Soil Quality - a critical review

Sampling and analysis or visual examination of soil to assess its status and use potential is widely practiced from plot to national scales. However, the choice of relevant soil attributes and interpretation of measurements are not straightforward, because of the complexity and site-specificity of soils, legacy effects of previous land use, and trade-offs between ecosystem services. Here we review soil quality and related concepts, in terms of definition, assessment approaches, and indicator selection and interpretation. We identify the most frequently used soil quality indicators under agricultural land use. We find that explicit evaluation of soil quality with respect to specific soil threats, soil functions and ecosystem services has rarely been implemented, and few approaches provide clear interpretation schemes of measured indicator values. This limits their adoption by land managers as well as policy. We also consider novel indicators that address currently neglected though important soil properties and processes, and we list the crucial steps in the development of a soil quality assessment procedure that is scientifically sound and supports management and policy decisions that account for the multi-functionality of soil. This requires the involvement of the pertinent actors, stakeholders and end-users to a much larger degree than practiced to date

Editorial overview: Sustainable intensification to feed the world: concepts, technologies and trade-offs

Agriculture puts an enormous pressure on available resources. This is likely to increase further due to the growing human population, increasing per capita consumption, and changes in diets. Agriculture itself is very sensitive to degradation of the resource base. It is mandatory that this pressure stays within the carrying capacity of Planet Earth and within specific sustainability thresholds. Sustainability thresholds are diverse and often normative. They can seldom all be realized to the full extent at the same time, and therefore trade-offs are unavoidable. Choices have to be made in the face of these trade-offs and often criteria upon which such choices are based not only depend on scientific or practical considerations, but also on norms and moral values. There is little consensus on how to make those choices nor is there consensus on the norms and moral values. This applies when local solutions are sought to realize ‘mindful agriculture’, but is further complicated when considering the global nature of agriculture and food production. There is certainly not enough consensus on how to realize sustainable intensification at the global scale as this requires trade-offs across borders. Both intensification and sustainability can be defined and interpreted in various ways. Because of the ambiguity of both concepts detailed contextualization is needed. Combining intensification and sustainability into sustainable intensification makes for an even more ambiguous and hence contested concept

Epilogue: global food security, rhetoric, and the sustainable intensification debate

The need to feed nine billion people in 2050 has given rise to widespread debate in science and policy circles. The debate is largely framed in neo-Malthusian terms, and elements of global food security (resilience of the food system, food quantity and quality, right to and access to food) demand equal attention. High-intensive agriculture, which enabled population growth and food for a large proportion of the global population, is often regarded as incompatible with current environmental (and social) sustainability. Because of the often problematic nature of high-intensive industrialized agriculture, sustainable agricultural intensification has been called an oxymoron. Pathways to sustainably intensify agriculture vary from business-as-usual to claims that a radical rethinking of our agricultural production is imperative. Three terms have been coined to differentiate such pathways. Whereas conventional intensification, that is business-as-usual, is uncontroversial (but often considered unlikely to be able to achieve environmental sustainability), the phrases sustainable intensification and ecological intensification both have a complex history. Although one could think that they have similar meanings, the phrases represent very different perspectives in discourses in science and policy circles. The terms Utopians and Arcadians are introduced for adherents of those perspectives. We observe that they both devote insufficient attention to inevitable trade-offs. Agricultural intensification in developing countries was greatly accelerated by the Green Revolution, which largely bypassed sub-Saharan Africa. Discontent with that outcome has led to a plethora of new terms to indicate more successful next steps for sub-Saharan agriculture. Industrialized agriculture as currently practised in developed countries will not provide a universal solution. This epilogue of the special issue and the literature herein show that intense debates on sustainable agricultural intensification are needed. Such debates on intensification demand reflection on the role of scientists with regard to their uses of current and the generation of novel knowledge

Soil biodiversity and nature-mimicry in agriculture; the power of metaphor?

Attention to soil biodiversity and its importance for sustainable food production has markedly increased in recent years. In particular, the loss of soil biodiversity as a consequence of intensive agriculture, land degradation and climate change has raised concerns due to the expected negative impacts on ecosystem services, food security and human health. The result is a strong demand for ‘nature-based’ practices that stimulate soil biodiversity or beneficial soil organisms and enhance soil health. Here, we examine the origin of popular ideas on the role of soil biology in sustainable soil management, as well as their potential to address key global challenges related to agriculture. Three examples of such ideas are discussed: 1) a higher fungal:bacterial (F:B) biomass ratio favours soil carbon storage and nutrient conservation; (2) intensive agricultural practices lead to a decline in soil biodiversity with detrimental consequences for sustainable food production; (3) inoculation with arbuscular mycorrhizal fungi reduces agriculture's dependency on synthetic fertilizers. Our analysis demonstrates how ecological theories, especially E.P. Odum's ( 1969) hypotheses on ecological succession, have inspired the promotion of agricultural practices and commercial products that are based on the mimicry of (soil biology in) natural ecosystems. Yet our reading of the scientific literature shows that popular claims on the importance of high F:B ratios, soil biodiversity and the inoculation with beneficial microbes for soil health and sustainable agricultural production cannot be generalized and require careful consideration of limitations and possible trade-offs. We argue that dichotomies and pitfalls associated with the normative use of nature as a metaphor for sustainability can be counterproductive given the urgency to achieve real solutions that sustain food production and natural resources. Finally, implications for soil ecology research and sustainable soil management in agriculture are discussed

Eco-functionality of organic matter in soils

BackgroundSoil organic matter (SOM) supports multiple soil ecosystem functions, underpinned by processes such as C sequestration, N mineralization, aggregation, promotion of plant health and compound retention. We know little about the relationship between these functions and SOM quality.ScopeWe aimed to develop “eco-functionality” as a framework to address questions on the relation between SOM properties and soil ecosystem functions.ConclusionsParadigm shifts in SOM research have not led to metrics for eco-functionality beyond decomposability and C:N ratio. Recalcitrant OM is under-researched despite its essential role in aggregation and C sequestration, especially in C-saturated soils. Most soil functions are dependent on SOM decomposition and require labile compounds. We conclude that eco-functionality is context-dependent and needs to take time scales into account. We plea for attempts to link operationally defined SOM fractions to functions in order to make SOM research more applicable

Loss of secondary-forest resilience by land-use intensification in the Amazon

Understanding how land-use intensification affects forest resilience is a key for elucidating the mechanisms underlying regeneration processes and for planning more sustainable land-use systems. Here, we evaluate how the intensification of a swidden cultivation system affects secondary-forest resilience in the Amazon. Along a gradient of land-use intensity, we analysed the relative role of management intensity, soil properties and landscape configuration in determining the resilience of early secondary forests (SFs). We assessed resilience as the recovery level of forest structure and species diversity achieved by SFs 5 years after abandonment. We used as a reference the recovery level achieved by SFs subjected to the lowest intensity of use, given that these SFs are part of a dynamic system and may not develop to old-growth forests. Therefore, we interpreted a deviation from this reference level as a change in forest resilience. The recovery of forest structure was determined by management intensity, while the recovery of species diversity was driven by landscape configuration. With increasing number of cycles and weeding frequency along with decreasing fallow period and patch area, SF basal area and canopy height decreased, regeneration shifted from a seed- to sprout-dependent strategy, and liana infestation on trees increased. With decreasing area covered by old-growth forest, species richness and Shannon diversity decreased. Secondary-forest resilience decreased with land-use intensification, mainly mediated by the effect of management intensity upon regeneration strategies. Our findings demonstrate the – many times overlooked – importance of previous management intensity in determining the structure of SFs and highlight the importance of regeneration strategy for forest resilience. Synthesis. Swidden cultivation supports people's livelihoods and transforms landscapes in the tropics. The sustainability of this system depends on ecosystem services provided by SFs that develop during the fallow period. Land-use intensification reduces the resilience of SFs and ultimately may drive the system towards an arrested succession state that holds a lower potential to deliver ecosystem services to the Amazonian people. Under an intensification scenario, the adaptation of management practices is needed to guarantee the resilience of swidden cultivation systems

Increased arbuscular mycorrhizal fungal colonization reduces yield loss of rice (Oryza sativa L.) under drought

Drought reduces the availability of soil water and the mobility of nutrients, thereby limiting the growth and productivity of rice. Under drought, arbuscular mycorrhizal fungi (AMF) increase P uptake and sustain rice growth. However, we lack knowledge of how the AMF symbiosis contributes to drought tolerance of rice. In the greenhouse, we investigated mechanisms of AMF symbiosis that confer drought tolerance, such as enhanced nutrient uptake, stomatal conductance, chlorophyll fluorescence, and hormonal balance (abscisic acid (ABA) and indole acetic acid (IAA)). Two greenhouse pot experiments comprised three factors in a full factorial design with two AMF treatments (low- and high-AMF colonization), two water treatments (well-watered and drought), and three rice varieties. Soil water potential was maintained at 0 kPa in the well-watered treatment. In the drought treatment, we reduced soil water potential to − 40 kPa in experiment 1 (Expt 1) and to − 80 kPa in experiment 2 (Expt 2). Drought reduced shoot and root dry biomass and grain yield of rice in both experiments. The reduction of grain yield was less with higher AMF colonization. Plants with higher AMF colonization showed higher leaf P concentrations than plants with lower colonization in Expt 1, but not in Expt 2. Plants with higher AMF colonization exhibited higher stomatal conductance and chlorophyll fluorescence than plants with lower colonization, especially under drought. Drought increased the levels of ABA and IAA, and AMF colonization also resulted in higher levels of IAA. The results suggest both nutrient-driven and plant hormone-driven pathways through which AMF confer drought tolerance to rice