Pasture-cropping: effect on biomass, total cover, soil water and nitrogen.
S.E. Bruce1, S.M Howden1, S. Graham1, C. Seis2, J. Ash3 and A.O. Nicholls1
1 CSIRO Sustainable Ecosystems, GPO Box 284, Canberra, ACT 2601.
2 ‘Winona’ via Gulgong. NSW 2852
3 School of Botany and Zoology, Australian National University, Canberra 200, ACT 2601.
A field experiment was conducted in the central west of New South Wales to determine the effect of pasture-cropping on biomass production, total cover, soil water and soil potentially available nitrogen compared to more conventional cropping and grazing enterprises. The experiment compared three treatments: (1) C4-dominant native perennial pastures; (2) winter oats crops; and (3) pasture-cropping (oats drilled into C4-dominant native perennial pastures). Biomass in the pasture-crop treatment was, on average, similar to biomass in the crop treatment during the oats growing season; and no different to biomass in the pasture treatment during the pasture growing season. The combination of retained stubble with crowns and shoots of perennial grasses resulted in greater ground cover in the pasture-crop treatment compared with the crop treatment. Soil water content for the 0-60 cm profile was reduced in the pasture-crop treatment compared to the crop or pasture treatment. Nitrogen availability was reduced and less variable in the pasture-crop treatment compared to the crop or the pasture treatment. Essentially, the oats added substantial biomass and litter to the pasture-crop system in spring without greatly altering biomass of the following pasture phase. The increased total biomass in the pasture-crop system caused a reduction in both soil water and nitrogen availability, and may reduce the risk of waterlogging, and loss of nitrogen through denitrification or nitrate leaching. Higher ground cover in the pasture-crop system may also reduce the risk of soil erosion.
Australian soils used for arable farming mostly developed under deep-rooted perennial cover, which was largely removed 50-150 years ago to reduce competition for annual crops and pastures (Lyon et al. 2004). Various adverse consequences can emerge in these soils because these contemporary winter-active, mainly shallow-rooted annual systems do not always use all the water available to them: increasing the likelihood of raised water tables and salinisation; and exacerbating or extending periods of waterlogging. Novel management systems, tailored to soil types, may be able to reduce these adverse conditions and increase yields.
Duplex soils are widely distributed in southern Australian farming systems, covering approximately 40 % of the cropping land (McCallum et al. 2004 recalculated from Chittelborough 1992), and are characterised by a texture contrast and, a distinct boundary between the A and B horizon (Chittelborough 1992). The B Horizons are generally dense and clayey with restricted hydraulic conductivity, leading to periodic water logging especially over winter when evaporation rates are low (McKenzie et al. 1999). Root penetration into the subsoil is often physically impeded due to the density of the clay and the poor aeration due to periodic saturation, leading to reduced yield potential. Erosion may also be a serious problem in these soils, particularly if sodic.
Changes in land-use and farm management practices form the primary means of addressing the challenges to crop production on these types of soils and have been the subject of previous research (Humphries et al. 2004; McCallum et al. 2004). A phase of perennial pasture, such as lucerne, can reduce the incidence of waterlogging due to improved infiltration into duplex subsoils and can increase subsoil water uptake of following winter crops leading to potential benefits for growth and yield (McCallum et al. 2004). However, phase farming systems rely on the removal of perennial species prior to planting annual crops such as wheat and may be costly due to the reliance on herbicides for perennial removal, and the establishment of the perennial phase. Lucerne may also be grown as an intercrop, where an annual crop is seeded over a permanent background of lucerne (Humphries et al. 2004). However, lucerne’s intolerance of acid and waterlogged soils (Humphries and Auricht 2001) and its poor persistence can limit its use or adoption.
Intercropping with native deep-rooted perennial grasses is one of the most promising innovations to reduce waterlogging and water tables resulting from high rainfall seasons. Native perennial pastures have evolved to grow in the weathered and often saline soils of Australia, show a considerable band of pH tolerance (Johnston et al. 2001), and are persistent if appropriate management practices are used. Pasture-cropping is one such system used by farmers in Australian farming systems, and involves direct-drilling winter crops (C3) into predominantly summer growing pastures (mostly C4 species). In comparison with C3 species, C4 species grow at higher temperatures and are more efficient in their use of water. From an ecological perspective, the key feature of this system is that there is a separation of the growing seasons of the crop and pasture with only limited overlapping ‘shoulder’ periods when competition is likely. This differs from other intercropping systems where growth tends to be more synchronous. The summer C4 pastures replace the fallow of typical cropping systems. What might this seasonal separation of resource use mean in terms of biomass production, water balance, and erosion?
Anecdotal reports of the benefits of pasture-cropping, such as: increasing soil cover, increasing biomass production, and reductions in soil acidity and transient waterlogging, prompted more detailed investigations of the impacts of pasture-cropping on biomass production, cover, soil nitrogen and water compared to more conventional cropping and grazing enterprises. This study examined the general impacts of pasture-cropping on biomass production, ground cover, soil water, and nitrogen compared to a C4 dominant perennial pasture and a direct-drilled winter oats crop in a replicated field experiment in the central-west wheat-belt of NSW.
The study was conducted at Winona near Gulgong, NSW (32°09’S, 149°34’E, altitude 500 m) with bi-monthly observations from September 2003 to January 2005. The soils had duplex profiles, ranging from Red Kurosols to Yellow Sodosols which are characterised by massive, strongly leached A horizons (pH 6.0, CaCl2), over a heavy red or yellow clay B horizon (pH 5-8, CaCl2), with grey mottles due to prolonged periods of anaerobic conditions associated with waterlogging. The top of the B horizon (0.3-0.4 m) had a hard layer (bulk density:1.7-1.9) with low hydraulic conductivity that resulted in temporary perching of water and saturation. The experiment had three replicate blocks along the soil gradient and since no block differences were detected in variables measured it was concluded that soil type was not having a significant effect, so this is not discussed further. The experiment had three treatments in each block: (1) direct-drilled oats crop with stubble retention (crop); (2) C4-dominant native perennial pasture (pasture); and (3) a C4-dominant perennial pasture with overcropped direct-drilled oats (pasture-crop). In January and April 2004 the crop treatment was sprayed with 1.5L/ha of glyphosate; in June 2004 the crop and pasture-crop treatment were sprayed with 800 mL/ha of sprayseed. In May 2003 and 14 June 2004, oats were direct-drilled with 70kg/ha of Granuloc 12 (11.9%N:17%P:5.5%K) in the crop and pasture-crop treatments; pastures were not fertilised. Each plot measured 12 m x 12 m. Shoot dry weight of C3 and C4 plant components (including oats seed yield), and plant cover (aerial and ground) were measured in four 0.25 m2 quadrats. Plant cover was estimated using a point quadrat method in ARCmap, using a 5cm point grid superimposed over digital images of the quadrats. Following each harvest the pasture and pasture-crop plots were cut to simulate grazing, except after stem elongation of the oats when only the pasture was cut. Soil water and potentially available nitrogen were measured in four soil cores in each plot. Soil cores (diameter 50 mm) were taken to a depth of 60 cm, and sectioned into 10 cm depth intervals. Gravimetric soil moisture content was determined on soil samples dried at 105oC for 48 hours. Bulk density was determined for each depth interval within the profile using the water displacement method (Cresswell and Hamilton 2002). Volumetric water (mm water per cm of soil) was calculated as the product of gravimetric water and bulk density. Potentially available nitrogen (Giannello and Bremner 1986) was determined on samples dried at 40oC for 72 hours. Data were analysed using the repeated measures ANOVA procedure in Genstat. In all analyses the residuals were checked and, if necessary, the data were transformed to achieve normality of variance using the natural log or arcsine transform. If necessary, means were back-transformed to the original scale of measurement. Due to the non-linearity of transformation, confidence limits reported in the scale of measurement are asymmetrical. All treatment differences referred to in the results are statistically significant, at least to the 5% level.
Results and Discussion
Biomass, yield and ground cover
Regardless of treatment, growth was lowest during summer months when water availability was low and temperatures high, and highest during November when water availability was adequate and temperatures were high. Live shoot biomass in the pasture-crop treatment was similar to biomass in the crop treatment during the oats growing season (June – November); and similar to biomass in the pasture treatment during the remainder of the year (Figure 1a). Oats seed yield was more variable under pasture-cropping but there was compensating pasture growth, resulting in no net increase in biomass variability (data not shown). Essentially, oats grew nearly as much in the pasture-crop as in the crop treatment, and pasture growth was similar in the pasture-crop and pasture. By having a mixture of C3 and C4 plants that are growing at all times of the year in the pasture-crop treatment there is an increase in the total light interception for photosynthesis leading to a greater useful biomass production per hectare (grain plus forage) than for either cropping or pasture treatments alone.
The combination of retained oats stubble with crowns and shoots of perennial grasses resulted in greater ground cover, on average, in the pasture-crop treatment compared with the crop treatment (Figure 1b). Just prior to harvest in November 2003, the pasture-crop treatment had greater ground cover than the crop, due to the presence of perennial species. During summer months, ground cover was similar in all three treatments, due to the presence of perennial species in the pasture and pasture-crop treatments and the presence of stubble in the crop and pasture-crop treatments. In winter, cover in the pasture-crop treatment was slightly less than that in the pasture treatment, which is likely to be a response to spraying and seeding of the pasture-crop treatment during the winter months.
By autumn and during droughts a pasture-crop system is likely to maintain substantially greater ground cover, due to perennials, than annual cropping. This could have the following benefits: a) reduced wind-induced soil erosion; b) reduced water-induced soil erosion due to potentially higher soil infiltration rates; c) reduced weed outbreaks due to competition from actively growing plants at all times of the year and fewer germination sites as a result of substantial ground cover; and d) increases in soil organic matter.
Soil water content
Soil water content of the 0-60 cm profile was less in the pasture-crop treatment compared to the crop or pasture treatment (Figure 1c). Soil water was highest in the crop treatment year-round, despite substantial plant growth during winter-spring, and may be a consequence of the shallow root systems of oats not accessing water deeper in the profile. The pasture-crop treatment had the lowest soil water content and is likely to be a response to the combination of both deep-rooted perennials and shallow rooted oats accessing water throughout the profile. The pasture treatment had an intermediate effect. Between September 2003 and March 2004 there was a large decrease in soil water content of the soil profile for all treatments. The drying was most pronounced in the pasture-crop and pasture treatments. Between May 2004 and September 2004 there was an increase in soil water content in all treatments. The wetting was most pronounced in the crop and pasture treatments.
Year-round plant growth and the mix of shallow-rooted and deep-rooted plants in the pasture-crop treatment are likely to be responsible for the reduced amount of water in the soil profile. In these strongly duplex soils the greater use of soil water reduces the risk of waterlogging, and in other soil types the frequency and size of drainage events is likely to be reduced, potentially decreasing the risk of dryland salinity.
Potentially available nitrogen
Potential nitrogen availability was less variable, and on average lower, in the pasture-crop treatment compared to the crop or the pasture treatment (Figure 1d), which may be a response to plant growth year-round. Peaks in potential nitrogen availability represent a complex interaction between dead root and litter inputs, previous soil temperature and moisture conditions and plant uptake. For example: the large peak in potential nitrogen availability in November 2003 in the crop treatment was associated with physiological maturity of the crop when N uptake was ceasing, root decay increasing, and limited nitrogen use over the previous year due to drought conditions (data not shown); in contrast, in November 2004 the plants were not yet at physiological maturity and were still taking up nitrogen at the time of sampling. The peaks in potentially available N in the crop treatment in July 2004 is likely to be due to limited plant use of N over the preceding months where plant growth was controlled by spraying. In contrast, the pasture-crop treatment had plant growth at this time.
In soils that are prone to waterlogging, large amounts of nitrogen can be lost through the process of denitrification (Moore et al. 2001). Active plant growth year-round in the pasture-crop system may reduce the risk of denitrification in two ways: through increased water use, reducing the risk of waterlogging; and through increased use of available nitrogen, reducing the amount of nitrate available for denitrification. A reduction in available nitrogen may also reduce the risk of soil acidification through reduced rates of nitrate leaching, particularly in free-draining soils.
Figure 1 here
The separation of C3 and C4 growing periods and the mix of shallow-rooted and deep-rooted plants in the pasture-crop treatment have a number of potential benefits compared to conventional cropping pasture management practices. In comparison with conventional cropping practices, pasture-cropping leads to higher total ground cover year-round and increased total biomass outside of the cropping season. This year-round increase in ground cover and growth is likely to result in reduced wind and water erosion, reduced weed outbreaks and increases in soil organic matter. Compared to the native pasture treatment, the pasture-cropping treatment results in less total ground cover but increased total biomass and litter, suggesting that the benefits of pasture-cropping compared to a native pasture treatment may be via greater productivity and soil organic matter but not greater erosion control. Nevertheless, the increased growth by the pasture-cropping treatment apparently resulted in reduced nitrogen availability and soil water content compared with both the native pasture and conventional crop treatments indicating that pasture-cropping may reduce the likelihood of water-logging, dryland salinity, loss of N through denitrification, and soil acidification developing. In an era when water-logging, dryland salinity, soil acidification and loss of soil carbon are having deleterious impacts on the productivity and sustainability of farming enterprises, pasture-cropping may provide one option for addressing these problems.
This research project was funded and supported by CSIRO Sustainable Ecosystems.
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Figure 1. Effect of three treatments crop, pasture and pasture-crop on: (a) biomass; (b) total cover; (c) soil water (0-60cm); and (d) potentially available N. 95% confidence intervals are presented.