Temporal Changes In Nutrient Concentration,stoichiometry And Soil PH In Montane Forests Across Southern China Under Global Changes

Soil acidification threatens terrestrial ecosystem functions and reduces biodiversity.Previous studies have confirmed the acidification of topsoil by acidic deposition.The temporal changes in soil p H and associated base cations,however,are largely unknown in deep soils,where global changes likely have an increasing impact.Moreover,controlled experiments have shown that global changes such as nitrogen(N)deposition and rising atmospheric CO₂decouple the biogeochemical cycles of carbon(C),N,and phosphorus(P),resulting in shifting stoichiometry that lies at the core of ecosystem functioning.However,these studies generally examined the response of topsoil stoichiometry to global changes,whether deep soil stoichiometry has changed or not remain unknown.Moreover,previous studies focused on one or two global-change drivers.However,most natural ecosystems have been subjected to multiple global-change drivers such as deposition of atmospheric N and sulfur(S),rising atmospheric CO₂ concentration,regional warming and altered water availability simultaneously.To investigate the effects of multiple global-change drivers will advance our understanding of how would natural ecosystems respond to environmental change.Natural ecosystems unusually cover large spatial scale,include diverse vegetation,soil,and parent material types across a wide range of climate gradients,therefore it is critical to examine temporal changes of soil acidification,nutrients concentrations and stoichiometry and assess the response to global-change drivers in different ecosystem types.This will help improve the results from global-change simulation experiments and advance our accuracy in predictions of global-change effects on terrestrial ecosystems.Here we used historical observations of soil and plant properties from 1955 to 2015at 251 montane sites in south China along soil profiles of 0-150 cm,as well as data of global-change drivers.The collected dataset includes 2,952 observations of soil p H and base nutrients,2,736 observations of soil C-N-P concentrations and ratios,and 1,922observations of plant nutrients and stoichiometry.We examined the temporal trends of soil acidification,soil and plant nutrients concentrations and stoichiometry,as well as their responses to atmospheric N deposition,atmospheric S deposition,rising atmospheric CO₂ concentration,regional warming and altered water availability.(1)We found significant trends of soil acidification,with significantly decreased p H(by 0.46 unit),increased concentrations of exchangeable H⁺,decreased concentrations of exchangeable Ca²⁺,Mg²⁺,Na⁺and base saturation during the last 60 years,after accounting for the effect of soil depth.Soil p H decreased more quickly in topsoil than deep soil,but the concentrations of exchangeable Ca²⁺and Mg²⁺decreased more quickly in deep soil than topsoil,suggesting an exacerbated depletion of base cations in deep soil layers.The temporal trends in soil p H and base cations differed among vegetation,soil,and parent material types and varied spatially across different climate zones,with significantly decreased p H and base cations for evergreen broadleaf forest and highly weathered Ultisols,and more pronounced temporal changes in soil p H and base cations at low elevations.Results from a sensitivity analysis indicated that deep soil acidification was more sensitive to nitrogen deposition and decreasing water availability,less sensitive to sulphur disposition and warming than topsoil,while the depletion of bases responded similarly to these drivers across the entire soil profile.(2)We found a significant increase in soil total C concentration and a decrease in soil total P concentration,resulting in increased soil C:P and N:P ratios during the past 60years across all soil depths.Though average soil N concentration did not change,soil C:N increased in topsoil,but decreased in deep soil.The temporal trends in soil C-N-P stoichiometry differed among vegetation,soil,and parent material types and varied spatially across different climate zones,with significantly increased C:P and N:P ratios for evergreen broadleaf forest and highly weathered Ultisols,and more pronounced temporal changes in soil C:N,N:P,and C:P ratios at low elevations.Our sensitivity analysis suggests that the temporal changes in soil stoichiometry resulted from elevated N deposition,rising atmospheric CO₂ concentration and regional warming.(3)We found significant decreases in leaf C,P and Mg concentrations,and increases in leaf N,K and Na concentrations.These changes in plant nutrients concentrations led to decreasing leaf C:N ratio and increasing N:P ratio during the past 60 years.There were significant effects of life-forms on temporal changes in plant nutrients,with broadleaf trees more pronounced temporal changes in leaf nutrients concentrations and stoichiometry than those of fern,herb,shrub and conifer tree.Our sensitivity analysis suggested that rising atmospheric CO₂ concentration and reduced water availability drived the temporal changes in plant leaf C:N and N:P ratios.The study suggested that the temporal changes in plant leaf nutrients concentrations and N:P stoichiometry reflect P deficiency in this low-P area.Rising atmospheric CO₂ concentration may intensify P-limitation across subtropical area.In summary,we found significant temporal decreases in soil p H and concentrations of P,exchangeable Ca²⁺and Mg²⁺,and increases in plant and soil N:P ratios in subtropical China.The temporal changes in soil properties have occurred across the whole soil depth and were driven by multiple global change drivers.The magnitudes of the changes in soil properties are dependent on vegetation types,soil types,and spatial climate variations.We highlight the dynamics of the deep soil profile,as well as multiple global change drivers,need to be included in future ecological predictions.