Direct anthoprogenic soil threats
Nutrient mining, surface sealing, pollution, biodiversity loss, compaction, landslides
Many human activities – particularly agriculture – can result in depleting the store of soil nutrients. “Nutrient mining” refers to agricultural practices that result in a negative nutrient balance, i.e. where losses are greater than gains. In cultivated areas, nutrient mining may occur when (a) no rest (fallow) period is used that allows nutrients to accumulate, e.g. via the decay of crop residues, (b) when there is no crop rotation (e.g. use of legumes to restore nutrients), or there is no application of fertlisers (organic or chemical). Nutrient mining results in reduced productivity, loss of soil biodiversity, and declines in biomass and cover, which may lead to other degradation processes, such as erosion and leaching.
The severity of nutrient mining depends on the balance between input and output which is determined by crop type and management, soil and climate and the technical and economic ability of farmers to adopt sustainable land management practices. Developed countries may often apply excess amounts of fertiliser that can carry heavy environmental consequences, while poorer communities often face net depletion of soil nutrients that threatens sustainability, economic viability and food security. Without fertiliser application, yields can be sustained for short periods only because the productivity consumes preexisting stocks of soil organic nutrients. Indian and sub-Saharan African smallholder systems are examples of this process. While soil nutrient depletion has severe economic impacts at global scales, they are most pronounced regionally in sub-Saharan Africa. The low and declining per capita food production in sub-Saharan Africa has been caused by soil nutrient depletion in smallholder farms over decades. As production declines, farmers are increasingly unable to restore soil nutrients through the application of manure or chemical fertilisers. Growing populations have increased local demand for agricultural land which made it nearly impossible to use extended fallow periods to regenerate natural soil fertility. Participatory programmes now reinstate site-adapted and economically sound crop rotations that can allow for fallow periods. In 2014, fertiliser use in sub-Saharan Africa, at 16 kg/ha, was far below the global average of about 138 kg/ha. While the increased use of fertiliser is imperative to avoid further land degradation by nutrient depletion, it also must be accompanied by a dramatic increase in the efficiency of their use. Some steps toward improving efficiency would include adaptive land management, such as conservation tillage, integrated pest management, high-yield crop varieties, agro-forestry and inclusion of livestock in the system. All of these are traditional ‘low tech’ solutions and, as such, have a greater potential for adoption through focused extension and education programmes. Other options for remediation and mitigation of nutrient mining include the use of chemical fertilisers and application of green manuring. Green manuring involves adding nitrogen-fixing crops and trees in cropping systems or by mixing of organic waste in the soil. Organic farming has additional benefits through the use of manures, because it enhances the development of microbial communities in the soil that are more complex than those that result from the use of synthetic fertilisers. These more intensive methods, however, should be complementary to more traditional management systems described above because none of them alone are likely to solve the problem of soil nutrient depletion.
To guide implementation of solutions, recently-developed methods are now used to map nutrient management zones based on spatial predictions of soil nutrient deficiency/sufficiency levels that can be linked to crop requirements. These maps can be used to target agronomic interventions to reduce nutrient depletion, such as adapted intensification, alternative crop varieties, fertiliser use efficiency and possible restoration of degraded areas. Around the globe, agriculture, increasingly driven by telecoupled needs, is tackling the enormous challenges of developing and implementing sustainable nutrient cycling.
The creation of impermeable layers over the soil, such as asphalt and concrete surfaces, isolates soil from the atmosphere and above-ground biosphere. Sealing is the most intense form of land “take” and is essentially an irreversible process. Direct consequences are the partial or total loss of living soil, often the most fertile, a reduction in plant production and habitat, the disruption of water and nutrient cycles caused by the interruption of soil functions and services and an increase in vulnerability to flooding. Soil is a porous medium that stores water, gases and heat but also harbours a large, diverse and vibrant biological community, all of which help contribute to and support the biosphere. Sealing interrupts or reduces storage and sink capacities, prevents the transport, infiltration and exchange of nutrients and eliminates the soil's biologically active agents. Sealing also influences the energy balance of an area by altering heat fluxes affecting the local and possibly the global climate. Altogether, sealing means that the soil, as an active and vast terrestrial medium that supports basic life and sustains global biogeochemical cycles, is not able to fulfil some of its most important ecosystem functions. On the other hand, when sealed for urban, transport and production infrastructures, these areas have considerable economic value as they contribute to the gross national product of countries. Soil sealing is directly associated with the scope of urbanisation, which is expected to rise significantly in the coming decades. Increases in lower atmosphere temperature due to the increased heat capacity of sealing substrate and reduction in overall evapotranspiration, creates ‘urban heat islands’. The natural process of soil crusting when a hard soil surface layer is formed that reduces porosity and permeability is sometimes also indicated as ‘sealing’and results in a low surface physical quality that reduces infiltration rates, increases runoff and adversely affects biomass productivity. Soil crusting is a degradation process, but adapted management techniques can prevent or limit it and it is therefore not as irreversible as man-made sealing. Loss of soil biodiversity
Soil biota provide a wide range of ecosystem services that are essential to the functioning of natural and managed ecosystems. Their impact is both direct (e.g. affects on crop yield) and indirect (e.g. the role of soil organisms in global carbon and nutrient cycles). In addition to the focused impacts of soil sealing, there are multiple pressures that threaten soil biodiversity. Some of these include the disruption of ecosystems by invasive alien species, acid rain, nutrient overloading, harmful agricultural practices, overgrazing and pollution.
The conscious or inadvertent deposition of anthropogenic materials from industry, agriculture or urban wastes poses a threat to soil health. Soil tends to act as a sink for almost all substances released in the environment by human activities. The effects of soil pollution can be directly toxic for the aboveand below-ground biota, but can also change the physical and chemical properties of the soil and indirectly affect the ecosystem. Some pollution can be reversed on- or off-site by soil decontamination techniques such as microbial and phyto remediation, soil washing, elektrokinetic remediation and thermal treatment. However, these treatments range in cost from EUR 25 to more than EUR 150 per cubic metre.
The use of heavy machinery in agriculture and forestry and the overload of livestock can reduce the water-holding and infiltration capacity of soil. These changes lead to increases in soil erosion and a general decrease in the land productivity. Soil compaction can be prevented by: reducing physical pressure on the soil, especially when wet; adopting minimum or no tillage cropping systems; increasing soil organic matter to improve its structure through in-field retention of crop and pasture residues; including plants with deep and strong taproots in crop rotations.
Overly steep and overburdened slopes can become unstable, leading to detachment and downslope movement of a mass of rock, debris or soil. Landslides can be caused naturally by intense rainfall, prolonged periods of wet weather and snowmelt, seismic activity, loss of vegetation cover after a fire and the undercutting of slopes by rivers or sea waves. Anthropogenic causes include deforestation, the removal of vegetation cover, and inappropriate agricultural practices and infrastructure construction that lead to slope cutting or loading. Landslide occurrence is expected where we find ill-planned urbanisation, land cover modifications and changing climate conditions.
2014 Washington State landslide
This landslide appears to have involved a complex sequence of events. About 10 million cubic metres of debris covered about 30 houses and 1 km of a State Route.
Source: Godt, J./USGS CC
Global biogeochemical cycles are processes of nutrient cycling in ecosystems. The atmosphere is the main reservoir for elements such as carbon (C), oxygen (O) and nitrogen (N), while the soil is the main reservoir for less mobile nutrients such as phosphorus (P), potassium (K), calcium (Ca). These nutrients are taken up by plant roots, stored for a period of time in biomass and eventually returned to the soil within the same ecosystem by soil decomposers.
The cycles that mediate levels of available P, C and N are coupled: P becomes available for uptake by plants and microbes through the action of enzymes in the soil that depend on N which is regulated by the availability of C. The presence of C and N in the soil is linked to biological processes such as photosynthesis, N fixation by plants and decomposition of litter, all of which tend to function at lower rates in drylands. Under arid conditions there is a decoupling of these cycles because phosphorus no longer relies entirely on levels of the other two nutrients, as it is also produced by rock weathering. The decoupling of cycles with increasing aridity can lead to an imbalance among nutrients.
An increase in aridity, as expected under climate change, could further disrupt C, N and P nutrient cycles in drylands. Imbalances between agronomic output (harvest) and input (manure and fertilisers) create nutrient deficiencies that reduce land productivity or excesses, and have environment consequences such as nitrification of water resources.