垫状点地梅对生长季土壤净氮矿化和酶活性的影响

Effect of a Cushion Plant,Androsace tapete,on Soil Net Nitrogen Mineralization and Enzyme Activities during the Growing Season

垫状植物是高寒生态系统中一类独特的物种,具有厚实的多年生垫状体,可以改善局部微环境,从而对生长于其内部的其他物种起到保育作用,被称为高寒生态系统的工程师,其中对土壤养分有效性的改变是其保育作用的重要途径,但关于这一过程的研究目前还很少。本研究选择青藏高原广泛分布的垫状点地梅(Androsace tapete),在西藏当雄念青唐古拉山脉南坡4500 m和4800 m两个海拔设置样地,通过动态测定垫状点地梅覆盖下土壤和无垫状点地梅生长的对照草地土壤在生长季中的无机氮含量、净氮矿化速率以及土壤酶活性,对比分析垫状点地梅对土壤氮素有效性的影响。结果表明:(1)土壤中硝态氮和铵态氮含量在4500 m样地无显著差异,但在4800 m样地,垫状点地梅覆盖下土壤的硝态氮与无机氮含量在生长季中期有显著增加,其中硝态氮比对照草地增加了56%,无机氮则增加了74.5%;(2)垫状点地梅还改变了土壤中氮的矿化趋势和速率。在4500 m样地,垫状点地梅覆盖下土壤净氮矿化在生长季中期为负值(氮固定),速率为-0.11μg g^(-1)d^(-1),而对照草地土壤则为正值(氮矿化),速率为0.61μg g^(-1)d^(-1),二者差异显著;在生长季初期和末期二者均为正值,但差异未达到显著水平;在4800 m样地,垫状点地梅覆盖下的土壤在生长季初期的净氮矿化为正值,速率为0.07μg g^(-1)d^(-1),而对照草地土壤则为负值,速率为-1.17μg g^(-1)d^(-1),差异达到了显著水平;生长季中期二者均为负值,但垫状点地梅覆盖下土壤净氮矿化速率(-1.95μg g^(-1)d^(-1))显著低于对照草地土壤(-0.02μg g^(-1)d^(-1));在生长季末期二者也均为负值,且差异不显著。(3)垫状点地梅覆盖下土壤中的硝酸还原酶与亚硝酸还原酶活性显著提高。与对照草地土壤相比,4500 m样地垫状点地梅覆盖下土壤中的硝酸还原酶活性提高了9.1%,亚硝酸还原酶活性提高了15.7%;4800 m样地两种酶的活性则分别提高了22.5%和16.1%,而这两种酶的活性与土壤中无机氮的动态变化显著相关。以上结果表明,与对照草地相比较,垫状点地梅会改变土壤中的氮矿化过程和无机氮含量,但存在着季节上的动态变化,且不同样地之间也存在差异,说明这种作用不仅受垫状植物生态工程师效应的影响,还会受到外部环境变化的影响。并且垫状点地梅的工程师效应在整个生长季出现了不同程度的变化,尤其是在如今全球变暖的大背景下,青藏高原也正在加速变化,因此需要更多的研究来阐明其变化过程。

Cushion plants are one kind of unique specie in alpine ecosystems.They have a compact perennial cushion structure and play a role in facilitating the survival of other associated species by improving the local micro-environment.They are called“engineers”in the alpine ecosystem and their enhancement of soil nutrient availability is one of the ways of their engineering effect.In this study,Androsace tapete,a species of cushion plant widely distributed on the Qinghai-Tibet Plateau,was selected to investigate the dynamics of this process during the growing season at elevations of 4500 m and 4800 m on the southern slope of the Nyenchenthanglha mountains in Damxung.The effects of A.tapete on soil nutrient availability were analyzed by comparing the inorganic nitrogen content,net nitrogen mineralization rate and soil enzyme activities during the growing season in the soil under A.tapete and ambient grassland(CK).The results showed three important aspects of this system.(1)Soil nitrate nitrogen and ammonium nitrogen did not show significant differences at 4500 m,but the contents of nitrate nitrogen and inorganic nitrogen under the A.tapete soil significantly increased in the middle of the growing season at 4800 m,with nitrate nitrogen increasing by 56%and inorganic nitrogen increasing by 74.5%.(2)The trend and rate of soil nitrogen mineralization were both changed under A.tapete.In the 4500 m sample site,soil net nitrogen mineralization under A.tapete was negative(nitrogen immobilization)in the middle of the growing season,and the rate was-0.11μg g^(-1)d^(-1),while that of CK was positive(nitrogen mineralization)and the rate was 0.61μg g^(-1)d^(-1);and the difference between them was significant.However,both were positive in the early and late growing season,and the difference did not reach a significant level.In the 4800 m sample site,the soil net nitrogen mineralization under A.tapete was positive in the early part of the growing season,and the rate was 0.07μg g^(-1)d^(-1),while the mineralization for CK was negative,and the rate was-1.17μg g^(-1)d^(-1);and the difference was significant.In the middle of the growing season,both of them were negative,but the soil net nitrogen mineralization rate under A.tapete(-1.95μg g^(-1)d^(-1))was significantly lower than that of CK(-0.02μg g^(-1)d^(-1)).At the late stage of the growing season,both of them were negative,and the difference was not significant.(3)Activities of nitrate reductase and nitrite reductase in the soil were significantly increased under A.tapete.Compared to CK at the 4500 m sample site,the activities of nitrate reductase and nitrite reductase under A.tapete were increased by 9.1%and 15.7%,respectively;and they were increased by 22.5%and 16.1%,respectively,at the 4800 m sample site.The activities of these two enzymes were significantly correlated with the dynamics of inorganic nitrogen in the soil.These results indicated that,compared to CK,the cushion plant A.tapete can change the process of soil nitrogen mineralization and the content of inorganic nitrogen,but that change had seasonal dynamics and spatial differences,which implies that this process was affected not only by the engineering of the cushion plant but also by changes in the local environment.Therefore,the engineering effect of cushion plant A.tapete was not constant during the growing seasons,and further studies are needed to clarify this process,especially considering the rapidly changing climate on the Qinghai-Tibet Plateau.