N₂ fixation by soybeans grown with different levels of mineral nitrogen, and the fertilizer replacement value for a following crop

Abstract

fixation by soybeans grown with different levels of mineral nitrogen, and the fertilizer replacement value for a following crop. Can. J. Soil Sci. 82: 139-145. A field experiment was established to study the impact of added mineral N on the prediction of N 2 fixation by soybean, and the consequences for the nature of any N credit that might be used to modify fertilizer recommendations to a following non-fixing crop. Nodulating and non-nodulating isolines of soybean were grown with five rates of N fertilizer, and in a second year corn was grown in the same plots and its yield compared with a response curve. Yield, total N content, amount of N derived from soil, and fertilizer utilization of the nodulating isoline of soybean were not affected by fertilizer N. In contrast, mineral N inhibited nodulation and led to a decrease in the amount of N fixed. The balance of N in the soil was more negative for lower levels of applied N, but by the following spring the amount of mineral N in the soil was the same in all plots. The yield of corn was greater in the plots that had grown nodulating soybean than the non-nodulating isoline. The N fertilizer replacement value of 25 ± 8 kg N ha -1 resulted from a greater amount of root residues in the nodulating soybean, together with a C:N ratio that would favour faster mineralization than in the non-nodulating isoline. Une expérience au champ a été réalisé pour étudier l'impact du N mineral ajouté sur la prévision de la fixation de N 2 par le soja, et les conséquences pour la nature de l'équivalent d'engrais azoté laissé par le soja pour la culture subséquente. Isolignes de soja capables ou non de noduler ont été cultivées avec cinq doses d'engrais, et dans l'année suivante le maïs a été cultivé dans les mêmes parcelles et son rendement comparé à une courbe de réponse. Le rendement, la teneur en N totale, la quantité de N dérivée du sol, et l'utilisation d'engrais du soja nodulé n'a pas été affecté par l'engrais de N. Par contre, la nodulation a été réduite par le N mineral et la quantité de N fixée a diminuée. Le balance de N dans le sol était plus négatif pour des niveaux plus bas de N appliqué, mais au printemps suivant la quantité de N mineral dans le sol était la même dans toutes les parcelles. Le rendement du maïs était plus grand dans les parcelles qui avaient eu le soja nodulé. L'équivalent d'éngrais azoté (25 ± 8 kg N ha -1 ) a résulté d'une plus grande quantité de résidu de racine dans le soja nodulé, ainsi qu'une proportion C:N qui favoriserait une minéralisation plus rapide que dans l'isoligne non-nodulée. Assuming the average grain yield of 2.42 Mg ha -1 they estimated that the N fixed was 98 ± 32 kg ha -1 . Both these values resulted from field experiments where no N fertilizer was applied.Legumes vary in their sensitivity to the effects of mineral N in the soil either as a result of the mineralization of organic matter or of fertilizer application, on N 2 fixation. While soybean is a relatively sensitive crop, both mineral and fixed N seem to be essential for maximum...

Understanding the variability in soybean nitrogen fixation across agroecosystems

“…While the average proportion of N 2 fixed by soybean plants in this study (60%) is similar to literature values (Unkovich and Pate 2000;Goss et al 2002), we observed significant variability in both the % N from fixation and total N 2 fixed across these fields. Both biomass production and the relative reliance on N 2 fixation contributed to the nearly 6-fold variation we observed in total N 2 fixed across the 13 fields at the end of the growing season.…”

“…This contrasts with results from fertilizer-based field studies and hydroponic greenhouse experiments, which have consistently reinforced the concept that soil N availability is the dominant edaphic regulator of BNF (e.g., Hardarson et al 1984;Streeter 1985;Waterer and Vessey 1993;Goss et al 2002). Field experiments usually are carried out in the same field and N availability is manipulated across a relatively constant background soil environment (Boller and Nosberger 1994;Goss et al 2002). We found that N 2 fixation was impacted by both the direct suppressive effect of soil N availability and other soil properties, such as soil texture that may indirectly impact N 2 fixation via oxygen and water saturation dynamics.…”

“…For example, we found weak evidence that BNF is regulated by soil N availability from the mineralization of endogenous soil N pools. This contrasts with results from fertilizer-based field studies and hydroponic greenhouse experiments, which have consistently reinforced the concept that soil N availability is the dominant edaphic regulator of BNF (e.g., Hardarson et al 1984;Streeter 1985;Waterer and Vessey 1993;Goss et al 2002). Field experiments usually are carried out in the same field and N availability is manipulated across a relatively constant background soil environment (Boller and Nosberger 1994;Goss et al 2002).…”

“…Second, the non-nodulating mutation may have broken down for some plants. We did not assess nodulation status of non-nodulating soybeans at R6 because nodulation status tends to decrease rapidly at the onset of senescence (Goss et al 2002). Third, high rates of denitrification during the saturated conditions that were frequent at the beginning of the growing season may have resulted in more enriched δ 15 N signature of reference plants early in the season.…”

Feedbacks between nitrogen fixation and soil organic matter increase ecosystem functions in diversified agroecosystems

“…For example, numerous greenhouse and field experiments have shown that increasing inorganic N reduced nodulation and legume N 2 fixation (e.g., Van Kessel and Hartley , Goss et al. , Salvagiotti et al. ).…”