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Agronomy Journal 93:207-215 (2001)
© 2001 American Society of Agronomy

PRODUCTION PAPERS

Residue Decomposition and Soil Nitrogen are Affected by Mowing and Fertilization of Marigold

Bonnie R. Ball-Coelho, L.Bruce Reynolds, Allison J. Back and John W. Potter

Agric. and Agri-Food Canada, S. Crop Protection and Food Res. Cent., Box 6000, Vineland Stn., ON, Canada L0R 2E0

Corresponding author (ballb{at}em.agr.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
To suppress root-lesion nematodes (Pratylenchus penetrans Cobb), marigold (Tagetes sp.) is grown as a rotation crop; however, little is known about its decomposition. The timing of N release to soil affects both the nutrition of the subsequent crop and also the environment, which could possibly be altered by biocides produced by marigold. Decomposition was quantified in the field by monitoring residues of marigold and cereal rye (Secale cereale L.), a common rotation crop, over time in litter bags subjected to different conditions. Marigold decomposition proceeded normally and without toxic effects on decomposers. In the fall of rotation years, topsoil NO3 concentration was usually higher under marigold (1.1 mg kg-1) than under rye rotation (0.3 mg kg-1), but this depended on the method of marigold management. In marigold plots, fall NO3 levels were greatest where plants were mowed early (August) or fertilized with 90 kg N ha-1 and lowest where plants were left standing over winter. In plots where marigold was mowed in September or left standing, fall NO3 levels were sometimes no higher than in rye plots. Overwinter N release from bags of marigold shoots (stems and leaves) on the soil surface (39 kg ha-1) was less than that from buried bags (119 kg ha-1). Together, these results suggest that a marigold rotation may be a viable alternative to rye, but to minimize N loss, marigold crops should be left standing over winter and preplant fertilized with 45 kg N ha-1.

Abbreviations: DM, dry matter • k, decomposition constant • Nini, initial total nitrogen concentration • Ninorg, soil inorganic nitrogen concentration


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
CROP ROTATION WITH MARIGOLD effectively controls damage from root-lesion nematodes (Reynolds et al., 2000). Marigold residues and their extracts are reportedly toxic to a variety of organisms, including nematodes (Kumari et al., 1986; Scramin et al., 1988; Siddiqui and Alam, 1988; Caswell et al., 1991), fungi (Makhatsa et al., 1993; Owino, 1992), insects (Perich et al., 1994; Weaver et al., 1994), and even germinating seeds (Tang et al., 1987). Tagetes patula L. roots exude thiophenes and benzofurans (Tang et al., 1987), but the specificity of the mechanism for nematode control is not well defined (Gommers and Bakker, 1988). Marigold could therefore also be toxic to soil organisms that are responsible for decomposing plant residues and cycling crop nutrients. Despite this possibility, the effect of decomposing marigold residue on the release of N has not been investigated. Litter decomposition rates are frequently considered to be regulated by substrate quality; and litter N content, C/N ratio, and lignin/N ratio have been shown to be of critical importance to litter decay (Gallardo and Merino, 1999) in systems where P is not limiting.

To develop best management practices for farming systems that utilize marigold for biological control, information on N cycling is needed. Specifically, an understanding of the timing of N release to the soil is required because this significantly affects both the nutrition of subsequent crops and the loss of N to the environment. In spring and fall, the potential for groundwater contamination by NO3 is extremely high, especially in sandy soils where NO3 is readily leached. When rye is planted as a rotation crop, it actively absorbs NO3 in late fall and early spring when most leaching occurs (Ball-Coelho and Roy, 1997), and thus minimizes the movement of NO3 to groundwater.

The overall goal of this study was to develop a marigold rotation cropping system to biologically control root-lesion nematodes while the main objective was to determine the effects of a marigold rotation on decomposition and soil nutrient dynamics. Specifically, the objectives were: (i) to quantify the decomposition and nutrient release from marigold residues relative to that of rye, a common rotation crop, and to determine what factors influence this release; and (ii) to compare the soil N levels over time under rye rotations vs. rotations using different marigold cultivation techniques. The first objective was addressed by measuring the standing crop nutrient content of both rye and marigold and also by monitoring changes in rye and marigold residue content over time in litter bags that were subjected to different situations in the field. The second objective was addressed through repeated soil sampling and analysis over time in both marigold and rye rotation systems. This study will help to predict the impacts of marigold rotations on subsequent crop nutrition and could also indicate whether nontarget soil organisms are affected by growing marigold. These factors will determine whether the proposed system is agronomically and environmentally sustainable.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Summary and conclusions
 REFERENCES
 
Plant and Soil Sampling
Two adjacent sets of plots (Exp. A and B) with treatments as outlined in Table 1 were located in southwestern Ontario, Canada (42°52' N, 80°31' W; 182 m above sea level). The mean annual temperature is 7.9°C, with an average of 136 frost-free days. The total annual precipitation averages 951 mm, with 52% received from April through September. The soil is a Psammentic Hapludalf (Fox loamy sand) with 85 to 90% sand in the A horizon. Selected soil chemical properties are listed in Table 2. Two sets of plots were established so that both a rotation crop (rye or marigold) and a tobacco crop (Nicotiana tabacum L. cv. Delfield) could be grown each year. The first rotation crop year was 1995 for Exp. A and 1996 for Exp. B. Both experiments were planted with tobacco in the final year of the study (1998).


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  		Table 1 Crop management practices of selected treatments in rotation years 1995 and 1997 (Exp. A) and 1996 (Exp. B)

 

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  		Table 2 Concentrations of available nutrients in the top 0.2 m of soil in the year of the first rotation for Exp. A (1995) and Exp. B (1996) (avg. of four blocks per exp.), and of the total C and N (means of five sampling dates from 1995–1997 for Exp. A and three dates from 1996–1997 for Exp. B)

 
Rye was drilled into all plots in the fall following tobacco harvest. In the spring of the rotation crop year, rye treatment plots received 45 kg N ha-1 broadcast as ammonium nitrate (NH4NO3). The rye treatment plots were disked in late July or early August to incorporate the mature rye, and an additional 20 kg N ha-1 was broadcast as NH4NO3. The disked rye self-seeded to provide a winter cover crop before the tobacco crop year. In the marigold rotation plot treatments, the winter rye cover crop was incorporated by plowing and secondary tillage immediately before drilling T. patula L. cv. Creole or T. erecta L. cv. CrackerJack seed. Forty-five kilograms per hectare of N as urea [(NH2)2CO] was applied as a preplant incorporated broadcast on 19 May 1995 (Exp. A), 22 May 1996 (Exp. B), and 26 May 1997 (Exp. A). For the Sept.-mow T + F treatment, an additional 45 kg N ha-1 was broadcast as NH4NO3 in late June or early July, and all plants in the Aug.-mow T and Sept.-mow T treatments were mowed down in August and September, respectively (Table 1). In early May of the following season (the tobacco year), rye rotation plots were plowed, and immediately before spring transplanting of the tobacco crop, marigold plants in rotation plot treatments that were left standing over winter (Stand T) were flail-mowed, and then all marigold treatment plots were plowed. Other cultural practices are detailed by Reynolds et al. (2000).

The mature rye shoot weight was determined each year by hand-cutting all plants from one 1-m2 area in each of the eight rye plots immediately before disking in the rotation crop year. The rye shoots were separated into straw and heads for dry matter (DM) determination and analyses. Additionally, the rye shoot regrowth the following spring was measured on 26 Apr. 1996 (Exp. A) and 21 Apr. 1997 (Exp. B) on one 0.252-m2 area per rye plot. The shoots were rinsed with distilled water before oven-drying. The marigold shoot weight was determined by cutting 20 plants at random immediately before the mowing operations. The shoots were separated into stalks and leaves, and DM production was calculated based on plants that were counted in 8-m sections of four rows. The standing dead T. patula (mainly stems by the spring) was measured on 14 May 1996 (Stand T, Exp. A) on one 0.252-m2 area per plot. Adhering soil was removed by brushing before DM determination.

Marigold and rye roots were sampled by taking one soil core adjacent to a plant per plot using a 0.1-m i.d. auger that was constructed with a low-compaction cutting head. The roots were sampled from Sept.-mow T. patula and T. erecta treatments from the top 0.3 m of the soil on 10 Sept. 1996 (Exp. B) and 15 Sept. 1997 (Exp. A). Rye roots were sampled from the top 0.3 m of the soil on 23 July 1996 (Exp. B) and 22 July 1997 (Exp. A). The roots were separated from the soil by hydropneumatic elutriation (Smucker et al., 1982) for DM determination. All plant material was oven-dried at 65°C for DM determination, and in cases where the samples were large, a subsample was taken for chemical analyses.

A composite of ten 0.02-m diam. soil cores per plot were taken from the top 0.2 m of the soil each spring and fall. Samples were obtained from Exp. A plots on 24 Apr. and 10 Nov. 1995; 2 May and 4 Nov. 1996; 23 Apr. and 5 Nov. 1997; and 28 Apr. and 4 Nov. 1998. Samples were collected from Exp. B plots on 24 Apr. and 6 Nov. 1996; 21 Apr. and 5 Nov. 1997; and 27 Apr. and 4 Nov. 1998. Soil inorganic N (Ninorg) was extracted by shaking 25 g of field moist soil in 25 ml of 2N KCl for 1 h. Soil NO3 and NH4 concentrations were determined from the filtered extract (Maynard and Kalra, 1993) by continuous-flow colorimetry (Tel and Rao, 1981). The soil water content was determined gravimetrically so that Ninorg concentrations could be converted to a dry-weight basis. Total and macroorganic (particulate + sand fraction) C and N were determined for spring and fall soil samples. The macroorganic matter (similar to the light fraction) was separated by shaking 12.5 g of air-dried soil in 50 ml of sodium hexametaphosphate (5 g L-1) for 60 min and then washing it through a 53-µm (270 mesh) sieve (Gregorich and Ellert, 1993).

Residue Decomposition in Litter Bags
To measure the release of N from marigold, shoots were collected before the September mowing on 27 Sept. 1995 (Exp. A), 23 Sept. 1996 (Exp. B), and 22 Sept. 1997 (Exp. A). The shoots were then separated into leaves and stems, and the stems were cut into ~8 cm lengths. Nylon mesh litter bags (0.13 by 0.13 m in 1995 and 0.13 by 0.26 m in subsequent years) were filled with ~10 g (fresh wt.) of stems or leaves per bag in 1995 and ~35 g of fresh leaves or 60 g of fresh stems per bag thereafter. The exact fresh weight of each bag was recorded. The bag mesh opening was 1.5 mm, which was small enough to prevent major losses of leaves but large enough to allow free entry of small animals (Gallardo and Merino, 1999). Four bags each of marigold leaves and stems were placed on the soil surface at the same plot where residue material was originally collected from on 28 Sept. 1995 (Exp. A), 25 to 26 Sept. 1996 (Exp. B), and 24 Sept. 1997 (Exp. A). Two leaf and two stem bags per plot were retrieved on 14 Nov. 1995 and 13 May 1996 (Exp. A); 21 Nov. 1996 and 12 May 1997 (Exp. B); and 18 Nov. 1997 and 5 May 1998 (Exp. A). The retrieved material was cleaned either by brushing, rinsing with distilled water, or by floatation over nested sieves, depending on the condition. In November 1995, the retrieved litter bags were bulked for chemical analyses, but thereafter each litter bag was analysed separately.

To determine whether marigold culture altered the activity of the organisms involved in residue decomposition, a switch-plot experiment was used. Rye residue litter bags (0.13 by 0.26 m with a 1.5-mm opening) were simultaneously placed in both rye and T. patula plots, and T. patula residue litter bags were simultaneously placed in both T. patula and rye plots. From Exp. A, 10 T. patula plants were cut before mowing from the Aug.-mow T treatment on 26 Aug. 1997 and separated into leaves and stems while rye shoots were collected on 1 Aug. 1997 and separated into heads and stems. The rye heads were microwaved for 5 min to prevent the seeds from germinating. After weighing, the following amounts were placed in 16 litter bags and then buried about 0.05 m deep on 27 Aug. 1997: T. patula leaves, 35; T. patula stems, 60; rye heads, 13; and rye straw, 10 g (fresh wt basis). In total, there were four replications of each rotation treatment (T. patula or rye), with each plot containing two buried litter bags of each of the four litter types (T. patula leaves, T. patula stems, rye heads, and rye straw). One litter bag per plot of each litter type was recovered on 17 Nov. 1997 and 27 Apr. 1998, and the residues were cleaned by brushing.

The dry weights of all residue samples were determined by oven-drying at 65°C. Before analysis, the litter samples with large particles were ground to <1 mm in a hammer mill. All plant and soil samples were then ground and mixed by rolling for 24 h on a conveyor in glass jars containing 5 (macroorganic fraction), 8 (whole soil), or 20 (plant material) sets of stainless steel rods. Each rod set was comprised of one 6-, two 4-, and three 3-mm diam. rods. The C, N, and S concentrations in the plant material and soil were determined by combustion analysis (LECO Corp., St. Joseph, MI).

The total DM weight of the residues that were initially placed in the field was determined for each bag by oven-drying a separate subsample of the same material, calculating the dry-weight/fresh-weight ratio for each subsample, and then multiplying this ratio by the fresh weight of each corresponding bag. The oven-dried residues were also ashed in a muffle furnace at 500°C for 4 h to minimize the effects of soil contamination in the litter bags. Ash-free initial DM weights for each bag were calculated by multiplying the percent ash by the initial DM weight for each bag and then subtracting this quantity from the initial uncorrected DM weight. The uncorrected DM weight of residue contained in each bag recovered from the field in November and May was determined by oven-drying and weighing all residue contained in a bag. The ash-free DM weight was then determined by ashing and weighing the oven-dried material and subtracting this quantity from the uncorrected DM weight for each bag. The relationship between the percent of initial residue weight remaining and time was determined using an exponential decay model

(1)
where k is the instantaneous fractional loss rate or decay constant, and t is the number of days. The percent of initial residue weight was calculated by dividing the ash-free DM weight that was measured t days after burial by the initial ash-free DM weight. The constant k is sometimes difficult to interpret because it is influenced by numerous environmental factors, especially in agroecosystems; but despite this, it allows for a general comparison of loss functions over a wide range of ecosystems and situations (Coleman et al., 1984).

The concentrations of C, N, and S in residue samples were also adjusted for the total C, N, and S content of soil accumulated within the residues. Soil contamination in the combusted sample was calculated from the sample ash weight minus the initial ash weight. The measured average concentrations in soil (7.0, 0.5, and 0.03 g kg-1 of C, N, and S) were then multiplied by the soil contaminant weight and subtracted from the measured C, N, or S concentrations. The lignin concentration was determined for selected plant samples that were ground to <2 mm using ANKOM fibre bags or crucibles with 72% sulfuric acid (H2SO4) for acidification of the acid-detergent fibre residue (Rowland and Roberts, 1994).

Data Analyses
Dry matter, C, N, and S variation in mature rye straw and heads and in marigold stems and leaves in September was compared using an analysis of variance (ANOVA) according to the randomized complete block design. Variation in the soil Ninorg concentration according to the treatment (Rye, Stand T, Aug.-mow T, Sept.-mow T, and Sept.-mow T + F) was also analyzed according to the randomized complete block design using an ANOVA. Whole soil and macroorganic fraction C and N were compared using repeated-measures analysis, with time as the repeated factor. The percent of ash-free residue remaining and the C, N, and S concentrations (initial and corrected) of marigold leaves and stems from bags placed on the soil surface were analyzed according to the randomized complete block design, with marigold species and plant part as factors. The percent of ash-free residue remaining and the C, N, and S concentrations of rye and T. patula shoots that were buried in the switch-plot experiment were analyzed according to the randomized block design, with location and plant part as factors. An ANOVA was performed using the General Linear Models Procedure (SAS Inst., 1989), and when factors in the analysis were significant, the appropriate means were compared using the protected LSD and a 0.05 probability level.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Summary and conclusions
 REFERENCES
 
Standing Crop
Neither the mow time nor the addition of extra N fertilizer affected marigold DM production or the population of soil root-lesion nematodes, other than the increase in stalk yield of 1 Mg ha-1 that occurred when mowing was delayed from late August to late September in 1996 (Reynolds et al., 2000). The amount of C in the standing crop was similar for Sept.-mow T and rye (Table 3) while the amount of N was usually nearly twice as large in marigold compared with rye. The shoot C/N ratio of marigold (40; calculated from fall 1995 data in Table 3) was more narrow than that of mature rye (55) but wider than that of the spring rye regrowth (17), indicating that the N contained in the tissues of the spring rye regrowth was being released relatively quickly. The rapid release of N from spring-incorporated rye regrowth has also been observed in other systems (Ball-Coelho and Roy, 1997). The amount of S in the standing crop was twice as large in marigold compared with rye in 1996 and more than three times as large in marigold compared with rye in 1995 and 1997 (Table 3). The S concentration was less in rye shoots than in marigold shoots and less in mature rye roots (1.7 g S kg-1) than in marigold roots (6.8 g S kg-1; data not shown). These trends are interesting, as Bakker et al. (1979) suggested that nematode control may be related to S-containing compounds in roots.


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  		Table 3 Dry biomass (DM) and C, S, and N content of marigold and rye in the fall and of rye shoot regrowth in April

 
Soil Carbon and Nitrogen
Neither the whole soil total C or N nor their macroorganic fractions varied with the rotation crop (data not shown). Typically, the total C and N in soil does not change with cropping practice over the short term. Macroorganic matter, a measure of root and litter fragments and some C associated with the sand fraction, can be a more sensitive indicator of short-term changes than the larger total C pool (Barrios et al., 1996). Angers et al. (1999) found that the total and light-fraction C measures were correlated with crop residue C inputs in fine sandy loam. In our study, treatments were applied over a short period of time, and the standing crop C content of rye in rotation plots tended to be similar to that of marigold (Table 3). Therefore, it is not surprising that whole soil and macroorganic C and N were the same under both rotation crops, and major changes in soil C are not expected over the long term.

The soil NO3 levels in the rye treatment plots were sometimes lower than those of certain marigold treatment plots (Table 4), especially in the fall of the rotation year (early November) and the following spring (late April). Although not always significant, the soil NO3 levels in the fall of the rotation years were always ranked: Rye < Stand T < Sept.-mow T < Aug.-mow T < Sept.-mow T + F. The marigold plots may have had slightly more NO3 in the soil than the rye plots by November (when marigold plants were dead) because self-seeded rye continued to grow and absorb NO3 into the late fall while marigold was killed by the first hard frost (early October), as is typical in southwestern Ontario. Furthermore, the amount of N in the mature standing marigold crop was twice that of rye 2 or 3 mo earlier (Table 3), and the C/N ratio was more narrow. These characteristics of marigold tissues may have led to a greater release of N to the soil in the marigold plots than in the rye plots. Despite the noted differences between marigold and rye, certain marigold treatments were better at minimizing fall soil NO3 than others. Some marigold treatments resulted in rotation-year fall (early November) soil NO3 levels that were no greater than the levels in rye rotation plots. For example, in the fall 1997 rotation year, the soil NO3 in rye plots was equivalent to that of both the Stand T and Sept.-mow T plots (Exp. A; Table 4). The soil NO3 levels in the Aug.-mow T or Sept.-mow T + F plots were always greater than in Stand T plots, and two-thirds of the time they were greater than in the Sept.-mow T plots in the fall of the rotation years (Table 4). Specifically, the NO3 levels in the fall 1995 rotation year (Exp. A) were greater in the Aug.-mow T and Sept.-mow T + F plots than in the Stand T or Sept.-mow T plots. The trend was repeated in the fall 1996 (Exp. B) and fall 1997 (Exp. A) rotation years (Table 4). Doubling the amount of fertilizer N with half applied later in the season, probably led to the greater soil NO3 levels in the Sept.-mow T + F treatment. Marigold does not regrow after cutting, but when mowing is delayed from August until September, marigold continues to grow (Reynolds et al., 2000). Therefore, early (August) mowing halted plant growth and N uptake earlier in the season than either later (September) mowing or leaving plants standing over winter. Early mowing also accelerated the transfer of N from residues to the soil, particularly from marigold leaves, which decompose rapidly (discussed below). This is because residues were returned to the soil surface earlier and the decomposition process was able to begin sooner.


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  		Table 4 Soil NO3–N in the top 0.2 m in spring (late April) and fall (early November) in the rotation (R) and tobacco (T) crop years for Exp. A and B

 
Some other soil NO3 trends were noted (Table 4). In the spring of tobacco crop years 1996 and 1998 (Exp. A) and 1997 (Exp. B), the soil NO3 levels were lowest in the rye plots. Also, soil NO3 was lower on these sampling events in Aug.-mow T than in Sept.-mow T or Stand T plots. This was probably the result of a greater N loss over winter when marigold had been mowed earlier in the growing season because the N uptake was similar for August and September mowed treatments (data not shown). In the Stand T plots, physical separation of residues from the soil delayed decomposition and allowed more N to be conserved within the system over winter. In the spring of the 1997 rotation year (Exp. A), the rye plots contained more NO3 (Table 4) and NH4 (data not shown) than all other plots because they had been fertilized with N before soil sampling. Otherwise, the NH4 levels usually did not vary by treatment, except in the fall of 1997 and the spring of 1998 when they were greater in the rye plots than in all others, and in the fall of 1998 for Exp. B when they were lower in the rye plots than in all others (data not shown). Soil NO3 was greater for both experiments in the fall of 1998 than at any other time, which was probably due to drier soil on 4 Nov. 1998 (0.05–0.06 kg kg-1) than on the other sampling events (0.09–0.14 kg kg-1). The differences noted in soil Ninorg had little impact on the tobacco yields the following year. The yields were greater in marigold rotation plots than in rye plots (mean yield increase was 197 kg ha-1), which was probably a result of the lower soil populations of root-lesion nematodes following marigold rotation (Reynolds et al., 2000).

Effects of Plant Part and Plant Type on Decomposition
The litter decomposition rates of the two marigold species (T. patula and T. erecta) did not vary greatly from one another, except in the 1996 to 1997 experiment when the stems of T. erecta decomposed slightly faster than those of T. patula (; data not shown). As expected, based on the known relationship between the C/N ratio and decomposition (Coleman et al., 1984), residue breakdown proceeded more rapidly for marigold leaves than for stems. This pattern was noted both in surface-placed and buried bags. For surface-placed bags (Fig. 1) , the average decay constant (k) for marigold stems was less than half of that for leaves, and by spring, the percent of initial residue remaining for stems (59%) was nearly double that of leaves (36%). Stem–leaf differences of similar magnitude were observed in buried bags (Fig. 2) . Mansoer et al. (1997) also noted this trend in a warmer climate for sunnhemp (Crotalaria juncea L.), with shoot DM losses from September to May (40%) that were very similar to those estimated in the present study (about 59% loss). Greater concentrations of lignin and lower concentrations of N in stems than in leaves were found both in their study and in ours. In both the surface-placed (Table 5) and buried-bag (Table 6) experiments, the lignin concentration was always greater and the N concentration lower in the stems than in the leaves. The initial N concentrations and N release measured in both studies were also comparable. From September to May, the N released from marigold leaves plus stems in the present study ranged from 17 to 50 kg N ha-1, or 25 to 47% of initial N (Table 5) while Mansoer et al. (1997) measured N releases of 75 kg ha-1, or 63% of initial . In spring, the DM remaining from the surface-placed bags of marigold stems contained 18 kg N ha-1, 2.5 kg S ha-1, and a C/N ratio of 69 (3-yr avg.; Table 5). This compared well with the overwinter loss for the T. patula that was left standing in the field where 47% of the DM remained by May 1996, with 30 ± 9.9 kg N ha-1, 3.9 ± 1.2 kg S ha-1, and a C/N ratio of 60 ± 11 (data not shown).



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  		Fig. 1 Decomposition of marigold (avg. of T. patula and T. erecta) stems and leaves placed in litter bags on the soil surface in September during Exp. A (1995–1996 and 1997–1998) and Exp. B (1996–1997). Bars are the SE

 


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  		Fig. 2 Decomposition of T. patula stems and leaves (lvs) and rye stems and heads in litter bags that were buried in rye and T. patula plots in August. Bars are the SE, and on each sampling date are the same ( for all treatments in November, and for all treatments in April), except for T. patula stems in rye plots in November , and T. patula leaves in T. patula plots in April

 

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  		Table 5 Initial lignin and dry matter (DM) and changes in C, S, and N concentrations and N amount (kg ha-1) over the winter in marigold leaves (lvs) and stems placed on the surface of marigold plots for the first (1995–1996) and second (1997–1998) rotations of Exp. A and the first rotation (1996–1997) of Exp. B

 

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  		Table 6 Initial lignin and DM and changes in C, S, and N concentrations and N amount (kg ha-1) over time in T. patula leaves (lvs) and stems and rye heads and stems in bags buried in rye and T. patula plots in 1997–1998

 
The decomposition constants (k) of the four residue types in the buried-bag experiment (Fig. 2) ranked the same as the initial N concentration (Table 6) or the inverse of the C/N ratio: T. patula leaves > rye heads > T. patula stems > rye straw. Interestingly, the decomposition constant of sugarcane (Saccharum officinarum L.) trash in the humid tropics fits into this ranking. The C/N ratio seemed to control the decomposition rate more than either the lignin content (Table 6) or the lignin/N ratio. The rankings for both were: Rye straw > T. patula stems > T. patula leaves > rye heads. This is not unusual, however, because the first phase of decomposition is mainly regulated by the nutrient content, with the carbohydrate and recalcitrant content only influencing the later stages of decomposition (Gallardo and Merino, 1999; Jama and Nair, 1996). In most cases, the N concentrations in T. patula and rye residues increased from late summer to late fall and again in spring (Table 6), indicating a narrowing C/N ratio, which typically occurs during decomposition (Blair, 1988; Schomberg et al., 1994; Mansoer et al., 1997; Cheshire et al., 1999). Decomposition was not limited by P or S because the soil available P was high (Table 2), and the decomposition rates were not related to the initial C/S ratio (Table 6).

Buried Bag Effects
The decomposition rates were much lower in bags that were placed on the soil surface (Fig. 1) than in buried bags (Fig. 2). The decay constants (k) for the surface-placed residues were half that of the buried residues in the 1997 to 1998 rotation year, and the percent of initial residue weight remaining by spring for the surface-placed residues (33 and 54% of initial leaf wt and stem wt) was nearly 1.5 times that of the buried residues (14 and 37% of leaf wt and stem wt). Also, less N was released from surface-placed than from buried bags. T. patula residues in bags buried from August 1997 to May 1998 released about 119 kg N ha-1, whereas only 39 kg N ha-1 (3-yr average) was released from bags left on the soil surface over roughly the same time period. The amount of rye straw remaining in buried bags after 8 mo (64%; Fig. 2) was similar to that observed by Cheshire et al. (1999) for wheat straw (50 to 75%) buried for 8 mo over the winter in Scottish and Danish soils. Schomberg et al. (1994) also observed greater decomposition constants for buried (0.005 and 0.007 d-1) than for surface-placed (0.001 and 0.001 d-1) residues of sorghum and wheat , respectively. In their study, lower breakdown of surface- as compared with below-ground residues was attributed to greater fluctuations in moisture and temperature and reduced nutrient availability on the surface.

Switch-Plot Effects
Decomposition rates of three of the four residue types were equivalent regardless of whether bags were buried in T. patula or rye plots. From August to November, only T. patula leaves decomposed slightly faster when buried in T. patula plots rather than in rye plots (Fig. 2). Decomposition for other residue types (rye stems, T. patula, and rye heads) also followed this trend but the difference was not significant. The average % DM remaining in November for all four residue types combined was lower in T. patula (42% ± 1.5) than in rye plots (49% ± 2.1). The weight loss constant was also greater in T. patula than in rye plots for all residue types, except rye stems (Fig. 2). By April 1998, plot type (rye or T. patula) in which residues were buried had little influence on % DM remaining (Fig. 2). The slightly more rapid residue breakdown noted in T. patula plots as compared with rye plots from August to November was likely due to greater N availability in T. patula plots, as litter recovered from T. patula plots in April 1998 had greater N concentrations than that recovered from rye plots (Table 6), and soil NO3 was usually greater in marigold than in rye plots in the fall of the rotation year (Table 4).

Changes in N concentration in residues over time depended to a slightly greater extent than weight loss on where litter bags were buried (Table 6). In November, corrected N concentrations were the same for residues buried in either rye or T. patula plots, but by April 1998, N concentrations of T. patula leaves and rye heads were both greater when buried in T. patula rather than in rye plots (Table 6). This could indicate that despite the application of 20 kg fertilizer N ha-1 to rye following its incorporation, decomposition in T. patula plots was less N-limited. Residue N concentration likely increased over time (Table 6) because, as is generally observed, the C/N ratio narrowed as decomposition progressed (Blair, 1988). For example, the C/N ratio of rye heads decreased from 32 to 22 to 19 during the August 1997 to April 1998 incubation period (data not shown). Despite this, N was actually released from buried residues over time. Net N release from rye residues in rye plots from August to November was 12 kg ha-1. From August to April it was 14 kg ha-1 (16 kg ha-1 from heads less 2 kg ha-1 immobilized in stems; Table 6). Net N release from T. patula residues placed in T. patula plots from August to April was 119 kg ha-1 (106 kg ha-1 from leaves plus 13 kg ha-1 from stems). All of the N loss measured for T. patula had occurred by the time the first set of litter bags were collected in November.

In another switch-plot study, Flemingia congesta leaves broke down faster in Dactyladenia barteri plots than in those of F. congesta (Henrot and Brussaard, 1997). This was attributed to a lower soil temperature (which would increase the decomposition rate in tropical systems) and a greater soil water content in D. barteri plots than in those of F. congesta. There was no evidence that the soil water content contributed to the difference in our study because it was greater in the rye plots (0.122 kg kg-1 ± 0.002) than in the marigold plots (0.114 kg kg-1 ± 0.002) on 5 Nov. 1997 in Exp. A and did not differ with rotation crop on 28 Apr. 1998 (data not shown). Water is usually not limiting in the fall and spring in southwestern Ontario.


   Summary and conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
REFERENCES
 
In this study, we did not find evidence that marigold was toxic to the soil organisms involved in residue decomposition. The plant biomass production of mature marigold was similar to that of rye, decomposition rates were similar in rye and marigold treatment plots, and residue breakdown rates were comparable for marigold and rye tissues. This corroborates findings that microbial numbers and activity were similar in rye and marigold plots (Topp et al., 1998).

There was little impact of the marigold rotation system on other indicators of soil health or quality. Certain marigold cultivation treatments led to slightly higher NO3 levels in the soils in the fall and spring following their rotation compared with the more common rye rotation system; despite this, N release could potentially be minimized in marigold rotation systems through proper management. Ideally, the time of N release from marigold residues should not coincide with fall and early spring rains when most of the NO3 movement to groundwater is likely to occur, and few roots are actively absorbing NO3. Our results consistently indicated that early (August) mowing or the addition of an extra 45 kg ha-1 fertilizer N to marigold rotation crops (90 kg N ha-1 applied in total) led to the highest fall soil NO3 levels while leaving marigold standing over winter led to the smallest increases in fall soil NO3 of any marigold cultivation treatment. The Stand T and Sept.-mow T treatments resulted in fall soil NO3 levels that were sometimes no greater than those in rye rotation plots. Decomposition and N release estimates from litter bags were much lower when they were placed on the soil surface than when they were buried. This further suggests that leaving marigold crops standing over winter, rather than mowing them and incorporating their residues, may help to minimize N loss and potential transfers to groundwater. Rotation with marigold has the advantage of providing biological control of nematodes (Reynolds et al., 2000), and if managed properly, could be a viable alternative to the traditional rye plus fumigation rotation system.


   ACKNOWLEDGMENTS
 
This study was supported by the Food Systems 2002 Program, Ministry of Agric., Food, and Rural Affairs, Educ., Res., and Lab. Division, Guelph, ON, Canada, and by the Canadian Tobacco Res. Foundation, Tillsonburg, ON, Canada. The authors gratefully acknowledge the technical assistance of A. White, A. More, L. Peterson, and D. Beaton.

Received for publication March 11, 2000.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 




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