Arable lands under the pressure of multiple land degradation processes. A global perspective

Highlights

• The analysis of the spatial footprint of the multiple forms of land degradation in global arable lands.

• Geostatistical analyses on aridity, soil erosion, vegetation decline, soil salinization and soil organic carbon decline.

• Analysis of the simultaneous pressure of the five major degradation processes in arable systems.

• About 40%/20% of the arable lands' area affected by the unilateral impact of aridity/soil erosion.

• An additional ~7% of global arable lands affected by the synergy of the two land degradation processes.

Abstract

While agricultural systems are a major pillar in global food security, their productivity is currently threatened by many environmental issues triggered by anthropogenic climate change and human activities, such as land degradation. However, the planetary spatial footprint of land degradation processes on arable lands, which can be considered a major component of global agricultural systems, is still insufficiently well understood. This study analyzes the land degradation footprint on global arable lands, using complex geospatial data on certain major degradation processes, i.e. aridity, soil erosion, vegetation decline, soil salinization and soil organic carbon decline. By applying geostatistical techniques that are representative for identifying the incidence of the five land degradation processes in global arable lands, results showed that aridity is by far the largest singular pressure for these agricultural systems, affecting ~40% of the arable lands' area, which cover approximately 14 million km² globally. It was found that soil erosion is another major degradation process, the unilateral impact of which affects ~20% of global arable systems. The results also showed that the two degradation processes simultaneously affect an additional ~7% of global arable lands, which makes this synergy the most common form of multiple pressure of land degradative conditions across the world's arable areas. The absolute statistical data showed that India, the United States, China, Brazil, Argentina, Russia and Australia are the most vulnerable countries in the world to the various pathways of arable land degradation. Also, in terms of percentages, statistical observations showed that African countries are the most heavily affected by arable system degradation. This study's findings can be useful for prioritizing agricultural management actions that can mitigate the negative effects of the two degradation processes or of others that currently affect many arable systems across the planet.

Introduction

Global soils are under the pressure of multiple threats due to population growth, economic development and climate change (Montanarella et al., 2016). Global arable lands currently face a major challenge, which entails a significant productivity increase in order to ensure global food security, in the current times marked by large-scale degradative conditions of lands. As 99% of food is produced on land and only 1% by aquatic systems, land degradation is strongly linked to food security (Pimentel, 2006). It is generally acknowledged that, in order to meet the food demand of over 9 billion people estimated for the year 2050, a remarkable increase is necessary in the global crop production, from 60–70% to 100% in the following decades (Tester and Langridge, 2010; Tilman et al., 2011; Foley et al., 2011; Pardey et al., 2014; Rockström et al., 2017). The effects of anthropogenic climate change are strongly negative for agricultural productivity and crop yields, and the projections of most models are pessimistic (Rosenzweig et al., 2014).

Among the major global environmental issues, population growth will accelerate the expansion of agriculture at the expense of forestlands (through deforestation, a major current perturbation of global forests) (Prăvălie, 2018), with the potential to intensify conflicts between food production and nature conservation (Laurance et al., 2014). In addition to agricultural expansion, an unsustainable agricultural intensification may accelerate land degradation through higher soil erosion rates, decline of organic carbon, heavy metal pollution or nutrient losses (Smith et al., 2016).

Ensuring global food security is however strongly threatened not only by the danger of global climate change, but also by the multiple forms of land degradation that currently affect many productive lands around the world (UNCCD, 2017; Montanarella et al., 2018). For example, global soil erosion – a major pathway of planetary-scale land degradation (Pereira et al., 2020) – has increased by 2.5% between 2001 and 2012 (mainly due to deforestation and cropland expansion), according to some recent estimations based on the RUSLE model (Borrelli et al., 2017). However, soil erosion varies significantly between regions and the soil erosion rates may be almost 100 higher than soil formation, in some cases of tropical areas (Hewawasam et al., 2003; Vanacker et al., 2007). Soil losses in arable lands, which total 17 billion tons globally, may incur a cost of more than 8 × 10⁹ US$ annually in agricultural productivity (Sartori et al., 2019).

Soil has been recognized as the second largest carbon pool after the oceans, and can contribute significantly to anthropogenic climate change mitigation (Lugato et al., 2014; Borrelli et al., 2018). As agricultural soils hold about 600 Gt of carbon in their first 1-m layer below the surface, an increase in carbon stocks of only 0.4% with certain management practices can offset about 30% of global anthropogenic greenhouse emissions (Minasny et al., 2017). However, this carbon pool has been significantly degraded globally, especially due to historical land use/cover changes (Sanderman et al., 2017).

In the future, the negative impact of land degradation on human society is likely to increase. For instance, it is estimated that between 50 and 700 million people could be forced to migrate by 2050 due to synergistic effects of land degradation and climate change (UNCCD, 2019). Also, the economic consequences of land degradation, which are already estimated to reach annual costs that range between 18 × 10⁹ US$ and the astonishing threshold of 20 × 10¹² US$, will be dramatic (UNCCD, 2019).

In order to tackle this major environmental threat, it is vital to globally implement the Sustainable Development Goals (SDGs), target 15.3 on Land Degradation Neutrality (LDN), which is an initiative of the United Nations Convention to Combat Desertification (UNCCD) that is already playing an important role both in maintaining the stability of productive lands, and in accelerating other SDGs, such as poverty eradication (SDG 1), food security (SDG 2) or climate change (SDG 13) (UNDP, 2017). However, for a more effective implementation of LDN, new knowledge and additional data on global land degradation patterns are needed. Although there are some previous studies addressing specific land degradation processes across the planet (e.g. Oldeman et al., 1990; Bai et al., 2008; Cherlet et al., 2018), there is a lack of comprehensive analyses focusing on the spatial pattern of multiple processes in global arable systems.

According to the identified research and knowledge gap, this study analyzes the incidence of land degradation pathways in global arable lands, starting from the premise that the presence of this disruptive environmental process in arable areas, with its various forms of manifestation, causes a harmful impact on agricultural systems. In this respect, five degradation processes (aridity, vegetation decline, soil erosion, soil salinization and soil organic carbon decline), which we consider to have a major influence on land productivity (considering five criteria presented in the methodology section), were selected and analyzed spatially strictly within the world's arable boundaries. Therefore, the research questions we raise in this study are: 1) What is the incidence of the main degradation processes in global arable lands, in terms of the number of occurrences within the world's arable land boundaries? 2) What is the incidence of the main degradation processes in global arable lands, in terms of the types of processes present in these anthropogenic systems? 3) What is the spatial footprint of degradation processes occurring in arable systems worldwide?

By answering these questions, this study is, to our knowledge, the first attempt to analyze the multidimensional presence of land degradation processes in global arable lands, using complex geospatial data that were explored globally and nationally. Although there are other global assessments of degradation processes, conducted based on important global databases/reports, such as GLASOD (Oldeman et al., 1990), FAO TerraSTAT (Bot et al., 2000), GLADA (Bai et al., 2008), IPBES (Montanarella et al., 2018) or WAD (Cherlet et al., 2018), they generally tackled land degradation processes individually and without geostatistical analyses focused on global arable lands. Given the complex and recent data explored for detecting the simultaneous presence of various land degradation processes in the world's arable landscapes, we consider this study's approach to be an important novelty of the paper, which can be essential for a deeper understanding of the current multiple threats of global food production systems.