Automatic compensation salinity refractometer

Salinity and pH were measured using fresh sediments, and nutrients were measured using air-dried sediments. Salinity was measured by an automatic compensation salinity refractometer (ATAGO Co., Japan). Sediment pH was determined by a pH meter (Mettler-Toledo Instruments Co., China). Sediments were air dried for a few days until the weight remained unchanged. Each sample was thoroughly mixed after passage through a 2-mm-pore-size sieve to remove roots and stones. Sediment ammonium, nitrite, and nitrate were extracted from air-dried sediments (10 g) with 100 ml of 1 M KCl by shaking at 180 rpm for 60 min. The filtered solution was determined by a continuous segmented flow analyzer (SEAL AutoAnalyzer 3 HR; Maquon, WI, USA). Sediments for total organic carbon (TOC) and total nitrogen (TN) measurements were ground to a fine powder (mesh number 100) using a mortar and pestle. Briefly, 2 g of mixed catalyst (K₂SO₄:CuSO4:Se at 100:10:1, passed through a no. 80 sieve) and 5 ml of concentrated H₂SO₄ were added into dried sediment (0.5 g) and then boiled at 150°C for 60 min and at 250°C for 240 min. Volume was fixed to 100 ml, and the concentration of the upper solution was determined using a TOC analyzer (Shimadzu, Japan).

Salinity and pH were measured in fresh sediments, whereas nutrients were measured in air-dried sediments. The interstitial water of the fresh sediment was collected by centrifugation, and the salinity and pH were measured using an automatic compensation salinity refractometer (ATAGO Co., Japan) and a pH meter (Mettler-Toledo Instruments Co., China). Sediments were air-dried for multiple days until their weight remained unchanged. Each sample was then thoroughly mixed after passing through a 2-mm sieve to remove roots and stones. Sediment ammonium, nitrite and nitrate were extracted from air-dried sediments (10 g) with 100 mL of 1 M KCl by shaking at 180 rpm for 60 min. The concentrations of NH₄⁺, NO₂⁻ and NO₃⁻ were determined using a continuous segmented flow analyzer (SEAL AutoAnalyzer 3 HR, Maquon, WI, USA). A Shimadzu TOC VCPH analyzer (Shimadzu, Japan) was employed to analyze the TN, TC and TOC as reported previously (50 (link)).