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To meet the challenge of feeding a growing world population with minimal environmental impact, we need comprehensive and quantitative knowledge of ecological factors affecting crop production. Earthworms are among the most important soil dwelling invertebrates. Their activity affects both biotic and abiotic soil properties, in turn affecting plant growth. Yet, studies on the effect of earthworm presence on crop yields have not been quantitatively synthesized. Here we show, using meta-analysis, that on average earthworm presence in agroecosystems leads to a 25% increase in crop yield and a 23% increase in aboveground biomass. The magnitude of these effects depends on presence of crop residue, earthworm density and type and rate of fertilization. The positive effects of earthworms become larger when more residue is returned to the soil, but disappear when soil nitrogen availability is high. This suggests that earthworms stimulate plant growth predominantly through releasing nitrogen locked away in residue and soil organic matter. Our results therefore imply that earthworms are of crucial importance to decrease the yield gap of farmers who can't -or won't- use nitrogen fertilizer.

Abstract. The study of soil N cycling processes has been, is, and will be at the centre of attention in soil science research. The importance of N as a nutrient for all biota; the ever-increasing rates of its anthropogenic input in terrestrial (agro)ecosystems; its resultant losses to the environment; and the complexity of the biological, physical, and chemical factors that regulate N cycling processes all contribute to the necessity of further understanding, measuring, and altering the soil N cycle. Here, we review important insights with respect to the soil N cycle that have been made over the last decade, and present a personal view on the key challenges of future research. We identify three key challenges with respect to basic N cycling processes producing gaseous emissions:1. quantifying the importance of nitrifier denitrification and its main controlling factors; 2. characterizing the greenhouse gas mitigation potential and microbiological basis for N 2 O consumption; 3. characterizing hotspots and hot moments of denitrification Furthermore, we identified a key challenge with respect to modelling: 1. disentangling gross N transformation rates using advanced 15 N / 18 O tracing models Finally, we propose four key challenges related to how ecological interactions control N cycling processes:1. linking functional diversity of soil fauna to N cycling processes beyond mineralization; 2. determining the functional relationship between root traits and soil N cycling; 3. characterizing the control that different types of mycorrhizal symbioses exert on N cycling; 4. quantifying the contribution of non-symbiotic pathways to total N fixation fluxes in natural systemsWe postulate that addressing these challenges will constitute a comprehensive research agenda with respect to the N cycle for the next decade. Such an agenda would help us to meet future challenges on food and energy security, biodiversity conservation, water and air quality, and climate stability.

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Intensive agriculture is dominated by monocultures of high‐yielding plants that receive large applications of nitrogen (N) fertilizers to boost plant productivity. However, these systems have low N use efficiency (NUE) as fertilized plants generally take up less than half of the N applied. A large fraction of the remainder N is susceptible to be lost from the agroecosystem generating a cascade of environmental and socio‐economic problems. Climate change and projected global increases in fertilizer use pose further risks to N losses and yield stability. We review and translate concepts from ecology in natural systems to demonstrate that NUE in intensive agroecosystems can be strongly increased by fine‐tuning the traits of the plant communities to the levels of N fertilization intensity. We present key plant traits of importance for N‐cycling (architectural, morphological and physiological traits, as well as symbiotic associations and exudation patterns); discuss ecological (with soil fauna and N‐cycling microbial communities) and agronomic interactions of this approach; propose interdisciplinary methodologies for future research ranging from pot to global scales; and highlight possible solutions leading to an optimal balance between N fertilizer use and productivity. Synthesis and applications. By showing the strong links between plant traits and nitrogen (N) cycling, our work opens possibilities to test ecologically informed hypotheses across gradients of soil fertility and N fertilizer management intensity, setting a research agenda for the coming years. Accordingly, the choice of plant species based on their functional traits will play a central role for the development of modern and productive agroecosystems that retain and use N more efficiently.