晋陕峡谷区坡耕地与撂荒地土壤水分对单次暴雨事件的响应[封面文章]

Response of soil moisture in sloping cropland and abandoned land to a single heavy rainfall event in the Jin-Shan Gorge region[Cover]

关键词:  坡耕地  撂荒地  水分运移  暴雨前后  晋陕峡谷区

Key words:  sloping cropland  abandoned land  soil moisture migration  before and after the downpour  Jin-Shan Gorge region

摘要:

为探究极端降水条件下不同土地利用类型土壤含水量（SWC）及酸碱性质的响应特征，以黄土高原晋陕峡谷区山西省吕梁市碛口镇李家山村坡耕地和撂荒地这两种土地利用类型为研究对象，基于单次暴雨事件前后0—300 cm土壤含水率和pH实测数据，探究不同土地利用类型下土壤水分与pH的变化规律及其耦合关系。结果表明：(1) 暴雨前后撂荒地土壤含水率整体高于坡耕地，但坡耕地对降水响应更强，坡耕地平均含水率增幅达40.4%，撂荒地增幅为20.1%，尽管暴雨后撂荒地整体含水率仍偏高，但两者含水率差值平均值由3.24%缩小至2.50%，表明暴雨在一定程度上削弱了两种土地利用类型间的土壤水分差异；(2) 坡耕地与撂荒地两种土地利用类型土层含水率变化主要集中在20—110 cm土层，而120 cm以下深层土壤变化幅度较小，表明单次暴雨对深层土壤水分补充作用有限；(3) 暴雨后两种土地利用类型土壤pH均呈下降趋势，其中坡耕地次要变化层酸化程度显著高于主要变化层（P=0.013），且坡耕地次要变化层酸化程度显著高于撂荒地（P=0.019）；(4) 坡耕地土壤含水率变化量（ΔSWC）和土壤pH变化量（ΔpH）呈显著正相关（r=0.373，P=0.039），而撂荒地两者相关性不显著（P=0.240）。研究表明，不同土地利用类型显著影响暴雨后土壤水分运移及酸碱度响应过程，坡耕地对极端降水事件更为敏感。本研究可为黄土高原地区土地利用优化及水土资源管理提供科学依据。

Abstract:

Background, aim, and scope In recent years, the frequency and intensity of extreme precipitation events on the Chinese Loess Plateau have increased under the influence of global climate change. As a major driving force of the regional hydrological cycle, precipitation infiltration directly affects soil moisture dynamics, nutrient migration, and soil chemical properties. Soil moisture is a key factor controlling vegetation growth and ecosystem stability, whereas soil pH strongly influences nutrient availability and microbial activity. Previous studies have mainly focused on the response of soil moisture to precipitation under different land-use types, while the coupling relationship between soil moisture variation and soil acid-base responses following heavy rainfall events remains poorly understood. Therefore, this study selected typical sloping cropland and abandoned land in the Jin-Shan Gorge region of the Chinese Loess Plateau as research objects. Changes in soil water content (SWC) and pH within a 0—300 cm soil profile before and after a single heavy rainfall event were investigated to reveal the effects of land-use type on soil moisture response and its relationship with soil pH dynamics, thereby providing a theoretical basis for land-use optimization and ecological restoration in the Chinese Loess Plateau region. Materials and methods The study was conducted in Lijiashan Village (37°38′09″N, 110°46′17″E), Qikou Town, Lüliang City, Shanxi Province, China, a typical hilly and gully region of the Chinese Loess Plateau. Sloping cropland and abandoned land were selected as representative land-use types. To minimize the influence of local microtopography, three spatially replicated sampling points (n=3) were established in an S-shaped pattern within each 10 m2 plot. Soil samples were collected before and after a heavy rainfall event. At each sampling point, soil samples were collected at 10-cm intervals from the surface (0 cm) to a depth of 300 cm, with three additional surface (0 cm) samples collected in each plot. A total of 372 undisturbed soil samples were obtained from the two land-use types before and after the rainfall event. For each soil layer, SWC was determined from the mean value of three spatial replicates, whereas soil pH was measured using a composite sample prepared by thoroughly mixing the three replicate samples. Consequently, statistical analyses were performed using data from 31 soil depths for each land-use type. Soil water content (SWC) was determined using the oven-drying method, and soil pH was measured using an FE28 benchtop pH meter. SWC and pH were measured, and their differences before and after rainfall were calculated as ΔSWC and ΔpH, respectively. Cluster analysis based on the magnitude of soil moisture variation was performed to classify the soil profile into drastic-response, primary-response, and secondary-response layers. Differences in ΔpH among response layers and between land-use types were evaluated using parametric or nonparametric statistical tests according to data distribution characteristics. Pearson correlation analysis and linear regression analysis were conducted to explore the coupling relationship between ΔSWC and ΔpH. Results Heavy rainfall significantly altered soil moisture in both land-use types, although the response intensity differed. Overall, SWC was higher in abandoned land than in sloping cropland, whereas sloping cropland exhibited a stronger response to rainfall. Specifically, the average SWC in sloping cropland increased by 40.4%, while that in abandoned land increased by 20.1%. Although SWC in abandoned land remained higher than that in sloping cropland after the rainfall event, the average difference in SWC between the two land-use types narrowed from 3.24% to 2.50%. Cluster analysis revealed similar vertical response patterns in both land-use types. The 20—110 cm soil layer was identified as the primary-response layer, where the largest increase in SWC occurred, whereas soil layers below 120 cm exhibited relatively small changes and were classified as secondary-response layers. Following the rainfall event, soil pH decreased in both land-use types. Further analysis based on moisture-response layers showed that ΔpH differed significantly between the primary-response and secondary-response layers in sloping cropland (P=0.013). The mean ΔpH values were −0.188±0.666 and −0.911±0.710, respectively. In contrast, no significant difference was observed between the two response layers in abandoned land (P=0.370). Comparisons between land-use types revealed no significant difference in ΔpH within the primary-response layer (P=0.674), whereas a significant difference was detected in the secondary-response layer (P=0.019), with sloping cropland exhibiting a stronger acidification tendency. Correlation analysis showed a significant positive relationship between ΔSWC and ΔpH in sloping cropland (r=0.373, P=0.039). Linear regression analysis also yielded significant results (R²=0.201, P=0.013). In contrast, the relationship between ΔSWC and ΔpH in abandoned land was weak and statistically insignificant (r=0.218, P=0.240). Discussion Heavy rainfall increased soil moisture in both land-use types, but the response was primarily concentrated within the 20—110 cm soil layer. This suggests that the wetting front generated by the rainfall mainly affected shallow and intermediate soil layers and was unable to penetrate deeply within a short period. This pattern indicates that the rainfall event mitigated the soil moisture disparities between the two land-use types to some extent. Such a pattern is consistent with the infiltration characteristics and soil structural properties commonly observed on the Chinese Loess Plateau. Long-term vegetation recovery in abandoned land has improved soil structure and increased organic matter content, resulting in higher overall soil moisture levels. In contrast, sloping cropland experiences stronger cultivation disturbance and possesses a relatively looser soil structure, leading to greater fluctuations in soil moisture following rainfall events. A single heavy rainfall event had limited effects on deep soil water recharge. The observed decrease in soil pH indicates that heavy rainfall can induce soil acidification, likely through the downward migration and leaching of soluble base cations during infiltration. The decrease in soil pH following the rainfall event indicates a general acidification trend. The significantly stronger acidification observed in the secondary-response layer of sloping cropland suggests that deeper soil layers are more sensitive to rainfall-induced ion transport processes. This suggests substantially stronger acidification in the secondary-response layer. Furthermore, the significant coupling relationship between ΔSWC and ΔpH in sloping cropland, but not in abandoned land, indicates that land-use type regulates the interaction between water movement and soil chemical properties. Improved vegetation cover and stronger buffering capacity in abandoned land may mitigate rainfall-induced pH fluctuations, resulting in a weaker acidification response. These findings indicate that soil moisture variation plays a stronger role in regulating soil pH dynamics in sloping cropland than in abandoned land. Conclusions The results indicate that sloping cropland is more sensitive to extreme precipitation events than abandoned land, exhibiting not only greater fluctuations in soil moisture but also a higher risk of rainfall-induced acidification. (1) A single heavy rainfall event noticeably increased soil moisture in both land-use types, with sloping cropland showing a stronger response than abandoned land. (2) Soil moisture variation was mainly concentrated within the 20—110 cm layer, whereas soil below 120 cm exhibited relatively weak responses, indicating limited deep-soil recharge from a single rainfall event. (3) Both land-use types experienced varying degrees of soil acidification after rainfall. Acidification in the secondary-response layer of sloping cropland was significantly stronger than that in the primary-response layer and also significantly stronger than that in the corresponding layer of abandoned land. (4) A significant coupling relationship between ΔSWC and ΔpH was observed in sloping cropland but not in abandoned land, demonstrating that land-use type strongly influences the coordinated response of soil moisture and soil acidity to heavy rainfall events. Recommendations and perspectives Therefore, soil and water conservation measures should be strengthened in sloping cropland to improve soil structural stability and reduce nutrient loss and soil degradation caused by heavy rainfall. Future studies should incorporate continuous rainfall events, soil organic carbon, soluble ions, and microbial community dynamics to further elucidate the mechanisms through which soil moisture changes drive soil chemical evolution under extreme precipitation conditions..