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Published in Soil Sci. Soc. Am. J. 68:162-167 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Biosolids Processing Effects on First- and Second-Year Available Nitrogen

Craig G. Cogger*,a, Andy I. Barya, Dan M. Sullivanb and Elizabeth A. Myhrea

a Washington State Univ. Puyallup Research and Extension Center, 7612 Pioneer Way E., Puyallup, WA 98371-4998
b Dep. of Crop and Soil Sci., Oregon State Univ., Corvallis, OR 97331

* Corresponding author (cogger{at}wsu.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Simple, reliable estimates of biosolids N availability are needed to develop land application programs that will benefit crops without risk of excess nitrate leaching. This study was conducted to assess the effect of biosolids processing on plant available nitrogen (PAN) release during the first and second growing seasons after application. We compared 14 sources of biosolids and a range of inorganic N rates in two replicated field experiments on established tall fescue (Festuca arundinacea Schreb.). The biosolids encompassed a range of treatment and dewatering/drying processes. A single biosolids application was made in May of the first year, and tall fescue yield and N uptake were measured by harvest for the next two growing seasons. Inorganic N was split across multiple applications each year. Fertilizer efficiency regression equations were developed for the inorganic N treatments, and used to calculate biosolids PAN from N uptake data. Year 1 PAN was similar across a range of biosolids treatment processes. For nonlagoon biosolids, PAN averaged 37 ± 5% of total biosolids N. Lagoon biosolids PAN ranged from 8 to 25% of total N, with the oldest, most stable biosolids having the lowest PAN. Year 2 PAN averaged 13 ± 2% for nonlagoon biosolids, excluding heat-dried materials, which were lower (5 to 8%). Our calculations indicated that about half of the Year 2 PAN became available during the cool season, suggesting that winter cover cropping may be needed to reduce the potential for nitrate leaching loss in summer annual cropping systems.

Abbreviations: PAN, plant available nitrogen


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
APPLYING MUNICIPAL wastewater biosolids to agricultural land is an effective way to recycle nutrients and organic matter from biosolids to benefit crop production. In the short term, biosolids application rates are usually based on N. Inadequate N supply can reduce crop yield, while surplus N can reduce crop quality and lead to accumulation of excess soil NO3, which is subject to leaching. Accurate predictions of N availability from biosolids are a key to recommending application rates that will benefit crops without risk of excess NO3 leaching.

In the 1980s, the USEPA developed guidelines for estimating mineralization of organic N based on the type of biosolids processing (USEPA, 1983). The USEPA recommended a mineralization rate of 20% of biosolids organic N the first year after application for anaerobically digested biosolids, and 30% for aerobically digested biosolids. Second-year mineralization estimates were 10% for anaerobic and 15% for aerobic biosolids. Data from laboratory mineralization experiments by Parker and Sommers (1983) and King (1984) supported those first-year estimates of N availability. Subsequent field studies have suggested greater N availability for anaerobically digested biosolids (Sullivan et al., 1997; Cogger et al., 1999). Little data has been available for other types of biosolids.

Field studies are a less-controlled but more agronomically realistic approach for assessing biosolids N availability. Nitrogen uptake by a field crop fertilized with biosolids can be compared with N uptake from the same crop fertilized with inorganic N to estimate the amount of plant available N derived from the biosolids during the growing season. Intensively managed, irrigated forage grass is an ideal crop for evaluating PAN. Forage grass has a linear response to N application across a wide range of rates during a long growing season (Whitehead, 1995), with little residual left in the soil, even at available N rates of 300 kg ha–1 or more (Steenvoorden et al., 1986; Cogger et al., 1999). In addition, forage grasses are harvested multiple times during the growing season, allowing evaluation of N availability across time.

To improve our ability to predict N availability from biosolids produced by a variety of treatment processes, we conducted a field N recovery experiment with irrigated tall fescue. Our specific objective was to assess the effect of biosolids processing on plant-available N in the first and second growing seasons after biosolids application.

This experiment was part of a national biosolids N availability assessment project. Summary data from the first year after biosolids application was combined with laboratory incubations, field data from several other locations across the USA, and model simulations to develop national estimates for biosolids N availability (Gilmour et al., 2000; Gilmour et al., 2003).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Site Description
Field plots were established in Puyallup, WA, located 55 km south of Seattle. The soil is mapped as a Puyallup fine sandy loam (coarse-loamy over sandy, isotic over mixed, mesic Vitrandic Haploxerolls), a deep, well-drained soil developed in recent alluvium consisting primarily of sand and silt. Mean surface soil (0–20 cm) texture at the experimental site is 470 g kg–1 sand, 460 g kg–1 silt, and 70 g kg–1 clay. At the beginning of the experiment, organic carbon was 18 ± 1.2 g kg–1 (0- to 30-cm depth) soil pH (1:2 suspension) was 5.8, Bray-1 P was 197 mg kg–1, and exchangeable K was 112 mg kg–1.

The climate is typical of the maritime Pacific Northwest, with cool, wet winters and mild, dry summers. Mean annual temperature is 11°C with a January mean of 4°C and a July mean of 18°C. Mean annual precipitation is 1020 mm, falling mostly as rain between October and May. Irrigation is necessary to maintain intensive forage production during the dry summer months. During the study period, precipitation for April through October was 188 mm in 1998, 228 mm in 1999, and 300 mm in 2000. The precipitation was supplemented by 285 mm of sprinkler irrigation in 1998, 280 mm in 1999, and 175 mm in 2000.

Forage-type tall fescue ‘AU Triumph’ was planted in June 1996 on ground that was previously cropped to silage corn (Zea mays L.). The tall fescue was maintained with inorganic fertilizers and irrigation until the experimental treatments were applied in May 1998 (Exp. 1) or May 1999 (Exp. 2). Tall fescue was harvested and removed from the plots six times per year during the establishment period and twice (April and May) before biosolids were applied in the first experimental year.

Biosolids
We obtained biosolids from 14 wastewater treatment plants (Table 1). The biosolids represented a range of wastewater plant sizes and biosolids treatment processes, including aerobic and anaerobic mesophilic digestion, thermophilic digestion, lime stabilization, lagooning, dewatering, air drying, and heat drying. All of the biosolids used were dewatered or dried materials. Solids content of the biosolids was determined on triplicate samples by drying at 60°C. Biosolids NH4 and NO3 were extracted with 1 M KCl. Ammonium-N was determined by automated colorimetric analysis with the salicylate method and NO3–N by cadmium reduction (Mulvaney, 1996). Nitrate-N was less than the detection limit of 0.01 mg kg–1 in all biosolids samples in 1998, and NO3 analyses were not run in 1999. Biosolids total N and C were determined with a LECO Total CNS 2000 elemental analyzer (Nelson and Sommers, 1996).


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  		Table 1. Descriptions and analytical data for biosolids materials.

 
Field Experiments
Two experiments were set up on adjacent fields; the first ran from May 1998 through October 1999, and the second from May 1999 through October 2000. Design and procedures for the two experiments were identical except where noted below. Both experiments had 14 treatments, each replicated four times in a randomized complete block design. Plot size was 1.5 by 6 m. All treatments received 212 kg ha–1 supplemental K2O and 45 kg ha–1 S as broadcast applications of KCl (0-0-62) and K2SO4·2MgSO4 (0-0-22-22S-11Mg) in the spring of each year.

Experiment 1 (1998-1999) included biosolids from eight sources (Table 1), five inorganic N treatments and a zero-N control. On 6 May 1998, a single surface application of each type of biosolids was made to the appropriate plots in Exp. 1. The target application rate was 400 to 500 kg total biosolids N per hectare. At this target rate, the expected plant available N would be well within the linear portion of the N uptake curve for intensively managed forage grass (Whitehead, 1995). Before applying the biosolids, the tall fescue was cut at a 5-cm height and the harvested forage removed. The biosolids were not incorporated.

The five inorganic N treatments received a total of 50, 100, 150, 200, and 250 kg N ha–1 in 1998 as ammonium nitrate (34-0-0 N-P-K). The inorganic N was split into three equal applications; one on the same date as the biosolids application, and the remaining two following the June and July harvests.

The tall fescue was harvested at the early boot stage with a small plot forage harvester. A 1- x 6-m swath was harvested from each plot at a 5-cm height. Harvests were on 3 June, 6 July, 5 August, 14 September, and 23 October. Because the tall fescue was harvested twice in 1998 before the biosolids application, the first harvest of the experiment was actually the third harvest of the season. The harvested forage from each plot was weighed wet, and a 500-g subsample was collected and oven-dried (60°C) for determination of dry matter and total N (LECO CNS 2000).

No biosolids were applied to Exp. 1 in 1999, but the tall fescue was harvested in the same manner as in 1998 to measure N uptake in the second year following biosolids application. The biosolids treatments received 150 kg ha–1 N as 34-0-0 split across five applications (one in March and the others following each of the first four harvests) to maintain stand vigor because greatly reduced N mineralization was expected during the second year. The five inorganic N treatments received 34-0-0 at rates ranging from 75 to 375 kg N ha–1 split across the same five application dates. Plots were harvested on 22 April, 24 May, 21 June, 21 July, 24 August, and 4 October. All other procedures were the same as in 1998.

Experiment 2 (1999-2000) included biosolids from nine sources (Table 1), four inorganic N treatments and a zero-N control. Three of the biosolids came from sources included in Exp. 1 (Baltimore, Everett, and Stayton), while the other six came from different wastewater treatment plants. The biosolids from Baltimore and Stayton were fresh material in 1999, while the Everett biosolids (long-term lagoon material) came from the same stockpile that was used in 1998. The biosolids were applied to freshly harvested plots on 25 and 28 May 1999. The four inorganic N treatments received ammonium nitrate at rates of 50, 100, 150, and 200 kg N ha–1, split into three equal applications. Plots were harvested on 21 June, 21 July, 24 August, and 4 October.

In 2000, the biosolids treatments received 150 kg N ha–1 as 34-0-0 and the inorganic N treatments received 75, 150, 225, or 300 kg N ha–1 as 34-0-0. All applications were split across five dates, the same as for Exp. 1. Harvest dates for 2000 were 12 April, 22 May, 21 June, 20 July, 24 August, and 4 October. All other procedures were the same as for Exp. 1.

Statistical Analyses and Calculations
Statistics for yield and N uptake were computed by ANOVA and Duncan's Multiple Range procedures (SAS Institute, 1996). Means were compared with Duncan's test following a protected (P < 0.05) F test.

Biosolids PAN was estimated by a two-step calculation. First, we used linear regression to calculate the fertilizer efficiency coefficient for the inorganic N treatments:

[1]
where Nrate is the amount of inorganic N (kg ha–1) applied for a given treatment and year, Nup is N uptake (kg ha–1) into the harvested portion of the tall fescue, eff is the fertilizer efficiency coefficient or the fraction of available N that is taken into the harvested portion of the tall fescue, and b is the N uptake intercept or the tall fescue N uptake (kg ha–1) from unamended soil.

Separate fertilizer efficiency regressions were calculated for each experiment each year. Year 2 regressions were calculated with and without the first harvest (April) of the year. Then the equation was rearranged and biosolids PAN calculated. For Year 1,

[2]
where Nupbs is N uptake (kg ha–1) for a given biosolids source, and eff and b are substituted from the fertilizer efficiency regression for the appropriate experiment. For Year 2:

[3]
where b150 is the N uptake at 150 kg ha–1 N from the appropriate fertilizer efficiency regression. This value is used for Year 2 rather than the y-intercept because all of the biosolids plots received 150 kg ha–1 inorganic N in Year 2.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Yield and Nitrogen Uptake
Dry matter yield and N uptake for Year 1 fit within a fairly narrow range for the treatments receiving nonlagoon biosolids (Tables 2 and 3). Applications of lagoon biosolids (Everett in Exp. 1 and 2, California and Iona in Exp. 2) resulted in lower dry matter yields and N uptake. Also, application of heat-dried biosolids from Baltimore resulted in significantly lower yield and N uptake compared with any of the other nonlagoon biosolids in Exp. 2.


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  		Table 2. Total N applied, total annual dry matter yield, and N uptake for first and second years following biosolids application, Exp. 1 (1998-1999).

 

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  		Table 3. Total N applied, total annual dry matter yield, and N uptake for first and second years following biosolids application, Exp. 2 (1999-2000).

 
In Year 2, yield and N uptake also fell into a narrow range for most biosolids, in part because all of the treatments were supplemented with 150 kg ha–1 of inorganic N. Total accumulated yields for Year 2 were greater than for Year 1 because the second-year yields were from the entire harvest season while the first-year experimental harvests did not begin until June.

Tall fescue N uptake for the inorganic N treatments had a linear response both years (Exp. 1 shown in Fig. 1) , validating the use of the linear regression model to calculate fertilizer N efficiency and plant available N. All of the inorganic N uptake data sets fit the linear model well (Table 4).



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  		Fig. 1. Harvested tall fescue N uptake vs. inorganic N rate, Exp. 1 (Years 1 and 2).

 

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  		Table 4. Linear regression equations for best fit of tall fescue N uptake vs. N rate for inorganic N treatments.

 
Plant Available Nitrogen—Year 1
Calculated PAN for Year 1 ranged from 29 to 40% of total biosolids N for the nonlagoon biosolids in Exp. 1, and from 28 to 45% of total biosolids N in Exp. 2 (Table 5). The PAN from lagoon biosolids was much lower, indicating reduced N availability from the more stable lagoon materials. The PAN for the lagoon biosolids ranged from 8 to 25%, with the oldest, most stable material (Everett) having the highest C:N ratio (13:1) and lowest PAN, and the youngest material (California) having the lowest C:N (8:1) and highest PAN. Aside from the lower PAN from the lagoon biosolids, we saw no clear effect of biosolids processing on first-year PAN. Similar yield, N uptake, and first-year PAN among the nonlagoon biosolids are not surprising, given their similar total N concentrations and C:N ratios (Table 1).


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  		Table 5. Biosolids plant available N for Year 1 following biosolids application.

 
To assess differences in the timing of biosolids N availability, we compared tall fescue N uptake by harvest in Year 1. Tall fescue grown with heat-dried biosolids (Baltimore and Milwaukee) generally had a higher proportion of N uptake in the first harvests after biosolids application compared with biosolids that were not heat dried, indicating greater early availability of N from the heat-dried biosolids. The fraction of seasonal N uptake recovered in the first two harvests was 0.65 to 0.67 for the heat-dried materials (Table 5). Most of the air-dried or dewatered materials had N uptake fractions ranging from 0.50 to 0.60 in the first two harvests. Exceptions were Stayton in Exp. 1 and California (short-term lagoon) in Exp. 2, which had N uptake profiles similar to the heat-dried materials.

Others have also reported rapid N availability from heat-dried biosolids in field trials (Goatley et al., 1998; Muchovej and Rechcigl, 1998). In 60-d laboratory incubations, Smith and Durham (2002) assessed the effect of heat drying on biosolids N release. Despite the loss of NH4 from heat drying, they found that heat drying increased the rate of incubation N release in three of five biosolids tested. Mineralizable N increased for all of the biosolids.

Plant Available Nitrogen—Year 2
Biosolids PAN was much less in Year 2 than in Year 1, although it was still substantial for most of the materials (Table 6). PAN from lagoon biosolids were generally lower than for the other materials, ranging from apparent immobilization for Everett to 8% of total N for Iona. The heat-dried materials also had low second-year PAN (5 to 8%). This indicates that after the initial period of rapid N release from heat-dried biosolids, subsequent N release is slow. Most of the dewatered and air-dried nonlagoon biosolids had Year 2 PAN ranging from 10 to 14% of total biosolids N, with the material from Bingen at 17% (Table 6). This second-year PAN is large enough to affect N application decisions for many crops.


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  		Table 6. Biosolids plant available N for Year 2 following biosolids application.

 
Since tall fescue is a cool-season perennial grass with a long growing season, it is able to use N that becomes available late in the fall and early in the spring. Because of potential leaching loss during the winter and early spring, much of this N may not be available to warm season annual crops that are not planted until May or June. To make a more reasonable estimate of the amount of PAN accessible to warm season crops in the second year after biosolids application, we recalculated second-year PAN based on the N uptake from the May through October harvests only. We did not include the April harvest in this calculation because it accounted for N uptake during the previous fall and early spring. Warm season PAN was 5 to 7% of total biosolids N for the dewatered and air-dried biosolids, and only 1.5 to 3% of total biosolids N for the heat-dried biosolids (Table 6). This averages less than half of the full-season PAN for the dewatered and air-dried biosolids, and less than a third of the full-season PAN for the heat-dried biosolids. Our calculations suggest that more than half of the second-year PAN would be subject to winter and spring leaching loss in some cropping systems. Where summer annual crops are grown, timely planting of a fall cover crop could capture some of this late-season biosolids N.

We did not measure NH3 volatilization in this project, nor did we attempt to separate the contributions of organic and NH4–N to PAN. Organic N was the predominant N form in all of the biosolids. All 14 biosolids had <25% of total N in NH4 form, and eight had <10% as NH4. With surface application of dewatered materials, significant NH3 loss is expected. It is interesting to note that biosolids that produced the greatest proportion of early season tall fescue N uptake (Stayton, Milwaukee, Baltimore) had low concentrations of NH4–N (Table 1). These results suggest that NH4 was a minor contributor to PAN in this study.

Summary and Implications for Biosolids Managers
The results of this study showed that first-year PAN was similar across a range of biosolids treatment processes. Lagoon biosolids were the exception. The PAN for Year 1 was 37 ± 5% averaged across all nonlagoon biosolids, which is higher than predicted with the standard USEPA guidelines (USEPA, 1983). The PAN from lagoon biosolids varied from 8 to 25%, depending on the age of the material. In the second year, PAN for heat-dried biosolids was lower than for dewatered and air-dried materials. Total Year 2 PAN for dewatered and air-dried nonlagoon biosolids was 13 ± 2%, while Year 2 PAN for heat-dried biosolids was about half as large. It appears that heat drying increases the short term availability of biosolids N, but decreases long term N availability.

Biosolids producers, users, and regulators can employ this information in developing biosolids application plans for a similar range of materials in similar climates. If biosolids are used in a summer annual cropping system, more than half of the expected second-year PAN may be susceptible to leaching before it is taken up by the crop. Timely planting of an N-scavenging cover crop during the fall after biosolids application could conserve some of this N and reduce the potential for leaching loss.


   ACKNOWLEDGMENTS
 
The Northwest Biosolids Management Association and the Water Environment Research Foundation provided financial support for this project.

Received for publication December 13, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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