Draft:Soil loss tolerance

Soil loss tolerance

Background Information

Soil loss tolerance, denoted as a T value, can be defined, broadly, as the maximum level of soil erosion that can occur annually before long-term natural soil productivity is adversely affected . Soil loss occurs naturally through geologic erosion, from wind and water, but can also be accelerated through anthropogenic activities, including intensive agriculture and land-use change . Soil loss tolerance acts as an important tool in assessing erosion risk of a particular soil and is crucial for the preservation of long-term soil productivity . The term first became popularized in 1962 by the U.S. Soil Conservation Service (SCS), now the Natural Resources Conservation Service, and stemmed from the soil conservation movement initiated in the United States in the 1930s . This earliest adoption of the term focused on physical and economic factors of soil loss and resulted in the establishment of T values for 12 soil orders in each region of the United States . Since then, the term has continuously evolved and been expanded upon, although the basic structure of the original criteria remains the same .

Evolving the Soil Loss Tolerance Concept

Interestingly, there is a lack of consensus that lends a singular definition, criteria, or calculation method to soil loss tolerance. The term has an extensive history, with numerous versions emerging as scientists continue to debate and challenge each other’s interpretations . More recently, definitions have shifted away from solely focusing on maximizing productivity for economic gain but have increasingly prioritized the maintenance of soil health and sustainable land use . To approach soil loss tolerance from a more holistic perspective, incorporating two components, referred to as T₁ and T₂, into soil loss tolerance calculations is now being emphasized . T₁ acts as the lower limit of the T value and represents the amount of soil loss that maintains desired soil productivity . Whereas T₂ acts as the upper bound which limits off-site impacts of erosion, including water pollution, flood control, gully erosion and reservoir or river sedimentation . This upper bound corresponds to the environmental, social, economic and political effects of erosion .  

Calculating Soil Loss Tolerance

Calculating soil loss tolerance is complex, making it difficult to represent with a singular equation. As a result, numerous methods have been developed. Additionally, the accuracy of T values is sometimes questioned owing to the spatial variability of soil across regions . Some researchers have proposed that recommended T values should be tailored to specific soil series, based on soil properties, or calculated as a function of the soil productivity index .  However, the three main approaches for estimating T values use the factors of soil formation rate, soil thickness, and soil productivity .

Influencing Factors

As well as these three main factors, additional influencers include:  

1. Topsoil formation rate  
2. Nutrient loss (by erosion)
3. Yield reduction (by erosion)
4. Maintenance of water retention structures  
5. Soil depth
6. Rill and gully formation
7. Maintenance of sedimentation rate

Therefore, we can expect that T values vary based on soil productivity, soil type, and location. Globally, T values range from 116 to 9,300 t km ^–2 yr^–1 . Often, a set of T values are applied as standards regionally across a country, however, these values are approximate and context specific, and thus, cannot be applied to other soils or land uses .  As an example, in carbonate soils, soil loss tolerance can be calculated based on the following formula :

$T=\nu Q\rho C+R(1-C)$,

where $T$ = soil loss tolerance (t km^–2yr^–1), $\upsilon$= dissolution velocity of carbonate rocks (m km^–2yr^–1),

$Q$ = content of acid-insoluble components (%), $\rho$= carbon density (t m^–3),

$C$ = proportion of carbonate,

$R$ = soil formation rate of other rock types

Importance

Since its origins, soil loss tolerance has gained significant attention considering the increasing issue of soil loss and degradation globally . Soil erosion has negative impacts on soil quality, agricultural productivity, biological diversity, and water quality . Additionally, the increasing frequency of extreme rainfall events under climate change scenarios, which can be directly linked to rates of erosion, bears consideration, especially given the connection between soil, carbon sequestration, and climate change mitigation . Soil loss tolerance can act as an essential tool in ecosystem restoration and erosion control, making it relevant for soil conservation and climate policies. For this reason, the use of accurate T values is extremely important .