Take home messages

• A wetter than average year across much of the Eyre Peninsula presented challenges and opportunities for growers
• Yields were substantially higher for many crops, requiring higher than typical nitrogen input and likely leaving a substantial stubble load
• Conversely, waterlogging-sensitive crops such as lentils were negatively affected
• With large swings in productivity, it is important to monitor N application and offtake in grain to minimise likelihood of ‘mining’ soil organic matter

Introduction to the season

The past season or so on the northern Eyre Peninsula has seen some exceptional yields as a result of full soil moisture profiles and above average in-season rainfall. Against a background of long-term yields in the 1.5-2t/ha range, crops around Minnipa yielded over 3.5t/ha for the 2022-3 season. Yields on the lower Eyre Peninsula of around 5t/ha again exceeded long-term averages of around 3.5t/ha. With clear expectations of higher than average rainfall, and likely high grain prices expected as a result of disrupted international markets due to the conflict in Ukraine, growers in the region did typically fertilise to the expected season, despite high fertiliser prices. Paddocks in the medic pasture phase of rotation likely performed well.

Notable exceptions to high yields were areas affected by the early 2022 floods that either remained inundated and untrafficable at sowing times or suffered from gully erosion. Here, areas of paddocks may not have been sown. In areas that were trafficable but received high in-season rainfall on top of good sub-soil moisture, crops sensitive to waterlogging such as lentils performed poorly with yield losses of around 20%.

Soil organic matter – what it is

The largest terrestrial stocks of both carbon (C) and nitrogen (N) are in the form of soil organic matter (SOM), and this includes soils under agricultural management. Between 1200—1550 gigatonnes (Gt) of C are stored in soils as soil organic carbon (SOC) globally, with estimates of 22.6–39.7Gt in the top 30cm of Australian soils (Viscarra Rossel et al. 2014). This equates to 1.92–3.36Gt N stored in the SOM of the top 30cm of all of Australia’s soils assuming a C:N ratio of 11.8 — an average of just over 4t N/ha. Most N is bound within SOM as organic N and is not immediately available. As plants can only take up mineral N and a small proportion of dissolved organic N (DON), SOM must be decomposed to release these compounds. A snapshot study found, on average, only 0.59—4.80% of total N was present in plant available forms in Australian agricultural systems (Farrell et al. 2016).

Soil organic matter and thus SOC is not homogeneous. Rather, it is a complex mixture of compounds of varying composition, stability, and protection. A classical viewpoint of stable (humic) and less stable (fulvic) substances that placed greater emphasis on the chemical stability of SOC fractions has largely been superseded by acknowledgement that much SOC is not particularly chemically stable. Instead, it is protected from decomposition by its association with clays and incorporation into aggregates (Lehmann & Kleber 2015). Projects such as the National Soil Carbon Programme (Baldock et al. 2013) and the current Soil Organic Carbon Monitoring (SOC-M) project seek to understand how the amount and composition of SOC changes as a result of management, soil type, climate, and time. The immediate management implication here is that most SOC is not necessarily highly resistant to loss due to its chemical composition, practices that protect the soil from unnecessary disturbance limit the opportunity for protection to be removed and decomposition by microorganisms to be accelerated.

Soil organic matter – where it comes from, and where it goes

The amount of SOC in any given systems is a function of its inputs and losses (Angers et al. 2022). Whilst external sources of C in the form of organic amendments such as poultry litter or biosolids may be locally available, this is not the case across a large proportion of land under cropping. In Australian broadacre systems, the only source of C inputs is from the photosynthesis of the plants grown in situ. This includes cash crops themselves (but not C removed in grain), grasses and/or legumes in a ley phase (noting export of C as feed), and cover crops which are then green- or brown-manured.

Typically, continuous cropping systems with a summer fallow input less C via photosynthesis of the annual grain crop than that of those in mixed systems with substantial ley phases. This is due to a number of factors:

• Grain crops are typically bred to maximise the conversion of resources (light, water, nutrients) to grain that is harvested and exported from the system.
• The amount of C partitioned below ground is typically much smaller in annual crops than perennial pasture species.
• Even over dry hot summers, pasture species may maintain a low level of C input though photosynthesis.

Thus, and the only feasible means of increasing inputs of C to soils in broadacre systems is to increase the amount of C inputs from plants, net primary productivity (NPP, the balance between plant inputs via photosynthesis and losses via respiration; Minasny et al. 2022)

As discussed in more detail in Macdonald et al. (2020) and Farrell et al. (2021), it is possible to estimate the inputs of plant C to the soil on the basis of observed crop yield. This can be done using several literature-derived figures for the key aspects of harvest index, C allocation within a crop, and retention factors for C additions to soil (Figure 1).

It is important to note that many of the factors used in the derivation of Figure 1 would be expected to vary in a non-linear manner with yield, and thus increases in yield in the higher ranges may not result in the same proportion of photosynthetically-fixed C being translocated to the non-grain pools. Further, retention factors of C in the soil are likely to be very soil-type dependent, and it is unlikely that such figures would apply equally across different soil classes and textures, with higher retention likely in heavier clay soils. Lastly, these figures do not consider losses of existing C, either through priming (Chowdhury et al. 2014) or as a result of disturbance in more energy-intensive amelioration activities, such as the deep ripping reported in Macdonald et al. (2021).

SOC is lost through a number of mechanisms. Firstly, erosion via wind or water can physically remove soil and its associated C from the paddock. Whilst the widespread adoption of no-till systems with stubble retention minimises these losses, excessive soil disturbance can result in greater erosion. High intensity rainfall events and resultant deep erosion can be harder to mitigate against, but stubble retention and intact plant roots can go some way towards reducing the risk.

Beyond erosion, SOC is also naturally lost through microbial respiration (Angers et al. 2022) where organic matter is turned over by microorganisms for energy and mined for nutrients such as nitrogen (N) and phosphorus (P). Microbial respiration can often considerably offset greater apparent C inputs from improved productivity due to warm and moist conditions also favouring microbial activity. Indeed, it is the same conditions that are known to increase N availability via mineralisation that also drive C loss via respiration of C from the same SOM. It is also important to understand that whilst slowed in dry hot conditions experienced over typical summer fallow period, microbial respiration still occurs, and is no longer being compensated for by plant inputs.

Managing nutrition and disease to maximise inputs in high rainfall years

Having established the link between plant productivity and inputs of C, we can consider which aspects of management require special consideration in highly productive seasons and the years that follow them. Limiting constraints to production is the key goal here, specifically biotic constraints such as soilborne disease impacts and nutrient mineralization, and this applies as much to crop management and choice as it does to management of the soil.

An important factor to monitor is N availability through deep N testing pre-season, and ideally through a combination of grain yield and protein monitoring on the header at harvest. Coupled with knowledge of how much pre- and in-season N fertiliser was applied, this N balance also provides an indication of likelihood of in-season SOC losses. Where more N is being taken off in the grain than was applied as fertiliser, the main source of this ‘extra’ N is SOM mineralisation, which necessarily means loss of SOC as CO2. In soils without other physical or chemical constraints, N availability is typically seen as the main limiting factor for grain yield, so a negative N balance is likely also indicative of missed yield potential (Lawes et al. 2021). However, substantial losses of N from fertiliser can occur through volatilisation and denitrification, particularly in warm and wet conditions and on calcareous soils. As the rates of N required increase in warm/wet years to meet increase crop demand, particular care is needed to limit these losses following the 4Rs principles (IPNI 2012).

Following high rainfall and high production years, substantial stubble loads may exist that could cause practical problems with sowing, interfere with seedling establishment, or harbour pests and disease. Because of the important role that stubbles play in protecting the soil surface from erosion, as well as providing a source of food, energy, and nutrients to soil micro and macro biota, it is highly preferable to retain stubble for as long as possible. In some circumstances, light incorporation may be appropriate, though this may temporarily increase N tie-up early in the crop season as the stubble decomposes and increases microbial N demand. Whilst this would be a transient effect, longer term benefits should accrue from providing a greater opportunity for more SOM to form from the decomposing and incorporated stubble.

Relative to higher yields in pasture legumes and cereals in the past year, grain legumes that struggle with wet or waterlogged conditions such as lentils were negatively affected. Though there are many factors to consider when planning crop rotations, if excessively we conditions are predicted for crops known to be sensitive to high soil moisture, a short season cover crop may provide an option to both add organic matter and bring down moisture levels. Whilst such a recommendation may appear counterintuitive, there are occasions when such an approach may be beneficial, as seen at some sites in the recent National Landcare Programme (NLP) Smart Farms project on mixed species cover crops (Farrell & Stanley 2023). Cover crops have been shown to contribute a number of benefits in wetter northern hemisphere systems, and the project demonstrated benefits in some situations.

Conclusions

Whilst overapplication of fertiliser can be economically inefficient and environmentally deleterious, underfertilisation runs down SOM and thus also the N it supplies through mineralisation. The wetter than average season on the Eyre Peninsula presented both opportunities and challenges for growers. Going forward, it will be important to monitor N levels in soil and grain to ensure that N mining is not taking place and any deficits are corrected.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC and I would like to thank them for their continued support. I also acknowledge valuable input from Brett Masters to provide local context.

References

Angers D, Arrouays D, Cardinael R, Chenu C, Corbeels M, Demenois J, Farrell M, Martin M, Minasny B, Recous S, Six J (2022) A well-established fact: Rapid mineralization of organic inputs is an important factor for soil carbon sequestration. European Journal of Soil Science. 73, e13242.

Baldock JA, Sanderman J, Macdonald LM, Puccini A, Hawke B, Szarvas S, McGowan J (2013) Quantifying the allocation of soil organic carbon to biologically significant fractions. Soil Research 51(8), 561-576.

Chowdhury S, Farrell M, Bolan N (2014) Priming of soil organic carbon by malic acid addition is differentially affected by nutrient availability. Soil Biology and Biochemistry 77, 158–169. doi:10.1016/j.soilbio.2014.06.027

Farrell M, Allen DE, Macdonald BCT (2016) Pools and fluxes: a snapshot of nitrogen dynamics in Australian soils. Proceedings International Nitrogen Initiative Conference, Melbourne, December 2016, pp. 1–4.

Farrell M, Gupta V, Macdonald LM (2021) Addressing the rundown of nitrogen and soil organic carbon. Proceedings GRDC Grains Research Update, Bendigo, February 2021, pp. 51-60.

Farrell M, Stanley M (2021) ‘Mixed cover crops for sustainable farming’

International Plant Nutrition Institute (IPNI) (2012) ‘4R plant nutrition manual: a manual for improving the management of plant nutrition, metric version.’ (Eds TW Bruulsema, PE Fixen, GD Sulewski). (IPNI: Norcross, GA)

Lawes R, Chen C, Whish J, Meier E, Ouzman J, Gobbett D, Vadakattu G, Ota N, van Rees H (2021) Applying more nitrogen is not always sufficient to address dryland wheat yield gaps in Australia. Field Crops Research 262, 108033.

Lehmann J, Kleber M (2015) The contentious nature of soil organic matter. Nature 528, 60-68

Macdonald LM, Farrell M, Baldock JA (2020) Soil organic matter – what the science tells us. Proceedings GRDC Grains Research Update, Barellan, March 2020, pp. 28–35.

Macdonald LM, Wilhelm N, Fraser M, Moodie M, Trengove S, Desbiolles J, Saunders C, Whitworth R, da Silva R, Llewellyn R, McBeath T (2021) Targeted amelioration in Mallee sands to maximise crop water use. Proceedings GRDC Grains Research Update, Adelaide, February 2021, pp. 37-42.

Minasny B, Arrouays D, Cardinael R, Chabbi A, Farrell M, Henry B, Koutika L-S, Ladha JK, McBratney A, Padarian J, Dobarco MR, Rumpel C, Smith P, Soussana J-F (2022) Current NPP cannot predict future soil organic carbon sequestration potential. Comment on “Photosynthetic limits on carbon sequestration in croplands”. Geoderma424, 115975.

Sanderman J, Farquharson R, Baldock JA (2010) Soil carbon sequestration potential: a review for Australian agriculture. CSIRO Sustainable Agriculture Flagship, Adelaide, Australia.

Viscarra Rossel RA, Webster R, Bui EN, Baldock JA (2014) Baseline map of organic carbon in Australian soil to support national carbon accounting and monitoring under climate change. Global Change Biology 20(9), 2953–2970. doi:10.1111/gcb.12569

Contact details

Dr Mark Farrell
CSIRO Agriculture & Food, Waite Campus, Urrbrae, SA 5064
08 8303 8664
Mark.farrell@csiro.au
@inverted_soil