Fertilizer nitrogen and stover removal effects on sorghum yields and nutrient uptake and partitioning

Agriculture, Ecosystems and Environment, 39 ( ! 992) 197-211

197

Elsevier Science Publishers B.V., Amsterdam

Fertilizer nitrogen and stover removal effects on sorghum yields and nutrient uptake and partitioning J. Mark PowelP and Frank M. Honsb ~lnternational Livestock Centerfor Africa, ILCA/ICRISAT, B.P. 12404, Niamey, Niger bDepartment of Soil and Crop Sciences, Texas A&M University, CollegeStation, TX 77843, USA (Accepted 8 November 1991 ) ABSTRACT Powell, J.M. and Hons, F.M., 1992. Fertilizer nitrogen and stover removal effects on sorghum yields an,J nutrient uptake and partitioning. Agrlc. Ecosystems Environ., 39: ! 97-2 ! !. In agricultural systems where large amounts of crop residues are produced, there may exist possibilities for removing part of the residues for alternative uses without detrimental effects to agricultural productivity and the environment. A 4 year field study was conducted in central Texas to evaluate the effects of sorghum (Sorghum bicolor L. Moench) genotype, fertilizer N application rate and stover removal on crop yields and plant nutrient uptake and partitioning. Fertilizer N application rates of 1 ! 2 kg ha- t were generally sufficient to produce maximum yields, to attain the highest fertilizer N uptake efficiency and for the grain sorghum cultivars to achieve the highest percentage partitioning of N, P and K into grain. Quadratic relationships best described the relationship between fertilizer N application rate and the amounts of N, P and K taken up by grain and forage sorghum cultivars. A strong interdependence between fertilizer N application rate and stover return on nutrient cycling appeared to be developing in the cropping system under study. Stover removal adversely affected yields, nutrient uptake and partitioning during the last 2 study years. The interactive effects of sorghum genotype, fertilizer N and stover removal on yields and soil productivity need to be assessed over a longer term if stover removal is to be developed into an environmentally sound and sustainable production strategy.

INTRODUCTION

Cereal stovers and other crop residues perform various functions in agricultural production systems. In most temperate zone cropping systems, crop residues are returned to fields after harvest, playing an important role in nutrient cycling, erosion control and the maintenance of favorable soil physical properties (Larson et al., 1972; Van Doren and Allmaras, 1978; Power et al., 1986). In minimum and no-tiUage systems, surface residues protect the soil surface from wind and water erosion, provide favorable seedbed conditions and conserve soil water. The magnitude of the beneficial effects associated Correspondence to: J.M. Powell, International Livestock Center for Africa, ILCA/ICRISAT, B.P. 12404, Niamey, Niger. © 1992 Elsevier Science Publishers B.V. All rights reserved 0167-8809/92/$05.00

198

J.M. POWELL AND F.M. HONS

with returning crop residues to fields depends on the quantity and quality of the residue, the subsequent crop to be grown, edaphic factors, topography, climate and soil management. Fossil fuel reserves are limited, necessitating a continuous search for alternative, renewable forms of energy. Crop residues have been considered as an alternative source of energy in the US (Burwell, 1978; Epstein et al., 1978; Wiedenfeld, 1984). In modern, highly technological agricultural production systems, crop yields are often much higher than in low-input production systems. Greater amounts of crop residues are returned to fields, resulting in increases in soil organic matter and available nutrient reserves (Dalai, 1986; Grove et al., 1986; Jenkinson et al., 1986). In agricultural systems where large quantities of crop residues are produced, there may exist possibilities for removing part of the residue from fields for alternative uses without detrimental effects to the soil environment and crop yields. Given the current high level ofsorghum (Sorghum bicolorL. Moench) production technology, the cultivation of "high energy" sorghum (HES) cultivats for the production of both grain and biomass (Miller and Creelman, 1980; Creelman et al., 1981 ) which can be converted to ethanol and methane, may offer promising future production alternatives for sorghum farmers in Texas. Removing crop residues from some fields, however, can adversely affect the soil environment, leading to a decline in soil productivity. If the cultivation of sorghum for methane and/or ethanol production is to become a viable strategy, the effects of stover removal on soil productivity must be assessed. The purpose of this study was to evaluate the effects of sorghum genotype, fertilizer nitrogen (N) application rate and stover removal on crop yields and plant nutrient accumulation and partitioning in a conventional tillage, continuous sorghum cropping system in central Texas. MATERIALS AND METHODS

Site description The experiment was conducted at the Texas Agricultural Experiment Station Research Farm in Burleson County on a Ships clay soil (very fine, mixed, thermic Udic Chromustert) during the 1985-1988 cropping seasons. The surface layer of the Ships soil (0-15 cm)" has a pH of 8.1 ( 1:1 water), an organic carbon content of 1.28% (Walkley-Black), very low total nitrogen, and available phosphorus and potassium as extracted by 1.4 M NH4OAc + 0.025 M ethylenediaminetetraacetic acid (EDTA) of 48 mg kgand 467 rag kg-', respectively (Powell, 1989). Average annual rainfall is 980 mm, 45% of which normally occurs between April and August. In order to avoid soil moisture deficits, two supplemental furrow irrigations were provided during the course of the study; one of approximately 50 mm 1 week

FERTILIZER N AND STOVER REMOVAL EFFECTS ON SORGHUM

199

before planting in 1986 and one of about 65 mm midway through the 1988 cropping season.

Experimental treatments Three sorghum (Sorghum bicolor L. Moench) cultivars ofdiffering harvest indices (ATx623 × RTx430, a high grain producing hybrid; ATx623 ×'Rio', a medium grain and high biomass producer; 'Grassr, a high biomass producing forage type), three fertilizer N levels (0, 112 and 224 kg N ha -~ year -I as NH4NO3) and three biomass return levels (0, 25 and 50% of total stover production in each plot) were arranged in a split-plot design with genotypes randomly assigned to main plots and N and stover rates assigned factorially to subplots. Each treatment combination was replicated four times in subplots consisting of six rows, 7.6 m long and 0.68 m apart. The cultivar ATx623 ×'Rio' was replaced by ATx623 × 'Hegari' in 1987 and 1988. During the previous two growing seasons, the former cultivar lodged severely, a trait considered undesirable for the mechanical harvest of either grain or biomass. Stover additions were applied uniformly to plot surfaces after the 1984, 1985, 1986 and 1987 grain harvests and incorporated into the top 10 cm of soil by three disk plowings followed by bedding. All plots received an annual application of 30 kg P ha- 1 as 0-46-0. Half of the N and all of the P was broadcast preplant and worked into beds with a rolling disk cultivator; the remaining N was side-dressed approximately 40 days post-emergence. Plantings occurred in late March or early April, and seedlings were thinned to attain a plant density of approximately 132 000 plants h a - I

Dry matter and nutrient yields To determine treatment effects on yields, grain and stover from the middle 3 m of the two innermost rows of each subplot were hand harvested and weighed. Stover subsamples were oven dried (70°C, 4 days) for dry matter (DM) determination. Grain moisture was determined at harvest and yields were standardized to 14% moisture content. Stover and grain subsamples were digested with H2504 (Nelson and Sommers, 1980), N and P were determined with an autoanalyzer (Technicon, 1977), and K by inductively coupled spectroscopy. Grain and stover nutrient concentrations were multiplied by their respective dry weights to determine annual amounts of each nutrient removed from fields at the various treatment levels. The effect of fertilizer N and stover return rates on fertilizer N uptake efficiency (FNUE) was calculated using the difference method (Parr, 1973 ). Total N uptake by above-ground DM at a particular stover return rate was corrected for N derived from stover return in the equation

200

J.M. POWELL AND F.M. HONS

Percent FNUE= [ (A-B)/C] X 100 where A is the total N uptake by above-ground DM at a fertilizer N rate C and at stover return rates of 0, 25 or 50%, B is the total N uptake by above-ground DM at a fertilizer N rate of 0 kg ha- ~ and at stover return rates of 0, 25 or 50% and C is 112 or 224 kg ha- ~year- t. RESULTS AND DISCUSSION

Morphologicaland productionfeatures of sorghum The four sorghum genotypes used in the study had distinctly different morphological and production features (Table 1). Average annual total aboveground DM yields ofATx623 × RTx430, the conventional grain sorghum, and ATx623 × 'Rio' and ATx623 ×'Hegari', the intermediate-type grain sorghum cultivars were 9.7 Mg ha -I, 10.8 Mg ha -t and 15.3 Mg ha -t, respectively. Approximately half of the total DM produced by ATx623×RTx430 compared with about one-fourth of the total DM of ATx623x'Rio' and ATx623 ×'Hegari' went into grain production. Total yields of 'Grassl', the forage sorghum, were 18.5 Mg ha- t averaged across all treatment levels during the 4 year study period. During the 1986 growing season, the short-day photoperiod sensitive cultivar 'Grassl' produced relatively small panicles which comprised approximately 7% of total dry matter. Stover production, and therefore absolute stover return, and leaf: stalk ratios were highest with 'Grassl' and ATx623×'Hegari', followed by ATx623X'Rio' and ATx623 × RTx430. TABLE I

Range of morphological and yield characteristics of selected sorghum genotypes* Genotype

Stover height (m)

Total dry matter (Mgha -~)

Harvest index (%)

Leaf: stalk ratio

ATx623 X RTx430 ATx623 X'Rio' ATx623 ×'Hegari' 'Grassl'

!.0- !.3 1.2-2.6 1.8-2.8 2.5-3.3

8.8- ! 0.4 9.9-11.6 ! 3.7-20.5 16.1-20.9

40-49 25-31 ! 8-26 0-5

1 : 2.6 1 : 5.0 ! : 5.7 1 : 5.2

'All measurements taken from plots of ! 12 kg N ha -I and 25% return rate. ATx623x'Rio' and ATx623 ×'Hegari' data for 2 years each, data for other genotypes for 4 years; leaf: stalk ratio only for first year for all genotypes.

FERTILIZER N AND STOVER REMOVAL EFFECTS ON SORGHUM

201

Sorghum response to fertilizer N and stover return

Differences in total yield and relative partitioning of dry matter into grain and vegetative material (Table 1 ) were not found to influence genotypic yield responses to fertilizer nitrogen. Application of 112 kg N ha- mappeared to be 16-

I o~

12

•

ATx623 x RTx430

R2ffiO.91 1966• y,.,5.38+76.96~-O.23x2 R2=0.87 1987" y-6.64+48.75x-O.15x 2 R2ffiO.67 1989- y-8.BO+49.43x-O.19x2 R2=0.45

r-,

O. 25, Io

I

I

I

ATx423x RIO (1985-88) ATx423x HEGARi (1987-88)

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0

I

30

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25 20,

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I

15

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5 ~_ 0

x

I 0

I 112 FERTlUZERNITROGEN(k9 ha-1)

I 224

Fig. 1. Sorghum response to fertilizer nitrogen (ATx623 X RTx430, grain hybrid; ATx623 x ' R i o ' and ATx623 ×'Hegari', intermediate-type grain sorghum cuitivars, 'Grassr, forage sorghum; all correlation coefficients significant at the P < 0.05 level).

202

J.M. POWELLAND F.M. HONS

sufficient to produce maximum yields for all genotypes during most years of the study (Fig. 1 ). However, yields of the conventional grain sorghum cultivar in 1985, and the intermediate-type sorghum cultivars in 1985 and 1987 responded positively and significantly (P<0.01) to 224 kg N ha-t. Coefficients of determination (R 2) for predicting DM from fertilizer N applications were lower, especially for the conventional grain sorghum cultivar, for the last 2 years of the study (Fig. 1 ). This indicates an increase in yield variability and a reduced ability of fertilizer N to affect yields. Stover return, in the absence of interactive effects with fertilizer N, had three contrasting effects on yields during the 4 year study period (Table 2). Grain yield of the conventional-type sorghum cultivar (ATx623 × RTx430) was significantly (P < 0.01 ) increased by stover return in the third year, grain and stover yields of the intermediate-type sorghum cultivar (ATx623 ×'Hegari') were decreased (P< 0.05 ) in the fourth year and yields of the forage sorghum cultivar ('Grassl') were unaffected by stover return during the 4 years of the study. Interactive effects of fertilizer N and stover applications included yield increases of ATx623 ×'Rio' and ATx623 × RTx430 in the second and fourth years of the study, respectively, when stover return was 25 or 50% in conjunction with 112 k8 N ha -t. Interactive effects between fertilizer N and stover return appeared to increasingly influence nutrient uptake and partitioning during the last two cropping seasons. Effect offertilizer N and stover return on FNUE

The analysis of variance (Statistical Analysis Systems Institute, 1982) for treatment effects on FNUE showed significant (P<0.0~) interactions between years, genotype and stover return and between years, fertilizer N and stover return. Stover return significantly decreased FNUE of TABLE 2 Summary of stover return effects on sorghum yields t Year

Genotype

Stover return effect on yia,ld

1985 1986 ! 98"l 1987 1987 1988 1988 i 988

All All ATx623 X RTx430 ATx623 X'Hegari' 'Grassr ATx623XRTx430 ATx623 X'Hegari' 'Grassr

None None Positive None None None Negative None

tAdapted from Powell (1989).

203

FERTILIZER N AND STOVER REMOVALEFFECTSON SORGHUM

ATx623×RTx430 in the second year, and increased FNUE of ATx623 x'Hegari' and 'Grassl' in the last year of the study (Fig. 2). In 1988, FNUE for 'Grassl' in plots receiving the 50% stover return rate was approximately three times greater than in plots where stover had been completely removed. The apparent decrease in FNUE, especially for the forage and the intermediate-type sorghum cultivars during the latter years relative to the beginning of the study (Fig. 2 ) may have arisen from cumulative adverse effects of stover removal. Stover return both increased and decreased FNUE depending on the fertilizer N rate and year of the study (Fig. 3). A fertilizer application rate of I 12 kg N ha- I resulted in the greatest FNUE for all genotypes during most years of the study. A decreased FNUE at 224 kg N ha- i probably occurred because i

ATx623 x RTx430

r--~ ATx623x Rio (1985-86) ~ ATx623x Hegor! (1987-88) 90.

801 1085

!-

5oi 40.

z

3oi

2oi

2o

80,

1U7

40'

30

20

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70,

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1986

8oi 7oi eol

7o

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•

0

20

25

Sl"OVERRETURN (~

50

oftotalproduced)

iiiilmil 0

25

b'rOVERR[TURN (~

50

oftotalproduced)

Fig. 2. Fertilizer nitrogen uptake efficiency (FNUE) of sorghum cultivars at various stayer return rates for 4 years (1= HSDo.o5 for yearly within genotype comparisons).

204

J.M. POWELL AND F.M. HONS lg8S

80"

;.

i

70.

'

1986 mm 112 N

801

[ = 9=, N

t ?o,

60

60

40, 30

40 30

20

.

,

,

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80,

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70

60

60 0

.

.I

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70

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50

40

40

3O

30

20

20 0

25

50

STOVERRETURN(111of totol produced)

0

25

50

S1'OVERRETURN(~ of total produced)

Fig. 3, Interaction of fertilizer nitrogen and stover return on fertilizer nitrogen uptake efficiency (FNUE) of sorghum for 4 years (l=HSDo.o5 for yearly within fertilizer nitrogen rate comparisons),

of low crop demand for this additional N increment. In plots where 224 kg N ha-i was applied in 1985, however, stover return increased FNUE from 47% in plots where stover was completely removed to 61% in plots where the stover return rate was 50% (Tukey's honestly significant difference ( H S D ) = 18%, calculated from Montgomery, 1984). During 1985, yields of both grain sorghum cultivars were increased significantly (P< 0.05), and larg~ yield increases of the forage sorghum cultivar were obtained when a fertilizer N application rate of 224 kg ha- ' rather than 112 kg ha- ' was used (Fig. 1 ). Although non-significant, stover return had negative effects on FNUE in 1986 and 1987 at fertilizer N applications of 112 kg ha-'. Tukey's HSD values for FNUE at 112 kg N ha -~ in 1986 and 1987 were 22.3% and 17.0%, respectively whereas actual maximum differences were 15% and 10%, respectively. In 1988, stover return doubled FNUE in 112 kg N ha- ~ plots and sig-

FERTILIZER N AND STOVER REMOVAL EFFECTS ON SORGHUM

205

nificantly increased FNUE in plots where 224 kg N ha-~ was applied (Fig. 3). The increased grain yield of ATx623 × RTx430 in 1988 in plots that received 112 kg N ha- i was because of the positive effect of stover return on FNUE. In another experiment, of the 112 kg N ha- ! applied to sorghum on Houston Black clays, it was estimated that 55% was taken up by the crop while 20% was immobilized and 17% was denitrified (Kissel et al., 1976). Fertilizer N uptake efficiency for sorghum receiving 112 kg N ha- ' in the present study ranged from 36 to 75% during the 4 year study period (Fig. 3). Approximately 28-72 kg h a - ' of applied fertilizer N applied at I 12 kg N ha- i was not, therefore, removed by the above-ground crop compared with 87-164 kg ha- ~when 224 kg N ha- i was applied. Some ofthe unrecovered N may have volatilized following the preplant surface application of NH4NO3 on an alkaline soil surface (pH 8.1 ). These losses were probably small, because the reaction product, Ca (NO3)2, is soluble and inhibits continued formation of volatile compounds (Fenn and Kissel, 1973), and fertilizers were incorporated immediately after application. The possibility of fertilizer N fixation within clay structures is remote given that Vertisols usually contain little vermiculite (Dudal and Eswaran, 1988). Most of the unrecovered N was probably immobilized by the stable organic matter fraction and/or denitrified in the heavy-textured soil. There were possibly two errors in FNUE estimations. First, soil N mineralization may have been greater in field plots where stover had been removed. Soil organic matter (SOM) levels declined in field plots where stover applications were less than 5 Mg ha- l causing a net increase in soil N mineralization (Powell, 1989). This increase in soil N availability perhaps increased soil N uptake from unfertilized plots, consequently underestimating FNUE. Second, differences in the amount of stover N applied to the plots may have caused FNUE values to be overestimated. At a given stover return rate, the amount of stover N returned to plots where fertilizer N was applied was greater than in plots where no fertilizer N was added. This additional stover N possibly increased N uptake in fertilized plots causing FNUE to be slightly overestimated. Fertilizer N and stover return effects on nutrient crop removals

Although there were distinct genotypic differences in dry matter yields and partitioning (Table 1 ) the four sorghum cultivars assimilated similar amounts of N, P and K. Estimated average annual uptake of N, P and K by the grain sorghum cultivars (ATx623 × RTx430, ATx623 ×'Rio', ATx623 ×'Hegari') averaged across all treatment levels during the study period was 112 kg ha- l,

206

J.M. P O W E L L A N D F.M. H O N S

22 kg ha- ' and 176 kg ha- t, respectively. Total annual analyzed nutrient uptake for the forage sorghum cultivar ('Grassl') was 120 kg ha- ', 22 kg haand 226 kg ha- ' of N, P and K, respectively. Fertilizer N highly influenced N, P and K removal and partitioning into grain and stover throughout the study period. The effects of stover return on nutrient uptake and partitioning varied according to the nutrient, genotype, fertilizer N level and year of the study. In order to facilitate description of 4 years of results, data will be presented on a yearly and a genotypic basis.

Effects offertilizer N on conventional grain sorghum nutrient uptake and partitioning The relationships between fertilizer N application and total N, P and K removal and partitioning by ATx623 × RTx430 were variable over years (Table 3). Except for linear crop N and P uptake functions in the first year of the study, quadratic equations best described the relationship between fertilizer N application rate and the amounts of N, P and K removed by the aboveground portions of the crop. Quadratic functions are often representative of the response of a cereal crop to various levels of a particular nutrient (Melsted and Peck, 1977; Marschner, 1986). The quadratic functions indicated that either maximum uptake had been achie~d, that other nutrient deficiencies may have impeded further uptake, or that nutrient imbalances may have occurred with increasing N. Linear relationships for total N and P uptake in 1985 indicated that ATx623 X RTx430 continued to remove these nutrients TABLE 3 Regression equations and partitioning percentages for predicting ATx623 × RTx430 N, P and K uptake from fertilizer N rate Year

Nutrient

Regression equation t

Rz

Partitioning percentage 2

1985 1985 1985 1986 1986 1986 1987 1987 1987 1988 1988 1988

N P K N P K N P K N P K

yffi 34.23 + 0.52x y - 9.98+0.04x

0.94 0.51 0.77 0.87 0.81 0.84 0.80 0.50

63 37 14 67 47 18 47

0.84 0.44 0.36

60

yf53.17+O.67x-O.OO13x 2 yf48.78+O.70x-O.OOI3x 2

y = 15.41 + 0 . 1 5 x - 0 , 0 0 0 4 x ' y = 6 7.30 + O.83x-O,OO2x 2

y--54.3 +0.63x-0,0012x' y = 12.80+0.07x-0,0001x 2

71 67 20 74 69 21 60

59 60 18 66 71 18 52

3

3

yffi60.38+O.92x-O,OO29x 2

y - 18.91 +0.09x-0,0004x 2 Yffigo.og+o.52x-O,OOlSx 2

ty, Nutrient; x, fertilizer N application rate (kg ha - ~). -'Mean percentage of total nutrient contained in She grain at 0, ! 12 and 224 kg N ha- *. 3Significant (P< 0.05 ) fertilizer N and stover return interaction. All regression coefficients significant at the P < 0.05 level.

69 3 3

53

FERTILIZER N AND STOVER REMOVAL EFFECTS ON SORGHUM

207

at the highest level of fertilizer N. Significantly greater ( P < 0.01 ) yields were obtained with 224 kg N ha -I in 1985 (Fig. 1 ). Lack of further yield and N uptake may have arisen from untimely fertilizer N application, nutrient imbalance (s) etc. Over the 4 study years, the quantity of N assimilated by ATx623 × RTx430 was closely related to fertilizer N rate. Except for the second year ofthe study, relationships between P and K uptake and fertilizer N were more variable. The reasons for the differences in N and P uptake may be associated with the relative accessibility of these nutrients to plant roots. Nitrate-N is transported with soil water by mass flow towards the roots at much faster rates than the advancement of root tips (Melsted and Peck, 1977), whereas soil P moves primarily by diffusive forces. Relative partitioning of total N, P and K into grain and stover depended largely on the fertilizer N level (Table 3). When fertilizer N was applied, 5274% of the total N removed by ATx623 × RTx430 above-ground DM was contained in the grain. The fertilizer application rate of 224 kg N ha- I resulted in lower nutrient partitioning into grain than when fertilizer applications were 112 kg N ha- i. Relative P partitioning into grain from plots receiving fertilizer N was similar to N partitioning and ranged from 60 to 71%, whereas almost all K (79-86%) was contained in stover. Prediction of K uptake, and partitioning of P and K into grain and stover depended on both fertilizer N and stover return rates during the last 2 study years. Fertilizer applications of 224 kg N ha-i in conjunction with stover return significantly ( P < 0.05) increased K uptake by ATx623 × RTx430 in 1987. Grain yields, and P and K uptake during 1988 were increased in plots where 112 kg N ha- ! and 50% stover were applied.

Effects offertilizer N on intermediate-type sorghum nutrient u ntake and partitioning Relationships between fertilizer N application and total N and K uptake by ATx623 ×'Rio' (the intermediate-type sorghum cultivar used in the first 2 study years) were fairly consistent, but were more variable for P uptake (Table 4). Linear relationships for N uptake during both years indicated that maximum N uptake may not have been attained. Because FNUE was relatively low for this intermediate-type sorghum cultivar (Fig. 2), and yields were increased significantly by fertilizer N applications of 224 kg ha- ~ (Fig. 1 ), it is possible that fertilizer N applications either did not coincide closely with crop demands and/or relatively large fertilizer N losses occurred via denitrification and/or immobilization. Although average harvest indices for this intermediate-type sorghum cultivar (28%) were much lo~er than for the conventional sorghum cultivar (44%), the relative partit:~oning of total N and P into grain and stover was

J.M. POWELLAND F.M. HONS

208 TABLE 4

Regression equations and partitioning percentages for predicting ATx623×'Rio' (i 985 and 1986) and ATx623×'Hegari" ( 1987 and 1988) N, P and K uptake from fertilizer N rate Year

Nutrient

Regression equation t

R2

Partitioning percentage 2

1985 1985 1985 1986 1986 1986 1987 1987 1987 1988 1988 1988

N P K N P K N P K N P

y= y= y= y= y= y= y= y= y= y=

0.86 0.53 0.74 0.90 0.76 0.67 0.96 0.77 0.80 0.85 0 37

34 16 3 47 32 6 16 II ! 24 15

K

3

23.41 +0.47x 7.86 +0.08x-0.0002x 2 60.52+0.8 l x - 0 . 0 0 0 2 x 2 54.14+0.52x 18.39+0.1 $x-0.0004x 2 119.6 +0.77x-0.0017x 2 59.08+0.55x 18.70+0.13x-0.0003x 2 185.3 + 1.41x-0.0042x z 40.82+0.65x-0.0013x 2 y = 14.94+0.08x-0.0003x 2

69 57 9 62 57 12 44 40 5 49 53

56 52 8 55 58 12 44 46 6 44 54

I.v, Nutrient; x, fertilizer N application rate (kg ha-I ). 2Mean percentage oftotal nutrient contained in the grain at 0, I ! 2 and 224 kg N ha- ,. -~Significant (P<0.05) fertilizer N and stover return interaction. All regression coefficients significant at the P < 0.05 level.

somewhat similar for both genotypes.Of the total N and P taken up in N fertilized plots, 55-69% (intermediate-type) and 52-74% (conventional) was removed in grain. Given the relatively large quantities of stover produced, most of the K assimilated by ATx623 ×'Rio' went into stover production. There were yearly differences in relationships between fertilizer N and N, P and K uptake and partitioning by ATx623 ×'Hegari', the intermediate-type sorghum cultivar used in the last 2 study years (Table 4). Correlations between fertilizer N application and N, P and K removal were high in 1987, indicating that in years when this genotype was responsive to the fertilizer N application rate (Fig. l ), nutrient uptake could be accurately estimated from the amount of fertilizer N applied. During the last year of the study, when stover return had a negative effect on yield, correlations between applied fertilizer N and nutrient uptake were poorer, especially for P. Potassium uptake was reduced (P<0.01) in plots where stover was applied in the absence of fertilizer N. Nutrient partitioning into grain was lower for ATx623 ×'Hegari' than for ATx623 ×'Rio'. Of the total N uptake by ATx623 ×'Hegari' in plots where fertilizer N had been applied, some 44-49% was assimilated by grain compared with 40-54% for P. Approximately 95% of total K uptake by N fertilized ATx623 ×'Hegari' was contained in stover.

Effects of fertilizer N on forage sorghum nutrient uptake and partitioning Relationships between fertilizer N applications and total N uptake by "Grassi' were linear during the first and third years of the study, indicating

209

FERTILIZER N AND STOVER REMOVALEFFECTS ON SORGHUM

TABLE 5

Regression equations and partitioning percentages for predicting 'Grassl" N, P and K uptake from fertilizer N rate Year

Nutrient

Regression equationt

R2

Partitioning percentage 2

1985 1985 1985 1986 1986 1986 1987 1987 1987 1988 1988 1988

N P K N P K N P K

y = 37.10+0.63x yffi 10.21 +0.10x-0.0003x 2 y = ! 13.3 + 1.10x-0.0025x' y-- 42.46+ 1.13x-0.0019x 2 yffi 15.65 + 0 . 2 4 x - 0 . 0 0 0 7 x ' y = i 15.8 + 2 . 6 6 x - 0 . 0 0 8 4 x ' yffi 61.05 +0.48x yffi 18.84+0.1 l x - 0 . 0 0 0 3 x ' 3

0.70 0.34 0.71 0.95 0.89 0.87 0.80 0.53

NR NR NR 19 19 4 NR NR

0.45

NR

N

3

P

y = 19.35 +0.14x-0.0004x 2

K

3

ty, Nutrient; x, fertilizer N application rate (kg ha= t ). 2Mean percentage of total nutrient contained in the grain at ! 12 kg N h a - i. NR indicates not recorded or no grain production in these study years. 3Significant (P<0.05) fertilizer N and stover return interaction. All regression coefficients significant at the P < 0.05 level.

that maximum N uptake was not attained during these study years (Table 5). Quadratic equations best described relationships between fertilizer N applications and P and K assimilated by 'Grassl'. Relationships between fertilizer N application and N, P and K taken up in 1986 were very strong, perhaps because this was the only year in which 'Gr~ssl' produced panicles. Panicle formation apparently greatly increased both the quantity and efficiency of N and P uptake. Although the total yields of'Grassl' in 1986 were lower than in other years (Fig. 1 ), N and P removal rates and the FNUE value (Fig. 2) were greatest during this year. Interactions between application rate of fertilizer N and stover return influenced N and K uptake by 'Grassl' in the last 2 years of the study. Although yields were not significantly increased during 1987, higher (P< 0.05) amounts of K were assimilated in plots receiving 224 kg N ha- i and 50% stover than in plots that received other fertilizer N and stover treatments. During the last study year, uptake of N and K was lower (P< 0.05) in plots where stover was added in the absence of fertilizer N, and K uptake was greater (P<0.05) in plots receiving both fertilizer N and stover than in other treatment plots. CONCLUSIONS

Sorghum yields, FNUE values and N, P and K uptake and partitioning var-

210

J.M. POWELLANDF.M.HONS

led with genotype, fertilizer N level, stover return and year of the study. Fertilizer N application rates of I 12 kg ha-, were generally sufficient to produce maximum yield, attain the greatest FNUE value, and for the grain sorghum cultivars, the highest percentage partitioning of N, P and K uptake into grain. The greatest FNUE value was attained by the forage sorghum cultivar which consistently produced greater total DM than the other sorghum genotypes. Intermediate-type sorghum cultivars, such as ATx623x'Hegari', may offer the best economic return in a biogas production system (Laceweil et al., 1986). Grain could be marketed for food or feed and a portion of stover returned to protect the soil environment, permitting some stover removal for energy conversion. Observed increases in yield variability, the decreasing ability of fertilizer N to affect yields, and reductions in FNUE during the latter part of the study may have arisen from the cumulative adverse effects of stover removal on the soil environment. Stover removal apparently caused an increase in soil N mineralization and availability (Powell, 1989), thereby affecting nutrient cycling. The effects ofstover removal on yields and nutrient cycling would probably be more pronounced in coarser-textured soils having lower soil organic matter and available nutrient reserves. The influence of stover removal on .,atrient cycling and overall soil productivity needs to be assessed over a longer term if stover removal is to become an environmentally sound and sustainable production strategy.

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