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Restorative crops for the amelioration of degraded soil conditions in New Zealand.

Abstract

The effects of restorative crops on the amelioration of a degraded soil were investigated in a 6-year field experiment. Treatments included perennial pastures, annual pastures, and arable crops. Improvements in some aspects of chemical, biological, and physical fertility were related to the amount of herbage dry matter returned to the soil and root production. Beneficial effects associated with returned organic matter were partly negated by the degradative effect of tillage. Treatments that returned most organic material to the soil showed the greatest increase in aggregate stability and supported the largest earthworm populations, especially without annual tillage. Differences between treatments in soil organic C content were not generally significant until the sixth year. In contrast, differences between treatments in microbial biomass C were apparent by the third year. Compaction by sheep during grazing appeared to result in a loss of soil macroporosity. In the sixth year, soil macroporosity was greatest in the annually cultivated, ungrazed treatments. The grazed perennial ryegrass and ryegrass/white clover treatments were the most effective in ameliorating degraded soil conditions. The rate of soil amelioration declined with depth, and was mainly confined to the top 10 cm of soil. The rate of amelioration was relatively slow, with, for example, 3 years needed for most of the increase in aggregate stability at 0-5 cm depth.

Additional keywords: aggregate stability, earthworms, hydraulic conductivity, pore size distribution, roots, soil organic matter.

Introduction

Mixed cropping is a major land use on the Canterbury Plains of New Zealand, a region that covers more than 750 000 ha. In this farming system, grazed ryegrass/white clover pastures are grown in rotation with arable crops, with the lengths of the pastoral and arable phases traditionally determined by the relative profitability of the 2 forms of farming and the need to maintain good soil physical conditions for plant growth (Haynes and Francis 1990). In traditional rotations, the pastoral and arable phases are of similar duration (each usually 2-4 years), and changes in soil organic matter content during the rotation are often not measurable (Haynes et al. 1991). Nonetheless, soil fertility and soil physical conditions decline during the arable phase and improve during the pastoral phase (Francis and Kemp 1990; Haynes and Beare 1996). These cyclical changes are largely due to the greater organic matter returns during the pastoral phase than during the arable phase. This results in a greater microbial biomass and soil aggregate stability under pasture than under arable cropping (Haynes and Swift 1990). However, the rate and extent of these changes during the cropping rotation are not well documented. Farmers are under pressure to increase the amount of cropping in their rotations, despite the degradation of soil physical conditions being a major risk to continued profitable cropping. Consequently, information is required on which management system is most effective in mixed cropping systems for ameliorating degraded soil conditions, so that the length of the restorative phase of the rotation can be minimised.

The objectives of this experiment were (i) to determine the extent and rate of improvement in soil fertility and physical conditions under a variety of restorative crops; and (ii) to explain the important mechanisms behind the improvements in soil conditions.

Materials and methods

Field experimental site

This long-term experiment was started in March 1989 at the AgResearch Lincoln farm in Canterbury, New Zealand (43 [degrees] 38'S, 172 [degrees] 30'E) on a Wakanui silt loam soil (Udic Dystochrept, USDA Soil Taxonomy; or Immature Pallic Soil, Hewitt 1993). The area had previously been intensively cropped for 11 years in a white clover-wheat-wheat-wheat rotation. Before the start of this experiment, a crop of linseed was grown. This soil was in a degraded state at the start of the experiment, as shown by the low initial values of some soil properties, especially microbial biomass C, earthworm populations, and aggregate stability (Table 1).

Table 1. Some soil properties (0-10 cm depth) at the start of the experiment in March 1989

Values in parentheses are standard deviations (n = 4)
Soil property Value

Sand content(A)(B) (%) 6
Silt content (%) 67
Clay content (%) 27
pH(B)(C) 5.7
Organic C (t/ha) 29.7 (1.22)
Total N (t/ha) 2.78 (0.111)
Microbial biomass C ([micro]g/[cm.sup.3]) 305 (27)
Earthworm population(D) (no./[m.sup.2]) 82 (45)
Aggregate stability(E) (mm) 1.08 (0.61)


(A) Particle size definitions from Milne et al. (1995).

(B) Particle size and pH analyses were single measurements of a bulked sample.

(C) 1: 5 soil: water.

(D) Hand sorting (Fraser et al. 1996).

(E) Mean weight diameter by wet sieving (Haynes 1993).

Field experimental design

Nine perennial or annual treatments were established in March 1989 and continued in the same plots for the next 6 years (Table 2). The perennial treatments were grazed ryegrass, mown ryegrass, white clover, ryegrass-white clover, and mixed herb ley. The annual treatments were: grazed ryegrass; grazed ryegrass established by direct drillings; barley; and lupin. Row spacings were 15 cm for all crops. Treatments were arranged in a randomised complete block design with 4 replicates of each treatment. Individual plots were 15 m by 15 m in area, with 5-m buffer strips between plots.

Table 2. Experimental treatments and their contrasting management practices from 1989 to 1995
Treatment Plant species Type of crop
code

PR Perennial ryegrass Perennial
PRM Perennial ryegrass Perennial
WC White clover Perennial
RC Perennial ryegrass/white clover Perennial
HL Mixed herb ley (white clover, Perennial
 red clover, chicory,
 prairie grass, timothy, lucerne,
 and tall fescue)
AR Perennial ryegrass Annual
ARDD Perennial ryegrass Annual
BAR Barley Annual
LUP Lupins Annual

Treatment Crop management
code

PR Grazed
PRM Mown
WC Grazed
RC Grazed
HL Grazed
AR Grazed; re-established each year
 using conventional cultivation
ARDD Grazed; re-established each year
 using direct drilling
BAR Planted in spring; residues burned
LUP Planted in spring; residues burned


Historically, ryegrass-white clover pastures have been used as restorative crops in New Zealand mixed cropping rotations. The use of pure swards of ryegrass and white clover in this study aimed to identify which of these plants was most effective in ameliorating soil physical conditions. Mown and grazed ryegrass treatments were included to investigate the effect of grazing animals on changes in soil properties. Mixed herb leys have recently received considerable attention in New Zealand as restorative crops, especially in organic fanning systems (Haynes and Francis 1990). Annual ryegrass crops were included to allow for a comparison between perennial and annual crops and to investigate the effects of tillage and direct drilling. Barley was included as an annual degradative crop while lupin was included because many New Zealand farmers regard this as an annual restorative crop due to both its nitrogen fixing ability and its reported effect as a biological plough (Henderson 1989; Haynes and Francis 1990).

Crop management

All annual crops were sprayed with herbicide (glyphosate at 3 L/ha; active ingredient 360 g/L) in the autumn. Conventionally cultivated plots were mouldboard ploughed to a depth of 15-20 cm, secondary cultivated, then rolled and harrowed before drilling. In direct drilled plots, seed was drilled into otherwise undisturbed soil using a triple disc drill. Annual crops were sown in autumn (April/May) each year. Postharvest residues of the annual arable crops were burned each year. Weed, pest, and disease control measures were applied to the different crops as necessary. In the first year, grass-based treatments were either grazed or mown twice. All clippings were returned as a surface mulch in the mown treatment. In the following years, the grass-based treatments were either grazed or mown every 4-6 weeks from spring to autumn. Grazing was usually completed within 1-2 days, at a stocking rate of about 30 ewes per plot. Temporary fences were erected during grazing to ensure that all plots received similar grazing pressure.

Superphosphate (150 kg/ha) was applied annually to all treatments. It was applied to the annual treatments at drilling and was surface-applied to the perennial treatments in winter. Nitrogen (100 kg/ha.year) was applied to the non legume-based treatments at a rate similar to the assumed N2 fixation in the leguminous pastures (Crush 1979). Nitrogen (as urea) was applied to the barley in one dressing at early tillering (spring), but was applied to the other treatments in 4-5 equal amounts from spring to summer. Soil moisture was measured using a combination of time domain reflectometry (0-20 cm depth) and neutron probe (20-1500 cm depth) methods. Sprinkler irrigation (50 mm per irrigation event) was applied to all treatments when soil moisture deficits in the top 60 cm reached 60-70 mm.

Plant measurements

In the grass-based treatments, quadrat cuts (0.25 [m.sup.2]) before each grazing were used to assess cumulative dry matter production. Grain yields of barley and lupins were determined by the hand harvesting of 2 quadrats (each 0.5 [m.sup.2]) per plot. Crop roots were sampled at flowering in the early summer of the fourth year (November 1993) of the experiment. Two soil blocks (each 30 cm by 25 cm) were excavated from each plot to a depth of 20 cm and the soil washed from the roots over a 300-[micro]m mesh sieve using running water. Root samples were dried between blotting papers, weighed and subsamples taken for root length measurement. The subsample was preserved in a solution containing 5% formaldehyde, 5% acetic acid, and 90% ethanol until required for analysis. The remainder of the sample was air-dried at 70 [degrees] C and weighed. Root length was determined using an image analysis technique (Harris and Campbell 1989). Root length density (m/m3) and specific root length (m/g) were also calculated.

Soil measurements

Soil samples were taken annually from all plots after the harvest of the lupin and barley crops. Care was taken to ensure that undisturbed sampling areas were used in all years. Samples for chemical analyses were only taken in years 3 (1992) and 6 (1995), and there was only limited sampling for the other analyses in the fifth year (1994) of the experiment.

For chemical analyses, 10 soil cores (2.5 cm internal diameter) were randomly taken from each plot and bulked. In year 3, samples were taken from 0-10 and 10-20 cm depths; in year 6 samples were taken from 0-5, 5-10, and 10-20 cm depths. Microbial biomass C was determined on field moist, sieved ([is less than] 2 mm diameter) subsamples stored at about 4 [degrees] C for [is less than] 48 h before analysis. Microbial biomass C was measured by the method of Vance et al. (1987), based on the difference between C extracted with 0.5 M [K.sub.2][SO.sub.4] from chloroform-fumigated and unfumigated soil, using a [k.sub.ec] factor of 0.38. Organic C and total N content were determined on air-dried, ground ([is less than] 150 [micro]m) samples. Organic C was determined colorimetrically by the Walkley and Black method (Blakemore et al. 1972) and total N was determined by semi-micro Kjeldahl digestion (Bremner and Mulvaney 1982).

Samples for aggregate stability determination were carefully collected from one location in each plot using a spade. Aggregates 2-4 mm in diameter were obtained by sieving field moist soil, and were then air-dried at 30 [degrees] C. Aggregate stability was determined by a wet sieving technique (Haynes 1993) using the equivalent of 50 g of oven-dried aggregates. Results are expressed as a mean weight diameter (MWD), the upper and lower limits of which are 3.00 and 0.25 mm, respectively.

Two intact soil cores (10 cm internal diameter by 5 cm deep) for pore size distribution determination were carefully removed from a soil pit in each plot in increments to a depth of 20 cm, with soil trimmed flush with the end of the containing cylinder. Both exposed surfaces were then peeled (using a 50:50 mix of cellulose acetate in acetone) to unblock any macropores that may have been smeared during trimming (Cameron et al. 1990). Cores were treated with formaldehyde to extract any earthworms that were present in the soil. Cores were transferred to tension tables and soil water desorption curves constructed (Topp et al. 1993), from which soil porosity parameters were calculated (Carter and Ball 1993).

A single, large, intact core (20 cm internal diameter by 20 or 30 cm deep) was collected from each plot as described by Cameron et al. (1990). Unsaturated hydraulic conductivity was measured at a matric potential of-25 mm (to exclude macropores [is greater than] 1200 [micro]m equivalent spherical diameter from flow), which was applied to the top and the bottom of the core (Cook et al. 1993). Saturated hydraulic conductivity was then measured on the same cores at a matric potential of +25 mm at the top of the core and a potential of 0 mm at the bottom.

Earthworms were counted and identified after hand-sorting from one soil block removed from each plot (0.04 [m.sup.2] area to 30 cm depth).

Statistical analyses

A standard 1-way analysis of variance (ANOVA) was used to determine treatment effects, with separate analyses conducted for each sampling depth where appropriate. Standard errors of means (s.e.m.) are presented in the Tables, with least significant differences (l.s.d.) presented in the Figures to indicate significant (P [is less than] 0.05) treatment differences. Linear regressions were determined using standard statistical techniques.

Results

During the trial, cumulative dry matter (DM) production in the grass-based treatments was generally greater for the perennial than annual treatments (Fig. 1a). In the perennial treatments, cumulative DM production was greatest for the grazed perennial ryegrass and least for the mown perennial ryegrass with other treatments intermediate. Annual DM production was similar for the grazed and mown perennial ryegrass in years 1 and 2 of the experiment, but production was much less in the mown than grazed treatment from year 3 onwards. In the annual grass-based treatments, DM production throughout the experiment was greater from the cultivated than from the direct drilled treatment. In general, annual DM production for each treatment did not show large variations between years. The exception was the mown perennial ryegrass, in which DM production declined markedly from year 2 to year 3. Cumulative harvest grain yield for lupin was much less than for barley throughout the experiment (Fig. 1b).

[Figure 1 ILLUSTRATION OMITTED]

For all the treatments in the early summer of the fourth year of the experiment, most of the root mass was present at 0-10 cm depth (Fig. 2). Except for the clover treatment, total root mass was greater in the perennial than annual treatments. Within the perennial treatments, the grazed perennial ryegrass and herb ley had the greatest root mass and clover had the least. Total root mass under perennial ryegrass was much less where plots were mown rather than grazed. There was a trend towards greater total root mass in the annual grass than arable treatments, although this was only significant for the lupins. Cultivation method had no major effect on total root mass of annual ryegrass, although with direct drilling there was significantly less root mass at 10-20 cm depth. Root mass at 10-20 cm depth was greatest for herb ley and least for clover and direct drilled annual ryegrass. Root length density (Table 3) varied between treatments in a similar way to root mass. Specific root length (Table 3) was similar in all the grass-based treatments, but was smaller in the clover, herb ley, and lupin treatments.

[Figure 2 ILLUSTRATION OMITTED]

Table 3. Treatment effects on root length density and specific root length at 0-20 em depth in November 1993 (year four)

For treatment descriptions, see Table 2
Treatment Root length density Specific root length
code (10-4 x m/[m.sup.3]) (m/g)

PR 41.5 65.5
PRM 28.6 84.9
WC 7.7 42.1
RC 29.0 69.8
HL 24.6 45.6
AR 17.0 88.3
ARDD 13.5 76.7
BAR 8.4 63.5
LUP 1.8 23.5
s.e.m. 3.06 7.7


After 3 years of the experiment, at 0-10 cm depth the annual cropped treatments (barley and lupin) had the smallest and the direct drilled annual ryegrass treatment had the greatest soil organic C (Table 4). Differences were much smaller at 10-20 cm depth. Broadly similar differences were measured for microbial biomass C. No clear trends between treatments were apparent for total N.

Table 4. Treatment effects on some soil chemical and microbiological properties after three years

For treatment descriptions, see Table 2
Treatment Organic C Microbial biomass C
code (t/ha) ([micro]g/[cm.sup.3])

 0-10 cm 10-20 cm 0-10 cm 10-20 cm

PR 31.3 31.9 488 408
PRM 32.2 31.9 555 492
WC 32.2 31.3 324 401
RC 33.4 32.8 521 401
HL 30.8 31.4 581 485
AR 31.4 30.1 567 439
ARDD 35.0 30.3 566 436
BAR 29.5 31.8 379 417
LUP 29.6 30.3 423 336
s.e.m. 0.96 1.66 68.9 27.8

Treatment Total N
code (t/ha)

 0-10 cm 10-20 cm

PR 3.25 3.29
PRM 3.29 3.34
WC 3.59 3.43
RC 3.46 3.56
HL 3.22 3.45
AR 3.33 3.28
ARDD 3.59 3.29
BAR 3.17 3.35
LUP 3.32 3.34
s.e.m. 0.139 0.116


In the sixth year of the experiment, the depth of measurement for chemical properties was changed (Table 5). Organic C, microbial biomass, and total N all increased between years 3 and 6. Organic C and microbial biomass C both showed similar trends, although differences between treatments were larger for microbial biomass C. Organic C and microbial biomass C were least in the annual cultivated treatments and greatest in the perennial treatments. Differences were greatest at 0-5 cm depth, but were still significant at 5-10 cm depth. Few differences between treatments were apparent at 10-20 cm depth. Organic C and microbial biomass C differed little with depth in the annually cultivated treatments. In contrast, organic C and microbial biomass C declined with depth in the perennial treatments and direct drilled annual ryegrass. Differences in total N were much smaller than for the other chemical parameters, although they showed similar trends.

Table 5. Treatment effects on some soil chemical and microbiological properties after six years

For treatment descriptions, see Table 2
Treatment Organic C
code (t/ha)

 0-5 cm 5-10 cm 10-20 cm

PR 20.1 19.8 37.5
PRM 16.5 18.5 36.7
WC 18.3 17.8 35.4
RC 19.6 19.4 38.6
HL 19.3 20.2 37.6
AR 16.9 16.1 33.7
ARDD 19.1 19.4 37.4
BAR 12.2 17.3 34.7
LUP 15.3 16.9 34.0
s.e.m. 0.80 0.52 1.18

Treatment Microbial biomass C Total N
code ([micro]g/[cm.sup.3]) (t/ha)

 0-5 cm 5-10 cm 10-20 cm 0-5 cm 5-10 cm 10-20

PR 1145 707 437 1.66 1.66 3.19
PRM 1038 711 504 1.51 1.58 3.23
WC 897 651 443 1.66 1.57 3.01
RC 1053 668 468 1.65 1.68 3.39
HL 1053 675 473 1.61 1.65 3.08
AR 773 522 440 1.43 1.43 2.89
ARDD 954 558 451 1.57 1.65 3.13
BAR 470 450 415 1.02 1.41 2.91
LUP 513 405 352 1.30 1.39 2.93
s.e.m. 48.6 24.7 24.6 0.068 0.044 0.118


Earthworm populations after year 1 (1990) were low in all treatments (Fig. 3) and were very similar to those present at the start of the experiment. From year 2 of the experiment onwards, earthworm populations were significantly different between treatments and were generally lower in the annual than the perennial treatments. Earthworm populations were consistently similar under barley and lupins (data not shown). These 2 treatments had the smallest populations throughout the experiment. Populations were similar under grazed and mown (data not shown) perennial ryegrass, and were similar under ryegrass-white clover and herb ley (data not shown). Earthworm populations increased in all perennial treatments during the experiment, reaching a maximum in 1993. Throughout the experiment, Apporectodea caliginosa comprised about 80-90% of the total earthworm population in all treatments (data not shown).

[Figure 3 ILLUSTRATION OMITTED]

Aggregate stability was low in the surface (0-5 cm) soil in all treatments in 1990 (Fig. 4). Aggregate stability increased in all treatments during the experiment, except for the barley and lupin treatments. In the perennial treatments, stability increased most rapidly in the first 3 years, although the maximum stability was measured in year 6. Except for the clover treatment, aggregate stability was generally lower in the annual than perennial treatments from 1991 onwards. Within the annual treatments, surface soil aggregates were most stable in direct drilled annual ryegrass and least stable under lupins, with cultivated annual ryegrass and barley intermediate. In the perennial treatments, stability increases were greatest for the grazed and mown (data not shown) perennial ryegrass treatments and least under clover. Stability in herb ley (data not shown) was similar to that under ryegrass/white clover throughout the experiment.

[Figure 4 ILLUSTRATION OMITTED]

Throughout the experiment, aggregates in the perennial and direct drilled annual ryegrass treatments showed a marked decline in stability with depth in the soil profile. In contrast, the stability of aggregates in annually cultivated soil was relatively constant to 20 cm depth. This is illustrated with data for selected treatments in 1995 (Fig. 5).

[Figure 5 ILLUSTRATION OMITTED]

Soil porosity parameters varied during the experiment, with data for 0-5 cm depth for selected years presented in Fig. 6. Treatment effects declined with depth in the profile, with only small differences between treatments below 10 cm depth (data not shown). At 0-5 cm depth, changes with time were inconsistent. Changes in total porosity between years were less marked than changes in particular pore size classes. Between 1990 and 1993, the volume of pores [is less than] 60 tam equivalent spherical diameter (e.s.d.) increased mainly at the expense of pores 60-1200 [micro]m and pores [is greater than] 1200 [micro]m e.s.d. A similar, but opposite trend was apparent between 1993 and 1995. Within the perennial treatments there was a trend towards lower total porosity under clover, especially at the 1990 and 1993 samplings. Total porosity of the mown perennial ryegrass treatment increased with time, relative to the other perennial treatments, mainly due to an increase in pores [is greater than] 1200 [micro]m e.s.d. Within the annual treatments, total porosity and the volume of pores 60-1200 [micro]m and [is greater than] 1200 [micro] m e.s.d, increased with time in the ungrazed barley and lupin treatments relative to the cultivated and direct drilled grazed annual ryegrass treatments.

[Figure 6 ILLUSTRATION OMITTED]

Unsaturated hydraulic conductivities were less than saturated hydraulic conductivities at all sampling times (Table 6). Differences between treatments were more pronounced for saturated than unsaturated conductivities. Differences between treatments for saturated conductivities increased with time. In 1995, the greatest saturated conductivity values were for the ungrazed annual treatments (barley and lupins) and the least values for the annual grazed and perennial clover treatments. The perennial grass based treatments were intermediate.

Table 6. Treatment effects on unsaturated ([K.sub.-25]) and saturated ([K.sub.sat]) hydraulic conductivity (mm/h) in years one (1990), four (1993) and six (1995)

For treatment descriptions, see Table 2
Treatment 1990 1993
code [K.sub.25] [K.sub.sat] [K.sub.-25] [K.sub.sat]

PR 19 25 26 70
PRM 41 51 19 29
WC 31 39 18 40
RC 14 20 17 43
HL 28 29 21 51
AR 19 25 14 39
ARDD 3 6 15 25
BAR 4 13 28 70
LUP 56 93 26 91
s.e.m. 8.6 16.4 3.0 11.9

Treatment 1995
code [K.sub.-25] [K.sub.sat]

PR 36 259
PRM 38 379
WC 25 185
RC 19 262
HL 28 331
AR 22 203
ARDD 43 103
BAR 26 596
LUP 28 982
s.e.m. 3.2 119.4


Discussion

Dry matter production in the perennial ryegrass grazed treatment was between 8 and 10 t DM/ha.year, which is common for irrigated pastures in this region of New Zealand (Perrott et al. 1992). The decline in DM production in the mown ryegrass treatment from the second year onwards probably resulted from the accumulation of a surface mulch of ryegrass residues returned during mowing. This surface mulch could well have suppressed growth by covering and shading the ryegrass plots. Dry matter production was less in the annual than perennial treatments as the annual treatments had considerably shorter growing seasons. Each autumn the annual treatments remained fallow for about 8 weeks between killing the existing vegetation (either by ploughing for the cultivated treatment or by spraying with herbicide for the direct drilled treatment) and the emergence of the next crop. The DM production was significantly less where grass was direct drilled rather than cultivated. This result was mainly due to the slower rate of DM production during winter and early spring in the direct drilled plots, as plant populations were similar for both cultivation treatments (data not shown).

Yields of winter barley were relatively constant with time. In contrast, yields of lupins declined with time, largely as the result of declining plant emergence and plant populations (data not shown). The poor emergence of lupins was the result of the soil's low surface aggregate stability (Fig. 4) and the high sensitivity of lupin seedlings to the presence of a surface crust (White and Robson 1989).

The return of organic material to the soil was much less for the annual arable crops than for the perennial treatments. For the grazed treatments, 60-90% of the above-ground DM consumed during grazing would have been returned to the soil as excreta (Haynes and Williams 1993), with 100% of the residues returned in the mown treatment. In contrast, returns of above ground organic matter to the annual arable crops were very low as the harvested grain was removed and post harvest residues were burned.

Differences between treatments were much more pronounced for root mass, length, and density than for the return of above-ground organic material. One measurement of root mass and length early in the summer does not give a measure of the total amount of below-ground organic material produced throughout the year. However, this measurement was useful for comparisons between treatments as, at this time of sampling, the root systems of the various plants would be expected to be at, or approaching, their maximum size (Russell 1973). Forage grass species characteristically have a high ratio of below to above ground biomass due to their dense, adventitious root systems (Haynes and Francis 1993). This is shown by the high root length density and high specific root length of the grass treatments (Table 3). In contrast, clover has a sparse, stoloniferous root system. Within the annual crops, lupins had significantly less root mass, root length, and root density due to its tap root system, in comparison with the adventitious root systems of the ryegrass and barley crops.

Increases in earthworm populations in the perennial treatments appeared to be related to the return of large amounts of organic material. Tillage did not appear to have a large effect on earthworm populations, as similar numbers were found under the conventionally cultivated and direct drilled annual ryegrass treatments. Earthworm populations remained at a low level throughout the experiment under barley and lupins, as organic matter returns were low. In the perennial treatments, maximum populations were recorded in the fourth year of the experiment. In the grazed perennial ryegrass and ryegrass/white clover treatments, populations were approaching the maximum values often measured for this soil type (Fraser et al. 1996). The decline in earthworm populations in years 5 and 6 was unexpected. This decline was probably the result of sampling 4-6 weeks earlier in the autumn in these years, when earthworm populations were likely to be smaller due to less favourable soil temperature conditions (Fraser and Piercy 1996). Regular irrigation applications during the growing season resulted in similar soil moisture contents at the time of sampling in all years (data not shown).

Aggregate stability values in all treatments were low for the first 2 years of the experiment. Soils in this region with values below 1.5 mm are regarded as unstable (Haynes and Francis 1990) and physical impediments to crop growth such as poor crop emergence (as shown by declining lupin emergence in this experiment), inadequate soil aeration, greater susceptibility to drought and nutrient deficiencies are likely to occur.

From the second year, soil aggregate stability was generally greatest in the treatments that returned the greatest amounts of herbage dry matter and root material to the soil. The stabilisation of soil aggregates by plant roots occurs through several mechanisms, the most important of which is the supply of organic matter to the soil. Organic matter supply is closely related to the root length of the plant (Shamoot et al. 1968) and occurs through root turnover and the exudation of organic substances. As a result, at 0-5 cm depth in year 6 there was a highly significant correlation between soil aggregate stability (expressed as MWD) and organic C content (MWD = -1.267 + 0.1912 x organic C; [R.sup.2] = 0.647, d.f. = 7). Plant roots may also stabilise aggregates by supporting a large microbial population in the rhizosphere, which itself produces exocellular polysaccharide material that can stabilize soil aggregates (Haynes et al. 1991; Haynes and Beare 1997). Indeed, in year 6 there was an even stronger correlation between aggregate stability and microbial biomass C content than between aggregate stability and organic C content (MWD = -0.017 + 0.002383 x microbial biomass C; [R.sup.2] = 0.980, d.f. = 7). Roots and associated mycorrhizal hyphae can also stabilise aggregates in a third way, through the formation of 2-dimensional networks that enmesh fine soil particles (Tisdall 1991).

In this experiment, increases in aggregate stability were most pronounced in the soil surface layer as this is where the greatest returns of organic matter occur. Similar trends in aggregate stability between treatments were measured deeper (5-20 cm) in the profile, although the size of these differences was much smaller. Other workers have reported results of greater stability with grasses than clover (Tisdall and Oades 1979; Haynes and Beare 1997) and with stability under perennial ryegrass increasing with increasing dry matter production (Douglas et al. 1992).

The rate of change in aggregate stability can be normalised with respect to the maximum value of aggregate stability by the equation (Kay 1990):

-ln[([MWD.sub.max] - MWD)/([MWD.sub.max] - [MWD.sub.t=0])] = bt

where [MWD.sub.max] is the maximum aggregate stability, [MWD.sub.t=0] is the aggregate stability at time zero, MWD is the measured aggregate stability, b is the rate constant and t is time in years. This function seems applicable for a broad range of soils, crops, and climatic conditions (Kay 1990). Data from 3 of our treatments are plotted in Fig. 7 and show widely different rate constants. The grazed perennial ryegrass clearly had a much faster effect on improving aggregate stability than white clover, whereas lupin had very little effect. Indeed, surface soil aggregate stability increased relatively consistently in the most effective treatments (grazed perennial ryegrass and ryegrass/white clover) for the first 3-4 years of this experiment. After that, any further increase in aggregate stability was relatively small. Increases in aggregate stability were greatest in the surface soil of the treatments that were undisturbed throughout the experiment. This is because organic C contents, and root mass and length, are greatest in the surface soil (Fig. 2) and because organic matter returns from grazing or mowing also occur at the soil surface.

[Figure 7 ILLUSTRATION OMITTED]

Although differences in the organic matter returns to the soil were large, differences in organic C and total N contents between treatments were generally not statistically significant until the sixth year of the experiment. This is because the large background level of stable organic matter (containing the majority of both soil C and N) makes it very difficult to measure relatively small changes that occur over short periods. Indeed, studies of mixed cropping rotations in this locality (e.g. 4 years pasture followed by 4 years arable) often show no appreciable changes in soil organic C content during the rotation (Haynes and Swift 1990; Haynes et al. 1991). Moreover, the soil organic C content is determined by both the amount of organic matter returned to the soil and the rate at which soil organic matter breaks down. A comparison of the cultivated and direct drilled annual ryegrass treatments shows that these treatments had similar root mass, root length density and specific root length. Nevertheless, there was a trend towards a greater soil organic C content for the direct drilled than the cultivated annual ryegrass from year 3 onwards, as tillage increased the rate of soil organic matter breakdown.

Although the changes in organic C levels only occurred slowly, larger differences in microbial biomass C content were apparent by the third year of the experiment. Similar results have been reported by other workers (Sparling et al. 1992; Haynes and Francis 1993). The microbial biomass C comprises only a small (1-2%) amount of the total organic C, but it has a rapid turnover rate and is often used as a more sensitive indicator of changes in soil organic matter dynamics than total organic C (Haynes and Beare 1996).

Within the perennial treatments, porosity was least under clover because of its low root mass and earthworm population. Earthworm populations were dominated by A. caliginosa, a species that creates semi-permanent burrows in the top 20 cm of these soils (Francis and Fraser 1998). Porosity was greatest in the mown perennial ryegrass treatment, probably because sheep treading compacted soil in the grazed treatments (Proffitt et al. 1995). Within the annual treatments, total porosity and macroporosity were greatest in the annual cropped treatments (barley and lupins). As these treatments had the lowest earthworm populations and root mass, the macropores in these treatments were likely to have been created through tillage at crop establishment. Both barley and lupins had very low aggregate stability, but it appears that without compaction by sheep during grazing and traffic over the plots during the growing season, at least some tillage-produced macropores persisted throughout the growing season. The low porosity in the annually cultivated ryegrass treatment was consistent with low root inputs, low aggregate stabilities and compaction by sheep treading.

Although soil porosity data gives no information on the continuity of macropores through the soil, measurements of hydraulic conductivity can provide this information. Continuous macropores are important in many soil processes, including root penetration, water infiltration and redistribution, and aeration. Consequently, crops that enhance the formation of continuous macropores are likely to be effective in creating good soil physical conditions. Differences between treatments were greater for [K.sub.sat] than [K.sub.-25] measurements, suggesting there is a greater difference between treatments in the number of continuous macropores [is greater than] 1200 [micro]m e.s.d, than in the number of continuous macropores [is less than] 1200 [micro]m e.s.d. In general, [K.sub.sat] increased slowly between 1990 and 1993, which is consistent with the slow change in other measured soil physical properties during this time. The large increase in [K.sub.sat] between 1993 and 1995 is partly due to the different depth of soil cores used in these years. In 1993, soil cores were 30 cm long, but in 1995 this was reduced to 20 cm as soil measurements in previous years had shown the treatment effects were limited to the topsoil. Since a comparison of 20 and 30 cm cores in 1995 showed that [K.sub.sat] values were only about twice as great when measured in the shorter cores (data not shown), a real increase in [K.sub.sat] still occurred between 1993 and 1995. The increase in [K.sub.sat] with time is due to the creation of more continuous macropores throughout the soil depth sampled. Except for the barley and lupin treatments, [K.sub.sat] increases were greatest for the treatments that had the highest earthworm populations, root densities, and aggregate stabilities.

In 1995, the lowest [K.sub.sat] values were measured for the grazed annual grasses and perennial clover treatments. This is consistent with other measured soil parameters. The greatest [K.sub.sat] values were in the barley and lupin treatments, probably due to the flow of water along empty root channels. Sampling occurred in autumn each year after the harvest of the crops, by which time a significant amount of the crops' roots would have decayed (Amato et al. 1987), leaving empty, continuous pores through the soil. These pores are likely to be very unstable (see Fig. 4) and may not persist for long after harvest. The unstable nature of these pores may explain the greater [K.sub.sat] values for these treatments in 1995. Samples were taken 4-6 weeks earlier in 1995 and so fewer macropores may have collapsed in the time since harvest. The [K.sub.sat] values were greater under lupin than barley, possibly because of the greater ability of lupin roots to penetrate through compact soil layers (Henderson 1989).

Conclusions

The results presented in this paper show that treatments which return the most above and below ground dry matter have the greatest beneficial effect on the amelioration of degraded soil. Perennial grass-based treatments were most beneficial because of their high dry matter production and large returns of organic matter to the soil. In the cultivated annual grass-based treatment, tillage partly negated the beneficial effects of organic matter returns. By year 6, the treatments that received the greatest organic matter returns had the greatest microbial biomass contents, organic C contents, earthworm populations and aggregate stabilities. In the most beneficial treatments, the rate of increase in aggregate stability at 0-5 cm depth was fastest in the first 3 years. Improvements in soil conditions were much slower deeper in the profile. The effect of these contrasting soil conditions on the growth and yield of subsequent arable crops will be reported in a following paper.

Acknowledgments

We thank Richard Gillespie for expert help in the running of this experiment, Lyndene Goodman and Yvette Le Warne for technical assistance, and David Baird for statistical advice. This work was carried out as part of Foundation for Research, Science and Technology contract number C02302.

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Manuscript received 3 March 1999, accepted 17 June 1999

G. S. Francis, F. J. Tabley and K. M. White

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Author:Francis, G. S.; Tabley, F. J.; White, K. M.
Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Geographic Code:8NEWZ
Date:Nov 1, 1999
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