Mechanism Of Species Loss In Grassland Communities Caused By Warming And Nitrogen Addition

Global warming and nitrogen enrichment are important driving forces for changes in grassland ecosystem structure and function.Many studies have found that nutrient enrichment,especially nitrogen enrichment frequently increases plant aboveground biomass but leads to intensified interspecific competition and species loss.Competition for light has been frequently proposed to explain such biomass-driven species loss,while this paper assumed that there may be other driving mechanisms,such as stronger water demand after increased plant biomass,will lead to water competition.To test this,a three-factor interactive field experiment on water addition(W),nitrogen addition(N),and increased temperature(T)was carried out in the semi-arid grassland communities from 2018 to 2022.The aboveground biomass,species richness were measured during the peak growth period of each year,and soil physical and chemical properties were also measured.At the same time,combined with the meta-analysis using published data from 276 experiments across grasslands worldwide,the mechanism of species loss caused by resource addition was explored.The main results are as follows:(1)Resource addition significantly increased aboveground biomass with the highest biomass under the three resources addition treatment.Nitrogen addition(N)and warming(T)significantly increased plant aboveground biomass(39.3%,37.9%,p<0.05)but reduced species richness(19.2%,13.9%,p<0.001).Water addition combined with nitrogen addition or warming(WN,WT),while maintaining higher plant biomass,did not cause decline in species richness,which is further supported by meta-analysis across grasslands worldwide:resources addition significantly increased aboveground biomass(except T treatment).Nitrogen addition and/or warming(N,T,NT)significantly reduced species richness,but water addition treatment(W,WN)increased species richness.Species richness under WNT was significantly higher than NT but still lower than the ambient level.(2)Species richness significantly decreased with increasing aboveground biomass,suggesting a strong biomass-driven species loss.Species richness was significantly higher and it decreased significantly slower with increasing aboveground biomass in treatments with water addition than those without watering,indicating that water can alleviate species loss caused by increased aboveground biomass.Species richness significantly increased with soil water content,further suggesting water as an important driver of species loss.(3)Both nitrogen addition and warming significantly increased soil nitrate nitrogen content(531%,p<0.001;44.2%,p<0.01).Aboveground biomass significantly increased with soil nitrate nitrogen content,and species richness significantly decreased with soil nitrate nitrogen content,indicating that nitrogen addition and warming increased soil available nitrogen,stimulated aboveground biomass accumulation,accelerated soil water consumption and canopy water interception,and then aggravated species competition.plants species characterized by low root biomass and thus low water uptake ability,such as Ixeridium dentatum,Poa annua and Plantago asiatica,rarely existed in plots with warming and nitrogen but no water addition.These species without a competitive advantage for water will be excluded from the community,then reduced species richness.These findings strongly suggest that increased aboveground biomass will aggravate plants competition for soil moisture,lead to the loss of species with smaller roots and weaker water competition ability after warming and nitrogen addition.The biomass driven-water competition hypothesis proposed in this study provides new insight into grasslands biodiversity conservation.Water addition can offset species loss caused by nitrogen enrichment or warming,especially on grasslands with annual precipitation below 600 mm or soil moisture below 30%,water competition is more important than light competition.Noteworthy,in areas where increased nitrogen deposition and warming occur simultaneously,water addition can only partially alleviate species loss caused by increased biomass.Therefore,species loss can be avoided to some extent in water-deficient areas where precipitation is expected to increase.Nitrogen utilization should be avoided in regions with rising temperature but decreasing rainwater.This study has deepened the understanding of the maintenance mechanism of species diversity in the context of global change,and has scientific significance for grassland ecosystem management and biodiversity conservation.