The status and drivers of soil-available nutrients in plant-recovered soils are not fully understood, limiting our ability to explore the role of soil-available nutrients in soil geochemical cycling and ecosystem sustainability. Here, we combined the spatial distribution of soil-available nutrients and chemical and aggregate properties from six soil types (subalpine meadow soil, meadow soil, dark brown soil, brown soil, yellow-brown soil, and cinnamon soil) and three horizons (a leaching horizon, sediment horizon, and parent material horizon) to study the status and drivers of soil-available nutrients. Our findings reveal that the soil-available nitrogen (AN) ranged from 72.33 to 169.67 mg/kg, the soil-available phosphorus (AP) ranged from 1.77 to 75.90 mg/kg, and the soil-available potassium (AK) ranged from 46.43 to 88.55 mg/kg in the six soil types. The subalpine meadow soil and the dark brown soil had the highest soil AN, with means of 169.67 and 139.35 mg/kg, respectively. The brown soil had the highest soil AP, with a mean of 75.9 mg/kg, and the dark brown soil and the brown soil had the highest soil AK, with means of 83.49 and 88.55 mg/kg, respectively. The results show that the soil types and soil depths had a significant impact on the status of AN, AP, and AK (p < 0.05). Moreover, a higher cation exchange capacity (CEC), the macro-aggregate contents (with 2–1 mm and 1–0.5 mm particle sizes) of the non-water-stable aggregates, and the macro-aggregate content and stability (2–1 mm particle size and geometric mean diameter (GMD) of the water-stable aggregates were deemed to facilitate soil-available nitrogen because of the positive correlations (p < 0.05). Lower exchangeable cations (ECs) and the micro-aggregate content (≤0.1 mm particle size) of the water-stable aggregates and higher soil cations helped in the accumulation of soil-available phosphorus and soil-available potassium, respectively. Moreover, the regulation of the soil chemical and aggregate properties was found to vary with soil type and horizon in a correlation analysis. Together, our results provide insights into the importance of chemical and aggregate properties in regulating soil nutrient availability across soil types, as well as providing strong support for the inclusion of soil resource utilization in regional forest restoration and management.
Nitrogen deposition has a considerable impact on biogeochemical cycling in terrestrial ecosystems. However, how litter production and element return respond to N addition remains poorly understood in nitrogen-rich subtropical regions. In this study, a 4-year nitrogen addition experiment explored its effects on foliar litter production and carbon, nitrogen and phosphorus in a subtropical Michelia wilsonii forest. A clear seasonal pattern in foliar litterfall was observed, regardless of nitrogen treatments, with a peak in spring and a smaller one in autumn. Foliar litter increased with increasing nitrogen but did not affect litter carbon concentrations and often decreased nitrogen and phosphorous concentrations. The effect of nitrogen addition was dependent on time (month/year). Carbon, nitrogen and phosphorous return showed similar bimodal seasonal patterns. Nitrogen addition increased carbon and nitrogen return but did not affect phosphorous. Our results suggest that the addition of nitrogen stimulates carbon and nutrient return via litterfall.
Afforestation is one of the most important forestry practices, but its impact on soil microbial communities remains poorly understood. In this study, we sampled the soil from 0–15 cm and 15–30 cm soil depths of 7-, 13-, 24-, 33-, and 53-year-old Chinese cedar (Cryptomeria japonica var. sinensis) plantations. To investigate the effect of stand age on soil microbial communities and their potential drivers, we measured phospholipid fatty acids (PLFAs) and soil physicochemical properties. At the 0–15 cm soil depth, the biomass of total PLFAs and functional microbial groups such as bacteria (B), fungi (F), Gram-negative bacteria (GN), Gram-positive bacteria (GP), actinomycetes (ACT), and arbuscular mycorrhizal fungi (AMF) increased sharply in 7- to 13-year-old stands, but then gradually leveled off in older stands. On the other hand, the biomass of total PLFAs and functional microbial groups at the 15–30 cm soil depth peaked in the 33-year-old stand. The biomass of total PLFAs and functional microbial groups was strongly influenced by stand age and soil depth, and was significantly lower at the 15–30 cm soil depth than at the 0–15 cm soil depth except for the 7-year-old stand. The F/B and fungi/total PLFAs ratios of both soil depths were markedly lower in the 13-year-old stand than in the remaining four stand ages, while the proportions of the bacterial group (GP and GN) showed contrasting trends. The biomass of all functional microbial groups and the GP/GN ratio were mainly mediated by soil organic carbon (SOC) concentration and the soil organic carbon to total phosphorus (C/P) ratio at the 0–15 cm soil depth, but primarily affected by ammonium nitrogen (NH4+-N) concentration at the 15–30 cm soil depth. The F/B ratio of the two soil depths was prominently affected by nitrate nitrogen (NO3−-N) concentration. Our results highlighted that SOC concentration and mineral N (i.e., NH4+-N and NO3−-N) concentration mainly drove changes in the soil microbial biomass and community composition with stand age in Chinese cedar plantations, and that the 13-year-old stand may be the key period for management.
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