Hydrological driving characteristics of soil carbon and nitrogen losses under different land use modes on karst slopes
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摘要:
喀斯特坡地壤中流与地表径流并存，共同驱动了土壤关键生源物质的迁移，造成该区土壤生产力下降、地下水污染。为揭示喀斯特坡地土壤碳氮流失途径及其水文驱动机制，本研究以喀斯特坡地径流微区（2 m×1.2 m）为研究对象，分析了火烧、轻度砍伐、重度砍伐、人工林、耕地、牧草6种不同土地利用方式对土壤碳氮流失途径、形态及通量的影响。结果表明：降雨是土壤碳氮流失的主要驱动因子，降雨产流阈值为16 mm，55 mm时达到产流峰值。各土地利用方式仅在暴雨下有显著差异，其中，人工林的土壤碳氮流失量较大，而重度砍伐的流失量较小。土地利用方式不改变土壤碳氮的流失途径，各土地利用方式均以地表流失为主（51.29%~75.15%），壤中流为辅，其中壤中流主要通过A层流失(65.20%~89.12%)。氮素流失形态以
${\rm{NO}}_3^{-}$ Abstract:There is no direct contact between shallow soil and underlying carbonate rock in karst slope. Due to the unique karst processes in the karst hillslopes, in the process of secondary rainfall, overland flow and subsurface flow jointly drive the migration of key biogenic materials in the soil rock profile, resulting in the decrease of soil productivity and groundwater pollution in this area. However, the main migration pathways and driving mechanisms of soil carbon and nitrogen loss under different land use modes remain unclear. Based on a complete hydrological annual rainfall runoff-nutrient monitoring from 6 different land use experimental hillslopes (burned land, light felling land, heavy felling land, planted forest, farmland and grassland), we took the slope runoff micro-area (2 m×1.2 m) of Huanjiang Karst Ecosystem Observation and Research Station of Chinese Academy of Sciences as the research object. This study analyzed the loss path, loss form and average loss of soil TN and Toc driven by the near-surface multi-interface lateral hydrological processes (surface runoff, lateral flow from Layer A and lateral flow below the layer). Results show that the karst slope has a high soil infiltration coefficient, and the soil carbon and nitrogen loss per unit area is positively correlated with rainfall. Rainfall is the main driving factor of soil carbon and nitrogen loss. The threshold rainfall of nutrient loss driven by runoff is 16 mm, and the peak rainfall is 55 mm, indicating that the karst slope shows the characteristics of nutrient loss driven by full runoff. There are significant differences of soil carbon and nitrogen loss among different land use modes, but only under heavy rainfall. Compared with other land use mods, planted forest can improve canopy interception, increase transpiration and infiltration, and thus weaken the surface flow production. However, research results also show that the soil carbon and nitrogen loss of planted forest is relatively large, which on the one hand is related to the single vegetation type, high surface exposure, and fast soil surface saturation; on the other hand, the channels formed by the interpenetration of plant roots have natural continuity and are interconnected with other channels in the soil to form a huge water transport network and promote the occurrence of runoff. Therefore, species suitable for karst areas should be cultivated; the allocation of shrub and grass species at the bottom of canopy should be strengthened; and moderate human interference should be carried out to optimize plant community structure and reduce the risk of soil carbon and nitrogen loss.
In this study, it is found that the soil carbon and nitrogen loss of heavy felling land is relatively small. During the process of heavy felling and root removal, the original pores of soil were destroyed, which had a negative impact on the occurrence of preferential flow and near-surface flow. The higher soil carbon and nitrogen loss in farmland and grassland is related to fertilization in tillage period, but the difference between them is not significant, indicating that conservation tillage is beneficial to soil microbial activities and thus improve soil fertility. Combined with the effect of fertilization, the difference in nutrient loss driven by hydrological processes may be concealed. Meanwhile, surface filling increase, thus reducing the near-surface loss. Land use modes do not change the path of soil carbon and nitrogen loss. The soil carbon and nitrogen in the slope of karst dolomite mainly loss through the surface (51.29%-75.15%), because the pores and joints in the dolomite area are evenly distributed and the water-barrier layer is easy to be generated and the permeability is good under the dual influence of physical weathering and tectonic forces. At the same time, the monitoring results of soil flow show that the flow loss in Layer A is much higher than that below Layer A, which is related to nutrient enrichment in Layer A on the one hand. On the other hand, the flow generation in karst areas follows the theory of interfacial flow generation, and the permeability of Layer A is much higher than that below Layer A.
${\rm{NO}}_3^{-}$ -
Key words:
- karst ecosystem /
- human disturbance /
- non-point source polution /
- carbon and nitrogen loss /
- soil flow.
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表 1 坡地径流小区处理方式
Table 1. Processing modes of the runoff micro-area
土地利用方式 坡形 坡度/° 处理方式 火烧 直形坡 34 每年一月份火烧一次 轻度砍伐 直形坡 23 每年一月份砍伐、搬移，不去除植物根系 重度砍伐 直形坡 24 每年一月份砍伐、搬移，去除植物根系 人工林 凹形坡 23 2006年挖坑种植枸骨木 耕地 凹形坡 25 顺坡种植玉米（每年三月份和六月份共施氮肥160 kg·hm−2） 牧草 凹形坡 24 种植牧草（桂牧1号；返青后施氮肥45 kg·hm−2，
每次刈割后施氮肥50 kg·hm−2）
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表 2 不同土地利用方式径流微区土壤基本理化性质
Table 2. Soil physical and chemical properties in runoff micro-area under different land use modes
土地利用
方式砾石覆
盖度/%全氮TN/
g·kg−1有机质/
g·kg−1容重/
g·cm−3土壤机械组成/% 0.02~2 mm 0.002~0.02 mm <0.002 mm 火烧 50 3.12 30.59 1.32 61.28 18.43 20.29 轻度砍伐 34 1.70 28.43 1.32 65.16 14.16 20.68 重度砍伐 49 1.77 27.26 1.34 62.93 15.82 21.25 人工林 28 2.23 30.00 1.26 48.90 15.81 35.29 耕地 44 2.18 22.27 1.40 64.48 14.47 21.05 牧草 40 1.69 25.37 1.29 70.58 11.07 18.35
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表 3 不同土地利用方式土壤TN流失量与流失浓度、产流量、降雨量的关系
Table 3. Relationship between the TN loss concentration and flux, runoff amount, and rainfall amount under different land use modes
土地利用方式 流失浓度 产流量 降雨量 拟合方程 R2 P 拟合方程 R2 P 拟合方程 R2 P 火烧 Y=0.701+0.875x−0.057x2+0.003x3 0.273 0.000 Y=0.011x0.770 0.830 0.000 Y=0.192x0.785 0.426 0.000 轻度砍伐 Y=0.923x0.684 0.125 0.017 Y=0.857+0.001x 0.812 0.000 Y=0.144x0.923 0.476 0.000 重度砍伐 Y=2.935-0.107x 0.200 0.004 Y=0.039x0.604 0.696 0.000 Y=0.247x0.707 0.314 0.000 人工林 Y=0.820x0.850 0.271 0.000 Y=0.006x0.846 0.724 0.000 Y=0.200x0.882 0.407 0.000 耕地 Y=3.920+0.489x+0.001x2 0.553 0.000 Y=0.008x0.868 0.624 0.000 Y=0.154x0.950 0.201 0.000 牧草 Y=-1.932+2.368x-0.202x2+0.007x3 0.393 0.000 Y=0.014x0.762 0.680 0.000 Y=0.285x0.711 0.219 0.000
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表 4 不同土地利用方式土壤TOC流失量与流失浓度、产流量、降雨量的关系
Table 4. Relationship between the TOC loss concentration and flux, runoff amount, and rainfall amount under different land use modes
土地利用方式 流失浓度 产流量 降雨量 拟合方程 R2 P 拟合方程 R2 P 拟合方程 R2 P 火烧 Y=76.329+8.766x+
0.372x2+0.004x30.125 0.122 Y=0.151x0.668 0.672 0.000 Y=30.937−3.556x+
0.162x2−0.002x30.552 0.000 轻度砍伐 Y=1.465x0.789 0.334 0.000 Y=8.448+0.007x 0.649 0.000 Y=22.743−2.594x+
0.138x2+0.001x30.578 0.000 重度砍伐 Y=5.099x0.319 0.067 0.069 Y=12.073+0.003x+
4.220E−7x20.912 0.000 Y=1.196x0.789 0.281 0.000 人工林 Y=2.633+0.897x 0.147 0.027 Y=−3.280+0.029x−
7.722E−6x2+
7.233E−10x30.918 0.000 Y=−42.504+8.174x−
0.311x2+0.004x30.603 0.000 耕地 Y=17.615+0.270x 0.218 0.001 Y=0.472x0.523 0.485 0.000 Y=6.332x0.370 0.074 0.061 牧草 Y=7.690x0.291 0.081 0.019 Y=0.428x0.539 0.514 0.000 Y=13.877−1.219x+
0.083x2−0.001x0.606 0.000
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