查看更多>>摘要：? 2021 Elsevier LtdSoil phagotrophic protists are highly abundant and play a vital role in nutrient cycling through feeding on microbes. Global change factors, individually or in combination, often affect soil bacteria and fungi, but whether and how the resulting changes may cascade to affect phagotrophic protists remain largely unknown. Combining direct microscopic counting and high-throughput sequencing of 18s rRNA gene, we examined effects of precipitation changes, warming and nitrogen (N) input on soil phagotrophic protists in a 3-yr manipulation experiment with a Tibetan alpine meadow. Precipitation addition (+30%) enhanced but precipitation reduction (?30%) and warming decreased the alpha diversity of phagotrophic protists, primarily through altering soil moisture. However, N input (12 g N m?2 y?1) increased protist abundance, and in particular, offset the negative effect of precipitation reduction on the relative abundance of phagotrophic protists through increasing the microbial biomass, implying a bottom-up trophic control. Together, these findings indicate that interactions of multiple global change drivers may affect soil protist communities directly by modifying the soil physiochemical environment and indirectly through trophic cascading, which have implications for the potential changes in their ecosystem functions in alpine meadow under future global change scenarios.
查看更多>>摘要：? 2021 The AuthorsUnderstanding phosphorus (P) dynamics in the rhizosphere is crucial for sustainable crop production. P mobilization processes in the rhizosphere include the release of plant and microbially-derived protons and extracellular phosphatases. We investigated the effect of root hairs and soil texture on the spatial distribution and intensity of P mobilizing processes in the rhizosphere of Zea mays L. root-hair defective mutant (rth3) and wild-type (WT) grown in two substrates (loam, sand). We applied 2D-chemical imaging methods in custom-designed root windows installed in the field to visualize soil pH (optodes), acid phosphatase activity (zymography), and labile P and Mn fluxes (diffusive gradients in thin films, DGT). The average rhizosphere extent for phosphatase activity and pH was greater in sand than in loam, while the presence of root-hairs had no impact. Acidification was significantly stronger at young root tissue (<2 cm from root cap) than at older root segments (>4 cm from root cap) and stronger in WT than rth3. Accompanied with stronger acidification, higher P flux was observed mainly around young, actively growing root tissues for both genotypes. Our results indicate that acidification was linked to root growth and created a pH optimum for acid phosphatase activity, i.e., mineralization of organic P, especially at young root tissues which are major sites of P uptake. Both genotypes grew better in loam than in sand; however, the presence of root hairs generally resulted in higher shoot P concentrations and greater shoot biomass of WT compared to rth3. We conclude that soil substrate had a larger impact on the extent and intensity of P solubilization processes in the rhizosphere of maize than the presence of root hairs. For the first time, we combined 2D-imaging of soil pH, phosphatase activity, and nutrient gradients in the field and demonstrated a novel approach of stepwise data integration revealing the interplay of various P solubilizing processes in situ.
查看更多>>摘要：? 2021 Elsevier LtdThis paper presents data on the net greenhouse gas balance (measured as CO2 equivalents) of N2O emitted and C fixed in nitrifying pastoral soils over a 10-week laboratory incubation period. In a low nitrogen environment without urine deposition, typical of large areas of grassland, archaeal-dominated nitrification increased soil organic carbon (SOC) to the point that these offset N2O emissions. In a very high N environment typical of urine patch, where bacteria dominated nitrification, N2O emissions were offset by C fixation for 3 weeks but were then overwhelmed by a rapid increase in N2O production. Given the very large area of grasslands globally and the importance of nitrification in grassland soils, our finding provides a basis for re-evaluating the budget for net greenhouse emissions from grassland.
查看更多>>摘要：? 2021Functional genes involved in nitrogen (N) cycling regulate soil nitrification, denitrification and N2O emissions. However, the general patterns and variability of N functional genes in response to N addition, and their association with N2O emission have not been synthesized for terrestrial ecosystems. We synthesized 2068 observations from 144 papers to explore the impact of N addition on the abundance and diversity of N functional genes, and their relationship to N2O emissions in croplands, grasslands and forests on a global scale. In croplands, N addition increased N2O emissions (109%), the abundance of ammonia-oxidizing archaea (AOA) (19%), ammonia-oxidizing bacteria (AOB) (95%), nirK (52%), nirS (40%) and nosZ (41%), and the diversity of AOB (15%), nirS (12%) and nosZ (11%). In grassland, N addition increased AOB abundance (130%) and decreased the abundance of nirS (?99%) and nosZ (?58%) genes, but in forests, significant effects were only found for the abundance of AOA (35%) and AOB (121%). N2O emission was negatively correlated with the abundance of nosZ, but positively correlated with the abundance of AOA and AOB. Apart from the abundance of functional AOA, AOB and nosZ genes, climate variables (precipitation and temperature), and available N concentrations were the main factors explaining the variation in N2O emission with N addition, as shown by random forest analysis. These findings indicate that impacts on N functional genes that encode enzymes involved in nitrification (AOA, AOB) and in the transformation of N2O to N2 (nosZ) are the main mechanisms behind the effect on N fertilizer-induced N2O emissions.
查看更多>>摘要：? 2021 Elsevier LtdHuman-induced global changes may significantly alter the structure and function of terrestrial ecosystems. Although nematodes play a critical role in material cycles and soil food webs, the overall trend and magnitude of changes in nematode responses to global change remain unclear. In this study, we synthesized nematode responses to the major global change factors (i.e., nitrogen (N) deposition, climate warming, elevated CO2, and drought) using data extracted from published global change experiments. We found that nematode and soil micro-food web responses differed among the global change scenarios. Specifically, N addition significantly decreased generic richness (?9%) and the abundance of fungivores (?14%) and omnivore-predators (?26%). Warming had minor effects on soil nematodes. Elevated CO2 significantly increased total nematode abundance (20%), the abundance of fungivores (42%) and herbivores (22%), and the ratio of fungivorous nematodes/bacterivorous nematodes (32%) but decreased nematode generic richness. Drought reduced total nematode abundance (?20%). Soil nematode responses to global change factors were influenced by the experimental system (i.e., ecosystem types and experimental duration) and environmental factors (i.e., mean annual temperature, mean annual precipitation, and latitude). Our synthesis indicates that soil pH, NH4+ content, N-application rate, ecosystem types, and experimental duration may be the main factors explaining the negative effects of N addition on soil nematodes, and that the warming effects may be best explained by soil moisture and ecosystem types. These findings can help to better predict how global change factors affect soil nematodes.
查看更多>>摘要：? 2021 The AuthorsTundra ecosystems hold large stocks of soil organic matter (SOM), likely due to low temperatures limiting rates of microbial SOM decomposition more than those of SOM accumulation from plant primary productivity and microbial necromass inputs. Here we test the hypotheses that distinct tundra vegetation types and their carbon supply to characteristic rhizosphere microbes determine SOM cycling independent of temperature. In the subarctic Scandes, we used a three-way factorial design with paired heath and meadow vegetation at each of two elevations, and with each combination of vegetation type and elevation subjected during one growing season to either ambient light (i.e., ambient plant productivity), or 95% shading (i.e., reduced plant productivity). We assessed potential above- and belowground ecosystem linkages by uni- and multivariate analyses of variance, and structural equation modelling. We observed direct coupling between tundra vegetation type and microbial community composition and function, which underpinned the ecosystem's potential for SOM storage. Greater primary productivity at low elevation and ambient light supported higher microbial biomass and nitrogen immobilisation, with lower microbial mass-specific enzymatic activity and SOM humification. Congruently, larger SOM at lower elevation and in heath sustained fungal-dominated microbial communities, which were less substrate-limited, and invested less into enzymatic SOM mineralisation, owing to a greater carbon-use efficiency (CUE). Our results highlight the importance of tundra plant community characteristics (i.e., productivity and vegetation type), via their effects on soil microbial community size, structure and physiology, as essential drivers of SOM turnover. The here documented concerted patterns in above- and belowground ecosystem functioning is strongly supportive of using plant community characteristics as surrogates for assessing tundra carbon storage potential and its evolution under climate and vegetation changes.
查看更多>>摘要：? 2021Plants with ammonium preference are able to exude biological nitrification inhibitors to reduce nitrification keeping the mineral N in NH4+ form. The question is whether plants with a NO3? preference are able to stimulate nitrification to shift mineral N towards NO3? production to meet their NO3? demand. In this study we attempted to solve this conundrum by conducting 15N tracing studies in a range of soils planted with wheat (Triticum aestivum L.), a typical NO3?-preferring crop, to quantify the gross rates of soil N transformations and the plant N uptake rates. Gross N mineralization rates (M) were stimulated by the presence of wheat in all studied soils, improving the mineral N supply. The wheat NH4+ uptake rates (UNH4) were significantly, positively correlated with M (p < 0.01). The wheat NO3? uptake rates (UNO3) were significantly higher than UNH4 confirming the NO3? preference of this plant. As NO3? production pathways we considered NH4+ oxidation (ONH4, the autotrophic pathway) and organic N oxidation to NO3?, ONrec) in this study. The stimulations of ONH4 were only observed in three out of five soils and, except one soil, ONH4 was much lower (average 1.29 mg N kg?1 d?1) than UNO3 (average 7.66 mg N kg?1 d?1) showing that the NO3? supply via this pathway was insufficient to meet the plants NO3? demand. In these soils, ONrec was significantly stimulated ranging from 0.86 to 5.52 mg N kg?1 d?1 and was responsible for 34%–74% of NO3? production during the 30 days experimental duration. Moreover, UNO3 was significantly, positively correlated with ONrec (p < 0.05), indicating a direct link between heterotrophic nitrification and plant NO3? uptake. One soil (SC2) exhibited a much higher ONH4 (>8.00 mg N kg?1 d?1) and only M was stimulated by the plants presence but not heterotrophic nitrification because the NO3? supply via ONH4 was sufficient to meet the plant NO3? demand. Heterotrophic nitrification was stimulated by NO3? preference plants when NO3? supply via oxidation of NH4+ to NO3? was insufficient to meet the NO3? requirements.
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