Estimation of Biological Nitrogen Fixation Associated with 11 Ecotypes of Panicum maximum Grown in Nitrogen‐15‐labeled Soil¹

Abstract

Contributions of plant-associated biological nitrogen fixation (BNF) may be important to N nutrition of grass~s. Eleven ecotypes of Panicum maximum were planted in the field in concrete cylinders containing soil (Typic Hapludult) that had a low but stable ' 5 N enrichment. Brachiaria radicans (cv. IRI 442) was used as the non-fixing control, and the plant aerial tissue was harvested eight times over a 390-day period. At the first harvest of the experiment (17 May 1984), the concentration of N in all the grasses was high and, subsequently, declined during the first 270 days of growth, reflecting the decreasing availability of soil N. Until soil N availability had become the main factor limiting plant growth, it was thought unlikely that any significant contribution of BNF would occur, and this was confirmed by the 15 N-enrichment data. At later harvests (February-April 1985), the N concentrations in the plants were lower and all the P. maximum ecotypes accumulated more than the B. radicans. Over this period, the '!N enrichment of all the P. maximum ecotypes was significantly lower than that of the B. radicans, indicating a contribution of plant associated BNF to the P. maximum ecotypes. Contributions of associated BNF to the P. maximum ecotypes were estimated to be between 24 and 38% of total N incorporated, equivalent to between S and 10 kg N ha -• per 30 days. The significant differences observed in 15 N enrichment between P. maximum genotypes suggest that further screening and selection of P. maximum ecotypes for high associated BNF is a worthwhile objective.

Elephant grass genotypes for bioenergy production by direct biomass combustion

“…However, the presence of N 2 fi xing bacteria of the genus Herbaspirillum inside this grass, detected by Kirchhof et al (2001), reinforces the possibility that BNF is an important process to the survival and productivity of this species. The importance of BNF to other grass species was already shown, with contributions of BNF of 30 to 40% registered for Brachiaria (Boddey & Victoria, 1986) and ecotypes of Panicum maximum (Miranda & Boddey, 1987), respectively. In sugarcane, Urquiaga et al (1992) found BNF could contribute up to 70% of the plant N needs, but the expected contribution of this process for sugarcane growing in farmer fi elds are of the order of 30%, ranging from 0 to 60% (Boddey et al, 2001).…”

Effect of arbuscular mycorrhizas on uptake of nitrogen by Brachiaria arrecta and Sorghum vulgare from soils labelled for several years with 15N

“…This constitutes strong evidence that there were negligible spatial or temporal variations of ''N enrichment of the mineral N pool during the 3 months of plant grow-th, as w-ould be expected, since all the soils had been labelled for more than 3 yr (Kohl & Shearer, 1981;Boddey & Victoria, 1986;Miranda & Boddey, 1987).…”

“…The '"N enrichment of the soil mineral N of such soils should be virtually stable with time (Kohl & Shearer, 1981;Boddey & Victoria, 1986;Miranda & Boddey, 1987),…”

Theoretical considerations in the comparison of total nitrogen difference and13N isotope dilution estimates of the contribution of nitrogen fixation to plant nutrition

“…Enrichment may be achieved in many ways; as additions of labelled fertilizer or organic matter (Chalk, 1985;Rennie, 1986) or soil may be used which has a stable residual ~SN label resulting from previous studies (Kohl and Shearer, 1981;Miranda and Boddey, 1987). The assumption is made that the control and test crops remove from the soil nitrogen containing the same ratio of labelled fertilizer N to non-labelled soil N even though the quantities of N derived from the soil plus fertilizer may be different.…”

“…In the case where the 15N enrichment in the plant-available N is constant with depth and does not change with time (Kohl and Shearer, 1981;Miranda and Boddey, 1987) the two crops cannot differ in the ratio R even if they recover different quantities of 15N-enriched nitrogen. However, the common (and often the most practical) method of adding 15N as soluble fertilizer does result in large variations of ~SN enrichment with both depth (Boddey and Victoria, 1986) and time (Fried et al, 1983;Witty, 1983;Witty and Ritz, 1984).…”