Effect of Brachiaria humidicola root exudates, rhizosphere soils, moisture and temperature regimes on nitrification inhibition in two volcanic ash soils of Japan.

摘要

Experiments were conducted to investigate the effects of soil types, moisture and temperature levels on nitrification inhibitory capability of Brachiaria humidicola root exudates on inorganic N applied as (NH₄)₂SO₄ in fresh volcanic ash soils of Japan collected from Tsukuba (Andisols) and Ishigaki (Terrace yellow soil), as well as in rhizosphere soils on which Brachiaria humidicola and Panicum maximum has been cultivated for 3 months. Root exudates collected by root washing method from B. humidicola supplied with NH+₄ or NO-₃ N in hydroponic media were separated into hydrophobic and hydrophilic phases. Soils were collected from the experimental sites of JIRCAS at Tsukuba and Ishigaki at 0-30 cm depth. The fresh soils were weighed into 204 centrifuge tubes each at 5 g dry soil weight equivalent per tube. The tubes were separated into 3 batches of 60, 72 and 72 tubes each per soil type. The hydrophobic and hydrophilic phases of B. humidicola rot exudates were each mixed with mixing solution [(NH₄)₂SO₄+NaClO₃] at 1:1 ratio and 0.6 ml of the mixtures was added to each tube of the first bath at 15 AT units g-1 soil, sealed with Para film and incubated at 30 degrees C. There were 5 treatments (control+4 root exudates) in 2 replicates per soil type for 6 periods. The second bath of tubes were added 0.6 ml of 1:1 mixture of mixing solutions+root exudates (hydrophobic+hydrophilic mixtures) or without exudates, soil moisture adjusted by addition of 0.6 ml or 1.2 ml, sealed with Para film and incubated at 30 degrees C. The third bath of tubes were added 0.6 ml of 1:! mixture of mixing solutions+root exudates (hydrophobic+hydrophilic mixtures) or without root exudates and incubated at 20 or 30 degrees C. Rhizosphere soils on which either B. humidicola or P. maximum have grown for 3 months and the control (no plant) were weighed at 5 g dry soil weight equivalent and added 0.6 ml of mixing solution [(NH₄)₂SO₄+NaClO₃] at equivalent of 12 g N kg-1 soil and incubated at 30 degrees C. The NO-1₂, NO-1₃ and NH+₄-N content of the soils were determined by auto-analyzer 2 after 1, 3, 7, 14, 28 and 56 days of incubation, data analyzed by ANOVA and treatment differences separated by LSD at 5%. Results showed that the hydrophobic, more than the hydrophilic phase of Brachiaria humidicola roots exudates, significantly reduced nitrification in soils, while their combined application was more potent than separate usage of the phases. Higher soil moisture and temperature enhanced nitrification, while nitrification was significantly reduced in B. humidicola rhizosphere soils than in P. maximum. Nitrification was generally higher in Andisol than Terrace yellow soil due to their inherent properties which significantly influences the functioning of B. humidicola root exudates in the level of nitrification inhibition in the two soils. The possibility of B. humidicola root exudates to inhibit nitrification showed that for optimal realization of the agronomy efficiency from applied nitrogen fertilizers on the field, root exudates of B. humidicola could be concentrated and mixed with fertilizers to checkmate nitrification level, increase N use efficiency (NUE) by crops and guide against N losses to the environment which could result to soil, water and air pollution.