Estimating N2-fixation in the field using 15N-labelled fertilizer: Some problems and solutions

The ¹⁵N-labelled fertilizer dilution technique provides a method of obtaining estimates of biological N₂-fixation in the field over the growing season. Field estimates of fixation obtained using peas, french beans, field beans and clover depended on the non-fixing control used. Differences in the N uptake patterns of the legume and control combinations, together with a decrease in the enrichment of plant available soil N with time, were major factors causing this dependency. A simple model of plant N accumulation at decreasing soil enrichment is presented, which explains these errors and allows a more rational choice of non-fixing control. The use of gypsum pelleted ¹⁵N fertilizer, or any other treatment which leads to a more stable soil enrichment, reduces errors caused by mismatched N uptake patterns in the two crops.     

Slow-release 15N fertilizer formulations to measure N2-fixation by isotope dilution

Pot experiments were carried out to examine the effects of slow-release fertilizer formulations on estimates of N₂-fixation determined by the isotope dilution method. Soybeans were used as the N₂-fixing plants, with non-nodulated soybeans and maize as the non-fixing controls. The ¹⁵N-fertilizer formulations used were (¹⁵NH₄)SO₄, K¹⁵NO₃, gypsum-pelleted K¹⁵NO₃, (¹⁵NH₄)₂SO₄ + glucose, ground plant material enriched with ¹⁵N or ¹⁵N-oxamide. The estimate of the amount of N₂ fixed by the nodulated soybean plants depended upon both the control plant and the fertilizer formulation used. Maize took up N later than non-nodulated soybean and estimates of soil N-pool (soil “A” value + fertilizer N added) calculated from the enrichment of this control were about twice as large as those calculated from the enrichment of non-nodulated soybean receiving the same fertilizer treatment. As a consequence, estimates of N₂-fixation relative to this control were lower than those relative to non-nodulating soybean (mean 140 mg N per pot compared with 292 mg N per pot). With unstablilized ¹⁵N salts errors were sufficient to produce negative estimates of fixation relative to maize. Even with a “well-matched” control (non-nodulated soybean) estimates of fixation varied with fertilizer formulation.     

Renal Replacement Therapy in Healthy Adult Horses

Background

Renal replacement therapy (RRT) has been implemented extensively in people to facilitate recovery from acute renal failure (ARF). RRT has not been explored in horses, but might provide a further treatment option in horses with ARF.

Objective

To investigate efficacy and safety of RRT in horses.

Animals

Five healthy adult horses.

Methods

A prospective study was performed on horses restrained in stocks and intravenously connected to a commercial RRT machine to allow continuous venovenous hemodiafiltration to be performed for 6 hours. The RRT machine was set at the following flow rates: blood flow rate 250 mL/min; dialysate rate 3,000 mL/h; prefilter replacement pump 3,000 mL/h; and postfilter replacement pump rate 2,000 mL/h. Balanced electrolyte solution was used as dialysate and replacement fluid. Heart rate, respiratory rate, body temperature, direct arterial blood pressure, urine output, and various clinicopathologic parameters were measured over the study period.

Results

Renal replacement therapy was successfully performed in horses, resulting in a mean creatinine clearance of 0.127 mL/kg/min (68.9 mL/min) and urea reduction ratio of 24%. No adverse effects were detected although a significant decrease in rectal temperature was observed (P ≤ .007). A significant increase in serum phosphorus (P ≤ .001) and decrease in BUN (P < .001) were also noted. A significant prolongation of prothrombin (P < .01) and partial thromboplastin time (P < .0001) were observed along with a decrease in platelet count (P ≤ .04).

Conclusions and Clinical Importance

Renal replacement therapy can safely and effectively be used in adult horses.     

The classification, distribution, and extent of soils with a xeric moisture regime in the United States

Soils with cool, moist winters and relatively warm, dry summers, a Mediterranean climate, are recognized as having a xeric moisture regime in Soil Taxonomy. These soils are classified mostly in taxa that use the formative element “xer” in the name. Soil series with either a xeric moisture regime or an aridic regime bordering on a xeric moisture regime make up more than 48,640,000 hectares in the western part of the United States. They are classified in the orders of Mollisols (20,080,000 ha), Aridisols (11,200,000 ha), Alfisols (5,320,000 ha), Inceptisols (4,800,000 ha), Entisols (4,400,000 ha), Vertisols (1,520,000 ha), Andisols (960,000 ha), and Ultisols (960,000 ha).