Responses of soil C,N,P,and enzyme stoichiometry to primary forest conversion
To elucidate the stoichiometric dynamics of organic C,N,P,and extracellular enzymes in soil in typical agricultural and forestry lands,primary evergreen broad-leaved forests and converted plantations,orchards,sloping croplands,and paddies with similar geographical backgrounds and clear land-use history in western Hunan Province were selected.The space-for-time substitution method was applied to reveal the effect of primary forest conversion on the contents of organic C,total N,and total P;activities of extracellular enzymes,including their:stoichiometric ratios;relationships with labile C,N,and P fractions at a soil depth of 0-40 cm.Compared to the primary forest,the contents of soil organic C,total N,and total P in the orchard and sloping cropland were much more greatly reduced(40%-79%)than that in plantations and paddies.The reduction was similar in topsoil and deep soil,indicating relatively unfavorable conditions for organic C and nutrient preservation in deep soil under orchards and sloping croplands.No significant differences in soil C-N-P ratios were observed among the orchards,sloping croplands,paddy fields,and primary forests.The C-N-P ratios in soil were 60%-124%higher in plantations than in primary forests,indicating co-limitation by N and P in the soil under the plantation.The β-glucosaminidase,β-N-acetyl glucosaminidase,leucine aminopeptidase,and acid phosphatase activities were reduced by 18%-64%in the converted land-use sites than in the primary forest.Compared to that of the primary forest,the stoichiometric ratios of C-and P-acquiring enzyme(EC/P)in the topsoil under the orchard and sloping cropland were significantly reduced,whereas the EC/P ratios in the 20-40 cm deep soil were significantly increased.The contents of organic C,total N,total P,enzyme activities,and their ratios were significantly correlated with the contents of the labile C,N,and P fractions in soil.Primary forest conversion resulted in substantial reductions in soil organic C,total N,and total P contents and extracellular enzyme activities in the topsoil and at depths of 20-40 cm deep.This indicates a reduction in microbial substrate availability and soil quality/health degradation.The responses of the stoichiometric ratios of soil organic C,total N,and total P contents and enzyme activities to primary forest conversion were relatively weak.Whether these stoichiometric ratios can effectively indicate the soil health change remains further exploration.
northwestern Hunan provincedeep soilsoil nutrientecological stoichiometryextracellular enzyme activityland use
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