Effects Of Stand Density On Soil Carbon Storage Of Manchurian Ash And Larch Plantations

Stand density,which is one of the key management methods for forest practices,has a great impact on soil carbon storage.So far,the knowledge of how stand density can affect soil carbon storage is scarce,and with discrepancies as well.Studing the change pattern of different stand density on soil carbon storage is significant for understanding the carbon cycling process in forest ecosystem.Manchurian ash(Fraxinus mandshurica)and hybrid Larch(Larix olgensis×L.kaempferi)are the commonly used species for afforestation in northeast China as they are highly economically and ecologically valued.In this study,the Manchurian ash and Larch plantations with different stand density were chosen for analysing the effects of stand density on soil carbon storage and two fractions of soil organic carbon.The impacts of stand density on carbon input from litter and root,as well as the carbon output caused by soil respiration are also discussed.Furthermore,the underlying mechanisms of influences of stand density on soil organic carbon is revealed by conducting an incubation experiment with the addition of cellulose and ammonium nitrate into soils from different stand density.The results are listed below.(1)The soil carbon storage increased with increasing stand density in both Manchurian ash and Larch plantations,as well as in each soil layer.The carbon storage in heavy fraction in both plantations increased with stand density in all soil layers.The carbon storage in light fraction in Manchurian ash plantations only increased in top soil layer,and stayed unchanged out of stand density in deep soil layers.In Larch plantations,on the contrary,the carbon storage in light fraction stayed unchanged out of stand density in top soil layer and increased with increasing stand density in deep soil layer.The increment of soil carbon storage in heavy fraction with increasing stand density in both Manchurian ash and Larch plantations were the responsibilities of total soil carbon storage change in all soil layers.The increment of soil carbon storage in heavy fraction in top-,middle-and deep-soil layer explained 76.64%-99.83%,91.53%-125.98%and 94.11%-98.20%of total soil carbon increment with increasing stand density,respectively.The increment of carbon storage in light fraction in each layer is slight.(2)The litter decomposition rate and decomposition amount do not change with changing stand density,while the litter annual production increased with increasing stand density in both Manchurian ash and Larch plantations.The decomposition rate of litter of Manchurian ash is faster than it of Larch,which is the annual mass loss and annual carbon loss of about 70%in Manchurian ash litter and about 30%in Larch litter.The carbon input in Manchurian ash plantations increased significantly with increasing stand density while in Larch plantations increased slightly.In below ground,the fine root biomass of Manchurian ash plantations declined with increasing stand density,while the fine root length density and surface area density did not change with stand density.In larch plantations,the fine root biomass,fine root length density and surface area density all presented the lowest in the low density site.This indicates that as the stand density increases,the carbon input from roots in Larch plantation significantly increases,while in Manchurian ash plantations it increases slightly.Thus,the soil carbon input increased with increasing stand density in both Manchurian ash and Larch plantations,which is contributed to the increment of litter carbon input in Manchurian ash plantations and of root carbon input in Larch plantations.(3)The average respiration in Manchurian ash plantations increased with increasing stand density while in Larch plantations declined.In Manchurian ash and Larch plantations,soil moisture showed no correlation to soil respiration while soil temperature had a significant positive correlation to soil respiraton.Meanwhile,the autotrophic respiration increased slightly with increasing stand density in Manchurian ash plantations and increased significantly with increasing stand density in Larch plantations,leads to the heterotrophic respiration in Manchurian ash plantatopns increased while in Larch plantation declined with increasing stand density.Thus,the carbon output in Manchurian ash plantations increased with increasing stand density while it declined in Larch plantations.(4)In the incubation experiment,the decomposition of added cellulose decreased with increasing stand density in each soil layer,as well as the cumulative soil derived CO2 emmission rate that also declined with increasing stand density in cellulose treatment and CK.With increasing stand density,the cumulative priming effect in top soil layer did not change with stand density,while in middle and deep soil layer it declined with increasing stand density.The addition of ammonium nitrate barely had.influences on the extracellular enzymes activity involved in carbon,nitrogen and phosphrate cycle.The addition of cellulose stimulated the extracellular enzymes activity involved in carbon,nitrogen and phosphrate cycle in top soil layer,but not in deep soil layers.The stimulation of cellulose to enzymes involved in carbon cycle also declined with increasing stand density.Therefore,we proposed that the effects of stand density on soil carbon storage could possibly be related to the inhibitation of soil microorganisms.In the higer density of forest,plants may import larger amount of microorganisms inhibitors into soil to restrain its activity,that caused the weaker soil output in high density sites.At the same time,more imported carbon was kept in soil rather than consumed by microorganisms whose activity is restrained.Thus,the soil carbon storage increased with increasing stand density.