Hydrated mucilage reduces post-dispersal seed removal of a sand desert shrub by ants in a semiarid ecosystem

Post-dispersal seed removal by animals can lead to extensive seed loss and thus is an important factor in structuring plant communities. However, we know much less about post-dispersal seed predation than about other forms of herbivory. Mucilage plays many ecological roles in adaptation of plants to diverse environments; nevertheless, until now the role of mucilage in ant-mediated seed movement remains largely hypothetical. We studied the role of mucilage in seed removal of Artemisia sphaerocephala by ants in Mu Us Sandland in Inner Mongolia, China. Messor aciculatus was the most active seed predator of Artemisia sphaerocephala. Time to first ant collecting (T _1st) of wet intact seeds was longest and significantly different from that for dry intact seeds, wet demucilaged seeds, and dry demucilaged seeds; number of seeds removed to ant nests was lowest for wet intact seeds. After they were collected by ants, 5 % of wet intact seeds were dropped during transport. Our results indicate that seed mucilage of Artemisia sphaerocephala may play a significant role in post-dispersal seed removal by (1) making seeds less attractive to ants, thus resulting in a delay of collection time; (2) forming a strong bond to soil particles, making it difficult for ants to remove seeds; and (3) making seeds more likely to be dropped during transport, thereby allowing them to escape from predation even after collection by ants. This study demonstrates the importance of mucilage in reducing seed removal by ants and thus in anchoring seeds of desert plants in the vicinity of mother plants.     

Responses of photosynthetic properties and chloroplast ultrastructure of Bryum argenteum from a desert biological soil crust to elevated ultraviolet‐B radiation

Our understanding of plant responses to enhanced ultraviolet‐B (UV‐B) radiation has improved over recent decades. However, research on cryptogams is scarce and it remains controversial whether UV‐B radiation causes changes in physiology related to photosynthesis. To investigate the effects of supplementary UV‐B radiation on photosynthesis and chloroplast ultrastructure in Bryum argenteum Hedw., specimens were cultured for 10 days under four UV‐B treatments (2.75, 3.08, 3.25 and 3.41 W m^–2), simulating depletion of 0% (control), 6%, 9% and 12% of stratospheric ozone at the latitude of Shapotou, a temperate desert area of northwest China. Analyses showed malondialdehyde content significantly increased, whereas chlorophyll (Chl) fluorescence parameters and Chl contents decreased with increased UV‐B intensity. These results corresponded with changes in thylakoid protein complexes and chloroplast ultrastructure. Overall, enhanced UV‐B radiation leads to significant decreases in photosynthetic function and serious destruction of the chloroplast ultrastructure of B. argenteum. The degree of negative influences increased with the intensity of UV‐B radiation. These results may not only provide a potential mechanism for supplemental UV‐B effects on photosynthesis of moss crust, but also establish a theoretical basis for further studies of adaptation and response mechanisms of desert ecosystems under future ozone depletion.     

The critical soil P levels for crop yield, soil fertility and environmental safety in different soil types

Background and aims

Sufficient soil phosphorus (P) is important for achieving optimal crop production, but excessive soil P levels may create a risk of P losses and associated eutrophication of surface waters. The aim of this study was to determine critical soil P levels for achieving optimal crop yields and minimal P losses in common soil types and dominant cropping systems in China.

Methods

Four long-term experiment sites were selected in China. The critical level of soil Olsen-P for crop yield was determined using the linear-plateau model. The relationships between the soil total P, Olsen-P and CaCl₂-P were evaluated using two-segment linear model to determine the soil P fertility rate and leaching change-point.

Results

The critical levels of soil Olsen-P for optimal crop yield ranged from 10.9 mg kg^?1 to 21.4 mg kg^?1, above which crop yield response less to the increasing of soil Olsen-P. The P leaching change-points of Olsen-P ranged from 39.9 mg kg^?1 to 90.2 mg kg^?1, above which soil CaCl₂-P greatly increasing with increasing soil Olsen-P. Similar change-point was found between soil total P and Olsen-P. Overall, the change-point ranged from 4.6 mg kg^?1 to 71.8 mg kg^?1 among all the four sites. These change-points were highly affected by crop specie, soil type, pH and soil organic matter content.

Conclusions

The three response curves could be used to access the soil Olsen-P status for crop yield, soil P fertility rate and soil P leaching risk for a sustainable soil P management in field.     

Soil CO2 efflux in a bioenergy plantation with fast-growing Populus trees – influence of former land use, inter-row spacing and genotype

Aims

In this study we quantified the annual soil CO₂ efflux (annual SCE) of a short rotation coppice plantation in its establishment phase. We aimed to examine the effect of former (agricultural) land use type, inter-row spacing and genotype.

Methods

Annual SCE was quantified during the second growth year of the establishment rotation in a large scale poplar plantation in Flanders. Automated chambers were distributed over the two former land use types, the two different inter-row spacings and under two poplar genotypes. Additional measurements of C, N, P, K, Mg, Ca and Na concentrations of the soil, pH, bulk density, fine root biomass, microbial biomass C, soil mineralization rate, distance to trees and tree diameters were performed at the end of the second growth year.

Results

Total carbon loss from soil CO₂ efflux was valued at 589 g m^?2 yr^?1. Annual SCE was higher in former pasture as compared to cropland, higher in the narrow than in the wider inter-row spacings, but no effect of genotype was found.

Conclusions

Spatial differences in site characteristics are of great importance for understanding the effect of ecosystem management and land use change on soil respiration processes and need to be taken into account in modeling efforts of the carbon balance.     

Main rhizosphere characteristics of the Cd hyperaccumulator Rorippa globosa (Turcz.) Thell

Aim

This article was aimed to explore the main rhizospherial properties of the Cd hyperaccumulator R. globosa compared to those of the non hyperaccumulator Rorippa palustris (Leyss.) Bess. representing the same genus (Rorippa) of Cruciferae.

Method

Pot culture experiments using soil spiked with Cd as CdCl₂·2.5H₂O and rhizobags were conducted to determine the differences in Cd accumulation vs. pH, dissolved organic carbon (DOC), Cd chemical fractionation, enzyme activities, and microorganism number in the rhizospheres of R. globosa and R. palustris, and in the bulk soils.

Results

Experiments on Cd uptake by R. globosa and R. palustris from soil spiked with different doses of Cd ranging from 0 to 40 mg?kg^?1, confirmed Cd-hyperaccumulating properties of R. globosa (Cd accumulation in the above-ground organs >100 mg kg^?1, enrichment factor EF> 1, translocation factor TF> 1, no significant biomass reduction at Cd doses >10 mg kg^?1) and the lack of such properties in R. palustris, which made these species suitable for comparative studies. The pH value was found to be a constant, specific property of the rhizosphere of R. globosa and R. palustris, and of the bulk soil, independent on the Cd dose, however the differences were rather small: by 0.2 unit lower in the rhizosphere of R. globosa, and only by 0.1 unit lower in the rhizosphere of R.. palustris compared to the bulk soil. Chemical fractionation of Cd, i.e. its affinity to pools of different binding strength, also appeared to be a specific feature of a rhizosphere and soil independent on the Cd dose. It exhibited a unique capability of the rhizosphere of the Cd-hyperaccumulator R. globosa to mobilize Cd, which enriched the most labile exchangeable fraction in 24.4 % and the immobile residual fraction in 42.3 %, compared to 19.3 % and 50.8 % in the bulk soil and in the rhizosphere of the non-hiperaccumulator R.palustris that did not show significant difference (p?<?0.05) from the bulk soil. In turn, DOC concentrations, enzymatic (urease and catalase) activity and microorganism (bacteria, fungi and actinomycetes) growth in rhizosphere soils were largely influenced by different Cd doses, although they were always considerably higher in the rhizosphere soils of R globosa, than in the rhizosphere of R. palustris and in the bulk soil, in particular at Cd doses ≥10 mg kg^?1.

Conclusion

pH and DOC changes in the rhizosphere of the Cd-hyperaccumulator R. globosa were found to be of a minor importance. The alteration of Cd chemical fractionation consisting in substantial reduction of the immobile residual pool and Cd enrichment primarily in the most labile exchangeable fraction, along with over 2-fold higher number of microorganisms was considered to be the driving force of Cd hyperaccumulation.     

Malleola tibetica sp. nov. (Aeridinae,Orchidaceae) from Tibet,China

Malleola tibetica, a new species from southeastern tropical Tibet, China, is described and illustrated. Morphologically, the new species is closely related to M. dentifera, but differs from it by having uniformly green leaves, flowers with entire lateral lobes of the lip and a basally thickened mid‐lobe, and a column that is densely cristaline‐papillose adaxially.