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Threats to Soils GLOBAL LAND OUTLOOK WORKING PAPER THREATS TO SOILS: GLOBAL TRENDS AND PERSPECTIVES A Contribution from the Intergovernmental Technical Panel on Soils, Global Soil Partnership Food and Agriculture Organization of the United Nations Editors: Gary Pierzynski (United States) and Brajendra (India) Secretariat: Lucrezia Caon and Ronald Vargas Contributing Authors: Amanullah (Pakistan), Talal Darwish (Lebanon), Gunay Erpul (Turkey), Ranier Horn (Germany), Nsalambi Nkongolo (Democratic Republic of Congo), Brajendra (India), Gary Pierzynski (United States), Peter De Ruiter (The Netherlands), and Miguel Taboada (Argentina) September 2017 DISCLAIMER The designations employed and the presentation of material in this information product do not imply the expression of any opinion whatsoever on the part of the United Nations Convention to Combat Desertification (UNCCD) concerning the legal or development status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. The mention of specific companies or products of manufacturers, whether or not these have been patented, does not imply that these have been endorsed or recommended by UNCCD in preference to others of a similar nature that are not mentioned. The views expressed in this information product are those of the authors or contributors and do not necessarily reflect the views or policies of UNCCD. CONTENTS 1. Introduction 03 2. Soil Erosion 03 2.1 Status of soil erosion 03 2.2 Policies and strategies for soil conservation and protection 04 3. Soil Organic Carbon (SOC) 06 3.1. Current status of SOC 06 3.2. Drivers of soil carbon loss 07 3.3. Regional trends in the condition of soils 08 3.4. Future perspectives of SOC 09 4. Soil Nutrient Balance 09 5. Soil Salinization and Sodification 10 6. Soil Biodiversity 11 6.1 Flora and fauna in soil 11 6.2. Status and loss of soil biodiversity 13 6.3. Major roles of soil biodiversity 13 7. Soil Contamination 14 8. Soil Acidity 14 9. Soil Compaction 17 10. Soil Sealing 20 10.1. Impact of chaotic urban sprawl on land resources 20 10.2. Land use planning for sustainable use of land resources 23 11. References 24 1. INTRODUCTION An important product of the United Nations pressures on soil resources are reaching Food and Agriculture Organization (UNFAO) critical limits. Further loss of productive soils Intergovernmental Technical Panel on Soils will amplify food-price volatility and send (ITPS) was the publication of the Status of the millions of people into poverty. This loss is World’s Soil Resources (SWSR; FAO & ITPS, avoidable. Careful soil management not only 2015). The report identified ten main soil secures sustainable agriculture, it also provides threats, globally. Soil erosion, soil organic carbon a valuable lever for climate regulation and a (SOC) change and nutrient imbalance were pathway for safeguarding ecosystem services” considered the most severe threats to soil; this (FAO, 2015). The SWSR addresses soil quality was followed by soil salinization and sodium status in relation to food security, fresh water affected soils (sodic soils), soil sealing and land availability, climate regulation, human health, take, loss of soil biodiversity, soil contamination, and biodiversity. acidification, compaction and waterlogging. Besides providing an overview on the status The terms land and soil are often incorrectly of soils at the global and regional level, the used as synonyms. Land is the solid surface report also contains technical recommendations of the earth that is not permanently under and suggested actions for the future. It is the water, and that supports agriculture, urban purpose of this chapter to discuss current trends living, habitats, and other uses. Soil is the regarding threats to global soils, and to debate unconsolidated material on the land surface that the available future scenarios. Additionally, the has been subjected to soil forming processes, chapter purports to be a platform from which to and that supports many ecosystem services to promote the development of future scenarios the benefit of society. Land use changes can relative to threats which still remain overlooked. influence soil, and soil supports many land uses. Soils play a critical role in delivering ecosystem 2. SOIL EROSION Soil erosion is broadly defined as the accelerated removal services. Healthy soils are a basic prerequisite of topsoil from the land surface through water, wind or to meeting varied needs for food, biomass tillage. Water erosion on agricultural land occurs mainly (energy), fiber, fodder, and other products, and when overland flow entrains soil particles detached by drop to ensuring the provision of essential ecosystem impact or runoff, often leading to clearly defined channels such as rills or gullies. Wind erosion occurs when dry, services in all regions of the world. However, soil loose, bare soil is subjected to strong winds. During wind resources are facing unprecedented pressures erosion events, larger particles creep along the ground or today, many of which are human-induced. Over saltate (bounce) across the surface until they are deposited the past three decades, there have been marked relatively close to field boundaries. Tillage erosion is the developments in our broader understanding direct down-slope movement of soil by tillage implements where particles only redistribute within a field. of humankind’s impacts on the earth, and of the frameworks with which to assess these 2.1 Status of soil erosion impacts. Specific soil processes are central to Over the last decade, the figures published for water earth-system processes that provide the safe erosion are of an order of magnitude ranging from ca. -1 -1 operating space for humanity – the concept 20 Gt (gigaton) yr to over 200 Gt yr . While this huge variation may at first seem to suggest that available of 'planetary boundaries' that cannot be estimates of global soil erosion are very uncertain, a more exceeded without causing potentially disastrous detailed analysis shows that estimates exceeding ca. consequences for humanity (Rockström et al., 50 Gt yr-1 are not realistic. The most likely range of global 2009; Steffen et al., 2015). The ITPS presented soil erosion by water is 20–30 Gt yr-1, while tillage erosion -1 a revised World Soil Charter, stating “Soils may amount to ca. 5 Gt yr . Total erosion rates for wind erosion are highly uncertain. are fundamental to life on Earth but human UNCCD | Global Land Outlook Working Paper | THREATS TO SOILS 3 Estimates of the total amount of dust that is yearly much higher than known soil formation rates, which are mobilized on land place an upper limit on dust mobilization typically well below 1 tonne ha-1 yr-1 with median values of by wind erosion on arable land at ca. 2 Gt yr1. Approximately ca. 0.15 tonnes ha-1 yr-1. Soil erosion has direct, negative 430 million ha of drylands, which comprise 40% of the effects for global agriculture. Soil erosion by water induces Earth’s surface (Ravi et al., 2011), are susceptible to wind annual global losses of 23-42 Mt (megaton) of nitrogen erosion (Middleton and Thomas, 1997). (N) and 14.6-26.4 Mt of phosphorus (P) to run off from agricultural land. These fluxes may be compared to annual Translating these global estimates into accurate local soil global fertilizer applications, which are of ca. 112 Tg erosion rates is not straightforward, since soil erosion (teragram) for N and ca. 18 Tg for P. These nutrient losses is highly variable, in both space and time. However, need to be replaced through fertilization at a significant typical soil erosion rates by water can be defined for economic cost. Using a United States farm price of ca. representative agro-ecological conditions. Hilly croplands USD 1.45 per kg of N and ca. USD 5.26 per kg of P implies under conventional agriculture and orchards in temperate an annual global economic cost of USD 33-60 billion for climate zones are subject to erosion rates of up to N, and USD 77-140 billion for P. It is therefore clear that 10-20 tonnes ha-1 yr-1, while average rates are often compensation for erosion-induced nutrient losses requires <10 tonnes ha-1 yr-1. Values during high-intensity rainfall a massive investment in fertilizer use. In poor regions such events may reach 100 tonnes ha-1 and lead to muddy as Sub-Saharan Africa, the economic resources needed flooding in downstream areas. Erosion rates on hilly to achieve such compensations for nutrient losses do croplands in tropical and subtropical areas may reach not exist. As a consequence, the removal of nutrients values of up to 50-100 tonnes ha-1 yr-1. Average rates, by erosion from agricultural fields might be much higher however, are lower and often around the benchmark than the amount of fertilizer applied; this also varies in of 10-20 tonnes ha-1 yr-1. The high rates found in such function of the erosion rates of given farming systems, hilly cropland tropical and subtropical areas occur due since the eroded soil often contains, per unit weight, about to the combination of an erosive climate (high intensity three times more nutrients than that which is left in the rainfall) and slope gradients, which are generally steeper remaining soil (Young, 1989). than those on cultivated land in temperate zones. The incidence of erosion on steep slopes is due not only to Soil erosion does not induce an important carbon (C) specific topographic conditions, but also to high population loss from the soil to the atmosphere; instead, erosion pressure combined with low-intensity agriculture, leading mostly induces a transfer of C from eroding locations to to the cultivation of marginal steep lands (Figure 1). depositional locations. Net losses are limited, since the C lost at eroding locations is partially replaced through Rangelands and pasturelands in hilly tropical and sub- dynamic replacement, whereas the soil C that is deposited tropical areas may suffer erosion rates similar to those of in colluvial and alluvial settings may be stored there for tropical croplands, especially when there is overgrazing.
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