| The semiarid region of Weibei is part of the Loess Plateau located in Shaanxi Province, Northwest China. Owing to the favorable climatic conditions, Weibei is regarded as one of the largest high-quality apple production bases in China. With the extension of cultivation years and shortening of the full-fruiting period, the aging of apple trees and other issues become increasingly prominent in Weibei Region. Consequently, the felling of over-mature apple trees has been common, while soil sickness is encountered in the replanting of apple trees. These conditions would negatively affect the production potential of apple orchards and sustainable development of the apple industry. Currently, there is an urgent need for research into the impact of farmland conversion to apple orchard and long-term cultivation of apple trees on soil carbon pools and soil phycial, chemical and biological properties in the vast area of Weibei, in order to promote the sustainable development of regional apple industry.In this study, representative apple orchards of different age groups were selected from two counties in Weibei, Luochuan and Binxian, with adjacent farmland as the control. Data were obtained by space-for-time substitution to identify the trends in soil carbon composition and stocks, basic physical-chemical properties, and enzyme activities in the vertical soil profile. The relationships between 16 indicators of soil properties were examined and the soil quality of apple orchards was fully assessed. Aggregate organic carbon(AOC) contents in different size classes were determined by dry and wet sieving. Further, functional groups in the fractions of soil organic carbon(SOC) and particulate organic carbon(POC) were qualitatively analyzed using Fourier transform infrared spectroscopy, in order to reveal the compositional and structural changes in SOC. This study provides evidence for accurately assessing the carbon sequestration potential of apple orchard ecosystems and the impact of long-term cultivation of apple trees on the soil quality. The main conclusions were as follows:(1) Farmland conversion to apple orchard improved the contents and densities of total SOC(TOC) under <10-year(<10a) orchards at the soil depths of 0~40 cm and 60~80 cm. With increasing age of apple orchard, the spatial distribution of TOC contents and densities varied in different trends. At depths of 0~10 cm and 40~100 cm, the two parameters increased with orchard age; at 10~40 cm depth, both parameters exhibited a turbulence trend, which in the 10~15-year(10~15a) orchards had a downward trend and then increased towards a higher age. When orchard age was greater than 25 yrs(>25a), the TOC contents and densities showed a deceasing trend at all soil depths. The SOC stock in the 0-100 cm soil profile increased with orchard age; however, significant increases were found only on the time scale greater than 15 yrs(P<0.05). The 0~40 cm soil depth was the main storage site of SOC pool, accounting for more than 49.9% of the total pool.(2) The carbon stock comprised primarily of soil inorganic carbon(SIC) in both farmland and orchard soils in the 0~100 cm profile. Farmland conversion to orchard had little impact on the SIC density. Nonetheless, the cultivation of apple trees was not conducive to accumulating SIC. A significant increase was found in the SIC stock only at the orchard age of greater than 25 yrs. In the vertical soil profile, the SIC densities generally increased with soil depth. Large variations were found in SIC densities at 0~40 cm depth between the orchards of different age groups; the soil depth of 40–100 cm was the main storage site of SIC pool, in which the distribution of SIC densities was relatively uniform.(3) Labile soil organic carbon(LOC), non-labile organic carbon(NLOC), and the proportion of LOC to TOC(LOC/TOC) decreased with soil depth in the 0~100 cm soil profile. Specifically, the distribution of LOC stock tended to be uniform at the depth of 40~100 cm. Farmland conversion to <10-year orchard showed minor impact on the LOC and NLOC, while it improved the stability of SOC at 0~80 cm depth, namely reduced the LOC/TOC ratio. In the 10~15-year and >25-year orchards, the LOC, SOC liability, liability index, carbon pool index, and carbon management index remained low in the soil profile, and significant decreases occurred especially at 40~80 cm depth. In the 15~25-year orchards, although the above five indicators increased with orchard age, there was a potential risk of SOC degradation.(4) Particulate organic carbon(POC), mineral-associated organic carbon(MOC), POC/TOC, and POC/MOC generally decreased with soil depth in the 0~100 cm soil profile. Farmland conversion to apple orchard improved the POC, POC/TOC, and POC/MOC levels at 0~20 cm and 80~100 cm depths, and long-term cultivation of apple trees was overall beneficial to the accumulation of POC. Similarly, in the 10~15-year and >25-year orchards, the POC contents and densities, as well as the POC/TOC and POC/MOC ratios, showed a downward trend at most soil depths; however, the POC stock remained increasing.(5) Soil compaction and bulk density generally increased with soil depth in the farmland and orchards, and both parameters below the soil depth of 20 cm surpassed the limits for root growth. Soil clays appeared to move down in the soil profile, mostly deposited at 20~40 cm depth. With the increase in orchard age, soil compaction and bulk density increased in the >20-year orchards, resulting in the leaching and deposition of soil clays at deeper soil depth(40~60 cm). As the orchard age increased, soil p H, Ca CO3 and HCO3- levels all fluctuated up and down, and then went up again; the orchard soils exhibited the trend of subface compactification, Ca CO3 degradation, and acidification.(6) The conversion of farmland into <10-year apple orchard enhanced soil invertase and alkaline phosphatase activities, while it reduced soil urease and catalase activities. With increasing age of apple orchard, soil invertase, urease, and alkaline phosphatase activities all fluctuated down and up, and then dropped again, with significant differences being observed between age groups(P <0.05); however, soil catalase activity only increased at 20~30 cm depth, with no clear trends over time at the other soil depths. In the vertical profile, soil invertase, urease, and alkaline phosphatase activities all decreased with depth; these enzyme activities were significantly higher at the depth of 0~20 cm than at below 20 cm, and there were minor changes in the deep soil. Soil catalase activity showed no clear trends in the vertical soil profile and the values were overall low at the depth of 20~60 cm.(7) Soil mechanical-stable aggregates were classified with microaggregates <0.25 mm as the superior class. The contents of >10 mm aggregates varied between different treatments in the order: 21-year orchards > 10-year orchard > farmland. In the orchard soils, the mean weight diameter(MWD) and geometric mean diameter(GMD) of mechanical-stable aggregates, as well as the content of >0.25 mm water-stable aggregates(WR0.25), increased with orchard age, whereas the fractal dimension(D) decreased with soil depth over time. The contents of water-stable aggregates increased with decreasing size, and <0.25 mm microaggregates were most abundant. In the farmland soils, the contents of water-stable macroaggregates in various size classes as well as their MWD, GMD, WR0.25, and aggregate stability index(ASI) were higher, whereas the D, percentage of aggregate disruption(PAD0.25), and unstable aggregate index(ELT) were lower, compared with the values of the orchard soils; the farmland soils showed higher water stability than the orchard soils.(8) Dry-sieving data showed that the aggregate associated organic carbon(AOC) contents varied significantly with size class(P <0.05) and remained relatively high in the <0.25 mm size class in both the farmland and orchard soils. At the 0–30 cm soil depth, the AOC contents in various size classes of mechanical-stable aggregates changed in the order: farmland > 21-year orchards > 10-year orchards. In each size class, the AOC contents generally decreased with soil depth. However, at the depth of 30–40 cm, the farmland soils contained lower AOC contents in the >3 mm size classes compared with the orchard soils. In both farmland and orchard soils, the lowest AOC stock and its contribution to TOC were found in the 3~2 mm size class, while the highest were attributed to the <0.25 mm and >7 mm size classes. An increase in orchard age improved the AOC stock and its contribution to TOC in macroaggregates of the >7 mm size class, and both were significantly higher at 20~40 cm than at 0~20 cm depth.Wet-sieving data showed that higher AOC contents occurred in the >1 mm size class in the farmland and 10-year orchards, as well as in the 2~0.5 mm size class in 21-year orchards. The lowest AOC contents were found in <0.25 mm microaggergates in all treatments. In each size class, the AOC contents decreased with soil depth. As the age of apple orchard increased, the AOC contents in the >5 mm size class remained decreasing at the 0~20 cm soil depth. Meanwhile, the AOC contents in the 5~2 mm and 1~0.25 mm size classes decreased at the 20~40 cm soil depth, whereas an upward trend was found in the other size classes. In both farmland and orchard soils, the AOC stock and its contribution to TOC in various size classes of water-stable aggregates increased with decreasing size, and the values were significantly higher in <0.25 mm microaggregates compared with other size classes(P <0.05). In the soil profile, the AOC stock in various size classes and their contribution to TOC varied in different trends between treatments.(9) In the farmland and orchard soils, the functional groups of SOC and POC mainly included hydroxyl, carboxyl, carbonyl, aliphatic, aromatic, and carbohydrate groups. Carbohydrates were identified to be the primary species of SOC in the apple orchards.(10) The relationships between 16 indicators of soil properties were examined by principal component analysis. The soil quality in the study area was assessed using two quantitative methods, the integrated soil quality index and the soil degradation index. The results showed that the soil quality deteriorated during the cultivation of apple trees; significant deterioration occurred particularly at the 0~40 cm soil depth in the 10~15-year orchards of the full-bearing stage. Meanwhile, the soil quality deteriorated with depth; the changes were significant at the 0~40 cm soil depth and minor at below 40 cm. |
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