Mulched drip irrigation: a promising practice for sustainable agriculture in China's arid region

Xinjiang is China’s primary cotton-producing region, delivering over 90% of national output and ~20% of global production. However, this arid oasis agroecosystem faces water scarcity, soil degradation, and extensive saline-alkali soils that threaten sustainable production. Mulched drip irrigation (MDI), introduced in the 1990s, has been widely adopted to improve water-use efficiency and manage salinity by reducing evaporation, promoting lateral salt movement away from roots, and preventing deep percolation. Despite its benefits, concerns persist regarding long-term suitability on reclaimed saline-alkali lands, including incomplete remediation of accumulated salts, residual plastic film pollution, and potential disruption of plow layer structure and microbiomes. A comprehensive, long-term sustainability evaluation has been lacking. This study addresses that gap with an extensive, multi-year assessment to elucidate mechanisms of long-term salt migration under MDI, successional changes in soil physicochemical properties and microbial communities, and the potential for sustainable agriculture in arid regions enabled by MDI.

Prior work has established MDI’s capacity for efficient water and salt management in arid systems, including desalination effects near the rhizosphere and reduced evaporation beneath mulch. Widespread adoption in Xinjiang has improved production efficiency and economic outcomes. Yet literature also flags risks: drip irrigation may not fully remediate salt accumulation over time; residual plastic mulch has generated rising pollution levels; and prolonged drip irrigation can alter plow layer structure and microbiome communities, potentially affecting long-term yield stability on saline-alkali soils. Studies have explored irrigation schedules with brackish water, biodegradable film alternatives, and strategies to balance leaching and salinity enrichment rates in arid regions. However, an integrated, long-term field evaluation of soil salinity dynamics, soil quality evolution, and microbiome succession under sustained MDI application remained limited, motivating the present 12-year observational study spanning up to 22 years of MDI chronology via space-for-time substitution.

Study area and design: From 2009 to 2020, research was conducted in the 18th regiment of the 121st battalion, Pao Tai Town, Shihezi City, Xinjiang, China, using a space-for-time substitution approach. Five cotton fields within ~2 km² were selected based on MDI initiation years (2012, 2008, 2006, 2002, 1998), yielding MDI durations ranging from 1 to 22 years. All plots originated from similar uncultivated saline lands, with uniform cotton variety, irrigation, and fertilization. Irrigation source was underground brackish water (average salt 1.65 g/L; ionic composition in Supplementary Table 1). Plots were located using Google Earth. Sampling scheme: Disturbed soil samples were collected mid-month from April to February each year (2009–2020). During the growth stage (April–October), samples (0–140 cm) were taken at three positions: under drippers within the membrane, the midpoint of the narrow cotton row, and the midpoint between two outer membranes. During non-growth (November–March), soil was sampled to 0–200 cm at plot centers and four corners for salinity analysis. Undisturbed 10×10×10 cm samples were collected annually after harvest at 10 cm intervals (0–40 cm and 0–100 cm), with five replicates at the center and corners, to assess soil stability, physicochemical properties (0–40 cm), and microbiome parameters (0–100 cm). Microbiome samples were kept cold in the field and stored at −80 °C in the laboratory. Salinity measurement: Air-dried, 1 mm-sieved soil was mixed with distilled water (1:5, mass ratio), shaken, filtered, and extract EC measured with DDS-11A. EC values were calibrated by drying and converted to salt storage (g/kg) using y = 0.008x + 0.876, where x is EC (µS/cm). Microbial analysis: DNA was extracted from 0.5 g fresh soil using DNeasy PowerSoil Kit. DNA quality/quantity were checked via NanoDrop ND-1000 and agarose gel electrophoresis. Bacterial 16S rRNA gene V5–V7 regions were amplified with primers 799F/1193R; fungal ITS1 with ITS1F/ITS2R. PCR: 98 °C 5 min; 25 cycles of 98 °C 30 s, 53 °C 30 s, 72 °C 45 s; final extension 72 °C 5 min. Amplicons were purified (Vazyme VAHTS DNA cleaning beads), quantified (Quant-iT PicoGreen), pooled equimolarly, and sequenced on Illumina MiSeq (v3 kits; 2×250 bp) by Shanghai Majorbio. Quality control used QIIME v1.9.1 (retain reads >200 bp, average Q ≥20); chimeras removed with UCHIME. OTUs were clustered at 97% similarity in Mothur v1.21.1 and classified against SILVA. Soil chemical and physical properties: Total C and total N via combustion oxidation/Dumas using CN-802 (VELP). Soil organic carbon determined similarly after removing inorganic carbon with 2 mol L−1 HCl. Available P via NaHCO3 method. Total porosity measured by a soil three-phase meter. Stable aggregates were separated by successive sieving; mean weight diameter (MWD) and geometric mean diameter (GMD) were computed. Statistics and indices: Analyses were performed in IBM SPSS Statistics 26: ANOVA, paired t-tests, regression, path analysis; LSD test for group differences; significance at P < 0.05. Microbial alpha diversity indices were computed in QIIME; community differences assessed via NMDS based on unweighted UniFrac; Chao1 index used for diversity. Soil quality assessment used minimum dataset (MDS) and total dataset (TDS). Max–min normalization was applied, with negatively correlated indicators (e.g., bulk density, total porosity, salt storage) normalized via Q(Xi) = (Ximax − Xi)/(Ximax − Ximin), and positively correlated indicators via Q(Xi) = (Xi − Ximin)/(Ximax − Ximin). PCA and correlation analysis identified MDS variables; weights (Wi) were based on commonality ratios. Soil quality index (SQI) was computed as SQI = Σ Wi × Q(Xi). Conceptual figures were prepared with Figdraw.

• Soil salinity dynamics: During non-growth periods, salinity decreased across 0–40, 40–100, and 100–200 cm, with greater reductions as MDI duration increased; a surface-to-deep gradient formed. During growth, salts migrated to the wetting front, creating a low-salinity root zone and deeper vertical displacement; lateral migration under mulch led to surface salt accumulation distinct from secondary salinization.
• Desalination rates over time: Average relative salt storage followed a power function decline with MDI duration. After 3 years of MDI, desalination exceeded 60% versus adjacent wasteland (rapid phase). From 3–8 years, desalination increased linearly to ~80% (steady phase). Beyond 8 years (up to 22 years), desalination stabilized at 80–90% (equilibrium phase).
• Mechanistic insight: Heterogeneous pore systems under long-term MDI drive convective salt migration, fastest under emitters; periodic irrigation promotes macro-scale uniformity and lateral dispersion, enabling deeper leaching with increased irrigation amount/frequency.
• Soil physical properties: Mechanical aggregate stability and macro-aggregate proportion increased across depths. Bulk density decreased initially after MDI adoption but increased after ~10 years; total porosity showed the opposite trend. Deep tillage (to 40 cm) contributed to reduced compaction early on.
• Soil nutrients: Total nitrogen in the cultivation layer steadily accumulated with MDI. In 0–40 cm, available P and total carbon were 2.86–7.66× and 3.81–5.53× higher, respectively, than native soils. Soil organic content under MDI was 3.92–6.03 g/kg versus 3.51 g/kg in unirrigated soil.
• Residual plastic film: In continuous MDI cotton fields at 0–30 cm depth, residual film ranged 121.85–352.38 kg/ha with an annual increase of 15.69 kg/ha, potentially contributing to later increases in bulk density.
• Microbial communities: Bacterial OTU sequences declined from 35,640 (saline wasteland) to 26,616 (22-year MDI). Dominant bacterial groups after 22 years included Gemmatimonadetes (5.83%), Actinobacteria (35.23%), Chloroflexi (24.34%), Proteobacteria (35.92%), and Acidobacteria (6.04–15.13%), with their average relative abundances increasing. Fungal OTUs numbered 628; sequences slightly decreased from 58,933 to 57,494 over 22 years; dominant fungal phyla (Ascomycota 84.84–98.72%, Mortierellomycota 4.24%, Basidiomycota 4.54%) remained stable. Major fungal genera (21.82–68.96% relative abundance) increased by 43.63% with MDI duration.
• Diversity: Chao1 indices for bacteria and fungi increased with years of MDI, reaching a stable stage after ~10 years; beneficial microbial richness stabilized after ~15 years, likely influenced by cotton root exudates and monocropping effects.
• Soil quality and yield: The soil microhabitat improved during the first 14 years and stabilized from years 14–22, with cotton yields showing consistent, gradual increases over 22 years.
• Management framework: A three-step water-saving, salt-control system was proposed: (1) optimize drip irrigation to move salt beyond the root zone during the growth stage; (2) winter/spring irrigation to mitigate deep-layer salt enrichment during non-growth; (3) integrate drainage (covered pipes, open ditches, shafts) to discharge salts from fields.
• Policy and mitigation: Regional measures (legislation, biodegradable films, mechanical recovery, thicker/reinforced films) have raised mulch film recovery rates in Xinjiang to >90%, with topsoil residual film recovery >20%, supporting long-term MDI sustainability.

The findings demonstrate that long-term MDI effectively desalinizes the root zone and maintains low salinity through controlled water delivery, lateral salt redistribution under mulch, and deeper leaching, directly addressing sustainability concerns in arid saline-alkali soils. The initial rapid, then steady and equilibrated desalination trajectory indicates that MDI drives soils toward a dynamic salinity equilibrium suitable for sustained cotton production. Concurrent improvements in soil structure (aggregate stability), nutrient accumulation (N, C, available P), and enhanced microbial diversity and functional groups collectively elevate soil quality, aligning with the study’s hypothesis that MDI improves the soil microhabitat over time. The subsequent stabilization of microbial diversity after ~10–15 years reflects a matured soil-plant-microbe system under consistent MDI and management, explaining the observed stabilization of soil quality and steady yield gains. Nevertheless, the increase in bulk density after a decade and the accumulation of residual plastic film highlight trade-offs that require targeted mitigation. Integrating MDI with complementary practices—refined irrigation scheduling, winter/spring leaching, engineered drainage, conservation tillage, cover cropping, and improved mulch film management—can safeguard soil structure and minimize pollution, ensuring ecological protection alongside productivity. The mechanistic understanding of salt transport and pore system evolution under MDI supports broader application to arid regions globally, while emphasizing the need to tailor irrigation regimes where salinity enrichment may outpace leaching.

Over a chronosequence up to 22 years, MDI in Xinjiang’s arid cotton systems consistently reduced root-zone salinity, enhanced soil structure and nutrient reserves, and fostered more diverse and stable microbial communities, culminating in improved soil quality and steadily increasing cotton yields. The study contributes a mechanistic framework for long-term salt migration under MDI, a data-driven assessment of soil physicochemical and microbiome succession, and a pragmatic three-step water-saving, salt-control strategy integrating growth-stage optimization, non-growth leaching, and drainage. Collectively, these outcomes confirm MDI as a sustainable, scalable solution for oasis agriculture. Future research should expand multi-site trials across soils and climates, quantify residual film dynamics and mitigation efficacy, and evaluate integrated agronomic practices (e.g., conservation tillage, cover crops, biodegradable mulches) to further enhance long-term sustainability.

• Residual plastic film pollution presents an environmental challenge; while mitigation measures exist, this study placed less emphasis on detailed dynamics of residual film accumulation and its direct impacts on soil properties.
• Bulk density increases after ~10 years suggest potential compaction linked to residual film and long-term monocropping, warranting targeted investigation.
• Generalizability requires validation across diverse soils and climates, especially in regions where salinity enrichment rates can exceed leaching capacity; rigorous multi-site field tests are needed.
• Continuous monocropping and agrochemical inputs may constrain microbial development and diversity; the study did not experimentally manipulate crop rotations or input regimes to isolate these effects.