Level Terrace Enhances the Distribution of Soil Organic Carbon Along the Depth Gradient in Mixed Forests: Microbial Mechanisms and Carbon Fraction Dynamics on the Loess Plateau

Soil and water conservation engineering measures exert a significant impact on soil microbial characteristics, soil factors, and soil organic carbon. However, in the Loess Plateau region of northwestern China, the influence of site preparation measures on the variations in microbial characteristics, soil nutrient availability regulators, and soil organic carbon fractions with soil depth remains insufficiently understood—and this knowledge gap is crucial for enhancing SOC sequestration. The present study measured soil organic carbon (SOC) and its fractions, soil nutrient availability regulators, microbial biomass, and enzyme activities at different soil depths in level terraces (LT) and natural slopes (NS). It aimed to reveal the regulatory mechanisms underlying the variation of SOC with soil depth under different site preparation measures. 1. Soil organic carbon and its fractions mineral-associated organic carbon (MAOC), particulate organic carbon (POC), soil organic carbon (SOC) 2. Soil nutrient availability regulatory factors Soil bulk density (BD) , Soil water content (SWC) , Soil pH, Total nitrogen (TN) , Total phosphorus (TP) , dissolved organic carbon (DOC) , dissolved organic nitrogen (DON), Nitrate nitrogen (NO3⁻-N) , ammonium nitrogen (NH4⁺-N) , Available potassium (AK) , available phosphorus (AP) 3. Microbial biomass and extracellular enzymes Microbial biomass carbon (MBC) , microbial biomass nitrogen (MBN) , Microbial biomass phosphorus (MBP) . The activities of soil β-glucosidase (BG), β-N-acetyl-glucosaminidase (NAG), leucine aminopeptidase (LAP), alkaline phosphatase (AlkP), and cellobiohydrolase (CBH) .水土保持工程措施对土壤微生物特性、土壤因子及土壤有机碳具有显著影响。然而，在中国西北黄土高原地区，整地措施对微生物特性、土壤养分有效性调控因子以及土壤有机碳组分随土层深度变化的影响仍未得到充分阐明——这一认知缺口对提升土壤有机碳（SOC）固存能力至关重要。本研究在水平梯田（LT）与自然坡面（NS）中，测定了不同土层深度下的土壤有机碳及其组分、土壤养分有效性调控因子、微生物生物量及酶活性，旨在揭示不同整地措施下土壤有机碳随土层深度变化的调控机制。 1. 土壤有机碳及其组分 矿质结合态有机碳（MAOC）、颗粒态有机碳（POC）、土壤有机碳（SOC） 2. 土壤养分有效性调控因子 土壤容重（BD）、土壤含水量（SWC）、土壤pH、全氮（TN）、全磷（TP）、溶解性有机碳（DOC）、溶解性有机氮（DON）、硝态氮（NO3⁻-N）、铵态氮（NH4⁺-N）、速效钾（AK）、有效磷（AP） 3. 微生物生物量与胞外酶 微生物生物量碳（MBC）、微生物生物量氮（MBN）、微生物生物量磷（MBP）；土壤β-葡萄糖苷酶（BG）、β-N-乙酰氨基葡萄糖苷酶（NAG）、亮氨酸氨基肽酶（LAP）、碱性磷酸酶（AlkP）及纤维二糖水解酶（CBH）的活性。