Effects Of Nitrogen Deposition On Plant Community Dynamics And Restoration Of Ecosystem Processes And Functions In Songnen Grassland With Saline-alkaline Patchness

Grasslands are the largest and most widely distributed ecosystems on Earth and provide not only high-quality livestock products for humans,but also a variety of ecological functions relating to soil and water conservation,climate regulation,and the maintenance of biodiversity.Due to global climate change and intensifying human activities(e.g.overgrazing),nearly half of the world’s grasslands are degraded to some degree,threatening the functioning of both ecosystem goods and services.Since the Industrial Revolution,the burning of fossil fuels and the use of fertilizers have triggered global nitrogen deposition.Numerous studies have shown that nitrogen deposition has important impacts on plant community composition,diversity,productivity,and the carbon service of grassland ecosystems.However,few experimental studies have considered and examined the potential impacts and ecological effects of global nitrogen deposition on degraded grassland ecosystems.Songnen Plain is one of the three major areas of saline-alkaline soil in the world.In recent years,overgrazing and mowing have led to extensive degradation of Songnen grassland by increasing soil salinity.The prominent problems of degraded grasslands are high saline-alkaline stress(high p H)and nitrogen limitation.We hypothesized that in degraded grasslands with high saline-alkaline stress,nitrogen deposition not only directly alleviates the nitrogen limitation,but also acidifies the soil(lowers soil p H),thereby alleviating the stress of plants and microorganisms,and promoting ecological restoration.Moreover,nitrogen deposition can directly affect the nitrogen cycle and further change the C:N:P stoichiometry of plants,soils,and microorganisms and the coupling of stoichiometric ratios among them,which may ultimately have significant impacts on the soil carbon service of the grassland ecosystem.In addition,patchy degradation is typical of degraded areas of Songnen grassland,and the plant and soil characteristics of different degraded patches varied greatly at fine spatial scales.Therefore,nitrogen deposition may have significant spatial effects depending on the degree of degradation.In order to test our hypothesis,five patches with different degrees of degradation were selected based on plant salt tolerance(multi-species mixed patches,Leymus chinensis patches,Puccinellia tenuiflora patches,Chloris virgata patches,and Artemisia anethifolia patches).We comprehensively studied the effects of nitrogen addition(simulated nitrogen deposition)on plant community dynamics,C:N:P stoichiometry,and soil carbon sequestration in patchy degraded grassland with saline-alkaline to reveal the mechanisms of nitrogen addition on the ecological restoration of structures,processes and functions.We report the main results and conclusions as follows:(1)Nitrogen addition promoted plant growth,increased plant community height and aboveground biomass in all degraded patches,and affected plant community dynamics spatially.Specifically,in multi-species mixed patches,nitrogen addition significantly increased the relative biomass of perennial grasses(L.chinensis),but decreased the relative biomass of legumes and species richness.In P.tenuiflora patches,elevated soil nitrogen availability reduced the relative biomass of perennial grasses(P.tenuiflora and Phragmites australis)and increased the relative biomass of fast-growing halophytes(A.anethifolia and Kochia scoparia),but did not change species richness.Nitrogen input had no significant effect on plant community composition in the patches of L.chinensis,C.virgata,and A.anethifolia.In addition,nitrogen addition reduced community stability in all degraded patches and the lightly-degraded multi-species mixed patches showed the most substantial decline.The effect of nitrogen addition on plant community dynamics was regulated by both soil p H and the competitive ability of plants.In multi-species mixed patches at soil p H< 8,increased soil nitrogen availability favored the growth of highly competitive L.chinensis with well-developed roots,and thereby caused species loss and reduced community stability.In P.tenuiflora patches at p H > 9,nitrogen addition accelerated the invasion of fast-growing halophytes(annuals)and was not conducive to vegetation restoration of degraded patches.(2)Patchy degradation strongly affected C:N:P stoichiometry in the studied saline-alkaline grassland ecosystem.As degradation increased,soil C:P and N:P significantly decreased,while C:N remained constant.Our results indicate that the loss of soil carbon and nitrogen from degradation is more than that of phosphorus,and degradation decouples the relationships of C-P and N-P.Consistent with soil C:N:P stoichiometry,leaf C:P,and N:P gradually decreased with degradation,while C:N did not change.Plant and soil C:P and N:P consistently maintained positive correlations across five degraded patches,which demonstrates that plants and soils change synchronously and that the retrogressive succession of plant communities is associated with soil nutrient loss.In contrast,microbial C:P and N:P increase and C:N decreases as degradation increases,which illustrates that microbes switch from a strategy of high-energy investment(low C:P and N:P ratios for reproduction)to one that prioritizes investing in structure(high C:P and N:P ratios for salt tolerance).Microbial C:P and N:P were negatively correlated with soil C:P and N:P,and the C:N:P stoichiometry between microbes and soil decoupled with patchy degradation.Our study suggests that the increased saline-alkaline stress in degraded patches reduced microbial biomass and activity,favoring a ‘resource-conservative’ strategy,based on optimizing structure,rather than a ‘resource-acquisitive’ strategy.In addition,it is stress rather than soil resources that has a stronger effect on microbial C:N:P stoichiometry in saline-alkaline grassland with patchy degradation.(3)Nitrogen input significantly altered the C:N:P stoichiometry in plants,soils,and microorganisms in patchy degraded grassland,and the effects were strongly dependent on patch types.In weakly alkaline soils at p H < 8,nitrogen addition enhances the dominance of perennial grass(L.chinensis)and fungi,and then may promote enhanced mutualism between phosphorus-limited plants and carbon-limited soil microorganisms.These enhanced mutualisms are likely to drive C-N-P cycling in multi-species mixed patches.In alkaline soils with p H 9-9.5,nitrogen addition triggered phosphorus limitation in both plants and microbes,leading to phosphorus competition between plants and microbes,and then drives N-P cycling in L.chinensis patches and P.tenuiflora patches.In strongly alkaline soils at p H > 9.5,nitrogen input only altered aboveground leaf nitrogen concentrations,C:N,and N:P in the patches of C.virgata and A.anethifolia,but had no significant effect on soil microbial biomass and C:N:P stoichiometry.Therefore,nitrogen addition had a limited influence on C-N-P cycling processes in highly-degraded patches due to the high soil p H and poor soil structure.(4)Grassland degradation was detrimental to soil carbon sequestration,not only affecting soil organic carbon from 0 to 40 cm,but also causing a large loss of soil inorganic carbon from 20 to100 cm,and the proportion of soil inorganic carbon loss far exceeded that of soil organic carbon.Compared with L.chinensis patches,soil inorganic carbon losses caused by degradation accounted for 84% and 86% of soil total carbon loss under P.tenuiflora patches and A.anethifolia patches,respectively.Moreover,the distribution of soil inorganic carbon from 20 to 60 cm was closely related to soil p H,while it was mainly regulated by electric conductivity from 60 to100 cm.In addition,soil p H,electric conductivity,and aboveground biomass were critical factors driving the profile distributions of soil organic carbon in the upper soil profile(0-40 cm).We also found that the profile distribution patterns of soil p H and electric conductivity in patchy degraded grassland with saline-alkaline were influenced by water-salt movement.Our study suggests that unique soil characteristics determine the stocks and distributions of soil carbon in different degraded patches by influencing plant growth and the dissolution and precipitation of soil inorganic carbon.(5)Nitrogen addition had a significant effect on soil organic carbon in surface soils(0-10 cm)in the patchy degraded grassland with saline-alkaline,and showed pronounced spatial effects depending on the characteristics of degraded patches.On weakly alkaline soils at p H < 8,nitrogen addition stimulated the decomposition of soil organic carbon by stimulating carbon limitation by microbes and reducing species richness,ultimately leading to a reduction in soil carbon stocks.However,nitrogen input had no effect on all other degraded patches with soil p H > 9.Although nitrogen addition promoted plant growth and increased aboveground biomass in all patches,the increased aboveground carbon input did not further contribute to soil carbon sequestration.Instead of aboveground biomass,soil p H played a critical role in regulating soil organic carbon stocks.However,alkaline soils can effectively resist soil acidification due to their high buffering capacity,and nitrogen addition had no significant effect on soil p H in all patches.Therefore,nitrogen addition did not break the abiotic limits affecting soil carbon sequestration and ultimately could not restore carbon-related ecosystem services in saline-alkaline grassland.In conclusion,nitrogen addition can drive the carbon-nitrogen-phosphorus cycle in patchy degraded grassland spatially by affecting the C:N:P stoichiometry of plants,soil,and microbes.We confirmed and highlight that soil p H plays a key role in limiting soil carbon sequestration in saline-alkaline grasslands.Short-term nitrogen addition cannot acidify alkaline soils,and therefore cannot rapidly restore soil carbon stocks in degraded patches.Moreover,nitrogen input decreased community stability,causing losses of plant species in lightly degraded multi-species mixed patches,and accelerating the invasion of halophytes into P.tenuiflora patches,which is detrimental to ecological restoration outcomes.We also provide strong empirical evidence that saline-alkaline grasslands have a completely different carbon-nitrogen-phosphorus cycling process from other grassland ecosystems and fill the gap in the understanding of plant-soil-microbial feedbacks in degraded saline-alkaline grasslands from atmospheric nitrogen deposition.Our study comprehensively assesses the effect of nitrogen deposition on the ecological restoration of diverse degraded patches and provides important theoretical implications for the management and ecological restoration of degraded saline-alkaline grasslands.