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Published online 13 May 2005
Published in Agron J 97:879-885 (2005)
DOI: 10.2134/agronj2004.0249
© 2005 American Society of Agronomy
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Manure Management

Extractable Soil Phosphorus and Inorganic Nitrogen following Application of Raw and Anaerobically Digested Swine Manure

Esteban R. Loriaa and John E. Sawyerb,*

a Ministry of Agric. (INTA), Dep. of Soil and Land Evaluation, Centro Colon, San Jose, Costa Rica (formerly Dep. of Agron., Iowa State Univ., Ames, IA)
b Dep. of Agron., Iowa State Univ., Ames, IA 50011-1010

* Corresponding author (jsawyer{at}iastate.edu)

Received for publication October 1, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Processing of swine (Sus scrofa domestica) manure in an anaerobic digester for biogas production is not a complete waste treatment process. Therefore, digested manure must be utilized in some manner, most likely as a source of plant nutrients. The objective of this study was to compare the effect of raw and digested liquid swine manure application on soil test P (STP) and inorganic N. A laboratory incubation study was conducted for 112 d, with a factorial combination of raw manure, digested manure, and inorganic fertilizer at five nutrient rates (0, 12.5, 25, 37.5, and 50 mg total P kg–1 and 0, 50, 100, 150, and 200 mg total N kg–1). Raw and digested swine manure produced the same NH4–N disappearance, NO3–N formation, net inorganic N, and increase in STP. Routine STP methods estimated similar P recovery with both manure sources, averaging 21% at the end of incubation. For the first 28 d of incubation, the STP levels were higher for fertilizer than manure; STP levels were similar for all P sources after 28 d. Nitrification of manure NH4 was rapid, reaching background concentrations by 14 d, with conversion rate similar to fertilizer NH4–N. By the end of incubation, maximum net extractable inorganic N, predominantly NO3–N, averaged 20% less than total applied N for both raw and digested manure. Anaerobic digestion did not substantially affect manure nutrient supply, and we conclude that anaerobically digested liquid swine manure can provide similar plant-available N and P as expected from raw swine manure.

Abbreviations: COD, chemical oxygen demand • STP, soil test phosphorus • TKN, total Kjeldahl nitrogen


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SWINE MANURE is an important source of N, P, and other nutrients for crop production. Processing of manure in an anaerobic digester is only a partial manure degradation process (Sweeten et al., 1990). During anaerobic digestion, chemical oxygen demand (COD) is reduced, but many organic compounds remain or are reduced to a lower molecular weight. Inorganic nutrients remain after digestion, with some, like NH4, increasing. Also, the volume remains constant (Kirchmann and Witter, 1989). Therefore, material remaining after digestion must be utilized in some manner, most logically land-applied as a crop nutrient resource.

Anaerobic digestion of animal manure has been extensively investigated during the last two decades (Williams, 1995). During anaerobic digestion, bacteria break down organic matter in an oxygen-free environment, resulting in fewer organic nutrient forms and more inorganic forms. Kirchmann and Lundvall (1993) reported increases of inorganic N (increase less than 10%); however, limited information is available regarding the effect of anaerobic digestion on nutrient content and plant nutrient availability.

Due to a potential increase in the use of anaerobic digestion systems for energy production, there is need for a reliable estimate of plant-available N and P in digested swine manure. Evaluation of digested manure as a crop nutrient source requires determination of the effects of digestion on nutrient content and the ability to provide plant-available nutrients when the material is applied to land.

Nutrient release from manure at a time when crops are not actively assimilating nutrients can cause low nutrient use and poor crop response. Binder et al. (1996) state the importance of synchronizing manure nutrient mineralization with crop use. Also, environmental loss of nutrients can occur when supply exceeds crop demand. One problem in manure management is the uncertainty of organic matter mineralization rate. Specific animal digestion processes (monogastric or ruminant), feed preferences, and rations of different species and overall handling of the manure are responsible for differences in manure nutrient concentrations and effects on availability to plants (Bailey and Buckley, 2001).

To evaluate anaerobically digested swine manure as a plant-available source of N and P, it is important to know how soil application affects STP and influences soil inorganic N levels and transformations, and how it compares with raw manure and inorganic fertilizer nutrient sources. Incubation studies provide information that can reflect field conditions, clarify soil mechanisms and nutrient transformations, and estimate nutrient concentrations in soil after application (Rogers et al., 2001). Our objective was to compare the effect of anaerobically digested liquid swine manure and raw liquid swine manure on extractable soil P and soil inorganic N.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil
A bulk amount (0- to 15-cm depth) of Webster silty clay loam soil (fine-loamy, mixed, noncalcareous, mesic Typic Haplaquoll) was collected in November from a field that had been in a corn (Zea mays L.)–soybean [Glycine max (L.) Merrill] rotation. After collection, the soil was partially dried for 1 d at room temperature (approximately 22°C), sieved (5 mm), and stored moist at 2°C until the beginning of the incubation period. Chemical characteristics of the soil before incubation are presented in Table 1.


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  		Table 1. Selected chemical characteristics of the soil before incubation.

 
Source of Nutrients
Three nutrient sources were used: raw swine manure, anaerobically digested swine manure, and inorganic soluble fertilizer N and P. The inorganic fertilizer N and P was a solution made of urea (44%), ammonium sulfate (44%), and ammonium phosphate (12%) containing 1000 mg P L–1 and 4000 mg N L–1. The raw and digested swine manures were collected from a commercial 5000 sow-gestation-farrowing production facility in southern Iowa. The manures were collected immediately before and after anaerobic digestion (15 d in a mesophilic anaerobic digester) and then stored in a plastic container at –5°C until application.

After thawing and thorough mixing, subsamples of each manure were chemically analyzed at the Iowa State University Analytical Services Laboratory for total Kjeldahl N (TKN), NH4–N (measured as NH3–N), total K, total P, total solids, volatile solids, COD, and pH (Am. Public Health Assoc., 1995). Chemical characteristics are listed in Table 2.


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  		Table 2. Selected chemical analyses of the swine manure sources. Values are means of four samples.

 
Five rates of total N and P from each nutrient source were applied to the soil. Rates were 0, 12.5, 25, 37.5, and 50 mg kg–1 for P and 0, 50, 100, 150, and 200 mg kg–1 for N. Phosphorus application rates were matched for each source. Due to differences in manure total N/P ratios, there were slight differences in the applied N amount between the manure sources. The treatments were a factorial combination of nutrient sources and rates, replicated three times in a completely randomized design.

Incubation
Treatments were applied to the soil using the following procedure. Moist soil (1 kg, oven-dry basis) was spread on top of a brown paper sheet (40 by 40 cm) in a thin layer. Each treatment was prepared by adding to the fertilizer or manure solution a calculated amount of distilled water that would bring the soil to approximately 80% water-holding capacity. The solution (treatment plus distilled water) was uniformly spread on top of the soil layer and then mixed with the soil by moving up and down the corners of the paper sheet. The treated soil was transferred to a polyethylene bag, mixed again by shaking the bag for about 1 min, and incubated at approximately 22°C on a laboratory bench. The polyethylene bag was left open (50%) to maintain air exchange. Soil moisture was maintained at 80% water-holding capacity by checking the weight each week and adding distilled water as needed.

Two soil subsamples, each 50 g, were collected from each bag by scooping out soil at 1, 7, 14, 28, 56, 84, and 112 d after treatment application. One sample was designated for inorganic N (NH4–N and NO2 + NO3–N) analysis and the other for routine soil P tests and pH. Hereafter, the NO2 + NO3–N concentration is referred to as NO3–N. The subsamples for N analysis were stored in polyethylene bags and frozen at –5°C. The other subsamples were air-dried and stored in paper bags at room temperature until analysis.

Soil Analyses
Extractable P was determined with the Olsen P, Bray-1 P, and Mehlich-3 P availability indices (Frank et al., 1998). Changes in extractable P concentrations were estimated by subtracting the STP measured in the control soil at each sampling date. For 7 and 112 d after application, the percentage of added P reflected in the STP increase was calculated as [100 x (STP treatment – STP control)/(P applied)], where STP treatment, STP control, and P applied are in mg kg–1.

Inorganic N was determined by extracting 10 g of soil (moisture adjusted) with 100 mL of 2 M KCl. The NH4–N and NO3–N concentration in the extract was determined by automated colorimetric flow-injection analysis (Lachat Instruments, Milwaukee, WI) (Mulvaney, 1996). Extractable NH4–N and NO3–N in treated soil were corrected by subtracting the concentrations measured in each control soil at each sampling date. Soil pH was determined on a 1:1 water soil paste using an electronic pH meter (Watson and Brown, 1998).

Data were analyzed by using the PROC MIXED procedure of SAS (SAS Inst., 2001). Repeated measure analysis was utilized to take into account sampling the same experimental unit in the comparison of treatment effects (Littell et al., 1998). Significant differences between treatments at a sampling date were determined using Tukey–Kramer least square means test.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nitrogen
The source x rate x time of incubation interaction was significant for extractable soil NH4–N and NO3–N concentrations (P ≤ 0.05). Soil NH4–N concentration was initially high with all N sources (measured at Day 1) and decreased rapidly for the first 14 to 28 d of incubation (Fig. 1) . During this period, NH4–N concentration was significantly higher with the fertilizer than both manure sources at rates of 150 and 200 mg N kg–1 but the same for all sources at the lower application rates. Raw and digested manure resulted in statistically the same NH4–N concentrations at each application rate and all sample dates. After 14 to 28 d of incubation, extractable soil NH4–N concentration returned to the background level and was the same for all N sources.



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  		Fig. 1. Extractable NH4–N and NO3–N for each nutrient source and application rate. At each sampling date, the NH4–N or NO3–N concentration in the control soil was subtracted from each sample. Means were determined to be significantly different (P ≤ 0.05) using Tukey–Kramer least square means test between N sources at each sampling date and are indicated by: z, fertilizer and digested manure; x, fertilizer and raw manure; and y, digested and raw manure.

 
Contrasting to NH4–N, extractable soil NO3–N concentration was initially low with all N sources but increased rapidly for the first 14 d of incubation for the 50 and 100 mg N kg–1 rates and for the first 28 d for the 150 and 200 mg N kg–1 rates (Fig. 1). The net increase in extracted NO3–N closely matched the net decrease in NH4–N. After 7 to 28 d of incubation, and depending on N rate, NO3–N concentration became significantly greater with fertilizer than both manure sources. After 28 d, NO3–N concentration remained relatively constant with the fertilizer but slowly increased with raw and digested manure until the end of incubation. Despite the continued increase in NO3–N with each manure source, NO3–N concentration was still significantly higher with fertilizer than manure. Maximum net extractable soil NO3–N measured with the fertilizer accounted for nearly 100% of applied N but only 80% of total applied N from the manure sources.

Soil pH
The source x rate x time interaction was significant for soil pH (P ≤ 0.05). Initially after application, soil pH was increased with application of raw and digested manure (Fig. 2) but not with fertilizer. Soil pH decreased rapidly after Day 1 for the first 14 to 28 d of incubation, depending on application rate. The soil pH decrease was the same for both raw and digested manure, but the decrease was not as large as with the fertilizer (Fig. 2). After 28 d, soil pH decreased slowly with each manure source, was relatively constant with the fertilizer source, and remained lower with fertilizer compared with manure.



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  		Fig. 2. Effect of nutrient source and application rate on soil pH. Means were determined to be significantly different (P ≤ 0.05) using Tukey–Kramer least square means test between nutrient sources at each sampling date and are indicated by: z, fertilizer and digested manure; x, fertilizer and raw manure; and y, digested and raw manure.

 
Phosphorus
For all soil P tests, the increase in extractable P was affected by the source and rate of P applied (P ≤ 0.05) (Fig. 3) . Also, the interaction with sample date was significant for the Olsen P and Mehlich-3 P tests (P ≤ 0.05) but not for Bray-1 P (P = 0.064).



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  		Fig. 3. Change in extractable P for each nutrient source and application rate. At each sampling date, the soil test P (STP) concentration in the control soil was subtracted from each sample to calculate the change in extractable P. A1, A2, A3, A4; B1, B2, B3, B4; and C1, C2, C3, C4 represent Olsen P, Bray-1 P, and Mehlich-3 P at 12.5, 25, 37.5, and 50 mg added total P kg–1, respectively. Means were determined to be significantly different (P ≤ 0.05) using Tukey–Kramer least square means test between P sources at each sampling date and are indicated by: z, fertilizer and digested manure; x, fertilizer and raw manure; and y, digested and raw manure.

 
The increase in STP was similar and remained fairly constant for both manure sources over the incubation period (Fig. 3). However, with inorganic fertilizer P application, there was an initial maximum STP increase in the first 14 to 28 d of incubation, and that STP increase was significantly larger than with manure application. After 28 d, STP declined with fertilizer P and was generally not significantly different from the STP increase associated with both manure sources.

Application rate did not affect P recovery from the three P sources (measured as percentage increase in extractable P). However, the source x sampling date x STP method interaction was significant (P ≤ 0.05). This is evident with greater recovery for the inorganic fertilizer with all three tests even though the percentage increase in the Olsen P test was lower than the other two soil P tests (Table 3). Also, the mean increase was the same for both manure sources, except for the Mehlich-3 test at Day 7 where the recovery was greater with digested manure.


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  		Table 3. Increase in extractable P for each soil test P (STP) method at 7 and 112 d, mean of all P application rates.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
No differences were observed between raw and anaerobically digested swine manure in regard to initial NH4–N supply, disappearance of NH4–N, NO3–N formation rate, pH decrease, or increase in STP. However differences between inorganic fertilizer and manure were found in all cases.

At 1 d after application, extractable NH4–N concentration was increased, but NO3–N concentration was not different than the control for all treatments. This indicates that the dominant inorganic N form applied for all sources was NH4, with no NO3 present in either manure source. Kirchmann and Lundvall (1993), working with raw and digested swine manure, observed no NO3–N and 75 to 85% of N as NH4–N, respectively. This is similar to our findings. The rapid decrease in extractable NH4–N and concurrent increase in NO3–N is a result of nitrification. The inorganic N transformation and time period for conversion are similar to those with liquid beef manure found by Schmitt et al. (1992) when incubated in a manure–soil mix and by Sawyer and Hoeft (1990a)(1990b) with simulated band injection. These results are also consistent with studies of N mineralization and transformation from sources with varying organic content (Chae and Tabatabai, 1986; Kirchmann and Lundvall, 1993; Duffera et al., 1999).

The maximum net extractable soil NO3–N with both raw and digested manure at all N application rates was only 80% of applied total N. However, the maximum net extractable soil NO3–N with fertilizer accounted for nearly 100% of applied N. This reflects the lower proportion of total N as NH4–N initially present in the manure sources (Table 2) and explains why the fertilizer responded with a greater immediate supply of plant-available N (Fig. 1). Despite high NH4 in both manures, a pool of organic N continued mineralizing during incubation, releasing NH4 that also quickly nitrified. Thus, extractable soil NO3–N continued to increase whereas NH4–N did not. However, in the 112 d of incubation, there was not an equivalent release of total N as inorganic N from the manure sources, as occurred for the inorganic fertilizer. Thus, some organic N apparently was not mineralized in 112 d. Other possible reasons for lower inorganic N accumulation include manure N volatilization during application and soil mixing, inorganic N assimilation into microbial biomass, or denitrification losses during incubation (Bernal and Kirchmann, 1992; Calderón et al., 2004; Chantigny et al., 2004). Change in extractable soil inorganic N in the first 7 d was the same for both manure sources and the control (+3 mg N kg–1 with manure and +4 mg N kg–1 for the control) and for fertilizer and manure after 112 d (+58 mg N kg–1 for manure and +61 mg N kg–1 for fertilizer). After 112 d of incubation, net inorganic N increase was 19 mg N kg–1 greater with manure and fertilizer N application than for the control. This indicates net mineralization with all N sources. Despite much lower COD after digestion (Table 2), there was no difference in inorganic N production or net mineralization between raw and digested manure.

The pH of soil treated with both manure sources showed an initial increase followed by a rapid decrease for the first 28 d of incubation (Fig. 2). Initial increase in soil pH has been reported with application of anaerobically digested and stored liquid swine manure, with increase due to manure alkalinity and carbonates (Chantigny et al., 2004). Higher pH would increase NH3–N and potential for surface volatilization. Soil pH decline paralleled the nitrification process (Fig. 1), during which an acidifying effect due to proton formation occurs (Bernal and Kirchmann, 1992). Soil pH was low enough at the highest application rates to potentially slow nitrification. The smaller decrease in pH found for soil treated with raw and digested manure (Fig. 2) could be caused by buffering from manure organic and inorganic components (Fordham and Schwertmann, 1977; Sommer and Husted, 1995), lower nitrification rate, mineralization of manure organic N, or use of ammonium sulfate in the fertilizer.

Swine manure contains a variety of inorganic and organic P compounds, with reported organic P fractions of 10 to 75% of total P (Gerritse and Zugec, 1977; Reddy et al., 1980; Sharpley and Moyer, 2000; He and Honeycutt, 2001). Organic P forms are also mineralized at different rates. Anaerobic digestion of the swine manure used in our study apparently did not alter the manure to an extent that affected P available to increase STP. Figure 3 shows only a few exceptions (A1, A2, A3, and C2) where differences in STP were evident between the two manure sources. Compared with inorganic fertilizer P application (where 100% of the P was orthophosphate), smaller increases in STP were measured within 28 d of application for both manures. A similar differential in initial STP increase (with greater increase in Mehlich-3 STP from inorganic P compared with manure P) was found by Griffin et al. (2003) with swine manure applied to a sandy loam soil.

Mineralization and adsorption govern net P trends and soil test increases when soils are amended with P (Taylor et al., 1978). Over time, manure P will undergo both processes whereas inorganic fertilizer (orthophosphates) would be influenced primarily by adsorption. The initial increase in STP was greater with the fertilizer P addition than with either manure. However, after 28 d, the extractable levels were similar for all sources. When inorganic P is added, all of the P is immediately available for soil chemical reactions or for soil test extraction; therefore, the speed of soil interactions will govern the STP measured. Manure has a mix of inorganic P and organic P forms (Poulsen, 2000; He and Honeycutt, 2001); therefore, P available to immediately influence STP is different than inorganic sources, and the P available to interact with the soil is also different than inorganic sources. From this, the initial increase in STP should be smaller with manure, as was measured in our study. However, the slower supply from the manure organic P pool may help to maintain a more constant level of plant-available P or STP, as compared with the inorganic fertilizer P, as also measured in the present study. Such a sustained contribution to STP over time was also reported for swine manure application in incubation studies by Griffin et al. (2003) and Laboski and Lamb (2003). In the Laboski and Lamb (2003) study, swine-finishing manure applied at rates simulating an injected band (144 and 288 mg P kg–1) resulted in sustained STP levels from 1 to 9 mo, contrasted to a decrease with fertilizer P.

As the rate of application increased, extractable P also increased among all sources (Fig. 3). This is consistent with work by Reddy et al. (1980) and Laboski and Lamb (2003) where increase in manure application rate increased plant-available P. However, in terms of percentage increase in extractable P, no consistent differences between application rates were found. The three soil P tests predicted similar estimates of the increase in STP with manure amendment (Table 3). Because of the greater inorganic P content, fertilizer application showed a greater increase in STP, similar to results of Baxter et al. (1998). After 112 d, the percentage of applied P measured in STP increase averaged 30 to 40% for the inorganic fertilizer and 15 to 30% for both manure sources. These are more similar than expected considering the differing P sources.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Both raw and anaerobically digested swine manure provided appreciable plant-available N and STP increase relative to inorganic fertilizer. Furthermore, anaerobic digestion did not affect manure N and P supply. On the short term after application, less than 28 d, plant P availability (as indicated by increase in STP) was equivalent for raw and digested manure but less than soluble inorganic fertilizer P. After approximately 28 d, similar plant-available P was measured with each manure source and fertilizer. Raw and digested swine manure provided the same and large amounts of inorganic N but, within 112 d of application, 20% less than total manure N and fertilizer N. Nitrification of raw and digested manure NH4 was rapid and paralleled nitrification of fertilizer NH4. This implies management for field application of each manure should minimize potential for losses of NO3 produced, as is done for fertilizer-based NH4–N. We conclude that digested swine manure is a valuable nutrient resource that producers can use for crop production and should be managed in the same way as for raw swine manure. Field studies with both manure sources would provide further confirmation for this suggestion.


   ACKNOWLEDGMENTS
 
Partial funding for this work was provided by the USDA-NRCS through cooperative agreement 6861149217.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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