Soil magnesium: Long-Term Nitrogen Deposition During Grassland Succession

The purpose of this experiment is to measure how initially disturbing an area and adding nitrogen over a long time will affect the number of species, the type of species present, the amount of annual growth, and the change from year to year in the growth of each species in a plant community which is also relieved of grazing by large and small mammals. This experiment is conducted within fields (A, B, and C) which were initially low in soil nutrients. The ground was disturbed by thoroughly disking the area prior to establishment of the experiment. There are 8 different levels of nitrogen addition with other nutrients added to ensure that nitrogen remains the limiting nutrient, and a control which receives no nutrients. Nitrogen fertilizer (NH4NO3) is applied twice per year, once in early May and once in late June. There are six replicates of each treatment per field. The treatments were randomly assigned to plots of size 4 by 4 meters. The plots are in 6 plot by 9 plot grids with 1 meter aisles between plots. The plot grids are enclosed by a fence to keep out mammalian herbivores. Gophers are trapped and removed as they appear. In the spring of 1992, subexperiments E097 and E098 were established. E097 is in fields A and C where randomly selected plots within each treatment no longer receive fertilizer. E098 is in field B where randomly selected plots within each treatment are burned. Note that the design of E002 is similar to E001 except E002 was thoroughly disked prior to establishment.本实验旨在探究初始生境扰动与长期氮素添加，对解除大、小型哺乳动物放牧的植物群落中，物种数量、现存物种组成、年生长量，以及各物种年际生长变化的影响。本实验设置于初始土壤养分含量较低的A、B、C三块样地中。实验开展前，通过圆盘耙翻耕对样地进行了彻底扰动。实验共设置8个氮素添加梯度，同时补充其他养分以确保氮素成为限制性养分 (limiting nutrient)，并设置不施加任何养分的对照组。氮肥（硝酸铵，NH4NO3）每年施用两次，分别于5月初与6月末施加。每个处理在每块样地内设置6个重复样方。所有处理随机分配至4米×4米的样方中。样方排布为6行×9列的网格，样方间预留1米宽的通道。整个样方网格均以围栏围合，以排除草食性哺乳动物的干扰；同时对出现的囊地鼠 (gophers) 进行诱捕移除。1992年春季，设立了两个子实验E097与E098：E097设置于A、C两块样地中，对应各处理下随机选取的样方将不再施加肥料；E098设置于B样地中，对应各处理下随机选取的样方将进行焚烧处理。需注意，E002的实验设计与E001近似，仅E002在实验设立前经圆盘耙翻耕彻底扰动。