Symbiotic N₂-Fixation Estimated by the¹⁵N Tracer Technique and Growth ofPueraria phaseoloides(Roxb.) Benth. Inoculated withBradyrhizobiumStrain in Field Conditions

Abstract

This field experiment was established in Eastern Cameroon to examine the effect of selected rhizobial inoculation on N2-fixation and growth of Pueraria phaseoloides. Treatments consisted of noninoculated and Bradyrhizobium yuanmingense S3-4-inoculated Pueraria with three replications each. Ipomoea batatas as a non-N2-fixing reference was interspersed in each Pueraria plot. All the twelve plots received 2 gN/m2 of 15N ammonium sulfate 10% atom excess. At harvest, dry matter yields and the nitrogen derived from atmospheric N2-fixation (%Ndfa) of inoculated Pueraria were significantly (P < 0.05) higher (81% and 10.83%, resp.) than those of noninoculated Pueraria. The inoculation enhanced nodule dry weight 2.44-fold. Consequently, the harvested N significantly (P < 0.05) increased by 83% in inoculated Pueraria, resulting from the increase in N2-fixation and soil N uptake. A loss of 55 to 60% of the N fertilizer was reported, and 36 to 40% of it was immobilized in soil. Here, we demonstrated that both N2-fixing potential of P. phaseoloides and soil N uptake are improved through field inoculations using efficient bradyrhizobial species. In practice, the inoculation contributes to maximize N input in soils by the cover crop's biomass and represent a good strategy to improve soil fertility for subsequent cultivation.

Comparison of 15N isotope methods to determine the recovery efficiency of nitrogen from green manure

“…In general, the recovery efficiency of N from green manure is low, rarely exceeding 20% in the first crop after application (Scivittaro et al, 2003;Ambrosano et al, 2005;Araújo et al, 2011), as also observed in the present study. Table 2.…”

“…The indirect method has been used to evaluate the biological N fixation rate in legumes and the transfer of N to crops (Sarr et al, 2016). However, the isotope dilution technique should be used with caution as it overestimates the N transfer rate from green manure to the commercial crop.…”

“…The same authors observed an increase in t 1/2 during a year with reduced precipitation, which shows that climatic conditions are another important factor. Ambrosano et al (2005) and Scivittaro et al (2003) found that the recovery efficiency of N from green manure varies, on average, from 5 to 24%, respectively. Therefore, information on the recovery efficiency of N from green manure by commercial crops is essential to ensure a proper planning and adjustment of legume biomass management practices in different edaphoclimatic conditions.…”

Nitrogen Uptake Efficiency and Total Soil Nitrogen Accumulation in Long-Term Beef Manure and Inorganic Fertilizer Application

“…If changes in the soil environment favor N immobilization, formation of soil organic N reduces crop uptake but favors TSN accumulation. Sarr et al [18] reported a loss of N fertilizer between 55 and 60% and soil immobilization between 36 and 40% in a tracer N study. In the same study, N fixation from the inoculated plants increased N recovery by 81%.…”

“…However, N from fertilizers in excess of plant requirements is not the only factor in the equation. Biological processes such as symbiotic and nonsymbiotic N 2 fixation in addition to atmospheric N in rainfall contribute a substantial amount of TSN [17,18]. ese biological or natural processes, which significantly influence soil N availability and subsequently crop yield, are independent of N fertilizer application rates and largely controlled by the environment [19].…”