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Effects of Soil Aggregate Stability on Soil Organic Carbon and Nitrogen

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Effects of Soil Aggregate Stability on Soil Organic Carbon and Nitrogen International Journal of Environmental Research and Public Health Article Effects of Soil Aggregate Stability on Soil Organic Carbon and Nitrogen under Land Use Change in an Erodible Region in Southwest China Man Liu 1, Guilin Han 1,* and Qian Zhang 2 1 Institute of Earth Sciences, China University of Geosciences (Beijing), Beijing 100083, China; [email protected] 2 School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, China; [email protected] * Correspondence: [email protected]; Tel.: +86-10-82323536 Received: 1 September 2019; Accepted: 8 October 2019; Published: 10 October 2019 Abstract: Soil aggregate stability can indicate soil quality, and affects soil organic carbon (SOC) and soil organic nitrogen (SON) sequestration. However, for erodible soils, the effects of soil aggregate stability on SOC and SON under land use change are not well known. In this study, soil aggregate distribution, SOC and SON content, soil aggregate stability, and soil erodibility were determined in the soils at different depths along the stages following agricultural abandonment, including cropland, abandoned cropland, and native vegetation land in an erodible region of Southwest China. Soil aggregation, soil aggregate stability, and SOC and SON content in the 0–20 cm depth soils increased after agricultural abandonment, but soil texture and soil erodibility were not affected by land use change. Soil erodibility remained in a low level when SOC contents were over 20 g kg 1, and it · − significantly increased with the loss of soil organic matter (SOM). The SOC and SON contents increased with soil aggregate stability. This study suggests that rapidly recovered soil aggregate stability after agricultural abandonment promotes SOM sequestration, whereas sufficient SOM can effectively maintain soil quality in karst ecological restoration. Keywords: soil nutrients; agricultural abandonment; soil aggregation; soil degradation 1. Introduction Agricultural abandonment is generally supposed to significantly increase the stocks of soil organic carbon (SOC) and soil organic nitrogen (SON) [1–3]. However, some studies report that the SOC and SON stocks show unaltered or decreased trends following agricultural abandonment, and are dependent on climate, prior land use type, and soil type [1,2]. In addition, many soil physical properties, for example, soil bulk density, porosity, and permeability, will be improved in the surface soils after agricultural abandonment [4], and these soil physical properties can affect the rate of soil organic matter (SOM) decomposition [5]. Soil microbial diversity and quantity generally increase after agricultural abandonment [6,7], which improves SOM stabilization through secretion of organic products. Increased vegetation abundance through agricultural abandonment aging enhances the input of SOM through leaf litters and root exudates [8]. The responses of the SOC and SON to land use change are largely unknown [3]. Soil aggregates are the basic unit of soil structure [9], and their stability is generally regarded as an indicator of soil quality. The mean weight diameter (MWD) of soil aggregates is often used to quantify soil aggregate stability [10].Soil aggregate stability is affected by a variety of binding agents, including organic materials, iron and aluminum oxides, carbonates, and metal cations [11]. Int. J. Environ. Res. Public Health 2019, 16, 3809; doi:10.3390/ijerph16203809 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2019, 16, 3809 2 of 14 In general, organic binding agents can significantly improve the water-stability of aggregates compared to inorganic binding agents [12]. Thus, these organic binding agents play important roles in resistance to soil erosion [13]. Stable aggregates provide a physical protection for SOM to reduce microbial attack [11]. Many studies in agroecosystems reported the distribution of soil aggregates, soil aggregate stability, and aggregate-associated SOC under different forms of land management, including no-tillage, fertilization, conversion of natural ecosystem into agriculture land, and vice versa [14–18]. Although soil aggregate stability and SOM sequestration are generally related phenomena [19], their interaction with land use change, including agricultural abandonment, remain unclear. Soil degradation has been the greatest threat to ecological stability, residential environment, and local economic development in the karst region of Southwest China [20]. Soil degradation, including soil erosion and nutrient depletion, severely restricts agricultural productivity after long-term tillage [21,22]. Soil erodibility is defined as the amount of soil loss caused by runoff, rainfall, or seepage within a standard unit [23,24]. K factor is an estimated value from a soil erodibility estimate model, such as the Universal Soil Loss Equation (USLE), Revised Universal Soil Loss Equation (RUSLE), Erosion Productivity Impact Calculator (EPIC), or Geometric Mean Diameter based (Dg) model [23]. K factor is a key parameter for assessing the susceptibility of soil to erode, and partly reflects soil quality [23]. Previous studies have contrasted K factors in different erosion prediction models [23–25] and spatial variability, as affected by soil type, geology, geomorphology, climate, and agricultural management [26]. Soil erosion evolution in typical karst region was less than in Loess Plateau because of more complex geomorphology [26,27]. Few studies have examined the relationship between soil aggregate stability and SOM sequestration under land use change in karst-based highly erodible soils [28]. The objectives of this study were to (1) assess the changes in soil aggregate distribution, soil aggregate stability, SOC and SON content, and soil erodibility (K factor) at different stages after agricultural abandonment; and (2) determine the effects of soil aggregate stability on SOC and SON at different stages after agricultural abandonment of erodible soils in this region. These studies will improve the understanding of mechanistic changes in SOM sequestration and soil quality after agricultural abandonment. This will be of benefit through improving land management measures for ecological restoration and more sustainable soil productivity. 2. Materials and Methods 2.1. Study Area The study sites (26◦15040”–26◦16010” N, 105◦46002”–105◦46040” E) were located in a karst region near Chenqi village in Puding county, Guizhou province, Southwest China (Figure1). The region has a humid sub-tropical monsoon climate; the average annual air temperature is 15.1 ◦C; the mean annual precipitation is 1314.6 mm. More than 80% of annual precipitation occurs in the rainy season from April to September, while less than 20% occurs in the dry season from October to March [29,30]. The altitude of the study area ranges from 1310 to 1524 m, and an average altitude of 1350 m above sea level (a.s.l.) [31]. This region has a typical karst hoodoo depression landform; the center depression of the study area is surrounded by many hills, and over 60% of the hill areas have a gradient over 30◦ [32]. The lithology is mainly Permian and Triassic limestone and dolomite [33]. The calcareous soils are Mollic Inceptisols [34]. Soil layer thickness is spatially variable, ranging from 10 to 160 cm with an average of 30 cm [31]. The soils are distributed fragmentarily within the cavities and grooves between rocks, thus supporting the growth of crops and native plants. Int. J. Environ. Res. Public Health 2019, 16, 3809 3 of 14 Int. J. Environ. Res. Public Health 2019, 16, x 3 of 13 FigureFigure 1. Location 1. Location of study of study area area and and sampling sampling sites. sites. InIn Chenqi,Chenqi, manymany arablearable soilssoils havehave erodederoded alongalong thethe cavitiescavities andand groovesgrooves becausebecause ofof intensiveintensive agricultural production in the pastpast 5050 yearsyears [[35].35]. Soil degradation, including both soil erosion andand nutrient depletion, threatens the residential quality of lifelife andand thethe locallocal economyeconomy [[35].35]. In order to curtail soil erosion in the karst region, the governmentgovernment implemented the “Grain for Green Green Program” (GGP)(GGP) atat thethe beginning ofof the 21st century [[23,36].23,36]. Many Many low low yielding, yielding, steeply steeply sloping croplands were abandoned, whichwhich were were gradually gradually covered covered by by native native vegetation vegetation [31 [31].]. In In the the study study area, area, native native plants plants are mainlyare mainly thorn thorn (Itea ( yunnanensisItea yunnanensis), hornbeam), hornbeam (Carpinus (Carpinus pubescens) pubescens), kembat, kembat (Lithocarpus (Lithocarpus confinis confinis), and), awnand (awnMiscanthus (Miscanthus sinensis sinensis) in shrub) in shrub lands lands and secondary and secondary forest forest lands. la Croplandsnds. Croplands have have been been cultivated cultivated over theover long-term the long-term with tillage with usingtillage moldboard using moldboard plows and plows nutrient and managementnutrient management using urea, using ‘complete’ urea, N-P-K‘complete’ fertilizers, N-P-Kand fertilizers, manure. and Crops manure. grown Crops are peanutgrown are (Arachis peanut hypogaea) (Arachis, potato hypogaea) (Solanum, potato tuberosum (Solanum), oilseedtuberosum rape), oilseed (Brassica rape napus (Brassica), and napus maize), (Zeaand maysmaize). Abandoned(Zea mays). Abandoned croplands are croplands chiefly abandonedare
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