Effects of different land-use types on soil microbial metabolic limitation and carbon use efficiency in the Yellow River floodplain
Description
Field investigations and sampling were conducted in April and September 2024 across three representative sites of the lower Yellow River floodplain: Taohuayu, Wantan, and Langchenggang. At each site, three distinct land-use types were selected: first floodplain , grassland, and farmland. Within each land-use type, three replicate plots were randomly established, yielding a total of 27 plots (3 sites × 3 land-use types × 3 replicates). In each plot, five soil cores (0–20 cm depth) were collected using a soil auger (3.6 cm inner diameter) and composited to form one representative soil sample. A total of 54 composite samples were collected across both sampling seasons. After removing visible roots and stones, fresh soil samples were sieved through a 2 mm mesh and divided into two subsamples: one stored at -20 °C for extracellular enzyme activity analysis (completed within one month), and the other air-dried for subsequent physicochemical property measurements.
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Soil water content (SWC) was determined by oven-drying fresh soil at 105 °C for 48 hours until a constant weight was achieved. Soil pH was measured using a pH meter in a 1:2.5 (w/v) soil-to-deionized water suspension. Total nitrogen (TN) was analyzed using elemental analyzer (Vario Macro Cube, Elementar, Germany). Total phosphorus (TP) was determined via the molybdenum blue colorimetric method following acid digestion, using a spectrophotometer (UV-1900, Shimadzu, Japan). SOC was measured by the dichromate oxidation spectrophotometric method. Dissolved organic carbon (DOC) was extracted using 0.5 mol L⁻¹ K₂SO₄ and quantified via TOC analysis. Available phosphorus (Olsen-P) was extracted with 0.5 mol L⁻¹ NaHCO₃ according to the Olsen method. Soil nitrate nitrogen (NO₃⁻-N) and ammonium nitrogen (NH₄⁺-N) were determined using an automated flow analyzer (AA3, SEAL Analytical, Germany). Microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP) were determined by the chloroform fumigation-extraction method. The quantification of glomalin-related soil proteins (GRSP) was conducted following the methodology of Wright et al. (1996), with procedural adaptations from Gałązka et al. (2017). Spectrophotometric analysis was performed to determine both difficult-to-extract GRSP (D-GRSP) and easily extractable GRSP (E-GRSP) fractions. Protein quantification was achieved via the Bradford assay using a bovine serum albumin standard (UV-1900, Japan).The extracellular enzyme activities involved in C-acquiring (β-1,4-glucosidase, BG), N-acquiring (β-1,4-N-acetyl-glucosaminidase, NAG), and P-acquiring (Alkaline phosphatase, AP) were determined using the standard fluorescence method (Saiya-Cork et al., 2002). BG, NAG, and AP activities were measured fluorometrically using a 200 μM solution of substrate labeled with 4-methylumbelliferone (MUB), and microbial biomass was determined by chloroform fumigation-extraction. All enzyme activities were expressed as nanomoles of substrate released per hour per gram of dry soil (nmol g⁻¹ h⁻¹). Molar ratios of dissolved organic C to available N [DOC : (NO3-N + NH4-N)] in soil and dissolved organic C to available P (DOC : Olsen-P) in soil were used as estimates of LC:N and LC:P, respectively (Sinsabaugh et al., 2009a). Microbial biomass BC:N, BC:P, and BN:P were calculated as molar ratios. TC:N and TC:P represent the molar ratios of SOC to TN, and SOC to TP, respectively. Non-normal data were log-transformed to achieve normal distribution before statistical analysis.