岩溶区不同植被类型下的土壤氮同位素分异特征

Differential characteristics of soil δ^(15)N under varying vegetation in karst areas

利用氮同位素自然丰度(δ15N)法研究了重庆青木关岩溶区不同植被类型下的土壤氮同位素分异特征及其影响因素。研究结果表明区内土壤1δ5N受不同的植被、地质和土地利用等影响而具有明显的横向和垂向分异特征,具体表现为:0—20 cm层土壤1δ5N与植物体1δ5N具有显著相关性,该层土壤1δ5N大小为草地土(4.79‰)>退耕还林土(4.77‰)>稻田土(4.28‰)>旱地土(4.25‰)>灌丛土(3.82‰)>针叶林土(3.81‰)>砂岩区针叶林土(3.40‰);20—40 cm层土壤δ15N大小为砂岩区针叶林土(5.21‰)>退耕还林土(5.10‰)>草地土(5.01‰)>稻田土(4.88‰)>灌丛土(4.70‰)>旱地土(4.55‰)>针叶林土(4.26‰);40 cm以下土壤1δ5N差异较小,灌丛最高,旱地最低。总体上,砂岩区林地土壤除表层贫化15N外,其余各层均富集15N,且分异较小。而岩溶区土壤δ15N具有明显的垂直变异特征,特别是灌丛林,表现为40 cm以上的土壤贫化15N,且变化较大,其中0—10 cm土壤δ15N受植物凋落物影响而较低,10—20 cm土壤δ15N受微生物作用影响稍显偏高,20—30 cm土壤δ15N随微生物活动减弱而有所降低;40 cm以下土壤矿化程度较大而富集15N,且变化较小。除受植被类型影响外,岩溶区土壤1δ5N还受其偏碱、富钙、高粘粒含量的物理化学特性以及农业施肥活动等因素共同影响。

Natural abundance δ15N can fingerprint the process of nitrogen biogeochemical cycling.By collecting soil samples under different vegetation types in Qingmuguan karst area,Chongqing,China,the spatial and vertical disparities of soil δ15N and its influencing factors were analyzed.The results showed that foliar δ15N values ranged from-3.85‰ to 2.25‰,while litter had higher δ15N values,ranging from-2.49‰ to 3.31‰.The soil δ15N values were much higher than those of plant leaves and litter,and showed significant spatial and vertical disparities due to varying vegetation,geological and land-use conditions.Within 0—20 cm layer,soils were generally 15N-depleted and showed positive relationship with those of covering plant leaves,indicating that the covering vegetation was the main source of soil organic nitrogen.The mean δ15N values of surface soil showed significant spatial disparities and a systematic control on land-use characteristics,and tended to decrease in the following order： grassland（4.79‰）,afforestation farmland（4.77‰）,paddy field（4.28‰）,day land（4.25‰）,shrub land（3.82‰）,coniferous forest land in limestone areas（3.81‰）,and coniferous forest land in sandstone areas（3.40‰）.However,in the layer of 20—40 cm,soil δ15N values were 5.21‰,5.10‰,5.01‰,4.88‰,4.70‰,4.55‰,and 4.26‰ in coniferous forest land（in sandstone areas）,afforestation farmland,grassland,paddy field,shrub land,day land,and coniferous forest land（in limestone areas）,respectively.Below 40 cm depth,soil δ15N values in different profiles showed slight spatial disparities,in which soils in shrub land had higher δ15N values and lower δ15N values were observed in the soils of dry land due to the application of chemical fertilizers.In general,variation coefficient of soil δ15N ranked in the order as： shrub land（31.36%） coniferous forest land in sandstone area（28.25%） dry land（19.28%） coniferous forest land in limestone area（12.97%） paddy field（9.88%） afforestation farmland（8.20%） grassland（6.82%）.Forest soils in sandstone area were 15N-depleted within 0—20 cm layer,while 15N-enriched below 20 cm depth.In limestone area,soil δ15N values were lower and showed significant vertical disparities between 0 and 40 cm,in which δ15N values were much lower within 0—10 cm layer due to the input of 15N-depleted litter,then increased obviously in the layer of 10—20 cm because of the intensive microbial activity,and decreased somewhat within 20—30 cm layer.Below 40 cm depth,soil δ15N values increased significantly due to intensive mineralization and decomposition of residual soil organic matter.Apart from the 15N-depleted plant litterfall,soil δ15N was also affected synthetically by soil texture,structure,pH,organic matter content,C/N,agricultural fertilizing activities,and so on.Karst soils,controlled by the chemical weathering of soluble rock,are often alkaline,calcium-enriched,clayey and thin,which makes the spatial and vertical disparities of soil δ15N more complicated than those in sandstone areas.In upper part of profiles,soils are loose,which are liable to migration of surface 15N-depleted organic nitrogen and result in decreasing of soil δ15N,while soils in lower part of profiles are relatively compact and 15N-enriched due to the greater mineralization degree of residual organic nitrogen.