Soil salinization is the accumulation of soluble salts in the soil, particularly in the root zone. The excess salt has severely constrained the agricultural productivity and the carbon sequestration capacity of the ecosystems in arid regions. Subsurface pipe drainage (SPD) technology can be expected to serve as an effective engineering measure to ameliorate the saline-alkali soils. It is often needed for its environmental sustainability in climate change mitigation. In this study, a systematic assessment was performed on its net environmental impact, specifically, the trade-off between its life-cycle carbon costs and the ecosystem carbon benefits. A field experiment was conducted in the Yanqi Basin of Xinjiang, from the representative arid inland area under severe secondary salinization. The carbon balance of the SPD system was quantified to clarify the synergistic water-salt regulation with the soil-crop-carbon sequestration. Four treatments were set: a control group (CK) under conventional flood irrigation and three SPD systems with the varying parameters (T1: burial depth 1.4 m, spacing 20 m; T2: depth 1.6 m, spacing 20 m; and T3: depth 1.6 m, spacing 40 m). Seasonal dynamics of the soil salinity were measured to evaluate the sunflower (Helianthus annuus L.) yield at harvest. The carbon sink of the vegetation was quantified after measurement. Crucially, a carbon accounting framework was applied to the full life cycle. The direct carbon costs were systematically integrated with the indirect carbon net in the ecosystem, including the emissions from the material production, on-site construction activities, and operation. The results demonstrated that all SPD treatments significantly reduced the rootzone (0-60 cm) soil salinity, compared with the CK. The high efficacy of the SPD was achieved to effectively break the salt stress on the crops. The rhizosphere environment directly improved the agricultural productivity. The underground pipeline treatment has significantly increased the yield of oil sunflowers. Consequently, the biomass production shared the major increase in the vegetation carbon sink, compared with the CK. Statistical analysis confirmed that a strong "desalination-yield increase-carbon sequestration" chain shared the significant negative correlations between soil salinity and both yield and carbon sink. The T1 treatment (1.4 m depth, and 20 m spacing) was consistently achieved in the best ecological-economic balance, indicating the effective desalination with the superior carbon efficiency among the tested designs. The carbon accounting revealed that the vegetation carbon sink increment of 10113 to 10421 kg/hm² was overwhelmingly offset in a strongly positive net carbon balance, while the SPD system incurred the direct carbon costs from 5310 to 5820 kg/hm². This finding can provide robust evidence to strategically design the SPD technology and synergistically enhance both the productive capacity and the ecological function, particularly the carbon sequestration potential of the saline-alkali lands. The vital technical parameters can be integrated into the scalable saline land remediation for the regional and national carbon neutrality.

Sediment deposition has posed a serious risk to the performance of emitter flow channels. This study aims to explore the influences of the emitter performance on sediment deposition, in order to reduce the risk of physical blockage occurrence. Different concentrations (1.8, 2.8, and 3.8 g/L) and particle sizes (0 ≤ d ＜ 0.054 mm, 0.054 mm ≤ d ＜ 0.075 mm, 0.075 mm ≤ d ＜ 0.1 mm) of sandy water were used in the emitter clogging test of intermittent drip irrigation. A comparison was also made on the particle size composition in the sediment output from the emitters that irrigated with the sandy water. A systematic analysis was then implemented for the sediment transport and siltation inside the emitters. A numerical simulation was also carried out to analyze the flow pattern in the emitter. The low-velocity or vortex zone was prone to clogging. An operation mode was finally recommended to improve the clogging-resistant performance of the emitters in the drip irrigation system, according to the flow pattern and the deposited sediments. The results show that: 1) There was a consistent effect of side-seam labyrinth emitters on the size of settled sediment, with a decrease in the specific gravity of clay and sand particles, and the increasing specific gravity of powder particles. The grain size composition of the input original sediment was compared with the output one. There was a decreased proportion of clay and sand particles with an increased proportion of silt particles. Since the water-following property of sand particles was weaker than that of clay particles and silt particles, there was a weak discharge from the emitter with water. Specifically, the proportions of clay particles and sand particles in the sediment decreased by 1.14-2.98 percentage points and 1.55-11.14 percentage points, respectively, whereas, the proportion of silt particles increased by 1.25-13.74 percentage points. 2) The low transport of large particle size was attributed to the small size of the labyrinth emitter flow channel. The clogging of the particle size was concentrated in the range of 0.054-0.1 mm particle size. Most sediment sizes were between 0.002 and 0.05 mm in the flow channel of the emitter, due to the agglomeration of sediment flocs. 3) Streamline in the mainstream area was moving forward in a wave-like manner. The flow velocity in the mainstream area was greater than that in the near-wall area, while the flow velocity at the central corner was large, and a vortex was generated in the upper and lower corners. The inner and outer water bodies in the centrifugal force and water pressure under the joint action of the inner water body flew to the lower wall of the lower runner unit, while the outer water body flew to the upper side wall of the upper runner unit, where the turbulence of the water flow diffusion rate was faster. A vortex area was formed in the inner and outer boundaries of the circulation in the labyrinth runner. Part of the particles was retained in the vortex area, thus causing the runner blockage. That was the main factor for the emitter clogging. After that, sand particles were very easy to settle and concentrate in the vortex center and the water surface. Some suggestions were proposed to reduce the number of right- and sharp-angled areas, in order to reduce the formation of low-velocity zones in the design optimization. The clogging of emitters can be prevented by the sand deposition. The long and narrow over-flow structures can also be avoided, according to the location of the clogging of sediments.