Microbial life-history strategies and nutrient limitation modulate soil organic carbon stability under nitrogen addition in subtropical plantations
Description
Continuous atmospheric nitrogen (N) deposition can substantially affect the stability of soil organic carbon (SOC) in forests. However, from the perspective of SOC fractions, the effects of different forms of N addition on SOC stability and the underlying microbial mechanisms remain unclear. In this study, an N addition experiment was conducted in subtropical plantations to investigate the effects of inorganic N (NH4Cl) and organic N (urea and glycine) on SOC fractions as well as their microbial drivers and pathways. The stable fraction was represented by microbial necromass carbon (MNC) and the labile fraction by cumulative CO2 emission (CCE). The results showed that organic and inorganic N additions reduced MNC by 20.7% and 18.8%, respectively, and increased CCE by 1.3% and 4.9%, while total SOC content remained unchanged. N addition increased microbial biomass, enhanced microbial carbon and phosphorus limitations, and shifted microbial communities from r- to K-strategists. Although different forms of N addition had consistent directions and pathways of effect on MNC and CCE, organic N exerted a stronger effect on microbial phosphorus limitation and a greater promotion of CCE. Furthermore, N addition significantly decreased the ratios of MNC:SOC and MNC:CCE. The shift of microbial communities toward K-strategists combined with intensified nutrient limitations represented the key regulatory pathway underlying the decline in MNC and increase in CCE, thereby weakening SOC stability. Taken together, short-term N addition markedly reshaped SOC fractions and weakened soil carbon stability, with microbial life-history strategies and nutrient limitations acting as central mediators. These findings highlight the necessity of integrating N forms and SOC fractions to more accurately assess the regulatory effects of atmospheric N deposition on critical carbon cycling processes in subtropical forests.
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In June 2019, a N addition experiment was conducted in a flat, homogeneous mixed conifer-broadleaf plantation. The dominant tree species in the plantation included 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 different N addition treatments: control (CK), NH4Cl, urea, and glycine. Inorganic N deposition was simulated using an NH4Cl solution, while organic N deposition was simulated using urea and glycine solutions, respectively. Considering that the average N deposition in subtropical China is approximately 3 g N m-2 year-1, the N addition rate was set at 6 g N m-2 year-1 to simulate future increases in N deposition and obtain more pronounced responses. 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. A total of 20 plots, each measuring 5 m × 5 m, were established, with five replicates per treatment. Buffer zones were set between plots to minimize potential interference. 2.2. Soil sampling and chemical properties characterization In June 2023, soil samples were collected from two depths (0–10 cm and 10–20 cm) using a five-point composite sampling method. Prior to sampling, surface litter and humus were removed. The collected soils were 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 then 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 carbon mineralization; the remaining portion was air-dried in a cool, shaded environment for soil total nutrient analysis. SOC was measured using an elemental analyzer (FLASH 2000 HT, Thermo Fisher Scientific, Germany). Total nitrogen (TN) and total phosphorus (TP) were determined using an automatic discontinuous chemical analyzer (SmartChem 210, Alliance, France). Dissolved organic carbon (DOC) was measured with a TOC/TN analyzer (multi N/C 2100S, Analytik Jena, Germany). Ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N), and available phosphorus (SAP) were determined using a spectrophotometer (UV-1601, Shimadzu, Japan). Soil available nitrogen (SAN) was defined as the sum of NH4+-N and NO3--N.
Institutions
- Jiangxi Agricultural University