Consistent effects of inorganic and organic N addition on microbial necromass carbon in subtropical forest soils: A mesocosm study
Description
Atmospheric nitrogen (N) deposition can substantially influence the accumulation of microbial residues in forest soils. However, the effects of deposited N forms on soil microbial necromass carbon (MNC) and the underlying microbial mechanisms remain unclear. In this study, an N addition experiment was conducted in a subtropical plantation to investigate the effects of inorganic N (NH4Cl) and organic N (urea and glycine) on soil microbial biomass, hydrolytic enzyme activities, and MNC at 0–10 cm and 10–20 cm depths. The results showed that, despite unchanged soil organic carbon (SOC) content and mineralization, additions of NH4Cl, urea, and glycine reduced soil MNC by 20.7%, 17.4%, and 20.1%, respectively. This decline in soil MNC was mainly attributable to a reduction in fungal necromass carbon, whereas bacterial necromass carbon remained unchanged following N addition. Regardless of N form, N addition increased microbial biomass, intensified microbial carbon and phosphorus limitations, and shifted microbial communities from r- to K-strategists. Furthermore, both inorganic and organic N addition decreased the MNC:SOC ratio, indicating a reduced contribution of microbial residues to the SOC pool. Shifts in microbial community composition toward K-strategists, together with aggravated phosphorus limitation, accounted for the decline in soil MNC under N addition. Overall, these findings suggest that, irrespective of N form, atmospheric N deposition weakens the contribution of MNC to SOC by regulating microbial life-history strategies and nutrient limitations in subtropical plantations.
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In June 2019, an N addition experiment was conducted in a flat, homogeneous mixed conifer-broadleaf plantation. The dominant tree species in the plantation are Pinus massoniana, Pinus elliottii, Liquidambar formosana, and Schima superba, with a canopy closure of 0.91, an average diameter at breast height of 20.3 cm, and an average tree height of 9.8 m. The experiment was designed as a randomized block, with four N addition treatments including control (CK), NH4Cl addition, urea addition, and glycine addition. Inorganic N deposition was simulated using an NH4Cl solution, and organic N deposition was simulated using urea and glycine solutions, respectively. A total of 20 plots (5 m × 5 m) were established and each treatment had five replicates. Buffer zones were set between plots to minimize potential interference. Considering that the average N deposition in subtropical China is approximately 3 g N m-2 year-1, N addition rate was set at 6 g N m-2 year-1 to simulate future increases in N deposition. N was dissolved in 4 L of deionized water and evenly sprayed over the plots in June and December each year, while the control plots received the same volume of deionized water. In June 2023, five soil cores per plot were collected from two depths (0–10 cm and 10–20 cm) after the removal of surface litter and humus were removed, passed through a 2 mm sieve to remove stones and visible plant and animal residues, and then thoroughly mixed to form a composite sample. Each composite sample was divided into three portions: the first portion was stored at −20 °C for phospholipid fatty acid (PLFA) analysis, the second portion was stored at 4 °C for the determination of soil enzyme activities, available nutrients, MNC, and C mineralization, and the remaining portion was air-dried and grounded to determination of soil total nutrient concentration. SOC concentration was measured using an elemental analyzer (Flash 2000 HT, Thermo Fisher Scientific, Germany). Total nitrogen (TN) content was determined using a fully automated discrete chemical analyzer (SmartChem 200, AMS Westco, Italy). Dissolved organic carbon (DOC) was extracted from fresh soil with deionized water and measured using a TOC/TN analyzer (multi N/C 2100S, Analytik Jena, Germany). Ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) were extracted from fresh soil with KCl solution, and available phosphorus (SAP) was extracted using NH4F–HCl solution, and determined with a spectrophotometer (UV-1601, Shimadzu, Japan). Soil available nitrogen (SAN) was defined as the sum of NH4+-N and NO3--N.