长期不施氮肥下稻麦轮作农田残留化肥氮的后效及去向

Long-term Fate and Availability of Residual Fertilizer Nitrogen in Rice-Wheat Cropping System in Taihu Lake Region of China

我国农田化肥氮用量高,造成较多肥料氮土壤残留,残留肥料氮既可被后季作物吸收利用,也可迁移进入环境。稻麦轮作是我国长江中下游农业区代表性种植制度,然而稻麦轮作农田土壤残留化肥氮的作物后效及去向目前尚不清楚。利用^(15)N示踪长期试验,连续追踪了2004年小麦季施用30%的^(15)N标记尿素后其土壤残留^(15)N在之后17个稻麦轮作年的变化动态及被后季作物吸收利用特征。试验起始小麦季设100 kg·hm^(–2)(N100)和250 kg·hm^(–2)(N250)两个施氮量处理,后续作物均不再施用氮肥。结果发现,34.5%~37.9%施入氮被当季小麦吸收,随后各轮作年稻麦作物吸收残留氮量随年限增加呈指数下降;17年中有12.2%~15.8%残留氮被后季作物吸收,其中,水稻对残留氮吸收能力较强,为9.2%~11.8%,小麦为3.3%~4.0%;观测期内化肥氮累积利用率为50.1%~50.3%。氮肥施入小麦当季,0~20cm土层残留为22.9%~33.5%,之后逐年减少;17年后降至7.8%~9.8%,但仍占0~100cm土层氮残留量(9.9%~13.4%)的73.5%~78.5%。同位素质量平衡估算的观测期内氮肥累积总损失率为36.3%~39.9%,与基于当季小麦氮肥利用率和0~20 cm土壤残留率计算得出的当季化肥氮总损失率32.0%~39.2%接近。作物籽粒、秸秆及土壤^(15)N丰度在观测期内均随时间呈指数递减;根据预测结果,不施氮下其降至^(15)N自然丰度背景值仍需28~37年。上述结果表明,稻麦农田化肥氮损失主要发生在当季,土壤残留后效持续时间长,但再迁移进入环境数量低。协同化肥氮当季损失的高效阻控和土壤残留的有效调控应是稻麦农田氮肥优化管理的关键环节。

【Objective】The high chemical nitrogen(N) fertilizer input in cropland soils of China has caused a large accumulation of residual fertilizer N in the soil in the current-season. This soil-residual fertilizer N can either be absorbed by subsequent season crops or lost to the environment through gaseous and hydrological pathways. The rice-wheat rotation is a dominant vital cropping system in the middle and lower reaches of the Yangtze River agricultural region in China. However, the residual effects and fate of the soil-residual fertilizer N in this cropping system remain unclear. 【Method】In this study, a^(15)N tracer long-term in-situ experiment was used to continuously monitor the fate and the residual effect of soil-residual fertilizer N in the following 17 years under non-fertilizer N application in a rice-wheat cropping system. The experiment had two N fertilizer treatments, with 100(N100) and 250(N250) kg·hm^(–2)of labelled urea(30 atom%) applied in the first wheat season, and no N fertilizer was added in the subsequent 17 years of the rice-wheat rotation. 【Result】 The results suggested that 34.5%–37.9% of the applied fertilizer N was taken up by the first wheat crop, and then the amount of residual N uptake by the rice and wheat decreased exponentially in the following rice-wheat rotation years. Over the following 17 years, 12.2%–15.8% of the applied fertilizer N was taken up by the subsequent crops(9.2%–11.8% for rice and 3.3%–4.0% for wheat), leading to the accumulative crop N recovery of 50.1%–50.3%, which was significantly higher than the in-season N use efficiency. We found that 22.9%–33.5% of the applied fertilizer N remained in the 0–20 cm soil after in-season wheat was harvested, which was then gradually decreased to 7.8%–9.8% after 17 years, but still accounted for 73.5%–78.5% of the total residual N in the 0–100 cm soil layer(9.9%–13.4%). The cumulative total loss of fertilizer N over the observation period estimated from the isotope mass balance was 36.3%–39.9%, which was close to the total loss of fertilizer N of 32.0%–39.2% calculated based on the N fertilizer use efficiency and the residual rate of 0–20 cm soil in the current season. The^(15)N abundance of crop grain, straw and soil all decayed exponentially with time during the observation period, which predicted that it would still take 28–37 years for the crop to decrease to the natural^(15)N abundance background value without N application.【Conclusion】Overall, fertilizer N losses in the rice-wheat cropping system mainly occurred in the current-season, and the residual effects of fertilizer N in soil lasted for a long time, but a negligible amount of this residual N can be lost to the environment. The keyways to optimal N fertilizer management in rice-wheat rotation are effectively reducing in-season fertilizer N losses and better utilizing soil-residual fertilizer N.