Repeated freeze–thaw events affect leaching losses of nitrogen and dissolved organic matter in a forest soil

Freezing and thawing may substantially influence the rates of C and N cycling in soils, and soil frost was proposed to induce NO losses with seepage from forest ecosystems. Here, we test the hypothesis that freezing and thawing triggers N and dissolved organic matter (DOM) release from a forest soil after thawing and that low freezing temperatures enhance the effect. Undisturbed soil columns were taken from a soil at a Norway spruce site either comprising only O horizons or O horizons + mineral soil horizons. The columns were subjected to three cycles of freezing and thawing at temperatures of –3°C, –8°C, and –13°C. The control columns were kept at constant +5°C. Following the frost events, the columns were irrigated for 20 d at a rate of 4 mm d^–1. Percolates were analyzed for total N, mineral N, and dissolved organic carbon (DOC). The total amount of mineral N extracted from the O horizons in the control amounted to 8.6 g N m^–2 during the experimental period of 170 d. Frost reduced the amount of mineral N leached from the soil columns with –8°C and –13°C being most effective. In these treatments, only 3.1 and 4.0 g N m^–2 were extracted from the O horizons. Net nitrification was more negatively affected than net ammonification. Severe soil frost increased the release of DOC from the O horizons, but the effect was only observed in the first freeze–thaw cycle. We found no evidence for lysis of microorganisms after soil frost. Our experiment did not confirm the hypothesis that soil frost increases N mineralization after thawing. The total amount of additionally released DOC was rather low in relation to the expected annual fluxes.     

Consistent profile pattern and spatial variation of soil C/N/P stoichiometric ratios in the subalpine forests

Purpose

The vertical patterns of soil carbon (C), nitrogen (N), and phosphorus (P) stoichiometry are still controversial, and relative contribution of their controlling factors also is rarely understood for the whole soil profile. This study aimed to assess the vertical variation of both C/N, N/P, C/P ratios and their determining factors along soil profiles in subalpine forests of the eastern Tibetan Plateau.

Materials and methods

Soil samples at five depths (0–10, 10–20, 20–30, 30–50, and 50–100 cm) were collected from 132 forest sites to evaluate the vertical distribution of soil C/N, N/P, and C/P ratios. Eleven relevant environmental factors (e.g., altitude, latitude, longitude, soil pH, soil bulk density, relative stone contents, soil order, slope, position, forest type, and dominant tree species) were measured to examine their relative contribution on stoichiometric ratios within each soil layer using boosted regression tree (BRT) analysis.

Results and discussion

Soil C/N, N/P, and C/P ratios consistently decreased with increasing soil depth. BRT models accurately predicted the soil C/N, N/P, and C/P ratios in the upper four layers (R ² = 49–97 %). For soil C/N and N/P ratios, altitude associated with latitude had the highest contribution across five soil layers, while the contributions of soil pH and bulk density were significant within soil layers closer to the surface. Independently, soil bulk density and altitude were the most important factors of C/P ratios in 0–30- and 30–100-cm soil layers.

Conclusions

This study indicated that soil C/N/P stoichiometric ratios, and the relative importance of their controlling factors, shifted within soil profiles across Tibetan Plateau forests. Further research will be needed to understand the regulatory mechanism of soil stoichiometry and biogeochemistry in response to environmental change at whole soil profiles.

     

Decline of soil fertility during forest conversion of secondary forest to Chinese fir plantations in subtropical China

The effects of forest conversion on soil fertility are still not well understood in subtropical zones. This issue was addressed by comparing chemical properties of soil in a secondary forest and a Chinese fir (Cunninghamia lanceolata Hooker) plantation at the Huitong Experimental Station of Forest Ecology. Total N, available P, ‐N, cation exchange capacity (CEC) and exchangeable Al³⁺ and H⁺ of soil were significantly lower in the pure Chinese fir plantation (PCP) than in the secondary forest while soil organic carbon (SOC), total K and exchangeable Na⁺ had a tendency to decrease in the PCP. In contrast, soil pH and percentage base saturation (PBS) significantly increased due to forest conversion, and available K, ‐N and exchangeable Ca²⁺, Mg²⁺ and K⁺ tended to increase in the PCP. Some underlying processes responsible for the differences in soil fertility between the secondary forest and the Chinese fir plantation were low litterfall and root input to soil and site preparation in coniferous plantations. There was no significant difference in the effect of slope position on chemical properties of soil in the PCP and the secondary forest. Results indicated that the conversion of secondary forests to coniferous plantations leads to a decline in soil fertility. Copyright © 2010 John Wiley & Sons, Ltd.     

Climate change effects on soil carbon dynamics and greenhouse gas emissions in Abies fabri forest of subalpine, southwest China

The Abies fabri forest on the eastern slope of the Gongga Mountain is a typical subalpine dark coniferous forest in southwestern China. The soil carbon dynamics and greenhouse gas emissions in the A. fabri forest in future climates were simulated by the Forest-DNDC (denitrification-decomposition) model. Three future climate scenarios (B1, A1B and A2) predicted by the Intergovernmental Panel on Climate Change (IPCC) were selectively investigated. The simulation showed that at elevated temperature and precipitation, the annual change of soil organic carbon (SOC) decreased in the forest floor pool but increased in the mineral soil pool. The increases in the CO₂, N₂O and NO emissions from soil were also quantified. The results indicated that elevated temperature and precipitation influenced the soil carbon dynamics, and significantly increased the greenhouse gas emissions from the soil in the A. fabri forest of subalpine.     

Effect of cultivation on microbial carbon and nitrogen in dry tropical forest soil

Summary Fifteen- and forty-year-old cropfields developed from a dry tropical forest were examined for soil organic C and total N and soil microbial C and N. The 15-year-old field had never been manured while the 40-year-old field had been fertilized with farmyard manure every year. The native forest soil was also examined. The results indicated that the native forest soil lost about 57% and 62% organic C and total N, respectively, in the 0–10 cm layer after 15 years of cultivation. The microbial C and N contents of the forest soil were greater than those of the cultivated soils. Application of farmyard manure increased the biomass-C and -N levels in the cultivated soil but the values were still markedly lower than in the forest soil. There was an appreciable seasonal variation in biomass C and N, the values being highest in summer and lowest in the rainy season. During an annual cycle, biomass-C contents varied from 180 to 727 g g^–1 and N from 20 to 80 g g^–1 dry soil, and both were linearly related. Microbial biomass C represented 1.6%–3.6% of total soil organic C and microbial biomass N represented 1.7% 1–4.4% of soil organic N.