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Published in J. Environ. Qual. 33:1098-1105 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Waste Management

A Comparison of In Situ Methods for Measuring Net Nitrogen Mineralization Rates of Organic Soil Amendments

Travis A. Hanselman*, Donald A. Graetz and Thomas A. Obreza

Soil and Water Science Department, 106 Newell Hall, University of Florida, Gainesville, FL 32611-0510

* Corresponding author (taha{at}mail.ifas.ufl.edu).

Received for publication June 10, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In situ incubation methods may help provide site-specific estimates of N mineralization from land-applied wastes. However, there are concerns about the reliability of the data generated by the various methods due to containment artifacts. We amended a sandy soil with either poultry manure, biosolids, or yard-waste compost and incubated the mixtures using four in situ methods (buried bags, covered cylinders, standard resin traps, and "new" soil–resin traps) and a conventional laboratory technique in plastic bags. Each incubation device was destructively sampled at 45-d intervals for 180 d and net N mineralization was determined by measuring the amount of inorganic N that accumulated in the soil or soil plus resin traps. Containment effects were evaluated by comparing water content of the containerized soil to a field-reference soil column. In situ incubation methods provided reasonable estimates of short-term (<45 d) N mineralization, but long-term (>45 d) mineralization data were not accurate due to a variety of problems specific to each technique. Buried bags and covered cylinders did not retain mineralized N due to water movement into and out of the containers. Neither resin method captured all of the mineralized N that leached through the soil columns, but the new soil–resin trap method tracked field soil water content better than all other in situ methods evaluated. With further refinement and validation, the new soil–resin trap method may be a useful in situ incubation technique for measuring net N mineralization rates of organic soil amendments.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CHEMICAL INDICES and laboratory incubation methods are frequently used for estimating the amount of N that mineralizes from land-applied wastes (Parker and Sommers, 1983; Chae and Tabatabai, 1986; Westerman et al., 1988; Douglas and Magdoff, 1991; Cabrera et al., 1994; Gordillo and Cabrera, 1997; Gilmour and Skinner, 1999; Qafoku et al., 2001). However, there is uncertainty regarding the extrapolation of laboratory-derived mineralization values to the field because the N mineralization process can be affected by a number of dynamic and site-specific factors (e.g., fluctuating temperature, water, and aeration) (Clark and Gilmour, 1983; Honeycutt and Potaro, 1990; Sims, 1990; Cabrera, 1993; Das et al., 1995; Sierra, 1997; Honeycutt, 1999). Therefore, some researchers have preferred to measure N mineralization rates in the field with in situ incubation devices. A variety of in situ procedures are described in the literature, but three methods are especially popular—the buried-bag, covered-cylinder, and resin-trap techniques (Westermann and Crothers, 1980; Distefano and Gholz, 1986; Adams and Attiwill, 1986; Raison et al., 1987; Goncalves and Carlyle, 1994; Subler et al., 1995; Hook and Burke, 1995; Stenger et al., 1996; Dou et al., 1997; Hatch et al., 1998; Honeycutt, 1999; Eghball, 2000).

The buried-bag method involves filling plastic bags with a soil sample and burying the bagged sample at a shallow depth in the field for several days or weeks. The method is appealing because it is easy to carry out and sensitive to on-site temperature fluctuations, and the water content of the bagged soil can be adjusted to a desirable condition (e.g., field capacity) at the start of the incubation (Eno, 1960). However, some limitations of the method have been reported. Physical damage to the bags from insects or plant roots may contribute to losses of mineralized N to the field soil via diffusion or mass flow (Eno, 1960). Elevated concentrations of nitrate and carbon dioxide inside the bags may promote denitrification (Subler et al., 1995).

The covered-cylinder method was developed as a more durable alternative to the buried-bag technique. Covered cylinders are usually constructed from PVC or metal pipes that are capped to exclude rainfall (Adams and Attiwill, 1986; Adams et al., 1989). Since the columns are open on the bottom, the soil is probably better aerated than bagged soil. Even so, perforations are sometimes added to the sidewall of the tubes to promote air exchange with the field soil (Dou et al., 1997). However, some problems with the covered cylinders were reported by Subler et al. (1995). They suggested that mineralized N could be lost though the aeration holes in the sidewall of the tubes or out of the bottom of the column by diffusion or mass flow. There is also a potential for plant roots to grow into or around the columns and absorb mineralized N from the containerized soil.

More elaborate in situ methods use ion-exchange resins to capture mineralized N as it leaches from soil contained within PVC or metal cylinders (Distefano and Gholz, 1986). The principal advantage of the resin-trap method over buried bags or covered cylinders is that temperature, moisture, and aeration of the containerized soil is supposed to fluctuate in a similar way as the field soil. Furthermore, since mineralized N is leached from the soil column during rainfall or irrigation, the technique may help prevent the artificial stimulation of denitrification. The drawback of the method is that a great deal of time and effort must be spent testing the resin-trap system in an empirical way to ensure that leached ions are trapped efficiently in the field because ion competition, resin saturation, and bypass flow are all factors that can reduce the effectiveness of the resins to capture mineralized N (Schnabel, 1983, 1995; Schnabel et al., 1993; Wyland and Jackson, 1993; Kjonaas, 1999a, 1999b).

Few studies have compared N mineralization estimates between in situ methods or compared in situ estimates of N mineralization to laboratory estimates. Subler et al. (1995) compared the covered-cylinder technique with a buried-bag method for measuring N mineralization of soils fertilized with inorganic N, a legume cover crop, and straw-pack manure. They concluded that the two methods generally gave similar seasonal estimates of net N mineralization. Adams and Attiwill (1986) compared a covered-cylinder procedure with an aerobic laboratory incubation for measuring the N mineralization rate of soil organic matter. They reported that in situ rates were less than for soils incubated in the laboratory. They attributed the discrepancy to temperature differences between the field and laboratory incubation.

It is clear that each of the various in situ methods may have unique limitations that can potentially affect the quality of the data produced. Thus, a potential exists for the various methods to give different and/or inaccurate results. However, resin-trap incubation devices appear to be the most promising in situ technology for measuring net N mineralization rates of organic soil amendments because field soil conditions are closely simulated. However, during preliminary testing of "standard" resin-trap (resin contained in nylon bags placed underneath soil columns) methods, we noticed a tendency for the containerized soil to be appreciably wetter than the surrounding field soil for several days following rain or irrigation events. Therefore, we felt that it was necessary to develop a "new" type of resin trap that improves the drainage characteristics of the soil columns. Since few comparative studies of the various in situ methods are available, we tested some well-known in situ techniques, a "new" situ device, and a conventional laboratory incubation procedure for measuring net N mineralization rates of organic soil amendments. The specific objective of this research was to compare estimates of net N mineralization from land-applied poultry manure, biosolids, and yard-waste compost obtained using buried bags, covered cylinders, standard resin traps, new soil–resin traps, and a conventional laboratory incubation in plastic bags.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Amendments
Three soil amendments were used in this study, poultry manure, biosolids, and yard-waste compost. Poultry manure was provided from Nutri-Source (Orlando, FL). Anaerobically digested biosolids were provided from Resource Reclamation Services (Miami, FL). Yard-waste compost was obtained from Enviro-Comp Services (Jacksonville, FL). Following collection, the amendments were air-dried for 2 wk, homogenized in a mixer, and passed through a 4-mm sieve to control variability. The total solids contents of the air-dried amendments were 84, 89, and 76% for the poultry manure, biosolids, and yard-waste compost, respectively.

Amendment Characterization
Total N and total C of the amendments was determined by oven-drying at 60°C for 48 h, grinding to a powder in a Spex 8000 Mixer/Mill (Spex Industries, Edison, NJ), and then analyzing the powder by combustion at 1010°C using a Carlo-Erba NA-1500 CNS Analyzer (Carlo-Erba Instruments, Milan, Italy). Inorganic N was extracted by shaking 10 g of the amendment with 100 mL of 2 M KCl for 1 h. The supernatant was filtered through Whatman (Maidstone, UK) no. 42 filter paper and analyzed for ammonia N, ammonium N, nitrite N, and nitrate N using USEPA Methods 350.1 and 353.2, respectively (USEPA, 1993a, 1993b). Selected characteristics of the amendments are shown in Table 1.


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  		Table 1. Mean values of selected chemical characteristics for the organic amendments.

 
Site Information and Soil Characteristics
Field experiments were conducted near the city of Gainesville, FL at the UF/IFAS Green Acres Agronomy Farm. Soil temperature (10-cm depth) was recorded every 10 min using an Optic StowAway Temp data logger (Onset Computer Corp., Bourne, MA). Rainfall was monitored with a Rain Collector II rain gauge (Davis Instruments, Hayward, CA) equipped with a HOBO Event (Onset Computer Corp.) data logger. The soil used for the incubation experiments was collected from the surface of a well-drained Kendrick sand (loamy, siliceous, hyperthermic Arenic Paleudults). The pH, cation exchange capacity, sand content, and water-holding capacity (0.01 MPa) of this soil were 5.9, 1.6 cmol+ kg–1, 956 g kg–1, and 0.048 g H2O g–1 soil, respectively.

Field Incubation Methods
Four in situ methods (buried bag, covered cylinder, ion-exchange resin coupled with a soil core, and a new ion-exchange resin procedure coupled with a soil core) were used. Field-reference soil columns, with and without amendments, were installed to compare the soil water content dynamics within the incubation vessels to field soil water content at each sampling time. All incubation cylinders were constructed from 5- x 35-cm PVC pipes. Storage-grade polyethylene bags (17 cm x 20 cm x 0.05 mm) with a slide-lock seal were used as the containers for the buried-bag procedure. For the perforated covered-cylinder method, eight holes (1-cm diameter) were drilled in the sidewall of the tubes (Dou et al., 1997). Standard resin traps were constructed from the combined use of nylon stockings and an outside-fitting PVC ring. The ring was placed inside the stocking and positioned to accept 10 g (oven-dry basis) of analytical-grade Rexyn-300 (Fisher Scientific, Pittsburgh, PA). The nylon material was stretched over the top portion of the PVC ring and tied along the side of the ring to form an approximately 1.5-cm-thick resin bed.

A new type of resin-trap device was constructed from an inside-fitting PVC coupling, which was filled with soil collected from the experimental site at the 30- to 35-cm depth and mixed with ion-exchange resin (Hanselman, 2000). Twenty grams of moist soil (water content, 0.10 g H2O g–1 soil) was pressed into the bottom of the soil–resin trap with a rubber stopper. Eighty grams of moist soil were mixed with 5 g (oven-dry basis) of Rexyn 300 using a glass rod. The mixture was pressed into the soil–resin trap. An additional 20 g of moist soil was added above the soil–resin mixture. Soil–resin traps were attached to the PVC cylinders with silicon sealant approximately 24 h before initiating the field incubations. A moist towel was placed inside the columns and both ends were sealed with parafilm to ensure that the soil–resin mixture did not dry out before installation.

Soil Collection and Amendment Procedure
A bulk soil sample was collected from the surface of the experimental plot to a depth of 30 cm and passed through an 8-mm screen in the field to improve homogeneity. Homogenized, field-moist soil was weighed into the in situ incubation devices and field-reference columns to an approximate bulk density of 1.5 g cm–3. Five grams (oven-dry basis) of organic amendment was mixed with the upper 15 cm of soil in each container. Amendments were applied at a rate that simulated a 25 Mg ha–1 surface application rate incorporated to a depth of 15 cm. The N application rate was 800, 1425, and 225 kg ha–1 for the poultry manure, biosolids, and yard-waste compost, respectively. The columns were placed upright in trenches (30 cm deep x 20 cm wide x 350 cm long) and pressed lightly to ensure good contact between soil and resin in the cylinders with the underlying soil. Soil was backfilled around the columns, so that the level of the soil within the columns and outside the columns was approximately the same. To avoid capturing surface water into the columns, 5 cm of each column (which did not contain soil) was left exposed above ground level. For the buried-bag procedure, 450 g of moist soil was weighed and transferred into bags that contained 5 g of the organic amendment. Soil water content was adjusted to approximately 0.01 MPa by adding deionized water. Air was forced out of the bags and the zip seal was closed. Bags were buried adjacent to the columns at a depth of 15 cm in trenches 20 cm wide x 350 cm long. Field plots were maintained by spraying with Roundup (Monsanto Technology, Marysville, OH) herbicide and by mechanical removal as needed to prevent plant growth in and around the in situ incubation devices.

Laboratory Incubation
A mass of soil equivalent to the amount used in the field was weighed into the same type of plastic bags and amended with the same amount of organic material, and water content was adjusted to the same level as for the field experiment. Bags were incubated at 30°C in an Environ-Shaker 3597 incubator (Lab Line Instruments, Melrose Park, IL). Water content of the bagged soil was checked every 2 wk and adjusted if necessary with deionized water to maintain initial soil water content.

Sample Collection
Field incubation was started 5 May 1999. Four replicate containers from each treatment (field and laboratory) were collected at the time of installation (5 May 1999) and after 45 (19 June 1999), 90 (3 Aug. 1999), 135 (17 Sept. 1999), and 180 d (1 Nov. 1999) of incubation. The initial sampling date roughly corresponded to a 3-d incubation period, because 2 d were required to complete the installation of the soil cores at the field site and 1 d was needed to process the samples in the laboratory.

The bags and soil columns were brought to the laboratory, emptied into a pan, mixed thoroughly, and stored at 4°C until inorganic N extraction (≤3 d). Resin traps from the standard incubation vessel trap were removed from the PVC cylinders and the entire resin trap was placed directly into a 500-mL Nalgene jar (Nalge Nunc International, Rochester, NY), sealed with the lid, and stored at 4°C until inorganic N extraction (≤3 d). Resin traps from the new soil–resin incubation vessel were detached from the PVC cylinders using a razor blade to cut the silicon seal. The contents were emptied into a 1000-mL Nalgene bottle, weighed, sealed with its lid, and stored at 4°C until inorganic N extraction (≤3 d).

Soil and Resin Analysis
Soil water content was determined by oven-drying at 105°C for 24 h. Soil inorganic N concentration was determined by shaking 10 g of field-moist soil with 100 mL of 2 M KCl for 1 h followed by filtration through Whatman no. 42 filter paper. Inorganic N extraction from the standard resin trap was done by swirling the entire resin trap (PVC ring, nylon material, and resin) with 100 mL of 2 M KCl for 1 h. Inorganic N was extracted from the new soil–resin trap by shaking the contents of the soil–resin trap with 500 mL of 2 M KCl for 1 h. Soil and resin extracts were filtered through Whatman no. 42 filter paper and analyzed using USEPA Methods 350.1 and 353.2, respectively (USEPA, 1993a, 1993b). Correction factors of 1.25 and 1.11 were used to account for incomplete recovery of adsorbed nitrate N and ammonium N from standard resin traps by a single extraction, respectively. No correction factors were needed for soil–resin traps, since preliminary experiments showed extraction efficiency of adsorbed nitrate N and ammonium N was approximately 99%.

Calculation of Net Nitrogen Mineralization
Net N mineralization of organic amendments was estimated by (i) subtracting the amount of inorganic N extracted from control soils and resins from the quantity of inorganic N extracted from amended soils and resins and (ii) subtracting the initial amount of inorganic N that was applied in the amendments from the difference from the first step. To express mineralization as a percentage of amendment organic N, the net quantity of N mineralized at each extraction time was divided by the initial amount of organic N applied in the amendment, with this result then multiplied by 100.

Experimental Design and Statistical Analysis
The experiment was a factorial design with the following treatment combinations: five methods (covered cylinder, buried bag, standard resin trap, new soil–resin trap, and laboratory incubation) x four amendments (control, poultry manure, biosolids, and yard-waste compost) x five sampling dates (initial, 45, 90, 135, and 180 d) with four replications. Data from the field and laboratory incubation studies were analyzed using the General Linear Models program of SAS (SAS Institute, 2000). Means separation was accomplished using Fisher's least significant difference (LSD) technique.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Temperature and Rainfall
Soil temperature ranged from 15 to 38°C during the field incubation studies. The mean soil temperature for each 45-d incubation period was 28, 30, 29, and 25°C, respectively, with an overall mean of 28°C (Fig. 1) . These data indicate that the 30°C temperature that was chosen for the laboratory incubation was nearly the same as average field soil conditions. Rainfall was 200, 230, 200, and 150 mm for each incubation period, respectively, with a total accumulation of 780 mm (Fig. 2) .



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  		Fig. 1. Soil temperatures (°C) measured 10 cm below surface during the field experiment (5 May to 1 Nov. 1999).

 


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  		Fig. 2. Daily rainfall accumulation (mm) during the field experiment (5 May to 1 Nov. 1999).

 
Soil Water Dynamics
The water contents of soils contained within the covered cylinders and the standard resin-trap method were much lower and much higher than field references, respectively, on all sampling dates (Table 2). A typical assumption of perforated covered-cylinder methodology is that water equilibration through holes in the sidewall or from the bottom of the incubation vessel will maintain soil water content similar to the surrounding field soil. However, the apparent lack of water equilibration demonstrates that such assumptions may not be valid. The standard resin-trap design produced a textural discontinuity between the soil and the resin trap, and thus produced a "container capacity" phenomena (White and Mastalerz, 1966; Schnabel, 1983). However, soils contained within the new soil–resin trap incubation vessel reliably mimicked field-reference soil water content (Table 2). The new soil–resin trap system eliminated most of the drainage impairment issues associated with the standard resin-trap design. Some small differences were noted, however, for soils amended with poultry manure, biosolids, and yard-waste compost on the last (180 d) sampling date. This difference is probably due to a rainfall event that occurred a few hours before the samples were collected (Fig. 2). This may indicate that the soil–resin traps did not drain as quickly as the field-reference soils, but overall, they were the best column incubation system in regard to soil water dynamics (Table 2).


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  		Table 2. Gravimetric soil water content as affected by amendment source (control, poultry manure, biosolids, and yard-waste compost) and method of incubation in columns.{dagger}

 
A problem with water equilibration was observed with buried bags. The initial soil water content of the bagged samples ranged from 0.063 to 0.073 g H2O g–1 soil for all amendment treatments. However, after 45 d of field incubation, about three fourths of the bags were perforated with small holes (presumably made by insects). The water contents of the bagged soils were not different (ranged from 0.064 to 0.075 g H2O g–1 soil) at 45 d from initial levels. However, at 90 d, nearly all of the bags were perforated and water content of the bagged soil deviated from the initial values. At the 135-d sampling event, soil water content was lower than initial values and ranged from 0.024 to 0.057 g H2O g–1 soil. Some water transfer into the bags apparently occurred during the 135- to 180-d incubation period, since water content of the bagged soils increased up to 0.072 g H2O g–1 soil.

The soil water content of laboratory-incubated soils ranged from 0.047 to 0.069 g H2O g–1 soil and averaged 0.061 g H2O g–1 soil for all amendment treatments over the duration of the study. This was slightly less than field-reference soil water content, which averaged 0.068 g H2O g–1 soil for all amendment treatments and times. These results indicated that the soil water content prescribed for the laboratory incubation study was nearly the same as average field soil water content.

Net Nitrogen Mineralization Estimates
Nitrogen Mineralization (0–3 d)
The initial measurements of N mineralization by method were not compared statistically since differences, if any, were probably due to rapidly decomposable fractions of the amendment and unavoidable short-term (a few hours) differences in incubation time during column installation. Nitrogen mineralization from poultry manure ranged from 38 to 47% of the added organic N for all methods within 3 d of application to the soil (Table 3). These data are similar to the results of Cabrera et al. (1994), who reported that 36 to 52% of poultry litter organic N had mineralized within 3 d. Similar results were also found by Westerman et al. (1988), who reported a large flush of ammonium N (approximately 50% of total Kjeldahl N) immediately following broiler litter application to a soil. The rapid mineralization of N from poultry wastes is probably attributable to the uric acid fraction of the organic N (Gordillo and Cabrera, 1997). Biosolids mineralization at 3 d ranged from 18 to 20% of added organic N (Table 3). Some researchers have noted the importance of rapidly mineralizable fractions of biosolids. For example, Lerch et al. (1992) reported 1-wk mineralization amounts that ranged from 16 to 33% of added biosolids organic N. They suggested that water-extractable amines are a labile fraction of organic N sources in biosolids. Yard-waste compost immobilized 1 to 4% of added organic N (Table 3). Labile C and N fractions are greatly reduced during the composting process, but some rapid consumption or fluxes of N may still occur. Li et al. (1997) found a rapid release of inorganic N from municipal solid waste compost over 5 d during a leaching study with Oldsmar sand (sandy, siliceous, hyperthermic Alfic Arenic Alaquods) from St. Lucie County, FL.


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  		Table 3. Net amount of organic N mineralized as affected by incubation method and amendment source (poultry manure, biosolids, and yard-waste compost) during a 180-d incubation.{dagger}

 
Nitrogen Mineralization (3–45 d)
Nitrogen mineralization estimates at 45 d varied greatly by method (Table 3). The observed differences are probably due to incomplete retention of mineralized N within covered cylinders and resin-trap incubation vessels and/or to changes in environmental conditions (i.e., water content within the incubation vessels). Several factors may be responsible for the loss of inorganic N from incubation vessels (e.g., gradient diffusion of ions out of buried bags and covered cylinders and insufficient N retention by the resin traps).

Buried-bag estimates of poultry manure, biosolids, and yard-waste compost mineralization were similar to laboratory-measured values. Covered cylinders underestimated poultry manure and biosolids mineralization, and overestimated yard-waste compost mineralization, relative to laboratory incubation. The predominance of dry soil conditions within covered cylinders is the likely explanation for the apparent reduction in mineralization or immobilization rates since gradient losses would be minimal if water did not transfer into or out of the columns. The standard resin-trap method gave the lowest estimates of poultry manure and biosolids mineralization at 45 d, but yard-waste compost mineralization estimates were similar to those found with buried bags (Table 3). Compared with standard resin traps, the new soil–resin trap gave higher estimates of poultry manure and biosolids mineralization but gave similar estimates for yard-waste compost mineralization. Compared with laboratory incubation, the new soil–resin trap incubation system underestimated poultry manure and biosolids mineralization, but gave similar estimates of yard-waste compost mineralization (Table 3). Neither resin-trap incubation method captured all of the initial flux of inorganic N that leached through the soil columns from poultry manure or biosolids amendments. However, the new soil–resin traps generally accumulated more inorganic N than did the standard resin traps (Fig. 3) . Perhaps, water channeled quickly through standard resin traps and bypassed the adsorption surfaces. The new soil–resin traps may have countered this "pulsing" type of hydraulic situation by enabling water to move evenly through the soil–resin matrix. Alternatively, electrical conductivity analysis of the soils contained in the different incubation systems suggested that large quantities of salts were leached from the soils amended with poultry manure and biosolids during 45 d of incubation (data not shown). These salts would have competed with inorganic N for adsorption surfaces. High flow rates and high salt levels in the leachate tends to result in poor retention of inorganic N on resins (Schnabel, 1983; Wyland and Jackson, 1993).



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  		Fig. 3. Inorganic N accumulation on standard resin traps (•) and the new soil–resin trap ({circ}) from unamended control soil and soil amended with poultry manure, biosolids, or yard-waste compost. The vertical line at each data point represents standard error of the mean.

 
Nitrogen Mineralization (45–180 d)
Nitrogen mineralization estimates of each of the amendments diverged greatly between 45 and 180 d of incubation, depending on the incubation technique used (Table 3). The unusual pattern of net N mineralization observed over time with each in situ method suggests inadequate retention of mineralized N within buried bags, covered cylinders, or on resin traps. Neither resin trap accumulated much inorganic N from poultry manure or biosolids after 90 d (Fig. 3).

Since large amounts of inorganic N were apparently lost from all of the field incubation devices, the amount of N mineralized from the amendments is underestimated as compared with laboratory incubation. This was especially apparent for soils amended with poultry manure and biosolids. Estimates of compost mineralization were similar using several of the methods, but the precision of these evaluations is questionable given the nature of problems identified with the in situ methods during field incubation.

Similar problems (i.e., bag damage and the loss of N from the containers) with these four in situ methods were found at two other research locations (UF/IFAS Horticultural Unit, Gainesville, FL, and the Southwest Florida Research and Education Center, Immokalee, FL) (Hanselman, 2000). At these sites, the buried bags became filled with water after 45 to 90 d in the field due to perforations and leakage through the zip-seal closure. In that case, gradient losses and denitrification probably prevented the accurate calculation of net N mineralization rates. The soil water content of covered cylinders at these sites fluctuated with the surrounding soil, but N losses as a result of this water movement into and out of the columns were severe. The soils contained in standard resin-trap incubation vessels were wetter than field-reference soils, but the new soil–resin trap tracked field conditions at both locations. Estimates of N mineralization between the resin methods were similar, but not consistent with N mineralization observed during laboratory incubation.

The laboratory incubation method showed typical patterns of net N mineralization for the poultry manure, biosolids, and yard-waste compost and thus appeared to give the least confounded estimates of N mineralization in this study (Table 3). The proportion of applied organic N that mineralized from poultry manure, biosolids, and yard-waste compost was 80, 66, and 2%, respectively, during 180 d of laboratory incubation (Table 3). Unfortunately, it is not known if similar amounts of N were mineralized from the amendments in the field due to the apparent lack of N retention in columns and bags. The laboratory-measured values of poultry manure net N mineralization reported in this study are within the range of estimates by Bitzer and Sims (1988) and Gordillo and Cabrera (1997). The biosolids mineralization estimates provided by the laboratory incubation are somewhat greater than values reported by other researchers using long-term incubation techniques. For example, He et al. (2000) reported 1 yr N mineralization estimates for pelletized biosolids amounted to 48% of added organic N and Whitehead (1985) reported N mineralization amounted to 47 to 60% of total N. Similar estimates of yard-waste compost N availability were reported by Hartz et al. (2000), who estimated net N mineralization was 1 to 2% after 24 wk of incubation.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The preservation of ambient soil water dynamics is a critical factor in the attempt to mimic field environmental conditions with in situ containment devices. Water content of soils contained within buried bags deviated from initial levels at time periods of >45 d, thereby violating the assumption that it would remain constant throughout the 180-d field exposure period. Water-content changes within buried bags were attributed to degradation of the bag material over time. Soil contained within perforated covered cylinders dried to considerably lower levels than did the field-reference soil column and did not respond to changes in water content of the surrounding field soil. The standard resin-trap design produced soil water conditions that were much wetter than field soil conditions, but the new soil–resin trap reliably tracked field soil water content. Thus, the soil–resin traps reduced the potential for soil water content–induced containment effects such as enhanced denitrification.

Compared with laboratory incubation, buried bags gave similar estimates of N mineralization before problems with bag degradation occurred in the field. Covered cylinders gave low estimates due to dry soil conditions and losses of N from the containers as water content equilibration occurred. Resin-based techniques underestimated long-term N mineralization rates of poultry manure, biosolids, and yard-waste compost due to incomplete N retention. Resin-trap adsorption efficiency was reduced beyond 45 to 90 d of incubation time, particularly for soils amended with poultry manure and biosolids. Soluble salts and organic compounds probably compromised the N retention capacity of the resin. The new soil–resin trap adsorbed a greater amount of the initial N flux compared with the standard resin procedure. Time-dependency and/or retention capacity limitations of resin-based methodology are problems that can be solved by increasing the amount of resin used in the traps and changing out the resin cartridges frequently (<45 d). Laboratory-incubated soils amended with poultry manure, biosolids, or yard-waste compost mineralized 80, 66, and 2% of organic N, respectively.

This study demonstrates three potential problems regarding in situ methods: (i) soil water content dynamics within the buried-bag, covered-cylinder, or standard resin-trap incubation systems are not fully reflective of ambient field soil water conditions; (ii) low recovery of mineralized N beyond 45 to 90 d due to bag degradation or resin-trap inefficiency underestimates N mineralization; and (iii) an improved incubation system, which maintains ambient field environmental conditions and provides better N recovery, is needed to ensure that mineralization estimates made under field conditions are accurate.

Of the in situ methods tested in this study, the new soil–resin trap incubation system maintained soil conditions that were most representative of actual field soil conditions, but recovery of mineralized N was insufficient during extended time periods for reliable measurements of net N mineralization from poultry manure, biosolids, and yard-waste compost. Simple refinements of the new soil–resin trap construction and a regular schedule of resin cartridge replacement (<45 d) would probably improve inorganic N retention considerably.


   ACKNOWLEDGMENTS
 
This research was supported by the Florida Agricultural Experiment Station and is approved for publication as Journal Series no. R-09539.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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JEQ 2004 33: 799-804. [Full Text]  




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