Nitrogen and soil organic matter decline - what is needed to fix it?

Take home messages

• Stocks of soil organic matter (SOM) and nitrogen (N) are limited resources and current trends across Australian agricultural soils indicate that these are declining (Luo et al. 2010).
• Soil derived N can contribute to the amount of N available to a crop. As the capacity of a soil to deliver N declines, increased rates of fertiliser N will be required and optimising profit (where marginal benefit=marginal cost) may move to lower yields.
• N balance calculations are essential to define how management is altering the stock of soil N. A range of indices exist that can be monitored over time to provide an indication of how management is affecting N stocks.
• Altering management practices to maintain SOM and N status are likely to be associated with increased costs (either increased expenditure or opportunity costs). Mechanisms for offsetting increased costs associated with applying management practices to accumulate organic matter and N exist, and more are under development.
• Taking a long term (decadal) view on the economic implications is critical to ensure future productivity will not be compromised in an effort to maximise short term (annual) profits.

Introduction

SOM and soil organic carbon (SOC) are sometimes used interchangeably and on average SOM contains 58% carbon (C) (Hoyle et al. 2011). The majority of the balance is made up of other elements including nutrients (N, phosphorus (P) and sulphur (S)) as well as oxygen and hydrogen. It is important to recognise that SOM contents are therefore 1.72 times greater than SOC contents and attention must be paid to soil test values to confirm what has been reported.

A simple organic carbon (OC) cycle for agricultural soils is provided in Figure 1. OC enters the soil through the capture of carbon dioxide (CO2) by crops and pastures and the subsequent deposition of residues on and within the soil. For surface deposited materials to contribute, the OC in the residues must be mixed into the soil or broken down and moved into the soil. Any removals of products or residues will reduce the flow of OC into the soil. Once in the soil, the activity of decomposer organisms will respire a portion of the OC back to CO2.

Soil erosion can also contribute significantly to SOC loss with practices such as maintaining ground cover reducing the impact. Although analysis laboratories typically provide values for SOC content, the actual amount of OC present in a soil is referred to as the stock of OC and is calculated by defining the tonnes of C present in a soil to a defined depth according to Equation [1] (Sanderman et al. 2011). Both the content and stock of OC content in a soil therefore represents the net balance between the rates of C addition and loss. Any alteration to management practices that can enhance rates of OC addition (flows with black circles in Figure 1) or reduce losses (flows with white circles in Figure 1) beyond that currently being attained have the potential to increase the amount of C in soil.

Figure 1. Carbon cycle in agricultural soils showing inputs and losses of organic carbon

Equation 1. Equation used to calculate the stock of organic carbon in the soil.

Declines in SOM and N status in agricultural soils

Conversion of native soils to agricultural production often results in a decline in SOC content or stock. Under Australian conditions, Luo et al. (2010) assembled data from 20 different studies indicating that cultivation of the 0cm-10cm soil layer resulted in a decline in SOC stocks to values approximately half those in soils in their native condition. However, the extent of loss did vary between 20% and 70% with similar, but more variable results when the 0cm-30cm soil layer was examined. The observation that significant amounts of SOC have been lost due to cultivation suggests that changes to management practices will be required to rebuild SOC. Although some changes have been implemented (e.g. reduced/zero tillage and reductions in stubble burning/removal), further change and the introduction of new approaches may be required.

The strong link between OC and N in SOM indicates that losses of SOC are also indicative of a loss of soil N. OC to N ratios of 10 to 12 are generally expected for mineral soils. Across a range of Australian soils varying in OC content from <1% to just over 14%, the OC to N ratio was found to be 11.1 on average (Kirkby et al., 2011). The implication of this is that where the OC content of the 0cm-10cm soil layer with a bulk density of 1.3 g/cm³ and no gravel, declines from 2% to 1% by weight, approximately 1081kg N/ha will have been mineralised. The possible fate of the mineralised N would be uptake and removal in agricultural products or loss from the soil. If the loss occurred over a 20-year period, then the reduction in OC has provided an average of 50kg N/ha/year. As the OC within the soil continues to decline, two outcomes will become evident: 1) the rate of OC loss decreases, hence, less N is mineralised and potentially made available to growing crops and 2) once a lower threshold value of OC is passed, little N will be mineralised and released and crop production will become much more reliant on fertiliser N additions.

Figure 2. Change in soil organic carbon stocks of the 0cm-10 cm layer with increasing duration of cultivation relative to the stock in soils under native condition.

Implications of declining organic matter and N status

SOM contributes positively to a range of soil properties and functions considered important to defining the potential productivity of soil. Across different types of soils (e.g. soils varying in clay content) the importance of organic matter contributions will vary. In Figure 3 a conceptual framework is presented that summaries the relative importance of organic matter; with the change in the width of the shapes providing an indication of the relative importance of organic matter to a particular function. As an example, for cation exchange capacity (CEC), organic matter will provide the only source of CEC in a sand and is therefore critical to the provision of this soil property. However, as clay content increases, the requirement for organic matter to provide CEC declines because the contribution of clay particles to CEC meets the needs of the soil. As a second example, consider the provision of energy for biological processes. Irrespective of clay content or nature of the minerals present in a soil, organic matter is the source of energy for organisms. Thus, the shape for this process remains wide across all clay contents.

With declining levels of organic matter in soil, the ability of the organic matter to contribute adequately to the functions identified in Figure 3 declines. If these contributions drop below the threshold values required to maintain adequate soil function, then soil productivity will be compromised. It is important to note however that for SOM to contribute to these properties and functions it needs to decompose and cycle. Thus, in attempts to build SOM, it is not desirable to stop its decomposition, rather attempts should be made to increase the rate of organic matter addition to result in a net gain and promote greater cycling and enhanced contributions to beneficial soil properties and processes.

Figure 3. Conceptual contributions of the contribution of organic matter to various soil properties and functions and how this varies across soils with changes in clay content. The width of the shape corresponds to the ascribed importance at a given clay content.

Under dryland growing conditions in Australia, yield potential is typically defined by the availability of water to grow grain crops by summing plant available water stored in the soil at sowing and predicting the amount of rainfall that will be received. Using the amount of water that is potentially available to set a yield target and assigning a protein content for the grain allows the derivation of N requirements. Achieving a good match between crop N demand and N availability requires the prediction of N delivery from the soil and the addition of an appropriate amount of fertiliser N. A declining soil N status means that to achieve yield and protein targets defined by the availability of water, additional fertiliser N will be required.

Fertiliser rate trials have demonstrated that the efficiency of fertiliser N use declines as fertiliser application rates increase (for examples see Bell et al. 2014; Lester et al. 2009). Each incremental increase in yield requires a larger addition of fertiliser N, and therefore, costs more; particularly in progressing towards the biological optimum yield (e.g. point B on the yield curve in Figure 4a). A contributor to this relationship resides in the mechanisms by which available N can be lost from the soil/crop system (e.g. volatilisation, denitrification and leaching), and the increased potential for these losses to occur as the concentration of available N in soil increases in response to increasing fertiliser addition. As a result, where fertiliser N application rates have to increase in response to a decreased ability of the soil to supply N, the cost of achieving an additional yield increment will increase and the profitability ($/kg of fertiliser N applied) of applying additional fertiliser N will decrease. Under such circumstances, and assuming all other variable costs remain fixed, the economic optimum yield (where marginal benefit = marginal cost, point A on the profit curve in Figure 4a) will decline as the ability of a soil to supply N decreases (point D versus point E in Figure 4b). It is important to note that the different responses presented for the soils with a low and high N supply capacity in Figure 4 are conceptual and have been accentuated to demonstrate the points being made. A more complete economic assessment is required to quantify the magnitude of the proposed profitability differences and fully assess the implications.

Figure 4. Changes in (a) the efficiency of fertiliser N use in terms of grain producing grain and potential relationship between biological and economic optimum yields (b) profitability of grain production with increasing fertiliser N application rates for soil with a low (solid line) or high (dashed line) N supply capacity. Note that these diagrams are conceptual and differences between low and high N supply capacity have been accentuated for the purpose of demonstrating potential differences.

Part of the benefit provided by soil N supply, relative to fertiliser N application, resides in the fact that N derived from organic matter decomposition is metered out over the growing season and responds positively to the same environmental conditions controlling crop growth and N demand (e.g. availability of water and temperature). With an increasing reliance on soil derived N, the supply and crop demand for N are likely to be better synchronised, leading to a lower chance of available N accumulating in the soil. However, if fertiliser N is added and creates an excess of available N, the slow release and more synchronous behaviour of N being mineralised from the SOM will be lost. This occurs because, when mineralised N enters the available N pool, it behaves in a manner similar to the added fertiliser N.

What is needed to fix it?

Increasing soil organic matter content or stock

Given that the amount of organic matter present in a soil results from the balance between inputs and losses (Figure 1), to shift SOM stocks to higher values will require an increase to the flow of OC into the soil. An exception to this may be where rates of SOM loss due to erosion can be reduced through maintaining a greater amount of soil cover. Questions that should be posed include:

1. Are organic materials being removed (e.g. crop residues) and can this practice be halted?
2. Are current management practices maximising water use efficiency (expressed in terms of dry matter production per mm of available water)? If not, are there alternative practices available that can be used to move towards greater water use efficiency and enhanced biomass production?
3. Is there scope to alter the production system to include a greater proportion of legumes, particularly legumes grown as a green or brown manure?
4. If erosion is an issue, can management practices be imposed that maintain a higher level of soil cover (for wind erosion) or can the movement of water over the soil be slowed (for water erosion).

Acknowledging that the current levels of SOM are a function of the history of management practices employed, if the answer to any of the above questions is yes, then there is scope to increase the storage of organic matter in the soil.

A tendency has existed to suggest that the adoption of defined management practices (e.g. reduced tillage, rotational grazing) can alter SOM stocks. Sampling many Australian grain growing soils has suggested that increasing stocks of SOM is less about the nature of management practice and more about whether C flow to the soil has been enhanced. Adopting a perceived ‘C friendly’ management practice provides no guarantee that soil C stocks will increase.  The manner in which the practice is implemented and its impact on C flow to the soil is critical. For example, a grower maximising productivity of grain crops (continually achieving close to the water limited yield) and retaining all residues may end up with a better SOC stock than a grazier operating with a stocking rate that is too high.

Maintenance of soil N

Most of the N contained in a soil (>95%) is found in the SOM. Rates of change of SOM are slow (often requiring >5 years to detect true change) and given the extent of spatial variability across paddocks and variations in seasonal conditions, it will be difficult to quantify the implications of growing single crops on soil N status through direct measurement. As a result, a number of agronomic indices have been developed and used to quantify the effectiveness of nutrient management based on yield responses, N extracted in grain and the difference between added and extracted N. These indices have been presented and discussed in a previous GRDC update paper (Baldock et al., 2018). In demonstrating the use of these indices, Norton (2016) obtained results across 4-5 years for 514 paddocks indicating on average that growers were mining N from the soil resulting in a decline in soil N status over time.

For growers to gain an appreciation of the implications of their management practices on soil N status, it is important to conduct N balance calculations. Given the different annual inputs, extractions and losses of N as a function of variations in applied management practices, soil properties and environmental conditions, growers are encouraged to complete annual N balance calculations (Equation [2] (Baldock et al. 2018)). Deriving values for all of the components of the N balance calculation may be difficult, particularly for some of the loss mechanisms; however, monitoring the N balance result obtained over time would remain useful and provide an indication of any trend. Although a trend to increasing N stocks is encouraged, it should be acknowledged that temporary periods of mining N stocks are acceptable, provided the extent of N mining is quantified and followed by a rebuilding phase in which N stocks are replenished. It is recommended that annual N balance calculations be performed; however, the values should be integrated and accumulated over time to define the full effect of applied management practices and temporal trends. Such information will allow grain growers to implement appropriate actions to maintain their production base into the future and continue to maximise profitable grain yield outcomes.

N_F=N added to the soil in the form of chemical fertilisers

N_OA=N added to the soil in the form of organic amendments (e.g. manure, composts, etc.)

N_dfa=N derived from atmospheric N2 by symbiotic and non-symbiotic fixation

N_dep=N deposition from the atmosphere

N_R=N removed in harvested products

N_L=leached from the root zone

N_V=volatilised as ammonia from fertilisers and soils

N_Den=N lost as N2 and N2O by denitrification

N_E=N lost by erosion

Equation 2. Calculation used to determine N balance.

Other than the application of fertiliser N, the main mechanism for growers to enhance N status is the inclusion of legumes in rotation with grain crops. This could include pulses and pasture options in rotation with grain production. To maximise N inputs, it may be appropriate to maximise the nodulation and biomass accumulation of a legume and retain all biomass (e.g. green manure). In essence, growers need to take a ‘crop management approach’ to growing a legume for augmenting the soil N status and contributing to SOM levels. Although this would be associated with a significant opportunity cost, the benefits to subsequent crops and long-term implications on soil N status and productivity may be positive. Longer term (>10 years) economic analyses of such options need to be considered since the most profitable short-term result will always be to maximise the extraction of N from the soil (i.e. mine the soil N reserve) thereby reducing the cost of production. Such analyses should also take into account other potential benefits including, but not limited to, diversification of the farm business, enhanced or additional weed control options and provision of crop disease breaks.

Options to offset the opportunity cost of maintaining SOM and N

Valuing SOM/C

Quantifying the value of SOM to production is essential. However, this is challenging, given the diversity of positive contributions SOM potentially makes to productivity and the different amount and types of organic matter required to achieve adequate functioning for different soils. Having such values will aid in the economic assessments of current investments or opportunity costs associated with management practices designed to maintain the SOM and N status. Where appropriate and consistent with farm business planning, entry into C markets may also contribute.

Valuing the natural capital of soil

Currently, the natural capital contained within soil does not contribute significantly to property valuation and little reward exists for the maintenance of natural capital. Based on the example provided earlier in this document, for every per cent by weight of OC in the 0cm-10 cm soil layer about 1000kg of N is present. Using a value of $1 to buy and apply a kg of N per ha, the N resource within the 0cm-10cm soil layer could be valued at $1000 per ha; however, such values rarely enter into the assessment of farm capital values. Movement by financial institutions towards valuing natural capital is now being discussed. Potential options include the provision of reduced interest rates on loans in response to being able to demonstrate that applied agricultural practices are maintaining or enhancing soil. Tools such as those being developed by Digital Agricultural Services will help facilitate natural capital valuation and its inclusion in financial decisions. Assessing the costs and benefits associated with changes in natural capital value will be required to clearly articulate the impact of such approaches on the farm business with both short- and long-term analyses being completed.

Accounting for the true cost of production

When the value of grain and products derived from grain are assessed, often little consideration is given to how their production has altered the resource base from which they were derived and the costs associated with maintaining the base. Interest exists in tracking the provenance of commodities and attaching information about how they were produced. Being able to demonstrate effective management practices that maintain or enhance the soil resource base may allow entry into markets that attract higher returns and help to offset any opportunity costs.

Conclusions

• Stocks of soil organic matter and N are limited resources and current trends across Australian agricultural soils indicate that these stocks are declining. Declines in SOM are likely to result in decreased productivity and sustainability into the future. Establishing threshold values of composition and stock appropriate to different combinations of soil type and climate is required.
• Soil derived N can make significant contributions to the amount of N available to a crop. As the capacity of a soil to deliver available N to crops declines, increased rates of fertiliser N will be required. As fertiliser N rates increase, the potential for N loss increases and typically leads to reduced fertiliser N use efficiency. As a result, with decreasing soil N supply capacity, optimised productivity (where marginal benefit=marginal cost) may move to lower yields.
• Completing N balance calculations is essential for grain growers to gain an understanding of how their management practices are altering the stock of N present in their soils. N balance calculations should be completed annually but integrated over time. Where negative N balances are obtained, the soil N resource is being mined. Under such circumstances, it is important to consider whether future long term (decadal) productivity and potential profit is being eroded to maximise short term (annual) values.
• Altering management practices to maintain SOM and N status are likely to be associated with increased costs (either increased expenditure or opportunity costs). Mechanisms for offsetting these costs exist and more are coming on line. Taking a long-term view on the economics of current management on future productivity is important.

References

Baldock, J., Macdonald, L., Farrell, M., Welti, N and Monjardino, M. 2018. Nitrogen dynamics in modern cropping systems. GRDC Grains Research Update Adelaide, 20 February 2018. (https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2018/02/nitrogen-dynamics-in-modern-cropping-systems)

Bell M, Lester D, Grace P (2014) Improving fetilizer nitrogen use efficiency in summer sorghum. GRDC Grains Research Update, Northern Region, 26 August, 2014. (https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2014/08/improving-fertilizer-nitrogen-use-efficiency-in-summer-sorghum)

Hoyle F.C., Baldock J.A., Murphy D.V. (2011) Soil organic carbon - role in rainfed farming systems: with particular reference to Australian conditions. In 'Rainfed Farming Systems.' (Eds P Tow, I Cooper, I Partridge and C Birch) pp. 339-361. (Springer Science)

Kirkby, C.A., Kirkegaard, J.A., Richardson, A.E., Wade,L.J., Blanchard, C and Batten, G. 2011. Stable soil organic matter: A comparison of C:N:P:S ratios in Australian and other world soils. Geoderma, 163, 197-208.

Lester DW, Birch CJ, Dowling CW (2009) Fertiliser N and P application on two Vertosols in north-eastern Australia. 3. Grain N uptake and yield by crop/fallow combination, and cumulative grain N removal and fertiliser N recovery in grain. Crop and Pasture Science 61(1), 24-31

Luo, Z., Wang, E. and Sun, O.J. 2010. Soil carbon change and its responses to agricultural practices in Australian agro-ecosystems: A review and synthesis. Geoderma, 155, 211-223.

Norton, R. 2016. Nutrient performance indicators from southern Australian grain farms. A report to the Grains Research and Development Corporation, Australia.

Sanderman, J., Baldock, J., Hawke, B., Macdonald, L., Puccini, A. and Szarvas, S. (2011) National soil carbon research programme: Field and laboratory methodologies. CSIRO Sustainable Agriculture Flagship.

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, the author would like to thank them for their continued support. Contributions made through the funding of research projects on soil organic matter and its cycling by the federal Departments of the Environment and Energy and Agriculture and Water Resources is acknowledged.

Contact details

Jeff Baldock
Locked Bag 2, Glen Osmond, SA 5064
08 83038537
Jeff.Baldock@csiro.au