Symbiotic nitrogen fixation by legumes in two Chinese grasslands estimated with the 15N dilution technique

Abstract

Symbiotic nitrogen (N) fixation by legumes was investigated using the 15 N dilution technique in two Chinese grasslands: one in the north-eastern Tibetan Plateau and the other in Inner Mongolia in China. A small amount (0.03 g N m -2 ) of 15 N labelled (NH 4 ) 2 SO 4 fertilizer was evenly distributed in two soils. One month after the 15 N addition, four legumes (Astragalus sp., Gueldenstaedtia diversifolia, Oxytropis ochrocephala and Trigonella ruthenica) in the alpine meadow and two legumes (Thermopsis lanceolata and Melissitus ruthenica) in the temperate steppe were collected. Several non-legume plant species were harvested as the reference. Above-ground biomass of legumes ranged from 8 to 24 g m -2 in the alpine meadow and from 11 to 35 g m -2 in the temperate steppe. The reference plants showed distinctly higher 15 N atom% excess than legumes (0.08% vs. 0.02% in the alpine meadow, 0.10% vs. 0.02% in the temperate steppe). The N derived from atmosphere (%Ndfa) ranged from 50 to 90% N in the alpine meadow, while it ranged from 85 to 92% in the temperate steppe. Based on the legume above-ground biomass, total symbiotic N 2 -fixation rate was estimated to be 1.00 g N m -2 year -1 in the alpine meadow and 1.15 g N m -2 year -1 in the temperate steppe. These N inputs by legumes can account for 9% of the gap between the N demand and the seasonal N release by mineralization in the alpine Kobresia grassland and 20% in the temperate Leymus grassland, respectively. Considering additional contribution of the root biomass, we suggest that biological N 2 -fixation by legumes plays an important role in the cycling of N in both Kobresia and Leymus grasslands on an annual scale.

Symbiotic nitrogen fixation and interspecific transfer by Caragana microphylla in a temperate grassland with 15 N dilution technique

“…We found that the %Ndfa of C. microphylla was about 90%, falling within estimated ranges (85-92) by previous observations in similar temperate grasslands (Yang et al, 2011). Given the %Ndfa reflects the proportion of N derived from the atmosphere through biological N fixation by legumes (Vallis et al, 1967;Temperton et al, 2007;Carlsson et al, 2009), our estimate indicate that the shrub legume mainly relies on its symbiotic fixation of atmospheric N. The choice of reference plants could influence the estimate accuracy of %Ndfa (Rennie et al, 1982;Holdensen et al, 2007), but a recent study proved the reference plants growing together with legumes can provide a more relatively reliable quantification of N 2 -fixation (Carlsson and Huss-Danell, 2014).…”

“…with mycorrhizal symbiotic systems). We inferred that the N fixed by C. microphylla in our studied temperate grassland would account for 75% of the gap of N demand and supply with the same method used by Yang et al (2011). This indicated that biological N 2 -fixation of C. microphylla plays a more important role in N dynamics in N-limited grasslands.…”

“…Despite the similar %Ndfa, the N fixed by C. microphylla is much larger than that of other legumes in temperate Leymus steppes (Li et al, 2002;Yang et al, 2011). This is mainly ascribed to the higher biomass of C. microphylla shrub.…”

Nutrient limitation of alpine plants: Implications from leaf N : P stoichiometry and leaf δ15N

“…This indicates that legumes strongly but not completely rely on N 2 fixation in alpine meadows. Direct estimation of biological N 2 fixation by legumes at the same site using the 15 N dilution approach showed that biological N 2 fixation represented approximately 40% of N assimilation in M. ruthenica and G. diversifolia, and 81% in O. ochrocephala (Yang et al, 2011). The strong but incomplete reliance on N 2 fixation might be associated with low temperatures, which are far below the optimum of 20-35°C for nitrogenase (Dart and Day, 1971).…”

“…However, their above-ground biomass was markedly reduced by the maximal N addition (Table 3). The legumes are adapted to low N conditions because they fix atmospheric N 2 via symbiotic rhizobia (Yang et al, 2011). However, they lose their advantage and are gradually outcompeted by grasses at high N level.…”