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a USDA-ARS, Sugarcane Field Station, 12990 U.S. Hwy. 441, Canal Point, FL 33438
b Everglades Research and Education Center, Univ. of Florida, 3200 East Palm Beach Road, Belle Glade, FL 33430
c USDA-ARS, North Central Soil Conservation Research Laboratory, 803 Iowa Ave., Morris, MN 56267
* Corresponding author (dmorris{at}saa.ars.usda.gov)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: EAA, Everglades Agricultural Area • EXC, extractable organic carbon (mg C g–1 soil) • EXN, extractable total nitrogen (mg N kg–1 soil) • MBC, microbial biomass carbon (mg C g–1 soil) • MBN, microbial biomass nitrogen (mg N kg–1 soil) • OP, organic matter oxidation potential (nmol CO2 kg–1 soil h–1) • RESP, microbial respiration (µmol CO2 m–2 s–1)
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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One way to increase soil C in mineral soils (<5% organic matter) is through minimum tillage practices, but organic matter decomposition in Histosols due to tillage may not be the same as in mineral soils. At >0.5 MPa water potential, a highly decomposed (saprist) organic soil may have 50% more pore volume compared with a mineral soil (Farnham and Finney, 1965), which provides a high potential for diffusion and storage of O2 for microbial utilization in the organic soil. Neller (1943) reported that soil O2 ranges between 15 and 17% at an 81-cm depth in Histosols during January and February. Water table levels were at 91 cm. When soil air contains 0.1% O2, microbial activity is only 20% less than microbial activity at a soil air concentration of 21% O2 (Broadbent (1960). Since O2 concentrations in drained Histosols may not be low enough to inhibit microbial activity, it is not certain if reducing soil tillage will have an impact on reducing soil subsidence.
In mineral soil, short term (within 24 h) gaseous C losses as CO2 can occur when surface soils are disturbed. Reicosky and Lindstrom (1995) used a large, portable photosynthesis chamber and reported substantial short-term gaseous losses of CO2 immediately after tillage to partially explain long-term CO2 losses from tilled mineral soils. The moldboard plow was the most intensive tillage implement and caused more CO2 loss than less-disruptive tillage methods. No-till or no soil disturbance lost the least amount of CO2, suggesting minimal environmental impact with these systems. Moldboard plowing was reported to have two major effects: to loosen and invert soil and allow a rapid CO2 loss and oxygen entry, and to incorporate and mix residues to enhance microbial attack (Reicosky and Lindstrom, 1995). Similar results for less intensive tillage were found by Ellert and Janzen (1999) and Rochette and Angers (1999). This interaction of soil and residue mixing enhances aerobic microbial decomposition of the incorporated residue to cause a net decrease in soil organic C (Reicosky et al., 1995).
Short-term soil C losses in Histosols could be different than in a mineral soil. An organic soil during the winter may contain 4 to 6% CO2 (Neller, 1943). The impact of tilling Histosols on increasing short-term gaseous C losses (CO2) compared with undisturbed soil that is already losing gaseous C by diffusion through the large soil pore spaces has not been investigated.
Soil organic matter formation and development is highly dependent on management decisions that influence intensity of tillage and the amount and placement of residue. Minimum tillage methods that leave most of the crop residue on the surface with limited soil contact preserves organic matter (Reicosky, 1997). Methods to maintain soil C levels and aggressive tillage are not compatible. Concern for soil conservation requires new knowledge to minimize agriculture's impact on the environment. With conservation tillage or no-till systems in some mineral soils, organic matter reaches equilibrium levels within 18 yr of constant use (Dick and Durkalski, 1998). To increase the soil organic matter, a delicate balance must be managed between the residue inputs of the previous crops and the tillage intensity associated with establishing the next crop. However, the contribution to C loss from recent (<1 yr) surface soil residue incorporation into a Histosol that already has a high organic matter content relative to C loss in an undisturbed condition is not certain.
Producers in the EAA commonly till soil before planting or subsoiling after a sugarcane harvest to improve water drainage in fields (Matherne et al., 1972). Additionally, frequent tine cultivations (shallow scratchings to a 2- to 4-cm depth) are made after planting to reduce weed competition and increase sugarcane yields (Glaz et al., 1989). Although minimum tillage would not be expected to eliminate C losses in organic soils of the EAA, reduced tillage could decrease the amount of organic matter oxidation (assuming the same tillage principles apply to Histosols as with mineral soils), because tillage aerates the soil and incorporates plant residues, both of which stimulates microbial activity. With high potential rates of C losses in drained organic soil (currently 14 Mg C ha–1 yr–1) (Stephens et al., 1984), the magnitude of the impacts of conservation tillage on C dynamics could be large. The objective of this experiment was to determine if increasing the surface soil disturbance through tillage significantly increases the potential for soil organic matter oxidation in Histosols.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Both experimental fields consisted of uniform organic soil and relatively flat topography. Tillage treatments consisted of (i) no-till control; (ii) switchplow (Model 3608, LMC Bainbridge Equip. Co., Bainbridge, GA)1; (iii) John Deere disk harrow (Model 425 offset disk; Deere and Co., Moline, IL); (iv) tine cultivation (custom built spring tine cultivator) one time, and (v) tine cultivation two times. Treatments were partitioned into four blocks with plot sizes of 4.3 by 7.6 m.
The switchplow is similar in its soil disturbance to a moldboard plow. It consists of eight curved plow blades. Blades were 60 cm wide by 56 cm tall and were spaced 50 cm apart on a tool bar. The average plowing depth for this implement in both fields was 29 cm. The disk harrow contained 16 coulter and 16 round disks (30-cm radius) that were spaced 25 cm apart, and mixed soil to an average depth of 14.5 and 7.8 cm in the fallow field and residue fields, respectively. The spring tine cultivator scratched the soil surface to an average depth of 4 and 2 cm in the fallow and residue fields, respectively, and provided small furrows about 19 cm apart from loosened soil deposited on both sides of the tines. The disk harrow and tine cultivator disturbed less soil in the residue field because dried plant material on the soil surface prevented deeper penetration. Tillage started at 0700 h on 22 and 24 Jan. 2002 in the fallow and residue fields, respectively. Both fields could not be tilled and sampled on the same day because of limitations of laboratory equipment that was available to process samples for analysis. However, general climate data from the weather station showed the 2 d of tillage had no rainfall and air temperature differences were <5°C (Fig. 1) .
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All samples were thoroughly mixed inside plastic bags and analyzed for OP by the 14C substrate induced respiration method (Tate, 1979). Analyses were performed by adding 0.5 mL of deionized water containing 12 nmol 14C (carboxyl)-benzoate (specific activity of 344 MBq mmol–1) to 10 g fresh soil inside a glass culture tube (20 x 150 mm). Benzoate was used because it is a major intermediate in the decomposition of aromatic compounds that are prevalent in organic soil, and it worked well in evaluating potential degradation of organic soil in Canada (Williams and Crawford, 1983). The culture tube was stoppered, and the contents were mixed end-over-end 20 times. The samples were then incubated for 2 h while a stream of air was passed over the soil surface and into 10 mL of 1 M NaOH trap solution. The apparatus was similar to that described by Zibilske (1994). At the conclusion of the incubations, the NaOH trap solution was analyzed for 14CO2 content with a scintillation counter. The quantity of CO2 evolved was calculated on the basis of 14C content in benzoate.
After 14C evolution analysis for microbial activities, all data were adjusted to the same substrate concentration (1.9 µM) because soil samples had different water contents and enzyme activity is related to substrate concentration. Adjustments could be made because substrate was applied at less than saturating concentrations, and at the levels applied in this study for all treatments there was a linear relationship between microbial activity and substrate concentration with origin at 0 (Morris and Snyder, 2002).
Water content was calculated after drying samples for 24 h at 105°C (Sanchez, 1990). Soil samples from each plot on Days 0 and 42 were air dried, sieved through a 2-mm screen, and analyzed for pH (2:1, soil to water), water-extractable P, and 0.5 M acetic-acid-extractable K by the standard tests used for organic soils in Florida (Sanchez, 1990). Extractable P and K levels at the beginning and ending of the experiment was determined so that nutrient release from organic matter oxidation or immobilization could be assessed.
In addition, microbial biomass carbon (MBC) and nitrogen (MBN) were determined by the fumigation/extraction method of Voroney and Winter (1993), except soil samples were filtered through a Whatman (Clifton, NJ) no. 42 filter paper, and the extractant (0.5 M K2SO4) before and after chloroform fumigation was analyzed for organic C and total N by high temperature combustion (USEPA, 1987) on a Shimadzu (Columbia, MD) CN Analyzer (Model TOC-VCSH). Sparling et al. (1990) suggested the extraction efficiency for MBC in high organic matter soils should be 0.35, but they did not suggest an extraction efficiency for MBN. To be consistent between relative extraction efficiencies between C and N, the extraction efficiencies of 0.25 and 0.15 for C and N, respectively, as reported by Voroney and Winter (1993), were used. In addition, the relative differences among tillage treatments were of greater interest than precise values for MBC and MBN. Extractable organic C and EXN were represented by the organic C and total N in the extraction before fumigation.
Soil RESP was measured on the same days and similar times as the soils were sampled using a soil respiration chamber (9.8-cm diam. by 10.2-cm-high chamber) attached to a photosynthesis meter (CID Inc., Vancouver, WA, Model CI 301PS). The system was closed (air outside the chamber was not allowed to enter during RESP measurements), air was circulated inside the chamber and photosynthesis meter by two circulating fans and one small pump, respectively, and CO2 fluxes were automatically calculated and recorded to a data logger after a 30-s stabilization period. To seal the chamber from outside air before each CO2 flux sampling, the chamber cylinder was gently pressed about 1 cm into the soil surface. Three RESP measurements were taken in each plot. In the residue field, the photosynthesis instrument malfunctioned on Day 0, so RESP data were not collected for that sampling. Air temperature (2-m height) at 0800 h and daily rainfall were obtained from a weather station located within 850 m of both fields.
For each field, data were analyzed as a randomized complete block design, assuming all treatments were random with sampling day as a repeated measure with the PROC MIXED statistical program (SAS Institute, 1999). The covariance structure was determined by the procedure of Tao et al. (2002). An LSMEAN test (P 
0.05) was used to determine differences in treatment means (SAS Institute, 1999). The average standard error for each parameter shown in Fig. 2 through 6
was obtained from the LSMEAN test (SAS Institute, 1999) of the tillage treatment by sampling day interaction. Correlations coefficients (r) among the parameter means as shown in Fig. 2 through 6 (n = 30) within each field type were calculated (SAS Institute, 1999).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Soil Respiration
Sample time, tillage treatment, and their interaction were significant for RESP in the fallow field. Immediately after tillage there was a very large increase in RESP in the switchplow treatment that remained higher than the other treatments up to 18 d after tillage (Fig. 2). At 42 d after tillage, less CO2 evolved from the switchplow treatment compared with the no-till treatment. However, when averaged across the 42 d after tillage, switchplow had RESP of 27 µmol CO2 m–2 s–1, while the other treatments averaged only 8 µmol CO2 m–2 s–1 (Table 1).
Similar to the fallow field, tillage and sampling date main effects and interaction were significant in the residue field. The increase in RESP of the switchplow was not as great as in the fallow field, probably because a sample was not taken on Day 0 in the residue field because of instrument malfunction. The RESP tended to decline from 3 to 42 d after tillage, but remained significantly higher than the other treatments after 42 d from tillage (Fig. 2). Averaged across the 42 d, switchplow RESP was 24 µmol CO2 m–2 s–1, while the other treatments were <25% of the switchplow, averaging 6 µmol CO2 m–2 s–1 (Table 1).
Extractable Carbon and Nitrogen
Extractable organic C and EXN represent readily available C and N for microbial utilization. Extractable C and N were significantly affected by main effects of sample time and tillage treatment and their interaction in the fallow field. The switchplow treatment tended to have more EXC throughout the 42 d after tillage, while the one-transit cultivation treatment tended to have more EXN up to 15 d after tillage compared with the other tillage treatments (Fig. 3). This pattern was maintained when averaged across the 42 d after tillage. The switchplow had the highest EXC (average 1.5 mg C g–1 soil), and the one-transit cultivation had the highest EXN (average 235 mg N kg–1 soil) of all treatments (Table 1). But, at 42 d after tillage, switchplow had significantly higher EXC and EXN than the other tillage treatments, possibly because of the switchplow bringing up undecomposed organic matter to the soil surface (Fig. 3). Evidence was indicated by small chunks (1 to 3 cm) of light brown peaty material being observed on the soil surface of the switchplow plots after tillage that were not seen on the other plots.
For the residue field, ANOVA was similar to the fallow field except the interaction was not significant for EXC. The switchplow tended to have higher EXC and EXN than the other treatments throughout the 42-d sampling period (Fig. 3). Averaged across the 42 d, the switchplow had EXC and EXN of 1.1 mg C g–1 soil and 144 mg N kg–1 soil, respectively, in contrast to no-till of 0.9 mg C g–1 soil and 11 mg N kg–1 soil, respectively (Table 1).
Microbial Biomass Carbon and Nitrogen
In the tillage fallow field (disk harrowed five times in the preceding 2 mo), MBC was significantly affected by tillage and sampling date treatments and their interaction, while MBN was only affected by the main effects. The most visible interaction was with the switchplow treatment, which showed a decline in MBC to levels lower than the other treatments by 42 d after tillage (Fig. 4). Averaged across the 42 d, the switchplow treatments had intermediate MBC and MBN (4.6 mg C g–1 soil and 285 mg N kg–1 soil, respectively), while the disk treatment was among the highest in MBC and MBN (5.4 mg C g–1 soil and 404 mg N kg–1 soil, respectively) (Table 2).
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Soil Temperature
Tillage and sample date main effects and their interaction were significant for soil temperature in the fallow field. The switchplow tended to have the warmest soil temperatures in three out of the six sample times (Fig. 5), averaging 19.6°C across the 42 d after tillage (Table 2). The lowest average temperature of all tillage treatments averaged 18.5°C in the two-transit cultivations and disk treatments (Table 2). For the residue field, soil temperature was only affected by sampling date (Fig. 5). Average temperature during the 42 d was 19°C (Table 2).
Soil Water Content
Similar to soil temperature in the fallow field, all main effects and their interaction were significant for soil water content. Even though switchplow tended to have less water 13 d after tillage (Fig. 5), probably because of greater evaporation, averaged across 42 d, it was among the highest with 1.4 g H2O g–1 soil (Table 2).
Likewise, in the residue field, all effects were statistically significant. The no-till treatment tended to have the highest and switchplow treatment the lowest quantity of soil water (Fig. 5) because of the mulching effect of preventing water loss and higher water-holding capacity of the plant residue on the soil surface of the no-till treatments and better drainage in the switchplow treatment. Differences in soil water content in no-till and switchplow averaged across the 42 d after tillage was also statistically significant (1.4 and 1.3 g H2O g–1 soil, respectively), but differences were small (<10%) among treatments (Table 2).
Rainfall and Air Temperature
Although the dew was very heavy the first few days after tillage, there was no rainfall from 7 d before to 18 d after tillage (Fig. 1). From 20 d after tillage there was standing water (1- to 2-cm depth) in both fields that declined during a 3-d period. Rainfall occurred almost weekly from Day 20 to the completion of the experiment.
Air temperatures varied from 8 to 22°C, with an average temperature of 16°C. The lowest temperatures were 8 and 9°C at 37 and 42 d after tillage, respectively.
In the residue field, there were fewer significant correlations than in the fallow field. Oxidation potential was positively correlated with RESP (r = 0.52, P = 0.01) and EXN (r = 0.73, P = 0.01) and negatively correlated with soil water content (r = –0.59, P = 0.01) (Table 3). Soil RESP was positively correlated with EXC (r = 0.54, P = 0.01) and EXN (r = 0.39, P = 0.05) and negatively correlated with soil water content (r = –0.47, P = 0.05). Microbial biomass C and N were not correlated with any of the parameters measured.
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Main effects of sampling time and tillage treatment were significant in the residue field for all soil chemical properties, but the interaction was only significant for soil pH and K. As in the fallow field, no consistent response of nutrient increase could be related to depth of tillage, and P showed greater changes during the 42 d compared with K (Fig. 6). In contrast to the fallow field, the trend was always for an increase in pH and nutrients across time due to mineralization of crop residues (Fig. 6). Average increase from Days 0 to 42 was pH 6.7 to 7.2, 1.9 to 4.7 mg P kg–1 soil, and 71 to 89 mg K kg–1 soil.
Correlation Analyses
The OP in the fallow field was negatively correlated with MBC (r = –0.58, P = 0.01) and MBN (r = –0.58, P = 0.01) and positively correlated with soil temperature (r = 0.55, P = 0.01), EXC (r = 0.52, P = 0.01), and EXN (r = 0.68, P = 0.01) (Table 3). Soil RESP was positively correlated with soil water content (r = 0.40, P = 0.05) and EXC (r = 0.41, P = 0.05). Microbial biomass C was positively correlated with soil MBN (r = 0.55**) and soil water content (r = 0.66; P = 0.01) and negatively correlated with EXN (r = –0.45, P = 0.05). Microbial biomass N was correlated with more parameters than MBC, having positive correlation with soil water content (r = 0.41, P = 0.05) and negative correlations with soil temperature (r = –0.37, P = 0.05), EXC (r = –0.47, P = 0.01), and EXN (r = –0.69, P = 0.01).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Our data demonstrate that deep tillage of muck soils can produce OP responses with 14C-labeled substrate (OP) that corresponds to responses in mineral soils (Fig. 2). There is no published information on use of OP in mineral soil, but Reicosky et al. (1997) showed that greater soil disturbance in mineral soils increased soil aeration and microbial activity, which in turn increased organic matter oxidation as measured by increased CO2 flux. In both fields of the present study, the switchplow had the highest OP, which persisted more than 42 d because of the greatest soil disturbance (Table 1). The switchplow promoted greater aeration for microbial activity and tended to bring more soil water content and EXC to the soil surface before 13 d after tillage (Fig. 3). Thus, greater EXC content in the soil surface, combined with increased oxygen and water allowed for higher OP.
Differences between the two field types occurred for EXN. In the residue field, there were greater amounts of EXN in the soil surface of the switchplow plots compared with the other tillage treatments (Table 1), which further contributed to increased OP (Table 1), since available N is essential for microbial growth. The switchplow treatment in the fallow field did not show greater amounts of EXN, but obviously N was sufficient for greater microbial activity (OP and RESP).
In both fields, no-till had the lowest EXN and EXC compared with the tilled treatments (Table 1) because there was less organic matter decomposition from not incorporating surface organic matter in the no-till treatment. These results correspond to those of Reicosky and Lindstrom (1995), with use of CO2 flux to measure organic matter decomposition in tilled and nontilled mineral soils. Our data show that even in high organic matter soils, leaving organic residue on the soil surface is a desirable management practice to conserve organic N and C.
However, OP responses due to tillage treatments that disturbed soil intermediate to no-till and switchplow treatments was not the same in both field types (Table 1). In the fallow field, OP for the disk treatment was similar to both tine cultivation treatments, while in the residue field, OP for the disk treatment was similar to the one-transit cultivation treatment and greater than the two-transit cultivation treatment. Differences between OP due to tillage treatment in both fields could not be explained by available C and N levels for microbial activity as expressed by EXC and EXN (Table 1). In the fallow field, the disk treatment had greater EXC and similar EXN compared with both tine cultivation treatments, while in the residue field, the disk treatment had less EXC compared with one-transit cultivation treatment and similar EXN compared with both tine cultivation treatments. Since OP did not correspond to EXC or EXN, the differences in OP among tillage treatments could also be related to changes in soil physical properties (not measured in our study) resulting from incorporation of fresh plant material, such as reduced bulk density or increased water percolation rate (Livingston et al., 1990), but are beyond the scope for interpretation in our study.
The switchplow treatments in both fallow and residue fields showed a larger CO2 flux than the other tillages on Days 0 and 1 (Fig. 2). Similarly, Reicosky and Lindstrom (1995) reported large fluxes within 24 h after tillage in mineral soils from the release of CO2 trapped in the soil pores and enhanced microbial activity. Furthermore, Reicosky (2002) indicated the CO2 flux in a moldboard treatment continued to be higher than the no-till treatment for up to 87 d after tillage. Unlike mineral soil, the large flux in our study persisted for approximately 20 d in the fallow field (Fig. 2). In the residue field, the flux persisted for a longer period (up to 42 d after tillage) (Fig. 2). Greater EXC and sufficient EXN in the switchplow compared with the other tillage treatments could have contributed to the persistence in the residue field, suggesting that the incorporated residue controlled the increased flux (Table 1).
Averaged across the 42 d, CO2 flux was 3.4 and 4.3 times greater from the switchplow treatment in the fallow and residue fields, respectively, compared with the average from the other tillage treatments (Table 1). The greater CO2 flux in the switchplow could not be attributed to greater MBC or MBN in the surface soil that immobilize both C and essential nutrients in the microbial cells, because the switchplow treatment had similar or lower microbial biomass than the no-till treatment in both fields. The reason for the lack of correspondence between RESP and either MBC or MBN may be because of large diffusion losses of trapped CO2 from the soil pores immediately after tillage (Fig. 2) (Reicosky and Lindstrom, 1995).
Reicosky et al. (1997) conducted a tillage experiment in a mineral soil and reported that CO2 flux immediately after moldboard tillage (25-cm depth) was about 17 µmol m–2 s–1, which is 4.7 times less than the switchplow treatment immediately after tillage in the fallow field of our experiment. Pioneering research of Broadbent (1960) and Waksman and Stevens (1929) indicated that C is the most limiting factor for microbial activity in organic soils. Even though C may be a limiting factor for microorganisms in Histosols, there are sufficient amounts of available C to result in greater microbial activities compared with some mineral soils.
The microbial biomass data in the fallow field conform to that of Follett and Schimel (1989) who compared no-till, mulch, and moldboard plow tillage in mineral soils and reported that greater soil disturbance decreased average microbial biomass. However, in our study, unlike the fallow field, the switchplow and no-till treatments had similar average MBC and MBN in the residue field (Table 2). There probably was sufficient available C and nutrients for microorganisms in the surface and subsurface soil of the residue field to minimize the influence of tillage practices on MBC and MBN.
The flux of CO2 in the switchplow treatment was reduced by an average of 80 and 50%, 20 d after tillage in the fallow and residue fields, respectively (Fig. 2). Part of the reduction in RESP across time is expected because of soil settling from dew, wind, and rain disturbance of the soil surface, which reduces O2 diffusion into the soil for RESP. Since cultipacking (compressing the soil with heavy steel rollers) accelerates the settling or soil particle consolidation process, future experiments should investigate cultipacking or other secondary tillage of Histosols to reconsolidate the soil particles soon after tillage as another option for reducing C losses.
Higher OP and RESP in the switchplow treatment compared with other treatments in both fields could not be explained by more favorable soil temperatures and soil water content contents. The fallow field had higher average soil water content and warmer temperatures in the switchplow treatment compared with the other treatments (Table 1), but the residue field's switchplow treatment had similar soil temperature and was among the lowest in soil water content compared with the other treatments (Table 1). Depth of soil mixing, presence of fresh plant residue, soil temperature, and soil water content had an interacting affect on organic matter decomposition.
Heavy rainfall on Days 18 and 19 that flooded the fields and very low air temperatures on Days 37 and 42 (Fig. 1) did not cause drastic and consistent shifts in periodic measurements of OP, RESP, MBC, and MBN (Fig. 2, 4) probably because flooding and low temperatures did not persist for a long enough period of time. Also, high rainfall did not pack the soil (according to visual observation in both fields of no ponding of water on the soil surface and loose soil structure) in the switchplow plots enough to reduce the OP compared with the other tillage treatments (Fig. 2). Strongly anaerobic conditions likely did not occur in the present study. Snyder et al. (2002), also working on Histosols in the same general area, did not detect significant amounts of CH4 evolution from flooded rice fields.
Nutrients are released as a result of microbial mineralization from soil organic matter. Annual nutrient release in Florida Histosols has been reported to be as high as 72 and 1200 kg ha–1 of P and N, respectively (Diaz et al., 1993; Terry, 1980). Nutrient release was not detected in the fallow field, as there was either a decrease or no change in nutrient content from Days 0 to 42 (Fig. 6). Also, these data were collected during the coolest season of the year. Low levels of mineralization and leaching losses could have minimized differences or reduced nutrient levels across time. The response was different in the residue field, which generally showed nutrient releases (Fig. 6). Even with the high rates of nutrient mineralization in Histosols and potential for leaching during the 42-d period, the fresh organic matter from the plants in the residue field decomposed faster than the native soil organic matter, resulting in detectable amounts of nutrient release in the surface soil. The reason for the pH decline in the fallow field and increase in the residue field may be related to addition of fresh residue, but will require further investigation.
Both radioactive C and soil RESP methods were effective in detecting differences in C loss due to tillage practice in Histosols, even though each method was measuring a different C substrate for microbial utilization; OP measures the potential decomposition of recalcitrant organic C compounds, and RESP measures potential decomposition of all available C sources. But in the fallow field OP and RESP were not correlated, while in the residue field there was a correlation between OP and RESP (Table 3). The reason for the lack of correlation in the fallow field is probably because of nutrient limitations for OP microorganisms, as indicated by a decline in soil P and K across 42 d and available C limitations for RESP microorganisms as indicated by a lack of a readily available C source (residue cover). In the residue field, there were adequate nutrients and C availability for all microbes as indicated by an increase in soil P and K across 42 d and the presence of a readily available C source.
Correlation analyses for the fallow field also showed a negative relationship between OP and MBC and MBN and no relationship between OP and MBC and MBN in the residue field (Table 2). We surmise that the microorganisms in the fallow field responsible for OP had to compete with other microbes for nutrients in the soil so that greater microbial biomass from soil populations resulted in fewer populations of microbes that degraded recalcitrant compounds. This is indicated in the fallow field by OP showing positive correlation between EXC and EXN, while MBC and MBN showed negative correlations with EXN. In contrast, the residue field had adequate amounts of nutrients for the microbial populations such that a relationship between OP, MBC, and MBN was not obtained. Consequently, there was no correlation between MBC or MBN and EXC or EXN; EXC and EXN were in adequate supply for microbial growth, so there was little competition for soil nutrients. However, higher EXN levels promoted greater OP, suggesting a N limitation for degradation of organic matter by microorganisms.
Higher soil temperatures resulted in greater OP in the fallow field as indicated by correlation analysis (Table 3), which is consistent with the finding that oxidation of organic matter in soil is increased with increasing temperatures (Volk, 1973). Since soil temperatures in the residue field were not correlated with OP (Table 3), and there were no significant differences among treatments in the residue field (Table 2), it appears that the unincorporated plant material on the soil surface may have helped maintain lower temperatures, thus leading to lower OP than the nonresidue field under various tillage practices.
Soil water content was not related to OP in the fallow field (Table 3) because the field drained within a short enough time period after rain to maintain favorable soil conditions for microbial growth. Microbial biomass C and N were increased with increased soil water content in the fallow field (Table 3). However, in the residue field, higher water contents resulted in lower OP levels (Table 3).
Soil RESP was increased with increasing soil water content in the fallow field and decreased with increasing water content in the residue field. In the fallow field, the soil drained sufficiently so that O2 diffused into the soil and CO2 diffused out of the soil into the atmosphere. In the residue field, the plant organic matter absorbed and retained some water, which reduced gaseous diffusion in the soil to a greater extent than in the fallow field. The differences in CO2 flux between the two fields was probably more of a physical effect (soil water content or other soil property) rather than a biological phenomenon due to microbial activity, because RESP was not related to MBC or MBN (Table 3).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication February 21, 2003.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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