Nitrogen dynamics in an Australian semiarid grassland soil

We conducted a four-week laboratory incubation of soil from a Themeda triandra Forsskal grassland to clarify mechanisms of nitrogen (N) cycling processes in relation to carbon (C) and N availability in a hot, semiarid environment. Variation in soil C and N availability was achieved by collecting soil from either under tussocks or the bare soil between tussocks, and by amending soil with Themeda litter. We measured N cycling by monitoring: dissolved organic nitrogen (DON), ammonium (NH4+), and nitrate (NO3"12) contents, gross rates of N mineralization and microbial re-mineralization, NH4+ and NO3"12 immobilization, and autotrophic and heterotrophic nitrification. We monitored C availability by measuring cumulative soil respiration and dissolved organic C (DOC). Litter-amended soil had cumulative respiration that was eightfold greater than non-amended soil (2000 compared with 250 ;Cg C/g soil) and almost twice the DOC content (54 compared with 28 ;Cg C/g soil). However, litter-amended soils had only half as much DON accumulation as non-amended soils (9 compared with 17 ;Cg N/g soil) and lower gross N rates (1 134 compared with 13 1326 ;Cg N 22[g soil]"121 22d"121) and NO3"12 accumulation (0.5 compared with 22 ;Cg N/g soil). Unamended soil from under tussocks had almost twice the soil respiration as soil from between tussocks (300 compared with 175 ;Cg C/g soil), and greater DOC content (33 compared with 24 ;Cg C/g soil). However, unamended soil from under tussocks had lower gross N rates (3 1320 compared with 17 1331 ;Cg N 22[g soil]"121 22d"121) and NO3"12 accumulation (18 compared with 25 ;Cg N/g soil) relative to soil from between tussocks. We conclude that N cycling in this grassland is mediated by both C and N limitations that arise from the patchiness of tussocks and seasonal variability in Themeda litterfall. Heterotrophic nitrification rate explained >50% of total nitrification, but this percentage was not affected by proximity to tussocks or litter amendment. A conceptual model that considers DON as central to N cycling processes provided a useful initial framework to explain results of our study. However, to fully explain N cycling in this semiarid grassland soil, the production of NO3"12 from organic N sources must be included in this model

Controls on soil nitrogen cycling and microbial community composition across land use and incubation temperature

We conducted a laboratory incubation of forest (Scots pine (Pinus sylvestris) or beech (Fagus sylvatica)), grassland (Trifolium repens/Lolium perenne) and arable (organic and conventional) soils at 5 and 25 °C. We aimed to clarify the mechanisms of short-term (2-weeks) nitrogen (N) cycling processes and microbial community composition in relation to dissolved organic carbon (DOC) and N (DON) availability and selected soil properties. N cycling was measured by 15N pool dilution and microbial community composition by denaturing gradient gel electrophoresis (DGGE), phospholipid fatty acid (PLFA) and community level physiological profiles (CLPP). Soil DOC increased in the order of arable-grassland-forest soil while DON and gross N fluxes increased in the order of forest-arable-grassland soil; land use had no affect on respiration rate. Soil DOC was lower, while respiration, DON and gross N fluxes were higher at 25 than 5 °C. Gross N fluxes, respiration and bacterial biomass were all positively correlated with each other. Gross N fluxes were positively correlated with pH and DON, and negatively correlated with organic matter, fungal biomass, DOC and DOC/DON ratio. Respiration rate was positively correlated with bacterial biomass, DON and DOC/DON ratio. Multiple linear modelling indicated that soil pH, organic matter, bacterial biomass, DON and DOC/DON ratio were important in predicting gross N mineralization. Incubation temperature, pH and total-C were important in predicting gross nitrification, while gross N mineralization, gross nitrification and pH were important in predicting gross N immobilization. Permutation multivariate analysis of variance indicated that DGGE, CLPP and PLFA profiles were all significantly (Plt;0.05) affected by land use and incubation temperature. Multivariate regressions indicated that incubation temperature, pH and organic matter content were important in predicting DGGE, CLPP and PLFA profiles. PLFA and CLPP were also related to DON, DOC, ammonium and nitrate contents. Canonical correlation analysis showed that PLFA and CLPP were related to differences in the rates of gross N mineralization, gross nitrification and soil respiration. Our study indicates that vegetation type and/or management practices which control soil pH and mediate dissolved organic matter availability were important predictors of gross N fluxes and microbial composition in this short-term experiment

Asociaciones de quistes de dinoflagelados de agua salobre a dulce de la formacion la colonia (Paleoceno?), Noreste de Patagonia, Argentina

A palynological analysis of the section of La Colonia Formation exposed at Estancia San Miguel yielded conspicuous assemblages of organic-walled dinoflagellate cysts (dinocysts) and green algae. The monotypic palaeoperidinioid dinocyst assemblage of ?Ginginodinium sp. in the basal beds of this section indicates low-salinity water conditions in a restricted shallow marine paleoenvironment. The green-algae-dominated assemblages together with specimens of ?Morkallacysta spp., Dinocysts type P and ?Vesperopsis sp. recorded in the middle and upper part of the San Miguel section indicate brackish to freshwater and freshwater depositional conditions, respectively. The changes in the composition of the palynological assemblages, in agreement with the analysis of the sedimentary facies, reflect a salinity-drop in the water bodies and a progressive upward-shallowing trend. The occurrence of specimens of the neritic open marine Cribroperidinium spp., Apteodinium sp., Circulodinium sp. and Areoligera sp. cf. A. circumsenonensis Fensome et al. is here considered as reflecting contemporaneous transported material from the adjacent shelf. The marine part of the La Colonia Formation is associated to Late Cretaceous and Paleocene ages. Nevertheless, palynomorphs together with a stratigraphical criterion suggest an age non older than Paleocene for the deposits of the unit at the San Miguel section