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The fate of nitrogen from legume and fertilizer sources in soils successively cropped with wheat under field conditions
Soil Biol. Biochem. Vol. 18, No. 4. pp. 417425,
0038-0717/8653.00+ 0.00 Pergamon Journals Ltd
1986
Printed in Great Britain
THE FATE OF NITROGEN FROM LEGUME AND FERTILIZER SOURCES IN SOILS SUCCESSIVELY CROPPED WITH WHEAT UNDER FIELD CONDITIONS J. N. LADD and M. AMATO Division of Soils, C.S.I.R.O., Glen Osmond, South Australia 5064 (Accepted
28 February 1986)
Summary-Using “N-1abelled legume material (Medicago littoralis) and fertilizers (urea, (NH&SO,, KNO,), a direct comparison has been made of the fate of nitrogen from these sources and their residues, in soils sown with two successive wheat crops. The availability of N from each source to both crops is discussed in terms of the release, movement and immobilization of N in the soil profiles. For fertilizer 15N, uptake by crops, distribution as inorganic 15N in soil profiles, total recovery and percentage recovery in organic residues in soil were not significantly influenced by the form of fertilizer applied. For both legume and fertilizer lSN, uptake by both crops was directly related to input; and uptake by the second crop was directly related to the amounts of lsN residual in soil after the first crop. About 17% of applied legume N was taken up by the tops of the first wheat crop, and, at the time of sowing of the second crop, about 62% remained as organic residues; total recovery in crop and soil averaged 84%. By contrast, about 46% of applied fertilizer N was taken up by crop 1, and at sowing in the following year 29% was present as organic residues, and total recovery in soil plus crop averaged 80% The availabilities of N from both legume and fertilizer residues to a second wheat crop declined markedly but continued to differ significantly (P 4 0.01) from each other. Expressed as percentages of total residual 15N present in soils at sowing, the second crop took up about 6% of legume-derived N and about 9% of fertilizer-derived N. Fertilizer N directly contributed 5% and 0.5% respectively of the N of first and second wheat crops, per 10 kg of fertilizer N applied ha-‘. Under the same conditions, legume N directly contributed about 2% and 1% respectively of the N of successive crops, per 10 kg of legume N applied ha-‘. The proportions of grain N derived from the applied sources were higher than those of straw N. For both legume and fertilizer ‘*N, the amounts of inorganic 15N present in soil profiles al sowing in successive years were directly related to 15Ninputs. A small but statistically-significant departure from linearity was observed for inorganic “N at sowing of crop 2 when related to total recoveries of 15Nin soils at that time; the higher the amount of 15Nrecovered, the greater the proportion present as inorganic 15N in the soil profile. The respective contributions of legume and fertilizer N to the total inorganic N pool in soil at sowing declined each year, but were similar to their contributions to the N of the following wheat crop. Concentrations of inorganic N and 15Nin soil profiles varied each year but their patterns of distribution in cropped soils were not influenced by the nature and amount of the initial amendments. The lSN atom% enrichments of the inorganic N at sowing in the cropped soils were relatively uniform throughout the profile.
INTRODUCTION
The wheat industry of the semi-arid regions of southern Australia has traditionally depended upon the process of N, fixation by legumes, grown in mixed pastures in rotations with cereals, to maintain adequate levels of nitrogen for crop growth. In more recent years, changes in farming practices (increased frequency of cereal cropping, retention of crop residues, harvesting of pasture- and grain-legume seed) and a deterioration of legume-based pastures have led to increased usage of fertilizer N to supplement inputs of N from symbiotic fixation. Ladd et al. (1981, 1983a) used “N-1abelled legume material (C-to-N ratios 11.1 and 14.9:1) to demonstrate that the proportions of legume-derived N taken up by first wheat crops, grown at several field sites and in different seasons, ranged from 1l-28%. Total recoveries in crop plus soil sampled to 90 cm depth generally exceeded 90% of legume N input. In anal-
studies other workers (Craswell and Martin, 1975; Olson et al., 1979; Riga et al., 1980; van Praag et al., 1980; Fredrickson et al., 1982; Olson, 1982; Olson and Swallow, 1984; Feigenbaum et al., 1984; Powlson et al., 1985) showed that the recoveries of fertilizer “N by first wheat crops ranged from 22 to 64%. Total recoveries of applied “N generally were ~80% although recoveries as high as 94% were reported when waterlogging of soils was prevented (Craswell and Martin, 1975). In these published studies climate and soil properties varied, as did experimental procedures (e.g. the use of confined and unconfined field microplots, type, amount and method of fertilizer application, etc.). Azam et al. (1985) followed the availability of N from NH: and legume sources to young maize plants grown in pots in the greenhouse. However, no previous study using 15N-labelled sources has permitted a direct comparison of the fate of legume and fertilogous
417
J. N. LADD and
418
izer N in soils cropped with cereals under field conditions. We describe the fate of “N from different sources (Medicago Iittoralis material, urea, NH:, and NO;), applied at several concentrations (for legume material and urea) to a soil which grew two mature wheat crops. The distribution in soil profiles of inorganic “N released from legume residues was determined at times of sowing in three successive seasons; inorganic 15N from fertilizer residues was determined at sowing in the years following crops 1 and 2. The availability of “N to both wheat crops, and the recovery of applied “N in soil organic residues were also measured.
MATJZRIALS AND METHODS
Field site
The field site was at Roseworthy, South Australia (lat. 34”32’S., long. 138”41’ E.) with a mean district rainfall of 440mm and a mean annual air temperature of 16.4”C. Annual rainfall for the 2 yr of cropping was 281 and 509 mm. The soil at the site was classified as a xerollic calciorthid. The topsoil f&7.5 cm) was a calcareous sandy-loam (pH 8.5, organic C 0.96%, organic N 0.098%); pH and CaCOJ content increased with depth (Ladd et al., 1981). Sources of added nitrogen
The soil was amended either with various iSN-labelled compounds viz. uYea (10.833 “N atom% enrichment), (NH&SO4 (10.808 “N atom% enrichment) or KNO, (10.712 “N atom% enrichment) or with ‘SN-labelled legume material. The latter was the same ground mixture of tops and roots of M. littoralis (C-to-N ratio 11.1:1, 3.17% N, 2.680 “N atom% enrichment) as described by Ladd et al. (1983a). Incubation of this legume material in attapulgite plus added mineral salts released, after 4 weeks, 31% of the plant N as inorganic N of a “N atom% enrichment of 2.330%. The lower enrichment of the released N as compared with that of the total N of the plant mixture was indicative of a preferential decomposition of the less-enriched tops component. Experimental
procedure
Two wheat crops were grown in successive years (1982, 1983) in soil amended either with the legume material (at the rates of 96.7 and 38.7 kg N ha-’ respectively), with urea (75, 50 and 25 kg N ha-’ respectively), or with NH: and NO; (each 50 kg N ha-‘). An eighth treatment involved the soil with no nitrogen amendment. The soils were confined within microplots consisting of reinforced, open-ended galvanized, steel cylinders (30 cm i.d.) installed to 90 cm depth (Ladd et al., 1981, 1983a). All treatments were randomised within each of three replicate blocks. At the commencement of the experiment in Spring (October) 1981, the soil had a good cover of green pasture containing the legume, N, littoralis, and the inorganic N content to 1m depth was < 10 kg ha-‘. Topsoil to 7.5 cm depth was removed from all cylinders. The field-moist soil was then sieved to remove coarse unlabelled plant materials, bulked and mixed, and subsamples (8.31 kg) were returned to the cylin-
M. AWATO
ders. At this time also 15N-labelled legume material (21.599 or 8.640 g plant dry matter) was mixed with the topsoils of appropriate cylinders. Immediately before sowing in May, 1982, solutions of either “N-1a~lled urea, ~H~)*SO~ or KNO, were added to topsoils of the appropriate remaining cylinders. Those cylinders containing either unamended topsoils or soils with legume residues received an equivalent amount of distilled water. The procedures adopted for the sowing, maintenance, harvest and analyses of the wheat crops have been described by Ladd et al. (1983a). In the present experiment wheat plants (thinned to 16 per cylinder shortly after germination) were harvested only at the ripe stage. Tops only were taken and were separated into grain and remainder (designated as straw), for measurements of yields of dry matter and contents of N and “N Concentrations of inorganic N and lSN in soil profiles were determined by analyses of subsamples of topsoils and of sections of soil cores (5 cm dia) taken within the microplots to 90cm depth. For those cylinders containing unamended soil or soil amended with legume material, the soils were sampled 3 times, viz. immediately before sowing the first and second crops and at a time corresponding to sowing in the third year of the experiment. For the remaining cylinders, which were to receive amendments of urea, NH: or NO;-N, sampling in the first year was omitted. Soils were air-dried (35”C, 7 days), ground (c 1.6 mm) and extracted (ZS’C, 1 h) with 2 M KC1 (125 cm3 KCl:50 g dry soil). Inorganic N (NH: + NO;) was determined as described by Bremner (1965a). Subsamples of those soils taken at the time of sowing of the second crop were ground more finely in a Tema mill, and analysed for total N (Bremner, 1965b). 15N atom% enrichments were determined on NH: samples using a Micromass 602C mass spectrometer, and adjusted for values of lSN atom% excess of crop N and soil N from unamended microplots. RESULTS
Release of inarganic residues
N from
legume and fertilizer
At sowing, year I. At the time of sowing the first wheat crop, inorganic lSN recovered in soil profiles to 90cm depth accounted for 16.6% (SE 3.5%) and 18.6% (SE 5.7%) respectively of input 15N in those cylinders which had received lSN-labelled legume material at rates equivalent to 96.7 and 38.7 kg N ha-‘. Thus despite the large sampling errors involved in determining soil NO; concentrations from analyses of extracts from soil cores, the recovered inorganic N from the decomposing legume residues was directly related to N input and contributed 13.0% (SE 2.7%) and 6.6% (SE 1.2%) respectively of the total inorganic N of the soil profile to 90 cm depth. At this time total recoveries of legume 15N in the profifes averaged only 75% of input. The inorganic N was not uniformly labelled with soil depth although the relative distribution of the released lsN at sowing was similar for both levels of amendment with legume material (Fig. la). Topsoils (O-7.5 cm) containing the incorporated legume resi-
419
Fate of nitrogen from legume and fertilizers
(cl
(b)
(al
t5N atom % enrichments of inorganic N 0.0
Depth -
0.2
0.4
20
40
0.0 II
0.2
0.4 0.0
0.2
20
400
20
I 0
40
tnorganic N of soil profiles (kg ha’)
Fig. 1. Amount (0-O) and rSNatom% enrichment (0) of inorganic N in profiles of soils amended with ‘5N-lahelled legume material. Soils sampled at sowing in (a) year 1, (b) year 2 and (c) year 3.
dues also contained inorganic N of relatively high lsN atom% enrichments, as did subsoils at 15-60 cm depth where most of the total inorganic N of the profiles resided. Between the times of incorporation of legume material and sowing of the first wheat crop (7months), there were 13 days when precipitation exceeded evaporation, the accumulative excess being 115.6 mm. The average value for the field capacity of the Roseworthy soil is 0.23 (Forrest et al., 1985). Using the simple leaching model of De Smedt and Wierenga (1978), a = I/&,, where I = total amount of water infiltrated and 0, is field capacity, a concentration peak of inorganic (NO;)-N was calculated to occur near 50 cm depth. When expressed as kg ha-’ per 7.5 cm depth of soil, two concentration peaks of inorganic N (and “N) were observed, one at O-7.5 cm depth and the other usually at 15-30 cm, although in some microplots at 30-60 cm. At sowing, years 2 and 3. Following successive wheat crops, the amounts of inorganic N from both legume and fertilizer residues decreased in the microplots at sowing, and contributed decreasing proportions of the total inorganic N of the profiles to 90 cm depth (Table 1). Nevertheless they continued, for each source, to be directly and linearly related to the amounts of legume and fertilizer “N added initially. The contribution of legume residues to soil inor-
ganic N halved on average between sowing in years 1 and 2, and further declined by a factor of 1.5 between years 2 and 3. For fertilizer residues, irrespective of the amount and nature of the compound applied initially, the relative contributions to the soil inorganic N pool at sowing declined by a factor of 1.6 (SE 0.08) between years 2 and 3. Based on climatic patterns occurring between crop harvest and sowing in the following year, the predicted depths to which NO; at the soil surface would be leached were 70 cm by sowing in year 2, and 17 cm by sowing in year 3. Observed peak concentrations of inorganic N (and “N) were generally at the O-7.5 cm and at the 15-30 cm depths in both years. Inorganic N which had accumulated in soil profiles to 90 cm depth at sowing in years 2 and 3 was more uniformly labelled than that present at sowing in year 1. This uniformity of labelling of soil inorganic N occurred with soils amended initially with either legume material or fertilizers (shown for legume material, Fig. lb, c; and for urea amendments, Fig. 2a, b). For the 5 sections of the soil profiles analysed to 90cm, average enrichments of inorganic N released from fertilizer residues exhibited coefficients of variation generally ~8% for years 2 and 3; C.V.s of the enrichments of inorganic N from legume residues were about 12% in years 2 and 3, which contrasted with values of about 60% at sowing in year 1.
Table 1. Recovery as inorganic N in cropped soils of nitrogen applied as legume material and as fertilizers source of applied N M. littoralis Urea NH: NO,
Amount of applied N (kg ha-‘)
Percentage of applied N as inorganic N at sowing Year 2 Year 3
96.7
5.9 (0.97)1
38.7 75 50 25 50 50
3.7 5.0 3.7 4.6 4.8 3.7
‘Standard errors of means in parentheses.
(0.31) (1.51) (0.34) (0.76) (1.16) (0.55)
4.2 3.6 3.3 2.6 2.9 2.5 2.3
(0.52) (0.33) (0.65) (0.39) (0.33) (0.22) (0.26)
Percentage of inorganic N at sowing from applied N Year 2 Year 3 7.9 2.3 4.4 3.0 1.5 2.9 2.5
(0.63) (0.16) (0.35) (0.22) (0.06) (0.19) (0.16)
4.6 1.9 2.3 1.8 1.0 1.9 1.7
(0.35) (0.05) (0.23) (0.22) (0.08) (0.19) (0.20)
420
J. N. LADD and M. AMATO
(a)
Table
(b)
3. Proportions
15N atom % enrichments of inorganic N Depth O;o (Cm) o-7 5 ‘7
0.2,
0.4,
of dry matter and nitrogen accounted for in grain
Lhea_, )
Dry matter Mean Range Nitrogen Mean Range Nitrogen from applied Mean Range
(75kgNha
3
tops
Proportion of dry matter and nitrogen of wheat tops accounted for in grain (%) Crop 1 crop 2
0.2 I
0.0 I
of wheat
0.40 0.39-0.41
0.37 0.35-0.39
0.76 0.74-0.79
0.73 0.70.75
0.77 0.7%x79
0.77 0.73xL80
sources
o-75
Urea
75-15
(50kgNha“)
,5_30
both crops was not influenced by the nature of the “N-1abelled source and the amounts applied (Table 3).
30-60 60-90
I’J
Uptake by crop 1 of N from fertilizers
legume residues and
Based on 15Natom% enrichments, the amounts of urea N and legume N recovered in the tops of the first wheat crop were proportional to the amounts of N of the respective sources added to topsoil (Fig. 3). About 77% (SE 0.5%) of the “N of the wheat tops was present in grain irrespective of the 15Nsource, i.e. the proportionality between 15N input and uptake was maintained for both grain and straw components I I L (Table 4). 0 20 40 0 20 Applied at 50 kg N ha-‘, N of urea, NH: and Inorganic N of soil profiles (kg ha’) NO; was of similar availability, and as a group Fig. 2. Amount (0-O) and 15Natom% enrichment (0) of inorganic N in profiles of soils amended with ‘5N-labelled fertilizer N was significantly (P < 0.001) more availurea. Soils sampled at sowing in (a) year 2 and (b) year 3. able than was the N of legume material. The percentages of applied fertilizer N in the tops of the first wheat crop averaged 46.7% (SE 1.55%; range 41.1-50.3%). By contrast, only 17.3% (16.1-l&5%) Dry matter and total N of wheat tops of the N of legume material (but equivalent to Despite some large yield differences no single 96.7-100.2% of inorganic N recovered at sowing and amendment, with one exception, significantly inderived from legume residues) was accounted for in creased the amounts of tops dry matter (grain or the tops of the first crop (Fig. 3, Table 5). straw), but most amendments increased total N of Urea contributed from 14.2 to 34.8% of the N of wheat tops (Table 2). The amounts of N taken up by tops, for N inputs ranging from 25 to 75 kg ha-‘. the first wheat crop were positively correlated Nevertheless, for this first crop only, fertilizers con(P < 0.01) with the rates of application of fertilizers, tributed smaller proportions of N to wheat tops than but not of legume material. they did to the total inorganic N of soil profiles at The soil amendments did not influence dry matter sowing (calculated range 25.2-49X%). By contrast production or the amounts of total N taken up by the legume material, incorporated at 96.7 kg N ha-‘, second wheat crop, or the proportions of dry matter supplied only 18.2% of the N in wheat tops, even and N of wheat tops accounted for in grain in though legume residues at both input levels coneither crop. The distribution of 15N within tops of tributed proportionately more to plant N than they I
Table 2.
Yields of successive crops of wheat grown in a soil amended with legume material and nitrogenous
source of applied N M.
liftoralis
Urea
NH: NO;
I
Amount of applied N (kg ha-‘) 96.7 38.7 75 50 25 50 50 0
Plant’ dry matter crop I 7.13 8.93 8.53 ‘9.20 7.14 8.78 8.03 6.47
(0.97)’ (0.74) (1.01) (0.76) (0.26) (0.86) (0.71) (0.89)
‘Total top growth at harvest. ‘Standard errors of means in parentheses. Significant increases in yield compared with unamended
(t ha -‘) crop 2 6.07 5.43 5.10 5.96 5.92 5.22 5.19 5.32
(1.16) (0.39) (1.05) (0.49) (0.54) (0.48) (0.29) (1.17)
Plant’ N (kg ha-‘) Crop I crop 85.7 ‘89.9 “103 ‘99.4 72.3 *98.4 *89.2 65.8
soil: *I’ < 0.05. **f < 0.01.
(10.4) (5.8) (7.3) (9.2) (1.7) (9.5) (7.2) (7.2)
54.7 45.5 48.2 52.3 49.8 43.4 44.8 45.0
fertilizers
2 (8.4) (3.6) (7.4) (3.8) (6.6) (2.5) (3.1) (8.6)
Fate of nitrogen
from
legume
421
and fertilizers Added N Source
0
Wheat Crop 1
11:
50
25
75
Added N (kg ha-j Fig. 3. Relationship
between
M. littoralis tops t roots
95% confidence levels
100
J
the amount of nitrogen added to soil as legume that taken up by a first wheat crop.
did to the pool of inorganic N at sowing (Table 4). Grain N tended to be more heavily labelied with 15N than was straw N, especially for wheat grown in the legume-amended soil. Recovery of applied “N at sowing of crop 2
At the time of sowing of the second wheat crop the total amounts of residual 15N in soil to 90cm depth plus those amounts removed in the tops of the first crop accounted for 78-U% of input. Total percentage recoveries were not influenced by amounts of input, but were slightly greater for those soils amended with legume material (Table 5). The
and fertilizer
sources
and
percentage recoveries (average 84.1%) of legumederived N following harvesting of crop 1 were also significantly (P < 0.01) greater than those obtained (75.0%) prior to sowing of crop 1. Of the residual 15Nin soil at sowing of crop 2, most was present as organic “N, accounting for about 62% of “N input as legume material and about 29% of fertilizer “N. Most was located in soils to 15 cm depth. When at the time of sowing in year 2, the inorganic 15Nfrom either legume or fertilizer residues was related to the total “N recovered in the profiles a small but significant (P < 0.01) curvilinear trend was obtained; the higher the amounts of total 15N
Table 4. Recovery in a first wheat crop of nitrogen applied as legume material and as fertilizers Amount of applied N (kg ha-‘)
Source of applied N M. littoralis
Percentage in applied N in crop I Grain Straw
96.7 38.7 75 50 25 50 50
Urea
NH: NO,
12.5 14.7 36.6 35.0 31.3 37.9 37.1
(1.4)’ (0.64) (2.1) (3.1) (1.6) (1.9) (2.2)
3.6 3.8 10.5 11.4 9.8 12.4 I I .5
Percentage of N in crop applied source Grain Straw
(0.11) (0.33) (0.69) (0.99) (0.65) (2.1) (0.68)
19.0 8.2 34.8 23.6 14.2 25.6 27.4
(1.8) (0.4) (1.0) (0.6) (0.8) (0.9) (1.4)
15.8 7.1 32.6 22.6 14.2 25.3 26.7
1 from Tops
(1.3) (0.1) (0.5) (0.3) (1.2) (0.2) (0.9)
18.2 8.0 34.3 23.3 14.2 25.5 27.2
‘Standard errors of means in parentheses. Table 5. Recovery in crop and soil of nitrogen applied as legume material and as fertilizers Percentage recovery of applied N Source of aoolied N M. littoralis
Urea
NH: NO;
Amount of aoolied N
In tops of first wheat cron
96.7 38.7 75 50 25 50 50
Soil sampled to 90 cm depth. ‘Standard errors of means in parentheses.
16.1 18.5 47.1 46.4 41.1 50.3 48.6
(1.5)’ (0.92) (2.7) (3.7) (0.47) (3.4) (2.9)
Inoreanic 5.9 3.7 5.0 3.7 4.6 4.8 3.7
In soil at sowing of second Organic 61.3 62.7 26.1 29.2 33.8 28.8 27.8
(1.2) (1.9) (0.69) (1.5) (1.9) (0.97) (2.2)
crop Total 83.3 84.9 78.2 79.3 79.5 84.2 80.1
(0.52) (2.5) (3.2) (2.3) (3.8) (2.4) (2.9)
(1.7) (0.3) (0.7) (0.4) (0.9) (0.7) (1.3)
J. N. LADD and M. AMATO
422
-2 2
5.0 -
Wheat Crop 2
x s ii f
4.0
.cY 5E 3.0
-
P E ; 2.0 2 5 P m 1.0 E
Residual N of amendments (kg ha’)
Fig. 4. Relationship between the amount of nitrogen from legume and fertilizer residues recovered in cropped soil and that taken up by a second wheat crop.
recovered the higher the percentage inorganic 15N.
recovered as
Uptake by crop 2 of N from legume residues
and fertilizer
For fertilizer residues and legume residues respectively, the amounts of “N taken up by the second wheat crop were proportional to the amounts present as organic plus inorganic “N in the soil profiles at sowing (Fig. 4). For soils which had received fertilizer “N initially, tops of the second wheat crop contained 3.1% (SE 0.17%) of the 15N input, or equivalent to 9.2% (SE 0.34%) of total residual “N (or 71.7% (SE 6.6%) of the inorganic “N) at sowing. For soils which had been amended with legume material, “N in tops accounted for, on average, 4.2% of 15Ninput, equivalent to 6.3% of the residual ‘*N (or 91.6% of the inorganic “N) at sowing. The availability of 15N in fertilizer residues to the second wheat crop was significantly (P < 0.01) greater than that of 15Nin legume residues; the linear regressions of uptake of 15Non residual 15Nbeing for fertilizer amendments y =0.091x, and for legume amendments, y = 0.061x. Thus the availabilities and the relative availabilities of residual “N from legume and fertilizers to a second crop were considerably less observed for applied fertilizer than those
Table 6. Recovery Source of armlied N .. M.
littoralis
Urea
NH; NO, ‘Standard
0, =0.464x) and legume (y =0.171x) to the first wheat crop. Legume N initially applied at 96 kg N ha-’ contributed only 8.0% to the N of the tops of the second wheat crop (cf. 7.9% of the inorganic N pool at sowing, and 18.2% of the N of the first crop). Urea N applied at 25-75 kg ha-’ supplied only l&4.4% of the N of the second crop, compared with 1.5-4.4% of the inorganic N at sowing (Table 6). The trend for grain N to be more highly labelled than straw N was again evident in the second crop, despite the greater uniformity of labelling of the soil inorganic N in the profiles at sowing compared with that observed in the previous year. Net mineralization of residual organic 15N during the last 12 months of the experiment were determined from changes in the amounts of inorganic “N in soil profiles at sowing in years 2 and 3, and the amounts of 15Nremoved in the tops of the second crop. Values are underestimated since they ignore the amounts of “N returned to soil in roots and other crop residues and also any losses of inorganic “N which may have occurred. Expressed as a percentage of the residual organic “N in soil at sowing in year 2, mineralization of fertilizer-derived residues was 4.8% (SE 0.98%), which did not differ significantly from the average (5.4%) mineralization of legume-derived residual N.
in a second wheat crop of nitrogen
Amount of applied N (ke ha-‘) .I
96.7 38.7 75 50 25 50 50
errors of means in parentheses
Percentage of applied N in crop 2 Grain straw 3.5 3.0 2.1 2.6 2.8 2.1 2.2
(0.94)’ (0.13) (0.32) (0.14) (0.22) (0.19) (0.14)
1.1 0.8 0.8 0.8 0.8 0.7 0.6
(0.21) (0.08) (0.06) (0.03) (0.05) (0.05) (0.02)
applied
as legume material
and as fertilizers
Percentage of N in crop 2 from applied source straw Tons Grain 8.4 3.4 4.6 3.3 I .9 3.4 3.3
(0.7) (0.1) (0.2) (0.2) (0.2) (0.3) (0.1)
7.1 3.1 3.9 2.8 1.6 2.6 2.5
(0.7) (0.2) (0.2) (0.1) (0.1) (0.3) (0.1)
8.0 3.4 4.4 3.2 1.8 3.1 3.1
(0.8) (0.1) (0.2) (0.2) (0.1) (0.3) (0.1)
Fate of nitrogen from legume and fertilizers DISCUSSION
Release and movement of inorganic N in soil The concentrations of NO; in similarly-treated soils may vary widely (Nielsen et al., 1982) even in soils from cores taken a few cm apart, and in the case of ‘rNO;, in soils confined within cylinders (Carter et aI., 1967; Ladd et al., 1981). Nevertheless despite some large differences at nominated profile depths in this experiment also, the average amounts of inorganic 15N in the total profile to 90cm were, at all sampling times, directly and linearly related to the initial inputs of legume and fertilizer lSN respectively. However, at sowing in year 2 there was a small but significant curvilinear trend which indicated a higher percentage recovery of inorganic “N (and a lower percentage recovery of organic “N), the greater the amounts of total 15N recovered. The trend was not great and did not affect the linear relationships established between input “N, inorganic “N recovered, and ‘jN taken up by crops. From the time of incorporation of legume material until sowing of the first wheat crop, inorganic N derived from soil organic matter in unamended cylinders increased from 9.5 to 77.7 kg ha-‘, a net gain equivalent to 4.7% of N to 15 cm depth. The distributions of inorganic N in the soil profiles at sowing in both unamended and legume-amended soils, and of inorganic 15Nfrom the legume material itself, were only approximately (and partly) described by a simple leaching model based on the climatic conditions experienced during this period. Despite the model output (which predicted a peak inorganic “N concentration at 50cm depth), and the absence of substantial amounts of 15Nin the lowest depth of soil sampled (60-90 cm), there is evidence to suggest some leaching may have occurred below 90 cm. The recovery of inorganic “N in soil to 90 cm depth was remarkably low, viz. only about 17.5% of the legume 15N input after 7 months decomposition compared with 32.5 and 27.2% respectively after incubating the same plant material with attapulgite and Roseworthy soil for only 4 weeks under laboratory conditions. Under field conditions, some losses of applied legume 15N may have been due to volatilization as “NH’ soon after the addition of the plant material to the soils, given that the soil is calcareous and would have been subjected to intermittent drying and remoistening under our summer climatic conditions. However nitrification is rapid. Losses due to run-off were unlikely since the rims of the microplot cylinders protruded about 5cm above the soil surfaces. That leaching below 90 cm is at least partly responsible for the deficit is suggested by the total 15N recoveries in crop 1 plus in soil sampled at sowing in year 2; these were about 10% higher than those at sowing in year 1. In the following years, predictions of inorganic N distributions were potentially confounded by the absence of information regarding the amounts of inorganic N not utilized by the preceding crop. Carry-over of nitrate from one growth season to the next is not an uncommon occurrence in soils cropped under the uncertain climatic conditions of a Mediterranean-type environment (Noy-Meir and Harpaz, 1977). We suggest that in our experiment
423
either no inorganic N had remained in the profile to 90 cm depth at the commencement of the crop ripening phase or that any remainder had been leached below the sampling depth by sowing time of the following year (especially by sowing time in year 2). This conclusion is based on the relatively uniform labelling of the inorganic N throughout the profile at sowing in years 2 and 3 respectively, even though in each case the atom% enrichment of the inorganic N was less than that of the preceding year. We suggest such results would be achieved if all of the inorganic 15N recovered in the soil profile to 90 cm depth were derived from the progressively more stable organic “N pool mixed (mainly in topsoil) with the major source of unlabelled inorganic N. Uptake of N by wheat crops The declining “N atom% enrichments with time of N released from labelled organic residues was predicted since organic “N from legume material, incorporated at the Roseworthy site (and elsewhere), decayed at relatively faster rates than did native soil organic N throughout an 8-yr experimental period (Ladd et al., 1985). Further, it was anticipated that because the net rates of decay of plant residues decline markedly within the first year of incorporation of the source materials, but slowly thereafter, then the 15Natom% enrichments of N of wheat crops grown subsequently would be similar to that of NOT-N in the soil profiles at sowing, as was observed (Tables 1 and 6). Nevertheless, the enrichments of NO;-N do fall slowly with time, even during a growth season, and analyses show that the N of field-grown wheat crops is not uniformly labelled. Consistently we have observed, as have others, that grain N is of slightly higher 15N atom% enrichment than is straw N (Tables 4, 6; Riga et al., 1980; Ladd et al., 1981, 1983a). In the experiments reported here these trends were obtained with both crops, even though in the case of the second crop, the ISNO; pool was relatively uniformly labelled at sowing. Indeed for those treatments in which “N-1abelled fertilizers were applied initially, the greater differences in the extent of labelling of grain N and straw N were achieved with the second crop rather than the first. Our results are consistent with the suggestions in an IAEA report (1974) that N taken up late in the vegetative growth phase is preferentially translocated to grain subsequently. Gass et al. (1971) demonstrated in experiments with corn grown in soil to which 15NO; was applied at different depths that grain N was preferentially derived from NO; obtained from the middle and lower parts of the soil profile. We suggest that in the cases of (a) both wheat crops utilizing 15NO; from legume residues and of (b) the second crop utilizing 15NO; from fertilizer residues, much of the available NOT-N at sowing was already distributed throughout the profile, and especially below 30cm, and such NO;-N would have had a slightly greater enrichment than that released from organic residues during the period of wheat growth. By contrast, in the case of the first wheat crops utilizing applied fertilizer N, all of the available 15Nwas present in the topsoil at sowing, with possibly less opportunity for more-highly enriched pools of
424
J. N. LADD and M. AMATO
NO; to be leached to depths below those of the active root zones, and therefore for late uptake and enhanced grain N enrichment. First wheat crops took up about 46% of applied fertilizer N. The efficiencies of fertilizer N use were not significantly affected by the form of the fertilizer and were comparable to those reported in the literature. Substantial proportions of applied fertilizer “N also became immobilized in organic residues in soils as a result of C metabolism during the period of wheat growth. By contrast, decomposition of applied legume material ensured that despite net mineralization of legume “N prior to and during cropping, the proportions of legume N immobilized in organic residues were greater than those of fertilizer “N. Thus the efficiencies of use of fertilizer 15N were markedly higher than those (17%) determined for applied legume N, but less than those (98%) based on the amounts of legume-derived inorganic N present in the soil profiles to 90 cm depth at the time of fertilizer application. The latter figure may be inflated if the first wheat crop had derived some of its nitrogen from NO; released from legume material and leached to below 90 cm before sowing. Our early studies on the supply of legume N to wheat crops grown under field conditions involved 3 sites; all soils were calcareous and of similar pH, two were sandy-loams, the third a heavy clay soil (Ladd et al., 1981). Wheat yields varied and clearly did not relate to the percentage of applied legume N taken up by the crops. Subsequent experiments at a fourth site with a soil of similar properties to the Roseworthy soil (Ladd et al., 1983a), together with experiments reported in this paper, have extended the data available. With this greater perspective, the early results obtained with the clay soil revealed a relatively low percentage uptake of legume N by a high-yielding wheat crop and are seen to be an exception. For all other soils, irrespective of season and levels of input of legume “N, there was a significant direct relationship between the percentage OJ) of legume N take up by the tops of first wheat crops and the dry matter yields (x, in t ha-‘). The relationship is described by y = 11.2 + 0.81x, for a range of yields from 4.6 to 16.6 t ha-’ (r = 0.88). The relationship appears to derive mainly from the capacity of the various crops to take up the released N, rather than from the net rates of N mineralization from the residues themselves. Ladd et al. (1981, 1983b, 1985) had shown that the percentage net decline of residual organic N from legume materials of similar low C-to-N ratios and which had decomposed for 0.5 years or more in unplanted soils at the same sites, may be significantly influenced by soil type, climate and input levels of legume N, but the effects are relatively small. Nevertheless, the effects for example of the decomposition and turnover of C of wheat roots on the net rates of N mineralization in soils (especially under water stress conditions which frequently limit crop yields in our environment), have yet to be assessed. The results also imply that the fate of N released from legume residues, but not taken up by a wheat crop, may in a given season be similar in soils of comparable physical properties. Second wheat crops took up 6.3% of legume residual “N and 9.2% of fertilizer residual 15N.Given
that for both treatments there had been no carry-over of IsNO; not utilized by the first crop, and knowing that inorganic lSN released at the time of sowing crop 2 was similarly distributed in soil profiles irrespective of the nature of the nitrogenous source applied initially, then the results are consistent with the notion that the greater stability of residual organic “N from legume material was due to it having resided in soil at least 7 months longer than had residual organic 15N of immobilized fertilizers. The results emphasize that the differences in availability of N from fertilizers and plant materials returned to soil occur mainly in the first year after their application only. The relative contributions of legume material and soil organic matter to the N of wheat crops will depend on the size and decay rates of each source. In studies reported here and earlier we have found that for legume materials of low C-to-N ratios (11 .l-14.9: 1) and decomposing in sandy-loam topsoils (of organic N contents of approximately 0.1%) a simple relationship has held irrespective of season and crop yield, viz. each 10 kg N applied to soils in legume materials contributed 2% of the N of a first wheat crop, and 1% of the N of the second crop (Ladd and Amato, 1985). We stress that this consistent relationship has been obtained using legume materials of similar low C-toN ratios, and soils of similar properties. In these circumstances similar proportions of legume N will mineralized from their readilybe readily decomposable components, and it can be expected that the further release and distribution of 15N0~ from 15N-labelled organic residues and of 14NO; from native soil organic matter, both sources residing principally in topsoils, will be similarly affected by climatic conditions. Whilst the range of inputs of legume N used in our experiments have covered most estimates of N gains due to legumes, as determined in long term rotation trials in southern Australia, nevertheless under normal farming conditions the C-to-N ratios of plant materials returned to soil and the timing of their return will vary widely. For example senesced legume materials, weathered and partly decomposed as surface residues over summer may be of higher C-to-N ratios than those of green legume materials, and may contribute less N for a following wheat crop than indicated from our “N based studies. Indeed incubation of brown weathered legume material mixed with soils may cause temporarily net immobilization of N (Ladd et al., 1986). By contrast, in grazed pastures some legume N will also be returned to soil in faeces and urine. Under field conditions much of the urea N in urine may be lost from the soil as NH3 by volatilization, but the remainder presumably would have an availability to the first succeeding wheat crops greater than the N of green legume materials and approaching that of the fertilizer N. Acknowledgements-We thank Mrs M. Bohnsack and Messrs J. M. Thompson and P. J. L. Williams for technical assistance, Mrs R. B. Anderson and Dr R. L. Correll for statistical analyses and Dr K. Smettem for calculations of NO< leaching. We are grateful to Mr H. J. Krieg of Roseworthy for his cooperation in our use of the experimental site.
Fate of nitrogen from legume and fertilizers REFERENCES
Azam F., Malik K. A. and Sajjad M. I. (1985) Transformations in soil and availability to plants of 15Napplied as inorganic fertilizer and legume residues. Plant and Soil 86, 3-13.
Bremner J. M. (1965a) Inorganic forms of nitrogen. In Methods of Soil Analysis (C. A. Black, Ed.), Part 2, pp. 1179-1237. American Society of Agronomy Monograph 9, Madison. Bremner J. M. (1965b) Total nitrogen. In Methods of Soil Analysis (C. A. Black, Ed.), Part 2, pp. 1149-1178. American Society of Agronomy Monograph 9, Madison. Carter J. N., Bennett 0. L. and Pearson R. W. (1967) Recovery of fertilizer nitrogen under field conditions using nitrogen”. Soil Science Society of America Proceedings 31, 50-56.
Craswell E. T. and Martin A. E. (1975) Isotopic studies of the nitrogen balance in a cracking clay. II. Recovery of nitrate lSN added to columns of packed soil and microplots growing wheat in the field. Australian Journal of Soil Research 13, 53-61.
De Smedt F. and Wierenga P. J. (1978) Approximate analytical solution for solute flow during infiltration and redistribution. Soil Science Society of America Journal 42, 407-412.
Feigenbaum S., Seligmar N. G. and Benjamin R. W. (1984) Fate of nitrogen-15 applied to spring wheat grown for three consecutive years in a semiarid region. Soil Science Society of America Journal 48, 838-843.
Forrest J. A., Beatty J., Hignett C. T., Pickering J. and Williams R. G. P. (1985) A survev of the nhvsical properties of wheatland ‘soils in eastern Australia. C.S.I.R.O.
Division of Soils Divisional Report No. 78.
Fredrickson J. K., Koehler F. E. and Cheng H. H. (1982) Availability of “N-1abelled nitrogen in fertilizer and in wheat straw to wheat in tilled and no-till soil. Soil Science Society of America Journal 46, 1218-1222.
Gass W. B., Peterson G. A., Hauck R. D. and Olson R. A. (1971) Recovery of residual nitrogen by corn (Zea muys L.) from various soil depths as measured by 15N tracer techniques. Soil Science Society of America Proceedings 35, 290-294.
International Atomic Energy Agency (1974) Isotope studies on wheat fertilization. Technical Report Series No. 157, IAEA, Vienna. Ladd J. N. and Amato M. (1985) Nitrogen cycling in legume-cereal rotations. In Proceedings of an Znrernational Symposium on Nitrogen Management in Farming Systems in Humid and Subhumid Tropics. pp. 105-127. IITA,
Ibadan, Nigeria. Ladd J. N., Amato M. and Oades J. M. (1985) Decomposition of plant material in Australian soils. III. Residual
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organic and microbial biomass C and N from isotopelabelled legume material and soil organic matter, decomposing under field conditions. Australian Journal of Soil Research 23, 603-611.
Ladd J. N., Butler J. H. A. and Amato M. (1986) Nitrogen fixation by legumes and their role as sources of nitrogen for soil and crop. In Proceedings of an International Conference on Biological Agricuhure, Wye College, England. In press. Ladd J. N., Oades J. M. and Amato M. (1981) Distribution and recovery of nitrogen from legume residues decomposing in soils sown to wheat in the field. Soil Biology & Biochemistry 13, 251-256.
Ladd J. N., Oades J. M. and Amato M. (1983a) Utilization by wheat crops of nitrogen from legume residues decomposing in soils in the field. Soil Biology & Biochemistry 15, 231-238.
Ladd J. N., Jackson R. B., Amato M. and Butler J. H. A. (1983b) Decomposition of plant material in Australian soils. I. The effect of quantity added on decomposition and on residual microbial biomass. Australian Journal of Soil Research 21, 5633570.
Nielsen D. R., Biggar J. W. and Wierenga P. J. (1982) Nitrogen transport processes in soil. In Nitrogen in Agricultural Soils (F. J. Stevenson, Ed.), pp. 4233448. American Society of Agronomy Monograph 22, Madison. Noy-Meir I. and Harpaz Y. (1977) Agroecosystems in Israel. Agroecosystems (Special Issue: Cycling of Mineral Nutrients in Agricullur~lEcosystems)
4; 143-167.
Olson R. V. (1982), Immobilization. nitrification. and losses of fall-applied, labelled ammonium-nitrogen during growth of winter wheat. Agronomy Journal 74, 991-995 Olson R. V. and Swallow C. W. (1984) Fate of labelled nitrogen fertilizer applied to winter wheat for five years. Soil Science Sociely of America Journal 48, 583-586.
Olson R. V., Murphy L. S., Moser H. C. and Swallow C. W. (1979) Fate of tagged fertilizer nitrogen applied to winter wheat. Soil Science Society of America Journal 43, 973-975.
Powlson D. S., Jenkinson D. S., Pruden G. and Johnston A. E. (1985) The effect of straw incorporation on the uptake of nitrogen by winter wheat. Journal of the Science of Food and Agriculture 36, 26-30.
van Praag H. J., Fischer V. and Riga A. (1980) Fate of fertilizer nitrogen applied to winter wheat as NarsNO, and (‘5NH,)2S0, studied in microplots through a fourcourse rotation: 2. Fixed ammonium turnover and nitrogen reversion. Soil Science 130, 10@105. Riga A., Fischer V. and van Praag H. J. (1980) Fate of fertilizer nitrogen applied to winter wheat as Nai5N0, and (“NH&SO, studied in microplots through a fourcourse rotation: 1. Influence of fertilizer splitting on soil and fertilizer nitrogen. Soil Science 130, 88-99.
0038-0717/8653.00+ 0.00 Pergamon Journals Ltd
1986
Printed in Great Britain
THE FATE OF NITROGEN FROM LEGUME AND FERTILIZER SOURCES IN SOILS SUCCESSIVELY CROPPED WITH WHEAT UNDER FIELD CONDITIONS J. N. LADD and M. AMATO Division of Soils, C.S.I.R.O., Glen Osmond, South Australia 5064 (Accepted
28 February 1986)
Summary-Using “N-1abelled legume material (Medicago littoralis) and fertilizers (urea, (NH&SO,, KNO,), a direct comparison has been made of the fate of nitrogen from these sources and their residues, in soils sown with two successive wheat crops. The availability of N from each source to both crops is discussed in terms of the release, movement and immobilization of N in the soil profiles. For fertilizer 15N, uptake by crops, distribution as inorganic 15N in soil profiles, total recovery and percentage recovery in organic residues in soil were not significantly influenced by the form of fertilizer applied. For both legume and fertilizer lSN, uptake by both crops was directly related to input; and uptake by the second crop was directly related to the amounts of lsN residual in soil after the first crop. About 17% of applied legume N was taken up by the tops of the first wheat crop, and, at the time of sowing of the second crop, about 62% remained as organic residues; total recovery in crop and soil averaged 84%. By contrast, about 46% of applied fertilizer N was taken up by crop 1, and at sowing in the following year 29% was present as organic residues, and total recovery in soil plus crop averaged 80% The availabilities of N from both legume and fertilizer residues to a second wheat crop declined markedly but continued to differ significantly (P 4 0.01) from each other. Expressed as percentages of total residual 15N present in soils at sowing, the second crop took up about 6% of legume-derived N and about 9% of fertilizer-derived N. Fertilizer N directly contributed 5% and 0.5% respectively of the N of first and second wheat crops, per 10 kg of fertilizer N applied ha-‘. Under the same conditions, legume N directly contributed about 2% and 1% respectively of the N of successive crops, per 10 kg of legume N applied ha-‘. The proportions of grain N derived from the applied sources were higher than those of straw N. For both legume and fertilizer ‘*N, the amounts of inorganic 15N present in soil profiles al sowing in successive years were directly related to 15Ninputs. A small but statistically-significant departure from linearity was observed for inorganic “N at sowing of crop 2 when related to total recoveries of 15Nin soils at that time; the higher the amount of 15Nrecovered, the greater the proportion present as inorganic 15N in the soil profile. The respective contributions of legume and fertilizer N to the total inorganic N pool in soil at sowing declined each year, but were similar to their contributions to the N of the following wheat crop. Concentrations of inorganic N and 15Nin soil profiles varied each year but their patterns of distribution in cropped soils were not influenced by the nature and amount of the initial amendments. The lSN atom% enrichments of the inorganic N at sowing in the cropped soils were relatively uniform throughout the profile.
INTRODUCTION
The wheat industry of the semi-arid regions of southern Australia has traditionally depended upon the process of N, fixation by legumes, grown in mixed pastures in rotations with cereals, to maintain adequate levels of nitrogen for crop growth. In more recent years, changes in farming practices (increased frequency of cereal cropping, retention of crop residues, harvesting of pasture- and grain-legume seed) and a deterioration of legume-based pastures have led to increased usage of fertilizer N to supplement inputs of N from symbiotic fixation. Ladd et al. (1981, 1983a) used “N-1abelled legume material (C-to-N ratios 11.1 and 14.9:1) to demonstrate that the proportions of legume-derived N taken up by first wheat crops, grown at several field sites and in different seasons, ranged from 1l-28%. Total recoveries in crop plus soil sampled to 90 cm depth generally exceeded 90% of legume N input. In anal-
studies other workers (Craswell and Martin, 1975; Olson et al., 1979; Riga et al., 1980; van Praag et al., 1980; Fredrickson et al., 1982; Olson, 1982; Olson and Swallow, 1984; Feigenbaum et al., 1984; Powlson et al., 1985) showed that the recoveries of fertilizer “N by first wheat crops ranged from 22 to 64%. Total recoveries of applied “N generally were ~80% although recoveries as high as 94% were reported when waterlogging of soils was prevented (Craswell and Martin, 1975). In these published studies climate and soil properties varied, as did experimental procedures (e.g. the use of confined and unconfined field microplots, type, amount and method of fertilizer application, etc.). Azam et al. (1985) followed the availability of N from NH: and legume sources to young maize plants grown in pots in the greenhouse. However, no previous study using 15N-labelled sources has permitted a direct comparison of the fate of legume and fertilogous
417
J. N. LADD and
418
izer N in soils cropped with cereals under field conditions. We describe the fate of “N from different sources (Medicago Iittoralis material, urea, NH:, and NO;), applied at several concentrations (for legume material and urea) to a soil which grew two mature wheat crops. The distribution in soil profiles of inorganic “N released from legume residues was determined at times of sowing in three successive seasons; inorganic 15N from fertilizer residues was determined at sowing in the years following crops 1 and 2. The availability of “N to both wheat crops, and the recovery of applied “N in soil organic residues were also measured.
MATJZRIALS AND METHODS
Field site
The field site was at Roseworthy, South Australia (lat. 34”32’S., long. 138”41’ E.) with a mean district rainfall of 440mm and a mean annual air temperature of 16.4”C. Annual rainfall for the 2 yr of cropping was 281 and 509 mm. The soil at the site was classified as a xerollic calciorthid. The topsoil f&7.5 cm) was a calcareous sandy-loam (pH 8.5, organic C 0.96%, organic N 0.098%); pH and CaCOJ content increased with depth (Ladd et al., 1981). Sources of added nitrogen
The soil was amended either with various iSN-labelled compounds viz. uYea (10.833 “N atom% enrichment), (NH&SO4 (10.808 “N atom% enrichment) or KNO, (10.712 “N atom% enrichment) or with ‘SN-labelled legume material. The latter was the same ground mixture of tops and roots of M. littoralis (C-to-N ratio 11.1:1, 3.17% N, 2.680 “N atom% enrichment) as described by Ladd et al. (1983a). Incubation of this legume material in attapulgite plus added mineral salts released, after 4 weeks, 31% of the plant N as inorganic N of a “N atom% enrichment of 2.330%. The lower enrichment of the released N as compared with that of the total N of the plant mixture was indicative of a preferential decomposition of the less-enriched tops component. Experimental
procedure
Two wheat crops were grown in successive years (1982, 1983) in soil amended either with the legume material (at the rates of 96.7 and 38.7 kg N ha-’ respectively), with urea (75, 50 and 25 kg N ha-’ respectively), or with NH: and NO; (each 50 kg N ha-‘). An eighth treatment involved the soil with no nitrogen amendment. The soils were confined within microplots consisting of reinforced, open-ended galvanized, steel cylinders (30 cm i.d.) installed to 90 cm depth (Ladd et al., 1981, 1983a). All treatments were randomised within each of three replicate blocks. At the commencement of the experiment in Spring (October) 1981, the soil had a good cover of green pasture containing the legume, N, littoralis, and the inorganic N content to 1m depth was < 10 kg ha-‘. Topsoil to 7.5 cm depth was removed from all cylinders. The field-moist soil was then sieved to remove coarse unlabelled plant materials, bulked and mixed, and subsamples (8.31 kg) were returned to the cylin-
M. AWATO
ders. At this time also 15N-labelled legume material (21.599 or 8.640 g plant dry matter) was mixed with the topsoils of appropriate cylinders. Immediately before sowing in May, 1982, solutions of either “N-1a~lled urea, ~H~)*SO~ or KNO, were added to topsoils of the appropriate remaining cylinders. Those cylinders containing either unamended topsoils or soils with legume residues received an equivalent amount of distilled water. The procedures adopted for the sowing, maintenance, harvest and analyses of the wheat crops have been described by Ladd et al. (1983a). In the present experiment wheat plants (thinned to 16 per cylinder shortly after germination) were harvested only at the ripe stage. Tops only were taken and were separated into grain and remainder (designated as straw), for measurements of yields of dry matter and contents of N and “N Concentrations of inorganic N and lSN in soil profiles were determined by analyses of subsamples of topsoils and of sections of soil cores (5 cm dia) taken within the microplots to 90cm depth. For those cylinders containing unamended soil or soil amended with legume material, the soils were sampled 3 times, viz. immediately before sowing the first and second crops and at a time corresponding to sowing in the third year of the experiment. For the remaining cylinders, which were to receive amendments of urea, NH: or NO;-N, sampling in the first year was omitted. Soils were air-dried (35”C, 7 days), ground (c 1.6 mm) and extracted (ZS’C, 1 h) with 2 M KC1 (125 cm3 KCl:50 g dry soil). Inorganic N (NH: + NO;) was determined as described by Bremner (1965a). Subsamples of those soils taken at the time of sowing of the second crop were ground more finely in a Tema mill, and analysed for total N (Bremner, 1965b). 15N atom% enrichments were determined on NH: samples using a Micromass 602C mass spectrometer, and adjusted for values of lSN atom% excess of crop N and soil N from unamended microplots. RESULTS
Release of inarganic residues
N from
legume and fertilizer
At sowing, year I. At the time of sowing the first wheat crop, inorganic lSN recovered in soil profiles to 90cm depth accounted for 16.6% (SE 3.5%) and 18.6% (SE 5.7%) respectively of input 15N in those cylinders which had received lSN-labelled legume material at rates equivalent to 96.7 and 38.7 kg N ha-‘. Thus despite the large sampling errors involved in determining soil NO; concentrations from analyses of extracts from soil cores, the recovered inorganic N from the decomposing legume residues was directly related to N input and contributed 13.0% (SE 2.7%) and 6.6% (SE 1.2%) respectively of the total inorganic N of the soil profile to 90 cm depth. At this time total recoveries of legume 15N in the profifes averaged only 75% of input. The inorganic N was not uniformly labelled with soil depth although the relative distribution of the released lsN at sowing was similar for both levels of amendment with legume material (Fig. la). Topsoils (O-7.5 cm) containing the incorporated legume resi-
419
Fate of nitrogen from legume and fertilizers
(cl
(b)
(al
t5N atom % enrichments of inorganic N 0.0
Depth -
0.2
0.4
20
40
0.0 II
0.2
0.4 0.0
0.2
20
400
20
I 0
40
tnorganic N of soil profiles (kg ha’)
Fig. 1. Amount (0-O) and rSNatom% enrichment (0) of inorganic N in profiles of soils amended with ‘5N-lahelled legume material. Soils sampled at sowing in (a) year 1, (b) year 2 and (c) year 3.
dues also contained inorganic N of relatively high lsN atom% enrichments, as did subsoils at 15-60 cm depth where most of the total inorganic N of the profiles resided. Between the times of incorporation of legume material and sowing of the first wheat crop (7months), there were 13 days when precipitation exceeded evaporation, the accumulative excess being 115.6 mm. The average value for the field capacity of the Roseworthy soil is 0.23 (Forrest et al., 1985). Using the simple leaching model of De Smedt and Wierenga (1978), a = I/&,, where I = total amount of water infiltrated and 0, is field capacity, a concentration peak of inorganic (NO;)-N was calculated to occur near 50 cm depth. When expressed as kg ha-’ per 7.5 cm depth of soil, two concentration peaks of inorganic N (and “N) were observed, one at O-7.5 cm depth and the other usually at 15-30 cm, although in some microplots at 30-60 cm. At sowing, years 2 and 3. Following successive wheat crops, the amounts of inorganic N from both legume and fertilizer residues decreased in the microplots at sowing, and contributed decreasing proportions of the total inorganic N of the profiles to 90 cm depth (Table 1). Nevertheless they continued, for each source, to be directly and linearly related to the amounts of legume and fertilizer “N added initially. The contribution of legume residues to soil inor-
ganic N halved on average between sowing in years 1 and 2, and further declined by a factor of 1.5 between years 2 and 3. For fertilizer residues, irrespective of the amount and nature of the compound applied initially, the relative contributions to the soil inorganic N pool at sowing declined by a factor of 1.6 (SE 0.08) between years 2 and 3. Based on climatic patterns occurring between crop harvest and sowing in the following year, the predicted depths to which NO; at the soil surface would be leached were 70 cm by sowing in year 2, and 17 cm by sowing in year 3. Observed peak concentrations of inorganic N (and “N) were generally at the O-7.5 cm and at the 15-30 cm depths in both years. Inorganic N which had accumulated in soil profiles to 90 cm depth at sowing in years 2 and 3 was more uniformly labelled than that present at sowing in year 1. This uniformity of labelling of soil inorganic N occurred with soils amended initially with either legume material or fertilizers (shown for legume material, Fig. lb, c; and for urea amendments, Fig. 2a, b). For the 5 sections of the soil profiles analysed to 90cm, average enrichments of inorganic N released from fertilizer residues exhibited coefficients of variation generally ~8% for years 2 and 3; C.V.s of the enrichments of inorganic N from legume residues were about 12% in years 2 and 3, which contrasted with values of about 60% at sowing in year 1.
Table 1. Recovery as inorganic N in cropped soils of nitrogen applied as legume material and as fertilizers source of applied N M. littoralis Urea NH: NO,
Amount of applied N (kg ha-‘)
Percentage of applied N as inorganic N at sowing Year 2 Year 3
96.7
5.9 (0.97)1
38.7 75 50 25 50 50
3.7 5.0 3.7 4.6 4.8 3.7
‘Standard errors of means in parentheses.
(0.31) (1.51) (0.34) (0.76) (1.16) (0.55)
4.2 3.6 3.3 2.6 2.9 2.5 2.3
(0.52) (0.33) (0.65) (0.39) (0.33) (0.22) (0.26)
Percentage of inorganic N at sowing from applied N Year 2 Year 3 7.9 2.3 4.4 3.0 1.5 2.9 2.5
(0.63) (0.16) (0.35) (0.22) (0.06) (0.19) (0.16)
4.6 1.9 2.3 1.8 1.0 1.9 1.7
(0.35) (0.05) (0.23) (0.22) (0.08) (0.19) (0.20)
420
J. N. LADD and M. AMATO
(a)
Table
(b)
3. Proportions
15N atom % enrichments of inorganic N Depth O;o (Cm) o-7 5 ‘7
0.2,
0.4,
of dry matter and nitrogen accounted for in grain
Lhea_, )
Dry matter Mean Range Nitrogen Mean Range Nitrogen from applied Mean Range
(75kgNha
3
tops
Proportion of dry matter and nitrogen of wheat tops accounted for in grain (%) Crop 1 crop 2
0.2 I
0.0 I
of wheat
0.40 0.39-0.41
0.37 0.35-0.39
0.76 0.74-0.79
0.73 0.70.75
0.77 0.7%x79
0.77 0.73xL80
sources
o-75
Urea
75-15
(50kgNha“)
,5_30
both crops was not influenced by the nature of the “N-1abelled source and the amounts applied (Table 3).
30-60 60-90
I’J
Uptake by crop 1 of N from fertilizers
legume residues and
Based on 15Natom% enrichments, the amounts of urea N and legume N recovered in the tops of the first wheat crop were proportional to the amounts of N of the respective sources added to topsoil (Fig. 3). About 77% (SE 0.5%) of the “N of the wheat tops was present in grain irrespective of the 15Nsource, i.e. the proportionality between 15N input and uptake was maintained for both grain and straw components I I L (Table 4). 0 20 40 0 20 Applied at 50 kg N ha-‘, N of urea, NH: and Inorganic N of soil profiles (kg ha’) NO; was of similar availability, and as a group Fig. 2. Amount (0-O) and 15Natom% enrichment (0) of inorganic N in profiles of soils amended with ‘5N-labelled fertilizer N was significantly (P < 0.001) more availurea. Soils sampled at sowing in (a) year 2 and (b) year 3. able than was the N of legume material. The percentages of applied fertilizer N in the tops of the first wheat crop averaged 46.7% (SE 1.55%; range 41.1-50.3%). By contrast, only 17.3% (16.1-l&5%) Dry matter and total N of wheat tops of the N of legume material (but equivalent to Despite some large yield differences no single 96.7-100.2% of inorganic N recovered at sowing and amendment, with one exception, significantly inderived from legume residues) was accounted for in creased the amounts of tops dry matter (grain or the tops of the first crop (Fig. 3, Table 5). straw), but most amendments increased total N of Urea contributed from 14.2 to 34.8% of the N of wheat tops (Table 2). The amounts of N taken up by tops, for N inputs ranging from 25 to 75 kg ha-‘. the first wheat crop were positively correlated Nevertheless, for this first crop only, fertilizers con(P < 0.01) with the rates of application of fertilizers, tributed smaller proportions of N to wheat tops than but not of legume material. they did to the total inorganic N of soil profiles at The soil amendments did not influence dry matter sowing (calculated range 25.2-49X%). By contrast production or the amounts of total N taken up by the legume material, incorporated at 96.7 kg N ha-‘, second wheat crop, or the proportions of dry matter supplied only 18.2% of the N in wheat tops, even and N of wheat tops accounted for in grain in though legume residues at both input levels coneither crop. The distribution of 15N within tops of tributed proportionately more to plant N than they I
Table 2.
Yields of successive crops of wheat grown in a soil amended with legume material and nitrogenous
source of applied N M.
liftoralis
Urea
NH: NO;
I
Amount of applied N (kg ha-‘) 96.7 38.7 75 50 25 50 50 0
Plant’ dry matter crop I 7.13 8.93 8.53 ‘9.20 7.14 8.78 8.03 6.47
(0.97)’ (0.74) (1.01) (0.76) (0.26) (0.86) (0.71) (0.89)
‘Total top growth at harvest. ‘Standard errors of means in parentheses. Significant increases in yield compared with unamended
(t ha -‘) crop 2 6.07 5.43 5.10 5.96 5.92 5.22 5.19 5.32
(1.16) (0.39) (1.05) (0.49) (0.54) (0.48) (0.29) (1.17)
Plant’ N (kg ha-‘) Crop I crop 85.7 ‘89.9 “103 ‘99.4 72.3 *98.4 *89.2 65.8
soil: *I’ < 0.05. **f < 0.01.
(10.4) (5.8) (7.3) (9.2) (1.7) (9.5) (7.2) (7.2)
54.7 45.5 48.2 52.3 49.8 43.4 44.8 45.0
fertilizers
2 (8.4) (3.6) (7.4) (3.8) (6.6) (2.5) (3.1) (8.6)
Fate of nitrogen
from
legume
421
and fertilizers Added N Source
0
Wheat Crop 1
11:
50
25
75
Added N (kg ha-j Fig. 3. Relationship
between
M. littoralis tops t roots
95% confidence levels
100
J
the amount of nitrogen added to soil as legume that taken up by a first wheat crop.
did to the pool of inorganic N at sowing (Table 4). Grain N tended to be more heavily labelied with 15N than was straw N, especially for wheat grown in the legume-amended soil. Recovery of applied “N at sowing of crop 2
At the time of sowing of the second wheat crop the total amounts of residual 15N in soil to 90cm depth plus those amounts removed in the tops of the first crop accounted for 78-U% of input. Total percentage recoveries were not influenced by amounts of input, but were slightly greater for those soils amended with legume material (Table 5). The
and fertilizer
sources
and
percentage recoveries (average 84.1%) of legumederived N following harvesting of crop 1 were also significantly (P < 0.01) greater than those obtained (75.0%) prior to sowing of crop 1. Of the residual 15Nin soil at sowing of crop 2, most was present as organic “N, accounting for about 62% of “N input as legume material and about 29% of fertilizer “N. Most was located in soils to 15 cm depth. When at the time of sowing in year 2, the inorganic 15Nfrom either legume or fertilizer residues was related to the total “N recovered in the profiles a small but significant (P < 0.01) curvilinear trend was obtained; the higher the amounts of total 15N
Table 4. Recovery in a first wheat crop of nitrogen applied as legume material and as fertilizers Amount of applied N (kg ha-‘)
Source of applied N M. littoralis
Percentage in applied N in crop I Grain Straw
96.7 38.7 75 50 25 50 50
Urea
NH: NO,
12.5 14.7 36.6 35.0 31.3 37.9 37.1
(1.4)’ (0.64) (2.1) (3.1) (1.6) (1.9) (2.2)
3.6 3.8 10.5 11.4 9.8 12.4 I I .5
Percentage of N in crop applied source Grain Straw
(0.11) (0.33) (0.69) (0.99) (0.65) (2.1) (0.68)
19.0 8.2 34.8 23.6 14.2 25.6 27.4
(1.8) (0.4) (1.0) (0.6) (0.8) (0.9) (1.4)
15.8 7.1 32.6 22.6 14.2 25.3 26.7
1 from Tops
(1.3) (0.1) (0.5) (0.3) (1.2) (0.2) (0.9)
18.2 8.0 34.3 23.3 14.2 25.5 27.2
‘Standard errors of means in parentheses. Table 5. Recovery in crop and soil of nitrogen applied as legume material and as fertilizers Percentage recovery of applied N Source of aoolied N M. littoralis
Urea
NH: NO;
Amount of aoolied N
In tops of first wheat cron
96.7 38.7 75 50 25 50 50
Soil sampled to 90 cm depth. ‘Standard errors of means in parentheses.
16.1 18.5 47.1 46.4 41.1 50.3 48.6
(1.5)’ (0.92) (2.7) (3.7) (0.47) (3.4) (2.9)
Inoreanic 5.9 3.7 5.0 3.7 4.6 4.8 3.7
In soil at sowing of second Organic 61.3 62.7 26.1 29.2 33.8 28.8 27.8
(1.2) (1.9) (0.69) (1.5) (1.9) (0.97) (2.2)
crop Total 83.3 84.9 78.2 79.3 79.5 84.2 80.1
(0.52) (2.5) (3.2) (2.3) (3.8) (2.4) (2.9)
(1.7) (0.3) (0.7) (0.4) (0.9) (0.7) (1.3)
J. N. LADD and M. AMATO
422
-2 2
5.0 -
Wheat Crop 2
x s ii f
4.0
.cY 5E 3.0
-
P E ; 2.0 2 5 P m 1.0 E
Residual N of amendments (kg ha’)
Fig. 4. Relationship between the amount of nitrogen from legume and fertilizer residues recovered in cropped soil and that taken up by a second wheat crop.
recovered the higher the percentage inorganic 15N.
recovered as
Uptake by crop 2 of N from legume residues
and fertilizer
For fertilizer residues and legume residues respectively, the amounts of “N taken up by the second wheat crop were proportional to the amounts present as organic plus inorganic “N in the soil profiles at sowing (Fig. 4). For soils which had received fertilizer “N initially, tops of the second wheat crop contained 3.1% (SE 0.17%) of the 15N input, or equivalent to 9.2% (SE 0.34%) of total residual “N (or 71.7% (SE 6.6%) of the inorganic “N) at sowing. For soils which had been amended with legume material, “N in tops accounted for, on average, 4.2% of 15Ninput, equivalent to 6.3% of the residual ‘*N (or 91.6% of the inorganic “N) at sowing. The availability of 15N in fertilizer residues to the second wheat crop was significantly (P < 0.01) greater than that of 15Nin legume residues; the linear regressions of uptake of 15Non residual 15Nbeing for fertilizer amendments y =0.091x, and for legume amendments, y = 0.061x. Thus the availabilities and the relative availabilities of residual “N from legume and fertilizers to a second crop were considerably less observed for applied fertilizer than those
Table 6. Recovery Source of armlied N .. M.
littoralis
Urea
NH; NO, ‘Standard
0, =0.464x) and legume (y =0.171x) to the first wheat crop. Legume N initially applied at 96 kg N ha-’ contributed only 8.0% to the N of the tops of the second wheat crop (cf. 7.9% of the inorganic N pool at sowing, and 18.2% of the N of the first crop). Urea N applied at 25-75 kg ha-’ supplied only l&4.4% of the N of the second crop, compared with 1.5-4.4% of the inorganic N at sowing (Table 6). The trend for grain N to be more highly labelled than straw N was again evident in the second crop, despite the greater uniformity of labelling of the soil inorganic N in the profiles at sowing compared with that observed in the previous year. Net mineralization of residual organic 15N during the last 12 months of the experiment were determined from changes in the amounts of inorganic “N in soil profiles at sowing in years 2 and 3, and the amounts of 15Nremoved in the tops of the second crop. Values are underestimated since they ignore the amounts of “N returned to soil in roots and other crop residues and also any losses of inorganic “N which may have occurred. Expressed as a percentage of the residual organic “N in soil at sowing in year 2, mineralization of fertilizer-derived residues was 4.8% (SE 0.98%), which did not differ significantly from the average (5.4%) mineralization of legume-derived residual N.
in a second wheat crop of nitrogen
Amount of applied N (ke ha-‘) .I
96.7 38.7 75 50 25 50 50
errors of means in parentheses
Percentage of applied N in crop 2 Grain straw 3.5 3.0 2.1 2.6 2.8 2.1 2.2
(0.94)’ (0.13) (0.32) (0.14) (0.22) (0.19) (0.14)
1.1 0.8 0.8 0.8 0.8 0.7 0.6
(0.21) (0.08) (0.06) (0.03) (0.05) (0.05) (0.02)
applied
as legume material
and as fertilizers
Percentage of N in crop 2 from applied source straw Tons Grain 8.4 3.4 4.6 3.3 I .9 3.4 3.3
(0.7) (0.1) (0.2) (0.2) (0.2) (0.3) (0.1)
7.1 3.1 3.9 2.8 1.6 2.6 2.5
(0.7) (0.2) (0.2) (0.1) (0.1) (0.3) (0.1)
8.0 3.4 4.4 3.2 1.8 3.1 3.1
(0.8) (0.1) (0.2) (0.2) (0.1) (0.3) (0.1)
Fate of nitrogen from legume and fertilizers DISCUSSION
Release and movement of inorganic N in soil The concentrations of NO; in similarly-treated soils may vary widely (Nielsen et al., 1982) even in soils from cores taken a few cm apart, and in the case of ‘rNO;, in soils confined within cylinders (Carter et aI., 1967; Ladd et al., 1981). Nevertheless despite some large differences at nominated profile depths in this experiment also, the average amounts of inorganic 15N in the total profile to 90cm were, at all sampling times, directly and linearly related to the initial inputs of legume and fertilizer lSN respectively. However, at sowing in year 2 there was a small but significant curvilinear trend which indicated a higher percentage recovery of inorganic “N (and a lower percentage recovery of organic “N), the greater the amounts of total 15N recovered. The trend was not great and did not affect the linear relationships established between input “N, inorganic “N recovered, and ‘jN taken up by crops. From the time of incorporation of legume material until sowing of the first wheat crop, inorganic N derived from soil organic matter in unamended cylinders increased from 9.5 to 77.7 kg ha-‘, a net gain equivalent to 4.7% of N to 15 cm depth. The distributions of inorganic N in the soil profiles at sowing in both unamended and legume-amended soils, and of inorganic 15Nfrom the legume material itself, were only approximately (and partly) described by a simple leaching model based on the climatic conditions experienced during this period. Despite the model output (which predicted a peak inorganic “N concentration at 50cm depth), and the absence of substantial amounts of 15Nin the lowest depth of soil sampled (60-90 cm), there is evidence to suggest some leaching may have occurred below 90 cm. The recovery of inorganic “N in soil to 90 cm depth was remarkably low, viz. only about 17.5% of the legume 15N input after 7 months decomposition compared with 32.5 and 27.2% respectively after incubating the same plant material with attapulgite and Roseworthy soil for only 4 weeks under laboratory conditions. Under field conditions, some losses of applied legume 15N may have been due to volatilization as “NH’ soon after the addition of the plant material to the soils, given that the soil is calcareous and would have been subjected to intermittent drying and remoistening under our summer climatic conditions. However nitrification is rapid. Losses due to run-off were unlikely since the rims of the microplot cylinders protruded about 5cm above the soil surfaces. That leaching below 90 cm is at least partly responsible for the deficit is suggested by the total 15N recoveries in crop 1 plus in soil sampled at sowing in year 2; these were about 10% higher than those at sowing in year 1. In the following years, predictions of inorganic N distributions were potentially confounded by the absence of information regarding the amounts of inorganic N not utilized by the preceding crop. Carry-over of nitrate from one growth season to the next is not an uncommon occurrence in soils cropped under the uncertain climatic conditions of a Mediterranean-type environment (Noy-Meir and Harpaz, 1977). We suggest that in our experiment
423
either no inorganic N had remained in the profile to 90 cm depth at the commencement of the crop ripening phase or that any remainder had been leached below the sampling depth by sowing time of the following year (especially by sowing time in year 2). This conclusion is based on the relatively uniform labelling of the inorganic N throughout the profile at sowing in years 2 and 3 respectively, even though in each case the atom% enrichment of the inorganic N was less than that of the preceding year. We suggest such results would be achieved if all of the inorganic 15N recovered in the soil profile to 90 cm depth were derived from the progressively more stable organic “N pool mixed (mainly in topsoil) with the major source of unlabelled inorganic N. Uptake of N by wheat crops The declining “N atom% enrichments with time of N released from labelled organic residues was predicted since organic “N from legume material, incorporated at the Roseworthy site (and elsewhere), decayed at relatively faster rates than did native soil organic N throughout an 8-yr experimental period (Ladd et al., 1985). Further, it was anticipated that because the net rates of decay of plant residues decline markedly within the first year of incorporation of the source materials, but slowly thereafter, then the 15Natom% enrichments of N of wheat crops grown subsequently would be similar to that of NOT-N in the soil profiles at sowing, as was observed (Tables 1 and 6). Nevertheless, the enrichments of NO;-N do fall slowly with time, even during a growth season, and analyses show that the N of field-grown wheat crops is not uniformly labelled. Consistently we have observed, as have others, that grain N is of slightly higher 15N atom% enrichment than is straw N (Tables 4, 6; Riga et al., 1980; Ladd et al., 1981, 1983a). In the experiments reported here these trends were obtained with both crops, even though in the case of the second crop, the ISNO; pool was relatively uniformly labelled at sowing. Indeed for those treatments in which “N-1abelled fertilizers were applied initially, the greater differences in the extent of labelling of grain N and straw N were achieved with the second crop rather than the first. Our results are consistent with the suggestions in an IAEA report (1974) that N taken up late in the vegetative growth phase is preferentially translocated to grain subsequently. Gass et al. (1971) demonstrated in experiments with corn grown in soil to which 15NO; was applied at different depths that grain N was preferentially derived from NO; obtained from the middle and lower parts of the soil profile. We suggest that in the cases of (a) both wheat crops utilizing 15NO; from legume residues and of (b) the second crop utilizing 15NO; from fertilizer residues, much of the available NOT-N at sowing was already distributed throughout the profile, and especially below 30cm, and such NO;-N would have had a slightly greater enrichment than that released from organic residues during the period of wheat growth. By contrast, in the case of the first wheat crops utilizing applied fertilizer N, all of the available 15Nwas present in the topsoil at sowing, with possibly less opportunity for more-highly enriched pools of
424
J. N. LADD and M. AMATO
NO; to be leached to depths below those of the active root zones, and therefore for late uptake and enhanced grain N enrichment. First wheat crops took up about 46% of applied fertilizer N. The efficiencies of fertilizer N use were not significantly affected by the form of the fertilizer and were comparable to those reported in the literature. Substantial proportions of applied fertilizer “N also became immobilized in organic residues in soils as a result of C metabolism during the period of wheat growth. By contrast, decomposition of applied legume material ensured that despite net mineralization of legume “N prior to and during cropping, the proportions of legume N immobilized in organic residues were greater than those of fertilizer “N. Thus the efficiencies of use of fertilizer 15N were markedly higher than those (17%) determined for applied legume N, but less than those (98%) based on the amounts of legume-derived inorganic N present in the soil profiles to 90 cm depth at the time of fertilizer application. The latter figure may be inflated if the first wheat crop had derived some of its nitrogen from NO; released from legume material and leached to below 90 cm before sowing. Our early studies on the supply of legume N to wheat crops grown under field conditions involved 3 sites; all soils were calcareous and of similar pH, two were sandy-loams, the third a heavy clay soil (Ladd et al., 1981). Wheat yields varied and clearly did not relate to the percentage of applied legume N taken up by the crops. Subsequent experiments at a fourth site with a soil of similar properties to the Roseworthy soil (Ladd et al., 1983a), together with experiments reported in this paper, have extended the data available. With this greater perspective, the early results obtained with the clay soil revealed a relatively low percentage uptake of legume N by a high-yielding wheat crop and are seen to be an exception. For all other soils, irrespective of season and levels of input of legume “N, there was a significant direct relationship between the percentage OJ) of legume N take up by the tops of first wheat crops and the dry matter yields (x, in t ha-‘). The relationship is described by y = 11.2 + 0.81x, for a range of yields from 4.6 to 16.6 t ha-’ (r = 0.88). The relationship appears to derive mainly from the capacity of the various crops to take up the released N, rather than from the net rates of N mineralization from the residues themselves. Ladd et al. (1981, 1983b, 1985) had shown that the percentage net decline of residual organic N from legume materials of similar low C-to-N ratios and which had decomposed for 0.5 years or more in unplanted soils at the same sites, may be significantly influenced by soil type, climate and input levels of legume N, but the effects are relatively small. Nevertheless, the effects for example of the decomposition and turnover of C of wheat roots on the net rates of N mineralization in soils (especially under water stress conditions which frequently limit crop yields in our environment), have yet to be assessed. The results also imply that the fate of N released from legume residues, but not taken up by a wheat crop, may in a given season be similar in soils of comparable physical properties. Second wheat crops took up 6.3% of legume residual “N and 9.2% of fertilizer residual 15N.Given
that for both treatments there had been no carry-over of IsNO; not utilized by the first crop, and knowing that inorganic lSN released at the time of sowing crop 2 was similarly distributed in soil profiles irrespective of the nature of the nitrogenous source applied initially, then the results are consistent with the notion that the greater stability of residual organic “N from legume material was due to it having resided in soil at least 7 months longer than had residual organic 15N of immobilized fertilizers. The results emphasize that the differences in availability of N from fertilizers and plant materials returned to soil occur mainly in the first year after their application only. The relative contributions of legume material and soil organic matter to the N of wheat crops will depend on the size and decay rates of each source. In studies reported here and earlier we have found that for legume materials of low C-to-N ratios (11 .l-14.9: 1) and decomposing in sandy-loam topsoils (of organic N contents of approximately 0.1%) a simple relationship has held irrespective of season and crop yield, viz. each 10 kg N applied to soils in legume materials contributed 2% of the N of a first wheat crop, and 1% of the N of the second crop (Ladd and Amato, 1985). We stress that this consistent relationship has been obtained using legume materials of similar low C-toN ratios, and soils of similar properties. In these circumstances similar proportions of legume N will mineralized from their readilybe readily decomposable components, and it can be expected that the further release and distribution of 15N0~ from 15N-labelled organic residues and of 14NO; from native soil organic matter, both sources residing principally in topsoils, will be similarly affected by climatic conditions. Whilst the range of inputs of legume N used in our experiments have covered most estimates of N gains due to legumes, as determined in long term rotation trials in southern Australia, nevertheless under normal farming conditions the C-to-N ratios of plant materials returned to soil and the timing of their return will vary widely. For example senesced legume materials, weathered and partly decomposed as surface residues over summer may be of higher C-to-N ratios than those of green legume materials, and may contribute less N for a following wheat crop than indicated from our “N based studies. Indeed incubation of brown weathered legume material mixed with soils may cause temporarily net immobilization of N (Ladd et al., 1986). By contrast, in grazed pastures some legume N will also be returned to soil in faeces and urine. Under field conditions much of the urea N in urine may be lost from the soil as NH3 by volatilization, but the remainder presumably would have an availability to the first succeeding wheat crops greater than the N of green legume materials and approaching that of the fertilizer N. Acknowledgements-We thank Mrs M. Bohnsack and Messrs J. M. Thompson and P. J. L. Williams for technical assistance, Mrs R. B. Anderson and Dr R. L. Correll for statistical analyses and Dr K. Smettem for calculations of NO< leaching. We are grateful to Mr H. J. Krieg of Roseworthy for his cooperation in our use of the experimental site.
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