Three scales of temporal resolution from automated soil respiration measurements

Soil respiration (R_s) is a combination of autotrophic and heterotrophic respiration, but it is often modeled as a single efflux process, influenced by environmental variables similarly across all time scales. Continued progress in understanding sources of variation in soil CO₂ efflux will require development of R_s models that incorporate environmental influences at multiple time scales. Coherence analysis, which requires high temporal frequency data on R_s and related environmental variables, permits examination of covariation between R_s and the factors that influence it at varying temporal frequencies, thus isolating the factors important at each time scale. Automated R_s measurements, along with air, soil temperature and moisture were collected at half hour intervals at a temperate forest at Harvard Forest, MA in 2003 and a boreal transition forest at the Howland Forest, ME in 2005. As in other temperate and boreal forests, seasonal variation in R_s was strongly correlated with soil temperature. The organic and mineral layer water contents were significantly related to R_s at synoptic time scales of 2–3 days to weeks, representing the wetting and drying of the soils as weather patterns move across the region. Post-wetting pulses of R_s were correlated with the amount of precipitation and the magnitude of the change from pre-wet-up moisture content to peak moisture content of the organic horizon during the precipitation events. Although soil temperature at 8–10 cm depth and R_s showed strong coherence at a 24-h interval, calculated diel Q₁₀ values for R_s were unreasonably high (6–74) during all months for the evergreen forest and during the growing season for the deciduous forest, suggesting that other factors that covary with soil temperature, such as canopy assimilatory processes, may also influence the diel amplitude of R_s. Lower diel Q₁₀ values were obtained based on soil temperature measured at shallower depths or with air temperature, but the fit was poorer and a lag was needed to improve the fit (peak R_s followed peak air temperature by several hours), suggesting a role for delayed substrate supply from aboveground processes to affect diel patterns of R_s. High frequency automated R_s datasets afford the opportunity to disentangle the temporal scales at which environmental factors, such as seasonal temperature and phenology, synoptic weather events and soil moisture, and diel variation in temperature and photosynthesis, affect soil respiration processes.     

美国路易斯安那州铁矿石土壤薄层磐层的表征特性

Placic horizons, defined as thin, wavy, hardened layers of iron and organic matter, are rare within the United States, occurring only in Washington, Alaska, and Hawaii. While ironstone is common in many soils of the southeastern United States, it is not known to contain appreciable organic matter. As a pilot study evaluating the justification for a larger study on ironstone in Louisiana, a 40 m lateral exposure with suspected placic horizons was evaluated in Vernon Parish, Louisiana. Results of laboratory analysis show elevated levels of iron and organic matter in the suspect horizons that meet the criteria of placic horizons as defined by the Soil Survey Staff. Based on the results of this study, additional evaluation of multiple pedons with similar features is warranted. Should additional pedons demonstrate similar properties, a new great group of ‘Petrudepts’ would be needed to describe both the placic horizons in the pedon and the udic moisture regime in which they occur.     

An Environmental Risk Assessment of Radon in Lantian Karst Cave of Shaanxi,China

Water, Air, & Soil Pollution - The radiation dose and environmental health risk of radon concentration in the Lantian karst cave of China to guides and visitors were estimated based on the...     

Simulating leaf area of corn plants at contrasting water status

An exponential decay function was fitted with literature data to describe the decrease in corn leaf expansion rate as predawn leaf water potential decreases. The fitted function was then applied to modify an existing leaf area simulation module in a soil–plant–atmosphere continuum corn simulation model (MaizSim) in order to simulate leaf area of corn plants at different water status. Data were collected from field for two years as well as from sunlit growth chambers located at USDA-ARS facilities in Beltsville, MD with different irrigation frequencies. Comparison among simulations and measurements indicated that the modified leaf area module improved leaf area simulation for corn plant under different drought stress. For plants under more severe drought stress, the improvement in leaf area simulation was more significant. These results suggested that the modified leaf area model presented an approach to mechanistically link corn leaf area with corn plant water status, and was suitable for integration with existing corn models that simulate corn leaf area.     

Aggregate stability in range sandy loam soils Relationships with runoff and erosion

The spatial variability of soil aggregate stability and its relationship to runoff and soil erosion were examined in a catena of soils and vegetation in a semiarid environment at the Rambla Honda field site (Tabernas, Almería, SE Spain) to evaluate the validity of structural stability as a soil erosion indicator in sandy loam range soils. The influence of soil properties and topography on the variability of aggregate stability was also examined. Methods include: 1) aggregate stability assessment at 12 sites (3 repetitions per site) on the hillslope by two methods: a) aggregate size distribution by dry sieving b) water drop test; 2) soil organic carbon content; 3) particle size distribution determination; 4) terrain attributes derived from a digital elevation model (1-m resolution); 5) monitoring runoff and erosion for nearly 3 years in eight (10 × 2 m) plots distributed over the hillslope. Results: 41% of the average soil mass is formed by > 2-mm aggregates. However, wet aggregate stability is poor, with a mean (of a total of 1440 aggregates) of only 26 drop impacts necessary to break up a wet aggregate (pF = 1). Significant relationships were found in the number of water drops required for aggregate breakdown and runoff and erosion rates. However, no significant relationships between the mean weight diameter of aggregates under dry conditions and runoff or erosion rates were observed. The relationships of aggregates with other soil properties, hillslope position and proximity to plants are also analysed. The most significant correlation found was between the number of drop impacts and soil organic matter content. The stability of topsoil aggregates seems to be a valuable indicator of field-assessed runoff and inter-rill erosion of sandy loam range soils under semiarid conditions.     

黄土高原不同植被类型对土壤养分、酶活性及微生物的影响

[目的]研究黄土高原不同植被类型对土壤酶活性、土壤养分和土壤微生物的影响,为该地区植被建设提供参考。[方法]根据研究区特点,选择4种(草地、柠条、杨树、蒿地)植被类型,采用SPSS软件分析各样地之间和同一样地土壤剖面不同层次的土壤酶活性和养分差异显著性和相关性,采用Canoco对微生物与环境因子进行分析。[结果](1)不同植被类型下,随土层深度的增加土壤酶活性与土壤养分均呈降低趋势。蒿地中酶活性最高,草地pH值随土层深度的增加而增加,表土层有机碳含量最高。硝态氮和铵态氮含量分别在杨树、柠条地块最高。(2)土壤养分间存在显著或极显著性相关,而土壤酶活性间存在极显著性相关。土壤酶活性与土壤有机碳、硝态氮之间都存在显著和极显著性相关。(3)表层(0—10cm)土壤微生物在门水平上多样性高。真菌第Ⅰ簇菌属与土壤pH值、硝态氮、铵态氮呈正相关,与土壤酶活性呈负相关;第Ⅱ簇菌属与土壤酶活性呈正相关关系,而与硝态氮、铵态氮、有机碳、pH值呈负相关关系;第Ⅰ簇与第Ⅱ,Ⅲ簇真菌群落间呈负相关关系。细菌与酶活性和土壤养分存在正相关和负相关。[结论]不同植被类型影响土壤酶活性、养分及微生物多样性,在植被建设过程中,宜种植蒿草来改善土壤的生态环境。     

Contribution and stability of forest-derived soil organic carbon during woody encroachment in a tropical savanna. A case study in Gabon

In this study, we quantified the contribution of forest-derived carbon (FDC) to the soil organic C (SOC) pool along a natural succession from savanna (S) to mixed Marantaceae forest (MMF) in the Lopè National Park, Gabon. Four 1-ha plots, corresponding to different stages along the natural succession, were used to determine the SOC stock and soil C isotope composition (δ¹³C) to derive the FDC contribution in different soil layers down to 1 m depth. Besides, to investigate changes in SOC stability, we determined the ¹⁴C concentration of SOC to 30 cm depth and derived turnover time (TT). Results indicated that SOC increased only at the end of the succession in the MMF stage, which stored 46% more SOC (41 Mg C ha^?1) in the 0–30 cm depth than the S stage (28.8 Mg C ha^?1). The FDC contribution increased along forest succession affecting mainly the top layers of the initial successional stages to 15 cm depth and reaching 70 cm depth in the MMF stage. The TT suggests a small increase in stability in the 0–5 cm layer from S (146 years) to MMF (157 years) stages. Below 5 cm, the increase in stability was high, suggesting that FDC can remain in soils for a much longer time than savanna-derived C. In conclusion, the natural succession toward Marantaceae forests can positively impact climate change resulting in large SOC stocks, which can be removed from the atmosphere and stored for a much longer time in forest soils compared to savanna soils.