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Paddock change for climate change

JN Tullberg

CTF Solutions and University of Queensland, Gatton

Abstract

"Plants grow best in soft soil, wheels work best on roads" is the self-evident conflict at the heart of Australian agriculture. Tyres on tractors and harvesters each represent about 20% of operating width, so with spraying and logistics, about 50% of paddock area is normally driven over in each zero till crop cycle.

The known, published and demonstrable consequences are:

- We use twice as much it energy as necessary in the processes of crop production.
- We cause major damage to infiltration rates, PAWC, soil health, WUE and yield.
- We restrict cropping opportunities and biomass production.
- N2O and CH4 balances are damaged, and N fertiliser efficiency is reduced.
- We damage productivity, increase GHG production and reduce system resilience.

Precision controlled traffic is the obvious answer, ticking all the boxes, economic and environmental. It opens many opportunities for better agronomy, such as split fertilizer application and relay cropping. This paper provides a brief review of these issues, and expands on the question of GHG impact. The end point is still a question – why is controlled traffic not the standard best practice recommendation?

Introduction.

Three important things happen when heavy equipment wheels drive on soft soil:

  • We use a lot more energy (i.e. tractor power) compacting soil under the wheels.
  • More energy is required to ameliorate compacted soil to its previous condition.
  • Important soil processes don't work as well until it has been fully ameliorated.

Productivity of non-wheeled soil increases as natural amelioration extends through the soil profile, improving soil's capacity to store and supply water and nutrients to plant roots, and improving the volume of soil available for root exploration. Costs are also reduced by systems that avoid the energy wasted in first compacting and then loosening soil. (Tullberg et al. 2007)

Natural amelioration spreads downwards under the influence of plant roots and soil biota, in addition to wetting/drying effects. In shrink/swell soils it occurs very rapidly at the surface, but happens more slowly as it moves downwards through the profile (McHugh 2004). The surface 2 cm could recover in two weeks, but it might take two years to get to 20 cm. Amelioration to 100 cm may take 10 years. This is why some of the benefits of controlling traffic are found immediately, but improvements continue for a number of years.

Unless we deliberately control field traffic, we can't avoid driving over at least 50% of land area per crop. Subsurface damage is not easily visible, but lasts several years. Wheel-induced subsoil degradation is universal in cropped paddocks, which goes a long way to explaining why farmers often don’t notice wheel track effects, at least until they start to controlled traffic.

The requirements for precise control of field traffic are simple in a zero tillage environment.

  • Modular working widths for spraying, planting and harvesting.
  • Common track widths and minimum tyre widths for all heavy axles, and
  • Accurate equipment guidance, with careful field layout.

The common Australian controlled traffic grain production systems use a 9 m or 12 m width module, 3 m track width, and 2 cm precision guidance, reducing the area affected by heavy wheels to <15%, and facilitating opportunity cropping. Setup costs are usually less than $40,000, and these are usually referred to as “CTF“ systems. Similar objectives can be achieved in low-technology environments using permanent bed systems (McHugh et al 2006).

The soil conservation/productivity impact of CTF systems is well known. Runoff is reduced and plant available water capacity increased, supporting better yields and/or greater cropping frequency. Less runoff, filtered through greater volumes of standing, anchored residue will reduce movement of soil, nutrients and agricultural chemicals, reducing erosion and improving water quality (e.g. Tullberg et al. 2007).

This paper focuses on the farming system effects relevant to climate change: fossil fuel energy incorporated in the fuel, herbicides and fertilisers, and on soil emissions and soil carbon.

Process and Results

A simple Excel spreadsheet approach has been used to compare greenhouse gas (GHG) emissions from different farming systems, and the printout of this spreadsheet appears here as figure 1, a series of tables. The first decision in this exercise is to decide what farming systems are reasonably typical of Australian broadacre cropping, and for the purposes of this paper, these have labelled:

  • Stubble Mulch -- minimum tillage, random traffic systems aimed at maintaining at least 30% residue cover, assumed here to involve 3 tillage, and 1 herbicide operation per crop.
  • Zero till -- random traffic, assumed here to involve 4 herbicide operations per crop, with some tillage required every third year on average (= 0.33 tillage operations per year).
  • CTF -- controlled traffic, zero tillage, opportunity cropping (= increased cropping frequency), reducing herbicide operations to 3/crop, and often with in-crop, placed fertiliser application.

Within these general headings, there is great variability depending on soils, geographical location and season. The outcome of any analysis obviously depends on the answers to questions such as "how many operations", or "how much of what fertiliser” or “which herbicide". Some of these options and have been incorporated in the spreadsheet to allow rapid examination of different options.

Energy-Related Inputs.

The assumptions used in the example and presented in these tables (Figue 1) are intended to be reasonably typical of broadacre cropping in eastern Australia, but the spreadsheet can be easily manipulated to illustrate other systems. The first assumptions, on the number of operations involved in the cropping system are set out in Table 1. These have been combined with values of fuel requirements for field operations (Table 2), calculated using data from DPIF (2008) modified by understanding of CTF energy efficiency (Tullberg 2001). The total fuel requirement of all field operations within each system, together with the CO2 released by burning that fuel appear in the right-hand columns of Table 2.

Replacement of tillage by herbicides is often assumed to improve energy efficiency (Wylie 1987), but the energy used in herbicide manufacture (or incorporated in the materials) is considerable. Glyphosate is the most commonly used herbicide, and also the most energy intensive. This information has been reviewed by Zentner (2004), the data source for Table 3. This information is shown, converted into diesel fuel and CO2 equivalents, for a number of common herbicides in the right-hand columns of Table 3. It also includes the author’s estimates of the relative frequency with which each herbicide is used, data needed as the basis for the “mean spray impact” calculation -- the CO2 equivalent of the average herbicide spray operation, set out beneath Table 3.

Nitrogen fertiliser is usually represents the largest single anthropomorphic energy input to cropping and nitrogen efficiency is generally poor (40 – 75%). Inefficient use of nitrogen is often associated with high rainfall and waterlogging, which is probably more frequent when fertiliser is deposited in a narrow slot cut into the compacted soil in (random traffic) systems. There are a number of claims of "greater yield with less fertiliser" in CTF, none of which have been subject to proper evaluation, but long-term CTF trials with minimal N input have demonstrated greater yields (often 10 -- 15%), without any increase in fertiliser input (e.g.Li 2007).

This author is not qualified to argue about nitrogen efficiency, but for the purposes of this paper is assumed that CTF requires approximately 10% less N, than zero tillage or stubble mulch, and this is the basis of the values quoted in table 4. Nitrogen fertiliser production requires approximately 75 MJ/kg ( energy per kg N), but the feedstock involved is almost always gas, which produces only 0.065 kg carbon dioxide per MJ energy. For most practical purposes, we can therefore assume that about 4.9 kg carbon dioxide is produced per kilogram of N fertiliser produced.

All input and energy-related emissions are summarised in the left-hand side of table 7.

Figure 1. Cropping system input and emission tables

Soil Emissions

Nitrous oxide (N2O) and methane (CH4 ) are both significant greenhouse gases having approximately 310 times and 23 times the greenhouse impact of carbon dioxide, so the global warming effect, is usually expressed in terms of carbon dioxide equivalent, “CO2 -e”. Nitrous oxide from agriculture is produced at high levels of water-filled porosity by denitrification of soil nitrates derived largely from fertilisers. Nitrogen loss, whether by denitrification, leaching or runoff, represents an environmental issue and loss of an expensive input, but each of these mechanisms occurs when soil is close to saturation. Relatively dry areas of natural vegetation usually oxidise small amounts of methane, but small amounts are often produced by cropped soils.

Research into nitrous oxide production from soils has shown large, apparently random, small-scale spatial variability (Ball et al. 1997). Many authors have demonstrated the association of emissions with soil compaction, porosity and the connectivity of soil pores (e.g. Ball et al.2008). Soil compaction reduces these, and the rainfall rate required to cause saturation. It will also reduce the rate of diffusion of oxygen into (and carbon dioxide out of) soil, promoting anaerobic conditions which might favour nitrous oxide production. Similar factors influence infiltration rates, where controlled traffic has been observed to reduce the small-scale spatial variability produced by random traffic, and the frequency of waterlogging.

Research into CTF impacts on nitrous oxide and methane production is rare, but recent work in Holland (Vermeulen et al. 2007) has compared emissions of these gases from random traffic and "seasonal" precision CTF (with annual mouldboard ploughing) in an organic vegetable production system. These results, summarised in Table 5, were obtained over three crops in two seasons and demonstrated a large and statistically significant reduction in nitrous oxide emissions from seasonal CTF. Methane was also absorbed by seasonal CTF, while random traffic emitted methane. Consistent and significant improvements were seen in total and air-filled porosity of CTF, and in crop yields.

The implications for Australian dryland production must be seen as speculative when citing results obtained in a European organic system with higher rainfall, annual soil disturbance and nitrogen supplied largely from manures. Improved porosity and pore continuity could be a major factor, which would be consistent with the demonstration of greater nitrous oxide and methane emissions from (random traffic) zero tillage by Aulakh et al. (1984), and with the general association of poor nitrogen efficiency with soil compaction. Zero till CTF, with the greatest porosity and infiltration rates (Li et al, 2001), might be expected to produce the least emissions.

Total emissions from random traffic and seasonal CTF have been estimated from the Dutch results on the basis of 30 and 150 days emission periods respectively for nitrous oxide and methane. A value for zero tillage has been estimated on the basis that—having much poorer porosity -- it is likely to be worse than tilled random traffic by the same amount that CTF is better than random traffic. This is arbitrary, but could well be of the right order. Values are set out in Table 6, which also provides the CO2 equivalent values.

Overall totals are given in Table 7, in which emissions related to inputs, based on reasonable evidence, are separated from the more speculative soil emission estimates. In both cases, controlled traffic zero tillage systems are clearly superior to those of random traffic cropping, zero till or stubble mulch. Stubble mulch appears superior to zero tillage in terms of total emissions. Speculative soil emission estimates are a major component of this total, but a very large change in magnitude would be needed to change this.

In moisture-limited environments, controlled traffic zero till systems can grow more biomass with less soil disturbance, to increase the rate of soil organic matter accumulation, or at least reduce its rate of loss. Water use efficiency and biomass production will be further increased when precision CTF allows relay crop planting (before harvesting the previous crop) to use any excess soil moisture for biomass production, prior to harvest. Cover crops which provide weed suppression could make good economic sense in this situation.

Improves environmental outcomes can be expected as cropping systems mimic processes of natural vegetation more closely. Precision controlled traffic with zero tillage is a significant step in this direction.

Conclusions

  • Emissions from inputs (fuel, herbicides and fertiliser) are substantially less in controlled traffic zero till systems, compared with (random traffic) zero tillage or stubble mulch systems.
  • Evidence on soil emissions indicates that these should also be much less from controlled traffic systems, but emissions from (random traffic) zero till should be greater than those from stubble mulch tillage.
  • Controlled traffic zero till increases productivity, reduces costs and improves all measures of environmental impact. It is difficult to understand why it is so rarely recommended by agronomists who sit in the mainstream of applied agricultural science.

References

Aulakh M. S. Rennie D. A. and Paul E. A. (1984) Gaseous Nitrogen Losses from Soils Under Zero-Till as Compared with Conventional-Till Management Systems J Environ Qual 13:130-136
Ball, B.C. Horgan, G.W. Clayton, H. Parker, J.P., 1997. Spatial variability of nitrous oxide fluxes and controlling soil and topographic properties. J. Environ. Qual. 26, 1399–1409.

Ball B.C., Crichton I. and Horgan G.W. (2008). Dynamics of upward and downward N2O and CO2 fluxes in ploughed or no-tilled soils in relation to water-filled pore space, compaction and crop presence. Soil and Tillage Research. (in press)

DPIF (2008) Selection and matching of tractors and implements. http://www2.dpi.qld.gov.au/thematiclists/9155.html

Li Y.X, Tullberg J.N, Freebairn D.M. (2001) Traffic and residue cover effects on infiltration. Australian Journal of Soil Research, 39 p239

Li Y.X, Tullberg J.N, Freebairn D.M. (2007) Wheel traffic and tillage effects on runoff and crop yield Soil & Tillage Research 97 282–292

McHugh, A, Tullberg, J.N. and Freebairn, D. (2004) The evolution of soil structural repair under zero till, controlled traffic, permanent bed farming. CIGR International Conference, China Agricultural University, Beijing, China. 11-14 October 2004.

McHugh, A.D. Li Hongwen, Zhang Liqin, E Shengzhe, Ma Zhongming, and Cao Xinhui (2006) Controlled traffic farming takes Conservation Agriculture into China. In Proceedings of China-Canada Conservation Agriculture Forum. September 21-23 Beijing, China. Pp: 75.

Tullberg J.N. (2000) Traffic Effects on Tillage Energy. Journal of Agricultural Engineering Research 75(4).375-382.

Tullberg J.N., Yule D.F. and McGarry D. (2007) Controlled traffic farming— From research to adoption in Australia. Soil & Tillage Research 97 272–281

Vermeulen, G.D., Mosquera, J., Wel, C. van der, Klooster, A. van der, Steenhuizen, J.W., (2007) Potential of controlled traffic farming with automatic guidance on an organic farm in the Netherlands. In: Stafford, J.V. (Ed.), Precision agriculture ’07. Papers presented at the 6th European Conference on Precision Agriculture, 3-6 June 2007, Skiathos, Greece, pp. 473-481.

Wylie, P. 1987. Conservation tillage for profit. Owen Art and Publishing. Pty Ltd: Brisbane, Qld.

Zentner,R.P., Lafond, G.P.,Derksen, D.A.,Nagy,C.N., Wall,D.D., May, W.E. (2004) Effects of Tillage Method and Crop Rotation on Non-Renewable Energy Use Efficiency in the Canadian Prairies. Soil and Tillage Research 77; 125 – 136.

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