Managing legume leys, residues and fertilisers to enhance the
sustainability of wheat cropping systems in Australia
1. The effects on wheat yields and nutrient balances
Anthony M. Whitbread
a,*, Graeme J. Blair
b, Rod D.B. Lefroy
caCSIRO, Tropical Agriculture PO Box 102, Toowoomba, Qld 4350, Australia
bDivision of Agronomy and Soil Science, University of New England, Armidale, NSW 2351, Australia cInternational Board for Soil Research and Management, PO Box 9-109, Jatujak, Bangkok 10900, Thailand
Received 22 April 1998; received in revised form 4 December 1998; accepted 6 December 1999
Abstract
Farming activities practiced on many Australian soils have resulted in substantial losses of soil organic matter (SOM), nutrient loss, soil structural degradation and declines in cereal yield and quality. Field trials, consisting of a legume or fallow phase followed by three wheat (Triticum aestivumL.) crops, were established on a degraded Ferric Luvisol (Red Earth) soil in New South Wales to investigate the effect of crop residue and fertiliser management on wheat yield and nutrient balances. There were no effects of a chickpea (Cicer arietinumL. cv Amethyst), barrel medic (Medicago truncatulaL. cv Sephi), or fallow phase on the grain yields of three subsequent wheat crops. Grain yield was depressed by 12% following a lucerne (Medicago sativa L. cv Trifecta) crop from which the plant residues had been removed, relative to when residues were returned or grazed. Consecutively, higher wheat grain yield losses of 7.4 and 8.6% in 1994 and 1995 were found on treatments from which wheat stubble was annually removed from the system. Grain yield losses of 6, 7 and 13% in three consecutive wheat crops were found where no fertiliser was applied at sowing. Nutrient balances, based on inputs of nutrients in fertilisers and residues, and the export of nutrients in grain and crop residue were found to be useful in describing the ¯ow of nutrients in a farming system and predicting possible soil nutrient depletion. Fallow systems provide no nutrient inputs and result in N losses of up toÿ189 kg haÿ1over three wheat crops. The balance of nutrients such as potassium (K), which are contained in larger proportions in stubble, were found to beÿ102 kg haÿ1on the wheat stubble removed treatments and8 kg haÿ1on the stubble retained treatments. Better recycling of crop residues and improving ley system to increase nutrient and C inputs have the potential to improve soil fertility and grain production.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Wheat yield; Nutrient balance; Legume rotation; Crop residue; Stubble management
1. Introduction
Many farming practices exploit soil organic matter (SOM) to provide essential nutrients for plant growth.
*Corresponding author. Tel.:61-74688-1200; fax:
61-74688-1193.
E-mail address: anthony.whitbread@tag.csiro.au (A.M. Whitbread).
The mineralisation of SOM releases nutrients to the soil which may become available for plant uptake, converted to unavailable forms, lost to the atmosphere or leached and eroded. The rate of this process depends on climatic conditions and the nature of individual nutrients. When a farming system is unbalanced, e.g., when the export of nutrients is greater than inputs, the demand for nutrients affects the SOM. As SOM is depleted, both chemical and physical soil fertility are lost and the required input costs are increased.
The long term effects of cropping and cultivation in Australian farming systems have been well documen-ted. The cropping of soil which had previously been under native vegetation has been shown to cause losses of SOM, C and nutrients (Dalal and Mayer, 1986; Blair et al., 1995), soil structural declines (Connolly and Freebairn, 1996) and soil degradation. Many Australian wheat growing systems are characterised by falling grain protein concentrations and land degra-dation, both of which are related to declines in soil chemical and physical fertility. It has been estimated that approximately 10 mha of agricultural land in Australia have been affected by some form of erosion and more than 30 mha have suffered fertility decline (Hamblin and Williams, 1995). Dalal et al. (1991) showed that in the once fertile soils of the Darling Downs and brigalow lands of subtropical eastern Australia, the protein concentration of wheat grain had dropped from 0.16 kg kgÿ1 to less than 0.11 kg kgÿ1after 25 years of cropping to less than 0.09 kg kgÿ1after 50 years of cropping and that yield declines of up to 50% had occurred. At a time when high grain protein concentrations and low production costs equate to better pro®tability, farming systems that optimise the ef®cient use of fertilisers and crop residues and ley phases need to be developed.
Grass and legume leys incorporated into a farming system can potentially improve soil fertility, crop yields and pro®tability (Holford, 1992; Armstrong et al., 1997). However, the effect of the commonly used short term (<1 year) ley phases and the manage-ment of crop residues and fertilisers on the overall fertility of the farming system are unclear. A ®eld trial was conducted in north-western New South Wales to examine the effects of short term legume ley crops and the management of wheat stubble and fertiliser on wheat yields. The aim of this experiment was to investigate the effects of management systems on
the level and nature of SOM, changes in soil chemistry and soil structural properties that result from changes in organic matter and the nutrient dynamics of these systems. This paper reports on the wheat yields and mass nutrient balance results while a companion paper (Whitbread et al., 2000) reports on the changes in soil C and physical properties.
2. Materials and methods
2.1. Site description
The experiment was carried out on a Ferric Luvisol (Red Earth) soil at the University of New England's McMaster Research Station, near Warialda in north-western New South Wales, Australia. This soil type is representative of large areas of eastern Australia where cereal-based farming occurs. Soil clay content increased gradually with depth from 153 g kgÿ1in the 0±0.1 m depth. Volumetric ®eld capacity (0±0.5 m) is 0.14 kg kgÿ1and wilting point is 0.05 kg kgÿ1. Aver-age annual rainfall is 642 mm of which approximately 265 mm is received in the June±December cropping period. Temperatures regularly rise above 308C in summer and fall to 5±108C in winter (Fig. 1).
2.2. Experimental design and layout
Treatments were laid out according to a split-split-split plot design, with main plots (chickpea (Cicer arietinum L. cv Amethyst), barrel medic (Medicago truncatulaL. cv Sephi), lucerne (Medicago sativaL. cv Trifecta) and fallow systems) being laid out in blocks. These were split according to management of residues which grew on the legume or fallow systems (removed, returned, grazed). These plots were then split into fertiliser (applied and not applied) or wheat stubble management (removed or returned).
A summary of the treatments and cropping sequence is presented below:
1993, 1994, 1995 Wheat phase 2 Fertiliser levels [(ÿF) 0, (F) 25,
7, 26.8 kg N, P, S haÿ1] 2 Wheat stubble management
[(ÿR) removed, (R) returned]
7.5 m2.5 m
The experimental area straddled an existing fence-line (slope of <5%) and had been conventionally farmed (multiple tillage operations to control weeds and prepare a seedbed) wheat±lucerne rotation (sev-eral cereal crops followed by a lucerne phase of 2±3 years) for at least 18 years. The ``wheat'' area had been sown to wheat for the previous 5 years. The other side of the fence, the ``lucerne'' area had been a grazed lucerne ley phase for 4 years preceded by cereal cropping for at least 16 years.
2.3. Legume phase
During May 1992, four cropping systems were established. On the ``wheat'' area, chickpea was sown at the rate of 55 kg haÿ1, barrel medic was sown at 10 kg haÿ1, and a long fallow was established. Each was a 15 m15 m plot randomly arranged in three replicates. No fertiliser was applied. The same sized plots were also marked out on the existing lucerne area
to provide three replicates. The long fallow is a farming system where no crops are grown and weeds are con-trolled by tillage or herbicides. The aim of the fallow is to increase stored water, and allow mineralisation to increase available soil nutrient concentrations.
These systems (hereafter referred to as chickpea, medic, fallow and lucerne) were grown during the winter and spring of 1992. In December 1992 the 15 m15 m plots were split into three 5 m15 m plots. The plant residue on the plots was either cut and removed, grazed in situ or left untouched. Due to the poor legume growth, as a result of dry seasonal conditions, medic, chickpea and lucerne hay was added at 4000 kg haÿ1 to the plots that were to be grazed or where the legume was to be retained. With-out this supplementation there would have been little or no difference between the pre-wheat cropping systems and the three methods of residue manage-ment. On the appropriate treatments all plant material which had grown was cut to ground level and removed. This plant material was weighed and sub-samples were collected and these, together with sam-ples of the added residues dried for nutrient analysis. The only plant material on the fallow plots were weeds that had grown during the season. There was insuf®-cient chickpea grain to warrant harvesting.
The stocked plots were grazed in situ with Merino sheep for 24 h. Sheep were con®ned to the plots using portable electric fences. This treatment was not repre-sentative of a normal grazing practice in Australia and sheep should have been con®ned to these treatments longer. Problems with con®ning sheep to plots and providing drinking water limited the period of grazing to 24 h.
2.4. Wheat phase
At sowing during June 1993, the 5 m15 m plots were divided into 2.5 m7.5 m plots. Wheat (Triticum aestivumcv. Janz) was planted into all plots with half the treatments receiving 144 kg haÿ1Crop King 600S fertiliser (25 kg N, 7 kg phosphorus (P) and 26.8 kg sulphur (S) haÿ1) and the other half receiving none. The crop was grown until maturity and harvested on November 28, 1993 using a plot header. After harvest, wheat stubble was either removed (ÿR) or returned (R). On the stubble returned treatment (R) wheat stubble, included standing stubble and husk and stub-ble which was deposited on the plots after the header had passed. Wheat was re-sown in 1994 and 1995 using the same treatments.
2.5. Water relations measurement
A single neutron probe access tube was installed into each of the 2.5 m7.5 m plots of replicate three of the chickpea, medic and fallow treatments prior to sowing in 1992. Dry soil conditions in the lucerne area excluded the installation of access tubes in this treat-ment. Soil water content was measured at 0.1 m intervals to 1.0 m depth using a neutron probe water meter and converted to volumetric water content following the calibration method of Carneiro and de Jong (1985). Soil water was measured at sowing and harvest, and at several intervals during the growing season. The extremely dry season of 1994 made emergency irrigation measures necessary to ensure crop growth.
2.6. Management of ®eld trials
Soil was cultivated to approximately 0.13 m depth 4 weeks prior to sowing by one pass with a tractor driven rotovator set at a low rotation speed. This ensured that
crop stubble was incorporated and remained within each respective plot. The soil water content at the time of cultivation was 0.10 kg kgÿ1. Ten soil samples (0± 0.1 m depth) were collected from each plot and bulked together. Wheat was planted at 0.2 m row spacing at 53 kg haÿ1using a planter. Fertiliser was banded with seed on the fertiliser applied plots.
The wheat crops were generally harvested in the last week of November. To collect grain and stubble samples for analysis and estimation of stubble yield, hand harvest samples (three 1 m rows per plot) were collected prior to harvesting with an autoheader. The autoheader remained on each plot long enough for the wheat trash to be expelled. Subsamples were collected to calculate and correct for water content. The sepa-rated stubble and wheat were subsampled and ground to 0.5 mm for chemical analysis.
2.7. Analyses of plant material
Plant samples, collected from the legume crops in 1992 and the three wheat crops, were dried, subsampled and ground to <0.5 mm. An ARL3560 Inductively Coupled Plasma Atomic Emission Spec-trometer (ICP-AES) was used to measure P, S and K after the plant material had been prepared by the sealed container digest procedure of Anderson and Henderson (1986). The measurement of N in the 1993 and 1994 harvest plant material was performed using an autoanalyser following a sulphuric acid/perchloric digestion procedure (Lindner and Harley, 1942). The N content of the 1995 plant material was analysed with a Near Infrared (NIR) scanning spectrophotometer 6500 (NIR Systems, Inc) with the standards being based on the wet digestion values obtained for the analyses of the 1994 plant material.
2.8. Analyses of soil
Soil samples (a composite mixture of 10 samples per plot) (0±0.1 m) were collected with a soil core (internal diameter 30 mm) in June 1993, prior to the wheat phase, and again in June 1996, after the wheat phase. The soil samples were analysed for available S using the KCl-40 extraction technique (Blair et al., 1991), for available P using the Colwell P method (Colwell, 1965) and for nitrate-N using the method of Adamsen et al. (1985).
2.9. Statistical analyses
Yield and nutrient data from the experiment were subjected to a split-split-split plot analysis of variance, with main plots (chickpea, medic, lucerne and fallow systems) in blocks then split according to management (removed, returned, grazed). These plots were then split into fertiliser applied and not applied and wheat residue removed or returned. Water relations in the medic, chickpea and fallow plots were statistically analysed using the legume manage-ment systems to estimate the error term. Residuals were examined for homogeneity, which generally indicated that data transformation was unnecessary. Mean separation was determined using Duncan's
multiple range test and is depicted in tables and using lower case letters. All means were separated at the 5% level.
3. Results
3.1. Rainfall, irrigation and soil water
The long term average monthly rainfall and the monthly rainfall received during the experiment are shown in Table 1. Good rainfall was received evenly throughout the 1993 growing season with no signi®-cant differences found in soil water content at sowing between the chickpea, medic and fallow treatments. Early rainfall in 1994 provided good pre-crop soil water. Rainfall received just prior to the 1994 crop was low and total natural rainfall received during the growing period was only 90 mm. Consequently, a total of eight irrigations of 15 mm each were applied at various intervals throughout the growing season. Although, the 1995 crop received almost the same amount of rainfall as the 1993 wheat crop, almost 90 mm fell a week prior to harvest, too late to bene®t the mature crop. Only one irrigation was applied during the 1995 season when soil water content was low.
Table 1
Long term average monthly rainfall (1969±1995), rainfall and irrigation received during the experiment at the McMaster Research Station, Northern New South Wales, Australiaa
Long term average rainfall (mm)
Monthly rainfall during the experiment (mm)
1992 1993 1994 1995
January 92 ± 24 46 165
February 77 ± 74 172 57
March 55 ± 25 34 19
April 41 ± 1 12 1
May 49 ± 58 2 38
June 31 ± 12 15 (15)b 57
July 45 32 62 4 (15) 6
August 33 60 26 26 (15) 1
September 37 15 62 14 (45) 59 (15)
October 55 42 78 11 (30) 47
November 62 52 14 35 137
December 63 51 56 52 98
Total 640 492 543 700
There was no difference in soil water between the treatments at sowing in 1993 and 1995 (data not presented). However, at sowing in 1994, soil water signi®cantly increased from 0.25 kg kgÿ1 where wheat stubble had been removed from the previous wheat crop, to 0.27 kg kgÿ1on the stubble retained treatments. There were no signi®cant differences in water content between the chickpea, medic and fallow systems at any time during the trial.
3.2. Wheat yields
Yearly differences in rainfall (Table 1) largely accounted for yield differences between the three wheat crops. The effect of the legume/fallow systems on wheat yield were restricted to the ®rst wheat crop following this phase. In 1993 wheat yields were lowest on the treatments following the lucerne phase on which residues had been removed rather than grazed (Fig. 2). Although, this trend continued into the second wheat crop, it was no longer a signi®cant effect. Whether the residues from the chickpea or medic systems were removed, returned or grazed resulted in no conclusive trends emerging.
The most signi®cant impact on wheat yields was due to the fertiliser and wheat residue management treatments. The absence of applied fertiliser at the sowing of the wheat crops generally resulted in a signi®cant decline in the grain and stubble yields
relative to the treatments with fertiliser applied (Table 2). The magnitude of this decline increased from 6.0 and 3.5% in 1993 to 12.9 and 13.8% in 1995 on the grain and stubble yields, respectively. The removal of wheat stubble at harvest resulted in declines in grain and stubble yield in excess of 7.4 and 6.6%, respectively (Table 2).
3.3. Nutrient content of wheat grain and stubble
The nutrient concentration and content of wheat grain and stubble is determined by soil fertility and crop yield. The effect of the pre-wheat legume/fallow phases on nutrient content of wheat was limited to the 1993 wheat phase and varied between wheat stubble and grain. The N content of wheat grain following a
Fig. 2. The effect of residue management of the legume/fallow systems grown in 1992 on the grain yield of wheat in 1993. Bar represents LSD atP0.05.
Table 2
The effect of fertiliser and residue management on grain and stubble yields and the decline in yields due to wheat stubble removal and no fertiliser application during 1993, 1994 and 1995a
ÿFb(kg haÿ1) Fc(kg haÿ1) Decline (%) ÿRd(kg haÿ1) Re(kg haÿ1) Decline (%)f
1993 Grain 3334 b 3547 a 6.0 ±g ± ±
Stubble 4152 a 4303 a 3.5 ± ± ±
1994 Grain 1983 b 2138 a 7.2 1981 b 2139 a 7.4 Stubble 2950 b 3238 a 8.9 2989 a 3199 a 6.6 1995 Grain 2209 b 2637 a 12.9 2266 b 2479 a 8.6 Stubble 3411 b 3953 a 13.8 3553 b 3812 a 6.8
aMeans followed by the same letter within rows within treatments are not signi®cantly different according to Duncan's multiple range test
atP0.05.
bNo fertiliser inputs at sowing. cFertiliser applied at sowing. dWheat stubble removed at harvest.
eWheat stubble remained on plots after harvest and incorporated before sowing.
fallow phase declined relative to the other legume treatments (Table 3). The P, S and K content of grain, however, was unaffected by the pre-wheat treatment. Nutrient content of the wheat stubble was generally highest following the chickpea treatment and N and K content was somewhat lower following the fallow treatment. The pre-wheat legume treatments where crop residues were removed resulted in signi®cantly lower N, P, S and K concentrations in wheat stubble (data not presented). The N content of grain in the legume residue removed treatments was 63 kg N haÿ1 and increased signi®cantly to 70 and 72 kg haÿ1when residues were returned or grazed. The effects of the
management of the pre-wheat legume/fallow phases were limited to the 1993 wheat crop.
The proportion of the nutrient contained in grain relative to the total nutrient content in the tops (grainstubble) ranged from greater than 84% for N and P, more than 60% for S to less than 23% for K (Table 3).
During the three wheat phases wheat residue man-agement and fertiliser application remained the domi-nant effect on grain and stubble nutrient content. A summary of the effect of the fertiliser and residue management on N, P, S and K content of the 1995 wheat crop is presented in Table 4 and represents a
Table 3
The effect of the legume or fallow systems on the N, P, S and K content (kg haÿ1) of wheat grain and stubble in 1993a
Nutrient Lucerne
aMeans followed by the same letter within rows are not signi®cantly different according to Duncan's multiple range test atP 0.05.
bThe proportion of nutrient as a percentage of the total contained in the grain stubble.
Table 4
The effect of fertiliser and stubble management on N, P, S and K content of grain and stubble at harvest 1995a
Plant
aMeans followed by the same letter within a column of either grain or stubble are not signi®cantly different according to Duncan's
multiple range test atP0.05.
bNo fertiliser inputs at sowing. cFertiliser applied at sowing. dWheat stubble removed at harvest.
similar pattern as the 1993 and 1994 wheat phases. The application of fertiliser and the conservation of crop stubble resulted in signi®cant increases in the content of N, P, S and K in grain (Table 4). Fertiliser application signi®cantly increased N and K content in stubble while residue retention signi®cantly increased S and K content.
3.4. Nutrient content of the pre-wheat legume/fallow phase
The form of residue grown during the pre-wheat phase determined the nutrient input into the system. The lucerne residues contained the greatest quantity of N, P, K and S followed by the medic and chickpea residues (Table 5). The fallow phase returned no nutrients to the system.
3.5. Nutrient balances
Nutrient balances were calculated for N, P, S and K based on all of the nutrient inputs in fertiliser and legume residues and nutrient removal in wheat grain and stubble. The nutrient balances have been calcu-lated for each wheat crop and these represent the cumulative effects of the legume phase and the pre-vious wheat crops.
The addition of fertiliser changed the balance of N, P or S in the system. In the case of S, the fertiliser S inputs exceeded removal in plant products and resulted in a positive balance of 22 kg S haÿ1. The addition of fertiliser signi®cantly increased the export of K fromÿ14 toÿ17 kg K haÿ1due to the increase in plant tissue K content.
For all the nutrient balances examined, there were signi®cant interactions between the legume/fallow systems and the management of the residues produced during this period (i.e., residues were either removed, returned or grazed) (Figs. 3±6). The nutrient balances were always signi®cantly lower when the residues were removed from the system rather than when they were returned or grazed. There were generally no signi®cant differences between the returned and grazed treatments. There was no attempt to estimate the removal of nutrients by the sheep in the grazing treatment as the time of grazing was short (<12 h) and potential for nutrient removal minimal.
The N balance indicates a net export of N from all treatments due to the high removal of N contained in grain (Fig. 3). The lucerne system contributed most N while the fallow system contributed none (Table 5). The retention of wheat stubble after harvest each year
Table 5
Nutrient additions from the residues added during the legume/ fallow systems grown in 1992 (kg haÿ1)
Legume/fallow system
Nutrient addition (kg haÿ1)a
N P S K
Lucerne 104.2 a 6.3 b 9.2 a 104.9 a Chickpea 71.4 c 6.0 c 5.3 c 56.2 b Medic 99.5 b 6.5 a 7.0 b 56.2 b Fallow 0 d 0 d 0 d 0 c
aMeans followed by the same letter within columns are not
signi®cantly different according to Duncan's multiple range test at
P0.05.
Fig. 3. The effect of residue management of the legume/fallow systems grown in 1992 on the ®nal N balance after three wheat crops in 1995. Soil N was not included in the N balance. Bar represents LSD atP0.05.
resulted in a balance ofÿ25 kg N haÿ1compared with ÿ32 kg N haÿ1when residues were removed.
The P balance was similar between the lucerne, chickpea and medic systems, but in the fallow system, where there was no initial input of P, balances were more negative (Fig. 4). The amount of P contained in crop stubble (11±14% as a proportion of the total contained in the plant tops) is relatively small com-pared with the proportion that is exported in grain (Table 3). There was therefore no signi®cant effect of wheat stubble management on the P balance.
The application of S rich fertiliser at the sowing of each wheat crop resulted in positive S balances for all systems (Fig. 5). The K balance was only positive when the lucerne residues, containing 104 kg K haÿ1 (Table 5) were returned to the system (Fig. 6).
There was a signi®cant interaction between the removal or return of wheat stubble after harvest and the yearly S and K balance. Since approximately 79% of K and 36% of S in the wheat crop was contained in
the straw, the management of wheat stubble signi®-cantly in¯uenced the K and S balances (Table 6).
3.6. Soil nutrients
Soil samples (0±0.1 m) collected prior to sowing in June 1993 and 1996 were analysed to determine concentrations of available N, P and S. Nitrate-N concentrations in 1993 were signi®cantly higher fol-lowing the lucerne and fallow phases, however, there was no signi®cant differences found for available P and S concentrations between the legume and fallow phases (Table 7). There was a minimal effect of the legume management on concentrations of available P and S, however, nitrate-N decreased from 35.7 and 33.3mg gÿ1when residues were returned or grazed, respectively, to 26.1mg gÿ1 when legume residues were removed.
After the three seasons of wheat, all available nutrient concentrations declined signi®cantly from
Fig. 5. The effect of residue management of the legume/fallow systems grown in 1992 on the ®nal S balance after three wheat crops in 1995. Soil S was not included in the S balance. Bar represents LSD atP0.05.
Fig. 6. The effect of residue management of the legume/fallow systems grown in 1992 on the ®nal K balance after three wheat crops in 1995. Soil K was not included in the K balance. Bar represents LSD atP0.05.
Table 6
The S and K balance of the wheat stubble removed (ÿR) and returned (R) treatments in each year of the wheat phase and the ®nal S and K balancea
Year S balance (kg haÿ1) K balance (kg haÿ1)
ÿR R ÿR R
1993 8.0 d 11.1a ÿ25.1c 23.8 a
1994 7.2 f 10.0 b ÿ45.9 e ÿ7.7 b
1995 7.6 e 9.5 c ÿ30.9 d ÿ8.3 b
Total nutrient exported 22.8 30.6 ÿ102 7.8
their initial levels. The effect of the fertiliser added treatments increased available P from 8.1 to 11.2mg gÿ1, however, N and S concentrations were unaffected. Wheat residue management resulted in minimal effects on concentrations of available soil nutrients.
4. Discussion
4.1. Wheat yields
The 1993 wheat grain yields were depressed fol-lowing the removed treatment of the lucerne phase. This was most probably due to the lower nutrient inputs and the removal of surface cover from this treatment. A number of other studies have resulted in con¯icting results regarding the in¯uence of legume leys on yields of subsequently grown cereal crops. Holford (1992) found that 12 months of lucerne growth caused dehydration of the soil to 200 mm and depressed wheat yields. Adequate rainfall between the lucerne crop and the next cereal crop is needed to rehydrate the soil to exploit the bene®t of the lucerne phase. McCallum et al. (1996) also reported yield declines in wheat production of 0± 827 kg haÿ1due to lower soil water content following a lucerne phase.
Holford (1992) reported bene®cial effects on wheat lasting for at least 9 years following a lucerne phase of 2.5±5.5 years on a Calcic Vertisol (Black Earth). Marcellos and Felton (1992) investigated the effect of one season of chickpea, wheat or long fallow on the grain yield of two subsequent wheat crops. The
increase in yield of the ®rst wheat crop following the chickpea phase ranged from 0.41 to 2.11 t haÿ1 with the response to the long fallow being similar. They also found that the level of soil nitrate at sowing of the wheat crop strongly in¯uenced wheat yields and there were no signi®cant bene®ts of chickpeas to subsequent wheat crops.
In the experiment reported here, the amount of N ®xed by the previous lucerne phase was unknown, but may represent a substantial additional input of N in these lucerne treatments. Hossain et al. (1995) showed that N ®xation was 56 kg N haÿ1per year for a medic ley and 83 kg haÿ1per year for lucerne ley on heavy clay soil at Warra, Qld. High available soil nitrate-N, P and S concentrations, which were measured prior to sowing of the ®rst wheat crop (Table 7), indicated that nutrient inputs during the legume phase were retained in the available soil nutrient pools.
The 1994 wheat crop was grown in a year where 90% of New South Wales was drought declared, including the district in which this experiment was located. Most of the 90 mm of rain that fell during the growing season was in small rainfall events and probably resulted in minimal bene®ts due to high evaporation losses. Irrigation supplied most of the crop's water needs, but evaporation rates were also high resulting in the loss of much of this water. The plots on which wheat stubble was retained had sig-ni®cantly higher soil water content at sowing in 1994, presumably due to better soil physical properties (Whitbread et al., 2000).
The decline in wheat grain and stubble yields as a result of no fertiliser addition increased consistently across the three wheat crops (Table 3). The removal of
Table 7
The effect of the legume/fallow systems on the concentration of available soil N, P and S (0±10 cm) at sowing in 1993 and 1996a
Legume/fallow system Nitrate-N (mg gÿ1)b Colwell P (mg gÿ1)c KCl-40 S (mg gÿ1)d
1993 1996 1993 1996 1993 1996
Lucerne
37.2 a 0.8 a 10.4 a 4.1 b 6.0 a 1.7 a Chickpea 25.0 c 1.1 a 21.7 a 12.7 a 6.1 a 1.6 a
Medic 29.9 b 1.3 a 20.3 a 11.9 a 5.2 a 1.2 b
Fallow 34.7 a 1.1 a 20.6 a 10.5 a 5.5 a 1.4 b
aMeans followed by the same letter within columns are not signi®cantly different according to Duncan's multiple range test atP0.05. bAdamsen et al. (1985).
wheat stubble after the 1993 and 1994 harvests also resulted in declining yields relative to the stubble retained treatment, especially on grain yields. An important observation was that the relative difference between the removed and returned treatment and the fertiliser applied and not applied treatments became larger each year. These can be related to several factors including decreases in the availability and concentration of soil nutrients and soil structural degradation which is reported in the following paper (Whitbread et al., 2000).
4.2. Plant nutrient dynamics
The legume/fallow system in¯uenced the content of nutrients in the plant tops during the 1993 wheat crop, but the predominant in¯uence on the 1994 and 1995 crops became the addition of fertiliser at sowing and the retention of the wheat stubble after harvest.
The proportion of nutrients in the various plant parts, which may be removed from a farming system in harvestable products, has implications to the nutri-ent balance of that system. More than 85% of N and P and 61% of S in the mature wheat plant was contained in the grain and was removed during harvest. In contrast, approximately 80% of K from the wheat plant was contained in the stubble. If the stubble is removed for hay or ethanol production, or burnt in situ, losses of K from the system are high. In the wheat cropping system, which was preceded by a fallow phase, the nutrient balances for N, P, S and K were generally negative, and this was especially so when stubble was removed and no fertiliser was applied. When legumes were used as the pre-wheat system, the addition of nutrients was higher.
Soil nutrient concentrations were found to decrease from the high concentrations following the legume phase to very low concentrations after the third wheat crop. It is likely that the low nitrate-N and S concen-trations in the 0±10 cm pro®le are to a large extent due to leaching. This would be most likely to occur over the summer rainfall period, during which time in this environment, heavy rainfall events are common. Although, the added fertiliser treatment resulted in positive S balances, S was probably lost through leaching. Available soil P concentrations were sig-ni®cantly higher on theF treatments but the wheat stubble management had no signi®cant effect. Despite the generally positive P balances in theF treatments, soil P concentrations continued to decline with wheat cropping.
The most exploitative cropping system was where pre-wheat legume residues and wheat stubble were removed and no fertiliser was applied. All nutrient balances in this system were negative showing a net export of nutrients (Table 8). The magnitude of N and K loss from the system was especially large and has deleterious implications for continued production. In the least exploitative systems (where legume residues and wheat stubble were retained and fertiliser applied) nutrient balances were positive for P and S. Where a fallow phase was used the K balance was negative and the N balance was only positive when the medic system was used as the pre-wheat phase. The gain in S was due to the high concentration of S in the fertiliser. Large exports of N and P in grain need to be replaced through fertiliser application, cover crops and N ®xation to stop the continued decline in soil fertility and grain protein concentrations.
Table 8
Nutrient balances for N, P, S and K (kg haÿ1) on the most exploitative and least exploitative management systems (the lucerne and fallow
systems presented only)
Most exploitativea Least exploitativeb
Lucerne (kg haÿ1) Fallow (kg haÿ1) Lucerne (kg haÿ1) Fallow (kg haÿ1)
N ÿ143.2 ÿ148.7 ÿ8.8 ÿ66.4
P ÿ15.2 ÿ23.2 6.7 0.6
S ÿ15.8 ÿ19.6 74.9 67.8
K ÿ106.6 ÿ134.9 75.4 ÿ28.7
5. Conclusions
The potential soil fertility and yield bene®ts from short term legume leys are dependent on the legume species, duration of the rotation, N-®xation and amount of plant biomass returned to the system. This study found limited effects of short term legume or fallow leys on wheat yields, however, the changes in the nutrient balances between the systems were sig-ni®cant. Although, crop yields were unaffected by a fallow phase and the nutrient content in plant tops was only in¯uenced in the ®rst wheat crop, the nutrient balance of the fallow treatment was signi®cantly more negative than the legume treatments. Farming systems that rely on fallow periods to conserve water and mineralise nutrients, are most likely to export large quantities of nutrients from the system, eventually resulting in low soil fertility.
By retaining crop stubble, nutrient exports are limited to that which is removed in the grain at harvest. Using nutrient balances to identify the ¯ow of nutri-ents into or out of a farming system will enable the better management of fertiliser to replace these losses. The retention of crop residues resulted in signi®cant increases in wheat yields soon after the treatment was imposed. This improvement was most likely a com-bination of the favourable effects of stubble on SOM and soil physical properties and is further investigated in the following paper (Whitbread et al., 2000).
Acknowledgements
The skilled technical help of Michael Crestani and Leanne Lisle is gratefully acknowledged. Financial support was provided by the Australian Centre for International Agricultural Research (ACIAR) through Projects 9102 and 9448 and the Grains Research and Development Corporation (GRDC).
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