Effects Of Cutting And Fertilization On The Chemical Characteristics Of N And P In Alpine Meadow Soil And Plants

Ecological stoichiometry, which is in combination with the basic principles of biology, chemistry and physics, is a tool used to study the relationship of the elements in the ecological process, especially relationships among organic carbon (C), nitrogen (N) and phosphorus (P) though analyzing and comparing the relative ratio of the elements of different level in ecological system. N and P are the fundamental elements of plant nutrient, whose recycle affects most of the ecosystem process. Plant N, P content and allocation are affected by soil nutrient availability, growth form, physiological characteristics, life history and other human activities, such as clipping, fertilizing. It is the result of the combination of environment and species system development. Community and different plant functional groups show diverse response such as plant height, biomasses, and nutrient limitation type to the disturbanced factors for their differences on nutrient acquisition, use efficiency, which may result in different adaptation to environment.In the natural terrestrial ecosystem, N:P ratio has been widely used to predict the N:P stoichiometry characteristics of different organizational levels at plant individual, community and ecosystem as well as the nutrient restriction. Gusewell (2005) argue that when plant N: P ratio<13, the vegetation is limited by N; when N:P ratio>16, the vegetation is limited by P; when13<N: P ratio<16, the vegetation is both limited by N and P. In addition to the use of plant N: P ratio, N, P content in plants is also used to judge plant nutrition status in ecosystems. Such as Duru (1997), they proposed standard equation:P%=0.15+0.065N%, and predicted the nutrient restriction. Plant nutrient has important role on plant growth and development. To a large extent, leaf N, P stoichiometric characters influence the plant growth rate, often we call it the growth rate hypothesis, which has the idea that the plant which grows faster usually has higher N, P contents and lower N:P ratio. However, the ecological stoichiometry theory is proposed, tested and enriched based on the research of aquatic organisms, the studies about terrestrial plants are not enough What is more, whether the ecological stoichiometry theory is applicable to all creatures or various ecological organization scale has yet to be further tested.Thus, a field manipulative experiment which includs clipping(unclipped, stubbled3cm, and1cm), watering(unwatered, watered)and fertilizing(unfertilized, fertilized) treatments was conducted in Haibei Research Station in2007to study the response of stoichiometric characteristics of soil, plant functional groups and community to clipping and fertilizing treatment in alpine meadow, and estimate the changes in aboveground biomass under different treatments, to explore the plant nutrition limiting types and that whether the growth rate hypothesis is also applicable at community level. The main results are as follows:1. Clipping and fertilizing have different effects on soil nutrient. Under non-cliped, stubbled3and stubbled1treatment, the soil total N content (mg·g-1) respectively are4.41,4.40,4.73; the soil total P content (mg·g-1) respectively are0.75,0.73,0.73; the soil available N content (mg·kg-1) respectively are32.62,29.71,30.29; the soil available P content (mg·kg-1) respectively are3.03,4.92,5.48; the soil available N:P respectively are11.04,6.22,5.70. Clipping has no significant effects on total soil nutrient, but the soil total N has a tendency to rise with long-term clipping processing; Clipping has no significant effect on soil available N while the soil available P rises with the increased clipping intensity significantly, that is to say clipping helps to stimulate the release of soil inorganic P. Neither the soil total nutrients nor the available nutrients change significantly under fertilizing treatment. That because soil available nutrient is instantaneous value, affected by absorption, consumption and return or recycle process of plants, the nutrients, worked as fertilizer, added into the soil is quickly absorbed by the plants rather than store in the soil nutrient depots.2. Clipping and fertilizing have a significant impact on community N contents, P contents and N:P ratio. Under non-cliped, stubbled3and stubbled1treatment, the community N content (mg·g-1) respectively are15.7,19.01,21.47; the community P content (mg·g-1) respectively are1.03,1.59,1.58; the N:P ratio respectively are15.91,12.54,13.82. Similarly, under non-fertilizing and fertilizing treatment, the community N content (mg·g-1) respectively are16.14and20.8, P content (mg·g-1) respectively are1.20and1.57; N:P ratio respectively are14.72and13.46. Clipping promotes the growth of younger plant tissue, increasing the absorption of N and P nutrients; Fertilizing can improve the soil nutrient availability, which is absorbed by plants. As whole, clipping and fertilizing increase community N content, P content, decrease the community N:P ratio.3. The N content (mg·g-1) of plant functional groups of legumes, grasses and sedges respectively are23.65,15.68,17.91; The P content (mg·g-1) of the three groups respectively arel.03,1.04,0.75; their N:P ratio respectively are24.82,15.59and27.18. The N content of legumes is significantly higher than that of grasses and sedges. The P content of legumes and grasses are significantly higher than the sedges. The N:P ratio of grasses is significantly lower than legumes and sedges. Clipping significantly increase the P content of grasses and N congtent, P content of sedges. Fertilizing has no significant influence on the N, P content of legumes, grasses and sedges; Both clipping and fertilizing has no significant influence on the N:P ratio of these three functional groups.4. In my study, mean N, P content (mg·g-1) of the community are18.73and1.38, N:P are13.98. Both the community N content and P content are lower than the results of Yang Kuo’s investigation on alpine meadow. According to the critical N:P ratio theory of community which is used as a predictor of nutrients limitation proposed by Gusewell (2005), the community is both restricted by N and P and more restricted by P. The plant functional groups of Legumes, grasses and sedges are restricted more severely by P because the N:P ratio of legumes and grasses are much higher, As a result, legumes increase P uptake efficiency while reduce the N uptake efficiency through negative feedback to alleviate the P restriction, showing a negative correlation between N content and P content in plants rather than positive correlation which is generally presented by other plants.5. The above ground biomass is significantly affected by clipping and fertilizing treatment. Under non-cliped, stubbled3and stubbled1treatment, the community above biomasses (g-0.25m-2) respectively are106.54,114.45,123.35. With the increasing clipping intensity, the community biomass increases, fertilizing significantly increases the community biomass too:the community above biomasses (g·0.25m-2) changes from87.971to142.78. However the response of legumes, grasses and sedges biomasses to clipping and fertilizing treatment are totally different:for legumes, the above ground biomasses (g·0.25m-2) respectively are1.60,7.05,11.53under the conditions of non-cliped, stubbled3and stubbled1treatment. Clipping significantly increases biomass of the group, while fertilizing has little effect on it, even the biomass declines after fertilizing. As to grasses and sedges, only clipping increases the biomass of sedges. Under non-cliped, stubbled3and stubbled1treatment, the biomass (g·0.25m-2) of sedges respectively are3.59,10.54,11.26. Fertilizing has little effect on the biomasses of grasses and sedges, but their biomasses has a trend to increase after fertilizing 6. In my study, at community level, the growth rate is significantly positive correlated with community N content and P content while significantly negatively correlated with community N: P ratio. That is to say that the growth rate hypothesis is also applicable at community level.In summary, within the range of clipping intensity and fertilizer level of my study, clipping and fertilizing have significant impact on alpine meadow soil and plant N, P stoichiometric characteristics. Moreover, clipping and fertilizing can affect plant above ground biomasses through changing soil and plant N, P stoichiometric characteristics. In order to improve the grassland productivity, we can fertilize more P element in future rangeland management. The widely used growth rate hypothesis is also applicable at community level.