What are the characteristics of degraded dryland soils, and what makes them unique?
Dryland soils are one of many classes of soil types each with unique features. To answer this topic question, let us consider the following sub-questions which dig deeper into the issues of soil conservation…
Does “degraded dryland soil” indicate that a region is too dry to support plant growth?
Dryland soils are periodically dry, but can receive significant spells of precipitation so that soil humidity is enhanced sufficiently to allow for high biological productivity. This however requires adequate nutrient resources, and the soil needs to be able to absorb rain water and transfer it to deeper soil layers, a property called the soil water infiltration rate. Furthermore the speed at which the moisture is evaporating from the soil depends on available shade or litter cover. Therefore the biological productivity is only in part determined by the amount of precipitation and depends to a significant part on additional soil and ecosystem parameters. The amount of biomass produced per unit of rainfall can be expressed as precipitation use efficiency (PUE) and can strongly vary between differently managed plots within the same area.
Degraded soils are characterized by low PUE, meaning only a low proportion of precipitation is being exploited for the production of biomass with the remainder running off or evaporating. In the Wadi Attir area PUE can vary by up to 10 fold, as determined experimentally.
Does “degraded dryland soil” imply that it has low soil organic carbon content, meaning that the soil is poor for supporting plant life?
Correct, soil organic matter content is the major parameter determining proper soil function in many ecosystems, and directly linked to function and quality of dryland soils such as those found near Hura and Project Wadi Attir (see scientific reference). Soil organic carbon content is affected by precipitation levels and the mechanical soil composition, but within given conditions biological productivity, and specifically the amounts of root biomass deposited by annual herbaceous vegetation is giving rise to an equilibrium concentration of Soil Organic Matter (SOM) which is to a large extent determining essential functions such as soil hydrology and water infiltration, soil microbial live (essential for nutrient fixation and mobilization), and soil respiration. Consequently SOM is the key interface between the biosphere and physical soil functions and the key parameter in determining soil quality and biological productivity.
This presentation delves deeper into the relationship between soil and carbon sequestration. Click the diagonal arrows to make the slideshow fullscreen.
SOM is being lost by overgrazing, and worse, by tilling or large scale soil movement resulting in complete vegetation depletion. Such degraded soils have less than 50% SOM compared to conserved soils and feature significantly reduced water infiltration and soil nutrient content. Often seed pools are depleted as well due to wind and water erosion, resulting in partial or complete loss of biological productivity. In drylands this can result in irreversible degradation or desertification, as soil properties deteriorate to the extent that doesn’t allow for significant plant productivity anymore. These mechanisms are schematically presented in figure 1.
Figure 1: Schematic representation of irreversible soil degradation, or rehabilitation by vegetation depletion or recovery and their relation to precipitation use efficiency. In drylands excessive loss of vegetation and SOM can create a novel irreversible low-productivity state because the lowered productivity can no longer induce restoration of adequate SOM concentrations.
What are some of the characteristics of degraded dryland soils?
Compaction and crusting are typical characteristics of degraded dryland soils. Similar to other soil quality impairments in drylands, soil crusting has self-amplifying negative impacts on ecosystem productivity by inducing a cascade of feedback mechanisms. Crusted soils allow for high water runoff, enhancing water erosion. Seed reserves are depleted due to wind erosion, and the full power of solar radiation is heating the topsoil to accelerate evaporation. Crusted soils especially in loess areas are being created by mechanical forces, turning exposed topsoil into fine dust that turns into a concrete like state when wetted by light rain or dew.
Similarly, standing water on loess creates extensive crusting, cracking and reduced water infiltration. Once dry, such crusted material turns back to highly erodible dust. Only a plant litter or vegetation layer can prevent such mechanisms. Project Wadi Attir is demonstrating this by recycling weed litter and application of litter producing dryland trees to rapidly restore annual vegetation, SOM and plant litter for avoiding those unfavorable soil crusting mechanisms
Degraded Shrubland
Fertile Former Goat Enclosure
On the other hand formation of microbial soil crusts is a desired mechanism to stabilize highly erodible slopes whereby adequate numbers of shrub or tree patches or ant nests are collecting the runoff water originating in crusted matrix to create the typical patchy dry shrubland ecosystems. Such microbial crusts are composed of cyanobacteria and other microorganisms and provide enhanced mechanical stability to prevent wind and water erosion where a stable protecting layer of plant litter cannot be established and maintained. Nevertheless, aridity levels as found at Project Wadi Attir are sufficient to ultimately allow for creation of a closed litter and vegetation cover by the help of silvipasture or strict conservation measures, which will potentiate productivity and resilience compared to patchy shrubland ecosystems.
What are the specific features of nutrient contents in degraded dryland soils?
Degraded dryland soils feature low to very low content of essential plant nutrients such as nitrogen, phosphate or potassium. Nevertheless this is not the primary cause for low productivity, which must be rather attributed to low water infiltration, high water runoff, and low SOM content. Consequently application of mineral fertilizer alone will not significantly improve productivity in degraded drylands. Phosphate will be lost by erosion, while nitrate can be lost by infiltration into deep soils ultimately polluting groundwater.
The difference between poor degraded soils, and optimally restored soils are best demonstrated in a patch overlayed by manure found at Project Wadi Attir which displays ten-fold higher productivity than surrounding degraded soil. Nitrogen and phosphate are clearly found in excess (table 1) but water infiltration is maximal, and evaporation is minimal thanks to the thick layer of organic matter.
| EC | Nitrate | K | P-PO3 | |
|---|---|---|---|---|
| Manure Covered | 1.68 | 30.8 | 117.5 | 243.2 |
| Degraded Soil | 0.53 | 3.3 | 8.7 | 6.2 |
Table 1: Nutrient content, specifically phosphate, in the manure overlaid plot is clearly excessive, but ten-fold nitrate content together with maximal water use efficiency can explain the ten-fold productivity observed in the manure treated plot.
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What are the characteristics of degraded dryland soils, and what makes them unique?
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