The fate of 15N-labeled cattle (Bos taurus) urine (52 g N m(-2)), applied to a 0.4-m2 surface area on three dates between May and October to three different pasture soils, was studied using 2-m2 lysimeters. Over a period of two years, the sward recovered most of the 15N, but the amount recovered decreased with application date (62% in spring to 17% in fall). However, N uptake by ryegrass (Lolium perenne L.) in Year 2 showed that some nitrogen came from the previous year's urine application. The largest leaching losses of urine N resulted from the late application date. These losses mainly occurred during the first winter despite the small amount of water drainage. Soil type largely determined 15N losses. The granitic Brunisol was the most freely draining and had the greatest leaching (up to 35% recovery of urinary N). In contrast, leaching in the silty loam Neoluvisol remained under 4% of 15N applied. The Calcosol appeared to be susceptible to all kinds of N losses with intermediate unaccounted-for N pool and leaching fractions and lesser utilization of urinary N by grass. Immobilization in soil organic matter, roots and litter, and stubble pools were not markedly influenced by the date of application or soil type. They amounted to 25 to 33, 2, and 2% of N applied as urine, respectively. In these climatic conditions with moderate drainage, leaching of water poor in quality for nitrate only occurred for late-season grazing or on the granitic Brunisol, which was very vulnerable to leaching.
Although both initial residual leaf area and initial N reserves influenced alfalfa regrowth, the residual leaf area had a greater effect on final forage production and N composition in the taproot, whereas the N uptake rate and final total N content in plant were more affected by the initial N reserve status than by the residual leaf area. Moreover, N storage as proteins (especially as vegetative storage proteins, rather than nitrate or amino acids) in the taproot allowed nitrate uptake to occur at significant rates. This suggests that protein storage is not only a means of sequestering N in a tissue for further mobilization, utilization for growth or tissue maintenance, but may also indirectly influence both N acquisition and reduction capacities.
This work assessed the central carbohydrate metabolism of actively photosynthesizing leaf blades of a C3 grass (Lolium perenne L.). The study used dynamic (13)C labelling of plants growing in continuous light with contrasting supplies of nitrogen ('low N' and 'high N') and mathematical analysis of the tracer data with a four-pool compartmental model to estimate rates of: (i) sucrose synthesis from current assimilation; (ii) sucrose export/use; (iii) sucrose hydrolysis (to glucose and fructose) and resynthesis; and (iv) fructan synthesis and sucrose resynthesis from fructan metabolism. The contents of sucrose, fructan, glucose, and fructose were almost constant in both treatments. Labelling demonstrated that all carbohydrate pools were turned over. This indicated a system in metabolic steady state with equal rates of synthesis and degradation/consumption of the individual pools. Fructan content was enhanced by nitrogen deficiency (55 and 26% of dry mass at low and high N, respectively). Sucrose content was lower in nitrogen-deficient leaves (2.7 versus 6.7%). Glucose and fructose contents were always low (<1.5%). Interconversions between sucrose, glucose, and fructose were rapid (with half-lives of individual pools ranging between 0.3 and 0.8 h). Futile cycling of sucrose through sucrose hydrolysis (67 and 56% of sucrose at low and high N, respectively) and fructan metabolism (19 and 20%, respectively) was substantial but seemed to have no detrimental effect on the relative growth rate and carbon-use efficiency of these plants. The main effect of nitrogen deficiency on carbohydrate metabolism was to increase the half-life of the fructan pool from 27 to 62 h and to effectively double its size.
We studied the effects of stubble carbon / nitrogen (C / N) reserves or residual leaf area (RLA) on the contribution of taproot C / N reserves to shoot regrowth of Medicago sativa L. after cutting. The study assessed the effects of two cutting heights (6 and 15 cm), two RLAs (0 or 100%), and two initial C / N reserve levels (high N or low N) on forage production, nitrogen (N) distribution, and C / N reserve dynamics within stubble and taproot. Alfalfa forage production was mainly affected by the initial taproot C / N reserve levels. However, stubble initial organic reserves (and to a lesser extent the RLA) were also of particular importance during early regrowth. The increase of cutting height led to increased stubble C / N supply to regrowing shoots, which partly offset the negative effect on forage production and on taproot C / N reserve depletion. Unlike taproot reserves, the positive contribution of stubble organic reserves to shoot C / N supply was effective for a single defoliation–regrowth cycle. Alfalfa management strategies that increase cutting height (and RLA) during the penultimate harvest in autumn should be considered in cold regions with significant winter stress in order to improve alfalfa winter survival and persistence, as well as spring herbage regrowth.
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