Addressing the rundown of nitrogen and soil organic carbon

Take home messages

• It is well known that stocks of soil organic carbon have declined in many Australian agricultural systems, including dryland grains production.
• This loss of carbon (C) has also resulted in a significant reduction in soil nutrient stocks, particularly nitrogen (N), that supply a significant proportion of a crop’s nutrition.
• Rebuilding soil C and N stocks is slow, and depends upon increased crop production to drive C inputs, supported by fertiliser requirements being met.
• Overcoming soil constraints, inclusion of cover crops and legume rotations, and a recognition of N budgeting requirements over multiple seasons should turn a system towards rebuilding C and N stocks.

Introduction

Soil organic matter (SOM) contains the largest stocks of both C and N in all terrestrial ecosystems, including those under agricultural management. Globally, 1200-1550 gigatonne (Gt) C is stored in soils, with estimates of 22.6 – 39.7 Gt in the top 30cm of Australian soils (Viscarra Rossel et al. 2014). Assuming a C:N ratio of 11.8 (Kirkby et al. 2011), this equates to 1.92 – 3.36 Gt N stored in the SOM of the top 30cm of all of Australia’s soils, an average of just over 4t N/ha. Soil organic matter is responsible for the provision of multiple ecosystem services important for agricultural production, including the provision of nutrients (particularly N), maintaining a diverse and heathy microbial community, infiltration and water retention, amongst others. Though often thought of as a single entity, it can be functionally broken down into discrete fractions that behave differently in terms of the agroecosystem services they provide, their vulnerability to loss, and their potential rates of accumulation.

The distinction between SOM and soil organic C (SOC) is that the latter refers solely to the C component of SOM. In modern analytical laboratories, SOC is commonly reported back to advisers and growers via a direct analysis by Leco® or equivalent instrument, rather than older outmoded techniques of ashing. Drawing this distinction allows clarity by separating the C from N and other nutrients contained within SOM. It is also important to differentiate between values reported as C or N content (usually as a percentage, or grams per kilogram) and those reported in a stock e.g., tons of C per hectare. Stock measurements implicitly factor in soil bulk density and gravel content, and are usually expressed to a defined soil depth.

In addition to the SOC itself and its fractions, two smaller but important pools of C exist in soil: microbial biomass C (MBC), which typically contains approximately 1% of the total C in a soil and represents the C stored in live microorganisms, and dissolved organic C (DOC) which is the soluble fraction (Gupta et al. 2019). This latter pool contains most of the C directly accessible by microorganisms for energy, but also more recalcitrant compounds that while soluble do not directly reflect availability of C. Though representing only a small percentage of the standing pool of C in the soil, the flux through these pools is high i.e., they turn over quickly, and thus, the flow of C through them belies their importance when measured as a stock. In many ways, this could be thought of in terms of a bath: If you were to run the tap with the plug out and measure the amount of water in the bath, you would find very little there at any point in time. However, you would be wrong to conclude that water is not important for the function of a bath. This same concept of ‘flux’ versus the ‘pool’ needs to be considered for N also.

Conversion of native ecosystems to managed agricultural production often coincides with a decline in SOC content and stock. Data from 20 different studies in Australian agricultural soils show that cultivation of the 0-10 cm layer had reduced C stocks to roughly 50% of those in their native condition, with similar but more varied results found when depths down to 30cm were considered (Luo et al. 2010). In Australian broadacre agriculture, no-till (NT) and stubble retention have been almost universally adopted over the past 20-30 years, with the general presumption that amongst other agronomic benefits, SOC stocks also increase. Contrasting findings in the literature, even between global meta-analyses (Kopittke et al. 2017; Powlson et al. 2014) suggest this outcome to be variable, and likely climate- and soil- specific (Ogle et al. 2019). Sanderman et al. (2010) reported improved cropping practices in Australia has the potential to increase SOC stocks by 0-2-0.3 t/ha/yr, though many of the improvements within that definition (e.g., NT, enhanced rotations, stubble retention) are now well established as best practice.

Recognising the intrinsic links between SOC and N, their loss in agricultural soils, and the opportunity to potentially replenish stocks, the aim of this paper is to build an understanding of how SOC and N availability are tied together, and the importance of seeking approaches that may address their decline. It provides a summary of the current state-of-the-art of knowledge on the mechanistic underpinnings of organic matter dynamics in soils, and uses this to construct actionable management possibilities to achieve these outcomes.

Linkages between the soil carbon and nitrogen cycles

Organic matter – form and function

To better understand how C accumulates, is lost and behaves in soil, a fractionation procedure has been developed to separate measurable fractions of discrete chemistry and functionality. This separates SOC into three fractions (Figure 1 and to follow):

• Particulate organic C (POC): The least decomposed fraction that is accumulated rapidly but also most vulnerable to loss, with an estimated turnover time of approximately 10 years. When considered beyond just its C component as SOM, this typically has a C:N ratio in the range of 20-40:1, still mostly resembles the crop biomass inputs from which it is derived, and is more responsible for the supply of energy to soil microorganisms than nutrients to crops.
• Humus-like organic C (HOC): Stabilised organic carbon that has undergone degradation and is often protected from loss due to binding to the soil mineral phase and protection within microaggregates. It has a decadal turnover time (up to approximately 100 years). This typically has a C:N ratio in the range of 8-14:1 as SOM, and is the likely source of most plant nutrients.
• Resistant organic C (ROC): This is a charcoal-like substance, typically with a very high C:N ratio >100:1 as SOM and a residence time of millennia. It occurs primarily because of the deposition of charcoal either from fires on the land (including from initial clearing) or deposition of soot. The soil amendment biochar also falls into this category. While not directly responsible for the delivery of nutrients, it influences the retention and exchange of cations including potassium and ammonium.



Figure 1. Electron micrographs of the three SOC fractions (courtesy of Jeff Baldock, CSIRO), and microbial biomass (courtesy of V. Gupta, CSIRO). Particulate carbon (POC) shows recognisable fragments of undecomposed plant material, humus carbon (HOC) is a waxy substance bound to mineral particles, and resistant carbon (ROC) shows a clear crystalline structure. The bacterial cells and fungal hyphae are clearly visible against the soil aggregates in the two lower pictures.

Inputs and retention of C in dryland soils

In Australian broadacre cropping systems, there are typically only two sources of C input to soil: the C fixed by plants in the paddock through photosynthesis, and the C contained in organic amendments such as composts and manures that may be applied. Though encouraged, if available at a reasonable price close to a source, the import of organic matter is not a viable option in many locations. Thus, the focus of this section is primarily on inputs from the crop or pasture plants grown in situ.

Carbon is fixed by plants through the uptake of carbon dioxide from the atmosphere and its subsequent conversion via glucose to compounds useful to the plant. As discussed in more detail in Macdonald et al. (2020), it is possible to estimate the inputs of plant C to the soil on the basis of observed crop yield. Further, using this estimation based upon data derived from the scientific literature, it is also possible to contextualise expectations for SOC increase in the context of likely required yield increase. This can be done on the basis of several literature-derived figures for the key aspects of C allocation within a crop, and retention factors for C additions to soil (Table 1).

Table 1. Calculation factors and their literature values used to estimate theoretical SOC stock change potential on the basis of crop inputs. Values are based upon literature (references in Macdonald et al. 2020) and can be updated with local expert knowledge for a given management scenario.

Calculation Factor	Value
Harvest index for cereal (HI)	0.37
Root:shoot ratio for cereal (RS)	0.50
Carbon content of cereal biomass (CC)	0.44
Retention factor of biomass (RFb)	0.30
Rhizodeposition ratio (proportion of root biomass, RR)	0.50
Retention factor of rhizodeposition (RFr)	0.57

It is important to note that these values are averages or in some cases estimates. The drive towards greater yielding varieties with increased harvest index (HI) often means that the proportion of photosynthetically fixed C in a plant which remains after harvest decreases as more resources are allocated by the plant to grain. Root:shoot ratios are less frequently measured, and the amount of C exuded via rhizodeposits (and thus not directly measured in studies quantifying root biomass) is even more poorly understood in Australian grain growing systems. The impacts of soil type, management and environmental factors on the retention of C either from above- and below-ground biomass, or rhizodeposits is also not well parameterised. Finally, any increased losses through priming processes whereby loss of SOC is accelerated by increased microbial activity is not considered (Chowdhury et al. 2014; Fang et al. 2020). Nonetheless, these numbers can be used to calculate possible increased in C inputs as a result of increased grain yield, assuming factors such as HI are unchanged.

First calculated is the likely change in above-ground biomass C ( ; Eqn 1), below-ground biomass C ( ; Eqn 2) and rhizodeposited C ( ; Eqn 3) which would accompany the change in yield.



[Equation 1]



[Equation 2]



[Equation 3]

Having calculated the changes in the plant three C components as a result of a change in yield, the retention factors RF^b and RF^r are applied to obtain an estimate in the change in SOC ( ):



[Equation 4]

The estimates calculated from Eqn 4 suggest for a 0.5t/ha increase in grain yield, an increase of 0.45t/ha SOC may be observed in the soil profile. This would be a 20% yield increase based on this year’s forecast average grain yield of 2.5t/ha across the Southern Region, according to ABARES’ current estimates. Assuming a bulk density of 1.3g/cm³ and a C content of 1.5% in the 0-30 cm layer to give a standing SOC stock estimate of 60t C/ha, this would be a change of less than 1% of that already in the soil i.e. a change of ~0.01% from 1.50% to 1.51% in the measured value. Such a change would be very difficult to quantify with any certainty, and thus measurements of SOC would need to be taken at intervals over a 5-10 year period to be certain that such an increase in inputs was indeed resulting in an increase in SOC stocks.

It is important to note that many of the factors listed (e.g., R:S, HI and RR) 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 as per the factors in Table 1. Further, retention factors are likely to be very soil-type dependent, and it is unlikely that such figures would apply equally across different soil classes and textures. 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) and shown in Figure 2.



Figure 2. Relationship between changes in grain yield ( ; 1^st x-axis) or aboveground biomass ( ; 2^nd x-axis) and changes in SOC ( ; y-axis) calculated using Eqns 1-4 and the literature derived estimates from Table 1. Values are estimates and per year. The greyed area shows the range for the published studies summarised by Sanderman et al. (2010) where ‘improved management practices’ resulted in an increase of up to 0.3t SOC/yr, a probable yield increase of approximately 0.4t/ha grain would be required. The grey dashed lines show the estimated change in SOC as a result of the average greatest yield increase realised as a result of deep ripping and associated activities in the current GRDC ‘Sandy Soils’ project (Macdonald et al., 2021).

Soil nitrogen cycling – processes, inputs and losses

In soil, most N is not immediately available, and is bound within SOM as organic N. 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).

The main input of N in broadacre cropping comes either from fertiliser or the inclusion of legumes in a rotation sequence, although a small proportion may also arrive through atmospheric deposition and fixation by free-living microbes in the soil. It is generally perceived that the efficiency of fertiliser N in Australian grains systems is low, with 40-50% of the N applied to a crop being recovered in that crop within the same season (Angus and Grace 2017). That does not however mean that the remaining N is lost from the system. In Australian dryland cropping systems losses of fertiliser N through leaching are low, especially in low rainfall zones. Further, while gaseous emissions of N2 and NH3 are less well quantified and may contribute significant losses of N in some situations (Harris et al. 2016), N2O emissions are amongst the lowest of any managed agricultural system.

Even in more heavily fertilised irrigated cotton systems of New South Wales and Queensland, the majority of N taken up by a crop is accessed via soil processes; primarily SOM mineralisation (Macdonald et al. 2017), and thus, the acknowledgement that efficiency of fertiliser N use sits at approximately 50% in a given season obscures the use of N supplied in previous systems.

The ‘elephant in the room’ when it comes to N export or loss from farming systems is actually the amount that is removed in the produce itself, or by livestock during grazing in mixed systems. As reported at previous GRDC Updates events, a comprehensive study by Norton (2016) found that the majority of properties studied were net exporters of N from the crop alone. If other losses (particularly N2 from denirtifcation, which is the least quantified) are also present in the system, this could contribute to significant N mining in the medium to longer term, with concomitant impacts on SOC stocks (Baldock et al. 2018).

Management to build carbon and nitrogen stocks

For both C and N, the same principle applies; the stock in the soil is a function of inputs and outputs. While the inputs of N are perhaps somewhat simpler to conceptualise and manage, it is primarily plant growth that results in C inputs to soil, and there are several means of manipulating this to improve the likelihood of increasing C stocks. And, as C stocks increase over the longer term, it is likely that the ability of the soil to supply N through the mineralisation of SOM will also increase, provided that the N balance remains positive.

Addressing soil constraints to increase soil organic matter inputs

The principles of conservation agriculture typically align with the goals of retaining and building SOM. These include minimising disturbance, maximising crop diversity/rotation, minimising fallow/bare ground, and integrating livestock where possible. However, these practices will only favour accrual of SOM where they increase inputs and/or decrease loss pathways.

Despite gains in productivity from broad adoption of NT and early sowing, there remains a yield gap between paddock production and the water limited yield potential. In many Australian cropping systems crop water use is limited by a range of surface and subsurface constraints which limit root growth and exploration. Common constraints include compaction, soil acidity and associated toxicities (aluminium, magnesium), alkalinity, sodicity and associated toxicity (boron, chloride, salt), and water repellence. While conservation practices and crop selection are useful tools in mitigating the impact of these constraints, they will not correct the physico-chemical condition of the soil.

Amelioration practices aim to overcome soil constraints for long-term improvement to crop growth and productivity. Under these scenarios, crop productivity and biomass production can be significantly increased, and will have subsequent impact on C and N flows through the soil profile. Examples include current research targeting subsoil acidity (Fleming et al. 2020), and deep ripping combined with the addition of organic amendments resulting in yield gains in some situations between 0.4 – 2 t/ha (Macdonald et al. 2021; Trengrove and Sherriff 2018). Research is ongoing to assess the longer-term benefits of these approaches to manage soil constraints, including developing a clearer understanding of scenarios in which they can be relied upon to deliver clear cumulative yield increases.

Nitrogen balance, excess and the ‘Nitrogen-bank’

Typical N fertiliser decisions focus either on rules of thumb for a district, or predictions of yield, and thus, likely N demand on the basis of model predictions e.g., Yield Prophet®. By and large, such predictions target maximum profit over yield. Importantly, they typically focus on returns within a single season. As Norton (2016) has shown, the net N balance of such practice is usually negative, meaning that N is being ‘mined’ from the SOM at a greater rate than it is returned, resulting in a concomitant loss of C. Thus, yields are effectively being ‘subsidised’ by SOM loss resulting in medium-long term reduction in the ability of soil to supply N through in-season mineralisation. This reduces the soil’s fertility in the longer term, and as mineralisation tends to release N at a rate closely matching the crop’s pattern of N demand, it is unlikely that extra fertiliser can simply offset lost N mineralisation potential in the longer term. Further, the main factor driving the yield gap in Australian grains systems is N limitation (Hochman and Horan 2018), and in wet and favourable seasons this conservative approach to N management may impact profit in the short-term.

Instead, growers and advisers could consider the N requirement of the system as a whole by 1) considering nutrient balance in the medium-long term i.e., N balance over a 5-10 year period, not just the season ahead; and 2) considering the need to ‘feed the soil’ via immobilisation of nutrients, as much as the crop itself. This second point explicitly accounts for the fertiliser N required to build SOM which is sometimes seen as a negative cost of building C stocks (Richardson et al. 2014), but allows for the replenishment of the store of N that is released slowly through mineralisation.

An emerging approach to slow and potentially reverse declines in SOC and N stocks is known as the ‘Nitrogen Bank’ strategy (Meier et al. 2021). Recognising that losses of N from dryland grains systems are often low, and thus, economic and environmental risks are minimal (Smith et al. 2019), we suggest that applying greater rates of N will increase profitability through addressing the main constraint to yield and reducing SOC run-down. A major limitation in calculating a crop’s N requirement is the ability to forecast rainfall and water-limited yield potential early in the season. A simple solution to this uncertainty is proposed whereby fertiliser application is calculated as the balance of crop N demand required to achieve the economic yield after subtraction of the available N stock at sowing, ignoring in-season mineralisation. If it is a dry season, excess N will mostly remain in situ and be captured in the next season’s pre-sowing N testing, and fertiliser application rates adjusted accordingly. This approach effectively removes much reliance on SOM to deliver N through in-season mineralisation, and SOM that is mineralised is likely replaced through greater plant C inputs and the higher N availability resulting from the increased fertilisation rates. It should be noted that whilst showing early promise with minimal fertiliser N losses in the drier systems that dominate the Southern Region (Smith et al. 2019), substantial losses through denitrification of larger up-front N additions have been documented in ex-pasture systems in the high rainfall zone (e.g., up to ~90% applied N, Harris et al. 2016). Further research is required to better understand the climatic and soil boundaries at which higher up-front N applications can be applied with minimal loss.

Legumes and nitrogen fixation

One of the key benefits of grain and pasture legumes in crop rotation is the N contribution through legume-rhizobia symbiosis that provides the legume N requirements and as an important contributor of N supply to subsequent crops. The effect of recent intensification of Australian cropping systems and the consideration of grain legumes as rotational crops has the potential to reduce N inputs and increase N use efficiency in following crops and improve overall soil quality. It is generally accepted that for many legume species, on average 20kg of shoot-N per tonne of dry matter is fixed by grain legumes, although the actual amount of N fixed can vary 15 to 25 kg N fixed per tonne depending on the legume type, field conditions including management practices applied and seasonal conditions (Peoples et al. 2009).

However, it should be remembered that N fixation provides the majority of the N demand of the grain legume crop itself, and a large part of the fixed N is exported in the grain. Hence, its contribution to the overall soil N supply may be limited. Despite this, grain legume crop residues generally have higher concentrations of N and lower C:N ratios than those of cereals. For example, with a harvest index of 34%, grain legume crops (pulses) add above-ground residues ranging from 1.4 to 10 t per ha which provides 1 to 3.9 t of C/ha for use by soil biota. As the quality and N content of pulse crop residues defines the amount of N added to the system, it influences the N mineralisation and tie-up (immobilisation) processes.

Despite the increased preference for cropping in recent years, pastures remain a dominant part of southern farming systems which can play a key role in sustaining and improving SOM and fertility. Nitrogen fixation from pastures provide an important component of the N supply to subsequent cereal crops, which are further complemented by the C inputs from above and below ground plant components (Peoples et al. 2012; Sanderman et al. 2014).

The amount of N fixed by various annual and perennial legumes in Australia can vary from <10 to >250 kg N/ha/year. Additionally, the below-ground pool of N in roots and nodules provides a significant source of N inputs e.g., 40-55% of total plant N estimated to be present below ground in pasture systems (Peoples et al. 2017). It is suggested that various factors including poor effectiveness of native rhizobial strains, little or no fresh rhizobial inoculation even with in sown pastures, nutrient disorders and soilborne diseases can reduce nodulation and contribute to the less-than-ideal contribution from N fixation by pasture legumes reported in recent decades (Peoples et al. 2012).

There is an opportunity to improve our understanding of the constraints to N fixation. Implementation of management strategies that can improve legume productivity and N fixation can not only arrest the decline in the N supply capacity of soils but also contribute to the improvement of overall SOM quantity and quality (Angus and Peoples, 2012; Sanderman et al. 2017). Given that the formation of new SOM is not only contingent on there being sufficient C and nutrients, but also the need for them to be co-located near clay minerals and in conditions suitable for microbial growth, conversion of legume root biomass to more stabilised SOM is likely to be more efficient than other plant inputs supplemented with nutrients supplied by fertiliser.

Break and cover crops

A final potential strategy to build C stocks, improve soil resilience and address N decline is the implementation of break or cover-cropping, either as green manure or to provide supplemental stock feed. Winter cover crops may be grown in lieu of a cash crop as part of a rotation sequence, or they may be established opportunistically during the summer fallow. With regards to managing the soil, their aim is to reduce erosion by maintaining a ground cover, increase C inputs and microbial activity, and potentially address nutrient stratification or subsoil constraints through deep roots.

A recent study in Europe found that length of vegetation cover was more important for grain yields and soil function than diversity within a rotation (Garland et al. 2021). If sown as a species mixture, the combination of species can be tailored to occupy multiple niches so that biomass production ‘overyields’ i.e., produces more biomass than that from an equivalent monoculture.

Cover cropping is an increasingly adopted strategy overseas, particularly in the USA. However, in Australia’s much drier climate, substantial questions remain as to whether any benefits derived offset potential loss of water through evapotranspiration of the cover crop, particularly in summer applications where the prevailing guidance is to manage weeds to maximise soil water retention. Current research led by Agex, SANTFA and CSIRO (Farrell and Stanley, 2021) is exploring these issues across 20 sites in the southern region, and is due to report in 2022.

Looking to the future

A growing body of evidence that suggests fertiliser strategies designed to maximise profit or offset financial risk in the short term do not meet the N demand of the system, and thus, invoke N-mining and resultant SOC loss. To arrest and reverse the loss of C and concomitant draw-down of N reserves in soils, the simple equation is that inputs need to be greater than exports and losses. There are several ‘levers that can be pulled’ on both sides of this equation, but it is important to understand that for the most part, the soil C and N cycles are intrinsically linked, as most N is bound in SOM, and the effectiveness of management efforts will be strongly influenced by climate and soil type (Hunt et al. 2020). Approaches that increase N inputs will both reduce N-mining and increase C inputs through greater plant productivity.

Recognising the monetary value of the N tied up in SOM (and indeed exploited through N-mining) suggests that a longer-term approach to N fertilisation strategies and legume rotations which result in a net import of N are required. Coupled with strategies that increase plant C inputs either through the alleviation of soil constraints or where appropriate, increased plant growth and time of soil cover through cover cropping, it is likely that over time SOM and thus N and C stocks will increase.

Many growers and advisers will ask ‘Why should we do this? Can we offset the rundown of soil N over the longer term by just increasing fertiliser rates once yields drop?’ The pragmatic answer is perhaps ‘maybe…’. However, mineralisation of SOM mimics N demand of crops and this is difficult to match with fertilisers, even advanced slow-release formulations. It seems highly unlikely that increased reliance on fertiliser N will improve the efficiency of N use by crops at the system level, with increased losses and lower efficiency of use in the longer term. Of course, the delivery of N is but one of the many ecosystem services we rely upon SOM to deliver.

Acknowledgements

This research was initiated by the GRDC project CSP00207.This is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support.

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Contact details

Dr Mark Farrell, Principal Research Scientist
CSIRO Agriculture & Food, Locked Bag 2, Glen Osmond, SA
08 8303 8664
mark.farrell@csiro.au
@inverted_soil