作物-土壤氮循环对大气CO2浓度和温度升高响应的研究进展

在地球化学元素循环中,氮素是最重要、最活跃的营养元素之一。农田生态系统中的氮素很大程度上决定农作物的产量和品质。然而,在全球气候变化背景下,随着大气CO₂浓度和温度升高,作物-土壤氮循环的变化可能显著影响农田生态系统中的作物生产。因此,研究作物-土壤氮循环对大气CO₂浓度和温度升高的响应,能够为科学合理地预测未来气候条件下,农田生态系统中作物的氮素需求,以及保障农作物产量的稳定供应提供理论依据,对于全面认识全球气候变化背景下的农田生态系统氮素循环过程及土壤可持续利用具有重要意义。本文综述了大气CO₂和温度升高对作物氮素吸收和分配,以及与氮有效性密切相关的土壤氮转化的影响,并系统总结了二者对作物-土壤氮循环过程产生的交互作用。总结以往研究发现,在大气CO₂浓度升高条件下,作物的蒸腾作用减弱,但光合作用增强,生物量加大,根系分支和根表面积增加,豆科作物的根瘤固氮能力提高,因此整体上促进作物对氮的吸收,并且增加作物向籽粒中分配氮的比例,但作物的平均氮浓度降低。此外,高CO₂浓度提高了土壤酶活性,增强了土壤有机氮矿化作用、硝化及反硝化作用,加速了土壤氮转化。升温和CO₂浓度升高对作物-土壤氮循环产生正向或负向的交互作用,主要表现在：高温和高CO₂浓度对作物的生物量、光合作用、地下部氮分配、根系分支以及根表面积具有协同促进作用,升高温度减轻了高CO₂浓度对作物蒸腾作用和作物氮浓度的抑制作用。然而,升温抑制了高CO₂浓度对作物向籽粒中氮分配、氮吸收以及产量的促进作用;升温虽然能进一步增强高CO₂浓度对土壤酶活性和有机氮矿化的促进作用,但是对于土壤硝化和反硝化作用,二者的交互作用以及相关的分子机制尚不明确。大气CO₂升高和温度升高对土壤微生物,以及微生物与作物之间的耦合关系的研究比较薄弱,特别是由微生物主导的氮循环过程及其对全球气候变化的反馈机制是未来研究的重点。本文提出利用16S rRNA、DGGE、T-RFLP、qPCR、RT-PCR技术、蛋白组学以及稳定性同位素探针原位研究技术,可以将复杂环境中微生物物种组成及其生理功能进行耦合分析,揭示大气CO₂浓度与温度对作物-土壤氮循环过程的交互作用机理,增强对气候变化下农田生态系统氮素循环响应的预测能力,为农田生态系统有效地适应气候变化提供科学的理论依据。

Nitrogen Cycling in the Crop-Soil Continuum in Response to Elevated Atmospheric CO2 Concentration and Temperature -A Review

In geochemical element cycling, nitrogen is one of the most important and active nutrient elements, determining grain yield and quality of crop. With the increase of atmospheric CO₂ concentration and temperature in global climate change, the changes in crop-soil nitrogen cycle may significantly affect crop production in agro-ecosystem. Therefore, studying the response of crop-soil continuum nitrogen cycle under elevated atmospheric CO₂ concentration and temperature could provide a theoretical basis for the scientific and reasonable prediction of crop nitrogen demand in farmland ecosystem and the guarantee of stable supply of crop yield under the future climate conditions. Effects of elevated CO₂ and temperature on nitrogen uptake and distribution in crop and soil nitrogen turnover were reviewed in this paper, and the interaction between elevated CO₂ and temperature on crop-soil nitrogen cycling processes in previous studies was systematically summarized. Under elevated atmospheric CO₂ concentration, although the transpiration of crop decreased, the photosynthetic rate, biomass, root branches and root surface area increased, and the root nodule nitrogen fixation ability of legume crop also increased. Thus, these factors overall promoted crop nitrogen uptake and increased crop yield and grain nitrogen allocation, but the average nitrogen concentration across crop decreased. Furthermore, the high CO₂ concentration increased soil enzyme activity, enhanced soil organic nitrogen mineralization, nitrification and denitrification, and accelerated soil nitrogen turnover. Warming and increasing CO₂ concentration interactively affected crop-soil nitrogen cycle. Warming and high CO₂ concentration synergistically promoted crop biomass, photosynthesis, underground nitrogen distribution and root branching and root surface area. Elevated temperature enhanced the elevated CO₂-induced inhibition on crop transpiration and nitrogen concentration. Warming inhibited the positive effect of high CO₂ concentration on nitrogen distribution, nitrogen absorption and yield of crop grains. Elevated temperature enhanced soil enzyme activity and mineralization at high CO₂ concentration, but molecular mechanisms of their interactive effect on soil nitrification and denitrification were still not clear. Research on soil microbes in relevant to aboveground crop, in particular, nitrogen cycling processes and their feedback mechanisms on global change remained unknown. The 16 S rRNA, DGGE, T-RFLP, qPCR, RT-PCR techniques, proteomics and in situ studies of stable isotope probes could be used to investigate the microbial species composition and physiological functions in complex environments. This review highlighted further investigations on the potential interaction between elevated CO₂ and temperature on nitrogen cycle in the crop-soil continuum in farming soils, and the microbial community in the rhizosphere that were involved in soil nitrogen cycle. Thus, the nitrogen cycle in agricultural ecosystem under climate change could be predicted, by which the adaptability of the ecosystems to climate change could be enhanced effectively.