Niklaus Pascal A, Leadley Paul W, Stöcklin Jürg, Körner Christian
Institute of Botany, Schönbeinstrasse 6, CH-4056 Basel, Switzerland e-mail:
Oecologia. 1998 Aug;116(1-2):67-75. doi: 10.1007/s004420050564.
Plant nutrient responses to 4 years of CO enrichment were investigated in situ in calcareous grassland. Beginning in year 2, plant aboveground C:N ratios were increased by 9% to 22% at elevated CO (P < 0.01), depending on year. Total amounts of N removed in biomass harvests during the first 4 years were not affected by elevated CO (19.9 ± 1.3 and 21.1 ± 1.3 g N m at ambient and elevated CO), indicating that the observed plant biomass increases were solely attained by dilution of nutrients. Total aboveground P and tissue N:P ratios also were not altered by CO enrichment (12.5 ± 2 g N g P in both treatments). In contrast to non-legumes (>98% of community aboveground biomass), legume C/N was not reduced at elevated CO and legume N:P was slightly increased. We attribute the less reduced N concentration in legumes at elevated CO to the fact that virtually all legume N originated from symbiotic N fixation (%N ≈ 90%), and thus legume growth was not limited by soil N. While total plant N was not affected by elevated CO, microbial N pools increased by +18% under CO enrichment (P = 0.04) and plant available soil N decreased. Hence, there was a net increase in the overall biotic N pool, largely due increases in the microbial N pool. In order to assess the effects of legumes for ecosystem CO responses and to estimate the degree to which plant growth was P-limited, two greenhouse experiments were conducted, using firstly undisturbed grassland monoliths from the field site, and secondly designed `microcosm' communities on natural soil. Half the microcosms were planted with legumes and half were planted without. Both monoliths and microcosms were exposed to elevated CO and P fertilization in a factored design. After two seasons, plant N pools in both unfertilized monoliths and microcosm communities were unaffected by CO enrichment, similar to what was found in the field. However, when P was added total plant N pools increased at elevated CO. This community-level effect originated almost solely from legume stimulation. The results suggest a complex interaction between atmospheric CO concentrations, N and P supply. Overall ecosystem productivity is N-limited, whereas CO effects on legume growth and their N fixation are limited by P.
在石灰性草原原位研究了植物养分对4年CO浓度升高的响应。从第2年开始，根据年份不同，在CO浓度升高时，植物地上部分的C:N比增加了9%至22%（P < 0.01）。前4年生物量收获中去除的N总量不受CO浓度升高的影响（在环境CO浓度和升高的CO浓度下分别为19.9 ± 1.3和21.1 ± 1.3 g N m），这表明观察到的植物生物量增加完全是通过养分稀释实现的。地上部分的总P和组织N:P比也未因CO浓度升高而改变（两种处理均为12.5 ± 2 g N g P）。与非豆科植物（占群落地上生物量的>98%）不同，在CO浓度升高时豆科植物的C/N没有降低，且豆科植物的N:P略有增加。我们将CO浓度升高时豆科植物中N浓度降低较少归因于几乎所有豆科植物的N都来自共生固氮（%N≈90%），因此豆科植物的生长不受土壤N的限制。虽然植物总N不受CO浓度升高的影响，但在CO浓度升高时微生物N库增加了18%（P = 0.04），而植物可利用的土壤N减少。因此，总体生物N库有净增加，主要是由于微生物N库的增加。为了评估豆科植物对生态系统CO响应的影响，并估计植物生长受P限制的程度，进行了两个温室实验，首先使用来自田间的未扰动草地整块土，其次在天然土壤上设计了“微观世界”群落。一半的微观世界种植了豆科植物，一半未种植。整块土和微观世界都采用析因设计，暴露于升高的CO和P施肥条件下。两个季节后，未施肥的整块土和微观世界群落中的植物N库均不受CO浓度升高的影响，这与在田间发现的情况类似。然而，当添加P时，在CO浓度升高时植物总N库增加。这种群落水平的效应几乎完全源于豆科植物的刺激。结果表明大气CO浓度、N和P供应之间存在复杂的相互作用。总体生态系统生产力受N限制，而CO对豆科植物生长及其固氮的影响受P限制。
