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Soil organisms are a crucial part of the terrestrial biosphere. Despite their importance for ecosystem functioning, few quantitative, spatially explicit models of the active belowground community currently exist. In particular, nematodes are the most abundant animals on Earth, filling all trophic levels in the soil food web. Here we use 6,759 georeferenced samples to generate a mechanistic understanding of the patterns of the global abundance of nematodes in the soil and the composition of their functional groups. The resulting maps show that 4.4 ± 0.64 × 10 20 nematodes (with a total biomass of approximately 0.3 gigatonnes) inhabit surface soils across the world, with higher abundances in sub-Arctic regions (38% of total) than in temperate (24%) or tropical (21%) regions. Regional variations in these global trends also provide insights into local patterns of soil fertility and functioning. These high-resolution models provide the first steps towards representing soil ecological processes in global biogeochemical models and will enable the prediction of elemental cycling under current and future climate scenarios. High-resolution spatial maps of the global abundance of soil nematodes and the composition of functional groups show that soil nematodes are found in higher abundances in sub-Arctic regions, than in temperate or tropical regions.

Partial substitution of chemical fertilizers by organic amendments is essential for improving the soil quality without yield loss. Fungi play an important role in soil quality because they decompose organic matter and cycle nutrients in the soil. However, there is limited information regarding the effect of different organic substitution rates (OSRs) on the soil quality and fungal community. This study investigated the relationship between the soil quality index and fungal community in a tea plantation under different OSRs of N, from a single application of synthetic fertilizer (NPK) to 100% N substitution with organic fertilizer (OM100). The OSRs were positively correlated with the soil physicochemical and biological soil quality index (SQI), but only the physicochemical SQI exhibited a significant relationship with tea production. The OSR also shifted the soil fungal community composition. Soil pH, soil organic C (SOC), microbial biomass C (MBC), and available potassium (AK) were the key characteristics that were significantly correlated with the variation of soil fungal community. Network analysis indicated that additional organic substitution can enhance the soil fungal network complexity, which also showed a positive correlation with the SQI. These results confirmed the feasibility of organic substitution for soil quality improvement, and implied that the soil fungal network complexity could be a new indicator for soil quality assessment.

In agriculture, winter cover crop (WCC) residues are incorporated into the soil to improve soil quality, as gradual litter decomposition can improve fertility. Decomposition rate is determined by litter quality, local soil abiotic and biotic properties. How these factors are interlinked and influenced by cropping history is, however, unclear. We grew WCC monocultures and mixtures in rotation with main crops Avena sativa (oat) and Cichorium endivia (endive) and tested how crop rotation influences WCC litter quality, abiotic and biotic soil conditions, and litter decomposition rates. To disentangle WCC litter quality effects from WCC soil legacy effects on decomposition, we tested how rotation history influences decomposition of standard substrates and explored the underlying mechanisms. In a common environment (e.g. winter fallow plots), WCC decomposition rate constants (k) correlated negatively with litter C, lignin and, surprisingly, N content, due to strong positive correlations among these traits. Plots with a history of fast‐decomposing WCCs exhibited faster decomposition of their own litters as well as of the standard substrates filter paper and rooibos tea, as compared to winter fallow plots. WCC treatments differentially affected soil microbial biomass, as well as soil organic matter and mineral nitrogen content. WCC‐induced soil changes affected decomposition rates. Depending on the main crop rotation treatment, legacy effects were attributed to biomass input of WCCs and their litter quality or changes in microbial biomass. Synthesis and applications. These results demonstrate that decomposition in cropping systems is influenced directly through crop residues, as well as through crop‐induced changes in soil biotic properties. Rotation history influences decomposition, wherein productive winter cover crops (WCC) with low lignin content decompose fast and stimulate the turnover of both own and newly added residues via their knock‐on effect on the soil microbial community. Thus, WCC have promise for sustainable carbon‐ and nutrient‐cycling management through litter feedbacks. Foreign Language In landbouw worden plantenresten van wintergroenbemesters in de bodem ondergewerkt om bodemkwaliteit te verbeteren. De afbraaksnelheid van groenbemesterstrooisel en het daarbij vrijkomen van voedingsstoffen wordt bepaald door strooiselkwaliteit en lokale abiotische en biotische bodemomstandigheden. Echter is het onduidelijk hoe al deze factoren worden beïnvloed door gewasrotatie. In een veldexperiment werden verschillende winterse groenbemesters verbouwd in monoculturen en mengsels, in rotatie met hoofdgewassen Avena sativa (haver) en Cichorium endivia (andijvie). De invloed van gewasrotatie op de kwaliteit van groenbemesterstrooisel, lokale abiotische en biotische bodemparameters en strooiselafbraaksnelheid werd getest. Om bovendien onderscheid te kunnen maken tussen effecten van strooiselkwaliteit en veranderingen in bodemomstandigheden, werden effecten van gewasrotatie ook getoetst op de afbraak van standaard substraten (filterpapier en rooibosthee). In eenzelfde omgeving, zonder specifieke groenbemestergeschiedenis (voormalig winter braak), waren afbraaksnelheden van groenbemesterstrooisels negatief gecorreleerd met concentraties van koolstof‐ (C), lignine‐, en tegen de verwachtingen in, stikstof‐ (N) in het strooisel. Dit resultaat werd verklaard door de onderlinge positieve correlaties tussen N, C en lignine. In vergelijking met voormalig winter braakvelden, toonden proefvelden met een geschiedenis van snel afbrekende groenbemesters een snellere decompositie van zowel eigen strooisels als ook van de standaard substraten. De verschillende erfeniseffecten op afbraaksnelheid konden worden gerelateerd aan de effecten van de groenbemesters op de bodem microbiële biomassa, bodem organische stof en minerale stikstofgehalten. Afhankelijk van het hoofdgewas, konden de erfeniseffecten worden toegeschreven aan de hoeveelheid plantenresten van de groenbemesters, de kwaliteit hiervan, alsmede aan veranderingen in de microbiële biomassa, maar niet aan veranderde abiotische bodemfactoren. Synthese en toepassing. Deze resultaten tonen aan dat decompositie in landbouwsystemen direct wordt beïnvloed door gewasresten en de bodem erfeniseffecten op biotische bodemomstandigheden. De volgorde van gewassen beïnvloedt afbraak, waarbij productieve groenbemesters met snel afbrekende strooisels de decompositie van nieuw materiaal stimuleren via de microbiële bodemgemeenschap en vrijgekomen stikstof. Winterse groenbemesters zijn daarom veelbelovende middelen om C‐ en N‐kringlopen duurzaam te beheren door middel van strooisel‐feedback. These results demonstrate that decomposition in cropping systems is influenced directly through crop residues, as well as through crop‐induced changes in soil biotic properties. Rotation history influences decomposition, wherein productive winter cover crops (WCC) with low lignin content decompose fast and stimulate the turnover of both own and newly added residues via their knock‐on effect on the soil microbial community. Thus, WCC have promise for sustainable carbon‐ and nutrient‐cycling management through litter feedbacks.

Population growth and climate change challenge our food and farming systems and provide arguments for an increased intensification of agriculture. A promising option is eco-functional intensification through organic farming, an approach based on using and enhancing internal natural resources and processes to secure and improve agricultural productivity, while minimizing negative environmental impacts. In this concept an active soil microbiota plays an important role for various soil based ecosystem services such as nutrient cycling, erosion control and pest and disease regulation. Several studies have reported a positive effect of organic farming on soil health and quality including microbial community traits. However, so far no systematic quantification of whether organic farming systems comprise larger and more active soil microbial communities compared to conventional farming systems was performed on a global scale. Therefore, we conducted a meta-analysis on current literature to quantify possible differences in key indicators for soil microbial abundance and activity in organic and conventional cropping systems. All together we integrated data from 56 mainly peer-reviewed papers into our analysis, including 149 pairwise comparisons originating from different climatic zones and experimental duration ranging from 3 to more than 100 years. Overall, we found that organic systems had 32% to 84% greater microbial biomass carbon, microbial biomass nitrogen, total phospholipid fatty-acids, and dehydrogenase, urease and protease activities than conventional systems. Exclusively the metabolic quotient as an indicator for stresses on microbial communities remained unaffected by the farming systems. Categorical subgroup analysis revealed that crop rotation, the inclusion of legumes in the crop rotation and organic inputs are important farming practices affecting soil microbial community size and activity. Furthermore, we show that differences in microbial size and activity between organic and conventional farming systems vary as a function of land use (arable, orchards, and grassland), plant life cycle (annual and perennial) and climatic zone. In summary, this study shows that overall organic farming enhances total microbial abundance and activity in agricultural soils on a global scale.

Global crop yields are limited by water and nutrient availability. Soil mulching (with plastic or straw) reduces evaporation, modifies soil temperature and thereby affects crop yields. Reported effects of mulching are sometimes contradictory, likely due to differences in climatic conditions, soil characteristics, crop species and also water and nitrogen (N) input levels. Here we report on a meta-analysis of the effects of mulching on wheat and maize, using 1310 yield observations from 74 studies conducted in 19 countries. Our results indicate that mulching significantly increased yields, WUE (yield per unit water) and NUE (yield per unit N) by up to 60%, compared with no-mulching. Effects were larger for maize than wheat and larger for plastic mulching than straw mulching. Interestingly, plastic mulching performed better at relatively low temperature while straw mulching showed the opposite trend. Effects of mulching also tended to decrease with increasing water input. Mulching effects were not related to soil organic matter content. In conclusion, soil mulching can significantly increase maize and wheat yields, WUE and NUE and thereby may contribute to closing the yield gap between attainable and actual yields, especially in dryland and low nutrient input agriculture. The management of soil mulching requires site-specific knowledge.

Plants release a wide set of secondary metabolites including volatile organic compounds (VOCs). Many of those compounds are considered to function as defense against herbivory, pests, and pathogens. However, little knowledge exists about the role of belowground plant VOCs for attracting beneficial soil microorganisms. We developed an olfactometer system to test the attraction of soil bacteria by VOCs emitted by Carex arenaria roots. Moreover, we tested whether infection of C. arenaria with the fungal pathogen Fusarium culmorum modifies the VOCs profile and bacterial attraction. The results revealed that migration of distant bacteria in soil towards roots can be stimulated by plant VOCs. Upon fungal infection, the blend of root VOCs changed and specific bacteria with antifungal properties were attracted. Tests with various pure VOCs indicated that those compounds can diffuse over long distance but with different diffusion abilities. Overall, this work highlights the importance of plant VOCs in belowground long-distance plant–microbe interactions.

Soil organisms have an important role in aboveground community dynamics and ecosystem functioning in terrestrial ecosystems. However, most studies have considered soil biota as a black box or focussed on specific groups, whereas little is known about entire soil networks. Here we show that during the course of nature restoration on abandoned arable land a compositional shift in soil biota, preceded by tightening of the belowground networks, corresponds with enhanced efficiency of carbon uptake. In mid- and long-term abandoned field soil, carbon uptake by fungi increases without an increase in fungal biomass or shift in bacterial-to-fungal ratio. The implication of our findings is that during nature restoration the efficiency of nutrient cycling and carbon uptake can increase by a shift in fungal composition and/or fungal activity. Therefore, we propose that relationships between soil food web structure and carbon cycling in soils need to be reconsidered.

Microorganisms associated with roots are thought to be part of the so-called extended plant phenotypes with roles in the acquisition of nutrients, production of growth hormones, and defense against diseases. Since the crops selectively enrich most rhizosphere microbes out of the bulk soil, we hypothesized that changes in the composition of bulk soil communities caused by agricultural management affect the extended plant phenotype. In the current study, we performed shotgun metagenome sequencing of the rhizosphere microbiome of the peanut ( Arachis hypogaea ) and metatranscriptome analysis of the roots of peanut plants grown in the soil with different management histories, peanut monocropping and crop rotation. We found that the past planting record had a significant effect on the assembly of the microbial community in the peanut rhizosphere, indicating a soil memory effect. Monocropping resulted in a reduction of the rhizosphere microbial diversity, an enrichment of several rare species, and a reduced representation of traits related to plant performance, such as nutrients metabolism and phytohormone biosynthesis. Furthermore, peanut plants in monocropped soil exhibited a significant reduction in growth coinciding with a down-regulation of genes related to hormone production, mainly auxin and cytokinin, and up-regulation of genes related to the abscisic acid, salicylic acid, jasmonic acid, and ethylene pathways. These findings suggest that land use history affects crop rhizosphere microbiomes and plant physiology.