根温与CO2浓度调控对设施黄瓜根际过程与氮肥利用率的影响及机制研究

The nitrogen use efficiency is relatively low in the greenhouse vegetable cultivation in our country. It is mainly caused by the inappropriate environment conditions for the vegetable growth in the greenhouse. Among them, the most important factors are the root temperature and CO2 concentration. However, it is still not clear how the root temperature and CO2 concentration regulate the nitrogen use efficiency of vegetables. In present project, the object of our interest is cucumber, which is the main greenhouse vegetable grown in the suburb of Yangtze River Delta. We focus on the effects of root exudates of cucumber on the concentrations of different forms of nitrogen in the greenhouse soil. Firstly, the composition and quantities of sugars, organic acids and amino acids in cucumber root exudates under different root temperatures and CO2 concentrations will be systematically analyzed in hydroponic condition, and the relationship between the releasing amounts of those compounds in root exudates and root extracts as well as root morphological traits will be established. Secondly, the releasing amounts of the compounds in root exudates of cucumber grown in soil will be estimated based on their concentrations in root and the root morphological traits. Then the artificial root exudates containing the same composition and quantities of compounds as the root exudates of cucumber grown in soil will be added in the incubated soil by a cellulose filter to mimic the root exudation. The effects of the root temperature and CO2 concentration on the concentrations of different nitrogen pools in soil will be investigated by comparing the gross nitrogen transformation rates between soil with cucumber growing and with artificial root exudates treating. This program will try to reveal the mechanisms by which the root temperature and CO2 concentration affect the nitrogen use efficiency of cucumber from three processes via which cucumber uptakes nitrogen nutrients: the root morphological traits and activities indicate root extension; the transpiration indicates mass-flow; the concentrations of different nitrogen pools in soil indicate diffusion. Finally, the appropriate root temperature and CO2 concentration for greenhouse cucumber growing will be suggested, and the nitrogen fertilizer application strategy which matches the environment conditions will also be established. This will be of great importance for both theoretical and practical applications on effective use of resources and sustainable utilization of greenhouse soil.

我国设施蔬菜氮肥利用率较低，往往是由于设施环境条件不能满足蔬菜生长需要，本项目重点关注的是根区温度和大气CO2浓度。目前尚不明确根温与CO2浓度是如何影响设施蔬菜氮肥利用率的。本项目重点研究设施黄瓜根系分泌物对土壤中各种形态氮素含量的影响：通过水培试验得到不同根温和CO2浓度下黄瓜根系分泌物中各组分分泌量与其根内含量和根系形貌参数间的相关性，再由土壤盆栽的根系形貌参数和根内含量推算出根系分泌物中各组分的分泌量，以模拟根系分泌物动态添加的土培试验与土壤盆栽相对比，验证不同根温和CO2浓度对土壤氮素转化过程和土壤氮库变化的影响。以根系形貌和活力解释截获过程，以蒸腾作用解释质流过程，以土壤氮库变化解释扩散过程，从黄瓜吸收氮素的这三个途径来阐明设施环境影响黄瓜吸收氮素的内在机制。在此基础上以设施大棚小区试验探究适宜的根温和CO2浓度范围，并制定与其相匹配的氮肥施用策略，以提升设施黄瓜的氮肥利用率。

我国设施蔬菜氮肥利用率较低，往往是由于设施环境条件不能满足蔬菜生长需要，本项目重点关注的是根区温度和大气CO2浓度。目前尚不明确根温与CO2浓度是如何影响设施蔬菜氮肥利用率的。针对此问题，本项目首先搭建了可准确控制根区温度和CO2浓度的研究平台，开发了同位素标记土壤的新技术，在此基础上开展了一系列根区温度和CO2浓度对设施黄瓜生长、矿质元素吸收途径、氮素利用率、根际微生物和设施土壤氮素转化过程的影响机制研究。结果表明：1）CO2浓度升高对设施黄瓜氮肥利用率的促进作用主要是依靠地上部生物量的提升，而根区温度可以通过同时提高黄瓜地上部生物量和植株氮浓度来提升黄瓜氮肥利用率。2）根区加温可以提高CO2施肥下叶片的光合速率和蒸腾速率，缓解高CO2造成的叶片部分气孔关闭；促进黄瓜根系生长和对氮的吸收，保持叶片氮浓度；有利于光合产物从叶片向根系转运，减少光合产物累积导致的叶绿体损伤等光合途径阻断问题，从而缓解CO2施肥导致的光合适应。3）根区加温增加了蒸腾速率，通过质流的增加和对根系生长的促进提高了黄瓜对硝态氮、钾、钙和镁的吸收。而磷、铁、锰和锌的吸收则主要依赖于CO2施肥和根区加温对根系生长促进，使其被截获的概率增加。根区加温还可以增加土壤矿质养分浓度和有效性，以促进黄瓜通过扩散吸收铵态氮、钾、磷、铁、锰和锌。因此，保持根区加温，可以促进CO2施肥条件下黄瓜对各种矿质养分的吸收量和利用效率。4）CO2施肥和根区加温通过增加相应的功能基因拷贝数和α多样性促进了土壤氮的初级矿化、硝化和固定速率，从而加速了氮循环的周转，为黄瓜生长提供了更多的无机氮，并降低了土壤无机氮浓度，可以实现在提高设施蔬菜生产力的同时保持设施土壤有机氮并减少土壤中的无机氮损失。本项目的研究结果可以从设施环境调控角度，为实现设施蔬菜氮素的高效利用和设施土壤的可持续利用提供科学依据。