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Is soil management the key to net-zero?

Of course, the simple answer is “No, not alone”, but it does have a role to play.

 
 

What is the link between soils and greenhouse gas emissions?


Agriculture releases significant amounts of the greenhouse gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) to the atmosphere (Figure 1).


Carbon dioxide is released largely from microbial decomposition of plant litter and soil organic matter or during burning of crop residues. Methane is produced when organic materials decompose in oxygen-deprived conditions, notably from fermentative digestion by ruminant livestock, stored manures and rice grown under flooded conditions. Nitrous oxide is generated by the microbial transformation of nitrogenin soils and manures, and is often enhancedwhere available nitrogen (N) exceeds plant requirements, especially under wet conditions. In fact, 70% of the total greenhouse gas (GHG) emissions from agriculture are associated with N fertiliser; the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) showed that this resulted from a combination of CO2 and N2O emissions during manufacture and N2O emissions, direct and indirect, from its use. Methane from livestock is the only agricultural GHG emission where soil condition and management play no role in the amount or timing of GHG emissions.

At the same time, there is a large quantity of carbon (C) held in the organic matter within the world’s soils. It is estimated that the global stock of soil organic C (SOC) is in the range 684-724 Pg to a depth of 30 cm. This quantity of SOC in topsoilis about twice the amount of C in atmospheric CO2 and three times that in global above-ground vegetation. Changes in this pool can contribute significantly to GHG emissions; for example, the IPCC estimated that the annual release of CO2 from deforestation (coming from both vegetation and soil) was about 25% of that from burning fossil fuels.

 

Figure 1. Agricultural greenhouse gas balance

 

Can agricultural soils lock away (sequester) more C?


During the last decade, much has been written about the possibility of slowing climate change through sequestering C in soil as soil organic matter. The “4 per 1,000” initiative was launched in December 2015 at COP21 and highlights the fact that a small increase (0.4%) in the total amount of C stored in soils would be larger than the annual increase in atmospheric CO2 in that year.

If environmental factors or management practices cause the SOC stock to increase over time, then the soil can be described as a sink because C is moving into it. On the other hand, if it is declining, it is a source because C is moving from SOC to the atmosphere. Bellamy et al. (2005; Nature 437: 245- 248) analysed changes in SOC content in soils of England and Wales by using data from surveys conducted on two occasions (approximately 1978 and 2003). Overall,there was a decline in SOC (the soils were a source of C) but in soils with the smallest C content during the first survey (mainly long-term arable soils) SOC had increased by the time of the second survey and so were a C sink. This increase was probably caused by increased organic C inputs resulting from additional returns of straw following the cessation of straw burning in the UK. We have seen the same small increase in the long-term straw incorporation experiment at Morley.

For areas with a low SOC levels there may be potential to accumulate SOC through altered management (e.g. including cover crops or leys in rotations) or land use (arable to woodland or grassland), thus creating a sink. However, in some cases,a low C content reflects a small potential for SOC accumulation, either because of soil type (for example, the sandy soils of Breckland have less capacity to stabilise C than heavier soils around Cambridge) or through limited plant growth resulting from climatic factors.

 

Figure 2. Levels of soil organic matter are a result of a balance between C inputs and outputs. A range of factors affects the processes of decomposition and stabilisation in different climates/soil types so the same inputs can lead to different amounts of SOC

 

The general principle that applies to all soils is that if you add more carbon to the soil, then you build more organic matter (Figure 2). The levels of SOC in any soil are a result of the equilibrium between the inputs of organic matter and the decomposition of this organic matter by soil organisms. The disruption of soil aggregates duringtillage changes the distribution and accessibility of SOC in soil and usually increase rates of decomposition; hence reducing tillage intensity can lead to more stabilisation of SOC.

In general, we see bigger changes where there is a change in land use or major rotational changes, rather than changes in management (e.g. reduced tillage, use of cover crops). But it is important to note that there are some general limitations to the effectiveness of C sequestration in soil or vegetation:

The amount of C locked up is finite:

  • the increase in SOC content ceases as a new equilibrium value is approached. The period of transition is often 25-40 years and is usually slower when SOC is increasing than when it is being lost. This principle is clear from long-term studies, which show that SOC does not accumulate indefinitely (Figure 3).


 

Figure 3. Changes in SOC are slow, finite and reversible. To maintain the new equilibrium level in SOC the changed management must be maintained, even when there are no further increases in SOC

 
  • The process is reversible: the change in land management leading to increased C in soil or vegetation must be continuedindefinitely to maintain the increased stock of SOC. For example, if a new woodland is established, the C accumulated in trees and soil will be lost if the trees are felled. Similarly, if a grass or legume ley is included in an arable cropping system at least part of the SOC accumulated during the ley period is lost after ploughing for the next arable phase. Though there will often be some overall increase in SOC in the long-term compared with continuous arable cropping if the ley- arable rotation is continued (Figure 3).

  • Land management changes leading to increased soil C may increase or decrease fluxes of the other more potent greenhouse gases: N2O or methane. Hence, it is essential to consider the full GHG budget not just the impacts on SOC.

Together with land use change and direct addition of OM, choice of crops and varieties may also affect the rate at which C accumulates in soil. Rooting structure, depth, patterns of root exudation and the extent of root associations with arbuscular mycorrhizal fungal all influence where SOC is stored in the soil profile and the balance between decomposition and stabilisation. PhD students (Emily Marr and GeorgeCrane) are working jointly with NIAB and the University of Cambridge to investigate some of these below-ground interactions.

Even with these limitations, sequestering additional SOC will contribute to climate change mitigation in the medium-term, depending on the options available for changes in management practices or land use.

Practices leading to SOC accumulation can also start immediately without the need for development of new technologies. Even where there is no net additional transferof C from the atmosphereto soil, and thus no climate change mitigation, increasing or maintaining the SOC is almost always beneficial for soil health and function, especially in agricultural soils.


However, experience has shown that improvement in productivity in arable systems after improved organic matter management takes some time to appear. Defra research has shown measurable benefits of improved organic matter management, in addition to any nutrient supply benefits, but these are often only realised afterat least six years of implementation. Increased SOC has positive impacts on soil physical properties, including increased stable aggregates, decreased risk of run-off, erosion or surface capping, increased rate of water infiltration and increased water retention. It has been shown that even small increases in SOC can have disproportionately large impacts on aggregate stability, infiltration and the energy required for tillage.

 

Figure 4. Sources and sinks of GHG in UK agriculture

 

To deliver these benefits for production, ensuring that there are regular additions of organic matter to “feed” the soil is more important than achieving any particular measured value of SOC.


In UK arable farms, practices with positive benefits on SOC include:

  • Reduced intensity of cultivation

  • Increasing tree cover on farm

  • Reduced area of cropping systems on peat and reversion to wetland

  • No bare soil – continuous green-cover cropping systems

  • Targeted steps to increase soil organic matter through managed additions of organic materials.

What about N fertilisers?


Work with farmershas highlighted that theeasiest soil management step to implement that will help mitigate climate change is, in fact, an increasedfocus on nutrientmanagement, in particular, steps to improve N use efficiencywithin a farm- specificN management plan.


Any benefitsof N fertiliser for crop productivity (and increased root and residue returns that increase C inputs to soil) can be offset by higher emissions of CO2 from fertiliser manufacture and losses of N2O from soils. Plants take up N from the soil solutionas ammonium (NH4+) and nitrate (NO-); these pools of N in soil are regularly replenished by the decomposition (mineralisation) of organic matter. In unfertilised systems, these pool sizes tend to be low, as plant and microbial uptake empty these pools, almost as fast as they are filled. N fertilisers add NH+ and NO- (or their pre-cursors e.g. urea, organic manures) and hence higher pools of mineralN (NH4+ and NO-) are found immediately (and for a few weeks) after fertiliser application. N2O is released during microbial processes of both nitrification and denitrification. Ammonium is rapidly nitrified to nitrate (NO3-) in a two-stage microbial process by the action of chemoautotrophic bacteria; this process releases some N2O as a by-product.


CH4 and N2O are very potent GHGs emitted during farming activities and much harder to offset


Denitrification, (first observed by Humphrey Davy at the beginning of the 19th century) is the release of nitrogen gas during microbial decomposition and is a relatively small flux in comparison with the rate of fertiliser addition in many aerobic agricultural soils. However, N2O is produced during the stepwise sequence of microbially mediatedreducing reactions and these losses are higher whensoils have high microbial activity and a high proportion of water-filled pores. So early spring fertiliser (or autumn N, e.g. to oilseed rape) is at increased risk of loss.

Practices that improve N use efficiency (and may also directly reduce N2O losses) include:

  • adjusting application rates based on precise estimation of crop needs (e.g. precision farming);

  • removing any other constraints to growth whether pH, other limiting nutrients or disease, or planning N applications to take accountof the change in yield potential (e.g. due to drought/ pest attack);

  • avoiding time delays between N application and plant N uptake (improved timing);

  • placing the N more precisely into the soil to make it more accessible to crops roots;

  • using slow-release fertiliser forms or nitrification inhibitors;

  • avoiding excess N applications, or eliminating N applications where possible;

  • imroving soil structure to improve both rooting and water holding capacity thereby increasing N use efficiency even when spring is dry (which seems to increasingly be the norm in the UK);

Together with agronomic practices, choice of varieties may also affect GHG emission. Cereal varieties that benefit more from higher N conditions tend to have been selected over the years, however adding N fertiliser is not always an efficientprocess. Work is underway in NIAB to investigate geneticdifferences in N responsiveness in cereal crops (Stephanie Swarbreck), which may inform variety selection in the future.

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