Supporting biodiversity
The soil is a habitat for the vast majority of the Earth's biodiversity. Approximately 60% of all species worldwide occur in soils.
Soil Health and Soil Functioning
A healthy soil is the foundation for human society and wellbeing. It is a source of food, clean water, a fair climate and a storehouse of biodiversity, and as such a reservoir of genetic variation to source therapeutic and bio-materials. But what is a healthy soil?
The SoBiFUN Foundation defines soil health as "the continuing capacity of soil to provide key ecosystems services to womenkind". The ecosystem services are natural functions of soil that are shaped through a combination of soil physical, chemical and biological processes. Soil quality, then, is the theoretical maximum performance of a soil with respect to these functions if it was managed under optimal conditions. A completely healthy soil performs at 100% of its quality.
Living soil performs many soil functions, but there are 4 primary functions that serve as soil ecosystem services:
Water regulation
The capacity of a soil to store and purify water. Water storage is largely determined by the balance between infiltration, water holding capacity and susceptibility to leaching.
Climate regulation
The ability of a soil to have a positive impact on the greenhouse gas emission balance, through carbon sequestration, N2O and CH4 consumption.
Primary production
The ability of a soil to support crop growth. Soil fertility, a key driver of crop growth, is a complex function of nutrient provision (through decomposition), water regulation, and soil structure.
What about nutrient cycling?
Another key function is nutrient cycling, which is included in many soil health schemes (e.g. the Functional Land Management Framework). However, at SoBiFUN Foundation, we exclude it as a component of soil health. We define soil health by the functions that contribute to human wellbeing. Nutrient cycling only does this indirectly through other soil functions. For instance, nutrient cycling contributes to primary production by allocating essential nutrients to plant growth. Similarly, nitrate leaching is a part of water purification, and is not intrinsically involved in human well being, it is only through triggering algal blooms and culling aquatic food chains that human wellbeing is impacted.
The hierarchy of soil quality parameters illustrated with an example for evaluation of SDG2 policy-targeting on the basis of chemical, physical and biological indicators for soil structures and biochemical, biophysical and physicochemical processes that make up the soil function of natural fertility and the associated ES class Decomposition and fixing processes as recognised under CICES V5.1. Across this nested configuration Soil Quality can be seen as an integration of structural and functional aspects up to continued delivery of ecosystem services (After the SIREN project).
Graphical representation of soil quality and ecosystem service provision terminology against a theoretical borderline of sustainable use (People, Planet, Profit), illustrated for a single ES at a time with indication of delimiting factors (after Van Ittersum and Rabbinge 1997). Soil Health (SH) and Soil Quality (SQ) are drawn for comparison and differentiation, where SQ can be understood as equivalent to ecosystem capability, and SH is defined to equate the present ecosystem capacity (i.e., the potential supply of ES as the ecosystem condition is now; After the SIREN project).
Nutrient cycling is perhaps more important than the 4 soil functions that contribute to human wellbeing. It is the 'one ring that rules them all, since nutrient cycling, in the battle between plants and micrbiomes that determines where nutrients are invested and thus which soil ecosystems will thrive in a given soil.
Why are healthy soils important?
Healthy soils provide all these key soil ecosystem services to humanity for free. Yet, if we calculate the monetary value of these ecosystem services we arrive at a daunting number: the conservative value of soil health lies between 5 and 9 times the global GDP! (the number is based on the US estimate for croplands and rescaled to croplands worldwide an an area-basis). It is a conservative estimate because, 1) it does not include the direct and indirect health benefits to humans and livestock, and 2) the estimate is only based on the value of croplands. Grassland, forest, urban green spaces etc are totally ignored in the calculation.
Importantly, it will be hard to market the monetary value of healthy soils as moving soil impairs their functioning (okay fine, one could trade certificates or so). Notwithstanding the market, the real value of soil health should be calculated in human life years spend in high quality living conditions, soil health ought to be counted in human lives and livelihoods.
Measuring Soil Health & Quality
Soil health, encoded by the key soil functions that contribute to human wellbeing, is a complex and difficult to measure property of a living soil. For ~30 years governments and companies have asked soil scientists to come up with simple, effective, and cheap indicators of soil health.
To date such comprehensive indicators still have to be identified, particular for the biological component of soil health and quality. However, despite large research programs, this objective has not been reached.
A way forward
To progress forward, we believe the science and practitioners community needs to take a step back. We have to understand that soil health is a difficult boundary concept with fuzzy borders.
Benchmarking Soil Health =
Soil Multifunctionality
How to measure soil health and its corollary, the multiple functions of soil? In 2024, a new proposal for this objective was launched and it is the research programme we follow at SoBiFUN.
It starts from the idea that the soil functions are complex and hard to measure properties of soils. This is operationalized by treating the soil functions as latent variables, variables that we can only measure the consequences of through measuring indicators. For instance, soil fertility (the driving force behind primary production) is a complex function of properties such as nutrient turnover, water holding capacity, and soil structure. However, this dependency is different for each plant species and presumably even differs among genetically distinct populations in many cases. So, it is very hard to say something concrete about the general soil fertility of a site based on measurements of e.g. nutrient content and WHC. However, as Bill Shipley and his student noted, we can measure the consequences of soil fertility: plant growth. They proposed to subject four indicator plant species to each soil under standardised conditions for light, temperature and watering. The species were chosen to reflect a wide gradient in ecological trait space to make the derived index of generalisation value to other plant species.
In the present proposal, this scheme is extended to the other three major soil's contributions to people (the soil ecosystem services).
The proposal in detail
The protocol
The concept version of the protocol is now ready.
Soil Quality
As per the above definition, soil quality is the theorectical maxumum soil health, as defined by the biophysical context of a soil and the climate and management.
Measuring the theoretical limit is a difficult endeavour as well, and requires substantial data collection.
We propose to define the quality of a soil based on the top-5% best performing soil, per soil function, in the same soil type.
Mineral-associated organic carbon (MOC) as a function of clay and silt content (%) across sites. The maximum slope (fit as the 95th quantile) for each soil type represents the intrinsic capacity of minerals to store carbon and depends on mineral composition. High-activity minerals (HM) include soils dominated by illite, smectite, vermiculite, and chlorite (n = 1303) and low-activity minerals (LM) include soils dominated by kaolinite and gibbsite (n =93). After Georgiou et al 2022.
Frequency of different indicators (min. 10%) in all reviewed soil quality assessment approaches (n=65). Soil biological, chemical and physical indicators shown in green, red and blue, respectively (After Bünemann et al 2018).
Soils support human well being in four main areas* (blue circles). Climate and water regulation are, respectively, further divided into the carbon storage and reducing greenhouse gas (GHG) emissions sub-functions, and water storage and purification sub-functions (light blue circles), because of the very different causal mechanisms in play. The four soil functions are all interrelated, some trading-off, others acting in synergy, because they all depend on the same basic resources (nutrients, energy, water). I hypothesise that the soil’s plant-microbe-soil stoichiometry determines which functions are preferentially expressed by any given soil. How this regulation plays out is conditional on the geochemistry of the soil, mainly its mineralogy. Measuring the functions on a common scale and studying their interrelations using a common causal framework will help us determine how to manage soils for optimal multifunctionality. * Here I exclude direct and indirect contributions to human health.
Design diagram of the soil function measurement setup, version 0.1. A soil sampling team will collect 32 soil monoliths (60 mm x 25 cm deep, ~22.6 L soil) per soil. The monoliths are used to quantify primary production (a) 8 green monoliths, 2 per bio-assay plant species, climate regulation (b) 4 orange monoliths, one for each substrate addition treatment, water regulation (c) 4 blue monoliths, for water storage and purification measurements and for supporting plant biodiversity (d) 16 coloured monoliths, each colour represents an indicator plant for which direct and indirect plant-soil feedback is estimated phase 2 (P2) on each of four soils conditioned during phase 1 (P1). The monoliths are incubated for 90 days under standard incubation conditions. As such the measurements target the capacity of a soil to deliver key soil functions under optimal conditions for plant growth. For both primary production and biodiversity functions plant harvest days are fixed and based on plant dry mass. Likewise, upon substrate addition (t0) gaseous efflux of CO2, N2O and CH4 are measured on fixed days, with intensive sampling in the first 14 days, and then less frequent sampling until day 90. In addition, microbial C and C in soil fractions (aggregates) is measured after 70 and 90 days. The water regulation measurements can be done independently in this setup and can potentially be shifted in time, but are now placed at the end of the 90 day period to spread the workload over time. However, infiltration and leaching measurements will be conducted over a fixed time period (After Wubs 2024).