Received 11 January 1997: accepted 17 May 1997. Abstract. The residual ... compacted Typic Haplustert (Vertisol) of north-western New South Wales, Australia.
Applied Soil Ecology ELSEVIER
Applied Soil Ecology 7 (1997) 11-30
Residual effects of tillage and crop rotation on soil properties, soil invertebrate numbers and nutrient uptake in an irrigated Vertisol sown to cotton N.R. Hulugalle b
a,b,*, L.A. Lobry de Bruyn c, p. Entwistle a.b
a Co-operatil'e Research Centre for Sustainable Cotton Production, Narrabri, Australia . Austrahan Cotton Research lnstttute, New South Wales Department of Agriculture PMB, Myall Vale Mail Run. Narrabri, NSW 2390, Australia c Department of Ecosystem Management, University of New England, Armidale, NSW 2351, Australia
Received 11 January 1997: accepted 17 May 1997
Abstract
The residual effects of tillage and cropping sequence on soil physical and chemical properties, surface-active and soil invertebrate composition and abundance, nutrient uptake, growth and yield of cotton were evaluated from 1994 to 1996 in a compacted Typic Haplustert (Vertisol) of north-western New South Wales, Australia. The experimental treatments from 1985 to 1992 were intensive tillage (disc-ploughing to 200 mm, chisel ploughing to 300 mm followed by ridging every year) sown with continuous cotton (Gossypium hirsutum L.); minimum tillage (planting on ridges retained intact from previous years with soil disturbance being limited to deepening of the furrows with disc-hillers and shallow cultivation on ridge surfaces) sown with either continuous cotton or a cotton-winter wheat (Triticum aestivum L.)-fallow rotation where wheat was sown with no-tillage. The tillage treatments were repeated in May 1993, and the plots were either fallowed or cropped by sowing either cowpea (Vigna unguiculata Walp.) or cotton. Cotton was sown with minimum tillage in 1994 and 1995 in all plots. Soil was sampled from the 0-150 mm, 150-300 mm, 300-450 mm and 450-600 mm depths, and analyzed for organic carbon, dispersion index, soil resilience (a measure of the self-mulching ability of the soil), plastic limit, soil strength, pH, exchangeable Ca, Mg, K and Na, and nitrate-N. Profile water content, nutrient uptake, numbers of soil invertebrates, cotton growth and lint yield, and fibre quality were also quantified. Soil strength was lowest where intensively tilled continuous cotton had been sown, whereas in plots where minimum tillage and cotton-wheat-fallow rotation were combined soil fertility was best (indicated by lowest values of pH, exchangeable Na, exchangeable sodium percentage and dispersion, and highest values of organic C) and water extraction by cotton greatest during periods of reduced water availability. The latter was attributed to cotton utilizing stable pores with a high degree of pore-continuity created by the root systems of preceding crops or associated macrofauna as 'by-pass channels' to avoid the restrictions of the soil matrix, thereby facilitating rapid access to sub-soil water. Cotton growth reflected these differences such that vegetative and reproductive growth, nutrient uptake and lint yield were greater and fibre quality superior wherever minimum tillage had been imposed, and best in plots under minimum tilled cotton-wheat-fallow rotation. Composition and abundance of surface-active and soil invertebrates were determined primarily by soil microclimate and pesticide application regime rather
* Corresponding author. 0929-1393/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0929-1393(97)00027-9
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N.R. Hulugalle et al. / Applied Soil Ecology 7 (1997) I I -30
than by tillage and cropping system. Ant numbers were lowest in intensively tilled plots whereas Collembola activity was limited to periods when the soil was moist. ~C)1997 Elsevier Science B.V. Kevwords: Minimum tillage; Soil properties: Vertisol: Cracking clay; Irrigation: Farming system: Cropping system: Tillage system: Soil invertebrates; Water use; Biopores: Cotton: Fibre quality: Biodiversity
1. Introduction
Minimum tillage and crop rotation have become a feature of many farming systems, including cotton ( G o s s y p i u m h i r s u t u m L.)-based farming systems, in eastern Australian Vertisols over the past decade (Constable and Forrester, 1995: Cooper, 1993; Freebairn et al., 1990). Benefits of minimum tillage and crop rotation in irrigated cotton-based farming systems are claimed to include reduction of soil erosion and physical, chemical and biological degradation: improved energy conservation and timeliness of land preparation: and improved water conservation (Constable and Forrester, 1995: Constable et al., 1992; Hulugalle, 1994; McGarry, 1990: Potter et al., 1995). However the above-mentioned benefits of minimum tillage and crop rotation may only become apparent, particularly with respect to sub-soil properties, some years after their imposition. Published data from rainfed and irrigated farming systems in clay soils suggest that the residual effects of intensive tillage, beneficial and detrimental, can persist for several years (Alakukku and Elonen, 1995; Colwick et al., 1981; Constable and Forrester, 1995; Eck et al., 1977; Daniells, 1989: Duval et al., 1989; Mahata et al., 1992). Among these studies the most degradative residual effects of tillage, usually in relation to increasing soil compaction, generally occurred after tillage was conducted in wet or moist soil. A few studies (Constable and Forrester, 1995: Daniells, 1989) also suggested that dry tillage of compacted soils can alleviate compaction for a number of years after tillage provided that on-site trafficking was controlled. Information on soil properties and crop growth is, however, sparse for situations where a long period of intensive tillage was followed by imposition of minimum tillage, which is the usual situation which occurs in commercial farming systems. Furthermore data on the residual effects of tillage on soil properties such as aggregate stability, aggregate size, organic matter, pH and ion-exchange
properties, and on soil invertebrate populations are few for cracking clay soils. With respect to the residual effects of crop rotations, research has generally been strongly focused on nitrogen in rotations which included a leguminous crop (Brown et al., 1993: Dalai et al., 1995; Garside et al., 1994: Pare et al., 1993). Research on nonleguminous rotation crops and soil properties other than nitrogen have been sparse, with alleviation of soil compaction, water use and N balance being the main focus (Constable and Forrester, 1995: Constable et al., 1992; Hulugalle and Entwistle, 1996. 1997). These studies concluded that, in the shortterm, sowing rotation crops benefited soil N (with respect to increasing N and its subsequent recycling) and alleviated soil compaction, although the longterm residual effects of rotation crops on soil properties were not addressed. The residual effects of tillage and crop rotations on surface-active and soil invertebrate composition and abundance do not appear to have been studied in irrigated Vertisols, although increasing tillage intensity is reported to dramatically reduce soil faunal numbers in semi-arid rainfed Vertisols (Radford et al., 1995). The objective of this study, therefore, was to quantify the residual effects of tillage systems and crop rotations which had been imposed for an extended period on soil physical and chemical properties, surface-active and soil invertebrate community composition and abundance, water use, nutrient uptake and cotton growth and yield in a minimum tilled, cotton farming system in an irrigated Vertisol. 2. Materials and m e t h o d s 2.1. Site d e s c r i p t i o n
The trial was located at the Australian Cotton Research Institute near Narrabri, north-western New South Wales, Australia (150°E, 30°S) which has a semi-arid climate. The experimental site experiences four distinct climatic seasons with a mild winter and
N.R. Hulugalle et al. / Applied Soil Ecology 7 (1997) 11-30
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Fig. 1. Variation of dispersion index with soil water content in the 0 - 1 5 0 mm, 150-300 m m and 4 5 0 - 6 0 0 m m depths in April 1995. []-intensively tilled continuous cotton; • - m i n i m u m tilled continuous cotton; • - m i n i m u m tilled cotton-wheat-fallow.
a hot summer. The hottest month is January (mean daily maxima and minima of 34°C and 19°C, respectively), whereas July is the coldest (mean daily maxima and minima of 18°C and 3°C, respectively). Mean annual rainfall is 616 mm. The soil at the experimental site is a deep uniform grey clay (Ug 5.25) (Northcote, 1979) and was classified as a fine, thermic, montmorillonitic, Typic Haplustert (Soil Survey Staff, 1994). The 0-300 mm depth has 53% clay and 26% sand, and the 300-600 mm depth has 60% clay and 22% sand. CaCO 3 concentration in the 0-300 mm depth is 0.5% and that in the 300-600 mm depth is 0.2%. Sub-surface compaction is high. Average bulk density of soil clods at a water content
of 20% in the 0-150 mm depth is 1.1 M g / m 3, and that in the 150-600 mm depth is 1.8 M g / m 3.
2.2. Experimental design and crop management There were three experimental treatments between 1985 and 1993 which were: (a) intensive tillage (disc-ploughing to 200 mm, chisel ploughing to 300 mm followed by ridging every year; deep ripping to 400 mm in 1987 when the soil was dry) with cotton (Gossypium hirsutum L.) sown in October every year; (b) minimum tillage (planting on ridges retained intact from previous years with soil disturbance being limited to deepening of the furrows with disc-hillers) with cotton sown in October every year;
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Fig. 2. Variation of soil water content and cone index with depth on 1 August 1995. D-intensively tilled continuous cotton; • - m i n i m u m tilled continuous cotton; • - m i n i m u m tilled cotton-wheat-fallow. Horizontal bars indicate s.e.m.
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Minimum tillage/ Continuous cotton
Intensive tillage/ Continuous cotton
17
Minimum tillage/ Cotton-wheat-fallow rotation
Fig. 3. Soil profile faces (150-550 mm) showing penetration patterns of 1:8 paint:water mixture on 27 July 1996. Due to similarity of data only profile faces in replicate 2 are shown. and (c) a cotton-winter wheat (Triticum aestivum L.)-summer (bare) fallow-rotation where cotton was sown with minimum tillage in October and wheat with no-tillage in May (with the last wheat crop being sown in the winter of 1991) (Constable et al., 1992). In northern New South Wales wheat is usually harvested in early December. Sowing cotton at this time would expose the crop to sub-optimal temperatures during boll formation and filling (Constable and Shaw, 1988). A fallow is, therefore, imposed until cotton is sown in October of the following year. Detailed information on site management history and layout have been reported elsewhere (Constable et al., 1992). Following harvest, the crops were slashed and shredded, and all residues retained in situ. The plots were furrow irrigated over a period
of approximately 8 h (equivalent to approximately 100 m m of water) when rainfall and profile water storage were insufficient to meet crop water requirements. In 1993 chickpea (Cicer arietinum L.) co. Barwon was sown in July; cotton co. Siokra 1 - 4 in October; and cowpea (Vigna unguiculata Walp.) cv. Coloona in December in all plots. Chickpea and cowpea were sown as green manure crops and cotton was harvested mechanically in late April. The experimental design used was a split-plot design with 4 replications with the three tillage system/cropping sequence combinations as main plots arranged in randomized complete blocks, and the crops sown in 1993 designated as sub-plots, which were completely randomized within each main plot (Petersen, 1994). Individual sub-plots consisted
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Fig. 4. Variation of profile water content with time during the 1994-1995 and 1995-1996 cotton growing seasons. D-intensively tilled continuous cotton; •-minimum tilled continuous cotton; •-minimum tilled cotton-wheat-fallow. Vertical bars indicate s.e.m.
18
N.R. Hulugalle el al. / A p p l i e d Soil Ecolo?y 7 (1997) I I 3(1
of 12-20 rows (ridges), 175 m long, spaced at l-m intervals. Accidental herbicide drift from an adjacent field in late July 1993 killed approximately 80% of the chickpeas sown. The remaining chickpeas were killed by an application of glyphosate at a rate of 2.5 I/ha, and the plots maintained thereafter as a (bare) tallow with weeds being controlled by a combination of herbicide (gtyphosate at a rate of 2.5 l / h a ) and shallow cultivation. Cotton was sown with minimum tillage in all plots in October 1994 and 1995. (NB. The last intensive tillage operations occurred in this site in May 1993). Cotton sown in 1994 and 1995 were harvested in May 1995 and 1996, respectively. Fertilizer (anhydrous ammonia) was applied in September to the cotton crops sown in 1994 and 1995 at a rate of 120 kg N / h a , respectively. Pesticides (commercial formulations of endosulfan, deltamethrin, thiodicarb, profenofos, propargite and bifenthrin) were applied from mid December to mid March when insect numbers exceeded recommended thresholds (Shaw, 1995). A limited irrigation policy was adopted during the 1994-1995 and 1995-1996 growing seasons. In 1994-1995 there was no irrigation between 70 and 155 days after sowing (DAS). Due to heavy rainfall, however, the plots remained wet until 108 DAS (Fig. 4). In 1995-1996 there was no irrigation after 81 DAS. Again, due to rainfall the plots remained wet until 110 DAS (Fig. 4).
2.3. Soil measurements Soil was sampled with a spade in April 1995 from all plots using a stratified random sampling design (Webster and Oliver. 1990) from the 0 - 1 5 0 mm. 150-300 mm, 300-450 mm and 4 5 0 - 6 0 0 mm depths and transported to the laboratory for analysis. The soil was air-dried and passed through a sieve with 2 mm diameter apertures, and plastic limit determined with a drop-cone penetrometer (Campbell, 1976) on 25 g samples pre-wetted to different water contents and equilibrated over a 2 day period without manual mixing in sealed plastic containers (25 cm3); pH (in 0.01 M CaC1 e); and exchangeable Ca, Mg, K and Na after extraction with alcoholic 1 M NH4C1 at a pH of 8.5 (Rayment and Higginson, 1992). Soil resilience, a measure of the self-mulching ability of the soil, was determined by puddling 25 g of air-dried soil
( < 2 mm) and oven-drying at 40°C for 72 h (Hulugalle et al., 1996). The size distribution of the aggregates formed (determined by dry-sieving on a mechanical shaker at 1440 vibrations per minute for 5 min) was expressed as the geometric mean diameter of the soil aggregates (Klute, 1986). Total soil organic carbon was determined by the wet oxidation method of Walkley and Black on soil which had been passed through a sieve with aperture diameters of 0.5 mm (Page et al., 1982). Dispersion (after immersion in bore water of EC = 0.06 d S / m . SAR = 4.2) was determined with a sediment densityspecific gravity meter (Mettler-Toledo I IOM) ~ on soil aggregates of 1-4 mm diameter previously wetted by evaporation in a humidifier, at soil water contents ranging from 40% to oven-dried value (0~). Dispersion index (in c/c) was expressed as: Dispersion index Mass of soil particles < 20 p.m released into the suspension due to immersion in water - Mass of soil particles < 20 p,m released into
X 100
suspension after complete dispersion of sample Dispersion index was plotted against soil water content, and the data fitted to equations of the form. Y=a+6 lnX or Y = a X ~', where Y = dispersion index and X - s o i l water content. The effects of experimental treatments on dispersion was evaluated by comparing best-fit dispersion curves for individual treatments and depths. Soil strength to a depth of 450 mm was measured with a 'Rimik' recording penetrometer on 1 August 1995. Measurements were made from the top of ridges with three profiles being measured from the four central rows of each sub-plot. Concurrently soil water content was measured gravimetrically from the same locations. In-situ soil structure was evaluated visually in sub-plots sown with cotton in 1993 during July 1996. The site was bare at the time and 103 mm of ram had been received during the preceding six weeks. Ten litres of 1:8 water soluble white acrylic paint:water mixture were applied to a 5 0 - 7 0 mm
I Trade names are given for the benefit ol the reader and does not mean endorsement of the product by the authors.
N.R. Hulugalle et aL / Applied Soil Ecology 7 (1997) 11-30
deep, 100 mm wide, 500 mm long trench dug within plant rows after excavation of the ridge to furrow level and allowed to infiltrate. One week later 600 mm deep soil pits were dug at right-angles to the trench. After smoothing, the exposed profile faces (150-550 mm) were photographed with a digital camera and the recorded images were downloaded onto a computer. Soil water content in the 300-400 mm depth at the time of measurement was measured gravimetrically. Soil water content in the 200-1200 mm depth interval was measured at regular intervals in all sub-plots from December until April during the growing seasons of 1994-1995 and 1995-1996 with a neutron moisture meter (CPN 503-DR Hydroprobe) which had been calibrated in situ (Greacen, 1981). Soil water content in the soil surface was measured gravimetrically at the same time. 2.4. Plant measurements
Plant samples were taken in December 1994 and 1995, and January, February and March 1995 and 1996 to evaluate leaf area index (which was measured on an LI-COR Model 3100 area meter), and boll number and weight. Plants sampled in early March 1995 and 1996 were used to evaluate nutrient uptake. N in plant tissues was measured with a near-infra red protein analyzer which had been precalibrated with the Kjeldahl method for tissue N (Handson and Shelley, 1993). Plant uptake of S, Mo, Zn, B, Mn, Cu, Mg, Ca, K and Na were evaluated by determining nutrient concentration in plant dry matter with an inductively coupled plasma-atomic emission spectrometer after microwave digestion with concentrated nitric acid (Handson and Shelley, 1993). After harvest in May 1995 and 1996, cotton lint fibre characteristics such as micronaire and length were measured with a Spinlab 900, and maturity and fineness with a Shirley FMT3 (Kohel and Lewis, 1984). 2.5. Soil invertebrate composition measurement
Invertebrate composition and abundance were evaluated in mid-December and late-March in the 1994-95 and 1995-96 seasons using pitfall trapping and soil cores. The pitfall traps measured surface
19
active invertebrates, whereas populations of soil invertebrates were quantified by soil cores (100 mm in diameter and 100 mm deep) which were driven into the ridges. In each plot 20 pitfall traps, and six soil cores were randomly allocated, resulting in a total of 80 pitfall traps and 24 soil cores per treatment. The pitfall traps were arranged in clusters of five with a trap located at each compass point and in the centre. The distances from the centre were allocated by random numbers as were the ridge lines in which the centre pitfall trap was placed. Two cores were sampled from either side of three central pitfall traps in each plot. Uniformity in sampling within individual plots was ensured by locating the clusters at least 25 m apart and the central pitfall trap not being located on the outer ridge lines of the plot. The pitfall traps were 25 mm in diameter and 115 mm long. They were placed flush with the soil surface and filled with a 70% ethyl alcohol and 10% glycerol mixture. The pitfall traps were left open for three days and nights, after which their contents were sorted and identified to order. The soil cores were placed on Tulgren funnels for seven days to extract any living invertebrates. The samples were sorted to order and the data converted to n u m b e r s / m 2. The furrows were not sampled because the soil in these areas was exposed to irrigation flooding. 2.6. Data analysis
The data presented in this paper pertain only to the residual effects of the tillage system/cropping sequence combinations imposed from 1985 to 1992 on soil properties, invertebrate numbers and cotton growth during the growing seasons of 1994-1995 and 1995-1996. Data were analyzed as a split-plot design with tillage system/cropping sequence combinations sown from 1985 to 1992 as main plots and crops sown in 1993 as sub-plots (Petersen, 1994). Data were also analyzed following univariate analysis of variance using the tillage systems and cropping sequences imposed from 1985 to 1992 as variables. Comparisons between tillage systems were limited to data obtained from continuous cotton plots (i.e., intensively tilled continuous cotton vs. minimum tilled continuous cotton), and between cropping sequences on data from minimum tilled plots (i.e., minimum tilled continuous cotton vs. minimum tilled
20
N.R. Hulugalle et al. / Applied Soil Ecology 7 ( 1997~ I 1-30
cotton-wheat-fallow rotation). The effects of the tillage system/cropping sequence combinations imposed from 1985 to 1992, and crops sown in 1993 on soil properties have been presented in separate reports (Constable et al., 1992; Hulugalle, 1994; Hulugalle and Entwistle, 1996, 1997).
3. Results and discussion
3.1. Soil physical and chemical properties The residual effects of tillage system were such that intensive tillage, in comparison with minimum tillage, resulted in lower organic C; and higher plastic limit, geometric mean diameter of soil aggregates after puddling and drying (GMD), pH, exchangeable Mg and Na, and exchangeable sodium percentage (ESP) (Tables I and 2). The residual effects of cropping sequence were also evident in this soil, with cotton-wheat-fallow rotation generally having lower pH and exchangeable Na than continuous cotton (Tables I and 2). Exchangeable Ca and K did not differ between treatments. Mean values of exchangeable Ca in the 0 - 1 5 0 mm, 150-300 ram, 300-450 mm and 4 5 0 - 6 0 0 mm depths were 23.5, 24.2, 23.8 and 22.3 cmol ( + ) / k g , respectively; and those of exchangeable K were 1.3, 1.0, 0.7 and 0.7 cmol ( + ) / k g , respectively. The above differences with respect to pH, plastic limit, G M D and exchangeable cations appear to be linked primarily to differences in organic C. Organic C was correlated to these soil properties as follows: pH = 9.08 - 0.42 X, r = - 0 . 5 3 ~* *, n = 144; GMD ( m m ) = 19.83 - 3.12 X, r = - 0 . 4 6 * * *, n = 144; Plastic limit (%) = 26.81 - 2.39X, r = - 0 . 5 7 * ~ *, n = 144; Mg (cmol ( + ) / k g ) = 25.72 - 3.34X, r = - 0 . 5 1 " * * , n = 144; Na (cmol ( + ) / k g ) = 6.24 - 1.23 X, r = - 0 . 6 5 ~* *, n = 144; ESP (%) = 16.23 - 3.17X, r = - 0 . 6 7 * * *, n = 144; where X = In 100 X organic C (%). Lower organic C with intensive tillage is frequently reported in the literature, and appears to be
due to rapid microbial decomposition by incorporation of crop residues during tillage (Doran et al., 1994; Dalal, 1989). Although tillage had not occurred in this site since May 1993, the differences in soil organic C suggest that tillage-induced long-term changes in soil conditions such as lower soil compaction and strength which occurred at this site favours enhanced cellulolytic microbial activity in intensively tilled plots (Fig. 2; Hulugalle and Entwistle, 1997: Torbert and Wood, 1992). Similar changes also occurred with continuous cotton in comparison with cotton-wheat-fallow rotation (Fig. 2; Hulugalle and Entwistle, unpublished data). Microbial decomposition and mineralization of organic matter result in the formation of organic acids, which in turn reduce soil pH in alkaline soils (Dalai, 1989: Hulugalle and Entwistle, 1997). Consequently, pH was lower in the treatments which resulted in higher organic matter contents. In comparison with maximum tillage, the lower exchangeable Na and Mg, and ESP with minimum tillage (Table 2) may be due to substitution of exchangeable Na and Mg by the H + (Hulugalle, 1996), which was released by organic matter mineralization. A similar substitution with respect to exchangeable Na also appears to have taken place in the 4 5 0 - 6 0 0 m depth of cottonwheat-lallow plots to a greater extent than in the same depth of continuous cotton plots. Dispersion in the 0 - 1 5 0 mm and 150-300 mm depths was greatest where intensive tillage had been combined with continuous cotton (Fig. 1, Table 3). This is presumably due to the residual effects of tillage-induced stresses on inter-particle bonds in the plough-layer (Emerson, 1991). Significant differences in dispersion did not occur between treatments in the 3 0 0 - 4 5 0 mm depth. In the 4 5 0 - 6 0 0 mm depth highest values of dispersion occurred where continuous cotton had been sown with minimum tillage. Cropping sequence in minimum tilled plots played a dominant role in determining dispersion in the 0 - 1 5 0 mm and 4 5 0 - 6 0 0 mm depths such that continuous cotton resulted in more dispersion than cotton-wheat-tallow rotation (Fig. 1, Table 3). This contrasts with observations made during 1985 to 1993 when tillage systems were the dominant factor in determining dispersion. This was such that intensive tillage resulted in higher dispersion than minimum tillage in the entire 0 - 6 0 0 mm depth (Hulu-
N.R. Hulugalle et al. / Applied Soil Ecology 7 (1997) 11-30
galle and Entwistle, 1997). With respect to dispersion, therefore, it appears that the residual effects of tillage systems are small in comparison with those of cropping sequences. Soil strength measured on 1 August 1995 was, in general, least in the 250-450 mm depth in plots where continuous cotton had been combined with intensive tillage (Fig. 2). The lower soil strength with intensive tillage appears to be due to loosening and 'shattering' of the soil by chiselling (Hulugalle and Entwistle, 1997). Sowing a cotton-wheat-fallow rotation also resulted in higher soil strength in the 250-450 mm depth when compared with sowing continuous cotton. Comparison between treatments at 50-100 mm and 200-250 mm depths was not possible because of significant ( P < 0 . 0 5 ) differences in soil water content. Penetration of paint/water mixture was greatest where minimum tilled cotton-wheat-fallow had been sown, and least in plots of intensively tilled continuous cotton (Fig. 3). Soil water content in the 0.300.40 m depth at time of measuring did not differ significantly between treatments and averaged 0.43 m 3 / m 3. (Soil water content at field saturation in the same depth is 0.46 m3/m3.) The high soil water content precludes any paint/water mixture flow via soil cracks, the main liquid flow pathway in dry cracking clays and which close at soil water contents near saturation (Constable and Forrester, 1995). This suggests that flow occurred through macropores which remained open in wet soil, presumably old root channels and macrofaunal burrows. The penetration pattern of the paint/water mixture in minimum tilled cotton-wheat-fallow rotation plots also suggest that numbers and continuity of these macropores were higher in comparison with continuous cotton plots. Profile water content monitored during the cotton growing seasons of 1994-1995 and 1995-1996 indicated that water depletion during extended drying cycles (108 to 155 DAS in 1994-1995 and 110 to 168 DAS in 1995-1996) was greatest where minimum tilled cotton-wheat-fallow rotation had been sown (Fig. 4). This was such that during the abovementioned extended drying cycles, mean daily water use by cotton sown in intensive tilled continuous cotton, minimum tilled continuous cotton and minimum tilled cotton-wheat-fallow rotation plots was
21
3.5, 3.3 and 4.0 mm, respectively, in 1994-1995 and 3.4, 3.5 and 4.1 mm, respectively, in 1995-1996. These results are surprising as both soil strength and bulk density were higher with minimum tillage and cotton-wheat-fallow rotation (Hulugalle and Entwistle, 1997; Fig. 2). At the same time, it has been widely reported that increasing soil compaction and strength reduce water extraction by cotton (Constable and Forrester, 1995; Constable et al., 1992; McGarry, 1990). We suggest, therefore, that greater water extraction by cotton in these plots is due to it utilizing stable pores with a high degree of pore-continuity created by the root systems of preceding crops or associated macrofauna as 'by-pass channels' to avoid the restrictions of the bulk soil. This may enable the crop to access sub-soil water more rapidly in comparison with cotton sown in intensively tilled plots which had a low frequency of such 'by-pass channels' (Dexter, 1991; Elkins et al., 1977; Stirzaker et al., 1996; Volkmar, 1996; Fig. 3). Digitized images of soil profile faces (150-550 mm) taken at this site suggest that, in comparison with minimum tilled cotton-wheat-fallow rotation, continuity and numbers of sub-surface macropores were lower where intensive tillage had been practised (see previous paragraph). The linkage between pore continuity and increased water extraction from deep sub-soil layers under conditions of limited water availability has been speculated upon by Dexter (1991) and Elkins et al. (1977), and studied under laboratory conditions and modelled by Stirzaker et al. (1996) and Volkmar (1996). These authors suggested that when surface water availability decreased, continuous biopores could facilitate water extraction from the sub-soil. The data presented in our paper appears to confirm their suggestions. Comparison with other field data is not possible due to the absence of reports which link soil structure, biopores and their distribution, and water extraction patterns of crops under conditions of reduced water availability. 3.2. Cotton crop growth
The cotton crop sown in 1994-1995 suffered moderate damage from a hail storm, primarily to the apical regions of cotton plants, which occurred at 40 DAS. Cotton vegetative growth, shown as seasonal
22
N.R. Hulugalle et al. / Applied Soil Ecology 7 (1997) I 1-30 3-
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Fig. 5. Variation of leaf area index with time during the 1994-1995 and 1995 and 1996 cotton growing seasons. []-intensively tilled continuous cotton; m-minimum tilled continuous cotton: •-minimum tilled cotton-wheat-fallow. Vertical bars indicate s.e.m
variation in leaf area index (LAI), was therefore characterized by relatively low values (in comparison with the undamaged crop of 1995-1996 they were 5 times lower) until 70 DAS, followed by a sharp increase in leaf growth and branching which resulted in relatively high values by 120 DAS (greater by 50% in comparison with the 1995-1996 crop) (Fig. 5). Growth curves (averaged among all treatments) for the 1994-1995 and 1995-1996 growing seasons were analyzed by regression analysis, which showed that they differed significantly ( P < 0.05). LAI of cotton did not differ significantly between treatments during the 1994-1995 growing season, although highest values occurred where minimum tilled cotton-wheat-fallow had been sown from 1985 to 1992. During 1995-1996, however, LAI of cotton at 115 and 143 DAS in plots which had been minimum tilled from 1985 to 1993 were significantly greater ( P < 0.05) than that in plots which had been intensively tilled (Fig. 5). There were no significant differences between minimum tilled plots which had been sown with either continuous cotton or cottonwheat-fallow during the same period. Lowest LAI was observed, therefore, where intensively tilled continuous cotton had been sown from 1985 to 1992. Reproductive growth of cotton, monitored as boll numbers per plant, did not differ significantly during the 1994-1995 season. Mean values at 124 and 137 DAS were 13.6 and 14.6 bolls per plant, respectively. During 1995-1996 bolls numbers per plant at 94 and 115 DAS did not differ significantly between treatments; mean values were 6.4 and 6.4 bolls per plant, respectively. At 143 DAS boll numbers per
plant were 6.6, 7.6 and 9.5 (+_ SE = 0.39, P < 0.0 l) where intensively tilled continuous cotton, minimum tilled continuous cotton and minimum tilled cottonwheat-fallow rotation had been sown from 1985 to 1992. Both tillage system ( P < 0.01) and cropping sequence ( P < 0.01) had significant effects on boll numbers per plant. The occurrence of higher vegetative and reproductive growth during 1995-1996 in plots where minimum tillage had been practised in the past, particularly in cotton-wheat-fallow rotation plots, appears to be a direct consequence of the greater water extraction therein (Fig. 4). The absence of any significant differences between treatments during the 1994-1995 growing season is probably due to confounding by hail damage and subsequent atypical crop growth patterns. 3.3. N u t r i e n t u p t a k e
Uptake of S, Ca, B, Mn and Cu by cotton, measured at crop maturity in March 1995, were significantly greater in minimum-tilled plots under a cotton-wheat-fallow rotation from 1985 to 1992 than in minimum-tilled continuous cotton plots (Table 4). Tillage system did not have any significant effect on nutrient uptake in March 1995. In March 1996, however, uptake of N, S, Mg, Ca, K, Na, Zn, B and Cu were greater in plots which were minimum tilled from 1985 to 1992 in comparison with plots subjected to intensive tillage at the same time. N uptake was also greater at the same time in minimum-tilled cotton-wheat-fallow plots in comparison with the
N,R. Hulugalle et al, / Applied Soil Ecology 7 (1997) 11-30
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