不同氮效率水稻生育后期根表和根际土壤硝化特征

通过田间试验研究了不同氮效率粳稻品种4007(氮高效)和Elio(氮低效)生育后期在N0(0 kgN hm-2)、N180(180 kgN hm-2)和N300(300 kgN hm-2)水平下根表、根际和土体土壤pH值、铵态氮(NH+4-N)和硝态氮(NO-3-N)含量、硝化强度和氨氧化细菌(AOB)数量.结果表明无论是齐穗期、灌浆期还是成熟期,根表土壤pH值均显著低于根际和土体土壤.土壤pH值范围在5.95至6.84之间变化.土壤NH+4-N含量随水稻生长显著下降,且随施氮量增加而显著增加.根表土壤NH+4-N有明显亏缺区,且随距水稻根表距离增加,NH+4-N含量逐渐升高.土壤NO-3-N含量随水稻生长显著增加,施氮处理均显著高于不施氮处理,但N180和N300处理差异不显著.NO-3-N含量表现为根际>土体>根表.水稻根表和根际土壤硝化强度随水稻生长显著下降,而土体土壤硝化强度随时间延长小幅增加.施氮显著提高4007水稻根表土壤在齐穗和收获期硝化强度以及Elio在齐穗期根际硝化强度,但在施氮处理N180和N300中无显著差异.在整个采样期间,土壤硝化强度均表现为根际>根表>土体.水稻根表和根际AOB数量随水稻生长而显著降低,而土体土壤AOB数量无显著变化.例如,根表土壤AOB数量在齐穗期、灌浆期和收获期分别为16.7×105、8.77×105个g-1 dry soil和8.01×105个g-1 dry soil.根表和根际土壤AOB数量无显著差异,但二者显著高于土体土壤AOB数量.就两个氮效率水稻品种而言,土壤pH值基本无差异.4007土壤NH+4-N含量均显著高于Elio.在齐穗期水稻根表、根际和土体土壤NO-3-N含量在N180水平下均表现为Elio显著高于4007.而在灌浆期和收获期,水稻根表、根际和土体土壤则表现为4007显著高于Elio.在所有采样期,两个水稻品种土体土壤硝化强度和AOB数量在3个施氮量下均无显著差异.Elio根表和根际土壤硝化强度和AOB数量在水稻灌浆期之前一直显著高于4007,而在灌浆期之后则显著低于4007,且最终产量和氮素利用率(NUE)显著低于4007,这可能是由于4007灌浆期后硝化作用强,根际产生的NO-3-N含量高,从而4007根吸收NO-3-N的量也高造成的.因此水稻灌浆期和收获期根表和根际硝化作用以及AOB与水稻高产及氮素高效利用密切相关.

Soil nitrification in root surface and rhizosphere at late growth stages of rice cultivars with different N use efficiency

Nitrification characteristics in rice rhizosphere were studied using two Japonica rice cultivars with different nitrogen use efficiency (NUE), 4007 (high NUE) and Elio (low NUE). We determined pH, ammonium (NH+4-N) and nitrate (NO-3-N), nitrification and ammonia-oxidizing bacteria (AOB) abundance in root surface, rhizosphere and bulk soil under the N0 (0 kgN hm-2), N180 (180 kgN hm-2) and N300 (300 kgN hm-2) levels at the late growth stages in the field conditions. The pH values in root surface soil were significantly lower than those in rhizosphere and bulk soil at heading, filling and harvesting stages, ranging from 5.95 to 6.84. NH+4-N concentration decreased but NO-3-N increased with the plant development. N application increased NH+4-N and NO-3-N concentrations in all soil samples. Depletion sections of both NH+4 and NO-3 were found in root surface soil. The NH+4-N concentration increased with increasing distance from the root surface. The maximal NO-3 concentration was in rhizosphere soil, then the bulk soil and the lowest was in root surface soil. Nitrification activities in both root surface and rhizosphere soils significantly decreased with the incubation time, but the reverse was true for the bulk soil. N application improved nitrification activities in root surface soil grown with 4007 both at heading and harvesting stages, and also improved nitrification activity in rhizosphere soil grown with Elio at heading stage. But there was no significant difference between N180 and N300 treatments. The nitrification activity showed such order as rhizosphere>root surface>bulk soil at the whole sampling stages. AOB abundance in both root surface and rhizosphere soils significantly decreased with the incubation time, while those in the bulk soil indicated no difference as the time passed. For example, the AOB abundances in root surface soil at heading, filling and harvesting stages were 16.7 ×105, 8.77×105 and 8.01×105 g-1 dry soil, respectively. There was no significant difference of AOB abundance between root surface soil and rhizosphere soil, but they were all significantly higher than those in the bulk soil. As far as the two rice cultivars were concerned, there was no difference with the soil pH values. The 4007 grown soil NH+4-N concentration was higher than Elio. NO-3-N concentrations in root surface, rhizosphere and bulk soils for Elio grown treatment under N180 level at heading stage were higher than those for 4007. But NO-3-N concentrations in root surface, rhizosphere and bulk soils for Elio grown treatment at filling and harvesting stages were significantly lower than those for 4007. Nitrification activities and AOB abundance in bulk soil had no difference among the three N treatments. Nitrification activity and AOB abundance in root surface and rhizosphere soil for Elio grown treatment were significantly higher than those for 4007 before filling stage, while the reverse was true after filling stage, and the rice yield and NUE for Elio were much lower than 4007. That might be due to the higher nitrification and higher NO-3-N concentration in thizosphere soil for 4007 than Elio after filling stage, which caused more NO-3-N absorption by 4007 than Elio. We concluded that nitrification and AOB abundance in root surface and rhizosphere soil at filling and harvesting stages were important to a high rice yield and high NUE.