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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]

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]

[Display omitted] • Water-nitrate coupling is explored under farmland and apple orchards • Large water deficit and nitrate accumulation are observed under apple orchards • Water deficit and nitrate accumulation are spatiotemporally asynchronous • Soil properties significantly affect water-nitrate coupling in deep soils Understanding the relationship between water and nitrate in soils is critical to agricultural production and aquifer vulnerability assessments, especially in regions with intensive agricultural activities and thick unsaturated zones. Deep soils usually record long-term variations in coupling water and nitrate processes, influenced by multiple environmental factors. As such, this study aims to investigate water and nitrate-nitrogen (NO 3 –−N) contents and their relationships in soil profiles of 13–23 m deep under farmland and neighboring apple orchards with different ages. Soil properties were considered to explore the individual effect of a single factor or combined effects of multiple factors by wavelet analysis and partial least squares path models. Compared with farmland, apple orchards had large water deficit and NO 3 –−N accumulation due to root water uptake and synthetic fertilizer inputs, with high spatiotemporal asynchrony in deep unsaturated zones. Specifically, water deficit and NO 3 –−N accumulation were respectively observed in relatively stable (5–15 m) and active layers (0–5 m, spatial asynchrony), with higher water deficit rates under young apple trees (e.g., 12 and 22 years old) but larger NO 3 –−N accumulation rates under old apple trees (e.g., 23–32 years old, temporal asynchrony), respectively. Under farmland and orchard, soil texture-organic carbon regulated soil water movement, while soil water-organic carbon affected nitrate transport and transformation processes, which further altered soil pH and electric conductivity conditions in deep vadose zones. The growth of apple trees leads to asynchronous water depletion and fertilizer accumulation in soils, and the > 20 years apple trees are limited by insufficient water despite the excessive fertilizer. The dried soil layers not only reduce potential groundwater recharge, but also prolong the residence time of nitrate bombs in thick unsaturated zones. This study provides essential information for sustainable management of vegetation and water resources, and benefits the complicated hydrological and biogeochemical cycles in thick vadose zones. [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]

The water and organic carbon stored within unsaturated zones are crucial in terrestrial ecosystems; however, the coupling effects of deep soil water and organic carbon are still poorly documented. To explore the water-carbon coupling effects within soils under tree plantations, we investigated the water and organic carbon content to 20 m deep under cultivated farmland and orchards with apple trees of varying ages (5, 10, 15, 20, and 24 years) in the loess-covered region of China. A conditional process model was used to explore the impacts of vegetation and edaphic factors on the water-carbon coupling processes. Compared with farmland, young apple orchards (0–10 years) showed lower water deficit, but the soil water storage reduced by 21%− 32% under old apple orchards (15–24 years). The organic carbon content exhibited negligible variation among apple trees of different ages. However, the water-carbon coupling effects varied with apple tree ages. A positive correlation existed between water storage and organic carbon density under young apple orchards (p < 0.05), but a negative correlation was found between water deficit and organic carbon sequestration under old apple orchards (p < 0.01). The conditional process model suggested that the water-carbon coupling relationship under young apple orchards was primarily controlled by clay content due to lower root biomass, while that under old apple orchards was concurrently shaped by silt content and coarse roots because of the developed root system of apple trees. This study offers novel insights into the sustainable management of agriculture and tree plantations in regions with deep-rooted vegetation and thick unsaturated zones. [Display omitted] • We analyzed water and carbon coupling effects within 0–20 m under apple orchards. • Apple trees triggered large water deficit but little organic carbon variation. • Soil texture led to positive water and carbon effects under young apple orchards. • Plant root caused negative water and carbon effects under old apple orchards. [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]

Highlights • Nitrous oxide emissions were higher in the apple orchard than in the wheat field. • Summer rainfall and winter snowfall were the principal controllers of N 2 O fluxes. • A moisture threshold associated with rainfall triggered higher N 2 O emissions. • Nitrous oxide emission due to land use change was altered by precipitation in arid region. Abstract The conversions of cropland to forest, or other tree-based systems, are considered to be important processes affecting regional and global greenhouse gas budgets, especially for nitrous oxide (N 2 O). From April 2007 to March 2009, in the rain-fed semi-arid climate of the Loess Plateau, China, soil N 2 O emissions were measured using static chambers from a winter wheat field and an apple orchard, which had been established in part of the wheat field 23 years earlier.Annual average N 2 O emissions from the apple orchard (2.40 kg N 2 O ha−1yr−1) were 12.15% higher than those in the wheat field (2.14 kg N 2 O ha−1yr−1). Seasonal rainfall, in combination with higher nitrogen fertilization, had a promoting effect on N 2 O emissions in the apple orchard compared with the wheat field. The amounts and patterns of summer rainfall and winter snowfall were the principal controllers of seasonal and annual N 2 O fluxes in these rain-fed semi-arid regions, likely through their influence on soil moisture content. Since there may be a moisture threshold associated with summer rainfall and winter snowfall triggering higher N 2 O emissions, climatic regimes should be taken into account when assessing the effects of land use on N 2 O emissions in the Loess Plateau. [ABSTRACT FROM AUTHOR]

Numerous studies have been conducted to understand the mechanism of groundwater recharge for water resources management. But the roles of wide-spread village ponds on the Chinese Loess Plateau on groundwater recharge remains uncertain. Here, to evaluate the groundwater recharge under village ponds, we measured environmental tracers (2H, 18O, Cl-) in rainwater, groundwater, and pore waters of the deep unsaturated zone under three land-cover types (cropland, a village pond, and two apple orchards with stand ages of 10 and 27 years). The results show that shallow groundwater is recharged by both diffusive recharge and focused recharge. Specifically, diffusive recharge occurs beneath farmlands, grasslands, orchards and similar land-cover types, but diminishes in old orchards due to deep root water uptake. The diffusive recharge beneath crop field had an infiltration velocity of 10.3–12.5 cm yr−1; the resulting recharge rate was 30–33 mm yr−1, accounting for 5–6% of the annual precipitation. However, focused recharge occurs mainly beneath village ponds with low-lying terrain, and had an infiltration velocity of 26 cm d−1 during the rainy season; the resulting mean recharge rate was 134 mm yr−1 in the catchment of the pond, accounting for 23.1% of the annual precipitation. Given that the water table is generally more than 30 m below the soil surface, it takes hundreds of years for rainwater to reach the water table via the diffusive recharge but only days to months via the focused recharge. Diffusive recharge contributed 55–68% to the total recharge while focused recharge contributed 32–45%. However, the conversion of croplands to apple orchards and the decrease in the number of village ponds are occurring, which diminishes diffusive recharge, and reduces focused recharge, respectively. These findings are critical to understanding hydrological processes and protecting groundwater resources. We should preserve, reclaim, and construct village ponds to ensure groundwater recharge, in addition to optimizing areal ratio of large farmland to apple orchards in the region for the sustainable utilization of groundwater resources. • Diffusive recharge and focused recharge coexist as processes of groundwater recharge in the Loess Tableland. • Diffusive recharge occurs in farmland and natural area and has a slow, steady velocity (10.3–12.5 cm yr−1). • Focused recharge occurs beneath village ponds and has a relatively rapid velocity (26 cm d−1) during the rainy season. • Diffusive recharge contributed 55–68% of the total recharge while focused recharge contributed only 32–45%. [ABSTRACT FROM AUTHOR]