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DIVISION S-8-NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Using Growing Degree Days to Predict Nitrogen Availability from Livestock Manures

^a Univ. of Maine, 495 College Ave., Orono, ME 04473-1294 USA
^b USDA-ARS, New England Plant Soil and Water Lab, Univ. of Maine, Orono, ME 04469-5753 USA

tgriffin{at}umext.maine.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Predictive tools are needed to better match N release from manure with crop demand. Growing degree days (GDD) have been successfully used to predict N release from crop residues and other amendments. A 112-d incubation experiment was conducted at 10, 17, and 24°C to evaluate GDD (0°C base temperature) predictions of N transformations from beef (Bos taurus), dairy, poultry (Gallus gallus), and swine (Sus scrofa) manure. Manure was incorporated at rates estimated to provide 150 kg N ha^-1 (or 75 mg N kg^-1 soil). Soil NO₃ and NH₄ concentrations were determined at weekly or biweekly intervals. The rate of NO₃ accumulation increased with increasing temperature, and could be predicted across temperature regimes using GDD. This predictive ability could be generalized across dairy, poultry, and swine manures using an exponential equation, , while N was immobilized by incorporation of beef manure. The disappearance of NH₄ was a linear function of time and of GDD. A single predictive equation was sufficient for dairy, poultry, and swine manures, in the form , with soil NH₄ reaching zero at 350 GDD. These laboratory data indicate that GDD can be used for predicting NO₃ accumulation and NH₄ disappearance from a range of livestock manures. If successfully extended to the field, this predictive capability may allow for improved management of N from animal manures.

Abbreviations: GDD, growing degree days • ICP, inductively coupled plasma emission spectroscopy • PAN, plant-available N

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
IMPROVED ability to predict the availability of N from organic sources, including livestock manures, plant residues, and industrial byproducts, would serve two complimentary purposes. First, at the farm level, supplemental fertilizer N application to crops could be restricted to those situations where a crop yield response is most likely, increasing productivity and avoiding unnecessary expense where a response is unlikely. Second, environmental loss of mineral N as NO₃ could be reduced by both eliminating unneeded application and better matching mineral N supply with crop demand.

Season-long mineralization constants for the organic N component of manure are broadly based on the recalcitrance of the organic N fraction. Mineralization constants for composted manures are commonly 5 to 10% yr^-1 (Hadas and Portnoy, 1994). Conversely, Bitzer and Sims (1988) found that an average of 66% of the organic N in poultry manures was mineralized in the first year. Cabrera et al. (1994) confirmed this rapid mineralization from poultry manures, estimating that 35 to 50% of organic N could be mineralized within 14 d of incorporation into soil. Estimated mineralization constants for dairy and beef manures usually fall between these extremes (e.g., 16–21% by Klausner et al., 1994). These estimated mineralization constants are useful in calculating N-based application rates. However, they do not provide sufficient detail to make supplemental N application decisions during the growing season.

Estimating the amount of mineral N available from manure during the growing season is further complicated by the presence of both mineral N (usually NH₄) and organic N fractions. Ammonium is subject both to rapid nitrification to NO₃ and to volatile loss as NH₃, while organic N requires an initial mineralization step to be utilized by plants. Because both nitrification and mineralization are microbially mediated in soil, they are influenced not only by substrate characteristics, but also by soil climate, including temperature, soil water status, and aeration.

In past research, thermal units or GDD have been used successfully to predict cumulative N mineralization through a growing season for carbonaceous wastes, biosolids, and plant residues of varying composition (Honeycutt et al., 1988, 1991; Honeycutt and Potaro, 1990), recognizing that the thermal unit relationship with mineralization is modified by soil water status (Doel et al., 1990). For livestock manures, a general relationship between N mineralization and temperature was demonstrated by Paul and Beauchamp (1994), and established temperature functions (e.g., Q₁₀ 2) have been used to predict NO₃ accumulation in the field (Cabrera and Kissel, 1988). Sims (1986) and Hadas et al. (1983) conducted laboratory incubations of soil amended with poultry manure at temperatures ranging from 0 to 35°C, finding that more N was mineralized at higher temperatures for a defined time period. Sims (1986) also clearly demonstrated that the rate and extent of manure N mineralized from the organic fraction varied among manures, even from the same animal species. While the general effect of increased temperature has been demonstrated, the predictive ability of GDD has not been evaluated for different livestock manures. This study was conducted to evaluate (i) the impact of temperature on N transformations for beef, dairy, poultry, and swine manures and (ii) the utility of GDD for predicting cumulative NO₃ availability, disappearance of manure NH₄, and mineralization of manure organic N.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soil and Manure Materials
Soil used for the laboratory incubation was collected from the USDA-ARS research site in Newport ME, in November 1998. The sandy loam soil (coarse-loamy, mixed, frigid Typic Haplorthod) has a particle-size distribution of 610, 290, and 100 g kg^-1 for sand, silt, and clay fractions, respectively, as measured by the pipette method (Gee and Bauder, 1986). Soil was collected from the Ap horizon (0–20 cm) in a field previously cropped to potato (Solanum tuberosum L.), sieved (2 mm), and stored at 4°C until the incubation was initiated. Field moisture level (0.19 kg kg^-1, determined gravimetrically) was maintained through the storage period. Selected soil properties include: soil (1:1, soil/water); cation-exchange capacity = 3.4 cmol kg^-1; P = 16.5 kg ha^-1; K = 303 kg ha^-1; Mg = 169 kg ha^-1; and Ca = 1130 kg ha^-1, as determined using a modified Morgan extraction (pH 4.8, 0.62 M NH₄OH + 1.25 M CH₃COOH) and inductively coupled plasma emission spectroscopy (ICP). Beef, dairy, swine, and poultry manures were collected from local commercial farms, transported in 20-L plastic buckets, and stored at 4°C until incubation was initiated. Manure analyses at the beginning of incubation are shown in Table 1 . Total C was determined by thermal conductivity detection following combustion at 1650°C on a CE Instruments NA2500 Elemental Analyzer (ThermaQuest Italia S.p.A., Rodano, Italy). Total N was measured by total Kjeldahl digestion (Wall and Gehrke, 1975). Manure P and K contents were measured by dry combustion, followed by digestion in HCl and ICP.

(1)

where f is the proportion of organic N fraction expected to mineralize within the first growing season. Coefficient f equals 0.25, 0.35, 0.60, and 0.50 for beef, dairy, poultry, and swine manures, respectively, from Klausner (1997) and Bitzer and Sims (1988). Soil water in all jars was maintained at 0.19 kg kg^-1 (80% of field capacity) by adding deionized water twice weekly for the first 4 wk, and once weekly thereafter. Jars were placed in incubators (10, 17, and 24°C) corresponding with their preincubation temperature and loosely capped with metal canning lids. To provide adequate aeration, lids were removed for 1 h daily during the first 4 wk and every 2 to 3 d thereafter.

Five-gram subsamples were removed from each jar at 0, 7, 14, 21, 28, 42, 56, 70, 84, and 112 d. Inorganic N (N_i: NO₃–N and NH₄–N) was determined colorimetrically on a Lachat Autoanalyzer (Lachat Instruments, Mequon, WI) following extraction of 3.5 g soil in 35 mL of 2 M KCl for 1 h on an orbital shaker. Gravimetric soil water was determined on each sampling date by drying 1.5 g of soil at 105°C for 24 h.

Cumulative nitrification (N_cum) for manure-amended soil, measured as soil NO₃–N concentration at time, t, was similar to Sims (1986) in correcting for unamended soil and initial soil NO₃

(2)

Soil concentration of manure-derived NH₄ at time, t, was defined similarly as

(3)

The amount of N mineralized from the manure organic N fraction at time, t, was calculated in several ways, taking the general form

(4)

with (NH₄)_t=0 being either manure NH₄ or soil NH₄ concentration and N_i equal to the sum of NO₃–N and NH₄–N. This calculation accounts for both indigenous soil N mineralized and manure NH₄ nitrified, both of which subsequently enter the cumulative NO₃ pool, N_cum.

Cumulative nitrification and organic N mineralization were fit to single exponential equations

(5)

(6)

as used by Deans et al. (1986). These equations were used to estimate nitrified or mineralizable N (N₀) and the associated rate constant, k, with time (Eq. [5]) and GDD (Eq. [6]), respectively. The disappearance of manure NH₄ was fit to linear functions

(7)

(8)

All equations were fit using all data points, although only mean values (of three observations each) are shown in the figures below. Equations were calculated using linear regression or nonlinear curve fitting via a Marquardt iteration (SYSTAT, Version 8.0; SPSS Corp., Chicago, IL). Regression equations were deemed significantly different if the 95% confidence intervals around the parameters (the rate constant, k, for example) did not overlap.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Net Nitrification
Manure-derived NO₃ accumulation (N_cum) was estimated by subtracting NO₃ concentration of unamended soil from NO₃ concentration of amended soil. This assumed that the manure amendment did not stimulate mineralization of indigenous soil organic N. It further recognized that the nitrified N could originate from two sources, manure NH₄ and manure organic N. Using dairy manure as an example, soil NO₃–N concentration reached levels of 50 to 70 mg kg^-1 during the 112-d incubation and was clearly influenced by incubation temperature (Fig. 1A) . Results were similar for soil amended with either poultry or swine manure (data not shown). The potential amount of N nitrified, defined as N₀ in the first-order exponential model (Eq. [5]), was not significantly different between temperature regimes, averaging 56.7 mg kg^-1 soil amended with dairy manure. However, the rate constant, k, was significantly higher at 24°C than at 10°C , indicating a faster rate of N accumulation with increasing temperature. This more rapid accumulation of NO₃ as temperature increases has been demonstrated by Honeycutt et al. (1991, 1993) for added plant residues, fertilizer, and papermill sludge. Organic N from different soils or from soil with different cropping histories also mineralized more rapidly as temperature increased (Stanford et al., 1973; Campbell et al., 1981; Addiscott, 1983; Gale and Gilmour, 1986). Sims (1986) measured net N mineralization from poultry manures from 0 to 40°C, and found both higher inorganic N accumulation and faster mineralization rates as temperature increased.

View larger version (23K): [in this window] [in a new window]   Fig. 1 Effect of temperature on NO3 accumulation in soil amended with dairy manure as a single exponential function of (A) time and (B) growing degree days (GDD). Data points are mean of three observations; t is time in days after incorporation, and s.e.k is the standard error of regression parameter k

To be useful in assessing N availability during the growing season, the GDD relationship would ideally be generalizable across manure types. In this incubation study, GDD could successfully predict N_cum for dairy, poultry, and swine manures using a single exponential regression, , as shown in Fig. 2 . The R² value for this regression, although not completely analogous to R² in linear models, was 0.75 when the equation was fit using all data points, and 0.85 when only mean values were used. The potential N nitrified (54.10 mg kg^-1) was only 75% of the 75 mg kg^-1 estimated by the PAN approach (the equivalent of 150 kg PAN ha^-1). This is very similar to the overestimation by the PAN approach (Eq. [1]) reported by Bitzer and Sims (1988).

View larger version (23K): [in this window] [in a new window]   Fig. 2 Nitrate accumulation in soil amended with beef, dairy, poultry, and swine manure as a function of growing degree days (GDD). Data points are mean of three observations; t is time in GDD after incorporation, and s.e.k is the standard error of regression parameter k

Ammonium Disappearance
As mentioned above, nitrification processes in manure-amended soil act on two primary pools of NH₄, namely (i) NH₄ originally present in the manure and (ii) NH₄ arising from mineralization of manure organic N. Because some manures contain high concentrations of NH₄ (i.e., 50–80% of total N), the ability to predict the disappearance of NH₄ after application would be valuable. For the amendments used here, we recognize that NH₄ disappearance might have occurred via nitrification or microbial immobilization. However, we have no clear method of discerning the amount of N transformed via each pathway, so instead we evaluated net NH₄ disappearance. It is also clear that N transformations were occurring even during the short period between manure incorporation and the subsequent sampling and extraction at ( 2 h later). This is most easily demonstrated by soils amended with beef manure, where NH₄–N concentration averaged 36.7 mg kg^-1 soil at the t = 0 extraction, even though this amendment contained no appreciable inorganic N (Table 1).

Dairy manure-amended soil is again used to illustrate the impact of temperature on NH₄ disappearance, and the effectiveness of GDD in predicting NH₄ disappearance. The disappearance of NH₄ within each temperature regime could be described using simple linear functions (Fig. 3A) . The decline in NH₄ through the first 28 d of the incubation was very similar to that shown by Schmidt et al. (1992), where soil NH₄ from injected dairy manure fell to zero even at high application rates. Both Addiscott (1983) and Zanner and Bloom (1995), in evaluating nitrification of NH₄-containing fertilizers in soils of varying cropping history, also noted linear reductions in NH₄ with time, with more rapid reductions at higher temperatures. Research by Grundmann et al. (1995) suggests that the nitrification rate would be maximized under conditions similar to those we used at 24°C. The differences in NH₄ at t = 0 due to incubation temperature are also clear in Fig. 3A. This may reflect very rapid microbial immobilization. It could also indicate very rapid nitrification; however, if this is the case then the lack of a corresponding increase in NO₃ at t = 0 with increasing temperature would suggest that this NO₃ must also be rapidly denitrified and lost.

View larger version (20K): [in this window] [in a new window]   Fig. 3 Effect of temperature on disappearance of NH4 in soil amended with dairy manure as a linear function of (A) time and (B) growing degree days (GDD). Data points are mean of three observations

View larger version (18K): [in this window] [in a new window]   Fig. 4 Ammonium disappearance in soil amended with dairy, poultry, and swine manure as a function of growing degree days (GDD). Poultry and swine treatments are represented by combined regression. Data points are mean of three observations

View larger version (18K): [in this window] [in a new window]   Fig. 5 Disappearance of manure NH4, as a percentage of manure NH4 input, from soil amended with dairy, poultry, and swine manure, as a function of growing degree days (GDD). Data points are mean of three observations

Manure Organic Nitrogen Mineralization
Unlike NO₃ accumulation and NH₄ disappearance, which were measured directly in this incubation, estimation of net mineralization of manure organic N (N_org) is a calculated value. We evaluated several approaches for calculating N_org. Two approaches take the form of Eq. [4]. The first, as used by Gordillo and Cabrera (1997) and Sims and Wolf (1994), subtracted the initial NH₄ content of the manure from N_i, under the assumption that all of the NH₄ in the manure is eventually nitrified and enters the NO₃ pool. Because the amount of manure-derived NH₄ extracted from the soil at was always less than 100% (averaging 70% for dairy, poultry, and swine manures; Fig. 5), this approach can consistently yield negative values for N_org, especially during the early phase of the incubation. The second approach, taken from Sims (1986), subtracts the soil NH₄ at from N_i to account for manure-derived NH₄ recovered in the initial extraction. While probably more realistic than the first approach, there is a problem in that NH₄ at was clearly influenced by preincubation temperature, as shown in Fig 3A for dairy manure. It is conceivable that higher preincubation temperatures resulted in greater population or activity of immobilizing microbes, which responded rapidly to manure addition.

The third approach tries to account for the fact that NH₄ concentration changes with time, until reaching zero (usually within 28 d). Weekly changes in soil NO₃ (NO₃) and NH₄ (NH₄) were calculated for . If NO₃ > NH₄, then (presumably) the difference for that time interval can be attributed to N mineralized from manure organic N by the equation

(7)

As the incubation proceeds, N_org for a specific time interval is essentially equal to NO₃ because NH₄ concentration becomes very small or zero.

Net mineralization could not be predicted from GDD using either approach based on Eq. [4]. An example of the lack of relationship (using data for poultry manure, corrected for soil NH₄–N at t = 0, as in Sims, 1986) is shown in Fig. 6 . Two things are apparent in this example. First, N_org stabilizes at around 28 mg N kg^-1 soil (equal to 48% of manure organic N input) as the incubation proceeds. And second, the distribution of N_org during the initial phase of the incubation (0–1000 GDD) is essentially random. This may be compounded by the fact that soil N_i follows a quadratic trend (increasing, then decreasing) during this initial phase, as shown in Fig. 7 , when NO₃ is accumulating rapidly but some manure NH₄ is still present. As Eq. [4] subtracts a constant value from N_i, this would mean that cumulative N_org increases quickly, but then declines as the incubation proceeds. Although this is possible, nearly all similar research in this area indicates that mineralization follows an increasing (e.g., exponential) function with time or GDD.

View larger version (15K): [in this window] [in a new window]   Fig. 6 Organic N mineralized from swine manure at different temperatures, as a function of growing degree days. Data points are mean of three observations

View larger version (18K): [in this window] [in a new window]   Fig. 7 Changes in inorganic N content (NO3 + NH4) in soil amended with swine manure between 0 and 600 growing degree days (GDD). Data points are mean of three observations

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The accumulation of NO₃ and disappearance of NH₄ from livestock manures was measured in a 112-d laboratory incubation at 10, 17, and 24°C. We were able to account for differences in incubation temperature and predict N_cum and N_amm through this period using GDD in single exponential and linear models, respectively. We were further able to predict these N transformations across animal manures that had widely different initial NH₄–N and organic N concentrations and application rates, if the manure contained appreciable NH₄. This predictive ability, which has now been demonstrated for numerous soil amendments and crop residues, is useful not only for estimating N availability from manures during the season, but also in improving the synchrony between N supply and crop N demand.Grundman Renault Rosso Bardin 1995

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Joint Contribution from University of Maine and USDA-ARS. Maine Agric. and Forest Experiment Station Journal no. 2394. Partial funding from USDA-SARE, Project ANE94-20.

Received for publication October 18, 1998.

	REFERENCES

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