1. College of Ecology, Resources and Environment, Dezhou University, Dezhou 253023, China; 2. Institute of Environental Research at Greater Bay, Guangzhou University, Guangzhou 510006, China; 3. School of Public Health, Shandong First Medical University (Shandong Academy of Medical Sciences), Jinan 250117, China
Abstract：To study the effects of nitrogen input caused by human activities on the release of plant root exudates and the nitrification process of soil, in-situ soil and reed seedlings of the Yellow River Delta wetland were collected for enrichment culture experiments, and the release rate of root exudates, nitrification, as well as related physicochemical factors were measured and analyzed. The results showed that the addition of different nitrogen had different effects on physicochemical factors of soil, among which NH4Cl had the most significant promoting effect on microbial biomass carbon and nitrogen and NH4+-N content, and KNO3 addition could significantly increase soil NO3--N content, and reed litter had a stronger promoting effect on soil organic carbon and total nitrogen. Both in rhizosphere and non-rhizosphere zone, the average ammonia oxidation rate (Va) was (5.16±1.37) and (3.62±0.80) μmol/(kg·d), respectively, and the content of NH4+-N was significantly positively correlated, while the average nitrite oxidation rate (Vn) was (6.01±1.42) and (4.45±1.08) μmol/(kg·d), and the content of NO3--N was significantly positively correlated. Soil SOC, TN and Va were significantly positively correlated, which may be explained by the fact that the increase of organic matters promotes the mineralization and then provides the reaction substrates for the ammonia oxidation process. The higher index of nitrogen balance in organic nitrogen addition group also indirectly proved the promotion of mineralization. Compared with the control group, exogenous nutrients can promote the secretion of total acids, total sugars and amino acids in reed root system, with the average release rates were (169.75±31.83), (116.72±21.76) and (9.52±3.31) μg/(g·d), but significantly inhibit or have no obvious effect on phenols and flavonoids. There was a significant negative correlation between the phenols secreted by roots and Va, which was mainly attributed to the selective inhibition of phenols to ammonia-oxidizing bacteria. However, amino acids were significantly positively correlated with Va and Vn, which could provide energy and nutrients for the metabolism of nitrifying bacteria and then promote the nitrification process. In general, on the one hand, the addition of exogenous nitrogen could provide a suitable growth environment for nitrifying bacteria by directly changing the physical and chemical properties of soil. On the other hand, exogenous nitrogen also stimulated the release of root exudates, increasing the content of nutrients and active substances, and then promoted the nitrification.
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