Where, when and how plant–soil feedback matters in a changing world

It is increasingly acknowledged that plant–soil feedbacks may play an important role in driving the composition of plant communities and functioning of terrestrial ecosystems. However, the mechanistic understanding of plant–soil feedbacks, as well as their roles in natural ecosystems in proportion to other possible drivers, is still in its infancy. Such knowledge will enhance our capacity to determine the contribution of plant–soil feedback to community and ecosystem responses under global environmental change. Here, we review how plant–soil feedbacks may develop under extreme drought and precipitation events, CO2 and nitrogen enrichment, temperature increase, land use change and plant species loss vs. gain. We present a framework for opening the ‘black box of soil’ considering the responses of the various biotic components (enemies, symbionts and decomposers) of plant–soil feedback to the global environmental changes, and we discuss how to integrate these components to understand and predict the net effects of plant–soil feedbacks under the various scenarios of change. To gain an understanding of how plant–soil feedback plays out in realistic settings, we also use the framework to discuss its interaction with other drivers of plant community composition, including competition, facilitation, herbivory, and soil physical and chemical properties. We conclude that understanding the role that plant–soil feedback plays in shaping the responses of plant community composition and ecosystem processes to global environmental changes requires unravelling the individual contributions of enemies, symbionts and decomposers. These biotic factors may show different response rates and strengths, thereby resulting in different net magnitudes and directions of plant–soil feedbacks under various scenarios of global change. We also need tests of plant–soil feedback under more realistic conditions to determine its contribution to changes in patterns and processes in the field, both at ecologically and evolutionary relevant time-scales

Transient negative biochar effects on plant growth are strongest after microbial species loss

Biochar has been explored as an organic amendment to improve soil quality and benefit plant growth. The overall positive effects of biochar on crop yields are generally attributed to abiotic changes, while the alternative causal pathway via changes in soil biota is unexplored. We compared plant growth effects of legumes in sterile soil inoculated with dilutions of soil and soil microbial suspensions to determine the direct effects of biochar-induced changes in soil biota on plant growth. Suspensions and soil were from soil amended with biochar and soil without biochar. By comparing consecutive plant growth phases on the same inoculated soils, we also determined the temporal effects of soil biota from biochar-amended and control soils. Biota from biochar-amended soil was less beneficial for Medicago sativa growth, especially with small amounts of inocula. Flowering was delayed in the presence of biota from biochar plots. Inoculum with either soil or soil suspension gave similar results for plant biomass, indicating that microorganisms play a major role. Vicia villosa growth did not respond to the various inocula, even though the inoculum quantity strongly affected nematode community composition and protozoan abundance. In a later growing phase the negative effect of biochar-associated biota on Medicago growth mostly disappeared, which leads to the conclusion that the benefits of biochar application via abiotic changes may outweigh the negative effects of biochar on soil biota.</p

Transient negative biochar effects on plant growth are strongest after microbial species loss

<p>Biochar has been explored as an organic amendment to improve soil quality and benefit plant growth. The overall positive effects of biochar on crop yields are generally attributed to abiotic changes, while the alternative causal pathway via changes in soil biota is unexplored. We compared plant growth effects of legumes in sterile soil inoculated with dilutions of soil and soil microbial suspensions to determine the direct effects of biochar-induced changes in soil biota on plant growth. Suspensions and soil were from soil amended with biochar and soil without biochar. By comparing consecutive plant growth phases on the same inoculated soils, we also determined the temporal effects of soil biota from biochar-amended and control soils. Biota from biochar-amended soil was less beneficial for Medicago sativa growth, especially with small amounts of inocula. Flowering was delayed in the presence of biota from biochar plots. Inoculum with either soil or soil suspension gave similar results for plant biomass, indicating that microorganisms play a major role. Vicia villosa growth did not respond to the various inocula, even though the inoculum quantity strongly affected nematode community composition and protozoan abundance. In a later growing phase the negative effect of biochar-associated biota on Medicago growth mostly disappeared, which leads to the conclusion that the benefits of biochar application via abiotic changes may outweigh the negative effects of biochar on soil biota.</p

The way forward in biochar research: targeting trade‐offs between the potential wins

Biochar application to soil is currently widely advocated for a variety of reasons related to sustainability. Typically, soil amelioration with biochar is presented as a multiple-‘win’ strategy, although it is also associated with potential risks such as environmental contamination. The most often claimed benefits of biochar (i.e. the ‘wins’) include (i) carbon sequestration; (ii) soil fertility enhancement; (iii) biofuel/bioenergy production; (iv) pollutant immobilization; and (v) waste disposal. However, the vast majority of studies ignore possible trade-offs between them. For example, there is an obvious trade-off between maximizing biofuel production and maximizing biochar production. Also, relatively little attention has been paid to mechanisms, as opposed to systems impacts, behind observed biochar effects, often leaving open the question as to whether they reflect truly unique properties of biochar as opposed to being simply the short-term consequences of a fertilization or liming effect. Here, we provide an outline for the future of soil biochar research. We first identify possible trade-offs between the potential benefits. Second, to be able to better understand and quantify these trade-offs, we propose guidelines for robust experimental design and selection of appropriate controls that allow both mechanistic and systems assessment of biochar effects and trade-offs between the wins. Third, we offer a conceptual framework to guide future experiments and suggest guidelines for the standardized reporting of biochar experiments to allow effective between-site comparisons to quantify trade-offs. Such a mechanistic and systems framework is required to allow effective comparisons between experiments, across scales and locations, to guide policy and recommendations concerning biochar application to soil