Man393.pdf

DIVISION S-4—SOIL FERTILITY
& PLANT NUTRITION

Short-Term Effects of Nitrogen Fertilization on Soil Organic Nitrogen Availability

M. B. Vanotti, S. A. Leclerc, and L. G. Bundy*

ABSTRACT

Long-term N fertilization affects soil organic N reserves, N mineral-
ization potential, and crop response to applied N, but little information
is available on the influence of short-term N fertilizer (STN) manage-
ment on soil organic N availability and crop response. This study was
conducted to determine if STN changes soil N supplying capability to
corn (Zea mays L.) after 3 yr of differential N fertilization on a Fayette
silt loam soil (fine-silty, mixed, mesic Typic Hapludalf) in Wisconsin.
Various rates of N fertilizer (0-402 kg N ha~') were applied to corn
in 1983, 1984, and 1985, and their residual effects on corn response
were evaluated in 1986. Soil profile NO3-N levels in spring 1986 were
very low in all plots (48° 4 kg ha~' [90 cm]~"), yet grain yields and
N uptake were significantly increased by STN applications. Corn N
uptake was linearly related to the total amount of N returned to
soil in crop residues during the previous 3 yr. Increased organic N
availability under high STN management was equivalent to a 78 kg
N ha™! rate, or 47% of the N fertilizer required for optimum crop
yields. In aerobic incubations (40 wk) of spring 1986 soil (0-30 cm),
STN additions increased N release only in the first few weeks. Kinetics
of N mineralization were best described by a two-component model
in which the active fraction (Na) of soil organic N was highly correlated
with corn N uptake (r = 0.88). Simulation of field conditions showed
that 95% of Na is available before crop maturity. A phosphate-borate
buffer organic N availability index was significantly and consistently
related to STN treatments. Relative increases in total soil organic N
corresponded with the 3-yr N balance between fertilizer additions and
grain removals, and were about 10 times larger than mineralizable
N. These results indicate that immobilization of excess mineral N into
stable soil organic N during decomposition of crop residues should be
considered in determining the environmental risk of N fertilization.
Although labile organic N is a small fraction of the total fertilizer N
contribution to soil N, its quantification should allow a more accurate

assessment of crop N needs.
Mx SITE-SPECIFIC factors influence the total
amount of N supplied by soil to a cereal crop,
but their relative importance depends on the specific
soil-crop system. For Midwest production situations
where corn follows corn, the most important sources
of available N are usually residual soil NO3-N and N
mineralized during the growing season from soil organic
matter including crop residues and recent organic amend-
ments such as manure (Meisinger, 1984; Schepers and
Mosier, 1991). Efficient use of N fertilizers requires a
careful accounting for all these sources on an annual basis
because they directly affect the amount of supplemental N
needed for optimum crop production, and the year-to-
year variation can be substantial (Bock and Hergert,

Department of Soil Science, 1525 Observatory Dr., Univ. of Wisconsin,
Madison, WI 53706-1299. Research supported by the College of Agricul-
tural and Life Sciences, Univ. of Wisconsin, Madison, through Projects
2764 and 3449, Received 10 Aug. 1994. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 59:1350-1359 (1995).

1350

1991; National Research Council Committee on Long-
Range Soil and Water Conservation, 1993; Vanotti and
Bundy, 1994a,b). Although substantial research effort
has been devoted to find a chemical or biological test
that accurately quantifies labile soil organic N pools
(Bremner, 1965a; Keeney, 1982; Bundy and Meisinger,
1994), a major problem is that the N mineralized or
extracted in laboratory tests is poorly correlated with
field measurement of N availability (Fox and Piekielek,
1984; Meisinger, 1984; McCracken et al., 1989; Hong
et al., 1990; Rice and Havlin, 1993). Inefficient use of
N fertilizer is likely when the N supply from organic
sources is unknown. Efficient N use is important because
N fertilization represents a significant cost to producers,
and because excess N may cause NO}; contamination of
groundwater.

Soil organic matter, including the microbial biomass,
is a dynamic nutrient reservoir that functions as both a
source of and sink for N through the competing effects
of mineralization and immobilization (Boone, 1990).
Addition of N fertilizers to corn under N-limiting condi-
tions leads to increased crop production and, in turn,
increased crop residues. Fertilizer N may enter the soil
organic fraction via crop residues, or by microbial immo-
bilization during decomposition of these residues (Stan-
ford, 1973; Hauck, 1981). When crop residues are re-
turned to the soil, N fertilization can increase organic
N reserves and N-supplying capacity of the soil. This
effect has been documented in long-term cropping studies
after many years of repeated N fertilization (El-Haris
et al., 1983; Janzen, 1987; McCracken et al., 1989;
Jenkinson, 1991; Vanotti et al., 1995). Mineralization
of stored N may in turn influence the N fertilizer needs
of subsequent crops. Results of a continuous corn X N
rate experiment in Wisconsin (Motavalli et al., 1992)
showed that the N furnished from organic sources after
discontinuing long-term (25-yr) N fertilizer additions
significantly increased corn N uptake and grain yield,
and decreased the response to added N during seven
consecutive years in which the effect was evaluated.
Olson and Swallow (1984) found that ~ 50% of the total
N fertilizer (50-100 kg N ha~' yr~') applied to wheat
(Triticum aestivum L.) in a 5-yr field experiment was
still accounted for within the organic matter fraction in
the soil system. Fertilizer N, once incorporated into soil
organic matter, becomes increasingly stable with time,

Abbreviations: STN, short-term N; Na, active N fraction; AC-N, auto-
clave N availability index; PBB-N, phosphate-borate buffer N availability
index; ANI-N, anaerobic incubation N availability index; NFEV, N fertil-
izer equivalent value.


VANOTTI ET AL.: SOIL ORGANIC NITROGEN AVAILABILITY 1351

but may need >5 yr to equilibrate with indigenous soil
N (Allen et al., 1973). Compared with native humus N,
a higher proportion of organic residual N during the
early stages of humification occurs in amino acid and
amino sugar forms (Allen et al., 1973, Smith et al.,
1978), and it is more susceptible to mineralization
(Freney and Simpson, 1969; Legg et al., 1971; Smith et
al., 1978). Though these studies have provided valuable
information on the fate of fertilizer N and potential
availability of organic residual N to crops, only limited
information is available on the short-term effect of N
fertilization on soil organic N availability and crop re-
sponse.

In this study, we evaluated the residual effects of
fertilizer N applied to corn during a 3-yr period on
subsequent corn grain production, N uptake, crop residue
turnover, and soil N mineralization potential. Further,
we evaluated chemical and biological indices for their
ability to detect and predict the effects of STN fertilization
and residue history on N availability and crop response.

MATERIALS AND METHODS
Field Study Description

Field studies to determine the residual effects of STN on
soil N availability and corn production were conducted from
1983 to 1986 at the Univ. of Wisconsin Agricultural Research
Station near Lancaster, WI (42°51'N, 90°42'W). The soil is
a well-drained Fayette silt loam. Corn was grown at the site
for 3 yr prior to initiation of this work.

Fertilizer N was applied for corn at rates of 0, 134, 268,
or 402 kg N ha™' before planting in 1983. The treatments
were arranged in a randomized complete block design with four
replications and included above-optimum N rates to evaluate the
extent of profile NO;-N accumulation and overwinter retention
in subsequent years (Bundy and Malone, 1988). In spring
1984, four N rates (0, 78, 156, and 234 kg N ha™') were
applied in each of the 1983 N treatments by splitting the
original plots. An additional split N treatment of 0, 78, and
156 kg N ha”! was superimposed on each of the 1984 treatments
in spring 1985. This provided a split-split plot design, with
1983 N treatments as main plots, 1984 N treatments as subplots,
and 1985 N treatments as sub-subplots. No fertilizer N was
applied in 1986, the residual year of the experiment, except
for three sub-subplots that were used to determine corn response
to N fertilizer applied in 1986. In this study, N rates of 78,
156, and 234 kg N ha™! were applied to plots that received
low STN treatments (134 kg N ha~' in 1983 and 0 or 78 kg
Nha“! in 1984 and 1985). Nitrogen treatments were broadcast
applied as NH«NO; and were incorporated into the top 15 cm
of soil by tillage within 24 h after application.

Corn production practices and dry matter, grain yield, and
total plant N measurement procedures were previously de-
scribed (Bundy and Malone, 1988). Grain yields are reported
at a grain moisture content of 155 g kg~'. Stover dry matter
was calculated as the difference between total aboveground
dry matter yield and grain yields (0% moisture). Aboveground
corn N uptake and grain N removal were calculated from
the total dry matter and grain yields, respectively, and the
corresponding N concentrations. The amount of stover N re-
turned to soil was calculated as the difference between total
aboveground N uptake at physiological maturity and grain N
removal from the field plot.

160
Ow uptake 140 138
140 130 130°
NITRATE N
ay
101 98 97 ae
8 100
2 80
2 60
La
40|| 44"
20
0

1983 oO i) 0 0| 402) 402) 402) 402
1984 OQ} 23) 234! t) 0} 234) 234)
1985 Oo} 156 ol 156 O| 156: O| 156

NITROGEN RATE (kg/ha)

Fig. I. Short-term N treatment effect on 1986 corn N uptake (zero.
N in 1986) and spring soil NOs-N content (0-90-cm depth). Each
value is the average of four field replicates.

°

Soil Inorganic Nitrogen Availability

To determine the amounts of inorganic N in soil profiles,
preplant soil samples were obtained from each plot in the
spring before N application (16-23 April) in 1983 through
1986, and in the fall after harvest (14-24 October) in 1983
through 1985. Soil samples were taken in 30-cm increments
to a total depth of 90 cm, using procedures described by Bundy
and Malone (1988). Soil samples were stored at —10°C until
they were dried at 30 to 35°C in a forced-draft dryer. The
dry soils were ground to pass a 2-mm screen and stored in
sealed plastic bags for further determinations. Inorganic N
was extracted with 2 M KCI (Bremner and Keeney, 1966),
and NO3-N and exchangeable NH.-N were determined by
automated analysis (Bundy and Meisinger, 1994). The amounts
of NO3-N and NH,-N in 30-cm profile depth increments were
calculated using the assumption that 1 ha (30 cm of soil) weighs
4.48 x 10° Mg. Values for exchangeable soil NHs-N are
not reported because the amounts detected each year were
uniformly low and were not affected by treatment. A detailed
analysis of residual profile NOs-N effects on corn response
to applied N for 1984 was previously reported (Bundy and
Malone, 1988).

Soil Organic Nitrogen Availability

The short-term effects of N fertilization on soil N availability
to corn were evaluated using chemical and biological methods
on surface (0-30-cm depth) soil samples collected in spring
1986 from eight field treatments that received the highest or
zero N rate in each of the previous 3 yr (1983-1985). These
STN rates are shown in Fig. 1. Laboratory analyses were
performed separately on the four field replicate samples from
each N treatment. The chemical methods used were: (i) total
soil organic N, (ii) NH,-N hydrolyzed by 0.01 M CaCb after
overnight (16 h) autoclaving at 121°C (Keeney, 1982), and
(iii) steam distillation with pH 11.2 phosphate-borate buffer
(Gianello and Bremner, 1988). The biological methods used
were: (i) NHs-N produced during a 1-wk waterlogged (anaero-
bic) incubation (Keeney, 1982), and (ii) inorganic N production
during long-term (40-wk) aerobic incubation (Bundy and Mei-
singer, 1994). Values reported for the autoclave (AC-N), the
phosphate borate buffer (PBB-N), and the anaerobic incubation


1352 SOIL SCI. SOC. AM. J., VOL. 59, SEPTEMBER-OCTOBER 1995

(ANI-N) indices were calculated by subtracting initial soil
exchangeable NH,-N from the total NH,-N extracted by these
methods. Inorganic N production during aerobic incubation
was determined by leaching with 100 mL of 0.01 M CaCh
and 25 mL of a minus-N nutrient solution (Stanford and Smith,
1972) after 0, 1, 2, 3, 4, 6, 8, 11, 14, 18, 22, 26, 30, 34,
and 40 wk of incubation at 35°C and a soil water tension of
80 kPa. Total soil organic N was measured on soil, crushed
to pass a 0,15-mm (100-mesh) screen, using tube digestion
(Nelson and Sommers, 1972) and colorimetric analysis of
NH.-N in the digest (Schuman et al., 1973).

We used the autoclave, phosphate-borate buffer distillation,
and anaerobic incubation procedures with the goal of obtaining
a rapid, relative indication of N availability that may be used
as a test to predict available N released from soil organic N
under field conditions. Long-term incubations were performed
to provide the base information needed to interpret data from
both field and N availability index measurements. Total soil
organic N, viewed as a net source of or sink for N, provided
valuable information on the fertilizer N balance in soil.

Several mineralization models were applied to the cumula-
tive net N mineralization data to describe changes in soil
organic N pools as affected by N fertilization histories. The
first-order model (Stanford and Smith, 1972), shown in prod-
uct-appearance form, is the single exponential equation

Nm = No[1 — exp(—kot)] (]

where Nm is the amount of N mineralized at time #, N, is the
initial amount of potentially mineralizable N, and k, is the
specific rate of decomposition. Because net N mineralization
may reflect the contribution of several soil organic N pools with
differing susceptibilities to decomposition, several researchers
have incorporated a more labile pool of N, into Eq. [1]. This
is illustrated by the double exponential equation (Molina et
al., 1980)

Nn = Nall — exp(—At)] + Nel — exp(-k)] [2]

and a special case of the two-component model (Bonde and
Rosswall, 1987)

Nm = Nall — exp(—Ad)] + Ct 3]

where Na and Ne represent the more available and more
recalcitrant soil organic-N fractions decomposing at specific
rates h and k, respectively, and C in the second term of Eq.
{3] is a zero-order rate constant corresponding to the resistant
fraction (C = Nek). Equation [3] is most appropriate for
describing the mineralization kinetics of data showing a rapid
curvilinear increase in cumulative N mineralization followed
by a slow linear increase that persists to the end of the incubation
experiment.

Statistical Analysis

Equations [1] to [3] were fit to N mineralization data by
nonlinear regression analysis (SAS Institute, 1988). The miner-
alization model giving a valid description of the data was
selected on the basis of mean square error and F tests as
described by Deans et al. (1986). Analysis of variance was
used to determine significant differences among the various N
rate treatments. Significant treatment effects were evaluated
using linear correlation and regression analysis (Draper and
Smith, 1981).

RESULTS AND DISCUSSION

Nitrogen Fertilization Effect on Crop
Productivity and Soil Nitrate

The effects of N applied during 1983, 1984, and 1985
on corn grain and stover dry matter production, stover
N concentration, and profile NO3-N levels found at
harvest (fall sampling) and in the following spring are
shown in Table 1. Grain yields were significantly in-
creased by applied N in all years. Total N uptake and
the amount of N returned to soil in aboveground crop
residues were usually increased by applied N. Although
substantial amounts of residual NO3-N were found in
soil at harvest time in all 3 yr, the relative amounts of
profile NO3-N that remained within the soil profile over
winter varied considerably among years. Equations in
Table 1 indicate that ~ 45% of the profile NO3-N found
in fall 1983 was recovered in spring 1984, but only ~ 14
and 6% of the fall NOs-N were recovered in the spring
of 1985 and 1986, respectively. The minimum profile
NOs-N level of 40 to 50 kg N ha~' observed in this
study is similar to a “background” residual N level usually
found in medium- to fine-textured soils (Schepers and
Mosier, 1991; Bundy and Meisinger, 1994).

Nitrogen losses through leaching and denitrification,
or transformation into organic compounds, may explain
the substantial decreases in profile NO3 observed during
the period between growing seasons. Wet conditions
during this overwinter period in all 3 yr of the study
favored NO; losses from the root zone (October-April
precipitation in 1983-1984, 1984-1985, and 1985-1986
was 368, 487, and 358 mm, respectively, relative to a
30-yr mean of 342 mm), especially during the fall 1984
to spring 1985 period. Alternatively, a significant fraction
of fall NO3-N may have been incorporated into the soil
organic N fraction during the decomposition of corn
residues, thus limiting the potential for NO3 leaching.
The possibility of immobilization of the residual mineral
N found in the fall and overwinter retention in soil
organic matter is supported by the relatively low N
concentration of the corn residues returned to the soil
in all 3 yr (Table 1). Although N fertilization increased
stover N concentration, the values found were usually
below the established threshold N content of 10 to 20 g
kg~! that leads to depletion of mineral N in the soil
during residue decomposition (Iritani and Arnold, 1959;
Bartholomew, 1965; Allison, 1973; Vigil and Kissel,
1991).

We estimated the amount of additional N needed for
decomposition of corn residues (above that contained in
these residues) based on the following assumptions: (i)
acritical residue N concentration of 16 g kg™' is required
to satisfy the needs of soil microorganisms during most
crop residues’ decomposition (Paul and Clark, 1989, p.
137), and (ii) roots comprise 20% of the total plant
biomass and N concentrations in roots and stover are
equal (Stanford, 1973). The results of these calculations
(Table 1) show that, except for the low N treatments,
the amounts of soil NOs available at the end of the 1983-
1985 growing seasons were adequate to meet the N
demands of microorganisms. These results also suggest


VANOTTI ET AL.: SOIL ORGANIC NITROGEN AVAILABILITY 1353

Table 1. Effect of N applied in 1983, 1984, and 1985 on corn grain yield, total N uptake, stover N returned to soil, and the amount of
soil profile NOs-N in the fall and the following spring.

Profile NO)-N

Stover . (0-90 cm)
Inorganic N
Grain Total Dry needs for residue Following
N rate yield N uptake matter N N concentration decompositiont Fall spring
kg ha-! Mg ha“! kg ha~' Mg ha-? kg ha-! ekg kg ha-* —— kg ha!
1983
0 5.15 118 7.08 43 6.1 98 52 63
134 6.64 119 6.15 26 3.8 120 93 17
268 5.80 134 8.33 49 38 119 193 167
402 6.13 162 8.26 70 8.0 93 358 213
P> Ft 0.02 0.11 0.08 0.22 0.27 0.01 0.02
CV, % 9 19 24 58 48 50 39
19848
0 7.63 137 8.02, 45 5.5 122 4 7
8 8.53 165 8.39 55 6.5 7 it 4
156 8.46 181 8.35 66 78 100 157 80
234 8.73 176 174 56 12 102 147 0
PDF 0.01 0.01 0.50 0.05 0.01 0.01 0.01
CV, % 8 u 17 35 28 az 39
19854
0 7.07 158 1.58 16 9.6 70 39 45
B 7.84 188 8.50 93 10.4 69 93 47
156 1:99 209 8.76 107 12.3 47 121 52
PDF 0.01 0.01 0.01 0.01 0.01 0.01 0.01
CV, % 15 2 25 41 35 10 35

Relationship between the amount of profile NOs-N found in spring and fall:
NOs-N spring 1984 = 61.2 + 0.448 NOs-N fall 1983 r? = 0.76** (n = 16)
NOs-N spring 1985 = 54.7 + 0.141 NOs-N fall 1984 r? = 0.14¢* (n = 62)
NOs-N spring 1986 = 41.9 + 0.062 NOs-N fall 1985 _r? = 0.14** (n = 192)

+* Significant at the 0.01 level.
+ Additional N required by soil microorganisms to decompose stover and roots.

+P > F = probability that tabular F ratio exceeds F ratio calculated by analysis of variance.

§ Averaged across 1983 N treatments.
{Averaged across 1983 and 1984 N treatments.

that the supplemental amounts of N required by microor-
ganisms for residue decomposition are similar across N
fertilization rates. Bartholomew (1965) noted that this
effect should be expected because, as shown in our data,
N fertilization of cereal crops generally increases both
the total residue production and the N content of the
residue material, which compensate each other. This
effect may limit residue N turnover and N fertilizer
availability to the following crop in soils with relatively
low soil NO;-N levels receiving low N rates (Table 1).

Residual Effects of Short-Term Nitrogen
Fertilization on Crop Productivity

The residual effects of 1983, 1984, and 1985 N fertil-
ization on 1986 corn response (unfertilized plots) are
shown in Table 2. Corn grain and N yields were signifi-
cantly increased by STN treatments. However, the quan-
tities of profile NO3-N found in 1986, and its variation
among STN treatments, were too small to account for
the observed crop responses (Fig. 1, Table 2), suggesting
that N fertilizer applied in the preceding 3 yr contributed
significantly to the soil’s mineralizable N fraction released
in 1986. Comparison between corn N uptake from fertil-
ized (Table 3) and unfertilized (Fig. 1) plots allowed
estimation of the fertilizer value of this labile pool of
soil organic matter. We found that the average net amount
of N mineralized during the 1986 corn growing season
in those soils that received high STN treatments was

equivalent to ~78 kg N ha“', or 47% of the observed
N rate required for optimum crop yields (167 kg ha~').

Table 2. Residual effects of 1983, 1984, and 1985 N rates on 1986
corn grain yield, plant N uptake and concentration, and spring
profile soil NO-N content.t

Grain Total N Plant N Profile NOs-N

N ratet yield uptake concentration _spring 1986§

kgha-'  Mgha"'—kg hat ekg" kg ha~!
Main effect of 1983 N rates!

0 105 16 45

134 114 19 46

268 127 8.5 52

402 126 8.4 48

P> Fe 0.01 0.01 0.01
Main effect of 1984 N rates§

0 5.56 116 18 44

8 5.43 112 19 47

156 5.48 114 8.0 49

234 6.38 129 8.5 50

P>F 0.01 0.01 0.01 0.02
Main effect of 1985 N rates!

0 5.10 105 18 45

8 5.62 417 8.0 47

156 6.32 129 8.5 52

P>F 0.01 0.01 0.01 0.01

+CV = 15, 21, 12, and 21% for corm grain yield, total N uptake, plant
N concentration and profile NO;-N, respectively.

No fertilizer N applied in 1986.

§ Profile NO;-N to the 90-cm depth.

{Main effects are averages across N rates applied in the other 2 yr.

#P > F = probability that tabular F ratio exceeds F ratio calculated by
analysis of variance.



1354

Table 3. Effect of N applied in 1986 on corn grain yield and total
N uptake.

1986 Profile NO;-N Grain Total N Plant N
N rate spring 1986 yield uptake concentration
kg ha~! kg ha! Mgha"! kg ha" pkg"

0 36 4.35 101 78

8 4 6.92 137 78
156 4B 8.97 206 10.0
234 40 9.39 223 11.4
P>Ft 0.01 0.02 0.01
CV, % 4 29 18

Selected regression equation:
Grain yield (Mg ha~!) = 4.43 + 0.0296 N rate (kg ha~') if N rate < 167
Grain yield (Mg ha~') = 9.37 if N rate = 167 7? = 1.00%" (n = 4)

** Significant at the 0.01 level.

+P > F = probability that tabular F ratio exceeds F ratio calculated by
analysis of variance.

} Linear-plus-plateau response model.

This observation illustrates the potential effect of short-
term N fertilization on site-specific variation of soil N
supplying capacity and fertilizer needs of subsequent
crops. It also emphisizes the need for a laboratory test
that can provide an index of the availability of soil N
and allow refined assessment of crop N needs.

Residual Effects of Short-Term
Nitrogen Fertilization on Soil
Nitrogen Mineralization

Table 4 shows the effect of STN treatments on N
mineralized during long-term aerobic incubation of
spring 1986 soil samples (0-30-cm depth). Significant
differences between STN treatments usually occurred
after the first 4 wk of incubation. The two-component
mineralization model (Eq. [3]) offered the best descrip-
tion of the kinetics of N mineralization on data for all
STN treatments evaluated. Optimized parameter values
thus obtained (Table 4) show that differences in N miner-
alization between STN treatments are largely due to
contributions from a readily available N pool (Na) of

SOIL SCI. SOC. AM. J., VOL. 59, SEPTEMBER-OCTOBER 1995

400
3 350 : } i
000 73 Fy
@ 2 i 3?
N 200 i i 3 :
E 150 at
B00 moe N treatments (3 yr)
5 a = High N rate
z 50; st © Low Nrate

0

0 10 20 30 40

INCUBATION TIME (weeks)

Fig. 2. Residual effects of short-term N fertilization on soil N mineral-
ization during aerobic incubation at 35°C and 80 kPa soil water
tension. Data show cumulative net mineralization (mean values +
standard error, n = 4) of soils (0-30-cm depth) sampled in spring
1986 that received the highest (792 kg ha~') or lowest (0 kg ha~')
N additions from 1983 to 1985.

uniform composition, as revealed by the absence of
significant differences in the associated rate of decompo-
sition, h. In contrast, the rate constant C, representing
the more resistant mineralizable N pool, was not affected
by STN treatments (Table 4). This is also illustrated in
Fig. 2, which shows that differences in soil N mineraliza-
tion between STN treatments became nearly constant
after about 8 wk of incubation. Bonde et al. (1988)
provided evidence that microbial biomass constitutes a
significant part of the potentially mineralizable N pool.
They reported mineralization rate constants for the avail-
able microbial biomass (hy = 0.36-0.61 wk~') similar
to those corresponding to the available fraction, Na, of
soil organic N (# = 0.45-0.56 wk"'). These rate con-
stants are essentially the same as those obtained in our
study using comparable methodology (Table 4).

Our finding that N fertilization contributes largely to
Na with little effect on the more resistant mineralizable

Table 4. Effect of 1983, 1984, and 1985 N fertilizer treatments on soil N mineralization in laboratory incubations and on the available

N fraction of soil organic matter.+

Cumulative net N mineralization (Noin)+

Nain = Na {1 — exp(— Ad] + C$

. N rate 1 wk 2 wk 3 wk 4 wk 6 wk 8 wk 40 wk Na h c
kg Nha“! wk-! kg N hat! wk-!

Main effect of 1983 N rates

0 37 6 4 % us 135 313 98 0.42 534

402 4 9 2 108 127 150 345 125 0.42 37

P> FR O11 0.14 0.08 0.02 0.01 0.01 0.01 0.01 0.98 0.38
Main effect of 1984 N rates

0 37 60 9 96 us 136 322 106 0.42 55

234 41 2 96 mi 128 150 336 uT 0.42 5.6

P>F 0.04 0.01 0.01 0.01 0.01 0.01 0.09 0.12 0.98 0.33
Main effect of 1985 N rates

0 38 “a 84 ” us 136 319 101 0.44 5.6

156 40 68 n 109 128 149 339 122 0.40 3.4

P>F 0.16 0.27 0.12 0.01 0.01 0.01 0.03 0.06 0.51 0.27

cv, % 15 17 15 u 6 8 7 21 4B 20

} Soil samples collected in spring 1986, 0- to 30-cm depth.

Cumulative net N mineralization by aerobic leaching-incubation procedure.
§ Equation [3).

{/Main effects are averages across N rates applied in the other 2 yr.

#P > F = probability that tabular F ratio exceeds F ratio calculated by analysis of variance.


VANOTTI ET AL.: SOIL ORGANIC NITROGEN AVAILABILITY 1355

Table 5. Linear correlation coefficients (r) between grain yield
and total N uptake of 1986 unfertilized corn or N residue
history, and soil N mineralization (x = 32).t

‘Total N returned to soil

in rep resid
Grain _N uptake ore

yield 1986 1986 1985 only 1983-1985

NarNt 0.85, 0.88 0.63 0.75
Cumulative net N mineralization

1wk 0.55 0.75 0.48 0.64

2wk 0.58 0.74 0.59 0.71

3 wk 0.71 0.78 0.66 + 0.81

4wk 0.78 0.79 0.68 0.80

6 wk 0.75 0.72 0.66 0.77

8 wk 0.74 0.76 0.59 0.76

40 wk 0.59 0.67 0.43 0.71

Selected regression equations:§
Grain yield = 1.796 + 0.0357 Na-N
N uptake = 24.5 + 0.855 Na-N
Grain yield = — 0.203 + 0.057 N mineralized at 4 wk
N uptake = — 36.0 + 1.492 N mineralized at 4 wk
NaN = 45.5 + 0.302 stover N (3 yr)
'N mineralized at 4 wk = 64.8 + 0.190 stover N (3 yr)

+ 1986 soil samples, 0- to 30-cm depth. All correlation coefficients significant
at the P = 0.01 level.

+ Available soil N fraction, Eq. (3]-

§ Grain yield in Mg ha~'; 'N uptake, N,N, stover N, and mineralized N
in kg ha~'.

N fraction (Table 4) is consistent with results obtained
by El-Haris et al. (1983) and Bonde and Rosswall (1987)
in studies of the influence of previous N fertilization
(4-8 yr) on soil mineralization. However, much longer
studies (25 yr) in Wisconsin (Vanottiet al., 1995) showed
a marked effect of N fertilization history on the resistant
mineralizable N pool as well.

The Na soil fraction derived from laboratory incuba-
tion data was highly correlated with the field-measured
N supplying capability of soil, which is represented by
the total N uptake of unfertilized corn (Table 5, Fig. 1
and 3). The slope coefficient in this linear relationship
suggests that 85% of the Na fraction contributed directly
to the N needs of the growing crop (Table 5). The role
that Na has in corn N nutrition was further evaluated

2
3

139 134.
125 320 Fo
109

ss é

87°90" 89

©
i]

POOL, 1986 (kg N/ha)
2
3

Na

1983 i) 0 0 0| 402) 402; 402| 402)
0} 234) 234) 0 O| 234) 234
156| O| 156) 0} 156 oO] 156

NITROGEN RATE (kg/ha)

Fig. 3. Short-term N treatment effect on the N content of a readily
‘available soil organic fraction (N,) derived from laboratory incuba-
tion data (0-30-em soil collected in spring 1986). Each value is the
average of four field replicates.

g

o
@
a
°

1.0
0.9} — N,pool mineralized
Zz 08 s+ corn N uptake
O 07
i
uw 06 , i
rol 1, Soil sampling
z 08 2, Planti
Ba ses
. Physiological
Q 03 maturity
& 0.2
oit 2 i
0
‘Apr | May’ Jun’ Jui ' Aug ' Sep
1986 CORN GROWING SEASON

Fig. 4. Computed N turnover of the readily available soil organic
'N (Na) pool vs. corn N uptake demands, based on mean daily
temperatures observed during 1986.

by simulating the dynamics of this component under field
conditions. For this purpose, the laboratory-obtained rate
constant of mineralization h (mean at 35°C = 0.42 wk"!
or 0.06 d=!) was adjusted for lower-than-optimum field
temperatures using a Qto (temperature coefficient) of 2
between 0 and 35°C (Stanford et al., 1973), with the
following equation:

h'= h 1O(~ 1-053605 + 0.0301037) {4]

where T is the observed mean daily temperature and
h’ is the corresponding (adjusted) mineralization rate
constant. The portion of Na mineralized under field
conditions during the 1986 growing season was estimated
on a daily basis using Eq. [4] and the computational
method described by Stanford et al. (1973). As reference,
we also estimated the N uptake demand function for
1986 using the equation given by Watts and Hanks
(1978), adjusted for the actual growing degree days
(base 10°C) observed from 1 May (planting) through
17 September (physiological maturity). Results of these
calculations (Fig. 4) indicate that 95% of the N contained
in the Ng fraction would be mineralized before the crop
reached physiological maturity, and most of the N re-
leased from this fraction was readily available to meet
crop demands during the period of rapid N uptake in
July and August.

The relationship between residue N returned to soil
and N mineralization in aerobic incubations or the Na
pool was better described when the residue contributions
from all 3 yr were considered (Table 5), and total N
recycled through crop residues was a good indicator of
N availability per se (Fig. 5). Cumulative N mineraliza-
tion was also highly correlated with the N supplying
capability of soil (N uptake in 1986), the correlation
improving with increased time of incubation up to 4 wk
(Table 5). Additional N released after 4 wk diminished
rather than augmented the predictability of N availability
under field conditions. Similar trends were noted for
1986 corn grain yield and for crop residue history (Table
5). These results were expected, based on the apparent
synchrony found between corn N needs and turnover of
the Na pool under field conditions (Fig. 4), and the fact
that this pool is a mathematical representation of the


re
3
s

a
3

2
3

— y=4440.97«
r= 0.82

CORN N UPTAKE, 1986 (kg ha”) g
a
3

50 100 150 = 200 250 300 350
STOVER N RETURNED, 1983-1985 (kg ha")

Fig. 5. Relationship between corn N uptake with no N applied in 1986
‘and the total amount of N returned to soil in the crop residues
during the previous 3 yr.

mineralization data obtained during the initial stages of
incubation (Fig. 2). Thicke et al. (1993) also reported
that N released during the initial 1 to 2 wk of incubation
was best correlated with N uptake of unfertilized corn
at four Minnesota locations. These findings support the
assertion (Bundy and Meisinger, 1994) that the N re-
leased during the first few weeks of incubation probably
reflects the readily mineralizable or active fraction of
soil organic matter, while that released later originates
from a much larger but more stable soil organic matter
pool. Therefore, the pattern of N mineralization during
the first few weeks of incubation is critical for understand-
ing the N availability status of soil.

Soil Organic Nitrogen Availability Indices

Low profile NOs levels in spring 1986 created optimum
conditions to evaluate soil organic N availability tests,
since these tests need to be calibrated against N-supplying
capability under field conditions, and the presence of

Table 6. Effect of previous N fertilization on three indices of soil
organic N availability, 1986.+

Organic N availability index}

Anaerobic

incubation Autoclave Phosphate-borate
N rate (ANLN) (AC-N) buffer (PBB-N)
Kg ha"!

Main effect of 1983 N rates§

0 139 356 n

402 194 347 90

P>FY 0.01 0.68 0.01
Main effect of 1984 N rates

0 167 354 8

234 168 349 87

P>F 0.99 0.81 0.01
Main effect of 1985 N rates§

0 175 351 16

156 158 351 86

P>F 0.37 0.99 0.04

+ Soil samples collected in spring 1986, 0- to 30-cm depth.

CV = 31, 18, and 15% for ANI-N, AC-N, and PBB-N, respectively.

§ Main effects are averages across N rates applied in the other 2 yr.

{P > F = probability that tabular F ratio exceeds F ratio calcuiated by
analysis of variance.

SOIL SCI, SOC. AM. J., VOL, 59, SEPTEMBER-OCTOBER 1995

high amounts of residual inorganic N in the soil may
overshadow the contributions of N mineralized during
the growing season. The only N availability index that
showed significant and consistent effects of past N fertil-
izer management was the PBB-N (Table 6). The ANI-N
test was significantly affected by 1983 N rate treatments,
but produced anomalous responses to 1984 and 1985 N
rate treatments in view of the significant N uptake re-
sponses found among these treatments (Table 2). Further,
the ANI-N and AC-N indices were not related with
crop yield, crop N uptake, residue history, or aerobic
mineralization measurements (Table 7, Fig. 6a). In com-
parison, the PBB-N index was highly correlated with both
field indicators of N availability and laboratory-measured
labile fractions of soil organic matter (Table 7, Fig. 6b).

The PBB-N values ranged from 32 to 107 kg N ha"!
(0-30-cm depth), and its linear relationship with crop
yield parameters reflects the fact that all observations
obtained in 1986 in the unfertilized plots occurred within
the responsive zone. For example, grain yield and N
uptake values in zero N rate plots ranged from 3.55 to
7.92 Mg ha”! and 68 to 193 kg N ha™', respectively,
which are lower than values obtained at non-limiting N
rates applied in 1986 (9.39 Mg ha“! and 223 kg N ha“ ';
Table 3). The N fertilizer equivalent value (NFEV) for
PPB-N was estimated from the yield response equation
in Fig. 7 and the yield response function for fresh N
rates in Table 3. The resulting algorithm, NFEV =
1,64(PBB-N — 54), provides a method for crediting
PBB-N test values against current N fertilizer recommen-
dations. For example, an N fertilizer credit of 75 kg
ha™! is anticipated for a PBB-N test value of 100 kg
ha~' (0-30 cm). This credit can then be used to adjust
N recommendations for soil organic N contributions. It
is not our intent to provide an algorithm with general
applicability, since information from numerous soils and
environments is required to develop such relationship,
but rather to indicate the feasibility of using a laboratory
soil test to predict N mineralization under field condi-
tions, and to provide a possible method of evaluation.
In this respect, the PBB-N test appears promising for
predicting changes in N-supplying capability of soil in-
duced by short-term N fertilizer management in corn
fields.

The PBB-N test (Gianello and Bremner, 1988) is based
on the findings (Tracey, 1952; Stevenson, 1982a) that
the N in glucosamine is converted quantitatively to NH3
when this compound is steam distilled with pH 11.2
phosphate-borate buffer for 3 min. Studies of N transfor-
mation during microbial decomposition of plant materials
have shown that this process is accompanied by synthesis
of hexosamines (Bremner, 1965b; Allen et al., 1973;
Namdeo and Dube, 1973), and that most of this substance
in soil is of microbial origin (Stevenson, 1982b). There-
fore, it is possible that PBB-N selectively measures the
microbial products of plant decomposition, including
the structural components of fungal mycelia (chitin, a
polymer of glucosamine). This possibility is supported
by the close relationship we obtained between PBB-N
test values and the Na fraction (Table 7), which has a


VANOTTI ET AL.: SOIL ORGANIC NITROGEN AVAILABILITY

1357

Table 7. Linear correlation coefficients (r) between yield and N uptake of unfertilized corn, N residue history, available soil organic N
fraction, or N mineralized at 4 wk (aerobic incubation), and three soil indices of N availability (2 = 32).7

Total N returned to soi

in crop residues Cumulative N

Grain yield N uptake mineralized
Availability index 1986 1986 1985 only 1983 to 1985 Na-NE at 4 wk
0.25(NS) 0.15(NS) 0,08(NS) 0.16(NS) 0.11(NS) 0.09(NS)
‘Autoclave (AC-N) 0.24(NS) 0.19(NS) 0.06(NS) 0.09(NS) 0.18(NS) — 0.08(NS)
Phosphate-borate
buffer (PBB-N) 0.76** 0.80" 0.67#* 0.79% one 0.79*

Selected regression equations:

Grain yield = 1.801 + 0.0484 PBB-N
12.9 + 1.327 PBB-N
Na-N = 20.1 + 1.086 PBB-N
Noe (4wk) = 48.0 + 0.697 PBB-N
PBB-N = 36.0 + 0.214 stover N (3 yr)

** Significant at P = 0.01; NS = not significant at P = 0.01.

+ Soil tests performed on 0- to 30-cm depth samples taken in spring 1986.
Na-N = available soil organic N fraction, Eq. (3].

§ Grain yield in Mg ha~'; N uptake, stover N, and soil tests in kg N ha~',

rate of N turnover similar to the microbial biomass
(Bonde et al., 1988).

Total Soil Organic Nitrogen Changes

Data in Table 8 show the 3-yr N balance between N
fertilizer inputs to corn and N removed in harvested
grain, and the total soil organic N measured at the
beginning of the fourth growing season (April 1986).
Total soil organic N levels were consistently increased

= 200} (A) .
g .
2 .
w 150 - oe
=
e e . . °
2 .
> tool ° . 8 . an
z . .
c °
8 ° 1 =0.14NS
50
50 100 = 180-200 280 300
ANI-N (kg ha’)
—~ 2007 (B) .
S .
= — y= 12,95 + 1.327x .
= 150 r= 0.80 .
WwW
x“ °
5
& os
S 100 ° a8
z e “0
Zz
& °
© 50
30 40 50 60 70 80 90 100 110

PBB-N (kg ha”)

Fig. 6. Relationships between corn N uptake with no N applied in
1986 and (A) anaerobically mineralized N or (B) NH,-N extracted
with the phosphate-borate buffer distillation method.

by 1983 and 1985 N rate treatments, but only the effect
of 1983 N rates was statistically significant. In general,
changes in soil organic N followed the N balance between
fertilizer additions and grain removals. For example,
the average difference in total organic N between soils
receiving the highest and zero N rate in 1983 through
1985 was 680 kg N ha~! (0.152 g kg~'), which is similar
to the 689 kg N ha“! difference in net N found between
the same treatments (Table 8). On the other hand, there
was little difference in total N returned to soil with crop
residues to account for the marked changes in soil organic
N. This implies that immobilization of excess fertilizer
N by soil microorganisms was the primary N cycle
process determining the relative increases in soil organic
N and the fate of fertilizer N in this study. Boone (1990)
suggested that only in a system with both energy-rich,
N-poor organic substrates and high inorganic N inputs
should soil organic matter be a net N sink, and such a
system perfectly describes the conditions encountered in
our study (Table 1). n

The relative increases in total organic N were about
10 times larger than the differences in mineralized N
found in the long-term incubation experiment. For exam-
ple, the average difference in N released during the

“B80

2

2

=

9 60

a

>
2

= 40

oO

z — y= 1.80 + 0.0484x
Oo 1=0.76

8 20

30 40 50 60 70 80 90 100 110

PBB-N (kg ha”)

Fig. 7. Relationship between corn grain yield with no N applied in 1986
and NH,-N extracted with the phosphate-borate buffer distillation
method.


1358

SOIL SCI. SOC. AM. J., VOL. 59, SEPTEMBER-OCTOBER 1995

Table 8. Changes in total soil organic N after 3 yr of differential N fertilization (1983-1985).

Total crop residue

N rate returned to soil

1986 soil
1983 1984 1985 Applied Nt Dry matter Grain N Net N§ organic N4

kg ha! Mg ha~! kg ha! g kg"!
0 0 0 40 20.7 (1) 125 (20) 200 (18) - 160 0.94 (0.07)
0 0 156 196 23.0 (0.5) 175 (18) 240 (9) 44 0.97 (0.05)
0 234 0 24 22.9 (1.5) 170 (5) 274 (7) 0 0.94 (0.03)

0 234 156 430 24.8 (1.6) 237 (23) 283 (17) 147 0.97 (0.
402 0 ) 442 23.7 (2.4) 182 (42) 283 (7) 159 1.03 (0.11)
402 0 156 598 24.7 (2.0) 220 (38) 315 (14) 274 1.10 (0.05)
402 234 0 676 26.7 (2.0) 249 (24) 300 (5) 376 1.06 (0.06)
402 234 156 832 26.7 (1.9) 263 (26) 303 (9) 529 1,09 (0.03)

Applied N = N rate plus starter fertilizer (13.3 kg N haé! yr~').
Aboveground measurements.

0- to 30-cm depth; main effects of 1983 N rates on soil organic N are significant at P = 0.01; effect of 1984 and 1985 N rates were not significant; CV.

t
t
iXe N = applied N — grain N harvest.
= 12%.

#

Standard error of the mean of four field replicate samples given in parentheses.

40-wk incubation from soils that received the highest
and zero N rates in 1983 through 1985 was 54 kg ha"!
(Fig. 2), which compares with the 680 kg ha”! difference
obtained for total soil organic N in the same soil samples.
These results are consistent with previous work (Allen
et al., 1973; Hauck, 1981; Olson and Swallow, 1984)
showing that most of the fertilizer N incorporated into
organic substances contributes mainly to a stable, passive
soil N pool where remineralization will probably proceed
for several decades.

CONCLUSIONS

Our results suggest that, in the long term, assessing
the potential of soil-crop systems to incorporate excess
N fertilizer into more stable organic fractions should be
an important consideration. This is critical because the
amount of N fertilizer currently used in U.S. corn produc-
tion exceeds the amount of N harvested in grain by
50 to 60% (National Research Council Committee on
Long-Range Soil and Water Conservation, 1993). Our
N balance shows that most of this net excess N during
a 3-yr period was probably sequestered into stable soil
organic forms during decomposition of plant residues,
limiting the risk of NOs leaching. Thus, to determine
the potential environmental risk of N fertilization, it is
necessary to know the capacity of a soil to immobilize
and store N. Our findings indicate that this capacity may
be enhanced by readily decomposable, N-poor organic
substrates such as corn residues.

To predict crop N fertilization needs, however, the
effect of past N fertilization on labile soil organic fractions
is much more important than its effect on total soil N,
even when labile pools probably represent a relatively
small fraction of the total fertilizer contribution to soil
N. Results from this work indicate that an enhanced
mineralizable N fraction due to short-term N fertilization
practices may substitute for up to one-half the N fertilizer
requirement for optimum yields. Thus, proper account-
ing for this N source could greatly reduce the uncertainty
and risk associated with N fertilizer applications. Our
results show that N released during the first few weeks
of aerobic incubations was well correlated with corn N
uptake. This relationship was not improved by consider-

ing N mineralized in longer term incubations. Our evalua-
tion of several N availability indices showed that the
PBB-N test was well correlated with labile fractions of
soil organic N, and this test may be useful for predicting
field changes in N-supplying capability of soil induced by
short-term N fertilizer management in corn production.
Additional research is needed to determine the suitability
of this test for other soils and environments.

ACKNOWLEDGMENTS

We are grateful to T.W. Andraski for technical assistance
in conducting the field studies. We also thank A.V. Kurakov
and P.C. Widen for their help in the laboratory work.

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