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Understanding the effects of long-term crop management on soil organic matter (SOM) is necessary to improve the soil quality and sustainability of agroecos

<p>Drought due to climate change will be more frequent and severe and expected to cause a loss of agricultural productivity in the next 30 years. Drought affects soil water availability, leading to several effects: reduction of crop growth, increases shoot/root ratio, decreases the uptake of nutrients by roots and translocation from roots to shoots, and availability of nutrients (N, P, S, Ca, Mg) in soil. The plant community regulates the composition and quantity of rhizodeposition, however, due to the impact of drought on plants functioning, allocation and fate of assimilated carbon (C) in the plant-soil-microorganisms system is changed.  Especially strong effects of drought on the functioning of soil microbial communities are seen in the crop monocultures, whereas the introduction of intercropping, especially with legumes, can potentially mitigate the drought impact. Thus, the present study aimed i) to trace the fate of rhizodeposits in the maize-soybean intercropping and compare with that in monoculture under drought vs optimal moisture conditions, ii) to reveal the differences in the rhizodeposits-C utilization by microorganisms under pulse water regime (wet to drought vs drought to rewetting).</p><p>The greenhouse experiment was conducted in June-August 2021. The soils were taken from the 10-old abandoned agricultural land in Witzenhausen, sieved through 4 mm, and 3 kg were placed into the incubation pots (15x15x20 cm). Maize (<em>Zea mays L.</em>) and soybean (<em>Glycine max (L.) Merr.</em>) were grown as monoculture (4 plants per pot) or intercropping (2+2 plants) until the vegetative stage (6-8 leaves), soil moisture was kept at 60% of WHC. Further, plants were divided into four treatments (each had four replicates), and each was labeled with K<sup>15</sup>NO<sub>3</sub> and followed by <sup>13</sup>C-CO<sub>2</sub> (one day later). The treatments were: i) constant wet (60% of WHC), ii) constant drought (30% of WHC), iii) wet to drought (labeling was done under 60% of WHC and soil dried till 30% of WHC and further kept for 14d), and iv) drought to wet (labeling was done under 30% of WHC and soil was slowly rewetted to 60% of WHC and further kept for 14d). Subsamples of soils and plants aboveground biomass were done 4 times: i) 1 day after <sup>13</sup>C-labeling, ii) after soil reached drought (30% of WHC) or 60% of WHC after rewetting, iii) in the middle of a drought or rewetting stage and iv) at the end of the experiment. Constant moisture and unlabelled treatments were sampled at the same time points; total and <sup>13</sup>C/<sup>15</sup>N were analyzed in plant biomass, dissolved organic matter (DOM), microbial biomass, and soil; NO<sub>3</sub><sup>-</sup> and NH<sub>4</sub><sup>+</sup> contents were measured in DOM extracts; additionally, activities of C, N, and phosphorus acquisition enzymes were measured. Thus, results from this experiment will shed light on the fate of rhizodeposits depending on the drought conditions and pulse water regime. The comparison of monoculture with intercropping will reveal how the presence of soybean can improve the nutrition of maize regarding N uptake, and whether microorganisms can be less affected by drought under intercropping.  </p>

Inputs of available organic materials into soil alter the decomposition of soil organic matter (SOM), a process called priming effect. Organic carbon (C) inputs in terrestrial ecosystems are common from various sources (e.g. rhizodeposits, plant residues, microbial necromass) simultaneously, but their interactions as well as mutual effects on SOM decomposition are unknown because multisource partitioning of pools and fluxes was not available. A dual-isotope approach (identical materials except for straw being possessed two 13C abundances) was adopted to partition total CO2 emission from three C sources: SOM, glucose and straw. Cumulative CO2 efflux was quantified into straw-derived (558 μg C g−1), glucose-derived (480 μg C g−1) and SOM-derived (58 μg C g−1) CO2 during the first 7 days of incubation. Glucose or straw addition induced positive SOM priming, whereas glucose combined with straw resulted in higher SOC loss than that induced by single addition of glucose or straw after day 7. The Spearman's correlation showed that the interactions between glucose and straw shifted from increased CO2 evolved during their intensive decomposition (days 1 to 3) to mutual constraint on mineralization during the late stage (days 5 to 7). This study provides evidences for the suitability of the dual-isotope approach to partition multiple sources of CO2 fluxes and C pools, and evaluates their individual or mutual contributions to SOM priming, thus, implicating C sequestration in terrestrial ecosystems.

No abstract is provided for this article.

Soils are one of the major players in the global carbon (C) cycle and climate change by functioning as a sink or a source of atmospheric carbon dioxide (CO2). The largest terrestrial C reservoir in soils comprises two main pools: organic (SOC) and inorganic C (SIC), each having distinct fates and functions but with a large disparity in global research attention. This study quantified global soil C research trends and the proportional focus on SOC and SIC pools based on a bibliometric analysis. Research on soil C pools started in 1905 and has produced over 42,000 publications (> 1.6 million citations). Although the global C stocks down to 2 m depth are nearly the same for SOC and SIC, the research has dominantly examined SOC (> 96% of publications and citations) with a minimal share on SIC (< 4%). Approximately 39% of the soil C research was focused on climate change. Despite poor coverage and publications, the climate change-related research impact (citations per document) of SIC studies was higher than that of SOC. Machine learning, biochar, soil properties, and climate change were the recent top trend topics for SOC research (2018-2022), whereas soil acidification, organic C, climate change, and Holocene were recent trends for SIC. SOC research was contributed by 150 countries compared to 85 for SIC. As assessed by publications, soil C research was mainly concentrated in a few countries, with only 10 countries accounting for 75% of the research. China and the USA were the major producers (44%), collaborators (36%), and funders of soil C research. SIC is a long-lived soil C pool with a turnover rate of more than 1000 years in natural ecosystems but intensive agricultural practices have accelerated SIC losses, making SIC an important player in global C cycle and climate change. The lack of attention and investment towards SIC research could jeopardize the ongoing efforts to mitigate climate change impacts to meet the 1.5-2.0 oC targets under the Paris Climate Agreement of 2015. This study calls for expanding the research focus on SIC and including SIC fluxes in C budgets and models, without which the representation of the global C cycle is incomplete.

The rhizosphere reflects a sphere of high substrate input by means of rhizodeposits. Active microorganisms and extracellular enzymes are known to be responsible for substrate utilization in soil, especially in rooted soil. We tested for microbial‐ and enzyme activities in arable soil, in order to investigate the effects of continuous input of easily available organics ( e.g. , root‐exudates) to the microbial community. In a field experiment with maize, rooted and root‐free soil were analyzed and rhizosphere processes were linked to microbial activity indicators such as specific microbial growth rates and kinetics of six hydrolytic extracellular enzymes: β‐glucosidase, β‐cellobiohydrolase, β‐xylosidase, acid phosphatase, leucine‐ and tyrosine‐aminopeptidase. Higher potential activities of leucine‐aminopeptidase (2‐fold) for rooted vs. root‐free soil suggested increased costs of enzyme production, which retarded the specific microbial growth rates. Total microbial biomass determined by the substrate‐induced respiration technique and dsDNA extraction method was 23% and 42% higher in the rooted surface‐layer (0–10 cm) compared to the root‐free soil, respectively. For the rooted soil, potential enzyme activities of β‐glucosidase were reduced by 23% and acid phosphatase by 25%, and increased by 300% for β‐cellobiohydrolase at 10–20 cm depth compared to the surface‐layer. The actively growing microbial biomass increased by the 17‐fold in rooted soil in the 10–20 cm layer compared to the upper 10 cm. Despite the specific microbial growth rates showing no changes in the presence of roots, these rates decreased by 42% at 10–20 cm depth compared to the surface‐layer. This suggests the dominance in abundances of highly active but slower growing microbes with depth, reflecting also their slower turnover. Shifts in microbial growth strategy, upregulation of enzyme production and increased microbial respiration indicate strong root effects in maize planted soil.

Crop diversity management is widely used to increase agricultural productivity and sustainability. Recent studies have demonstrated that maize root exudates can drive interspecific facilitation to enhance N 2 fixation of bean in intercropping systems. However, the mechanisms of N 2 fixation enhancement stimulated by root exudates in the intercropping systems remain unclear. Four experiments were designed to provide a progressively deeper understanding of how root exudates stimulate microbial‐mediated N 2 fixation. First, the effects of faba bean/maize intercropping on yields and soil microbial communities were determined in a field experiment. Second, root‐derived interspecific facilitation was evaluated using a root partitioning approach. Third, the key microbial taxa in the faba bean rhizosphere were traced using 13 C‐labelled maize root exudates. Fourth, the co‐driven mechanism of maize root exudates and micro‐organisms in the faba bean rhizosphere were explored. Faba bean/maize intercropping with maize residue return increased the yields of faba bean (26%), maize (27%) and broccoli (9.1%) compared to that under monocropping. Nodulin‐like 4 (NODL4), chalcone‐flavanone isomerase (CFI) and early nodulin‐like (ENODL2) gene expression in faba bean roots intercropped with maize increased by 1.5–2.3‐fold compared to that observed under monoculture. More than half of the N 2 fixation of faba bean increase under intercropping was due to interactions with micro‐organisms. Nine key bacterial genera in the faba bean rhizosphere were identified by 13 C‐DNA‐based stable isotope probing analysis. Among them Agromyces , Arthrobacter , Bacillus , Lysobacter and Paenibacillus directly fix N 2 , while Gemmatimonas , Heliobacillus , Natronocella and Sorangium increase the N 2 fixation capacity of Azotobacter by providing additional carbon sources. These key bacteria triggered by maize root exudates played an important role in the rhizosphere facilitation of intercropping. Synthesis and applications . We demonstrated a novel root–root facilitation of N 2 fixation and increased crop yields co‐driven by root exudates and rhizosphere bacteria under faba bean/maize intercropping, and nine key bacteria associated with this process were identified by 13 C‐DNA‐based stable isotope probing. We recommend the adoption and optimization of intercropping systems with residue return to reduce the shortcomings of continuous cropping and to increase the sustainability of crop production.

Soil acidification influences competitive N uptake between plants and microorganisms. The mechanisms by which soil acidification affects competition betwee