Search

The Chinese Loess Plateau has undergone extensive revegetation to restore degraded land and enhance carbon sequestration. However, soil organic carbon (SOC) sequestrated in the soil profiles of deep-rooted plants has not been fully studied. Here, we investigated the SOC within a 0–23 m profile in farmlands and apple orchards converted from farmlands with different ages (A5, <5 years; A10, ~10 years; A15, ~15 years; A20, >20 years) and the controlling factors on three loess tablelands (Changwu, Qingyang, and Luochuan). The results show that SOC stocks among farmlands and orchards showed no significant difference (p = 0.88); however, SOC stocks showed a trend with tree ages, i.e., a decrease for A5 and A10 but an increase for A15 and A20. For the vertical variability, the SOC stock was the highest within 0–1 m, regardless of the standing age; however, the SOC stock in this layer only accounted for 8.8% of the total SOC stock (97.93 ± 9.18 kg m−2). Climate accounted for 82% of the variations and controlled the changes in SOC in the 0–1 m range, while soil texture dominated the SOC in the soil below 1 m, accounting for 57% of the SOC variations. The variations in SOC in the thick, unsaturated zones provide implications for future land use management and the sustainability of apple orchards in arid regions. [ABSTRACT FROM AUTHOR]

Afforestation of degraded land significantly influences soil organic carbon (SOC) and total nitrogen (STN) sequestration. The interaction effects of plantation age and climate gradient on SOC and STN changes following afforestation are not well understood. In this study, five sites were selected along a precipitation gradient (410–600 mm yr−1) in the Loess Plateau. The SOC and STN stocks at a depth of 0–200 cm were measured in cropland and black locust (Robinia pseudoacacia L.) forests with different plantation ages, that is, young forest (<15 years), middle‐aged forest (15–25 years), and old forest (>25 years). The SOC and STN stocks in the 0‐ to 200‐cm profiles of young forest, middle‐aged forest, and cropland increased significantly with mean annual precipitation (p <.05), whereas the increasing trend of the SOC stocks of old forest was not significant, indicating an age‐dependent change in the SOC and STN stocks across the precipitation gradient. The SOC stock change (∆SOC) following afforestation increased with mean annual precipitation in young forest, but it had a decreasing trend in middle‐aged and old forests. The STN stock changes (∆STN) in the three forests were negative at most sites, and they all decreased along the precipitation gradient. There were significant positive correlations between ∆SOC and ∆STN (p <.01), and 1‐g STN stock accumulation was accompanied by 8.40‐, 6.10‐, and 10.48‐g SOC accumulation for young forest, middle‐aged forest, and old forest, respectively. The different patterns of SOC and STN stock changes should be incorporated into soil C and N modelling and estimation. [ABSTRACT FROM AUTHOR]

• Root water uptake depth increased in "S" shape with tree age and stabilized at 20 m after 20 years in an apple orchard. • Soil water deficit increased in depth of 3–21 m at a rate of 5.8 mm m−1 yr−1 from 10 to 24 years of tree age. • Apple trees could draw water horizontally up to 7 m from deeper soil layers of adjacent cropland. • Orchard-cropland mosaic structure is able to optimize the use of soil water resources in the Loess Plateau. It is of great significance to explore soil water distribution and consumptions of the agroforestry systems in water-deficient areas. In recent decades, continuous development of apple orchards in the southern Chinese Loess Plateau has formed the orchard-cropland mosaic vegetation pattern. In this study, soil water content (SWC) in two series were measured, (1) 0–21 m soil profiles of five apple orchards with different stand ages (5, 10, 15, 20 and 24 years) and one cropland; (2) 0–6 m soil profiles at different sites within the orchard-cropland border zone (with apple orchard aged 10 and 24 years old, denoted as AC10 and AC24, respectively). Results showed that (1) the water consumption depth of apple orchards increased in an "S"-shaped pattern with stand age and reached 20 m after 20 years, while the average SWC within 3–21 m layer decreased and the soil water deficits increased at a rate of 5.8 mm m−1 yr−1; and (2) the average SWC in 3–10 m layer of 15, 20 and 24-year-old orchards accounted for 69.3%, 64.2% and 57.9% of the field capacity, respectively, these proportions came to 71.2%, 67.4% and 61.1% in the 3–21 m layer, respectively; and (3) AC24 had greater influence on SWC of adjacent cropland than AC10. The soil water storage of 2–6 m layer in AC24 ranged from 713 mm to 879 mm within 7 m away from the orchard, which were significantly lower than those of 7–15 m away from the orchard (933–1042 mm). Overall, the apple tree root system can absorb soil water from a depth of 20 m in the vertical direction and up to 7 m from the adjacent cropland in the horizontal direction. Therefore, a reasonable orchard-cropland mosaic pattern should be arranged in regional agricultural planning, so as to promote the optimized utilization of regional soil water resources, and to coordinate the development of fruit industry and food security, in which the width of a cropland can't be < 14 m. [ABSTRACT FROM AUTHOR]

Determining changes in soil organic carbon (SOC) and inorganic carbon (SIC) stocks following ecological restoration is important for estimating the regional carbon budget and evaluating ecological effects. However, there is limited understanding on the interacting effects of vegetation restoration and climate gradients on SOC and SIC stocks in shallow and deep soil layers over large scales. This study selected seven sites along a climate transect from the east to the west of the Chinese Loess Plateau, and the SOC and SIC stocks were measured to a depth of 300 cm in sites covered by cropland and three vegetation restoration types (grassland, shrubland and woodland). The spatial variations and controlling factors of the SOC and SIC stocks at different depths following vegetation restoration along the precipitation gradient were investigated in detail. The results indicated that the SOC and SIC stocks in the 100–300 cm layers accounted for more than 50% of the total values in the 0–300 cm profile. The total SOC stock in the 0–300 cm profiles significantly increased along precipitation gradient ( p <  0.01) in all vegetation types except woodland. The total SIC stock decreased, but the change was not significant, which caused little variation in the total carbon stocks along the precipitation gradient ( p >  0.05). Shrubland and woodland plantation following cropland abandonment resulted in soil carbon accumulation, whereas grassland represented carbon loss in sites with mean annual precipitation (MAP) greater than 470 mm. The changes in the SOC stock (ΔSOC) in the surface layer (0–20 cm) and those in the SIC stock (ΔSIC) in the deep layers (100–300) following revegetation significantly decreased along precipitation gradient ( p <  0.05). The interactions between ΔSOC and ΔSIC stocks were evident, especially in the upper soil layers. An accumulation of 1 kg SOC was accompanied by 0.73 kg loss of SIC in the 0–40 cm layer and 1.26 kg increase of SIC in the 40–300 cm layer per square meter following revegetation. MAP and mean annual temperature (MAT) mainly affected the spatial patterns of SOC and SIC in the upper layers, while land use and soil texture mainly affected soil carbon in the deep layers. This study indicates that vegetation restoration does not always result in soil carbon sequestration at every depth after cropland abandonment, which depends on climatic conditions and varies among different vegetation types. [ABSTRACT FROM AUTHOR]

ABSTRACT Potential evapotranspiration represents the maximum evaporative capacity for a region, which underscores the importance of analysing its changes and attribution to provide information for water resource management. This study focused on the northern Loess Plateau of China, investigated the changes in potential evapotranspiration ( ET0) using data from 34 stations during 1960-2013 and conducted its temporal and spatial attribution through the differentiation equation method. The results show that the annual mean ET0 is 1004.9 mm with a decreasing trend at the rate of 0.33 mm year−1. The lowest values were found in the northeast and southwest regions, while the greatest ET0 was in the northwest region. An 'evaporation paradox' existed as a whole, but not obviously. The ET0 was most sensitive to actual vapour pressure ( ea), followed by solar radiation ( Rs), mean temperature ( Tmean) and wind speed ( U2). The inter-annual variation in the sensitivity of ET0 in relation to climatic factors showed that ET0 became more sensitive to U2 and Tmean but less sensitive to Rs and ea from 1960 to 2013. Temporally, the increasing Tmean resulted in the increase of ET0; while its effect was weakened by the other three factors, especially by decreasing U2. The contributions of climatic factors to the ET0 spatial variation trends were also analysed along two transects, but the dominant factors were different for each transect. Furthermore, the possible impacts of human activities on ET0 changes were also discussed. [ABSTRACT FROM AUTHOR]

Soil water has become a major limiting factor in agriculture and forestry development on the Loess Plateau of China. In the past 20–30 years, large areas of apple orchards have been built in this region, which have resulted in excessive consumption of deep soil water and soil desiccation. To evaluate the effects of orchard development on deep soil water content (SWC), a meta-analysis of 162 sampling sites on the loess tableland from 44 peer-reviewed publications was conducted in this study. The results showed that the deep SWC in orchards depended on stand age, planting density and annual precipitation. In regions with 550–600 mm precipitation, the orchard with lower planting density showed no soil desiccation in young and early fruiting stages, while deep soil (>2 m) desiccation occurred in full fruiting and old orchards. The effect of planting density on deep SWC varied with stand age. There were significant differences in SWC among different planting densities in early fruiting orchards (p < 0.05), in which soil desiccation occurred in orchards with higher planting density. However, with the continuous consumption of soil water by apple trees, deep soil desiccation occurred in old orchards regardless of planting density. Further, affected by the spatial variation of annual precipitation, deep SWC in orchards significantly decreased with annual precipitation from 650 to 500 mm among the 44 study sites (p < 0.05). Our results suggest that the planting density should be reasonably regulated on the level of annual precipitation, and apple trees need to be pruned appropriately with a goal of moderate productivity, so as to achieve the sustainable use of regional water resources, food security and economic development. [ABSTRACT FROM AUTHOR]

• Deep soil and adjacent farmland were both important water sources for apple trees. • SWD increased at a rate of 5.94 mm m−1 yr−1 in apple orchard before 30 years of age. • The migration and accumulation of NO 3 –-N confirmed the soil water exchange. • The orchard-farmland pattern can promote efficient water and nutrient use in the CLP. With the vigorous promotion of apple tree planting on the Chinese Loess Plateau (CLP), a mosaic pattern of apple orchards and traditional farmland has emerged in the region. However, the interaction between adjacent farmland and apple orchards in terms of soil water and nutrient migration remains unclear. This study aimed to investigate the spatial distribution of the soil water content (SWC) and soil nutrients, as well as their interactions, in 0–10 m soil layers of apple orchards of different stand ages and adjacent farmland under the orchard-farmland land use pattern. The results revealed a significant decrease in average-depth SWC in the orchards in the 0–10 m soil profile (R2 = 0.90) with increasing orchard age, accompanied by a significant increase in soil water deficit (SWD, R2 = 0.94). The annual growth rate of SWD was found to be as high as 5.94 mm·m−1·y−1. Deep soil and surrounding farmland were identified as important sources of soil water for apple tree growth, particularly in older orchards; a 25-year-old orchard absorbed approximately 10.8 % of soil water towards farmland with a horizontal distance of 7 m. The distribution of soil organic carbon (SOC) and total nitrogen (TN) in the 0–10 m soil profile exhibited a similar pattern, decreasing and then stabilizing with increasing depth. However, the peak depth and accumulation of soil nitrate (NO 3 –-N) significantly increased with orchard age. The migration and accumulation of NO 3 –-N in the soil provided a scientific basis for the transport of soil water, with NO 3 –-N accumulation (405.32 mg kg−1) observed in the deep soil (3–6 m) of farmland located 5 m away from a 30-year-old orchard, confirming the exchange of soil water and nutrients between the orchard and the farmland. The stand age of the orchards was identified as the main environmental factor influencing soil water transport under the orchard-farmland land use pattern. Overall, our findings provide valuable insights into the interaction between apple orchards and farmland under the mosaic pattern, which can contribute to a more rational layout of apple orchards and farmland in the future, promoting the efficient utilization of soil water and nutrients on the CLP. [ABSTRACT FROM AUTHOR]