Net Primary Productivity and Rainfall Use Efficiency of Pastures On

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Tropical Grasslands (2004) Volume 38, 47Ð55 47

Net primary productivity and rainfall use efﬁciency of pastures on reconstructed land following open-cut coal mining in central Queensland, Australia

S.A. BISRAT1, B.F. MULLEN1, A.H. GRIGG2 use in central Queensland, but investigations into AND H.M. SHELTON1 the grazing capacity of these rehabilitated 1 School of Land and Food Sciences, pastures have only recently begun. Given the The University of Queensland, Australia high cost of rehabilitation (AUD25 000/ha) and 2 Centre for Mined Land Rehabilitation, the relatively small areas involved, Grigg et al. The University of Queensland, Australia (2000) suggested that grazing management should be directed to preventing degradation of the plant-soil system rather than achieving high Abstract production from cattle. Guidelines for sustainable The development of improved pastures for cattle grazing management are therefore essential. grazing is a logical post-mining land-use option Long-term experience and research in for the Bowen Basin coalfields of central northern Australia have indicated that safe Queensland, but little is known of the potential stocking rates require that 30% or less of average productivity or livestock carrying capacity of seasonal pasture production is utilised (McKeon these systems. A research program was instigated et al. 1990). This strategy requires the determina- to determine key indicators of grazing capacity, tion of net above-ground primary productivity viz. above-ground net primary productivity (NPP) (NPP) of pastures, a key measure of the potential and rainfall use efficiency (RUE), at 18 plots productivity of a pasture system (Redman 1992). across 3 mines. NPP ranged from 3000Ð11 000 Given the stochastic climate in central Queens- kg/ha/yr and RUE ranged from 4Ð21 kg/ha/mm. land (Willcocks and Filet 1993), and the variable These results are comparable with those for nature of rehabilitated pastures in terms of slope, buffel grass pastures on unmined lands in the aspect and spoil chemical and physical properties region. Multivariate linear regression was used to (McLennan 1994), considerable variation in develop a model for prediction of RUE from plot pasture productivity is expected among sites and topographic and edaphic characteristics. The over years. Rainfall use efficiency (RUE), deter- variables of slope, magnesium concentration and mined from seasonal NPP and rainfall, may be a exchangeable sodium percentage most effectively valuable tool for predicting pasture yields in 2 predicted RUE (cumulative r = 0.77). Predicted environments when moisture is the major limita- RUE and average seasonal rainfall were then tion to growth (Grigg et al. 2000). The NPP and used to predict NPP for the 3 mines. Safe RUE of improved grass pastures on unmined stocking rates were calculated based on 30% lands in central Queensland have been deter- utilisation of seasonal NPP. mined (Willcocks and Filet 1993), but are not Introduction available for rehabilitated pastures on mined lands in the region. This paper reports prelimi- Around 8000 ha of land disturbed by open-cut nary research to determine the NPP and RUE of coal mining in the dry, subtropical region of pastures on reconstructed lands following coal central Queensland, Australia, has been returned mining in central Queensland. Climatic and bio- to pastures since large-scale operations com- physical characteristics influencing NPP and menced in 1961. Cattle grazing is a logical post- RUE were examined and used to develop an mining land-use option, being the dominant land empirical model to predict RUE and, from seasonal rainfall records, NPP. Safe stocking rates Correspondence: B.F. Mullen, School of Land and Food Sciences, The University of Queensland, Qld 4072, Australia. for the 3 pasture systems were calculated from Email: [email protected] NPP estimates.

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48 S.A. Bisrat, B.F. Mullen, A.H. Grigg and H.M. Shelton

Materials and methods 1992). Total cover and green cover were estimated visually. Green cover was the area covered Experimental plots by attached, standing vegetation as a percentage Eighteen plots were selected for the measurement of total surface area. Total cover included both of NPP on areas of established pasture on recon- green cover and litter cover. Peak green cover structed lands at Blackwater, Norwich Park and was the highest percentage of green cover Goonyella/Riverside mines in the Bowen Basin recorded for each plot over the growing season. coalfields in central Queensland, Australia Standard errors for the means of peak DM yield (20¡Ð25¡S; 148¡Ð150¡E). The Bowen Basin has a and peak green cover were determined for each subhumid climate with approximately 650 mm plot. annual rainfall, 70% of which falls over the summer (NovemberÐApril). Mean monthly Plot characterisation maximum/minimum temperatures range from 34¡/21¡C in January to 23¡/7¡C in July. Two soil cores were collected from each plot at All areas were rehabilitated to pastures of depths of 0Ð15 and 15Ð50 cm at each pasture Cenchrus ciliaris (buffel grass), Chloris gayana sampling for determination of physical and (rhodes grass) and a range of legumes, notably chemical properties. At the completion of the Macroptilium atropurpureum (siratro). At the sampling program, cores were bulked within Blackwater mine, pasture was sown directly on to plots for each depth, air-dried and ground to pass spoil in the mid-1970s. At the Norwich Park and through a 2 mm sieve. Total soil nitrogen (N) and Goonyella/Riverside mines, top-soil was applied total carbon (C) concentrations were determined to a depth of 20Ð30 cm prior to sowing in the by combustion analysis (LECO CNS 2000, Leco mid-1990s. Corporation, St Joseph, MI, USA). Phosphorus Six plots (12 × 15 m) were fenced to exclude (P) and exchangeable cation (Ca, K, Mg, Na) grazing animals at each of the 3 mines. The plots concentrations were determined by inductively were selected to represent the range of variability coupled plasma atomic emission spectroscopy of the rehabilitated pastures at each mine and to (SPECTRO P+M, Spectro Analytical Instru- minimise variation within each plot. All plots had ments, Kleve, Germany). Cation exchange a uniform cover of buffel grass, the dominant capacity (CEC) and exchangeable sodium per- pasture species. Daily rainfall and maximum and centage (ESP) were calculated from exchange- minimum temperatures were recorded at each able cation analysis. Soil pH (1:5 in H2O) and site over the experimental period. electrical conductivity (EC) were also determined. Net primary productivity sampling and analysis Particle size distribution (proportion of clay, Plots were slashed to a height of 2.5 cm above silt, fine sand and coarse sand) was determined ground level in early spring (SeptemberÐOctober) using the pipette method on the <2 mm fraction of 2000, prior to the commencement of the (Klute 1982). Water holding capacity (WHC) was growing season, and plant material removed. determined at field capacity (P = 10 kPa) and at Plots were then sampled monthly until late April permanent wilting point (P = 1500 kPa) from 2001. At each sampling, dry matter (DM) yields 3 replicates for each sample at each depth using a of regrowth were measured from 3 quadrats pressure plate apparatus (Klute 1982). (1m × 1m) in each plot, by cutting to a height of Slope was determined for each plot using an 5 cm and weighing after oven-drying at 80¡C for inclinometer and aspect was determined using a 48 hours. At the initial sampling, placement of compass. quadrats was at random. At subsequent samplings, quadrats were placed 50 cm from the Determination of rainfall use efficiency previously cut location, so that successive samplings sampled progressively older regrowth. Rainfall use efficiency (RUE), defined as the At each sampling, care was taken to avoid distur- amount of DM biomass produced on 1 hectare bance of areas scheduled for future cutting. per millimetre of rain (Le Houerou 1984), was Peak DM yield, the highest yield recorded for calculated as follows: each plot over the growing season, was used to RUE = Peak DM yield (kg/ha)/ determine the primary productivity (Redman cumulative rainfall (mm)

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NPP and RUE of pastures on mined lands 49

Cumulative rainfall refers to that falling from well below the long-term average of 640 mm. initial slashing to achievement of peak yield. Goonyella/Riverside mine received 744 mm of RUE values were then regressed against the rainfall, well above the long-term average of range of measured plot parameters in order to 608 mm, although most of the rain fell at the investigate the factors most affecting RUE. beginning of the growing season between Parameters signiﬁcantly related to RUE were October and December 2000 in heavy, short- then used in a multiple step-wise linear regres- duration events. Rainfall at Norwich Park mine sion to predict RUE. Where parameters were (824 mm) exceeded the long-term average of correlated with each other, the parameter least 723 mm. The JulyÐSeptember quarter was the related to RUE was omitted from the analysis. driest period at all 3 mines.

Determination of safe stocking rates Pasture DM yield, surface cover and rainfall use Average long-term annual pasture growth for efﬁciency pasture systems at the 3 mines was calculated Of the 18 experimental plots, one exclosure at from the mean RUE value and the mean seasonal Norwich Park was grazed by cattle in November rainfall (OctoberÐApril). Safe stocking rates were 2000 and was excluded from subsequent calculated for each mine by assuming a sustain- measurements and analyses. Seasonal peak able pasture utilisation rate of 30% of seasonal DM yields among the 17 plots ranged from forage growth (McKeon et al. 1990). Pasture 3040Ð11 080 kg/ha (Plot 5 at Blackwater and Plot removal by stock was calculated based on a daily 16 at Norwich Park, respectively), with a mean intake of 2.5% of body weight and trampling peak yield of 5350 kg/ha (Table 1). Mean peak losses of 30% of intake. An animal equivalent DM yield at each mine was highest at Norwich (AE) was considered to be a 450 kg steer. Park (7550 kg/ha) followed by Goonyella/River- side (4380 kg/ha) and Blackwater (4130 kg/ha). Peak green cover varied from 45Ð100%, (Plot Results 6 at Blackwater and Plots 14 and 16 at Norwich Park, respectively), with a mean cover of 68% Rainfall (Table 1). DM yield was signiﬁcantly related to Blackwater mine received 458 mm of rainfall for green cover (R2 = 0.75) throughout the regrowth the 12-month period from May 2000ÐApril 2001, phase (Figure 1).

12 000

10 000 y = 0.57x2 + 27.7x R2 = 0.75 8 000

6 000

4 000 DM yield (kg/ha)

2 000

0 020406080100 Green cover (%) Figure 1. Relationship between green cover and total DM yield during regrowth of rehabilitated pastures for 17 plots in central Queensland (n = 96).

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50 S.A. Bisrat, B.F. Mullen, A.H. Grigg and H.M. Shelton

Rainfall use efﬁciency (RUE) values ranged concentrations were highest at Blackwater mine. from 4Ð21 kg/ha/mm among the 17 plots Similarly, CEC was highest at Blackwater, with (Table 1). The lowest RUE value (4 kg/ha/mm) the lowest CEC observed at Goonyella/Riverside. was recorded at Goonyella/Riverside mine Mg and Ca concentrations and C:N ratios were whereas the highest value (21 kg/ha/mm) was lowest, and salinity (EC) and sodicity (ESP) were recorded at Norwich Park mine (Table 1). highest at Goonyella/Riverside. WHC and silt proportion were relatively low at Norwich Park. Soil chemical and physical characteristics Coarse sand proportion was relatively low at Blackwater. There were no major differences in Soil chemical and physical properties for the 3 ﬁne sand proportion, clay proportion, soil pH, or mines are summarised in Table 2. C and N P concentration among the 3 mines (Table 2).

Table 1. Peak DM yield and standard error of the mean (s.e.), peak green cover and s.e., slope, cumulative rainfall to achieve peak yield and rainfall use efﬁciency for 17 plots at 3 open-cut coal mines in central Queensland, Australia.

Mine/plot Peak DM yield Peak green cover Slope Cum. RUE at peak rainfall yield

(kg/ha) s.e. (%) s.e. (%) (mm) (kg/ha/mm) Blackwater 1 4870 789 75 18.0 5 293 17 2 3260 233 63 1.7 11 361 9 3 3410 671 60 5.8 17 293 12 4 5670 1937 87 8.3 12 316 18 5 3040 468 52 16.4 18 316 10 6 4550 236 45 7.6 9 316 14 Goonyella/Riverside 7 5390 805 75 7.6 13 716 8 8 5220 996 60 15.3 15 716 7 9 4130 797 73 6.0 18 716 6 10 3220 1013 52 16.1 17 711 4 11 3770 453 63 4.4 17 711 5 12 4550 362 70 2.9 13 711 6 Norwich Park 13 6850 500 92 8.3 14 582 12 14 7440 550 100 0.0 3 582 13 15 5670 1701 85 7.6 4 502 11 16 11 080 85 100 0.0 5 533 21 17 6700 859 72 11.7 9 453 15

Mean 5350 68

Table 2. Chemical and physical characteristics at two depths for reconstructed soils following mining. Data are averages from 6 plots at Blackwater mine, 6 plots at Goonyella/Riverside mine and 5 plots at Norwich Park mine.

Mine Blackwater Goonyel1a/Riverside Norwich Park

Soil depth (cm) 0Ð15 15Ð50 0Ð15 15Ð50 0Ð15 15Ð50

C (%) 3.3 2.4 1.2 1.5 1.0 1.1 N (%) 0.16 0.14 0.07 0.07 0.07 0.07 C:N ratio 21 17 17 21 14 16 P (cmol/100g) 19.4 19.3 17.4 19.9 20.2 17.7 Ca (cmol/100g) 17.9 11.9 6.5 5.4 11.2 11.4 K (cmol/100g) 0.54 0.48 0.29 0.28 0.26 0.29 Mg (cmol/100g) 12.2 12.4 5.8 5.7 8.5 10.1 Na (cmol/100g) 1.4 1.0 3.1 3.2 1.2 2.0 CEC (cmol/100g) 32 26 16 15 21 24 ESP (%) 4 4 18 20 5 8 pH 8.8 9.0 8.5 8.6 8.7 8.7 EC (dS/m) 0.15 0.13 0.45 0.32 0.10 0.15 Clay (%) 46 37 40 36 43 45 Silt (%) 22 27 17 23 8 10 Fine sand (%) 20 23 26 18 25 25 Coarse sand (%) 12 13 17 23 24 20 Water holding capacity (%) 25 23 23 22 18 20

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NPP and RUE of pastures on mined lands 51

RUE model (15Ð50 cm) and CEC (15Ð50 cm) were omitted, being related to Mg (0Ð15 cm). Ca (15Ð50 cm), Of the 38 plot parameters measured, 11 were ESP (15Ð50 cm) and EC (15Ð50 cm) were signiﬁcantly (P<0.05) correlated with RUE omitted, being related to ESP (0Ð15 cm). (Table 3). Slope, EC and ESP were negatively The remaining 5 parameters were then used correlated with RUE, whereas surface cover, Ca, for multiple regression analysis. The ﬁnal regres- Mg and clay content were positively correlated. sion model used 3 parameters and explained 77% Individually, slope and surface cover were the of the variation in RUE in the equation: best predictors of RUE (Table 3). However, surface cover was excluded from the multiple RUE (kg/ha/mm) = regression analysis because it was not inde- Ð0.49*Slope + 0.59*Mg (0Ð15) Ð 0.12* pendent of slope (Figure 2). Similarly, Mg ESP (0Ð15) + 12.78

Table 3. Correlation matrix (r) for variables with signiﬁcant individual relationships with RUE. Critical r value (P<0.05) is ±0.47 (n = 17).

Variable RUE Cover Slope Mg ESP EC Ca Mg CEC ESP EC Clay 0Ð15 cm 0Ð15 cm 0Ð15 cm 15Ð50 cm 15Ð50 cm 15Ð50 cm 15Ð50 cm 15Ð50 cm 15Ð50 cm

RUE 1 Cover 0.66 1 Slope Ð0.67 Ð0.72 1

Mg 0Ð15 cm 0.57 0.20 Ð0.10 1 ESP 0Ð15 cm Ð0.57 Ð0.24 0.35 Ð0.26 1 EC 0Ð15 cm Ð0.47 Ð0.45 0.34 Ð0.27 0.42 1 Ca 15Ð50 cm 0.61 0.35 Ð0.43 0.57 Ð0.61 Ð0.43 1 Mg 15Ð50 cm 0.57 0.38 Ð0.37 0.78 Ð0.29 Ð0.45 0.74 1 CEC 15Ð50 cm 0.57 0.37 Ð0.43 0.72 Ð0.24 Ð0.42 0.85 0.93 1 ESP 15Ð50 cm Ð0.48 Ð0.22 0.25 Ð0.28 0.94 0.34 Ð0.65 Ð0.38 Ð0.30 1 EC 15Ð50 cm Ð0.56 Ð0.46 0.43 Ð0.42 0.70 0.43 Ð0.79 Ð0.65 Ð0.62 0.76 1 Clay 15Ð50 cm 0.50 0.30 Ð0.53 0.08 Ð0.26 Ð0.36 0.41 0.33 0.43 Ð0.12 Ð0.26 1

100

y = Ð0.127x2 + 1.00x + 93.3 70 R2 = 0.57 Surface cover (%) cover Surface 60

50 051015 20 Slope (%) Figure 2. Relationship between slope and total surface cover at peak yield for 17 plots on rehabilitated pastures in central Queensland.

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52 S.A. Bisrat, B.F. Mullen, A.H. Grigg and H.M. Shelton

Safe stocking rates by reducing photosynthetic area and ground cover, thereby reducing rainfall retention and Predicted safe stocking rates based on average infiltration. Pearson (1965) reported lower RUE rainfall are presented in Table 4. Plot 16 was values for moderately grazed paddocks in com- excluded from determination of mean RUE for parison with lightly grazed paddocks in Eastern Norwich Park as it was of considerably higher Idaho. On the other hand, if paddocks are yield (Table 1) compared with the surrounding ungrazed for a long time, the problem of stand pasture and therefore not representative of the stagnation may arise leading to lower RUE values rehabilitated area. (Myers and Robbins 1991). Appropriate grazing management is therefore necessary to maximise Discussion RUE. However, the RUE data suggest that pastures on reconstructed lands following coal Mean peak DM yield recorded across the 3 mines mining in the Bowen Basin can be as productive (5350 kg/ha) approximated mean DM yields as those on unmined lands. recorded for mined and unmined lands in central Queensland in other studies. Grigg et al. (2000) Factors affecting growth and RUE reported a mean total DM (including litter) of 5000 kg/ha for buffel grass-dominant pastures at Plant growth is driven by solar radiation, temper- Moura, Blackwater, Gregory and Goonyella/ ature, soil moisture and essential nutrients. Being Riverside mines. DM yields of 6610 kg/ha were an index of plant growth (NPP), RUE is similarly reported for buffel grass pastures on unmined influenced by these factors. Little variation existed land in central Queensland (Willcocks and Filet in solar radiation levels among plots in the current 1993). study. Incident radiation was relatively high RUE values were also similar to those in the throughout the growing season (c. 19 MJ/m2/d, literature. Many authors have calculated water use data not presented) and therefore solar radiation efficiency (WUE) values for pastures by was not included as a variable for prediction of accounting for initial and final soil moisture and RUE. Temperature was also excluded from exam- assuming negligible run-off and deep drainage. In ination as it did not limit plant growth during the such studies, WUE and RUE are generally very growing season (mean day/night temperatures similar as differences between initial and final were 33o/20oC) and did not vary among plots. In soil water are small in comparison with total contrast, soil moisture is commonly limiting in seasonal rainfall. WUE values for buffel grass central Queensland grazing lands, where seasonal pastures on unmined land ranged from 6.7 rainfall of approximately 500 mm is vastly lower kg/ha/mm in south-west Queensland (Johnston than the seasonal evaporation of approximately 1996) to 15 kg/ha/mm (RUE = 14.6 kg/ha/mm) in 2000 mm. central Queensland (Willcocks and Filet 1993). A hierarchy of factors affects the ability of pas- These values are comparable with the mean RUEs ture to utilise rainfall: 1) incident rain must be for Goonyella/Riverside mine (6.0 kg/ha/mm) retained within the landscape long enough for and Blackwater and Norwich Park mines (13.3 adequate infiltration to take place; 2) the rate of and 12.8 kg/ha/mm, respectively). Some caution infiltration must be sufficient for water to pene- is required when comparing RUE values, as the trate into the root zone; and 3) losses from deep data in the current study were derived from percolation must be minimised. The strong corre- ungrazed pastures. Heavy grazing reduces RUE lations between slope, cover and RUE in the

Table 4. LongÐterm seasonal rainfall, mean RUE, potential forage yield and calculated safe stocking rate for rehabilitated pastures at 3 open-cut coal mines in central Queensland, Australia.

Blackwater mine Goonyella/Riverside mine Norwich Park mine

LongÐterm average seasonal rainfall (mm) 497 492 579 Mean RUE (kg/ha/mm) 13.3 6.0 12.8 Potential forage yield (kg/ha) 6600 3000 7400 Calculated safe stocking rate (ha/AE1) 2.7 5.9 2.4

1 AE = animal equivalent (450 kg steer).

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NPP and RUE of pastures on mined lands 53

current study indicate that the ability to trap or that the resistance to water movement through a slow the passage of incident rain for subsequent soil could be described by a combination of infiltration has a strong influence on pasture pro- sodicity (ESP), clay content and clay mineralogy. ductivity. These factors affect soil porosity, clay dispersion, The predominance of slope in influencing RUE pore spaces and hydraulic conductivity. Soils has important implications for mine operators with clay contents in the range 30Ð60% were at considering grazing as a post-mining land use, or greatest risk of reduced hydraulic conductivity in indeed for the establishment and maintenance of the presence of sodium, a situation evident at a grass cover for surface stability alone. Recon- Goonyella/Riverside mine. The moderate clay structed lands following open-cut coal mining are content and high ESP at the Goonyella/Riverside generally steep, with slopes in excess of 10%, due plots would be expected to reduce water infiltra- to the considerable costs (up to AUD25 000/ha) tion. Surface ESP was individually correlated involved in regrading the waste rock typically with RUE (r = Ð0.57) and contributed to the dumped at the angle of repose of c. 75%. multivariate model. Sodicity at the surface causes Although not measured, surface roughness sealing and crusting which directly inhibits water created by deep ripping during the rehabilitation penetration. Elevated salinity is a common fea- process probably influenced retention of rainfall ture of waste rock from these mines and the at the study plots, notably at Norwich Park mine. downward migration of salts beneath rehabili- Carroll et al. (2000) reported substantial reduc- tated pastures provides an indication of the tions in runoff with increasing surface roughness ability of water to infiltrate. for a given slope, as long as rills did not breach The dry climate of central Queensland the ridge-furrow relief. It is interesting to note, imposes strong constraints on pasture growth. however, that the effect of surface roughness was Annual evaporation exceeds rainfall by a factor most pronounced early after establishment, fol- of 3 and mean monthly evaporation is greater lowing which vegetation cover became the pre- than mean monthly rainfall throughout the year. dominant factor affecting runoff. In the current The greatest deficit between evaporation and study, surface cover declined markedly once slope rainfall occurs between October and December increased above approximately 14% (Figure 2), (Willcocks 1993), early in the growing season. indicating a possible over-riding influence of Approximately 60% of the annual average rain- slope. The significance of surface roughness and fall is estimated to occur in events of less than its relationship with slope will be investigated in 10mm, the minimum considered necessary for future research. adequate plant growth (Willcocks 1993). In Vegetation cover, which was strongly related addition, larger falls frequently occur in short- to RUE as an individual parameter (Table 3), has duration storms of high intensity, with concurrent consistently been the main factor affecting runoff low infiltration and high runoff. These patterns and erosion in these landscapes (McIvor et al. explain the prominence in the RUE model of fac- 1995; Carroll et al. 2000; Loch 2000). Cover has tors affecting the ability of the pasture to access a two-fold effect in reducing runoff. It impedes incident rainfall. The results are also confirma- overland flow and increases infiltration via root tion of the utility of RUE as a measure of pasture macropores (Loch and Orange 1997). In this productivity for these lands. study, cover was positively related to DM yield Factors related to soil fertility were of lesser (R2 = 0.75, Figure 1) and negatively related to importance in determining RUE in comparison slope (R2 = 0.57, Figure 2). Lower levels of cover with factors associated with water retention and were generally associated with steeper slopes, infiltration (Table 3). Despite being positively indicating a possible ‘feedback’ effect on pasture related to RUE, absolute levels of Mg did not productivity. Similar negative relationships with appear to pose any direct nutritional limitations slope have been reported for rangelands else- on pasture growth (Aitken and Scott 1999). where (Holecheck et al. 1995). Whilst Mg is an essential plant nutrient, its inclu- Factors affecting potential infiltration (ESP, sion in this preliminary model was considered to EC), as distinct from surface retention of water, be a surrogate for the more important soil fertility were also important in this study, although their factors, N and CEC. Mg concentrations were significance needs to be confirmed with data strongly related to soil surface N (r2 = 0.53) and from an expanded set of sites. Shaw (1995) found soil surface CEC (r2 = 0.74) concentrations (data

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54 S.A. Bisrat, B.F. Mullen, A.H. Grigg and H.M. Shelton

not presented). Suboptimal N concentrations in central Queensland are comparable with those of central Queensland soils are known to signifi- the surrounding unmined lands. Using multivar- cantly limit growth of pastures on unmined lands iate linear regression, slope, soil Mg concentra- in the region (Burgess and Barrett 2000). CEC is tion and ESP were selected from a range of also an important measure of fertility, by indi- topographic and edaphic characteristics for pre- cating the soil’s ability to retain major plant diction of RUE. The model suggested that high cations. Mg concentration was a significant com- RUE was primarily dependent on factors that ponent of CEC in the current study, so that CEC increased rainfall retention and infiltration, rather made no significant contribution to predicting than soil fertility limitations. Incorporation of a RUE in the multiple linear regression in addition broader range of sites, and validation using inde- to Mg. The apparent importance of Mg may be a pendent data, will strengthen the model. result of the small sample size to date and N, P or CEC may feature in a model incorporating data Acknowledgements from an expanded set of sites. P concentrations were not limiting for pasture We thank site environmental officers Bernie growth (Peverill et al. 1999) and were compara- Kirsch, Jorrit Vochteloo and Glenn McDonald for tively similar among the study plots (Table 2), so their support and assistance with field programs; that P was poorly related to RUE. Johnston Bruce Black and David Frank for their assistance (1996) reported soil P as an important factor with cattle management; and Narelle McCallum, affecting RUE in south-west Queensland, but soil Lachlan Crawford, Steve Williams and Kamal P concentrations were generally lower and more Yatapanage for technical support. This study was variable than in the current study. Concentrations funded by the Australian Coal Association of other plant nutrients were not limiting for Research Program. growth of pasture grasses (Peverill et al. 1999). References Safe stocking rates AITKEN, R.L. and SCOTT, B.J. (1999) Magnesium. In: Peverill, K.I., Sparrow, L.A. and Reuter, D.J. (eds) Soil analysis: an The calculated safe stocking rates for Blackwater interpretation manual. pp. 255Ð262. (CSIRO Publishing: Melbourne). and Norwich Park mines (Table 4) were similar to BISRAT, S.A. (2002) Primary productivity and grazing capacity safe stocking rates recommended for buffel grass of rehabilitated pastures on the open-cut coal mines in pastures on unmined sites in central Queensland, central Queensland. M. Thesis. University of Queensland. BURGESS, J.W. and BARRETT, C.M. (2000) Soil fertility and 3 ha/head (Lambert and Graham 1996) and animal productivity in the Nebo-Broadsound district of 2Ð3 ha/head (Partridge 2000). However, the central Queensland. Tropical Grasslands, 34, 139Ð146. Goonyella/Riverside mine had considerably lower CARROLL, C., MERTON, L. and BERGER, P. (2000) Impact of vegetative cover and slope on runoff, erosion, and water carrying capacity, apparently due to its steep quality for field plots on a range of soil and spoil materials slopes and high soil ESP. Rehabilitated pasture on central Queensland coal mines. Australian Journal of systems on mined lands are highly variable in Soil Research, 38, 313Ð327. GRIGG, A., SHELTON, M. and MULLEN, B. (2000) The nature topography and substrate, and thorough sampling and management of rehabilitated pastures on open-cut coal is required to confidently predict pasture produc- mines in central Queensland. Tropical Grasslands, 34, 242Ð250. tion over extensive reconstructed landscapes. HOLECHECK, J.L., PIEPER, R.D. and HERBEL, C.H. (1995) Other pastures at Goonyella/ Riverside mine exist Range management principles and practices. 3rd Edn. on lands of more gentle slopes and these may (Prentice Hall: New Jersey). JOHNSTON, P.W. (1996) Grazing Capacity of Native Pastures support higher stocking rates. in the Mulga lands of South Western Queensland: A Model- Longer-term grazing studies investigating ling Approach. Ph.D. Thesis. University of Queensland. pasture sustainability and animal production KLUTE, A. (1982) Methods of Soil Analysis, Part 1:— Physical and mineralogical methods. (American Society of potential at a range of stocking rates are Agronomy: Madison, Wisconsin). underway at the 3 sites (Bisrat 2002) to validate LAMBERT, G. and GRAHAM, G. (1996) Sown Pasture Notes, Central Queensland. (Queensland Department of Primary the outcomes of the current study. Industries: Brisbane). LE HOUEROU, H.N. (1984) Rain use efficiency: a unifying con- cept in arid land ecology. Journal of Arid Environments, 7, Conclusions 213Ð247. LOCH, R. J. (2000) Effects of vegetation cover on runoff and erosion under simulated rain and overland flow on a rehabil- The study showed that the NPP and RUE values itated site on the Meandu Mine, Tarong, Queensland. of rehabilitated pastures in the Bowen Basin of Australian Journal of Soil Research, 38, 299Ð312.

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(Received for publication October 1, 2002; accepted December 13, 2002)