Soil nitrogen: Interactive Effects of Fertility and Distribution on Plant Community Diversity and Structure

There are four levels of nitrogen, corresponding to treatments A, C, F and G in E001, applied at the same time as in that experiment. For a description of fertilizer added to E052, see file fertilization details. There are four levels of soil disturbance designated 1, 2, 3 and 4. Level 1: undisturbed Level 2: 1 pass with a 7 HP Honda rear-tined rototiller with the elevator set to till to a depth of 9 inches Level 3: 2 passes or however many required to produce about 50% bare ground Level 4: 3 passes or however many required to produce 100% bare ground. This requires 3 passes in some plots but 5 or 6 in others. In addition, all woody vegetation not destroyed by tilling is cut at the base. Rototilling is applied in late April. Each fertilization treatment receives each disturbance treatment, for a total of sixteen treatments. There are four replicates of each of the sixteen treatments. In addition, the four extreme ends (lowest N, lowest disturbance; highest N, lowest disturbance, etc. ) are replicated an additional ten times. Treatments are applied in a completely randomized design. Each of the 104 plots is 5m x 5m. Measurements taken at E052 will include: 1) species abundances, 2) community biomass allocation to leaves/roots/stems/flowers, 3) above and below ground net primary production and 4) rates of nitrogen mineralization. For a list of treatments, see the treatment layouts in file trmte52. The plots in E052 are enclosed by a fence to exclude mammalian herbivores. Galvanized welded-wire hardware cloth with 6mm x 6mm openings was buried to a depth of 50cm. Additional hardware cloth extends 60cm above the ground and poultry netting extends to 2m above the ground. In 1990, ten plots of each of four treatments (N1D1, N1D4, N4D1, N4D4, where N is the level of nitrogen added and D is the disturbance treatment) were randomly selected for the competition experiment. The above and belowground effects of neighbors on transplanted grass seedlings were measured using three competition treatments: transplants were grown in 50cm diameter subplots 1) with no neighbors, 2) with the roots but not shoots of neighbors, 3) with all above and belowground parts of neighbors present. One transplant was grown in the center of each subplot. Competition treatments were applied to subplots within the main plots during 14-18 May, 1990. Competition treatments were replicated three times in each of the ten replicate plots for each of the four combinations of nitrogen supply rate and disturbance. Thus, the 40 plots of the nitrogen and disturbance combinations (2 nitrogen levels x 2 disturbance levels x 10 replicates) each contained 9 subplots (3 competition treatments x 3 replicates), each subplot comprising one transplant in one competition treatment. A total of 360 seedlings were transplanted. Competition treatments were assigned randomly to subplots within each plot. In 1991, three species of contrasting sizes, life histories, growth rates and positions along gradients of nitrogen and disturbance in the experiment were used: Schizachyrium scoparium, Agropyron repens, and Setaria viridis. Transplants were grown individually in subplots, with 1) all neighbors and 2) no neighbors. The experiment was conducted in high and low nutrient environments to test for the interaction between the environment, the size and the species. There were ten replicate plots of each of the two environments, one receiving no additional nitrogen and the second receiving nitrogen at 17g/m2/yr. In total, there were 32 transplants (3 species x 3 size classes x 2 competition treatments x 1, 2 or 3 replicates) in each plot, and 20 plots (2 environments x 10 replicates), or 640 transplants. Seedlings were transplanted into the field during 3-6 June, 1991.本实验设置4个氮素水平，对应E001实验中的A、C、F、G处理，施加时机与E001实验一致。关于E052的施肥方案详情，请查阅文件`fertilization details`。实验共设4级土壤扰动处理，编号分别为1、2、3、4： 水平1：未扰动 水平2：使用1台7马力（HP）本田后旋式旋耕机，将入土深度调至9英寸进行单次旋耕 水平3：进行2次旋耕，或旋耕至约50%地块成为裸地所需的次数 水平4：进行3次旋耕，或旋耕至100%地块成为裸地所需的次数。部分样地仅需3次旋耕即可达标，部分样地则需5至6次。此外，所有未被旋耕清除的木本植被，需从基部切断。 旋耕作业于4月下旬开展。 每种施肥处理均与每种扰动处理组合，共计16种处理组合。16种组合各设置4次重复。此外，4组极端处理（最低氮素+最低扰动、最高氮素+最低扰动等）额外增设10次重复。所有处理采用完全随机设计。实验样地均为5m×5m的正方形地块，共计104块。 E052实验的测定指标包括：1）物种多度；2）群落生物量向叶、根、茎、花的分配格局；3）地上与地下净初级生产力；4）氮矿化速率。处理组合列表详见文件`trmte52`中的样地布置图。 E052的样地均加装围栏以排除哺乳动物食草动物的干扰。围栏基础为孔径6mm×6mm的镀锌电焊网，埋深50cm；地面以上部分延伸60cm高度，上方加装2m高的家禽防护网。 1990年，本实验从4种处理组合（N1D1、N1D4、N4D1、N4D4，其中N代表氮素添加水平，D代表土壤扰动处理）的样地中各随机选取10块，开展邻体竞争实验。实验设置3种竞争处理，以测定邻体对移植禾本科幼苗的地上与地下作用效应：1）无邻体植株；2）保留邻体根系但去除地上部分；3）保留邻体地上与地下全部器官。每个副样地中心定植1株幼苗。竞争处理于1990年5月14日至18日施加于主样地内的副样地中。针对4种氮素-扰动组合的10次重复样地，每种竞争处理均重复3次。因此，40块氮素-扰动组合样地（2个氮素水平×2个扰动水平×10次重复）各包含9个副样地（3种竞争处理×3次重复），每个副样地对应1株处于某一竞争处理下的幼苗。本次实验共计移植360株幼苗，各主样地内的副样地随机分配竞争处理。 1991年，实验选取3个在植株大小、生活史、生长速率以及沿氮素与扰动梯度的生态位位置均存在显著差异的物种：须芒草（Schizachyrium scoparium）、匍匐冰草（Agropyron repens）与狗尾草（Setaria viridis）。实验设置2种竞争处理：1）保留全部邻体植株；2）无邻体植株，幼苗均单独定植于副样地中。本实验设置高、低两种养分环境，以探究环境、植株大小与物种三者间的交互效应。两种养分环境各设置10次重复样地：一组不额外添加氮素，另一组氮素添加量为17g/m²/年。每个主样地内共计定植32株幼苗（3个物种×3个大小等级×2种竞争处理×1、2或3次重复），共计20块样地（2种环境×10次重复），总移植幼苗数为640株。幼苗定植工作于1991年6月3日至6日完成。