Differential responses of grasses and forbs led to marked reduction in below-ground productivity in temperate steppe following chronic N deposition

Enhanced deposition of atmospheric nitrogen (N) has profound impacts on ecosystem processes such as above-ground productivity and community structure in grasslands across the globe. But how N deposition affects below-ground processes of grasslands is less well known. Here, we evaluated the effects of chronic N amendment at a relatively low rate (20kgha(-1)year(-1)) on root traits (root productivity, root biomass, root/shoot ratio) in Inner Mongolia steppes by rhizotron and ingrowth core and soil monolith techniques at levels of individual species, functional groups and ecosystem. For 8years, N amendment suppressed above-ground net primary production (ANPP), photosynthetic rates and root biomass of forbs, but enhanced ANPP and root biomass of grasses. This led to an overall reduction in below-ground productivity of the grassland by 24-33%, while ANPP remained unchanged. Nitrogen amendment acidified soil and subsequently increased extractable soil manganese (Mn) concentration. Nitrogen amendment increased foliar Mn concentrations in forb, but not grass species, leading to a significant inhibition of photosynthetic rates in forb species.Synthesis. These findings highlight the importance of the differentiating responses of plant functional groups to long-term N deposition and the important consequences of these responses for below-ground productivity and long-term soil C sequestration.

大气氮（N）沉降的增加对全球草原的生态系统过程，如地上生产力和群落结构，有着深远的影响。但是氮沉降如何影响草原的地下过程却鲜为人知。在此，我们通过根窗、根生长芯和土壤整段标本技术，在个体物种、功能群和生态系统水平上，评估了相对较低速率（20千克/公顷·年）的长期施氮对内蒙古草原根系性状（根系生产力、根系生物量、根冠比）的影响。8年来，施氮抑制了非禾本科草本植物的地上净初级生产力（ANPP）、光合速率和根系生物量，但提高了禾本科植物的ANPP和根系生物量。这导致草原地下生产力总体下降了24 - 33%，而ANPP保持不变。施氮使土壤酸化，进而增加了土壤可提取锰（Mn）的浓度。施氮提高了非禾本科草本植物叶片的锰浓度，但禾本科植物没有，这导致非禾本科草本植物的光合速率受到显著抑制。综合。这些发现强调了植物功能群对长期氮沉降的不同响应的重要性，以及这些响应对地下生产力和长期土壤碳固存的重要影响。