Effects Of Warming,nitrogen Input And Altered Precipitation On Soil Organic Carbon Persistence And Soil Respiration

Soil is the largest terrestrial reservoir of active carbon(C),and small changes in the processes regulating soil organic carbon(SOC)cycling may significantly affect the atmospheric CO2 concentration.Global changes,such as elevated CO2 and temperature,nitrogen(N)addition,and changes in precipitation,may affect soil C cycling by altering soil abiotic and biological processes.The ongoing global change is multi-faceted and interactions of multiple environmental change factors may amplify or offset the effects of individual factors on soil C cycling.However,few studies have so far explored the effects of multiple global change factors on soil organic C dynamics,and the underlying mechanisms.The Loess Plateau in Northwest China is one of the largest loess regions in the world.This area is a typical fragile ecosystem with high altitude,water limitation and severe soil erosion.Thus,current climate change may strongly affect ecosystem C and N cycles on the loess Plateau.With the aim to investigate effects of warming,reactive N input,altered precipitation on soil C persistence and soil respiration,and the potential mechanism in a semi-arid grassland on the Loess Plateau,we conducted meta-analysis combined with a multi-factorial field experiment established in Yunwu Mountain National Nature Reserve of Ningxia Hui Autonomous Region on the Loess Plateau.The manipulative experiment included two warming levels(ambient and OTC warming),two N levels(ambient and 12 g N m-2 yr-1 N input)and precipitation alteration(30%precipitation reduction,ambient precipitation and 30%precipitation increase).The main results are as follows:1.The results of simulated global change experiment on the loess Plateau showed that in the absence of N input,precipitation additions significantly enhanced soil aggregation and promoted the coupling between aggregation and both soil fungal biomass and exchangeable Mg2+.However,N input negated the promotional effects of increased precipitation,mainly through suppressing fungal growth and altering soil pH and clay-Mg2+-OC bridging.Warming increased C content in the mineralassociated fraction,likely by increasing inputs of root-derived C,and reducing turnover of existing mineral-associated C due to suppression of fungal growth and soil respiration.Together,my results provide new insights into the potential mechanisms through which multiple global change factors control soil C persistence in arid and semi-arid grasslands.These findings suggest that the interactive effects among global change factors should be incorporated to predict the soil C dynamics under future global change scenarios.2.I examined soil respiration(Rs)response to multi-factor global change over two consecutive growing seasons in the field experiment.Our results showed that soil respiration rate increased with precipitation gradients.In addition,warming and N input interactively affected soil respiration.Warming alone did not significantly impact Rs,but it significantly suppressed Rs under reactive N input by reducing fungal biomass and oxidase production.Together,these results indicate that soil N availability affect Rs response to climate warming,highlighting the urgency to incorporate soil N availability into Earth system models to better predict C-climate feedbacks in a warmer world.3.Meta-analysis of N input experiments(87)showed that N input increased the proportion of soil macroaggregates and the mean weight diameter of aggregates at the global scale,mainly due to the promotion of plant root growth.Form of N input dominates N effects on soil aggregation.Urea application significantly improved the soil aggregate stability,while inorganic N(combined NH4+-N,NO3--N and NH4NO3)input had minor effect.Further,N-promotion of soil aggregation occurred mainly in croplands under low to moderate N input(<200 kg N ha-1 yr-1).Considering the important role of plant roots in N-promoting soil aggregation,combination of urea fertilizers and reduced perturbations(e.g.,reduced-tillage)may be key to enhance soil aggregation and organic C retention and persistence in vast agroecosystems.4.I meta-analyzed the results from 165 warming experiments and found that climate warming had minor effect on soil C storage across the globe,but long-term warming can exacerbate soil C loss.Soil N availability is an essential driver mediating soil C input,output,and transformation response to warming.High N availability(soil C:N ratio<15)suppressed root growth response to warming,but promoted the coupling between lnRR of root biomass and soil C.When N was relatively limited(soil C:N ratio>15),climate warming significantly enhanced root biomass,but did not lead to soil C accumulation,possibly because enhancement of CO2 release offset the soil C input.These results suggest that soil N availability(C:N ratio)may be an important parameter for predicting the response of soil C input,output and transformation under future climate warming scenarios on a global scale.Together,results from field experiments showed that 1)in a semi-arid grassland of the Loess Plateau,precipitation increase promoted soil aggregate stability and N input negated this precipitation effects and,2)warming had minor effect on Rs under ambient N level,but significantly reduced Rs under reactive N input.Meta-analysis results showed that 3)on a global scale,reactive N input improved soil aggregate stability,and N form dominated this promotion effect,and 4)soil N availability(C:N ratio)is an important factor that mediates soil C input,output and transformation response to climate warming.Together,these results suggest that results from singlefactor experiments may not be reflective to the field reality of multi-faceted global change,highlighting the need to incorporate the interactive effects of multiple global change drivers into the C-climate models to better predict the soil C dynamics under future climate change scenarios.