Long-Term (13 Years) Decomposition Rates of Forest Floor Organic Matter on Paired Coniferous and Deciduous Watersheds with Contrasting Temperature Regimes

Abstract

Two sets of paired watersheds on north and South facing slopes were utilized to simulate the effects of temperature differences that are on the scale of those expected with near-term climatic warming on decomposition. Two watersheds were pine plantations (Pinus strobus L.) and two were mature deciduous forests established at similar elevation ranges and precipitation at the Coweeta Hydrologic Laboratory, but they differed in slope aspect (north vs. South facing), solar radiation, and litter temperature by about 2.0 • C. Nylon netting was placed on plots each year for 13 years and litterfall was measured. This time span in which decomposition rate was measured encompassed the time until less than 8% of the initial C remained. Decomposition rates of foliar litter were significantly faster on the slightly warmer watersheds, in both the coniferous and deciduous forests (Analysis of Variance). The turnover rate (year −1 ) was 0.359 (±0.006) for the South facing vs. 0.295 (±0.011) for the North facing coniferous watersheds, and 0.328 (±0.011) vs. 0.297 (±0.012) for the corresponding deciduous watersheds. Turnover rates of pine vs. deciduous broadleaf litter over 13 years were not significantly different because of the high proportion of relatively refractory Quercus spp. in the deciduous litterfall and because of a trend towards convergence of the rates after two years. After a greater decomposition rate in the first year or two, years 2-13 fit a negative exponential curve well (a timespan not well represented in literature) and there was only a small accumulation of humus older than 13 years. The fate of C in litterfall in the South facing deciduous forest was as follows: 14.3% was lost as leaching of dissolved organic C, 2.2% was lost as downward fine particulate matter flux from the bottom of the forest floor, 78.2% was mineralized (by mass balance), leaving only 5.4% of foliar litter after 13 years of decomposition. In these soils with a mor type O horizon, there was evidence that translocation of DOC and in-situ root production must be more important sources of mineral soil organic matter than downward migration of particulate humus.

The effect of stand structure on litter decomposition in Pinus sylvestris L. stands in Turkey

“…Many researchers have reported that the facing slope affects the decomposition rate, being faster on the northern slope than on the southern slope (Sariyildiz and Kucuk 2008;Jasińska et al 2019). On the other hand, some researchers have reported a faster decomposition of litterfall on the southern aspects than on the northern (Mudrick et al 1994;Qualls 2016), which is in line with our results. Furthermore, in open-canopied mature stands, the decomposition rate was found to be faster on the southern slopes in comparison to the north in contrast to the case of the dense canopy, likely due to age-and light-related changes in litterfall composition or balance between temperature and moisture on the forest floor.…”

Temporal Variations in Soil Profile Carbon and Nitrogen during Three Consecutive Years of 15N Deposition in Temperate Oak and Pine Forest Stands

“…Finn et al [44] reported that the amounts of soil C and N pools that remained in coniferous forest floors after decomposition at a constant temperature were larger than those in deciduous forest floors. In particular, even under similar environmental conditions (climate region, temperature, rainfall, and soil moisture), it is well-known that soil CO 2 efflux is highest from deciduous forests, followed by mixed forests, and lowest from coniferous forests [44][45][46], as a result of the higher lignin content of their leaf-litter [47,48] and lower SOM degradation [46]. Compared with the control soils, N deposition obviously increased soil total C contents (p < 0.001) ( Figure 6), and this increase in SOC pools was well supported by the increase in 15 N recovery ( Figure 5) and the concurrent decrease in soil-δ 13 C to 0-10 cm soil depth in both forest soils ( Figure 6).…”

Soil Microbiomes Under Climate Change and Implications for Carbon Cycling

“…A related area of uncertainty is how the type of plant-derived carbon enhances microbial SOC storage or alternatively accelerates SOC decomposition (12). For example, leaf litter and needle litter serve as sources of carbon for microbial growth in forest soils, but litter chemistry and pH varies by vegetation type [e.g., between root and foliar litter (13) or between deciduous and coniferous forest litter (14)]. In turn, these biochemical differences influence SOC levels through changing decomposition dynamics (14).…”

“…For example, leaf litter and needle litter serve as sources of carbon for microbial growth in forest soils, but litter chemistry and pH varies by vegetation type [e.g., between root and foliar litter (13) or between deciduous and coniferous forest litter (14)]. In turn, these biochemical differences influence SOC levels through changing decomposition dynamics (14). Also, increased diversity of plant communities increases rates of rhizodeposition, stimulating microbial activity and SOC storage (15), although soils eventually reach a saturation point beyond which they cannot store additional carbon (16).…”