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Straw deep returning as an interlayer is a novel practice to enhance soil carbon and nutrients. However, the impact of applying various amounts of straw as an interlayer on soil quality still remain unclear in the saline soil. Therefore, a field experiment was carried out over four years (2015–2018) in Hetao Irrigation District, China. The aim was to evaluate the impact of four straw interlayer rates (i.e., 0, 6, 12, and 18 Mg ha−1) applied at 40 cm depth on soil quality index (SQI) and its relationship to sunflower yield in saline soil. Our results showed that, in comparison to no straw interlayer (CK), straw interlayers applied at rates of 6, 12, and 18 Mg ha−1 improved SQI on average by 2.0, 2.7, and 3.0 times in four years, respectively (p < 0.05). This suggested that straw deep returning as an interlayer improved SQI, especially for middle and high amounts (12 and 18 Mg ha−1). Partial least squares path model (PLSPM) illustrated that the improvement of SQI was due to the high-moisture and low-salt environment created by straw interlayer in the early two years (2015–2016), while the higher soil nutrients released from straw decomposition in the subsequent years (2017–2018). The improvement of SQI contributed to sunflower yield, which was related to the decrease of soil salinity, the increase of soil moisture, soil organic carbon (SOC), total nitrogen (TN), and available nutrients under straw interlayers. The sunflower yield was increased by 8.7–13.4% under straw interlayers (p < 0.05), following the order of 18 = 12 > 6 >0 Mg ha−1. The greater increment of yield was detected during the initial phase of burying straw interlayers, which indicated that straw as an interlayer played a more important role than nutrient supply from straw decomposition. The findings highlighted that appropriate straw return amount (i.e., 12 Mg ha−1) as an interlayer is an economic practice to benefit soil quality and crop yield synchronously in salt-affected soils. [Display omitted] • Straw deep returning as interlayer improved soil quality. • Great soil quality was due to the high moisture and low salt in the first two years. • Great soil quality was due to the higher soil nutrients in the last two years. • The improvement of soil quality significantly increased sunflower yield. [ABSTRACT FROM AUTHOR]

Limited water resources have severely restricted saline soil amelioration and utilization in the Hetao irrigated area, China. Burying straw interlayer is an effective management for salt-affected soil amelioration. However, little is known whether the straw interlayer could boost soil fertility and improve bacterial community, while reducing irrigation water consumption by 10% in the third and fourth years after application. Therefore, the legacy effect of the combination of straw interlayer and irrigation water reduction was investigated following the three treatments: (i) no straw interlayer plus 100% irrigation (CK); (ii) straw interlayer plus 100% irrigation (SI+W100), and (iii) straw interlayer plus 90% irrigation (SI+W90). SI+W90 significantly increased 0–40 cm soil organic carbon (SOC) content and soil SOC/TN (C/N) ratio than CK by 12% and 13%, respectively. In addition, SI+W90 significantly decreased 0–40 cm soil total nitrogen (TN) and SOC contents by 10% and 8% respectively, but significantly increased 0–40 cm soil C/N ratio by 9%, compared to SI+W100. Furthermore, SI+W100 and SI+W90 significantly increased the relative abundance of Proteobacteria , Bacteroidetes , and Nitrospirae than that of CK, while SI+W90 significantly decreased the relative abundance of Acidobacteria compared to SI+W100, and these bacterial phyla were significantly correlated with 0–40 cm SOC content. Notably, there was no significant correlation between crop yield and soil parameters in 2018 with the relatively low yield, while crop yield was significantly correlated with 0–20 cm SOC content and the relative abundance of Proteobacteria and Firmicutes in 2019 with relatively high yield. These results indicated that both soil fertility and certain bacterial phyla may contribute to yield variation. Therefore, SI+W90 maintained crop yield in the two years. In conclusion, the combination of straw interlayer and irrigation water reduction may be more recommended for agricultural sustainable development under the condition of limited irrigation water supply in the arid and salt-affected regions. [Display omitted] • 100% irrigation with buried straw increased the TN relative to 90% irrigation. • Buried straw with 100% and 90% irrigation considerably increased SOC and affected bacterial community. • 90% irrigation with buried straw was more conducive to increase soil C/N ratio. • 90% irrigation with buried straw was suggested in arid saline soil. [ABSTRACT FROM AUTHOR]

Burying straw deeply to establish interlayer is an effective strategy for saline soil improvement, and affects deep soil organic carbon (SOC) sequestration due to the landfill of exogenous carbon. However, the legacy effects of straw interlayer on deep SOC pools and its biotic regulatory mechanism remain unclear. Therefore, a field experiment was conducted with the buried interlayer at straw application rates of 0 Mg ha−1 (CK), 6 Mg ha−1 (SL6), 12 Mg ha−1 (SL12) and 18 Mg ha−1 (SL18). The straw interlayer was buried at 40 cm soil depth. Soil samples at the interlayer area (35–40 cm and 40–45 cm layers) and below the interlayer (45–55 cm, 55–65 cm and 65–75 cm layers) were collected to quantify changes in SOC fractions, as well as the mechanisms of soil microbial community and enzyme after a 6-year burying straw interlayer. Straw interlayer increased particulate organic carbon (POC), mineral-associated organic carbon (MAOC) and SOC contents, which were regulated by the straw application rate and soil depth. High amount of straw caused the positive effect of straw interlayer on SOC pools to extend deeper soil layer. Specifically, SL18 treatment with more straw inputs increased POC content by 33%-212%, MAOC content by 13%-28%, and SOC content by 22%-51% in relative to CK treatment, which was an effective practice to increase SOC sequestration in saline-alkali soil. Meanwhile, straw interlayer increased the POC/SOC, resulting in POC contribution to SOC greater than MAOC. Partial least squares path modeling showed that at the interlayer area, soil microbial community promoted the increases in POC and MAOC, contributing to SOC sequestration. Below the interlayer, soil enzymes promoted an increase in POC, contributing to SOC sequestration. These findings suggested that the regulation of SOC pool shifted from soil microorganisms to enzymes with soil depth, which will help us understand the regulatory mechanism of deep SOC pools after building straw interlayer in saline-alkali soil. [Display omitted] • High straw amount extended positive effect of straw interlayer on SOC to deeper soil. • Straw interlayer sequestrated SOC in deep soil mainly by increasing POM-C. • Soil microbe increased POM-C and MAOM-C to sequestrate SOC in interlayer area. • Soil enzymes promoted an increase in POM-C to sequestrate SOC below the interlayer. [ABSTRACT FROM AUTHOR]

• Soil water significantly decreased with rates of buried straw during the later stage of sunflower. • Salt leaching flux increased with increases in thickness of buried straw. • Adoption of straw barrier affected saturated water conductivity and porosity among layers. • As the years went by, the thicker straw layer increased the upward mobility of salt. • The 5 cm thickness straw layer is more practical for field work in dry saline soil. Salt stress severely constrains crop productivity in arid lands of the world. Burying straw at the 40 cm soil depth plus plastic film mulching could mitigate root zone salinity, but little is known about how the thickness of buried straw affects soil water and salt transport. Therefore, a three-year field experiment was conducted from 2010 to 2013 to address this issue, with treatments including: compacted straw thickness of 3 cm (T3), 5 cm (T5), or 7 cm (T7) (corresponding to straw application at rates of 6, 12 and 18 t ha−1, respectively). In addition, a supplementary experiment, which included treatments of no buried straw layer (CK) and 5 cm of compacted straw layer thickness (t5) in the same micro-plot experiment, was carried out from 2014 to 2016 to identify soil pore structure and hydraulic parameters after three years of deep straw burial. Results showed that the initial soil water content increased with increasing thickness but significantly (P < 0.05) decreased in later periods. Soil salinity consistently decreased with increasing straw thickness, and T7 was most effective for improving salt leaching. However, the water and salt regulation under T5 gradually decreased mainly due to the change in saturated water conductivity and porosity among layers. After irrigation, the flux of salt leaching (FL) increased with straw thickness, and the FL under T7 significantly exceeded that under T3 by 105, 89 and 33% and that under T5 by 84, 66 and 33% in 2011, 2012 and 2013, respectively. The salt flux during evaporation under T7 was much higher (P < 0.05) than that under T3 and T5 by 92 and 10% in 2012 and 38, 44% in 2013. At harvest, salt storage within the soil consistently ranked as T3 > T5 > T7. Although T7 had the most pronounced effect on salt mitigation, it was difficult to implement under normal field conditions. Thus, for relatively good water infiltration, salt leaching and inhibition of salt return, straw buried to a thickness of 5 cm is recommended. [ABSTRACT FROM AUTHOR]