Effects of N fertilization on N2 fixation and N balances of soybean grown after lowland rice

Abstract

We report a study in northern Thailand to examine the effects of fertilizer N, applied both to paddy rice and to a subsequent soybean crop on symbiotic and yield characteristics of soybean and on the differences between inputs of fixed N 2 and the removal of N as harvested product. Treatments were a factorial arrangement of 0, 100 and 300 kg N ha -1 applied to the rice (designated R0, R100 and R300, respectively), and 0, 25 and 50 kg N ha -1, applied as 'starter' fertilizer to the soybean ($0, $25 and $50, respectively).Nitrogen applied to the rice increased rice yields by up to 74% but proportions recovered by the rice were low (45% [R100] and 14% [R300]). The rice N treatments had only marginal effects on soybean nodulation (up to 17% reduction in early growth) and above-ground dry matter (up to 9% increase). Effects on soybean seed yield and total N 2 fixed were insignificant. Starter N, applied to the soybean at sowing, also marginally reduced nodulation and enhanced above-ground dry matter. Total N 2 fixed was unaffected but seed yield was increased by up to 6%. For all treatments, total above-ground N ranged from 145 to 179kgha -~ with 72 to 85% (122 and 140kgha -~) derived from N 2 fixation. When harvested product consisted of seed only, differences between inputs of fixed N 2 and removals of seed N were close to zero (-10 to +9 kg N ha -l) with little effect of fertilizer N. The N balances were reduced by an average of 18 kg N ha -1 when straw was included as harvested product. We concluded that N applied to the rice and to the following soybean was inefficiently used by those crops and had only marginal effects of symbiotic activity of the soybean. Furthermore, the benefit of the N 2 fixing soybean in this system was to slow the decline of, rather than enhance, the N fertility of the soil

The tripartite symbiosis between legumes, rhizobia and indigenous mycorrhizal fungi is more efficient in undisturbed soil

“…Goss and de Varennes (2002) used soybean as the experimental plant, a legume native to Eastern Asia grown for its seed which has a very high protein and oil content. Soybean establishes a symbiosis with B. japonicum and can derive from 25% to 85% of its N from atmosphere (Jefing et al, 1992;Vasilas et al, 1995), although the average value in central Ontario is only approximately 40% (Ravuri and Hume, 1992). Annual medics (Medicago spp.)…”

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

“…Nonetheless, the presence of a high level of mineral N adversely affects N 2 fixation. The number and dry weight of nodules decrease with increasing mineral N (Chen et al 1992;Jefing et al 1992;Koutroubas et al 1998), and the percentage of N derived from atmosphere decreases, with a concomitant increase in the percentage of N derived from fertilizer (Deibert et al 1979;Hardarson et al 1984;Herridge and Brockwell 1988;George and Singleton 1992). …”

“…] can obtain N from the soil solution, or from the atmosphere through symbiosis with Bradyrhizobium japonicum. Estimates of the amount of N 2 fixed range from 25 to 85% of the total N present in soybean shoots (Deibert 1979;Jefing et al 1992;Vasilas et al 1995). The average amount of N fixed by a soybean crop and present in the aboveground tissues was about 80 kg N ha -1 in central Ontario (Ravuri and Hume 1992).…”

“…An intense transfer of N from vegetative parts to seeds occurs during the reproductive phase (Zapata et al 1987;Vasilas et al 1995;Koutroubas et al 1998). By harvest about 40-80 kg N ha -1 are left in the stover (Zapata et al 1987;Jefing et al 1992;Koutroubas et al 1998), and 20-40 kg N ha -1 in the roots, assuming that roots contain 11% of total plant N (United Co-Operatives of Ontario 1981). There is usually a net depletion of soil N after a soybean crop.…”

“…There is usually a net depletion of soil N after a soybean crop. Depending on whether or not the stover is removed, and on the amount of N fertilizer applied, the balance of soil N between sowing and harvest can vary from +9 to -133 kg N ha -1 , excluding the root system (Zapata et al 1987;George and Singleton 1992;Jefing et al 1992). However, even if the crop fails to provide a clear net benefit in terms of total N in the soil, the N balance is usually assumed to be more positive than with a non-legume crop.…”