Yuzhe Wang

δ 15N of soil nitrogen pools and their dynamics under decomposing leaf litters in a suburban native forest subject to repeated prescribed burning in southeast Queensland, Australia

PurposeGlobal environmental changes could affect forest productivity and thus organic matter input to soil via litterfall. We conducted a 9-month litter decomposition experiment to examine the effect of litter removal and addition on soil nitrogen (N) dynamics in a subtropical eucalypt forest subject to prescribed burning.Materials and methodsTwo litter treatments were applied: addition of double litter rates and without any litter addition. In situ 15N pool dilution method was used to determine soil gross N transformation rates, and δ15N of soil inorganic N pools and their dynamics were also measured.Results and discussionThe results showed that the addition of decomposing litters had no significant effect on soil net and gross N mineralization and nitrification rates. Soil N rates showed a pronounced seasonal pattern with higher rates in summer and lower rates in winter, which could be explained by the seasonal variation of environmental conditions such as temperature but not soil water content in this study. The soils had significantly higher δ15N in NO3−-N than in NH4+-N after removing the forest floor, which might result from gaseous N losses via nitrification, denitrification and NO3−-N leaching during the experimental period.ConclusionsSoil N transformations were not significantly affected by decomposing litters during the 9-month of decomposition. Our findings demonstrate that long-term studies of litter decomposition combined with repeated measurement of soil N transformation rates are needed to fully understand the seasonal patterns of soil N cycling and its response to decomposing litters in forest ecosystems.

Effects of inundation and stranding on leaf litter decomposition and chemical transformation

Inundation and stranding are important processes of the riparian ecosystem due to water level fluctuation. Plant litter decomposition is a key process that determines the accumulation of soil organic matter in riparian ecosystems, but little is known about the alternating effects of inundation and stranding on this process. Using litters of the grass species Heteropogon contortus, we studied how the remaining mass and nitrogen (N), δ13C and δ15N, and 13C-CPMAS NMR spectra responded to permanent inundation, temporary inundation and drying over a period of twelve months. Inundation (permanent or temporary) and stranding altered litter C and N dynamics. The δ13C declined in the immersed litters and was stable after the litters were transported to the grassland plots, while δ15N in the litters that were decomposing continually in the water rapidly increased during the earlier stage of decomposition. We observed a significant increase in the proportion of ketone, carboxyl, and alkyl in the permanently inundated litter samples compared with those of litters decomposed at terrestrial habitats at the final harvest. These results indicated that the effects of inundation on the decay of labile and recalcitrant litter components were asynchronous. The decomposing litters in the inundation treatment differed chemically from those in the terrestrial habitat treatments and were characterized by greater relative abundances of ketone C and carboxyl C. The higher values of alkyl/O-alkyl for the stranding litters that had higher mass remaining and C/N as compared to those of inundated litters that had lower mass remaining and C/N in the final harvest, suggested a relatively higher contribution of the recalcitrant components to the litter residues. Likewise, the effects of transient inundation depend on the timing of immersion and stranding.

Ploidy variation and karyotype analysis in Hemerocallis spp. (Xanthorrhoeaceae) and implications on daylily breeding

Daylily or Hemerocallis spp. (Xanthorrhoeaceae) is one of the most economically important flowering crops in the world. Ploidy level of 149 genotypes both local to China and introduced from the United States and New Zealand were assessed with an outcome of 79 diploids, 22 triploids and 48 tetraploids. A large proportion of introduced cultivars were tetraploid (42%), half were diploid and the rest, 8%, were triploid. Most Chinese cultivars were diploid; only one was tetraploid. Among the 29 wild genotypes collected from the Taihang Mountain range, 13 (45%) were triploid and 16 (55%) were diploid; no tetraploids were identified. For the 31 genotypes with known ploidy, 22 matched previous counts with the rest showing a lower ploidy. As different ploidy is common in daylily, we suggest that ploidy levels should be assessed in breeding programmes for specific breeding purposes. Karyotypes of three diploid, three triploid and two tetraploid genotypes were constructed and they were assigned to ‘3A’, ‘2B’ and ‘3B’ categories based on Stebbins’ karyotype classification.