Longbench mastersizer 2000

SOC was determined by the Walkley–Black method [28 ], TN by the Kjeldahl method [29 ], and TP by molybdenum antimony blue colorimetry [30 (link)]. Soil PSDs were analyzed by laser diffraction using a Longbench Mastersizer 2000 (Malvern Instruments, Malvern, UK). PSD was described based on the percentages of clay (<0.002 mm), fine silt (0.002–0.02 mm), coarse silt (0.02–0.05 mm), and sand (0.05–1 mm). Clay and fine silt were considered fine soil particles and coarse silt and sand were considered coarse particles.

The soil bulk density (BD) of each soil layer was measured with the cutting ring method (Ding et al., 2019 (link)). SOC was determined using the dichromate oxidation method (Nelson & Sommers, 1982 ), and TN was determined using the Kjeldahl method (Bremner & Mulvaney, 1982 ). For soil PSD (particle-size distribution) and micro-aggregate analysis, soil samples were analysed by a laser diffraction technique using a Longbench Mastersizer 2000 (Malvern Instruments, Malvern, England) (Xiao et al., 2014 (link)). There are some differences between the pretreatment methods for determining soil PSD and micro-aggregate. For soil PSD, soil samples were pretreated with 6% H₂O₂ and 10% HCL to remove organic matter and carbonates and oxides and were soaked in distilled water for 24 h, then mechanically dispersed with 0.4% Calgon by an ultrasonic bath for 5 min. For micro-aggregate determination, the soil samples were soaked in distilled water for 24 h and mechanically dispersed in ultrasonication for 5 min (Xiao et al., 2014 (link)). Soil PSD was described in terms of the percentage of sand (0.05–2 mm), fine silt (0.002–0.020 mm), coarse silt (0.02–0.05 mm) and clay (<0.002 mm). The size grades of the micro-aggregates were classified to be the same as that of the PSD.

Soil samples were collected from four plots during July 2012 to May 2013. After carefully removing surface organic materials and fine roots, soil samples were collected at three soil depths: 0–10 cm, 10–20 cm and 20–30 cm. The five replicate samples were homogenized mixed. Visible roots and rocks were removed prior to further processing.
All the air-dried soil samples were dispersed by sodium hexametaphosphate (NaHMP) and ultrasonic lasting 30 s after manually sieved (2 mm) and homogenized. The particle fractions of samples were analyzed with Longbench Mastersizer 2000 (Malvern Instruments, Malvern, England). Soil salinity was quantified as soil electrical conductivity (EC, μS cm^{− 1}) by measuring the EC of suspension with a 1:5 soil water ratio after 1 h suspension shaking at 25°C. Soil pH was measured using a glass electrode. Soil organic matter (SOM) content was determined by K₂CrO₇ routine colorimetric method [22 ].