CLONING,EXPRESSION, AND CHARACTERIZATION OF A HISTONE‐LIKE PROTEIN FROM THE MARINE DINOFLAGELLATE LINGULODINIUM POLYEDRUM (DINOPHYCEAE)1

Although nucleosomes and histones are lacking in dinoflagellate nuclei, small basic histone‐like proteins have been reported, but their function(s) is unknown. In this study we cloned and sequenced a gene for a histone‐like protein from the dinoflagellate Lingulodinium polyedrum (Stein) Dodge (HLp) (formerly Gonyaulax polyedra Stein) and investigated its post‐translational modification and DNA‐binding activities. HLp appears to be acetylated in L. polyedrum, and we identified several L. polyedrum proteins that possess histone acetyltransferase activity and may be responsible for this modification. HLp binds weakly to L. polyedrum DNA but to certain specific sequences with higher affinity, consistent with its having a regulatory function.     

Spatial distribution and turnover of root-derived carbon in alfalfa rhizosphere depending on top- and subsoil properties and mycorrhization

Aims

This study analyzed the extent to which root exudates diffuse from the root surface towards the soil depending on topsoil and subsoil properties and the effect of arbuscular mycorrhizal fungal hyphae on root-derived C distribution in the rhizosphere.

Methods

Alfalfa was grown in three-compartment pots. Nylon gauze prevented either roots alone or roots and arbuscular mycorrhizal fungal hyphae from penetrating into the rhizosphere compartments. ¹⁴CO₂ pulse labeling enabled the measurement of ¹⁴C-labeled exudates in dissolved (DOC) and total organic carbon (TOC) in the rhizosphere, distributed either by diffusion alone or by diffusion, root hair and hyphal transport.

Results

Root exudation and microbial decomposition of exudates was higher in the rhizosphere with topsoil compared to subsoil properties. Exudates extended over 28 mm (DOC) and 20 mm (TOC). Different soil properties and mycorrhization, likely caused by the low arbuscular mycorrhizal colonization of roots (13?±?4 % (topsoil properties) and 18?±?5 % (subsoil properties)), had no effect.

Conclusions

Higher microbial decomposition compensated for higher root exudation into the rhizosphere with topsoil properties, which resulted in equal exudate extent when compared to the rhizosphere with subsoil properties. Higher ¹⁴C activity used for labeling compared with previous studies enabled the detection of low exudate concentrations at longer distances from the root surface.     

Effect of C3–C4 Vegetation Change on δ13C and δ15N Values of Soil Organic Matter Fractions Separated by Thermal Stability

Carbon isotopic composition of soils subjected to C₃–C₄ vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter (SOM) pools with fast and intermediate turnover rates. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability, so that thermogravimetry can be used to separate SOM pools with contrasting turnover rates. Soil samples from a field plot cultivated for 10.5 years with the perennial C₄ plant Miscanthus×gigantheus were analyzed by thermogravimetry coupled with differential scanning calorimetry (DSC). Three SOM fractions were distinguished according to the differential weight losses and exothermic or endothermic reactions measured by DSC. The δ¹³C and δ¹⁵N values of these three fractions obtained by gradual soil heating were measured by IRMS. The weight losses up to 190 °C mainly reflected water evaporation because no significant C and N losses were detected and δ¹³C and δ¹⁵N values of the residual SOM remained unchanged. The δ¹³C values (−16.4‰) of SOM fraction decomposed between 190 and 390 °C (containing 79% of total soil C) were slightly closer to that of the Miscanthus plant tissues (δ¹³C = −11.8‰) compared to the δ¹³C values (−16.8‰) of SOM fraction decomposed above 390 °C containing the residual 21% of SOM. Thus, the C turnover in the thermally labile fraction was faster than that in thermally stable fractions, but the differences were not very strong. Therefore, in this first study combining TG-DSC with isotopic analysis, we conclude that the thermal stability of SOM was not very strongly related to biological availability of SOM fractions. In contrast to δ¹³C, the δ¹⁵N values strongly differed between SOM fractions, suggesting that N turnover in the soil was different from C turnover. More detailed fractionation of SOM by thermal analysis with subsequent isotopic analysis may improve the resolution for δ¹³C.     

Dramatic loss of inorganic carbon by nitrogen‐induced soil acidification in Chinese croplands

Intensive crop production systems worldwide, particularly in China, rely heavily on nitrogen (N) fertilization, but left more than 50% of fertilizer N in the environment. Nitrogen (over) fertilization and atmospheric N deposition induce soil acidification, which is neutralized by soil inorganic carbon (SIC; carbonates), and carbon dioxide (CO₂) is released to the atmosphere. For the first time, the loss of SIC stocks in response to N‐induced soil acidification was estimated for Chinese croplands from 1980 to 2020 and forecasts were made up to 2100. The SIC stocks in croplands in 1980 were 2.16 Pg C (16.3 Mg C/ha) in the upper 40 cm, 7% (0.15 Pg C; 1.1 Mg C/ha) of which were lost from 1980 to 2020. During these 40 years, 7 million ha of cropland has become carbonate free. Another 37% of the SIC stocks may be lost up to 2100 in China, leaving 30 million ha of cropland (37.8%) without carbonates if N fertilization follows the business‐as‐usual (BAU) scenario. Compared to the BAU scenario, the reduction in N input by 15%–30% after 2020 (scenarios S1 and S2) will decrease carbonate dissolution by 18%–41%. If N input remains constant as noted in 2020 (S3) or decreases by 1% annually (S4), a reduction of up to 52%–67% in carbonate dissolution is expected compared to the BAU scenario. The presence of CaCO₃ in the soil is important for various processes including acidity buffering, aggregate formation and stabilization, organic matter stabilization, microbial and enzyme activities, nutrient cycling and availability, and water permeability and plant productivity. Therefore, optimizing N fertilization and improving N‐use efficiency are important for decreasing SIC losses from acidification. N application should be strictly calculated based on crop demand, and any overfertilization should be avoided to prevent environmental problems and soil fertility decline associated with CaCO₃ losses.     

Dominant plant species shift their nitrogen uptake patterns in response to nutrient enrichment caused by a fungal fairy in an alpine meadow

Niche partitioning by time, space and chemical forms has been suggested as an important mechanism to maintain species coexistence. Climate warming is assumed to increase soil nutrient availability through enhancing mineralization of soil organic matter in a variety of terrestrial ecosystems. However, few studies have yet examined how dominant plant species contribute to species coexistence when nutrient enrichment occurs in native ecosystems. We studied a single fairy ring (5 m diameter) in a Kobresia meadow in the Tibetan Plateau. This kind of rings is caused by a basidiomycete fungus Agaricus campestris, and is evidenced by dark-green vegetation boundaries. Nutrient enrichment occurs due to enhanced decomposition of soil organic matter (SOM) in the fungus growth zone of these rings. We conducted a short-term ¹⁵N labelling experiment and found that dominant plant species shifted their N uptake patterns and preferred N form (NO ₃ ^? , NH ₄ ⁺ , and amino acid N) in response to nutrient enrichment in an N-limited alpine meadow. The legume Gueldenstaedtia diversifolia had the lowest aboveground biomass among the five plant species studied at low available N level, although it mainly utilized ammonium (the most abundant N form). The two graminoids (Elymus nutans and Stipa aliena) demonstrated similar aboveground biomass at low and high available N levels, showing a similar pattern switching from NH ₄ ⁺ /NO ₃ ^? uptake outside the ring to glycine uptake in the annulus zone of the ring. The biomass of the forb Gentiana straminea differed significantly at low and high available N levels, but its N uptake pattern almost remained unchanged. Species therefore differed in their response to nutrient enrichment, most species showing chemical niche shifts instead of niche conservatism. This finding has important implications with regard to understanding the mechanisms responsible for species coexistence when natural nutrient enrichment is induced by climate warming in terrestrial ecosystems.