Department of Integrative Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
Ecology. 2013 Jan;94(1):106-16. doi: 10.1890/11-2070.1.
Given the regulatory impact of resources and consumers on plant production, decomposition, and soil carbon sequestration, anthropogenic changes to nutrient inputs and grazing have likely transformed how grasslands process atmospheric CO2. The direction and magnitude of these changes, however, remain unclear in this system, whose soils contain -20% of the world's carbon pool. Nutrients stimulate production but can also increase tissue palatability and decomposition. Grazing variously affects tissue quality and quantity, decreasing, standing biomass, but potentially increasing leaf nutrient concentrations, root production, or investment in tissue defenses that slow litter decay. Here, we quantified individual and interactive impacts of nutrient addition and simulated grazing (mowing) on above- and belowground production, tissue quality, and soil carbon inputs in a western North American grassland with globally distributed agronomic species. Given that nutrients and grazing are often connected with increased root production and higher foliar tissue quality, we hypothesized that these treatments would combine to reduce inputs of recalcitrant-rich litter critical for C storage. This hypothesis was unsupported. Nutrients and defoliation combined to significantly increase belowground production but did not affect root tissue quality. There were no significant interactions between nutrients and defoliation for any measured response. Three years of nutrient addition increased root and shoot biomass by 37% and 23%, respectively, and had no impact on decomposition, resulting in a -15% increase in soil organic matter and soil carbon. Defoliation triggered a significant burst of short-lived lignin-rich roots, presumably a compensatory response to foliar loss, which increased root litter inputs by 33%. The majority of root and shoot responses were positively correlated, with aboveground biomass a reasonable proxy for whole plant responses. The exceptions were decomposition, with roots six times more decay resistant, and grazing impacts on tissue chemistry, with shoots undergoing significant alterations, while roots were unaffected. Because neither treatment affected concentrations of decay-resistant compounds in roots, the implied net effect is higher soil C inputs with potentially longer residency times. Areas managed with nutrients and moderate grazing in our study system could thus accumulate significantly more soil C than unmanaged areas, with a greater capacity to serve as sinks for atmospheric CO2.
鉴于资源和消费者对植物生产、分解和土壤碳固存的调节作用，人为改变养分输入和放牧可能改变了草原对大气 CO2 的处理方式。然而，在这个土壤中含有全球 20%碳储量的系统中，这些变化的方向和幅度尚不清楚。养分刺激生产，但也可以增加组织的适口性和分解。放牧通过各种方式影响组织的质量和数量，减少地上生物量，但可能增加叶片养分浓度、根系生产或增加组织防御投资，从而减缓凋落物分解。在这里，我们量化了养分添加和模拟放牧（刈割）对北美西部草原地上和地下生产、组织质量和土壤碳输入的单独和交互影响，该草原具有全球分布的农业物种。由于养分和放牧通常与增加根系生产和更高的叶片组织质量有关，我们假设这些处理方法将结合起来减少对 C 储存至关重要的富含难分解物质的凋落物的输入。但这一假设并未得到支持。养分和刈割相结合，显著增加了地下生物量，但没有影响根系组织质量。在任何测量的响应中，养分和刈割之间都没有显著的相互作用。三年的养分添加分别使根和茎生物量增加了 37%和 23%，对分解没有影响，导致土壤有机质和土壤碳增加了 15%。刈割引发了富含木质素的短命根系的大量爆发，这可能是对叶片损失的补偿反应，使根凋落物输入增加了 33%。大多数根和茎的反应呈正相关，地上生物量是整个植物反应的合理代表。例外是分解，根系的抗分解能力是其 6 倍，而放牧对组织化学的影响，地上部分发生了显著变化，而根系则不受影响。由于两种处理都没有影响根中抗分解化合物的浓度，因此隐含的净效应是土壤 C 输入增加，潜在的停留时间更长。在我们的研究系统中，用养分和适度放牧管理的区域因此可以比未管理的区域积累更多的土壤 C，具有更大的吸收大气 CO2 的能力。
