Soil nitrate and ammonium: 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实验中同期施用的处理A、C、F及G。关于E052实验中施用的肥料详情，请参阅文件fertilization details。
本实验设置4个土壤扰动等级，分别标记为1、2、3、4：
等级1：无扰动（undisturbed）；
等级2：使用1台7马力本田（Honda）后旋式旋耕机（rototiller）作业1次，旋耕机提升装置设置为耕作深度9英寸；
等级3：作业2次，或根据实际情况调整作业次数，直至约50%的地表裸露；
等级4：作业3次，或根据实际情况调整作业次数，直至100%的地表裸露。部分样地仅需作业3次，部分样地则需5至6次。
此外，所有未被耕作破坏的木本植被均需从基部砍伐。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种处理组合各设置4次重复。此外，4组极端处理组合（最低氮素水平+最低扰动、最高氮素水平+最低扰动等）额外增设10次重复。所有处理均采用完全随机设计进行布置。
104个样地的规格均为5米×5米。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、花的分配比例；3）地上与地下净初级生产力；4）氮矿化速率（nitrogen mineralization）。处理组合列表请参阅文件trmte52中的处理布置图。
E052实验的样地均设置围栏以排除哺乳动物食草动物的干扰：孔径为6毫米×6毫米的镀锌焊接铁丝网被埋设于地下50厘米处，额外的镀锌铁丝网延伸至地面以上60厘米处，家禽防护网则延伸至地面以上2米处。
1990年，从4种处理组合（N1D1、N1D4、N4D1、N4D4，其中N代表氮素添加水平，D代表扰动处理等级）中各随机选取10个样地开展种间竞争实验。本实验通过3种竞争处理测定邻体对移栽禾本科幼苗的地上与地下影响：将幼苗种植于直径50厘米的小样区中，分别设置1）无邻体、2）仅保留邻体根系而移除地上部分、3）保留邻体所有地上与地下部分三种处理。每个小样区的中心位置种植1株移栽幼苗。竞争处理于1990年5月14日至18日施加于主样地内的小样区中。针对4种氮素供给水平与扰动组合，在每种组合的10个重复样地中，竞争处理各设置3次重复。因此，40个对应氮素与扰动组合的样地（2个氮素水平×2个扰动等级×10次重复）中，每个样地均包含9个小样区（3种竞争处理×3次重复），每个小样区对应1种竞争处理下的1株移栽幼苗。本次实验共计移栽360株幼苗。每个样地内的小样区均随机分配竞争处理。
1991年，本实验选用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：Schizachyrium scoparium（小须芒草）、Agropyron repens（匍匐冰草）、Setaria viridis（狗尾草）。移栽幼苗单独种植于小样区中，分别设置1）保留所有邻体、2）无邻体两种处理。本实验分别在高营养与低营养环境下开展，以探究环境、植株大小与物种三者间的交互效应。两种营养环境各设置10个重复样地：一组不额外添加氮素，另一组按17克/平方米/年的速率添加氮素。每个样地共计种植32株移栽幼苗（3个物种×3个大小等级×2种竞争处理×1/2/3次重复），共计20个样地（2种环境×10次重复），整体移栽幼苗总数为640株。幼苗于1991年6月3日至6日移栽至田间。