Drying and rewetting of forest floors: dynamics of soluble phosphorus,microbial biomass-phosphorus,and the composition of microbial communities

Drying and rewetting (D/W) of soils often leads to a pulse of total dissolved phosphorus (TDP) by lysis of sensitive microorganisms. The relevance of D/W on the P cycle in ecosystems depends on the duration of the TDP release. In forest soils, the forest floor represents a hotspot of microbial activity and is often prone to D/W. Here, we investigated the dynamics of TDP, the microbial P pool (Pmic), and the composition of microbial communities after D/W. Samples were taken from Oi and Oe layers of a European beech and a Norway spruce site and desiccated up to ??100 MPa (pF 6) at 20 °C, while controls were kept moist. TDP and Pmic were measured 0, 1, 3, 7, and 14 days after rewetting and the composition of microbial communities was analyzed by automated ribosomal intergenic spacer analysis after 14 days. After D/W, the largest TDP net release (D/W-control) was from Oe layers with 40–50 mg P kg^?1 and inorganic P as the dominant fraction. The TDP concentrations decreased strongly in Oi layers within 1 (beech) to 4 (spruce) days, while remaining stable in Oe layers. The TDP dynamics were linked to the decrease and recovery of Pmic after D/W. Pmic dynamics differed between layers and stand types, suggesting the influence of microbial communities with different D/W sensitivities. The composition of microbial communities varied strongly among sites and layers, while D/W only affected the composition of bacterial and fungal communities in the spruce Oe layer. D/W of forest floors increases the plant available P and affects the P cycle in forest ecosystems.     

Effects of drying and rewetting on soluble phosphorus and nitrogen in forest floors: An experiment with undisturbed columns

Drying and rewetting (D/W) of soils often resulted in the release of soluble phosphorus (P) and nitrogen (N), thereby changing the availability of both nutrients. Most experiments on D/W have been conducted with disturbed mineral soil samples and with rewetting of the soil samples by abrupt change in the water potential. Here, we studied the effect of D/W on the leaching of P and N from undisturbed forest floors of a European beech and a Norway spruce site under near field conditions of desiccation and rewetting. We hypothesized that even under realistic rewetting of undisturbed forest floors, the leaching of P and N is increased after D/W and that the effects are less pronounced for spruce than for beech because of the larger hydrophobicity of the spruce forest floor. Undisturbed forest floor columns were subjected to desiccation at 20°C until a matrix potential of –100 MPa (pF 6.0) was reached, while controls were kept at moist conditions. Columns were irrigated by 22 mm day^?1 from day 1–3 and by 10 mm day^?1 from day 4–14 given in automated short pulses. Leachates from the soil columns were analyzed for orthophosphate, total P, NH₄, NO₃, and total N. In the spruce forest floor the concentrations of total P in leachates and the leachate fluxes were strongly increased after D/W. The increase of solute P was less for beech than for spruce coinciding with less actual rewetting of the beech forest floor. Leaching of total N from the spruce forest floor was not affected by D/W, however, concentrations and leaching of NH₄ increased, while leaching of NO₃ decreased. For beech the leaching of total N and NH₄ increased after D/W, while NO₃ leaching decreased. The results indicate that also under realistic conditions, D/W of forest floors increases solute P and leads to changes in the ratio of NH₄/NO₃ in solution, thereby altering the availability of the nutrients.     

Response of enzyme activities to nitrogen addition in forest floors of different C-to-N ratios

The effect of different inputs of mineral N on several enzyme activities involved in the C and N cycles was investigated using Oa material of forest floors from four Norway spruce [ Picea abies (L.) Karst.] sites with different C-to-N ratios. The samples from each site were treated with five different concentrations of mineral N (as liquid NH ₄NO ₃). All samples were incubated aerobically for 15–20 weeks at 15°C and at field capacity. Respiration was measured weekly. At the end of the incubation period, four enzyme activities (endoglucanase, ß-glucosidase, polyphenol oxidase and ß-glucosaminidase) and microbial biomass were determined. Endoglucanase activity was increased and ß-glucosidase activity was decreased by N additions only in Oa material having a wide C-to-N ratio. In N-supplemented samples of low C-to-N ratio, increased polyphenol oxidase activities were often detected as a consequence of N addition. ß-Glucosaminidase activity responded positively to mineral N additions, particularly in Oa samples with low internal N concentration. The results of the present study indicate that the effects of N additions on enzymatic activities of organic matter in late stages of decomposition are related to the C-to-N ratio. Increasing inputs of mineral N to spruce ecosystems may especially affect C-hydrolyzing enzyme activities in soils with wide C-to-N ratio leading to an incomplete degradation of cellulose and thus reduced C availability to micro-organisms.     

Antibiotic prophylaxis of lower respiratory tract contamination in horses confined with head elevation for 24 or 48 hours

Objective To evaluate the administration of procaine penicillin prior to or during confinement with head elevation as a means of reducing the associated accumulation of inflammatory lower respiratory tract secretions and increased numbers of bacteria within the lower respiratory tract of confined horses. Design and Procedure Two experiments were conducted to evaluate the efficacy of different dose rates and dosing frequencies. In experiment A a single low dose (15,000 IU/kg) of procaine penicillin was administered to four horses immediately prior to confinement with head elevation for 48 hours. The systemic leucocyte response, gross and cytologic characteristics of transtracheal aspirate and bacterial numbers in lower respiratory tract samples were compared with corresponding samples from two horses confined with heads elevated but not given penicillin. The efficacy of higher dose rates (20,000 IU/kg and 40,000 IU/kg) given before and during confinement with heads elevated for 24 hours was evaluated in experiment B. Results Treatment with procaine penicillin had no effect on the systemic leucocyte response or on the accumulation of inflammatory lower respiratory tract secretions at any of the dosing schedules evaluated. The number of bacteria isolated from trans-tracheal samples was reduced at 12 hours for treated horses in experiment A and at 24 hours for experiment B. β-haemolytic Streptococcus spp were not isolated from treated horses in either experiment. Bacterial species isolated from treated horses were predominantly Pasteurella and/or Actinobacillus spp, however, members of the family Enterobacteriaceaé and a Staphylococcus sp were isolated from treated horses. One treated horse in experiment A developed clinically apparent pulmonary disease. Conclusions The prophylactic administration of penicillin before or during confinement did not reliably reduce bacterial numbers or prevent the accumulation of purulent lower respiratory tract secretions in horses confined with their heads elevated. Numbers of β-haemolytic Streptococcus spp were reduced following treatment, suggesting that the repeated administration of procaine penicillin may have some merit as part of a strategy to prevent transport-associated respiratory disease. However, methods directed at minimising the duration of confinement with head elevation, augmentation of the clearance of accumulated secretions and prompt identification of animals in which airway inflammation has extended to the pulmonary parenchyma remain the best ways of minimising transport-associated respiratory disease.     

Effect of transportation on lower respiratory tract contamination and peripheral blood neutrophil function

Objective To evaluate the effect of transportation on lower respiratory tract contamination and peripheral blood neutrophil function in horses and to compare results from transported horses with those obtained in earlier experiments from horses confined with heads elevated. Design A prospective study. Procedure Six horses were transported by road for 12 h. Clinical and haematological examination, transtracheal aspiration and cell function studies were conducted before and after transportation. Results obtained after transportation were compared to pre-transportation values. Results After transportation, peripheral blood leucocyte and neutrophil numbers were increased and rectal temperatures were elevated. Transtracheal aspirates showed an accumulation of purulent respiratory tract secretions with increased numbers of bacteria, particularly β-haemolytic Streptococcus spp and members of the Pasteurellaceae family. Three horses also had increased numbers of bacteria from the Enterobacteriaceae family relative to corresponding samples from earlier studies. Phagocytosis by peripheral blood neutrophils was significantly reduced, while the oxidative burst activity of peripheral blood leucocytes was either unchanged or enhanced. Clinical Implications Bacterial contamination of the lower respiratory tract occurs as a routine consequence of transportation of horses and is likely to be an important determinant in the development of transport-associated respiratory disease. Inflammatory airway secretions and increased numbers of bacteria were rapidly cleared, without clinical evidence of significant pulmonary disease and without additional treatment, in normal horses that were allowed to lower their heads after transportation. Peripheral blood neutrophilia and a reduction in neutrophil phagocytic function were evident for at least 36 h after transportation, suggesting that horses may require a number of days to recover from the stress of transportation. As the potential role of bacteria from the Enterobacteriaceae family in the development of transport-associated respiratory disease has not been elucidated, horses which develop clinical disease following transportation should undergo thorough bacteriological investigation to ensure appropriate treatment.     

Global resilience of tropical forest and savanna to critical transitions

It has been suggested that tropical forest and savanna could represent alternative stable states, implying critical transitions at tipping points in response to altered climate or other drivers. So far, evidence for this idea has remained elusive, and integrated climate models assume smooth vegetation responses. We analyzed data on the distribution of tree cover in Africa, Australia, and South America to reveal strong evidence for the existence of three distinct attractors: forest, savanna, and a treeless state. Empirical reconstruction of the basins of attraction indicates that the resilience of the states varies in a universal way with precipitation. These results allow the identification of regions where forest or savanna may most easily tip into an alternative state, and they pave the way to a new generation of coupled climate models.     

The relationship of scrotal circumference to testicular weight in rams

Scrotal circumference (SC), testicular diameter (TD) and testicular weight (TW) were measured at necropsy of 148 Merino rams of varying age (yearling: 110, 2- to 3-year-old: 5, 4-year-old: 28, greater than 7-year-old: 5). These rams, although culled from flocks in north western Queensland for poor conformation or advancing age, had no clinically palpable or gross necropsy lesions of the genitalia. Despite the fact that SC measurement was performed at necropsy on scrota with greatly varying wool cover and its diverse content of plant material (burrs and grass seeds), the correlation of SC with TW was positive, high and significant (r = 0.92, P less than 0.01). This correlation was greater than that between TD and TW (r = 0.91, P less than 0.01) even though both were measured after dissection of the scrotal contents. The clinically convenient SC measurement deserves wider application as an estimate of testicular weight and therefore fertility in rams.