Bioresource Technology 44 ( 1993) 165-173

NITROGEN RECOVERY FROM BROILER LITTER IN A WHEAT-MILLET PRODUCTION SYSTEM

R. P. Flynn, C. W. Wood & J. T. Touchton

Department of Agronomy and Soils, 202 Funchess Hall, Auburn University, Auburn, Alabama 36849-5412, USA

(Received 12 June 1992; revised version received 27 October 1992; accepted 28 October 1992)

Abstract Nitrogen (N) released through mineralization from broiler fitter can supply the N requirements for crops, but litter may cause yield reductions and loss of fertifizer value if applied in excess of crop needs. Afield study was conducted over two years at Crossville, AL, to determine the fate of N in a winter-wheat (Triticum aestivum L.) -- pearl-millet (Pennisetum americanum L.) crop rotation. Soil and plant N contents were determined after fall application of O, 9, or 18 Mg ha i fitter supplemented with 0 or 34 kg ha- i fall-appfied N and O, 22, 44, or 66 kg ha- 1 of spring-applied N fertilizer. Millet followed wheat harvest with no additional N added to the soil. Broiler fitter applied at 9 Mg ha i eliminated the need for fall- appfied N fertilizer and reduced the need for spring- applied N to 22 kg ha-1. The 18-Mg ha-i fitter rate reduced grain yield, and the reduction increased as spring-applied N rates increased. In the first year, N removed in wheat grain plus millet tissue averaged 44 and 16% of N from 9 and 18 mg ha i litter, respectively. In the second year, N removal from the litter treatments was 17 and 14% of N contained in 9 and 18 Mg ha- 1 litter, respectively. The results of this study suggest that 9 Mg ha 1 of broiler litter with approximately 22 kg ha-1 of supplemental spring-applied N may be optimum for winter-wheat production. A summer annual such as pearl millet following winter wheat can utilize residual N and decrease the potential for N loss via leaching.

Key words: Winter wheat, pearl millet, nitrogen recovery efficiency, broiler litter.

INTRODUCTION

Land-applied broiler litter can supply nutrients neces- sary for crop growth, the most prevalent being nitrogen (N) (Bitzer & Sims, 1988; Sims, 1987). However, N in broiler litter is present in both inorganic and organic forms that are subject to volatilization, denitrification, immobilization, mineralization, leaching, and plant uptake (CasteUanos & Pratt, 1981; Liebhardt et al.,

Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain

1979; Sims, 1987). The extent to which these pro- cesses occur determines the potential that broiler litter has as an environmental contaminant. The preferred process to avoid contamination and garner some eco- nomic benefit is plant uptake. Unfortunately, broiler litter can be easily applied in excess of crop needs resulting in injury, reduced yields, and increased non- point pollution hazards (Sims, 1987; Weil et al., 1979).

Nitrogen will not become a pollutant if plants can convert N to protein soon after it is released from broiler litter. This requires an application rate that will supply adequate N for early plant growth and release sufficient N from the organic fraction for optimum crop performance throughout the growing season. In some situations, broiler litter can eliminate the need for N fertilizers (Ketcheson & Beauchamp, 1978; Sims, 1987), whereas, in other situations, supplemental N may be needed for optimum yields (Oyer et al., 1987). Mineralization of N from the organic fraction of broiler litter, however, can extend beyond the period in which crops can actively remove N. Castellanos and Pratt (1981) estimated that about 60% of the organic N in broiler litter was available over a 300-day greenhouse study. Sims (1987) suggests that corn (Zea mays L.) grain and stover will remove roughly 16% of the N per year from the slowly mineralized fraction of broiler litter, which leaves a considerable pool of soil N subject to leaching, denitrification, volatilization, or plant uptake.

One potential cropping system for the south-eastern USA is a winter wheat (Triticum aestivum L.) -- pearl millet (Pennisetum americanum L.) rotation. The region has relatively warm winters, and a winter annual, such as wheat, may prevent leaching losses during the winter months when nitrates are most likely to move out of the soil profile (Bielby et al., 1973; Bolton et al., 1970). Since the organic N in broiler litter may become available over an extended period, a winter-wheat crop followed by a summer annual, such as millet, may provide the crop manager with an opportunity to remove N from the soil as it is released from the organic fraction.

The objectives of this study were (i) to evaluate two rates of broiler litter as a N source for wheat growth, (ii) to determine if supplemental N is needed for optimum wheat yield, and (iii) to evaluate the agronomic conse-

165

166 R. P. Flynn, C. W. Wood &J. T. Touchton

quences of residual N as the only nutrient source for millet-forage production following a wheat crop.

M E T H O D S

The study was conducted twice at the Sand Mountain Substation, Crossville, Alabama (34°18'N, 86°01'W). The first test was conducted from October 1988 to February 1990 on a V~nville fine sandy loam (sili- ceous, thermic, Typic Fragiudult). The study was repeated on the same soil series but at a different loca- tion on the substation from October 1989 to February 1991.

Treatments consisted of a 3 x 2 factorial of litter and spring-applied N (spring-N) fertilizer rates arranged in whole plots and fall-applied N (fall-N) rates in split plots. There were four replicates of each treatment. Individual plot size was 3 x 9 m. Wheat (Pioneer 2550) was planted on 7 October 1988 and 27 October 1989; the row spacing was 18 cm. Litter rates of 0, 9, and 18 Mg ha-~ were applied and incorporated to a depth of 10 cm nine days before planting wheat. A fall applica- tion of 34 kg ha- 1 N as ammonium nitrate was incor- porated to 10 cm in half of all plots at planting. Spring-N, as ammonium nitrate, was applied at rates of O, 22, 44, and 66 kg ha -~ in 1988 and 1989 on 22 February. Wheat grain was harvested with a plot combine from a 10"4-m 2 area within each plot. Grain moisture content and weight were recorded in the field, and a sub-sample was taken for chemical analysis. Grain yields were corrected to 13% moisture. All vegetative weights are reported on a moisture-free basis. Wheat-straw yield was determined from a 0"33- m 2 area in each plot at harvest. Wheat straw was chopped but not removed from the field prior to plant- ing pearl millet. Millet was planted on 18-cm row- centers after wheat harvest with no additional N applied to the soil. Millet forage was harvested from a 0"33-m 2 area within each plot on 30 August and 18 October 1988 and on 1 September and 14 October 1989. No chemical or physical weed control was necessary owing to adequate competition by the desired crops.

Broiler litter used in this study was a mixture of wood chips, chicken excreta, and wasted chicken feed. Characteristics of broiler litter are listed in Table 1. Broiler litter applied in 1987 supplied 203 kg organic N, 26 kg NH4-N, and 10 kg NO3-N at 9 Mg ha 1 litter

and 407 kg organic N, 52 kg NH4-N, and 20 kg NO3-N at 18 Mg ha-1 litter. Treatments amended with 9 Mg ha 1 litter in 1988 received 258 kg organic N, 36 kg NH4-N, and 7 kg NO3-N ha-1. The 18-Mg ha-~ litter rate supplied 518 kg organic N, 72 kg NHa-N, and 13 kg NO3-N ha- t

Nitrogen content of wheat grain, straw, and millet forage was determined by utilizing a LECO CHN-600 analyzer (LECO Corporation, St Joseph, MI, 49085- 2396). Soil samples were taken to a depth of 61 cm and analyzed for total N via Kjeldahl methods (Bremner & Mulvaney, 1982) after the final millet harvest.

Data analysis on yield components and N removal was performed by using the Statistical Analysis System (SAS, Cary, NC) for regression and general linear models (Freund & Littell, 1981). Regression proce- dures with stepwise addition of significant independent variables was utilized. The proposed linear model for the study was:

E(Yx) = (t3o + AlfloU1)+ (31 + Alfll U1 )Xl

where,

E ( Y x ) =

-{- (32 + A132 U1 ) x2

the expected value of Y due to the influence of independent factors,

x i = Spring-N rate in kg ha- ~, fl0 = the Y-axis intercept, Ajfl 0 =correction to the intercept due to use of

broiler litter or fail-N, fll = slope coefficient of the linear-rate term, A lfl I = correction to the linear slope coefficient due

to use of broiler litter or fall-N, f12 = slope coefficient of the quadratic-rate term, Alfl 2 = correction to the quadratic slope coefficient

due to use of broiler litter or fall-N, UI = 1 if fall-N alone or in combination with

broiler litter causes a deviation away from the combine response to spring-N, other- wise U~ = 0.

A combined analysis of variance with year showed significant year-X-response variable interactions; dependent variables were analyzed separately by year. The probability level used for insertion of an inde- pendent variable in the statistical model was a = 0.10.

Our statistical approach allowed for separation of the dependent-variable response due to broiler litter in

Table 1. Selected chemical characteristics of broiler litter used in this study"

Total

(g kg - ~) (mg kg - I) (g kg- 9

1988 194 268 33 1400 3 600 16 23 15 0"23 294 1989 183 296 41 900 4900 14 18 4 0-27 242

"All analyses based on wet weight.

Year Moisture Total C Total N NO3-N NH4-N P K Mg Cu Ash

Nitrogen recovery from broiler litter in wheat-millet system 16 7

combination with or without fall-N, which was an advantage over a standard analysis-of-variance ap- proach. The separation of dependent variables was important because it delineated the difference in yield response to spring-N between litter rate and the prac- tice of fall applications of N. The regression procedure also allowed generation of plotting equations so that response differences could be visualized. The ap- proach further allowed for the inclusion of higher- order (quadratic) terms in the statistical models for a check of deviation from linearity.

Cumulative N-recovery efficiency was calculated from plant-N-uptake data as:

N recovery t rea tmen t - N recoveryno N applied X 100 N applied

(Bock, 1984). Negative efficiency values imply that more N was removed from the system when no N had been applied than when N was applied. Nitrogen not recovered by the harvested plant is subject to loss from the soil-plant system by leaching, volatilization, and/or denitrifica- tion. Unrecovered N (EX) was estimated from the recovery efficiency and N-application rate

[( 1 - NRE/100)*(N applied)].

Soil-N data allowed for a partial accounting of N that might remain in an immobilized or inorganic form. A bulk density of 1-60 g c m - 3 , which is typical of the Wynville soil series, was used to calculate the concen- tration of total N in the soil left after wheat- and mil- let-N removal. Any N that could not be accounted for from applied N was assumed to be lost to leaching or volatilization.

RESULTS AND DISCUSSION

Overall, wheat-grain yields in 1989 were lower than in 1988 (Fig. 1) owing to a high regional-disease inci- dence (T hurlow & Johnson, 1988, 1989 ). In both years of our study, when no litter had been applied, the fall application of 34 kg ha 1 N as ammonium nitrate improved grain yield when no N had been applied in the spring. According to the regression models, one 66-kg ha- t N application in the spring with no fall-N applications of litter or ammonium nitrate was best for grain yield in both years (Fig. 1, Table 2). Touchton and Hargrove (1983) demonstrated in 15 of 24 experi- ments that a combination of ammonium nitrate applied in the fall and spring can significantly improve wheat- grain yield. In 8 of 15 experiments, fall-N was required for optimum yield regardless of spring-N rate. In the other seven experiments, fall-N was required for opti- mum yield, but a fall-N application reduced the amount of spring-N needed. Touchton and Hargrove (1983) found that a fall application of 34 kg ha -1 N followed by 54 kg ha- 1 N in the spring had a beneficial effect on wheat yield.

Grain yield was not positively influenced in 1988 or 1989 by the combination of broiler litter and fall-N.

However, a fall application of litter at 9 or 18 Mg ha- 1

in combination with supplemental spring-applied N had both positive and negative effects on grain yield. Generally, increasing the broiler litter rate to 18 Mg ha-~ had no beneficial effect on grain yield with one exception in 1989 when no supplemental spring-N had been applied (Fig. 1). Shortall and Liebhardt (1975) noted that wheat-grain yield can be depressed with high application rates of broiler litter. Yield reductions with high application rates of broiler litter that are usually due to soluble-salt or ammonia-toxicity effects have been demonstrated (Shortall & Liebhardt, 1975). Salt effects may have been responsible for the yield decline in 1988 with 18 Mg ha t litter, although it is more likely that lodging, owing to excessive N (Peter- son & Voss, 1984) contributed to the lower grain yield, since straw production in littered plots was greater than in no-litter plots (Fig. 2). Grain yields suffered a 1400-kg ha -1 yield decline in 1988 with 18 Mg ha litter when compared with 9 Mg ha- 1 litter with no fall- N. Grain yield in 1989 was not negatively influenced by an 18-Mg ha ~ litter rate when compared with the 9-Mg litter rate with fall-N and no supplemental spring-N. Wheat-grain yield was greatest in littered plots in 1988 when a spring-N application of 22 kg ha -1 supplemented a fall application of 9 Mg ha -~ litter. Spring-N rates above 22 kg ha -1 in 1988 depressed yield, and in 1989 spring-N applications with 9 Mg ha-l litter had no significant effect. These results indicate that a moderate rate of litter (9 Mg ha-~ litter) can substitute for fall-applied ammonium nitrate and reduce or eliminate the need for supple- mental spring-N.

Nitrogen partitioned to wheat grain is positively correlated to carbon partitioning and does not vary outside a narrow ratio range of carbon to nitrogen (19-21), despite differences in the N status of the whole plant (Simpson, 1986). Hence, if N removal from the soil system is desired, wheat grain has a limited capacity for removing N. Nitrogen uptake by wheat grain in our study followed the same pattern as grain yield with one exception in 1988 (Fig. 1, Table 2). Generally, if grain yield was low, grain-N removal was low. However, N removal in non-littered treatments with no spring-N was lower than in the 9-Mg ha ~ litter treatment with no spring-N. The N content of the grain is only a portion of the total N within a wheat plant. Wheat straw is another plant component where N is used and provides another avenue for removal from the soil system if it is removed from the field.

Too much N often results in excess straw production leading to increased lodging (Peterson & Voss, 1984). In 1988, wheat-straw production was greatest when broiler litter had been applied (Fig. 2, Table 3). Both rates of litter produced the most straw, and the effect of litter was independent from that of spring-N. Spring-N, however, did increase straw production if a fall applica- tion of ammonium nitrate had been made and no litter had been applied. There were no spring-N effects on straw yield if fall-N had not been applied. In 1989,

1 6 8 R. P. Flynn, C. W. W o o d &J. T. Touchton

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Fig. 1. Wheat-grain yield and N-uptake response to spring-N, broiler litter, and fal l -N in 1 9 8 8 and 1 9 8 9 . - - o , zero; - - e , fal l -N only; - - v , 9 M g h a - ] broiler litter; - - V , 9 M g ha J broiler litter and fall-N; . . . . % 18 M g h a - ~ broiler litter; . . . . i , 18

M g h a - J broiler litter and fall-N.

Table 2. S t e p w i s e regress ion analysis ( a = 0 . 1 0 ) o f wheat-grain yie ld and N uptake by grain for 1 9 8 8 and 1 9 8 9 °

Treatment flo A g3 o 161 A i16 t 162 A 1162

(kgha -I)

1988 Grain Yield (R 2 = 0.84)

Z e r o 3 051 0 0 0 - 0 .10 0-30 FalI-N only 3 0 5 1 0 0 16.5 - 0 -10 0 9 M g ha -1 B L 3 0 5 1 0 0 37 .9 - 0 .10 - 0-67 9 M g h a - l B L and fal l -N 3 051 0 0 0 - 0 .10 - 0 .24 18 M g h a - j BL 3 0 5 1 - 1 3 9 4 0 0 - 0 - 1 0 0 18 M g h a - ~ B L and fal l -N 3 051 - 1 4 7 6 0 0 - 0 .10 0

1989 Grain Yield (R 2 = 0.59)

Z e r o 2 183 - 1 2 6 3 0 27 0 FalI-N only 2 183 - 1 0 4 8 0 17 0 9 M g h a - 1 B L 2 183 2 9 0 0 0 0 9 M g h a - 1 B L and fal l -N 2 1 8 3 0 0 0 0 18 k g h a - 1 B L 2 1 8 3 0 0 - 15 0 18 M g h a - 1 B L and fal l -N 2 183 0 0 - 22 0

1988 Grain-N Uptake (R 2 = 0.67)

Z e r o 63 .4 - 26 .4 0 0 .64 - 0 . 0 0 2 Fall-N only 63"4 - 12.6 0 0 - 0"002 9 M g h a - l B L 63-4 0 0 0 - 0 . 0 0 2 9 M g h a - ] B L and fal l -N 63-4 0 0 0 - 0"002 18 M g h a - ] B L 63"4 - 24 .0 0 0 - 0"002 18 M g h a - 1 B L and fal l -N 63-4 - 25 .3 0 0 - 0"002

0 0 -007 0

- 0 -004 0 0

1989 Grain-N Uptake (R 2 = 0.57) Z e r o 42-0 - 24 .5 0 0 0 FalI-N on ly 42 .0 - 25 .7 0 0 .36 0 9 M g h a - 1 BL 42"0 4.2 0 0 0 9 M g ha -1 B L and fal l -N 42"0 0 0 0 0 18 M g h a - ~ B L 42"0 0 0 - 0 -20 0 18 M g h a - ] B L and fal l -N 42"0 0 0 0 0

0"007 0 0 0 0

- 0 ' 0 0 5

~For abbreviat ions refer to and Methods .

Nitrogen recovery from broiler litter in wheat-millet system 16 9

moderate disease pressure (Thurlow & Johnson, 1989) in northern Alabama hindered wheat growth, which resulted in no significant treatment effects and an aver- age production of 4437 kg ha- ~ straw.

Nitrogen uptake by straw in 1988 was positively influenced by broiler-litter and fall-N treatments (Fig. 2, Table 3). As the broiler-litter rate increased, N uptake increased, which indicated that N mineralized

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S p r i n g N A p p l i e d (kg ha -1) Fig. 2. Wheat-straw yield and N-uptake response to spring-N, broiler litter, and fall-N in 1988 and 1989. 1989 Straw biomass: no variable met the 0.10 significance level for entry into the model. - - o, zero; - - *, fall-N only; -- ~7, 9 Mg ha- J broiler litter; --

V, 9 Mg ha- ' broiler litter and fall-N; . . . . n, 18 Mg ha- ' broiler litter; . . . . m, 18 Mg ha- ~ broiler litter and fall-N.

Table 3. Stepwise regression analysis (a = 0"10) of wheat-straw biomass and N uptake for 1988 and 1989 a

Treatment flo 1~ 1~0 J~l A I~1 ~2 A 1~2

(kg ha- 9

1988 Straw Biomass (R 2 = 0.33) Zero 5 409 - 2 210 0 0 0 0 FalI-N only 5 409 - 1458 0 21 0 0 9 Mg ha- 1 BL 5 409 0 0 0 0 0 9 Mg ha- 1 BL and fall-N 5 409 0 0 0 0 0 18 Mgha -1BL 5 4 0 9 0 0 0 0 0 18 Mg ha- 1 BL and fall-N 5 409 0 0 0 0 0

1989 Straw Biomass No variable met the a = 0-10 significance level. Mean straw biomass was 4 4 3 7 kg ha- 1.

1988 N Uptake by Straw (R 2=0.62) Zero 87 - 64 0 0 0 0 FalI-N only 87 - 48 0 0 0 0 9 Mg ha- 1 BL 87 - 29 0 0 0 0 9 Mg ha- 1 BL and fall-N 87 0 0 0 0 0 18 Mgha - 1 B L 87 0 0 0 0 0 18 Mg ha- 1BL and fall-N 87 13 0 0 0 0

1989 N Uptake by Straw (R 2 = 0"45) Zero 53 - 22 0 0 0-002 0 FalI-N only 53 - 23 0 0 0"002 0 9 Mgha - 1 B L 53 - 13 0 0 0.002 0 9 Mg ha- 1BL and fall-N 53 - 8 0 0 0.002 0 18 Mg ha- 1 BL 53 0 0 0 0.002 0 18 Mg ha- 1 BL and fall-N 53 0 0 0 0"002 0

aFor abbreviations, refer to Methods.

170 R. P. Flynn, C. W. Wood &J. T. Touchton

from the litter accumulated in wheat straw. Broiler litter at 9 Mg ha-1 with no fall-N increased straw-N content by 19 kg ha- 1 over that of a single fall-N treat- ment with no litter in 1988. Fall-N plus 9 Mg ha- ~ litter was equivalent to the straw-N content attained with 18 Mg litter with no fall-N in 1988 (Fig. 2, Table 3). Appli- cation of 18 Mg ha- ~ litter resulted in 45 kg ha- 1 more straw-N than a single application of fall-N in 1988. Spring-N had no effect on straw-N uptake in 1988, but spring-N in 1989 caused a straw-N accumulation in all treatments. Differences in N uptake between litter treatments could be discerned in 1989, despite the treatments having no effect on straw-biomass produc- tion. Nitrogen-uptake patterns presented in Fig. 2 indicate that N not required for grain production accu- mulates in straw and provides an opportunity for addi- tional N recovery from litter-treated fields.

Mineralization of N originating from broiler litter may not be complete at the end of the wheat-growing season (Brinton, 1985). Moreover, N contained in wheat straw returned to the soil is slowly mineralized owing to its relatively high C: N ratio (Simpson, 1986). Thus, a potential exists for N from litter and wheat straw to be mineralized and available for further crop production. A summer millet crop grown for forage could take advantage of this mineralized N.

Millet dry-matter yield for both years increased as the litter-application rate increased (Fig. 3, Table 4). Millet biomass also responded to fall-N and spring-N applications but varied between year and treatments. Overall, millet-biomass yield in 1988 was greater than in 1989. However, in both years, N carry-over from the broiler-litter applications was apparent. FalI-N applica-

tions made in combination with litter and no spring-N apparently increased N availability for crop production after wheat harvest. In 1988, spring-N had no effect on yield except when applied in combination with 18 Mg ha 1 with no fall-N application (Fig. 3, Table 4). In 1989, fall-N had no effect on millet biomass when 9 Mg ha- t litter had been applied. However, fall-N had a positive effect on millet yield when applied in combina- tion with 18 Mg ha- 1 litter and when applied in combi- nation with spring-N above 22 kg ha-t and no litter. Millet yield increased in 1989 when fall-N was applied in combination with 18 Mg ha- l litter and no spring-N. Millet yield in 1989 was lowest with the combination of no litter, no fall-N, and 66 kg ha- 1 spring-N. Millet- biomass yield in 1989 increased as spring=N increased, except when no-litter or fall-N had been applied.

Millet removed 88 kg ha-1 N from the soil with no ammonium nitrate fertilizer or broiler litter in 1988 and 50 kg ha -1 N in 1989 (Fig. 3, Table 4). In 1988, millet removed an additional 53 kg ha -1 N from the 9-Mg broiler-litter rate and an additional 95 kg ha- ~ N from the 18-Mg broiler-litter rate in 1988. In 1989, millet removed an additional 24 and 72 kg ha-1 N from the 9- and 18-Mg litter rates, respectively (Fig. 3, Table 4). A significant litter x fall-N interaction sug- gests that fall-N in conjunction with litter increasd the amount of residual-N available for removal at the 9-Mg and 18-Mg ha -1 rates in 1988. However, in 1989, the effect of fall-N was significant but variable in its effect. Millet-N uptake increased when fall-N was applied in conjunction with 18 Mg ha -~ litter but fall-N had no effect on millet-N uptake when applied with 9 Mg ha- 1 litter. Millet-N uptake in 1989 was lowest when the

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. . . . m, 18 Mg ha- ~ broiler litter and fall-N.

Nitrogen recovery f r o m broiler litter in whea t -mi l l e t system

Table 4. Stepwise regression analysis (a = 0" 10) of pearl-millet biomass and N uptake after winter wheat °

171

Treatment flo A lflo fl l A tilt t32 A 1fl2

(kg ha -I)

1988 Millet Biomass (R 2 = 0.67) Zero 5 562 0 0 0 0 0 FalI-N only 5 562 0 0 0 0 0 9 Mg ha- 1 BL 5 562 2 434 0 0 0 0 9 Mg ha- ~ BL and fall-N 5 562 2 968 0 0 0 0 18 Mg ha- ~ BL 5 562 4610 0 48 0 0 18 Mg ha- 1BL and fall-N 5 562 5 541 0 0 0 0

1989 Millet Biomass (R 2 = 0.67)

Zero 5 174 - 1474 0 - 29 0.228 0 FalI-N only 5 174 - 2 125 0 0 0.228 0 9Mgha -1BL 5 174 0 0 0 0.228 0 9 Mg ha- l BL and fall-N 5 174 0 0 0 0.228 0 18 Mgha -1BL 5 174 0 0 33 0.228 0 18 Mg ha- ~ BL and fall-N 5 174 2 744 0 0 0.228 0

1988 Millet-N Uptake (R 2 = 0.71)

Zero 88 0 0 0 0 0 FalI-N only 88 0 0 0 0 0 9 Mgha -1BL 88 53 0 0 0 0 9 Mg ha- 1 BL and fall-N 88 63 0 0 0 0 18 Mgha -1BL 88 95 0 1 0 0 18 Mgha -1BL and fall-N 88 115 0 0 0 0

1989 Millet-N Uptake (R 2 = 0"73)

Zero 74 - 24 0 0 0"005 0 FalI-N only 74 - 36 0 0 0"005 0 9 Mg ha- 1 BL 74 0 0 0 0"005 0 9 Mg ha- 1 BL and fall-N 74 0 0 0 0"005 0 18 Mg ha- 1 BL 74 32 0 0 0"005 0 18 Mg ha- ~ BL and fall-N 74 63 0 - 0"50 0-005 0

"For abbreviations, refer to Methods.

crop was produced on soil amended with one fall-N application and no litter or supplemental spring-N (Fig. 3, Table 4). In general, spring-N application to the previous wheat crop influenced millet-N removal in the same manner as millet-biomass product ion with one exception. In 1989, 18 Mg ha-1 litter with no supple- mental fall-N or spring-N accumulated more N in the tissue than the 9-Mg ha - ~ fitter treatment, while millet biomass at 18 Mg ha - 1 litter could not be distinguished from the 9-Mg ha -~ fitter treatment. These results indicate that N can accumulate in the millet tissue as it did in the wheat straw and provide an avenue for N removal f rom the soil.

Examination of N recovery efficiency for this crop- ping system points to areas of concern and also opti- mism with regard to management of broiler litter as an N fertilizer (Table 5). Nitrogen-recovery efficiency was dependent on the rate of broiler litter applied, fall-N, and the number of crop components removed from the field; spring-N or its interaction with other independ- ent variables had no influence on N-recovery efficiency. Efficiency was reduced when fall-N was applied, which suggests that fall-N was not beneficial f rom either a crop-product ion or environmental perspective. Pro- duction and removal of wheat grain alone resulted in

an extremely low recovery efficiency when litter was applied. Negative efficiency values occurred when more N was removed from the zero-N plots than from treated plots. Millet following wheat enhanced N-recovery efficiency and reflects previously reported efficiency values (Brinton, 1985; Sims, 1987).

Unrecovered N (EX) from broiler litter can be subject to leaching, denitrification, and further mineral- ization (Sims, 1987). A total of 162 kg ha -l N with 9 Mg ha -1 broiler fitter and 387 kg ha -1 N with 18 Mg ha - 1 broiler litter was left in the soil system subsequent to the 1988 millet harvest (Table 6). Total soil N after the millet harvest in 1988 was 315 kg ha ~ in the 9Mg broiler-litter plots and 444 kg ha-1 in the 18-Mg litter plots. All the N is accounted for in soil or plant uptake, which suggests that no N had been leached by the end of the 1988 test. This pool of N, however, is still sub- ject to the processes of mineralization, leaching, deni- trification, and plant uptake.

Less N was removed in 1989 owing to lower wheat- grain and straw yield when compared with 1988 N removal (Figs 1 and 2, Tables 2 and 3). At least 87% of the N from the 9-Mg h a - 1 broiler litter treatment was unaccounted for in 1989. In 1988, 68% of the N from 9 Mg ha-1 litter was unaccountable. Soil-N levels in

Table 5. Cumulative N-recovery efficiency for wheat grain (NREg) and grain plus millet (NREgm) for 1988 and 1989

Litter Fall-N NREg (%) NREgm (%) rate rate

(Mg ha- i) (kg ha- i) 1988 1989 1988 1989

18

0 56.4 43.0 88.6 67.2 34 32.5 18.3 35"2 32.3

Mean 42.8 28.9 58.1 47.2

0 8"8 9'5 29.0 17.5 34 5"1 7'2 26.8 15"2

Mean 7.0 8.4 27.8 16.3

0 - 0"13 3"4 25"4 13"0 34 - 0 ' 3 1 2"8 21-5 16"5

Mean - 0'22 3.1 23-5 14.7

Analysis of Variance Source Pr> F LSDo. t Pr> F LSD~ I Pr> F LSD~I Pr> F LSDo. I

Broiler litter (BL) 0.0001 7.5 0.0001 6.3 0"0174 22.4 0.0100 18.7 Spring-N (SN) 0.6804 0.8131 0.7248 0"7578 BL× SN 0.9949 0"6406 0.7877 0"8065 FaI1-N (FN) 0'0001 2.2 0"0001 2-0 0'0006 9.4 0"0601 10.2 BLx FN 0"0001 2"3 0"0001 3.4 0"0004 16.3 0"0332 17.7 SN × FN 0"5035 0"8764 0"2216 0-6533 SN x FN x BL 0"5638 0"9039 0.1526 0-5766

Table 6. Unrecovered N from wheat grain (EX~) and grain plus millet (EXgm) as influenced by litt :r rate, spring-N, and fall-N"

Litter rate Spring-N EX b EXam (Mg ha- 9 rate

(kg ha- ~) 1988 1989 1988 1989

FaH-N (kg ha- 9 Fall-N (kg ha - 9 0 34 Mean 0 34 Mean 0 34 Mean 0 34 Mean

(kg N ha- 9

0 0 22 31 8 17 22 9 41 25 14 46 30 - 7 50 21 0"8 33 17 44 18 49 33 24 62 43 14 68 41 11 71 41 66 33 69 51 35 73 54 23 58 40 46 71 59

mean 20 45 34 24 53 41 10 46 31 19 48 36

18

0 217 250 233 273 318 295 162 190 176 262 291 276 22 231 274 253 291 324 307 186 221 204 262 303 283 44 253 296 274 308 348 328 181 214 200 281 310 298 66 294 344 319 337 377 257 242 274 258 295 343 319

mean 249 291 270 302 341 322 193 225 210 274 312 294

0 474 508 491 573 604 589 387 403 395 527 531 529 22 504 543 523 598 638 618 368 423 395 553 540 547 44 523 554 539 633 667 650 375 438 402 560 577 568 66 550 586 568 654 698 676 394 450 422 574 590 582

mean 513 548 530 614 652 633 381 428 404 553 560 556

Analysis of Variance Source Pr> F LSDo. w Pr> F LSD~w Pr> F LSD~w Pr> F LSD~w

Broiler litter (BL) 0.0001 9 0.0001 6 0.0001 24 0.0001 26 Spring-N (SN) 0.0001 9 0.0001 6 0.0004 24 0.0007 23 BLx SN 0.0001 9 0-0001 6 0.2421 0.9724 FalI-N (FN) 0"0001 3 0-0001 2 0"0001 11 0-0001 9 BLK x F N 0-0325 5 0-6490 0.4606 0.0365 16 SN x FN 0.2652 0.4592 0.5326 0.6304 B L x SN x FN 0.7412 0-1640 0.8263 0.8374

aFor abbreviations refer to Methods. bEX~ = ( 1 - NREJ100)*(N applied) CEXgm = ( 1 - NRI~g m / 100 )* (N applied).

Nitrogen recovery from broiler litter in wheat-millet system 173

1989 after millet harvest accounted for most of the applied N not removed by wheat and millet.

The fate of residual N from broiler-litter application depends on the recovery of inorganic N and on the rate of mineralization of the organic fraction (Bitzer & Sims, 1988). Adding fall-N and spring-N to the litter- treated plots only served to increase the pool of resi- dual N. Significant differences among spring-N rates with residual N after grain product ion indicate that spring-N applications also increase amounts of residual N (Table 6). The millet crop, however, took advantage of some of this residual N.

SUMMARY

Fall application of 9 Mg ha l broiler litter provided winter wheat and a subsequent millet crop with suffi- cient N. Wheat receiving 9 Mg ha-1 litter plus 22 kg ha - l N in 1988 produced yields similar to those attained with 44 kg ha 1 N of spring-applied ammo- nium nitrate. Wheat-grain yield in 1989 under the 9-Mg ha-J broiler-litter rate plus fall-N was similar to yields attained with 66 kg ha -L spring-applied N. Millet-dry-matter yields increased with increasing broiler-litter rates, which reflected the amount of N still available for plant growth after wheat harvest. Produc- tion of wheat alone did not take full advantage of the mineralized organic N from broiler litter that occurred over the summer and fall months. Supplementing wheat with fall-N or spring-N above 22 kg h a - l was unnecessary when litter at 9 Mg ha - l had been applied and resulted in increased residual-soil-N levels. Further gains in N-recovery efficiency could occur with remo- val of wheat straw from the field. Based on N uptake, an additional 58 kg ha 1 N with 9 Mg ha - broiler litter or 87 kg ha -I N with 18 Mg broiler litter could have been removed in wheat straw after the first year. Millet growth in 1989 was not as good as that in 1988, which resulted in less N removal f rom the soil system. How- ever, residual N from litter treatments was enough to produce a forage-millet crop with no additional N application.

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