热带滨海区不同土地利用背景下土壤溶解性有机氮的组成与粒径分布特征

溶解态有机氮(DON)是土壤中活跃的氮库,其生态环境行为与它的化学组成和粒径分布密切相关。为评估热带滨海区不同土地利用方式对不同粒径中土壤溶解性有机氮组成特征的影响,从水稻田、橡胶园、菜园和果园采集土壤样品,通过一系列微滤和超滤(0.7,0.45,0.2,0.1μm,100,10,1 kDa)对土壤溶解性有机氮分级,并使用连续流动分析仪、三维荧光光谱和红外光谱研究了滤液中溶解态有机氮、无机氮的含量及荧光组分和有机官能团特征。结果表明,4种土地利用背景下土壤DON值的范围为5.25-10.88 mg·kg^-1,其大小顺序为水稻>菜园>果树>橡胶,且DON与溶解性总氮(DTN)的比值范围为26.08%-67.11%,其中橡胶土最高,水稻土最低;不同粒径下4种土地利用类型土壤DON主要集中在<100 kDa的粒径中,其值范围为4.85-9.48 mg·kg^-1,占全量的85.89%-92.41%。三维荧光光谱(3D-EEMs)及平行因子分析表明,4种土地利用背景下土壤DON含有两种类腐殖质组分及一种类蛋白质组分,且以类腐殖质组分为主,占比54.00%-77.67%;类蛋白组分对土地利用变化敏感,且随着粒径的减小,类蛋白组分占比增加,在<1 kDa组分中比例最高。红外光谱结果表明,4种土地利用背景下土壤DON主要在6个位置有相似的吸收峰,包含3410 cm^-1、1636 cm^-1、1402 cm^-1、1138-1035 cm^-1、673 cm^-1、602 cm^-1,不同土地利用背景下各吸收峰的透光度不同,强度最大的吸收来自游离的胺类N-H伸缩振动;水稻、菜园土壤DON芳香物质含量较高,结构较复杂。了解DON的组成与粒径分布对土地利用的响应,对进一步研究其生态环境行为具有重要意义。

Composition and Size Distribution Characteristics of Soil Dissolved Organic Nitrogen under Different Land Use Types in Tropical Coastal Areas

Dissolved organic nitrogen(DON)is an active nitrogen pool in soil environments,and its ecological and environmental fate are highly related to the chemical molecular composition and size distribution.To estimate the effects of different land use types on size-dependent DON quantity and quality in tropical coastal areas,soil samples were collected from paddy fields,rubber plantations,vegetable gardens and orchards,and soil dissolved organic nitrogen was fractionated through a series of micro-and ultra-filtrations with different cutoffs membranes,including 0.7μm,0.45μm,0.2μm,0.1μm,100 kDa,10 kDa and 1 kDa.Abundance of dissolved organic nitrogen,inorganic nitrogen,organic functional groups and fluorescent components in the filtrates were investigated by continuous flow analyzer,three-dimensional fluorescence and near-infrared spectroscopy.Results showed that the quantity of dissolved organic nitrogen(DON)under different land use types ranged from 5.25?10.88 mg·kg^-1,and they followed this order:paddy soil>vegetable soil>orchard soil>rubber soil.The ratios of DON to DTN ranged from 26.08%to 67.11%,with rubber soil being the highest and paddy soil being the lowest.In addition,the quantity of soil DON under four land use types was mainly concentrated in the size of<100 kDa,ranging from 4.85 to 9.48 mg·kg^-1,accounting for 85.89%?92.41%.EEM with Parallel factor analysis(PARAFAC)revealed that the fluorescence of DON under different land use could be separated into three components:2 humic-like and 1 protein-like signals.And the fluorescence of DON was mainly composed of humic-like components,accounting for 54.00%?77.67%.Protein-like component were sensitive to land use changes,the percentage of protein-like component generally declined from 0.7μm to 1 kDa fractions,being highest in the<1 kDa fraction.Fourier transform infrared spectroscopy showed that the soil DON had similar absorption peaks in six regions including 3410 cm^-1,1636 cm^-1,1402 cm^-1,1138?1035 cm^-1,673 and 602 cm^-1.Their relative intensities differed between samples,and the strongest absorption in all cases arising from stretching vibration of free amine N-H groups.The DON in paddy soil and vegetable soil had more aromatic substances and the structure was more complex.Understanding the response of the composition and size distribution of DON to land use was of great significance to further study its ecological and environmental behaviors.