Hamilton Iii E William, Giovannini Michele S, Moses Stephanie A, Coleman James S, McNaughton Samuel J
Department of Biological Sciences, Syracuse University, Biological Research Laboratories, 130 College Place, Syracuse, NY 13244-1220, USA e-mail:
Oecologia. 1998 Sep;116(3):407-418. doi: 10.1007/s004420050604.
Large mammalian herbivores in grassland ecosystems influence plant growth dynamics in many ways, including the removal of plant biomass and the return of nutrients to the soil. A 10-week growth chamber experiment examined the responses of Sporobolus kentrophyllus from the heavily grazed short-grass plains of Serengeti National Park, Tanzania, to simulated grazing and varying nitrogen nutrition. Plants were subjected to two clipping treatments (clipped and unclipped) and five nitrogen levels (weekly applications at levels equivalent to 0, 1, 5, 10, and 40 g N m), the highest being equivalent to a urine hit. Tiller and stolon production were measured weekly. Total biomass at harvest was partitioned by plant organ and analyzed for nitrogen and mineral element composition. Tiller and stolon production reached a peak at 3-5 weeks in unclipped plants, then declined drastically, but tiller number increased continually in clipped plants; this differential effect was enhanced at higher N levels. Total plant production increased substantially with N supply, was dominated by aboveground production, and was similar in clipped and unclipped plants, except at high nitrogen levels where clipped plants produced more. Much of the standing biomass of unclipped plants was standing dead and stem; most of the standing biomass of clipped plants was live leaf with clipped plants having significantly more leaf than unclipped plants. However, leaf nitrogen was stimulated by clipping only in plants receiving levels of N application above 1 g N m which corresponded to a tissue concentration of 2.5% N. Leaf N concentration was lower in unclipped plants and increased with level of N. Aboveground N and mineral concentrations were consistently greater than belowground levels and while clipping commonly promoted aboveground concentrations, it generally diminished those belowground. In general, clipped plants exhibited increased leaf elemental concentrations of K, P, and Mg. Concentrations of B, Ca, K, Mg, and Zn increased with the level of N. No evidence was found that the much greater growth associated with higher N levels diminished the concentration of any other nutrient and that clipping coupled with N fertilization increased the total mineral content available in leaf tissue. The results suggest that plants can (1) compensate for leaf removal, but only when N is above a critical point (tissue [N] 2.8%) and (2) grazing coupled with N fertilization can increase the quality and quantity of tissue available for herbivore removal.
草原生态系统中的大型哺乳食草动物通过多种方式影响植物生长动态，包括去除植物生物量以及将养分返还至土壤。一项为期10周的生长箱实验研究了来自坦桑尼亚塞伦盖蒂国家公园重度放牧的矮草平原的肯氏鼠尾粟对模拟放牧和不同氮营养水平的响应。将植物进行两种修剪处理（修剪和未修剪）以及五个氮水平处理（每周施加相当于0、1、5、10和40克氮/平方米的水平，最高水平相当于一次尿液冲击量）。每周测量分蘖和匍匐茎的产量。收获时的总生物量按植物器官进行划分，并分析其氮和矿质元素组成。未修剪植物的分蘖和匍匐茎产量在3 - 5周时达到峰值，然后急剧下降，但修剪植物的分蘖数持续增加；这种差异效应在较高氮水平下增强。随着氮供应的增加，植物总产量大幅增加，以上部生物量为主，修剪和未修剪植物的总产量相似，除了在高氮水平下修剪植物产量更高。未修剪植物的现存生物量大部分是死亡的茎；修剪植物的现存生物量大部分是活叶，修剪植物的叶显著多于未修剪植物。然而，仅在施氮量高于1克氮/平方米（对应组织氮浓度为2.5%）的植物中，修剪会刺激叶片氮含量增加。未修剪植物的叶氮浓度较低，且随氮水平增加而升高。地上部分的氮和矿质元素浓度始终高于地下部分，虽然修剪通常会促进地上部分的浓度，但一般会降低地下部分的浓度。总体而言，修剪植物的叶片钾、磷和镁元素浓度增加。硼、钙、钾、镁和锌的浓度随氮水平增加而增加。未发现有证据表明与较高氮水平相关的更大生长量会降低任何其他养分的浓度，并且修剪与氮肥施用相结合会增加叶片组织中可利用的总矿质含量。结果表明，植物能够（1）补偿叶片去除，但仅当氮含量高于临界点（组织氮含量2.8%）时；（2）放牧与氮肥施用相结合可以提高可供食草动物食用的组织的质量和数量。
