Soil and Vegetation Patterns Along a Toposequence in a Dolomite Peak-cluster Depression Catchment


 AimA deeper understanding of relationships between soil and vegetation is a prerequisite for accelerating karst area vegetation restoration. Remarkable achievements have been made at regional and individual plant scales, but research on the relationship between soil and vegetation is insufficient at the hillslope catena scale in karst areas.MethodsSoils and vegetation were investigated along a toposequence (upper-, middle-, lower-slope, and depression) of a dolomite peak-cluster depression catchment.ResultsA continuous soil catena pattern was developed along the toposequence. From the top to bottom of soil catena, soil thickness, fine soil mass ratio, nutrient stocks, and epikarst thickness gradually increased, while gravel mass ratio, pH, and saturated hydraulic conductivity gradually decreased. However, nutrient contents showed no significant change trends along the soil catena. There was a strong spatial association between soil types and dominant vegetation communities. The associations were as follows: herbs associated with entisols in the upper-slope; herbs and shrubs with inceptisols in the middle-slope; shrubs with semi-alfisols in the lower-slope; and trees with alfisols in the depression. ConclusionsThe dolomite rocks displayed an evenly progressive karstification process. This led to an undeveloped underground karstic network incapable of transporting soil materials into underground. Soil materials still accumulated at different topographic positions surface and formed a continuous catena. Parameters for nutrient stock may be more suitable for assessing soil productivity and to guide vegetation restoration key factors in karst regions than nutrient content parameters.

form a catenary pattern. However, for the karst landscape, knowledge about the soil-63 vegetation relationships were mostly relied on the results carried out in regional or plant 64 individual scales. Related studies in catchment or catenary scales are relative rare due 65 to the ubiquitous extremely high structural heterogeneity in these scales. For instance, 66 in karst regional scale, Jiang et al. (2014) found that bedrock geochemistry via 67 influencing the regolith water-retention capacities determined the karstic vegetation rocks. Borehole analysis suggests that karstification degree decreases as depth increases. 135 Study area karstification is characterized by penetrating dissolution pores, micro-tensile 136 dissolution fractures, and local dissolution fractures. Its highly weathered dolomite is 137 mostly lost its original rock structure and even deconstructed into dolomite sand. There 138 is no obvious bubble reaction obtained after dropping dilute hydrochloric acid on it.  Due to the strong spatial heterogeneity in the karst area, a total of 14 soil pits were 149 used: three for both upper-slope and depression positions, and 4 for both mid-slope and 150 lower-slope positions (Table S1), for soil profiles description and soil samples 151 collection. Soil pits were excavated to the C horizon or the epikarst zone. The main 152 topographic variables, including altitude, slope gradient and gravel content, and soil 153 profile morphological characteristics for each soil pits were recorded and described 154 according to FAO-ISRIC-ISSS (2006) (Wrb 2006 weigh to obtain total soil cylinder mass. This was the total soil material mass ( -g).

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Gravels, defined as having a diameter > 2 mm were selected and provided the gravel 181 component total mass ( -g). What remained was the fine soil component which were 9 components <2 mm. Fine soil mass was expressed as (g). This was ground and 183 sieved through a 2 mm sieve. Dried cylinders sample gravel was water-soaked for 12 184 hours fully saturation. Using drainage method determined gravel volume ( -cm 3 ) 185 (Wang et al. 2017). Gravel density ( -g/cm 3 ) could be calculated as： The values determined above were used to calculate fine soil mass ratio ( -%), 188 gravel mass ratio ( -%), fine soil volume ratio ( -%), and gravel volume ratio is fine soil volume (cm 3 ), the formula was as follows: Fine soil texture was determined using the hydrometer method.  .

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The PVC tubes, with a volume of 1178 cm 3 , described above to collect soil cylinders  According to the gravel height and fine soil height for each soil layer, calculate the 235 gravel volume ( 1 -cm 3 /m 2 ) and the fine soil volume ( 2 -cm 3 /m 2 ) of the soil layer per 236 unit area. Gravel density ( -g/cm 3 ) were used to calculate the mass of gravel per unit 237 area of soil ( 1 -kg/m 2 ). Fine soil bulk density ( -g/cm 3 ) were used to calculate the 238 mass of fine soil per unit area of soil ( 2 -kg/m 2 ).
represents the total mass of soil Pedon scale nutrient stocks is equal to the sum of gravel nutrient stocks and fine soil 250 nutrient stocks for each soil layer, but the gravel nutrient content is extremely low 251 (Table S2), resulting in the low nutrient stocks of gravel, and negligible. Therefore, the  Table S3.  for 64% and 27% of the catchment area. The peak-cluster depression subdivides into 270 four topographic positions. 1) exposed bedrock cliffs with a slope close to 90°. 2) an 271 mostly exposed rock upper-slope with a slope of 32°; a greater exposed rock mid-slope 272 with of 23°; and, less exposed rock lower-slope of 18°. weakly-weathered rock mass changes to a moderately-weathered rock mass, then to 276 strongly-weathered clastic rock ending in dolomitic sand layer (Table 1).

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The drilling cores's average mass contents for CaO and MgO were about 31% and 278 21%, respectively (Table 1). Average mass content of acid-insoluble account for about 279 0.17% of total rock mass. This suggests a very pure dolomite rocks in this study 280 catchment. As the slope decreases, the soil pH value of the soil gradually decreased 281 (Table 1). Upper-slope and mid-slope soil pH inherited the high alkaline characteristics 282 of the underlying weathered dolomite. Lower-slope soil pH, particularly in depressions, 283 reduced to neutral, or acidic. That soil's properties were less affected by the underlying 284 bedrock.  "A" layer is black surface layer. "AC or B" is a yellow illuvial horizon and "C" is a 289 white dolomite strongly weathered layer (Table 1). There is a deep dolomite weathering 290 layer in the study catchment. The degree of weathering decreases with depth (Table 1).

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The differences in the degree of dolomite weathering at different topographic positions 292 are that weathering weakest on the upper-slope where the weathered layer was 1.5 m.   316 Upper-and middle-slope soil profiles were composed mainly of gravel. Upper slope 317 average gravel mass and volume ratios were 73% and 64% respectively (Fig. 2). In the 318 middle slope these were 78% and 47% respectively. Lower-slope and depression soil 319 profiles were mainly fine soil. Lower-slope gravel mass and volume ratios were 1.30% 320 and 1.09%, respectively. Depression ratios were 0.21% and 0.24%. Gravel mass and 321 gravel volume proportions decreased as slope decreased. Fine soil mass and fine soil 322 proportion volumes showed the opposite trend which are obvious slope colluvium 323 characteristics (Fig. 2).

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There was a high average clay content of between 32%-52% in lower-slope and 325 depression profiles (Fig. 3). Lower-slope clay content relatively smoothly, with depth 326 (Table 2). Upper and middle slopes have higher sand content which increases with depth.
The average profile sand content was between 68% and 81% (Fig. 3). This shows that 328 lower-slope and depression belong to the clay group. Saturated hydraulic conductivity 329 decreased as the soil depth increase (   335 If there is a "rule of nutrient content change" along a slope it was not obvious (   344 This study determined that soil nutrient surface accumulation was significant. A layer 345 nutrient, alone, are not an accurate measure of reflect soil fertility and productivity.

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Middle-slope accumulation ranges from 60%-76%. Lower-slope accumulation was 348 between 33%-56% and for depressions it was 22%-26% (Fig. 5). A layer of depression 349 accumulated relatively few nutrients. Along a slope, nutrient accumulate of A layer 350 gradually lessens. From the characteristics of the black soil layer (Table 1) of upper-slope soil profile, it could also be derived that slope pedon scale surface nutrient 352 accumulation was significantly stronger than that of the depression.

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Surface accumulation increased as the amount of fine soil increased except for lower-356 slopes.  (Table   360 4). On the mid-slope, shrubs were 58% and herbs 26%. On lower-slopes shrubs and 361 herbs were 62% and 23%. In depressions, herbs were 55% and trees were 42%. Number   369 All measured factors were divided into three categories: 1) topographic factors which    This result provided field solid evident to confirm that, in a karst area, the chemical 396 dissolution in carbonate weathering process is dominated.

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The dolomite showed an overall uniform weathering pattern. The rock outcrops in 398 the hillslopes are relatively low, thus bare rocks are no barriers to the continuous soil 399 distribution. Even where soil was shallow and the dolomite strongly weathered and the 400 weathering layer was thick, the dolomitic weathering was characterized by diffuse and 401 integral dissolution. As a result, karst fissures and conduits were not well developed.

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Soil was mainly distributed on the surface. Vertical, water-driven migration into 403 underground karst voids was not detected. This suggests that even in strongly-   437 Vegetation, and soil, patterns of the dolomitic peak-cluster depressions present a clear 438 spatial correspondence. This is revealed soil catena distribution patterns being 439 compared to vegetation spatial distribution pattern. (Fig. 8). Vegetation patterns can be   465 Soil productivity is the ability of soil to support plant growth. Where climates are 466 similar, highly abundant vegetation and biomass usually indicates high productivity 467 (Wang et al. 2015). Non-karst area soil, generally, is deeper and less gravelly. Soil 468 amount maybe not an important factor limiting its productivity. Therefore, scholars  In this study, it was found that the soil nutrient content was high, but the vegetation 478 in the area was low in number and richness, and low in plant species and diversity. This 479 shows that the nutrient content does not accurately reflect the vegetation characteristics.

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In this regard, we believe that in this karstic shallow soil zone, the nutrient content may 481 not accurately reflect soil fertility. On the contrary, the nutrient stocks that varied along 482 the toposequence showed the same regularity as the vegetation, and the correlation 483 between them was significant. In addition, the indicators affecting nutrient stocks, such 484 as slope position, soil thickness, sand content, clay content, gravel mass ratio, and fine 485 soil mass ratio, showed the same consistent pattern of variation with vegetation, and 486 the correlation analysis also reflected the significant relationship between the above 487 indirect indicators and vegetation. In summary, comparing nutrient content and nutrient 488 stocks, we propose that nutrient stocks is more applicable to assess soil productivity.  These differences lead to apparent different soil-epikarst structural differences between 510 them. Weather soil-epikarst-vegetation catenary is also obvious in peak-cluster 511 depression catchment developed from limestone but this remains to be elucidated.    BD is total soil bulk density. is fine soil bulk density. m1 is gravel mass per unit area. m2-fine 616 soil mass per unit area. mt is soil mass per unit area. 617 n.a.: not analysed. 618 The statistics of the above values were on a 400m 2 sample. Percentage is the ratio different 620 vegetation types to the total plant number. 621  geohydrologic background of the study catchment. 1, 2, 3, 4, and 5 indicate the karst aquifer; the 638 sandstone aquifer, a relatively impermeable layer; the porous quaternary aquifer; the spring and 639 ground water flow paths, respectively. P1 is is early Permian. Q is Quaternary. C1, C2, and C3 640 indicate the early, middle and late Carboniferous, respectively. 641 In Figures a, b, c, and d, the left is part represents the nutrient content and the right part represents 654 the nutrient stocks. Note: Sp is slope position. Sd is Soil depth. Gc is gravel coverage. BD is bulk density.
is fine 664 soil BD. Cn is nitrogen content. Cp is phosphorus content. Ck is potassium content. Co is organic 665 matter content. MF is fine soil content. MG is gravel content. VF is fine soil volume ratio. VG is 666 gravel volume. m1 is gravel per unit area mass. m2 is fine soil per unit area mass. mt is the total 667 mass of soil per unit area, Pn is the pedon total nitrogen stocks, Pp is the pedon total phosphorus 668 stocks, Pk is the pedon total potassium stocks, Po is the pedon organic matter stocks, Sn is the 669 total nitrogen stocks of A layer, Sp1 is the total phosphorus stocks of A layer, Sk is the total 670 potassium stocks of A layer, So is the stocks of organic matter in A layer, Tt is tree types, Nt is the 671 number of trees, Ts is the type of shrub, Ns is the number of shrub, Th is the type of herb, Nh is 672 the amount of herb. 673