SSSAJ Grow Your Career with SSSA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (23)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Kristensen, H.L.
Right arrow Articles by Meisinger, J.J.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Kristensen, H.L.
Right arrow Articles by Meisinger, J.J.
Agricola
Right arrow Articles by Kristensen, H.L.
Right arrow Articles by Meisinger, J.J.
Soil Science Society of America Journal 64:371-378 (2000)
© 2000 Soil Science Society of America

DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Effects of Soil Structure Disturbance on Mineralization of Organic Soil Nitrogen

H.L. Kristensenb, G.W. McCartya and J.J. Meisingera

a USDA-ARS Environmental Chemistry Lab., Beltsville, MD 20705 USA
b Dep. of Terrestrial Ecology, National Environmental Research Inst., Vejlsøvej 25, DK-8600 Silkeborg, Denmark

gmccarty{at}asrr.arsusda.gov


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Disturbance of soil structure by tillage operations is thought to make soil organic N accessible for mineralization which was otherwise protected from degradation. The origin of N released by disturbance of soil structure is, however, poorly understood and needs to be related to microbial activity. This study was performed to investigate the effect of soil structure disturbance on the release of active or protected organic N pools in surface soils (0–2 cm) under plow- (PT) or no-tillage (NT) management. Active soil N was defined as the pool participating in mineralization–immobilization turnover during short-term incubation (6 d) while protected pools were considered inactive during this period. The active pool of soil N was labelled with 15N in intact samples of PT and NT soils. The samples were either kept intact or sieved and repacked, and then leached weekly during a 35-d incubation period. The disturbance of soil structure increased mineral N release from 6 to 15 mg kg-1 in the NT soil within the first week after disturbance. This release was found to originate from both active and physically protected N pools as could be assessed by the relative differences in 15N content of mineralized N by intact and disturbed soil samples. In contrast, the release from the PT soil was 7 to 9 mg N kg-1 after disturbance, with only a minor contribution from protected N pools. These results support the theory that disturbance of soil structure by tillage may destabilize and release protected pools of soil N. Over the entire period of incubation, protected N accounted for 27% of total N release in the NT soil and 12% in that of PT. The calculation of availability ratios, defined as the ratio between the 15N enrichment of mineralized N and that of total soil N, showed that recently added 15N was less available for mineralization in the NT soil as compared to that of PT. The probable cause for this difference was the higher C/N ratio of organic matter in NT surface soil indicating more nonhumified organic matter when compared to PT organic matter.

Abbreviations: PT, plow-tillage • NT, no-tillage • MIT, mineralization–immobilization turnover • AR, availability ratio


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
SOIL MANAGEMENT PRACTICES THAT MINIMIZE SOIL DISTURBANCE have been introduced and gained acceptance in crop production with the purpose of reducing soil erosion and conservation of soil organic matter. There is substantial evidence that tillage management influences the dynamics of C and N in agricultural soils. For example, NT management generally increases the amount of active soil N and organic and microbial biomass C and N, and results in substantial stratification of these pools within the profile surface soil (Doran, 1987; Follett and Schimel, 1989; Beare et al., 1994a; McCarty and Meisinger, 1997).

Greater retention of organic C and N in soils under NT management has been associated with increased soil aggregation (Beare et al., 1994a, 1994b). It is thought that disruption of soil aggregates by tillage operations causes physically protected pools of organic C and N to become available for microbial degradation (Waters and Oades, 1991; Ladd et al., 1993). The physically protected organic matter is thought to be less available for microbial degradation than unprotected matter because of location inside micro- and macro-aggregates and small pores, or because of encrustation of organic matter by clay minerals (Tisdall and Oades, 1982; Gupta and Germida, 1988; Hassink, 1992). Another source of protected organic N, however, may consist of N pools that are immobilized within living but inactive microorganisms or within components of the active microbial community with low rates of turnover. For example, organic N may be immobilized in fungal hyphae which have been proposed to act as binding agents of microaggregates into macroaggregates (Tisdall and Oades, 1982). With disturbance of the hyphal network, the fungal tissue could be subjected to microbial degradation. This kind of protection would be dependent on the ability of the living cell to maintain the integrity of the cellular environment and, with death of the organism, the protecting mechanism would be lost. This could be considered to be a form of biological protection.

Investigations of the influence of tillage on soil N mineralization have often involved the use of soil samples prepared for investigation by either fractionation or sieving (Craswell and Waring, 1972; Gupta and Germida, 1988; Follett and Schimel, 1989; Beare et al., 1994a; McCarty and Meisinger, 1997). Bundy and Meisinger (1994), however, stressed the importance of using experimental techniques with minimal disturbance of the soil in studies where the management systems under evaluation are themselves characterized by differences in the degree of soil disturbance. Most studies of disturbance effects of soil structure on mineral N release from intact samples of agricultural soil have made no attempt to differentiate the origin of released N (Rice et al., 1987; Cabrera and Kissel, 1988; Stenger et al., 1995). By use of 15N techniques, Grace et al. (1993) found simulated cultivation increased the release of recently synthesized microbial metabolites, while Hassink (1992) reported indirect evidence for pools of physically protected N in soils by relating N release to pore size distribution. Disturbance of soil structure may influence mineralization–immobilization turnover by increasing availability of protected N to microbes or by stimulating protozoa–soil fauna grazing on microbes (Ladd et al., 1993). Thus it is likely that released N may originate both from formerly inactive (protected) and active soil N pools. There is a need to relate N release caused by soil structure disturbance to microbial activity to enable assessment of protection mechanisms.

The purpose of this study was to investigate the effect of soil structure disturbance on mineralization and immobilization of N in the surface layer of soils subjected to long-term PT or NT management, with particular focus on the release of protected N.

For this study, we posited that the protected pool of soil N does not participate significantly in short-term biological cycling such as occurs in mineralization–immobilization turnover (MIT) (Jansson, 1958). Thus, the protected N pool was operationally defined as that which was not isotopically enriched by immobilization of 15NH4+ during an incubation designed to label the pool of active soil N (active phase, Follett et al., 1989). The contribution from protected soil N pools to mineralization after disturbance of soil structure could then be detected as a dilution of the 15N enrichment of N released from the active N pool, the dilution being caused by release of 14N from the protected pools. By use of this dilution of mineralized N in disturbed relative to intact samples, different pools of protected soil N could be characterized according to the mechanism of protection. For example, we also considered biological protection to be a potentially significant mechanism. In the assessment of types of protected soil N, no assumption was made regarding the extent by which they were mutually exclusive or additive.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Study Site and Sampling
Intact soil samples were collected from experimental field plots established in 1976 to study the influence of annual moldboard PT and NT management on fertilizer uptake by a continuous monoculture of maize [Zea mays (L.)]. The field was located in the Piedmont region in central Maryland on a moderately well drained Delanco silt loam (fine-loamy, mixed mesic Aquic Hapludults). The experimental plots were established in a split-plot design with different N fertilizer rates nested within tillage treatments. Soil samples used in the present study were from two field replicates of PT and NT plots that received annual surface applications of 135 kg ha-1 fertilizer N. Previous work showed that this rate represents the sufficiency rate of N fertilization for maize production (Meisinger et al., 1985). In March 1996, intact soil samples were collected from each field plot by pressing polycarbonate plastic tubes (41 mm diam.) against the soil surface that gave minimal compaction of the soil. Tillage had not been performed since the previous harvest and fresh litter remained on the surface of both PT and NT plots. Prior to sampling, pieces of fresh litter were therefore removed from the soil surface. The samples were brought to the laboratory and remained in the sampling tubes throughout the experiment to ensure minimal disturbance of soil structure. The excessively moist samples (25–35% H2O of soil dry weight) were partially dried for 3 d at 22°C. Each of these intact soil samples were then cut to an approximate dry weight of 40 g (2.0–2.5 cm length) by gently pushing the sample partly out of the tube and cutting it from the lower end of the sample. The dry weight obtained was based on bulk density measurements of three extra samples collected from each field site. Each tube containing soil was then placed in the top part of a Büchner-type filtering funnel with a two component filter. All details on the materials used in the setup are provided in Fig. 1 . The average pH in the PT and NT soil samples was 5.8 and 5.3, respectively (soil/water ratio 1:2).



View larger version (25K):
[in this window]
[in a new window]
  		Fig. 1 One subunit of the system for leaching intact soil samples with unsaturated flow. Components of assembled subunit: (A) funnel bottom (Nalgene part no. 4280-0550, 45 mm i.d.); (B) No. 9 rubber stopper; (C) three hypodermic needles (0.8 mm o.d., 2.5 cm length); (D) air hole (3 mm); (E) polycarbonate tube (41 mm i.d., 1.6 mm wall); (F) soil core; (G) Büchner funnel (Nalgene part no. 4280-0550, 45 mm i.d.); (H) 2.5 µm glass fiber filter (Fisher Scientific); (I) 0.45 µm cellulose nitrate membrane filter (Whatman) prerinsed with water; (J) vacuum manifold (-80 kPa); (K) 120 mL bottle. The leaching solution (0.01 M CaCl2) was supplied to each subunit of the system by use of a peristaltic proportioning pump (Technicon Model III, pump tubing 0.51 mm i.d.)

 
Labeling the Active Nitrogen Pool
The active microbial N pool in the intact soil samples was labeled with 15N by injection of a mixture of 15(NH4)2SO4 (5 mg N kg-1 soil, 99% enriched) and glucose (150 µg C g-1 soil) followed by an incubation period of 6 d. This permitted incorporation of the label into the portion of active N involved in mineralization–immobilization turnover (MIT) within the period of incubation. A 1-mL hypodermic syringe fitted with a needle (0.51 mm o.d., 16 mm length) was used to make a series of 5 injections of the labeling solution to a depth of 1.5 cm. With each injection, the needle was pulled out evenly as the solution was dispensed. A total of 0.35 mL of solution was injected into each sample giving a final soil water content of approximately 19 and 24% of soil dry weight for PT and NT soil, respectively (70% of field capacity). The treated soil samples were placed together with a wet paper towel into 0.9 L bottles and incubated for 6 d at 25°C.

The Leaching System
The soil samples in the tubes were leached by an unsaturated flow of solution; a leaching method that prevents ponding at the soil surface and thus reduces the risk of preferential flow through soil pores as well as of NO-3 loss by denitrification. The leaching system was composed of multiple subunits of which details are given in Fig. 1. A multichannel proportioning pump supplied leaching solution (60 mL 0.01 M CaCl2, flow rate 8.9 ± 0.14 mL h-1) to the surface of each sample and a vacuum manifold system acting on membrane filters supplied tension to soil water (-80 kPa) which enabled collection of leachate in a bottle. After each leaching event, the vacuum was maintained for approximately 2 h until the soil water content in each sample reached the content previous to leaching (19–24% of dry weight). The collected leachates were diluted with 4 M KCl to a final concentration of 1 M KCl, shaken, and stored at 3°C for a maximum of 14 d until analysis for amount and isotopic content of inorganic N.

The efficiency of the leaching system for removal of NO-3 from intact and disturbed samples was tested and the use of 60 mL of leaching solution removed an average of 96 to 98% of the NO-3 in the soil cores during incubation. This efficiency was acceptable for use of the system in this study. The amount of NH+4 in leachates or soil extracts was below detection limits.

Structral Disturbance Experiment
To study the effect of soil structure disturbance on mineralization of active and protected soil N pools, intact samples of PT and NT soil were sieved after their active soil N pool had been labeled. With this study, three replicate samples were taken from each of two replicate field plots, sieved (mesh size 2 mm), and repacked to the original bulk density, while three other replicate samples from each plot were left intact. Both disturbed and intact samples were leached on a weekly basis during a 35-d incubation with the first leaching occurring the day before the disturbance of soil structure by sieving to remove any initial soil NO-3.

Between leaching events, the soil samples in the subunit components of the leaching system were removed from the vacuum manifold and placed in 0.9-L bottles with a wet paper towel and incubated at 25°C. The bottles were opened for aeration every third day of the incubation.

Biomass Perturbation Experiment
To study the possible origin of protected N from biologically protected pools in the soil, the microbial biomass was perturbed in samples without disturbing the soil structure. This was done by fumigation with chloroform to perturb the biomass in intact samples of PT and NT soil. Six replicate soil samples were taken from one of each field plot and labeled with 15N in the active microbial N pool as previously described. Before start of incubations, the samples were placed on the leaching system to remove any initial soil NO-3. Half of these intact replicate samples were fumigated with ethanol-free CHCl3 for 20 h and evacuated repeatedly to remove residual chloroform. All samples were then placed in 220-mL bottles, sealed with rubber stoppers, and treated with 2 mL acetylene to inhibit nitrification. The samples were incubated at 25°C for 14 d to allow equilibration of 15N enrichment between the perturbed biomass and the released NH+4. Then the samples were extracted in 1 M KCl (soil/solution ratio 1:10), and soils and extracts analyzed for amount and isotopic content of N. The acetylene treatment was included to eliminate the influence of nitrification on MIT processes because the fumigated soils would have reduced or no populations of nitrifying bacteria in contrast to control soils without fumigation. This procedure was judged effective as the amount of NO-3 in the samples at the end of the experiment was below detection limits.

Sample Analysis
Concentrations of NO-3 and NH+4 in leachates and soil extracts were determined colorimetrically by flow-injection analysis (Lachat Instruments, 1989, 1990). The samples were also prepared for analysis of isotopic content of NO-3–N and NH+4–N by use of the diffusion method by Brooks et al. (1989) prior to analysis by an isotope mass spectrometer interfaced with an automated N-C analyzer (Europa Scientific Ltd., Cheshire, UK). The soil water content was obtained by drying samples for 24 h at 105°C.

After termination of the structural disturbance experiment, each sample was sieved (mesh size 2 mm), mixed, and subsampled for analysis of pH, total soil C, and microbial C content as well as the amount and isotopic content of total soil N and microbial N pools. The total C and N content was measured by dry combustion on a C and N analyzer (Leco CNS - 2000). The isotopic content of total N in soil samples was determined by dry combustion of soil using the same N-C analyzer interfaced with the mass spectrometer for N isotope analysis (Hauck et al., 1994). Microbial biomass C and N pool sizes were estimated by use of the chloroform fumigation–incubation technique (Voroney and Paul, 1984) and with and . Soil (14 g) from each sample was fumigated with ethanol-free CHCl3 for 20 h followed by incubation for 10 d at 25°C in 250-mL bottles. Headspace gases were sampled with a gas syringe and analyzed for CO2 content by gas chromatography (McCarty and Blicher-Mathiesen, 1996). The fumigated soil samples were extracted in 1 M KCl (soil/solution ratio 1:10) and analyzed for amount and isotopic content of NO-3 and NH+4.

Data Analysis
The contributions of active and protected pools of soil organic N to the mineral N pool formed during incubation of the disturbed soil samples were calculated using amounts and 15N enrichments of the NO-3 pools obtained from both intact and disturbed samples. The data were analyzed using the principle of isotope dilution (Jansson, 1958) which can be expressed in the following equation:

(1)
which can be solved for both X and Y when

(2)
where .

The mineralization rate of recently 15N-labeled relative to indigenous organic soil N was determined by calculating the availability ratio (AR) as originally defined by Broadbent and Nakashima (1967):

(3)

Potential Errors
An assumption made with use of the above equations is that the 15N label was evenly distributed in the soil sample. This may have been violated to some degree since the distribution was a result of injection of solution and subsequent diffusion of the label into the soil matrix during the preincubation. Sensitivity analysis performed by Davidson et al. (1991) showed, however, that this assumption may not be critical in this type of labeling procedure as long as the distribution is not biased concurrently to a nonrandom occurrence of the processes under study.

The fact that the 15N-labeling procedure was performed prior to imposition of the structural disturbance treatments on the soil ensured that the label was incorporated under similar conditions for all samples resulting in similar distributions of label in the active N pool. The treatment, imposed by sieving and repacking half of the samples involved in the structural disturbance experiment, probably redistributed the labeled pool more uniformly than that originally in the intact samples. Because mineralization of soil organic N can be described by a decay function with first-order kinetics, the simple imposition of a more uniform distribution of the 15N labeled soil organic matter should have little or no influence on the kinetics of subsequent N mineralization. Hence, little or no bias in 15NO-3 production should have resulted from the redistribution of 15N labeled organic matter in these experiments.

The average recovery of added 15N at the end of the soil structural experiment was high and of equal size irrespective of soil type and treatment (90 and 89% for disturbed PT and NT samples, 90 and 92% for intact PT and NT samples, respectively). This indicates that losses of 15N due to denitrification and leaching of dissolved organic N compounds, were of minor importance during the experiment.

Statistical Analysis
Statistical significance of differences in soil properties and distribution of 15N between PT and NT soils were tested by analysis of variance (F test). Differences in mineralization rates and availability ratios between disturbance treatments and PT and NT soils were tested by analysis of variance followed by pairwise comparisons by Tukey's studentized range test (t test). Relationships between soil properties were investigated by simple linear regression modeling and homogeneity of slopes were tested using general linear models procedures (SAS Institute, 1988). In the assessment of statistical significant differences between results, tests with P < 0.05 were considered statistically significant and tests with 0.05 < P < 0.15 were considered as trends.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
Mineralization of Nitrogen
Comparison of amounts of 15NO-3 and total NO-3 collected in leachates from the intact and disturbed NT soil samples showed that soil structure disturbance caused increased release of 15NO-3 within the first week after treatment but had little influence on release in subsequent weeks (Fig. 2) . The results obtained with the PT soil samples showed that structure disturbance had no effect on the amount of 15NO-3 released with any of the leaching events. Comparison of data from corresponding PT and NT soil samples showed that the amount of 15NO-3 released from the PT soil was generally higher than that from the NT soil for all periods of incubation. However, the total amounts of NO-3 mineralized from the intact samples of PT and NT soil did not differ significantly (F test P < 0.22). But, disruption of the NT soil structure caused a two- to threefold increase in the total amount of NO-3 mineralized within the first week after the disruption. This influence decreased markedly in the subsequent weeks of incubation. Thus soil structure disturbance seemed to increase availability of organic N for mineralization in the NT soil with little corresponding influence in PT soil. The increased release of 15N in the NT soil did indicate, however, that at least part of the released N came from a pool of recently synthesized organic N.



View larger version (23K):
[in this window]
[in a new window]
  		Fig. 2 The amount of 15NO-3–N and NO-3–N collected during leaching of incubated soil samples obtained from fields under plow- (PT) or no-tillage (NT) management. The soil structure of the samples had been disturbed or was kept intact, and the arrow indicates the time of the disturbance. Different letters indicate significant differences between collected amounts within day of leaching ( ; t test, P < 0.05)

 
Mineralization of Active and Protected Nitrogen
The involvement of different soil N pools in mineralization was investigated further by calculation of the portions of NO-3 mineralized from active and protected N pools with disruption of soil structure (Fig. 3) . In the mathematical treatment of data (Eq. [1] and [2]), we assigned production of NO-3 in the intact soil samples as being from the active N pool with the assumption that the 15N enrichment of the mineralized NO-3 reflected the 15N enrichment of the active N pool for both intact and disturbed soil samples. With this it can be calculated that soil structure disturbance significantly increased the amount of active N mineralized in the NT samples by 3 mg kg-1 soil within the first week (F test P < 0.02) while no corresponding increase was detected in the PT soil samples. The amount of physically protected soil N released from the NT soil only tended to be higher than from the PT soil due to variability among replicate soil samples (F test P < 0.12). This tendency is consistent with the findings of Beare et al. (1994b), who found that soil under long-term NT management had a better soil structure with a higher number and increased stability of macroaggregates than under PT. Macroaggregates are believed to be closely associated with the pool of protected organic matter in the soil (Tisdall and Oades, 1982; Gupta and Germida, 1988). When calculated for the entire period of incubation after soil disturbance (Days 7–35), the protected N accounted for 27% of total N release in the surface layer of the NT soil and for 12% of total N release in the PT soil. These results are comparable to those reported by Beare et al. (1994a) (29% and 6.5%, respectively), who used a nonlabeling technique with measurements of N release from crushed macroaggregates of the 0 to 5 cm surface layer of soil.



View larger version (30K):
[in this window]
[in a new window]
  		Fig. 3 The amount of NO-3–N released from active or protected soil N pools during incubation of intact and disturbed plow- (PT) or no-tillage (NT) soil samples. The arrows indicate the time of soil structure disturbance. The bars indicate standard errors of the total amount released at each leaching event ( )

 
Origin of Protected Nitrogen
The rate for release of N in the disturbed NT soils increased rapidly with a subsequent corresponding decrease to approximately that of intact samples within the first two weeks after the treatment (Fig. 2). This indicated that readily decomposable organic matter was made accessible for degradation by soil microorganisms by structure disturbance. To determine if this may be related to perturbation of the soil microbial biomass resulting in release of biologically protected N, we studied the flush of soil N mineralization and the 15N isotope dilution of this pool resulting from chloroform fumigation in intact samples of NT and PT soils. It is well established that fumigation of soil causes release of N from the microbial biomass which is then subject to mineralization (Voroney and Paul, 1984). The fumigation of soil samples caused a 3.2 to 3.6 factor increase in amount of soil N mineralized when compared to untreated samples (Table 1) . The 15N enrichment of the N released from the fumigated PT samples tended to be lower than that from the untreated samples (F test P < 0.06) indicating that the PT soil contained a pool of biologically protected N. This contrasts the results for the NT soil where no significant difference was found in the 15N enrichment between the fumigation treatments (F test P < 0.22). This is surprising given the fact that the NT soil had a much higher pool of microbial biomass C and N (Table 2 ; F test P < 0.001) and may, for example, be expected to have greater fungal biomass than the PT soil due to a lack of soil disruption and the involvement of hyphae in macroaggregate formation (Gupta and Germida, 1988; Beare et al., 1992).


View this table:
[in this window]
[in a new window]
  		Table 1 Effects of perturbation of microbial biomass by chloroform fumigation on amount and 15N content of NH+4 released during incubation of plow-tillage (PT) and no-tillage (NT) soil samples. The P value of F tests ( ) of differences between fumigated and intact samples are indicated

 

View this table:
[in this window]
[in a new window]
  		Table 2 The properties of the soil organic C and N pools of the plow-tillage (PT) and no-tillage (NT) soil samples and the range and coefficient of variation (CV) of these properties. The P values give significance of the differences between tillage means (F test, )

 
Distribution and Availability of Nitrogen-15
Figure 4 shows the distributions of added 15N label and indigenous N in the PT and NT soils at termination of the 35-d incubation in the soil structural disturbance experiment. As can be expected, a single event of structural disturbance had no measurable influence on the distribution of N in the different pools of soil N (comparison not shown). By contrast, the distribution of the 15N label was substantially different in the PT and NT soils. The proportion of label that was mineralized in PT soil was significantly greater than that of the NT soil (F test P < 0.001). Correspondingly, the 15N content of the NT soil (residual + biomass) remained significantly higher (F test P < 0.001) as well as, with slight approximation, that portion in the biomass fraction alone (F test P < 0.06).



View larger version (72K):
[in this window]
[in a new window]
  		Fig. 4 The distribution of 15N and N after the 35 day incubation of plow- (PT) or no-tillage (NT) soil samples ( ). The values indicate size of the specified N pool on a mg N kg-1 soil basis

 
The distribution of total soil N also differed substantially between the PT and NT soils when viewed on absolute and relative bases (Fig. 4). For example, the average size of total N and biomass N pools in the NT soil was approximately twice those of the PT with greater absolute amounts of soil N being mineralized in the NT soil (42 mg kg-1) when compared to that of PT (30 mg kg-1) (F test P < 0.04). When viewed on a basis relative to total soil N, however, mineralization was greater in the PT soil (2.8%) as compared to NT soil (2.0%) (F test P < 0.001). Follett and Schimel (1989) suggested that it is likely that the differences in distribution of 15N label are at least partly due to the differences in the total amounts of N in the surface layer of PT and NT soils.

To further investigate this hypothesis, the 15N mineralization data were normalized on a basis of amounts of total N in the incubated samples of PT and NT soils. This was done by calculation of the AR of 15N enrichment of the NO-3 produced relative to the 15N enrichment in the overall soil N pool (see Eq. [3]). Figure 5 shows the AR calculated for N mineralized at various times during the incubation. The resulting values of AR for all soils were substantially greater than unity which is expected as the recently added 15N has been immobilized in a pool of active soil N which is more available for mineralization relative to other indigenous soil N. It can be seen that structural disturbance of soil caused a decrease in the AR relative to that of the intact samples at the first leaching event after disturbance for the PT soil (Fig. 5). This decrease is indicative of the release of physically protected N from an unlabeled pool of soil N as also shown in Fig. 3. Interestingly, the adjustment of 15N mineralization data on the basis of total soil N did not bring equivalence between PT and NT soils with respect to availability of 15N label for mineralization. This finding indicates that factors other than soil N pool size influence the availability of recently added N for mineralization by soil microorganisms.



View larger version (21K):
[in this window]
[in a new window]
  		Fig. 5 The availability ratio for N in leachates collected during the incubation of plow- (PT) or no-tillage (NT) soil samples. The soil structure of the samples had been disturbed or was kept intact. The arrow indicate the time of disturbance. Different letters indicate significant differences between availability ratios within day of leaching ( ; t test, P < 0.05)

 
Relationship Between Availability Ratio and Soil Properties
The characterization of the individual soil samples, collected as intact cores from the field, showed not only wide divergence in properties of the soil surface layer between the tillage treatments but also within the replicate samples from each treatment (Table 2). Both the size and coefficient of variation (CV) for the total and microbial soil N pools were two to four times higher in the NT soil as compared to that of PT, and similar pattern of divergence was found for total and microbial C in these samples.

Linear regression analysis between the total N content of all soil samples and the AR of added 15N, as calculated for the entire experimental period after disturbance, (Fig. 6A) shows a strong negative correlation between AR and total N ( ). In this analysis, both disturbed and intact samples were pooled even though other analysis showed that there was good probability that this treatment caused variation in the slope (P < 0.06). An initial conclusion from these results may be that the use of availability ratios was not effective for adjusting the 15N mineralization data to the differences in size of N pools within soils. Further analysis of data, however, provides another explanation.



View larger version (17K):
[in this window]
[in a new window]
  		Fig. 6 Linear regressions between properties of plow- (PT) or no-tillage (NT) soil samples that were incubated and leached weekly during a period of 35 d. The soil structure of the samples had been disturbed or was kept intact. The linear regressions shown are (A) the availability ratio against the soil N content; (B) the soil N content against the soil C/N ratio; and (C) the availability ratio against the soil C/N ratio

 
For this study, soil samples were collected within the top 2 cm of surface soil. McCarty and Meisinger (1997) found substantial stratification with depth of the composition of soil organic matter with considerable increase in the C/N ratio at the surface layer of NT soil in contrast to that of PT. It was concluded that this increase was due to annual deposition of fresh plant residue with high initial C/N ratio, which is left on the soil surface of the NT soil, while being mixed into deeper soil layers of the PT soil. Characterization of the soil samples used in this study showed this same pattern with higher C/N ratios in the surface soils under NT than PT management but also with substantial variation in this ratio within replicate soil samples (Table 2). Regression analysis shows a strong positive relationship between total N content and C/N ratio in the samples ( ) which indicates that nonhumified plant residue is an important component of soil organic matter in the surface layer (Fig. 6B). We surmise that the strong covariance between total N content and C/N ratio accounts for the apparent relationship between total N and AR. This indicates that the variation in the C/N ratio is the primary factor inducing variation in AR for these soils. Regression analysis provides evidence for a negative correlation ( ) between AR and the C/N ratio of the samples (Fig. 6C). It is thus suggested that the C/N ratio and the degree of decomposition of the organic matter in the soils were important factors determining the availability of recently added N to microorganisms. This conclusion gains support from work by Blackmer and Green (1995), who found that 15N added to fresh corn stover was quickly immobilized and not readily released with the residue decomposition. Another factor, which may contribute to the observed differences in AR, is that the microbial community of the NT surface soil may be relatively more dominated by fungi when compared to the PT soil (Beare et al., 1992). A microbial community with a higher proportion of fungi relative to bacteria may be expected to have a slower turnover of biomass and a higher immobilization of added N in fungal components with a low potential for mineralization (Holland and Coleman, 1987; He et al., 1988). It is also possible that differences in soil structure between PT and NT soils may have influenced 15N availability for mineralization, for example, due to protection of microbes from grazing by protozoa and soil fauna (Ladd et al., 1993)

Relevance of Experimental Approach
Sieving was used here as a treatment for disturbing the soil structure and the results obtained have two implications for the experimental approach in studies of soil N dynamics. The sieving treatment was found to release mineral N from protected N pools in the soils, however, this release was not well reflected by the flush in total mineral N release after sieving. This flush in net N mineralization has been recommended by Hassink (1992) as a measure of protected N release from soils. But, the current experimental approach with use of sieving and 15N-labeling techniques showed that the flush in mineral N release may originate from both active and protected N pools as in the case of the NT soil, or that protected N may be released despite the lack of a flush in mineral N release as found in the PT soil (Fig. 3). Thus, the use of 15N labeling of the active soil N pool can be a useful approach for studying the involvement of protected organic N pools in MIT activities. The sieving treatment was found, however, to influence net N mineralization rates to a different degree in the soils under study, with the influence declining within 7 to 14 d after the treatment. These findings add support to the conclusion of other studies (Craswell and Waring, 1972; Hassink, 1992); that use of freshly sieved soil samples (through 2-mm mesh size or less) should be avoided when preparing samples for studies of N metabolism in soils which are characterized by different degrees of disturbance (Bundy and Meisinger, 1994).


   ACKNOWLEDGMENTS
 
This work was supported in part by a fellowship from the National Environmental Research Institute of Denmark, the Danish Research Academy, and Aalborg University.

Received for publication March 22, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




This article has been cited by other articles:


Home page
 Soil Sci.Home page
A. Bono, R. Alvarez, D. E. Buschiazzo, and R. J. C. Cantet
Tillage Effects on Soil Carbon Balance in a Semiarid Agroecosystem
Soil Sci. Soc. Am. J., June 18, 2008; 72(4): 1140 - 1149.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (23)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Kristensen, H.L.
Right arrow Articles by Meisinger, J.J.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Kristensen, H.L.
Right arrow Articles by Meisinger, J.J.
Agricola
Right arrow Articles by Kristensen, H.L.
Right arrow Articles by Meisinger, J.J.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome