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a USDA-ARS, Integrated Farming and Natural Resources Research Unit, 2413 E. Hwy 83, Weslaco, TX 78596-8344
b The Univ. of Texas-Pan American, Biology Dep., 1202 West University Drive, Edinburg, TX 78539
* Corresponding author (lzibilske{at}weslaco.ars.usda.gov)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AEP, alcohol-extractable polyphenolics • DOM, dissolved organic matter • SOM, soil organic matter • WEC, water-extractable organic C • WEP, water-extractable polyphenolics • *, significant at the 0.05 probability level • **, significant at the 0.01 probability level • ***, significant at the 0.001 probability level
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil C decomposition and retention also depend on residue biochemical quality (Melillo et al., 1982; Heal et al., 1997; Seneviratne, 2000), soil factors (Alvarez and Lavado, 1998; Bayer et al., 2001) and on the environmental conditions (Meentemeyer, 1978) under which the residues are decomposed by microorganisms. Both residue placement and quality contribute to stratification and accumulation of C and nutrients in reduced-tillage soils.
Relatively little is understood about relative C inputs to soil from roots and shoots (Gale and Cambardella, 2000), and little is known specifically about the contribution of roots to soil C (Soon and Arshad, 2002). Preserving crop residues where they are physically produced (i.e., shoots above ground, roots below ground) further differentiates inputs based on residue quality compared with the mixing of residues and soil during tillage.
Roots dominate organic matter inputs to the upper soil horizons while aboveground residues provide the surface inputs in no-till systems. Carbon inputs from roots can be substantially different from that of shoots (Puget and Drinkwater, 2001) resulting in different decomposition and nutrient mineralization patterns. Gale and Cambardella (2000) concluded that root-derived C was more responsible than surface residue-derived C for soil C gains in a no-till soil. The influence of roots on SOM transformations may be greater than that of aboveground residues (Milchunas et al., 1985; Norby and Cotrufo, 1998).
Plant polyphenolics are important factors in C and N transformations and nutrient fluxes in soil (Palm and Sanchez, 1990; Martens, 2002), and play an important role in aggregate stability (Martens, 2002). Residues high in lignin and polyphenols tend to extend C residence time in soils (Tian et al., 1993). Using crop residues high in polyphenolic content may encourage C retention (Tian and Brussard, 1997) and counteract the more rapid SOM losses in tropical climates (Sanchez and Logan, 1992; Shang and Tiessen, 1998; Jenkinson and Anayaba, 1977).
Soil pH has been found an important factor in the solubility of phenolic acids (Whitehead et al., 1983). Whitehead et al. (1981) found that water-extractability of phenolic acids increased with increasing pH, with the solubility threshold between pH 7.5 and 10.5. However, in a review of N release patterns from a broad range of plant litters and leaves in tropical systems, Seneviratne (2000) found that all of the reviewed studies were conducted in soils with pH ranges from 4.5 to 6.5. This indicates a lack of information regarding the relationships between residue phenolics and humification processes in alkaline soils.
Decomposition models developed for cooler climates have met with limited success when applied to tropical systems (Gijsman et al., 1997). This may be due in part to differences in C cycling and nutrient transformations in the warmer systems. The combination of warmer climate and alkaline soils constitutes a unique agricultural system producing cotton, sorghum [Sorghum bicolor (L.) Moench], corn, vegetables, and citrus. Much more information concerning C retention and nutrient cycling is needed to ensure long-term productivity of these systems.
The objective of this research is to test the hypothesis that plant tissue type (stems and roots), key chemical components of plant tissues (polyphenolics, soluble C) and placement (surface or buried at 10 cm) affect the kinetics of SOM loss in hot climates.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Roots were excavated by shovel, collecting roots within a radius of 20 cm from the standing stalk stub and between 10 and 20 cm deep. Plant parts were brushed gently to remove most adhering soil and were cut to 10-cm lengths. Corn stem sections ranged from 25- to 33-mm diam., corn roots from 3 to 7 mm; diameter of cotton stem sections ranged from 15 to 20 mm, and roots from 3 to 5 mm. Only cotton root laterals were used. Nylon mesh bags (15-cm square with 1.0-mm openings) were prepared and 20 g of air-dried plant material were placed into separate bags. The bags were sewn shut with nylon thread. Care was taken to leave enough room in the filled bags so that when compressed by the soil, the bags would collapse around the plant matter, increasing the contact between soil and plant residues.
Bags were placed within the experimental plot (10 by 10 m) in a completely randomized design with four replications of tissue and placement. The bags and were horizontally separated by 20 cm. A location was chosen for the placement of litterbags adjacent to the plots on which the crops were grown. The site was fallowed in the spring of the year and kept weed-free with glyphosate [N-(phosphonomethyl) glycine] sprays as needed until bag placement in August 2002.
A trowel was used to excavate soil to 10 cm and bags were placed horizontally into the holes and covered with soil. Identification tags were attached with thread to the bags such that the tag remained above the surface of the soil when the bags were covered. For surface placement, bags were secured to the soil surface by a thread attached at each bag corner to rigid aluminum wires pressed into the soil. This effectively pressed the bags against the soil surface.
Four replicate bags of each plant material and soil placement combination were collected after 0, 3, 6, 9, and 12 mo. The bags were opened in the laboratory, the contents removed and gently brushed to remove adhering soil. This process removed all but a small amount of soil (53–115 mg bag–1, data not shown). Brushed plant materials were used in all analyses. Subsamples were used to determine the oven-dry (65°C for 3 d) mass remaining. Field-moist material was used for all biochemical tests with the exception of AEP, which was oven dried (65°C) and ground (0.5-mm screen) before extraction. Soil samples were taken near the buried bag position to determine soil moisture content at the sampling times.
Data reported here derive directly from plant material instead of surrounding soil and, therefore, relate more to the narrower subject of plant material decomposition than to broader topics of soluble C effects in soil. The term, WEC will be used instead of dissolved organic matter (DOM) for this discussion. The method used to generate the samples probably removed colloidal C as well as dissolved C; therefore WEC may be a more accurate description in the context of the current experiment. Water-extractable C in plant tissues was determined by mechanically shaking 5 g of plant material in 50 mL of deionized water for 30 min. in a conical centrifuge tube. Tubes were centrifuged at 250 x g, membrane-filtered (0.45 µm), and the liquid was either analyzed immediately or frozen (–20°C) until analysis. The extracts were analyzed for total organic C with a Dohrmann DC-190 Total Organic Carbon Analyzer (Tekmar-Dohrmann, Rosemount Analytical, Inc., Santa Clara, CA).
Water-extractable polyphenolic compounds were determined in the extracts by colorimetric reaction with Folin-Denis reagent (King and Heath 1967) using an aqueous tannic acid standard line. For AEP, plant tissue was milled to 0.5 mm and 100 mg was extracted three times with 20 mL of 50% (v/v) aqueous methanol at 75°C for 1 h (Osono and Takeda 1999). Polyphenolics were quantified with Folin-Denis reagent as described above. The standard curve was prepared with tannic acid.
Data were analyzed by repeated measures ANOVA (Systat 8.0, Systat Software, Inc., Point Richmond, CA). The Pearson correlation matrix was calculated on the dataset, along with Bonferroni probabilities for multiple comparisons. Bonferroni probabilities are reported as *, **, and *** to denote significance at the 0.05, 0.01, and 0.001 levels, respectively. Standard errors were calculated using the appropriate error terms.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Differences in mass loss rates can be partly attributed to differences in environmental conditions between the soil surface and 10 cm beneath the surface. Surface residues are more exposed to extremes of wetting and drying that can modify the kinetics of decomposition (Franzluebbers et al., 1994). Seneviratne and Wild (1985) found that mild wet/dry cycles could enhance CO2 emission from soil, suggesting that variation in environmental conditions affects decomposition rates.
Generally, more than 75% of residues remained after 1 yr when placed on the surface, compared with near 40% when the residues were buried. This supports the conclusions of Douglas et al. (1980) that surface residues decompose less rapidly than buried residues. Evaluated over the year, plant tissue type and placement were strong predictors for mass loss (Table 1). First-order rate constants calculated with mass loss data (Table 2) that quantify the relationships are shown in Fig. 2. Differences between surface-placed and buried residues, as well as between the four plant tissues evaluated in the experiment support the finding that tissue type and placement strongly affect decomposition.
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Water-Extractable Carbon
The roles of DOM in soil systems are complex (Zsolnay, 2003) and include its use as an energy source for decomposition of other substrates (Reinertsen et al., 1984), and for use by other microbes not located near organic substrates (Zsolnay and Görlitz, 1994). The transient nature of DOM in soil suggests that it may be more useful for evaluating current biological activities.
Concentrations of WEC (Fig. 3) were generally greater than those reported in the literature for whole soil. Zsolnay and Görlitz (1994) found a mean of 9.4 µg WEC g soil–1 for intensively used agricultural soils. Wang et al. (2003) found WEC from 23 soils to average 294 µg WEC g soil–1. The difference is undoubtedly due to extracting plant material directly, instead of extracting from soil. Beginning levels of soluble C were very low since whole residues were extracted to evaluate a more realistic situation in the field. Significant increases were detected in WEC concentrations after the first three months of field exposure for all treatments (Fig. 3). At the end of the experiment, WEC in all treatments was greater than at the beginning, indicating that polymer hydrolysis was still a prominent activity. The purpose of monitoring WEC was not to quantitatively determine the rates of release from decomposing tissue, but to correlate WEC to other measures of decomposition, since it has been proposed that DOM (WEC) may be the principal C source for soil microbes (Marschner and Kalbitz, 2003).
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Corn residue WEC (Fig. 3a,b) also increased in the first three months. Surface-placed stems contained significantly higher amounts than buried stems (Fig. 3a) at 6 mo (P = 0.029), 9 mo (P = 0.038), and 12 mo (P = 0.022). At three months, surface-placed roots contained significantly more (P = 0.033) WEC than buried roots (Fig. 3b). However, the effect was not maintained as root WEC content fell significantly below stem content at 9 mo (P = 0.027) and 12 mo (P = 0.017). Evaluated over the entire experiment, placement and time were the only significant influences on corn WEC content (Table 1).
Water-Extractable and Alcohol-Extractable (Tissue) Polyphenols
Multiple roles for polyphenolic compounds in soil processes have been documented. Polyphenolic binding of protein can lead to N immobilization (Tian et al., 2001), which can slow decomposition and promote SOM accumulation. In addition, polyphenolics affect other nutrient transformations in soil (Hättenschwiler and Vitousek, 2000), and contribute directly to humus formation by reacting with microbially produced molecules (Martens, 2000), forming stable macromolecules. Whitehead et al. (1983) found water-soluble forms of phenolic acids were <0.7% of the total extracted with 2 M NaOH. In another study (Whitehead et al., 1981) water-extractability of phenolic acids was found to increase with increasing pH, with the solubility threshold between pH 7.5 and 10.5. This suggests that WEP in the present experiment may have been reduced by soil pH.
Corn tissues (Fig. 4a,b) generally released more than twice the polyphenolics than cotton tissues (Fig. 4c,d) during decomposition. Corn stems and roots released most of the polyphenolics within the first three months of the experiment. At three months, significantly greater (P = 0.003) WEP was found in surface-placed corn stems than in buried (Fig. 4a). The effect was maintained for all following sampling times. For roots, however, a significant increase (P = 0.044) in WEC was present only at three months for surface-placement compared with buried roots (Fig. 4b). Water-extractable polyphenolics were higher in surface residues compared with buried residues (Fig. 4a,c) between 3 and 12 mo, except for the last observations on cotton roots (Fig. 4d). Evaluated over the 1-yr period, cotton tissue, tissue placement, and time significantly affected WEP (Table 1). For corn, however, tissue type did not affect WEP, but position did have a significant effect. This indicates that there may be significant differences between cotton and corn that result in differential decomposition dynamics.
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Different methods are commonly used for determining plant polyphenolic content, which makes direct comparisons of results from this study with others more difficult. Most reported data concerning polyphenolics were determined for fresh plant tissue. This is important because plant polyphenolics can change substantially in chemical composition and effects during senescence (Hättenschwiler and Vitousek, 2000).
Corn stem polyphenolic (AEP) concentration trended slightly upward during the first three months, regardless of placement (Fig. 5a). Surface-placed stems were significantly greater (P = 0.034) in AEP than buried stems only at 12 mo. For corn roots (Fig. 5b), AEP in buried tissue was significantly decreased (P = 0.026) at three months. This significant difference was maintained for the duration of the experiment. This was the only corn tissue-placement combination that showed appreciable net change in concentration in polyphenolics during the experiment.
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These results indicate greater initial polyphenolic content in cotton residues, which might have been expected to slow mass loss of cotton tissues. However, buried corn and cotton tissues appeared similar in the pattern of mass loss (Fig. 2). The greater amount of polyphenolics in cotton apparently may not have been a limiting effect on decomposition. Since no fertilizer was added to the soil or residues, N availability for residue decomposition may have been more limiting. Further work is needed to determine the relative effects of N availability and polyphenolic content of residues. Evaluated over the 1-yr period, position significantly affected AEP, and interactions between time, position and tissue were determined (Table 1). This supports the marked differences in AEP dynamics between cotton stems and roots (Fig. 5c,d, respectively).
Synthesis
For buried corn stems and roots, the highest rates of mass loss occurred during the first three months of decomposition (Fig. 2a,b). Stem mass loss was significantly correlated to WEC, whether surface-placed or buried (r = –0.62* and –0.74*, respectively, Fig. 3a,b) for the whole year. This indicates, not surprisingly, a correspondence between decomposition rate and C released from stems during decomposition. During the same period, large increases in WEP were seen for both corn stems and roots (Fig. 4a,b) of which only the buried placement was significantly correlated (r = –0.69*) to mass loss. Water-extractable C and WEP were strongly correlated for corn stems and roots, with correlation coefficients ranging from 0.98*** to 0.70* for surface and buried placements, respectively. This suggests that C compounds released from tissues originated in multiple pools of compounds in the tissues. The desiccation of surface residues, regardless of tissue type, has undoubtedly affected the decomposition characteristics of the residues.
Little change in corn tissue polyphenolics during the first three months was noted for either stem or root tissues, regardless of residue placement. Corn polyphenolics were significantly correlated only to WEP, with coefficients ranging from 0.84* to 0.69* for the entire incubation. Correlations tended to be negative between mass loss and tissue polyphenolics, but the relationship was not as strong (r = –0.44 to –0.37, corn stem and root, respectively) as was the relationship to WEP. This is probably due in part to the large pool of polyphenols contained in these tissues that changed little, except for buried roots (Fig. 5b) during the incubation.
Cotton mass loss was negatively correlated to WEC for buried stems (r = –0.95***) and roots (r = –0.88**), but was insignificantly correlated to surface-placed cotton stems (r = –0.42) or roots (r = –0.51). Not surprisingly, the relatively unfavorable conditions of the soil surface appear to contribute more to residue retention than to decomposition. Cotton stem mass loss was also correlated to WEP for both surface-placed (r = –0.79*) and buried (r = –0.94**) tissues. Root mass loss, however, correlated with WEP only when they were buried (r = –0.91**). This might be explained by a different capacity of stem and root tissues to absorb and retain water. Accordingly, it would appear that stem tissue absorbs or retains more water than root tissue. Moreover, point-in-time measurements of biological and biochemical properties are likely to be strongly affected by the amount of moisture available to the decomposing microbes around the time of sampling. Table 3 shows the content of water in the residues sampled over the experimental period. Expectedly large differences in surface-placed versus buried residue moisture content were observed. Surface-placed stem residues never contained more than about 20% moisture, while buried residues contained from a low of about 15% to a high of about 77% moisture. Interestingly, cotton roots contained more moisture than corn roots under the same environmental conditions. Plant residue moisture content was correlated to several indices. Buried corn stem WEC and mass loss correlated well to plant tissue water content (0.68**, –0.79**, respectively). Neither surface nor buried placements of corn roots correlated to tissue water contents. For cotton, surface-placed stem mass loss was correlated to tissue water content (–0.71*) and to WEP (0.83**). Buried stems showed no response, indicating that subsurface soil moisture was probably not limiting the tissue properties measured. For cotton roots, tissue moisture was correlated only to WEC (0.75*) for the surface-placement treatment. No buried cotton root parameter was significantly correlated to tissue water content, again indicating that moisture was apparently not limiting the decomposition.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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For cotton, residue remaining was negatively correlated to WEC in the tissues, indicating not only the link between solubilization of C polymers and decomposition, but that diffusion of soluble C away from these residues and into the soil may not be rapid, thus creating a gradient. Residue remaining was also negatively correlated to WEP and AEP polyphenolics in the remaining residues. However, since WEC and WEP were generally positively correlated, a clear role for soluble polyphenolics in controlling decomposition is not apparent.
Results for corn were generally similar to those for cotton. Again, most mass loss occurred within the first three months of the incubation. Residue remaining was negatively correlated to WEC and to WEP, but direct correlations between WEC, WEP, and AEP make it difficult to resolve a role of polyphenolics for depolymerization in this system.
These results emphasize the importance of residue moisture content during decomposition, and indicate that different residues may have different capacities to hold moisture that may affect the biochemical characteristics and kinetics of decomposition.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication February 16, 2004.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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This article has been cited by other articles:
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L. M. Zibilske and J. M. Bradford Oxygen Effects on Carbon, Polyphenols, and Nitrogen Mineralization Potential in Soil Soil Sci. Soc. Am. J., January 1, 2007; 71(1): 133 - 139. [Abstract] [Full Text] [PDF]
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