摘要
为了解区域典型农林用地土壤碳、氮、磷、胞外酶生态化学计量特征,采用时空代换法研究湘西北本底一致、土地利用史清晰的常绿阔叶原始林及其转换而来的人工林地、果园、坡耕地和水田0～40 cm土层中有机碳含量、全氮含量、全磷含量、胞外酶活性、计量比值及其与活性碳、氮、磷组分的关系.结果表明,与原始林相比,果园和坡耕地土壤有机碳、全氮、全磷含量的降幅(40%～79%)较高,且深层土壤(20～40 cm)和表土(0～20 cm)的降幅类似,表明原始林转换为果园、坡耕地不利于深层土壤有机碳和养分的保存.果园、坡耕地、水田和原始林土壤碳、氮、磷含量的比值无显著差异.杉木人工林土壤碳、氮、磷含量比值较原始林高60%～124%,说明人工林土壤存在较强的氮、磷素共同限制作用.与原始林相比,土地利用方式转换后,土壤胞外酶活性降低 18%～64%;果园和坡耕地表土碳磷获取酶计量比(EC/P)也显著降低,而深层土壤EC/P显著升高.土壤中碳含量、氮含量、磷含量、酶活性及其计量比值与活性碳、氮、磷组分含量显著相关.原始林转换导致表土和深层土壤中的碳含量、氮含量、磷含量、胞外酶活性大幅下降,微生物底物有效性降低,表明土壤质量/健康退化.土壤碳、氮、磷含量计量比和酶计量比值对原始林转换的响应较弱,对土壤健康的指示作用有待进一步探索.
Abstract
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.
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