fig. 1) may have increased PNM and inorganic N, thereby increasing tomato yield and N uptake in 1996 compared with 1997. In contrast, greater rainfall from May to July in 1997 than in 1996 may have increased soil moisture content, thereby increasing potential for N leaching as inorganic N level from May to August was relatively lower in 1997 than in 1996 (Fig. 4). Increased soil moisture also may have reduced tomato root growth and N uptake capacity in the anaerobic condition, thereby influencing tomato yield.
Significant correlation between soil PCM, PNM, and inorganic N and tomato yield and N uptake as opposed to nonsignificant correlation between soil organic C and N and tomato yield suggests that labile pools of soil C and N are better indicators of crop productivity than labile + recalcitrant pools. The correlation was greater with inorganic N (r = 0.71–0.78, P 0.01, n = 18) than with PCM and PNM (r = 0.55–0.63, P 0.05, n = 18), indicating that N availability in soil is a better predictor of tomato yield and N uptake than PCM and PNM.
The increased soil organic C and N levels in NT with hairy vetch after 3 yr suggests that NT with legume cover cropping can be successfully used to conserve organic C and N levels, especially in southeastern USA and in regions where organic matter level is low (Doran, 1987) and where mild winter supports cover crop growth (Sainju et al., 2000a, 2000b). As a result, it will help to reduce the deleterious effects of global warming by sequestering atmospheric C and N in soil. Because hairy vetch also supplies substantial amount of N for summer crop (Hargrove, 1986; McVay et al., 1989; Kuo et al., 1997b) and increased soil inorganic N level and tomato yield similar to that increased by 90 and 180 kg N ha-1 (Fig. 4; Table 4), it has the additional advantage of substituting or reducing the rate of N fertilization without substantially reducing tomato yield (Table 3), thereby decreasing the potential for N leaching in ground water (Sainju and Singh, 1997). Because tomato yield and N uptake were lower in NT compared with CP and MP but were similar in CP and MP, and because organic C and N levels in CP with vetch were in between the levels in NT and MP with or without vetch, reduced tillage, such as CP, with hairy vetch may be a best management practice to sustain tomato yield without using N fertilizer and to conserve soil organic matter level compared with MP with or without vetch. The potentials for soil erosion and N leaching can also be reduced using this practice because of decreased soil disturbance and N mineralization. As tillage had greater influence on organic C and N levels in soil previously cropped with alfalfa for 8 yr compared with cover cropping or N fertilization, conservation tillage practices are needed to sustain soil quality and productivity when cash crops, such as tomato, are produced.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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