N2 fixation and nitrogen allocation to above and below ground plant parts in red clover-grasslands

Abstract

Leys, used for grazing or production of forage to be conserved as silage or hay, are very important crops in northern areas. In order to measure the N 2 fixation in leys of varying ages and during different parts of the season, detailed measurements were taken of yield, N 2 fixation and the amounts of N remaining in the field after harvesting red clover (Trifolium pratense L.)-grass leys at a site in northern Sweden, where they are generally harvested twice per growing season. Entire plants, including stubble and roots, were sampled at the time of first and second harvest and, in addition, at the end of the growing season in three neighbouring fields, carrying a first, a second and a third year ley, respectively. N 2 fixation was measured by both 15 N isotope dilution (ID) and 15 N natural abundance (NA) methods. The proportion of clover dry matter (DM) in the stands increased from the first to the second harvest, but the grasses dominated throughout the entire season, especially below ground. The N concentrations, in both herbage and whole plants, were about twice as high in the clover as in the grasses. Seasonal variations in N concentrations were minor, and total N contents followed the same trends as DM. The clover acquired nearly all of its N from N 2 fixation: the proportion of N in clover herbage derived from N 2 fixation was often >0.8 throughout the season. The variations in the amounts of N 2 fixed during the course of the season corresponded well to the seasonal changes in clover biomass. Amounts of fixed N 2 allocated to clover herbage during the whole season were in the range 4 to 6 g N m −2 in this unusually rainy year. Calculations of daily N allocation rates to herbage showed that N uptake rates were similar, and high, in grasses during May-June and July-August, while N 2 fixation rates in clover were about 10-fold as high in July-August as in May-June, reflecting the need for N in clover growth. The proportion of N remaining in clover stubble and roots after the first and second harvests was about 60 and 25%, respectively, while about 60% of the N in grasses remained in stubble and roots after both harvests. The considerable amounts of biomass and N that were left in field after harvesting red clover-grass leys are important for regrowth of the plants and provide substantial N fertilization for the next crop in the crop rotation.

Underestimated role of legume roots for soil N fertility

“…In case of pasture legumes, the N concentration in harvested roots and nodules was 19.5 g N kg −1 DM, quite similar to 19.3 g N kg −1 DM determined by Huss-Danell et al (2007) for red clover (Trifolium pratense L.) cultivated in Sweden. Fixation rate was above 90 % Nda in both grain legumes and did not differ significantly between above-and belowground plant tissues in both cases (Table 1).…”

“…Estimates for both lupine species (7-9 %) were smaller than the values (13-14 %) quantified by Unkovich et al (1997) and Walley et al (2007) for lupine roots under the field condition. However, they were in the range of values (9-12 %) reported by Fillery and McNeill (2001) for Lupinus angustifolius L. The value determined in the present pasture legume (11 %) is also within the last range, but is smaller than the estimate of 12.6 % reported by Huss-Danell et al (2007) for the red clover in a mixed stand under rainy conditions and even smaller than the range of values (17-24 %) reported by McNeill et al (1997) for subclover grown in intact cores of soil-plant systems. Present data were determined under subhumid dry climate in rainfed conditions whereas most data in the literature were obtained under soil moist and controlled air temperature or under higher rainfall and lower temperature.…”

“…Plant N from symbiotic fixation of atmospheric N 2 is added to the ecosystem mainly via the decomposition of legume parts or via urine and feces from grazing animals in pastures. In addition, rhizodeposition, i.e., exudates and products of root and nodule necrosis, may also add N to the soil (Fustec et al 2010;Herridge et al 2008;Huss-Danell et al 2007;Høgh-Jensen and Schjoerring 2000;Walley et al 2007). Thus, N 2 -fixing plants can make important contributions to increase the sustainability of agricultural ecosystems by reducing the use of energetically costly mineral N fertilizers.…”

Grassland carbon sequestration and emissions following cultivation in a mixed crop rotation

“…Also, root production in annual crops occurs within certain growth stages of crops and ceases, whereas perennials have a more continuous root production. Root biomass in the present study was recovered by wet sieving, which can be biased to the loss of fine roots and root hairs (Huss-Danell et al, 2007).…”

Nitrogen losses from grass ley after slurry application surface broadcasting vs. injection

“…A significant amount of NH 4 -N in slurry might also have been microbially immobilized soon after application due to decomposition of fatty acids in slurry (Kirchmann andLundvall 1993, Sørensen andAmato 2002). According to the results of Huss-Danell and Chaia (2007), over 30 kg N ha -1 can be incorporated into grass roots in the northern part of Sweden. Pierzynski and Gehl (2005) showed that some of the N saved from NH 3 emissions may have been lost as N 2 O from slurry injected fields.…”