Seasonal dynamics of soil CO<Subscript>2</Subscript> effluxes with responses to environmental factors in lower subtropical forests of China

Seasonal metrics and environmental responses to forestry soil surface CO₂ emission effluxes among three types of lower subtropical forests were consistently monitored over two years with static chamber-gas chromatograph techniques among three types of lower subtropical forests. Results showed that annual CO₂ effluxes (S+L) reached 3942.20, 3422.36 and 2163.02 CO₂ g·m^?2·a^?1, respectively in the monsoon evergreen broadleaf forest, mixed broadleaf-coniferous forest and coniferous forest. All the three types of forests revealed the same characteristics of seasonal changes with the CO₂ effluxes peaking throughout June to August. During this peaking period, the effluxes were 35.9%, 38.1% and 40.2% of the total annual effluxes, respectively. The CO₂ emission process responding to the environmental factors displayed significantly different patterns in forestry soils of the three types of forests. The coniferous forest (CF) was more sensitive to temperature than the other two types. The Q ₁₀ values were higher, along with greater seasonal variations of the CO₂ efflux, indicating that the structurally unique forestry ecosystem has disadvantage against interferences. All the three types of forestry CO₂ effluxes showed significant correlation with the soil temperature (T _s), soil water content (M _s) and air pressure (P _a). However, stepwise regression analysis indicated no significant correlation between air pressure and the soil CO₂ efflux. With an empirical model to measure soil temperature and water content in 5 cm beneath the soil surface, the CO₂ effluxes accounting for 75.7%, 77.8% and 86.5% of the efflux variability respectively in soils of BF, MF and PF were calculated. This model can be better used to evaluate the CO₂ emission of soils under water stress and arid or semi-arid conditions.     

Seasonal dynamics of soil CO2 effluxes with responses to environmental factors in lower subtropical forests of China

Seasonal metrics and environmental responses to forestry soil surface CO₂ emission effluxes among three types of lower subtropical forests were consistently monitored over two years with static chamber-gas chromatograph techniques among three types of lower subtropical forests. Results showed that annual CO₂ effluxes (S+L) reached 3942.20, 3422.36 and 2163.02 CO₂ g·m⁻²·a⁻¹, respectively in the monsoon evergreen broadleaf forest, mixed broadleaf-coniferous forest and coniferous forest. All the three types of forests revealed the same characteristics of seasonal changes with the CO₂ effluxes peaking throughout June to August. During this peaking period, the effluxes were 35.9%, 38.1% and 40.2% of the total annual effluxes, respectively. The CO₂ emission process responding to the environmental factors displayed significantly different patterns in forestry soils of the three types of forests. The coniferous forest (CF) was more sensitive to temperature than the other two types. The Q ₁₀ values were higher, along with greater seasonal variations of the CO₂ efflux, indicating that the structurally unique forestry ecosystem has disadvantage against interferences. All the three types of forestry CO₂ effluxes showed significant correlation with the soil temperature (T _s), soil water content (M _s) and air pressure (P _a). However, stepwise regression analysis indicated no significant correlation between air pressure and the soil CO₂ efflux. With an empirical model to measure soil temperature and water content in 5 cm beneath the soil surface, the CO₂ effluxes accounting for 75.7%, 77.8% and 86.5% of the efflux variability respectively in soils of BF, MF and PF were calculated. This model can be better used to evaluate the CO₂ emission of soils under water stress and arid or semi-arid conditions.

     

Estimate of productivity in ecosystem of the broad-leaved Korean pine mixed forest in Changbai Mountain

We measured soil, stem and branch respiration of trees and shrubs, foliage photosynthesis and respiration in ecosystem of the needle and broad-leaved Korean pine forest in Changbai Mountain by LI-6400 CO₂ analysis system. Measurement of forest microclimate was conducted simultaneously and a model was found for the relationship of soil, stem, leaf and climate factors. CO₂ flux of different components in ecosystem of the broad-leaved Korean pine forest was estimated based on vegetation characteristics. The net ecosystem exchange was measured by eddy covariance technique. And we studied the effect of temperature and photosynthetic active radiation on ecosystem CO₂ flux. Through analysis we found that the net ecosystem exchange was affected mainly by soil respiration and leaf photosynthesis. Annual net ecosystem exchange ranged from a minimum of about ?4.671 μmol·m^?2·s^?1 to a maximum of 13.80 μmol·m^?2·s^?1, mean net ecosystem exchange of CO₂ flux was ?2.0 μmol·m^?2·s^?1 and 3.9 μmol·m^?2·s^?1 in winter and summer respectively (mean value during 24 h). Primary productivity of tree, shrub and herbage contributed about 89.7%, 3.5% and 6.8% to the gross primary productivity of the broad-leaved Korean pine forest respectively. Soil respiration contributed about 69.7% CO₂ to the broad-leaved Korean pine forest ecosystem, comprising about 15.2% from tree leaves and 15.1% from branches. The net ecosystem exchange in growing season and non-growing season contributed 56.8% and 43.2% to the annual CO₂ efflux respectively. The ratio of autotrophic respiration to gross primary productivity (R _a:GPP) was 0.52 (NPP:GPP=0.48). Annual carbon accumulation underground accounted for 52% of the gross primary productivity, and soil respiration contributed 60% to gross primary productivity. The NPP of the needle and broad-leaved Korean pine forest was 769.3 gC·m^?2·a^?1. The net ecosystem exchange of this forest ecosystem (NEE) was 229.51 gC·m^?2·a^?1. The NEE of this forest ecosystem acquired by eddy covariance technique was lower than chamber estimates by 19.8%.     

Soil water depletion depth by planted vegetation on the Loess Plateau

Evapotranspiration of much planted vegetation exceeds precipitation, and this can deplete soil water and cause a deep dry layer in the soil profile, which is a serious obstacle to sustainable land use on the Loess Plateau, China. This study aimed to determine water depletion depth of planted grassland, shrub, and forest in a semiarid area on the Loess Plateau. Soil moisture of five vegetation types was measured to >20 m in depth. The vegetation types were crop, natural grasse, seven-year-old planted alfalfa (Medicago sativa L.), 23-year-old planted caragana (Caragana microphylla Lam.) shrub, and 23-year-old planted pine (Pinus tabulaeformis L) forest land. Through comparing moisture of planted alfalfa grass, caragana shrub, and pine forest to crop and natural grassland, the depth and amount of soil water consumed by grassland, caragana brush and pine forest was determined. The depth of soil water depleted by alfalfa, caragana brush, and pine forest reached 15.5, 22.4 and 21.5 m, respectively. Supported by National Basic Research Program of China (Grant No. 2007CB407204) and National Natural Science Foundation of China (Grant No. 40471082)     

Seasonal variation in carbon exchange and its ecological analysis over <Emphasis Type="Italic">Leymus chinensis</Emphasis> steppe in Inner Mongolia

Eddy covariance technique was used to measure carbon flux during two growing seasons in 2003 and 2004 over typical steppe in the Inner Mongolia Plateau, China. The results showed that there were two different CO₂ flux diurnal patterns at the grassland ecosystem. One had a dual peak in diurnal course of CO₂ fluxes with a depression of CO₂ flux after noon, and the other had a single peak. In 2003, the maximum diurnal uptake and emitting value of CO₂ were ?7.4 and 5.4 g·m^?2·d^?1 respectively and both occurred in July. While in 2004, the maximum diurnal uptake and release of CO₂ were ?12.8 and 5.8 g·m^?2·d^?1 and occurred both in August. The grassland fixed 294.66 and 467.46 g CO₂·m^?2 in 2003 and 2004, and released 333.14 and 437.17 g CO₂·m^?2 in 2003 and 2004, respectively from May to September. Water availability and photosynthetic active radiation (PAR) are two important factors of controlling CO₂ flux. Consecutive precipitation can cause reduction in the ability of ecosystem carbon exchange. Under favorable soil water conditions, daytime CO₂ flux is dependent on PAR. CO₂ flux, under soil water stress conditions, is obviously less than those under favorable soil water conditions, and there is a light saturation phenomena at PAR=1200 μmol·m^?2·s^?1. Soil respiration was temperature dependent when there was no soil water stress; otherwise, this response became accumulatively decoupled from soil temperature.     

Seasonal variation in carbon exchange and its ecological analysis over Leymus chinensis steppe in Inner Mongolia

Eddy covariance technique was used to measure carbon flux during two growing seasons in 2003 and 2004 over typical steppe in the Inner Mongolia Plateau, China. The results showed that there were two different CO₂ flux diurnal patterns at the grassland ecosystem. One had a dual peak in diurnal course of CO₂ fluxes with a depression of CO₂ flux after noon, and the other had a single peak. In 2003, the maximum diurnal uptake and emitting value of CO₂ were −7.4 and 5.4 g·m⁻²·d⁻¹ respectively and both occurred in July. While in 2004, the maximum diurnal uptake and release of CO₂ were −12.8 and 5.8 g·m⁻²·d⁻¹ and occurred both in August. The grassland fixed 294.66 and 467.46 g CO₂·m⁻² in 2003 and 2004, and released 333.14 and 437.17 g CO₂·m⁻² in 2003 and 2004, respectively from May to September. Water availability and photosynthetic active radiation (PAR) are two important factors of controlling CO₂ flux. Consecutive precipitation can cause reduction in the ability of ecosystem carbon exchange. Under favorable soil water conditions, daytime CO₂ flux is dependent on PAR. CO₂ flux, under soil water stress conditions, is obviously less than those under favorable soil water conditions, and there is a light saturation phenomena at PAR=1200 μmol·m⁻²·s⁻¹. Soil respiration was temperature dependent when there was no soil water stress; otherwise, this response became accumulatively decoupled from soil temperature.

     

Soil pore characteristics of evergreen and deciduous forests of the tropical monsoon region in Cambodia

Both evergreen and deciduous forests (Efs and Dfs) are widely distributed under similar climatic conditions in tropical monsoon regions. To clarify the hydraulic properties of the soil matrix in different forest types and their effects on soil water storage capacity, the soil pore characteristics (SPC) were investigated in Ef and Df stands in three provinces in Cambodia. Soils in the Ef group were characterized in common by large amounts of coarse pores with moderate pore size distribution and the absence of an extremely low K_s at shallow depths, compared to Df group soils. The mean available water capacity of the soil matrix (AWC_sm) for all horizons of the Ef and Df group soils was 0·107 and 0·146 m³ m^?3, respectively. The mean coarse pore volume of the soil matrix (CPV_sm) in the Ef and Df groups was 0·231 and 0·115 m³ m^?3, respectively. A water flow simulation using a lognormal distribution model for rain events in the early dry season indicated that variation in SPC resulted in a larger increase in available soil water in Ef soils than in Df soils. Further study on deeper soil layers in Ef and each soil type in Df is necessary for the deeper understanding of the environmental conditions and the hydrological modelling of each forest ecosystem. Copyright © 2010 John Wiley & Sons, Ltd.     

Epigenetic salt accumulation and water movement in the active layer of central Yakutia in eastern Siberia

Observations of soil moisture and salt content were conducted from May to August at Neleger station in eastern Siberia. Seasonal changes of salt and soil moisture distribution in the active layer of larch forest (undisturbed) and a thermokarst depression known as an alas (disturbed) were studied. Electric conductivity EC_e of the intact forest revealed higher concentrations that increased with depth from the soil surface into the active layer and the underlying permafrost: 1 mS cm^?1 at 1·1 m, to 2·6 mS cm^?1 at 160 cm depth in the permafrost. However, a maximum value of 5·4 mS cm^?1 at 0·6 m depth was found in the dry area of the alas. The concentration of ions, especially Na⁺, Mg²⁺, Ca²⁺, SO₄^2? and HCO₃^? in the upper layers of this long‐term disturbed site, indicates the upward movement of ions together with water. A higher concentration of solutes was found in profiles with deeper seasonal thawing. The accumulation of salts in the alas occurs from spring through into the growing season. The low concentration of salt in the surface soil layers appears to be linked to leaching of salts by rainfall. There are substantial differences between water content and electric conductivity of soil in the forest and alas. Modern salinization of the active layer in the alas is epigenetic, and it happens in summer as a result of spring water collection and high summer evaporation; the gradual salt accumulation in the alas in comparison with the forest is controlled by the annual balance of water and salts in the active layer. Present climatic trends point to continuous permafrost degradation in eastern Siberia increasing the risk of surface salinization, which has already contributed to changing the landscape by hindering the growth of forest. Copyright © 2006 John Wiley & Sons, Ltd.     

A comparison between simulated and measured CO2 and water flux in a subtropical coniferous forest

Using data from eddy covariance measurements in a subtropical coniferous forest, a test and evaluation have been made for the model of Carbon Exchange in the Vegetation-Soil-Atmosphere (CEVSA) that simulates energy transfers and water, carbon and nitrogen cycles based on ecophysiological processes. In the present study, improvement was made in the model in calculating LAI, carbon allocation among plant organs, litter fall, decomposition and evapotranspiration. The simulated seasonal variations in carbon and water vapor flux were consistent with the measurements. The model explained 90% and 86% of the measured variations in evapotranspiration and soil water content. However, the modeled evapotranspiration and soil water content were lower than the measured systematically, because the model assumed that water was lost as runoff if it was beyond the soil saturation water content, but the soil at the flux site with abundant rainfall is often above water saturated. The improved model reproduced 79% and 88% of the measured variations in gross primary production (GPP) and ecosystem respiration (R _e), but only 31% of the variations in measured net ecosystem exchange (NEP) despite the fact that the modeled annual NEP was close to the observation. The modeled NEP was generally lower in winter and higher in summer than the observations. The simulated responses of photosynthesis and respiration to water vapor deficit at high temperatures were different from measurements. The results suggested that the improved model underestimated ecosystem photosynthesis and respiration in extremely condition. The present study shows that CEVSA can simulate the seasonal pattern and magnitude of CO₂ and water vapor fluxes, but further improvement in simulating photosynthesis and respiration at extreme temperatures and water deficit is required.

     

Spatial and temporal variations of pCO2, dissolved inorganic carbon and stable isotopes along a temperate karstic watercourse

This study investigated CO₂ degassing and related carbon isotope fractionation effects in the Wiesent River that drains a catchment in the karst terrain of the Franconian Alb, Southern Germany. The river was investigated by physico‐chemical and stable isotope analyses of water and dissolved inorganic carbon during all seasons along 65‐km long downstream transects between source and mouth. Calculated pCO₂ values at the source were 21 400 ± 2400 µatm. The pCO₂ rapidly decreased in the river water and dropped to an average of 1240 ± 330 µatm near the mouth. About 90% of this decrease occurred within the first 6 km of the river. The river was supersaturated with respect to CO₂ over its entire course and must have acted as a continuous year‐round CO₂ source to the atmosphere. The average CO₂ flux from the karst river was estimated with 450 mmol m^?2 day^?1 with higher fluxes up to 5680 mmol m^?2 day^?1 at the source. At the source, δ¹³C_DIC values showed no seasonal variations with an average of ?14.2 ± 0.2‰. This indicated that groundwater retained high pCO₂ mainly from soil CO₂. The contribution of soil CO₂ to dissolved inorganic carbon was estimated at 65% to 72%. The downstream CO₂ loss caused a positive shift in δ¹³C_DIC values of 2‰ between source and mouth because of the preferential loss of the ¹²C isotope during degassing. Considering the findings of this study and the fact that carbonate lithology covers a significant part of the earth's surface, CO₂ evasion from karst regions might contribute notably to the annual carbon dioxide release from global freshwater systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Soil Microbial and Enzyme Activities Response to Pollution Near an Aluminium Smelter

The aim of this research was to assess the impact caused by a long‐term pollution by fluoride and heavy metals in two soils (PS1 and PS2) near an aluminium smelter in Slovakia, on soil microbial biomass C (MBC), basal respiration, metabolic quotient (qCO₂) water‐soluble organic C (WSOC) and enzymes activities involved in the C, N and P biogeochemical cycles. An unpolluted soil was used as control (C0). Results obtained for soil fluoride content reflected a gradient of fluoride exposure in topsoils of contaminated sites. Decreases in microbial and enzymatic activities and in MBC to organic C ratio were found in PS2 site, which is closer to the smelter and exhibited the highest fluoride content. PS1‐soil showed an extreme alkaline pH caused by leaching of waste effluents from the smelter dumping site, higher contents of Zn, Cu, Pb and Cd, significantly larger MBC, qCO₂ and catalase and urease activities, and much larger basal respiration and dehydrogenase activity than PS2 and C0‐soil. Phosphatase, β‐glucosidase and BAA‐protease were negatively correlated with WSOC, basal respiration and dehydrogenase activity, and showed some degree of inhibition in polluted sites. These results may indicate different responses of microbial communities to ecosystem disturbances, caused by the drastic changes in soil's physicochemical properties as result of the long‐term emissions of fly ash with high levels of contaminants that are still affecting soil microbial and enzymatic activities.     

Impact of forest degradation on streamflow regime and runoff response to rainfall in the Garhwal Himalaya,Northwest India

Baseflows have declined for decades in the Lesser Himalaya but the causes are still debated. This paper compares variations in streamflow response over three years for two similar headwater catchments in northwest India with largely undisturbed (Arnigad) and highly degraded (Bansigad) oak forest. Hydrograph analysis suggested no catchment leakage, thereby allowing meaningful comparisons. The mean annual runoff coefficient for Arnigad was 54% (range 44–61%) against 62% (53–69%) at Bansigad. Despite greater total runoff Q_t (by 250 mm year^–1), baseflow at Bansigad ceased by March, but was perennial at Arnigad (making up 90% of Q_t vs. 51% at Bansigad). Arnigad storm flows, Q_s, were modest (8–11% of Q_t) and occurred mostly during monsoons (78–98%), while Q_s at Bansigad was 49% of Q_t and occurred also during post-monsoon seasons. Our results underscore the importance of maintaining soil water retention capacity after forest removal to maintain baseflow levels.

EDITOR D. Koutsoyiannis; ASSOCIATE EDITOR D. Gerten     

Soil Seed Banks and Forest Succession Direction Reflect Soil Quality in Ziwuling Mountain,Loess Plateau,China

The paper presents a study by taking the soil seed banks and vegetation successions of the forests in Ziwuling Mountain as indicators to analyze the effects of the ages, and the litter layers and soil depths at growing locations in seven types of forest communities on their seed bank formations and soil quality. The results showed that the seed banks at different growing locations in the communities increased in the order of upper slope, middle slope, and lower slope; the seed storages of the seed banks in the different layers of the communities varied, much more higher in the litter layers than in 0–15 cm, and the seed storages of the seed banks in the seven types of forest communities ranked in the increasing order of Pinus tabulaeformis forest, Pinus shenkaneusis forest, Quercus liaotungensis forest, Populus davidiana forest, Betula platyphylla forest, scrub communities, and grassland communities; in the meantime, the seed storage of seed banks peaked in 30–50 years old P. shenkaneusis forest, 30–40 years old P. tabulaeformis forest, 15–30 years old Q. liaotungensis, and P. davidiana and B. platyphylla forests, and 10–15 years old scrub and grassland communities, and the ages of the communities varied with the seed storages of the seed banks in a significantly correlative manner following a fitted exponential equation. In addition, the soil seed banks of the seven types of communities consisted of rich and diverse species with the herbaceous and shrub species greatly outnumbering the arbor species; in general, the coniferous forests were composed of 31 kinds of plants, the deciduous and broadleaf forests consisted of 20–29 plant species, the shrubs contained 27 plant species, and the herbaceous plants numbered 20 plant species; The various species compositions contained only 4–6 arbor species with most being foreign species. In each of the compositions, Bothriochloa ischemum was the grassland plant with the highest occurrence frequency, Sophora viciifolia and Hippophae reamnoides were the shrub plants with the highest occurrence frequencies, and Q. liaotungensis was the arbor plant with the highest occurrence frequency, and they followed by P. shenkaneusis and P. tabulaeformis. These results showed that soil seed banks and forest successions are better indicators for soil quality from natural successions.     

Improving Soil Enzyme Activities and Related Quality Properties of Reclaimed Soil by Applying Weathered Coal in Opencast‐Mining Areas of the Chinese Loess Plateau

There are many problems for the reclaimed soil in opencast‐mining areas of the Loess Plateau of China such as poor soil structure and extreme poverty in soil nutrients and so on. For the sake of finding a better way to improve soil quality, the current study was to apply the weathered coal for repairing soil media and investigate the physicochemical properties of the reclaimed soil and the changes in enzyme activities after planting Robinia pseucdoacacia. The results showed that the application of the weathered coal significantly improved the quality of soil aggregates, increased the content of water stable aggregates, and the organic matter, humus, and the cation exchange capacity of topsoil were significantly improved, but it did not have a significant effect on soil pH. Planting R. pseucdoacacia significantly enhanced the activities of soil catalase, urease, and invertase, but the application of the weathered coal inhibited the activity of catalase. Although the application of appropriate weathered coal was able to significantly increase urease activity, the activities of catalase, urease, or invertase had a close link with the soil profile levels and time. This study suggests that applying weathered coals could improve the physicochemical properties and soil enzyme activities of the reclaimed soil in opencast‐mining areas of the Loess Plateau of China and the optimum applied amount of the weathered coal for reclaimed soil remediation is about 27 000 kg hm^?2.     

Soil CO2 flux baseline in an urban monogenetic volcanic field: the Auckland Volcanic Field, New Zealand

The Auckland Volcanic Field (AVF) is a dormant monogenetic basaltic field located in Auckland, New Zealand. Though soil gas CO₂ fluxes are routinely used to monitor volcanic regions, there have been no published studies of soil CO₂ flux or soil gas CO₂ concentrations in the AVF to date or many other monogenetic fields worldwide. We measured soil gas CO₂ fluxes and soil gas CO₂ concentrations in 2010 and 2012 in varying settings, seasons, and times of day to establish a baseline soil CO₂ flux and to determine the major sources of and controlling influences on Auckland's soil CO₂ flux. Soil CO₂ flux measurements varied from 0 to 203 g m^?2 day^?1, with an average of 27.1 g m^?2 day^?1. Higher fluxes were attributed to varying land use properties (e.g., landfill). Using a graphical statistical approach, two populations of CO₂ fluxes were identified. Isotope analyses of δ¹³CO₂ confirmed that the source of CO₂ in the AVF is biogenic with no volcanic component. These data may be used to assist with eruption forecasting in the event of precursory activity in the AVF, and highlight the importance of knowing land use history when assessing soil gas CO₂ fluxes in urban environments.     

Evaporation Process in Soil Surface Containing Calcic Nodules on the Northern Loess Plateau of China by Simulated Experiments

Soil containing calcic nodules is widely present on the northern Loess Plateau of China owing to soil genesis under local climate conditions. In most studies, little attention is payed to the effect of calcic nodules on soil evaporation and ecoenvironment, resulting in inaccurate evaporation estimation in this kind of soil and further improper field water management measures and irrigation effects. In this paper, soil column experiments were conducted in order to investigate evaporation process in soil containing calcic nodules and the effect of calcic nodules on soil evaporation was determined. The results indicated that evaporation reduction was positively related to calcic nodule content (CNC = mass of calcic nodules/total mass), and could be estimated by the experiential equation: E_soil = E₀ (1 – 0.4 CNC) (E_soil = actual evaporation, E₀ = theory evaporation in soil without calcic nodules). When CNC was below 0.2, the impact could be neglected. While, as CNC exceeded 0.2, the impact needed to be considered during soil evaporation estimation. As CNC reached 0.5, soil evaporation could be reduced by 7.5 mm, accounting for around 10% of the total soil water. Water balance calculation in soil columns showed that water absorbed by calcic nodules was partially available to evaporation. Water available to evaporation was positively related to CNC, and this water could not exceed 63% of the water absorbed by calcic nodules. Generally, evaporation behavior was dominated by calcic nodule quantity and its water absorption. These results provide new ideas for irrigation measures in arid areas of the globe.     

Carbon dioxide exchange and the mechanism of environmental control in a farmland ecosystem in North China Plain

CO₂ flux was measured continuously in a wheat and maize rotation system of North China Plain using the eddy covariance technique to study the characteristic of CO₂ exchange and its response to key environmental factors. The results show that nighttime net ecosystem exchange (NEE) varied exponentially with soil temperature. The temperature sensitivities of the ecosystem (Q ₁₀) were 2.94 and 2.49 in years 2002–2003 and 2003–2004, respectively. The response of gross primary productivity (GPP) to photosynthetically active radiation (PAR) in the crop field can be ex-pressed by a rectangular hyperbolic function. Average A _max and α for maize were more than those for wheat. The values of α increased positively with leaf area index (LAI) of wheat. Diurnal variations of NEE were significant from March to May and from July to September, but not remarkable in other months. NEE, GPP and ecosystem respiration (R _ec) showed significantly seasonal variations in the crop field. The highest mean daily CO₂ uptake rate was ?10.20 and ?12.50 gC·m^?2?d^?1 in 2003 and 2004, for the maize field, respectively, and ?8.19 and ?9.50 gC?m^?2·d^?1 in 2003 and 2004 for the wheat field, respectively. The maximal CO₂ uptake appeared in April or May for wheat and mid-August for maize. During the main growing seasons of winter wheat and summer maize, NEE was controlled by GPP which was chiefly influenced by PAR and LAI. R _ec reached its annual maximum in July when R _ec and GPP contributed to NEE equally. NEE was dominated by R _ec in other months and temperature became a key factor controlling NEE. Total NEE for the wheat field was ?77.6 and ?152.2 gC·m^?2·a^?1 in years 2002–2003 and 2003–2004, respectively, and ?120.1 and ?165.6 gC·m^?2·a^?1 in 2003 and 2004 for the maize field, respectively. The cropland of North China Plain was a carbon sink, with annual ?197.6 and ?317.9 gC·m^?2·a^?1 in years 2002–2003 and 2003–2004, respectively. After considering the carbon in grains, the cropland became a carbon source, which was 340.5 and 107.5 gC·m^?2·a^?1 in years 2002–2003 and 2003–2004, respectively. Affected by climate and filed managements, inter-annual carbon exchange varied largely in the wheat and maize rotation system of North China Plain.     

Energy flux partitioning and evapotranspiration in a sub‐alpine spruce forest ecosystem

In this study, we examined the year 2011 characteristics of energy flux partitioning and evapotranspiration of a sub‐alpine spruce forest underlain by permafrost on the Qinghai–Tibet Plateau (QPT). Energy balance closure on a half‐hourly basis was H + λE = 0.81 × (R_n ? G ? S) + 3.48 (W m^?2) (r² = 0.83, n = 14938), where H, λE, R_n, G and S are the sensible heat, latent heat, net radiation, soil heat and air‐column heat storage fluxes, respectively. Maximum H was higher than maximum λE, and H dominated the energy budget at midday during the whole year, even in summer time. However, the rainfall events significantly affected energy flux partitioning and evapotranspiration. The mean value of evaporative fraction (Λ = λE/(λE + H)) during the growth period on zero precipitation days and non‐zero precipitation days was 0.40 and 0.61, respectively. The mean daily evapotranspiration of this sub‐alpine forest during summer time was 2.56 mm day^?1. The annual evapotranspiration and sublimation was 417 ± 8 mm year^?1, which was very similar to the annual precipitation of 428 mm. Sublimation accounted for 7.1% (30 ± 2 mm year^?1) of annual evapotranspiration and sublimation, indicating that the sublimation is not negligible in the annual water balance in sub‐alpine forests on the QPT. The low values of the Priestley–Taylor coefficient (α) and the very low value of the decoupling coefficient (Ω) during most of the growing season suggested low soil water content and conservative water loss in this sub‐alpine forest. Copyright © 2013 John Wiley & Sons, Ltd.     

Soil degradation in central Spain due to sheet water erosion by low‐intensity rainfall events

Runoff and sediment lost due to water erosion were recorded for 36 (1 m²) plots with varying types of vegetative cover located on sloping gypsiferous fields in the South of Madrid. 75% of the events had maximum 30‐minute intensity (I₃₀) less than 10 mm h^?1 in the period studied (1994–2005). As for the vegetative cover, maximum correlation between runoff and soil loss was found in the least protected plots (0–40% cover) during the most intense rainfall events; however, a significant positive correlation was also observed in plots with greater coverage (40–60%). If coverage exceeded 60%, rainfall erosivity declined. The average amount of sediment produced in high‐intensity events was significantly greater (approximately 7 g m^?2 per I₃₀ event >10 mm h^?1) than that produced in the rest of the moderate‐intensity events (approximately 3 g m^?2 per I₃₀ event <10 mm h^?1), but due to the high rate of occurrence of the latter throughout the year sediment loss during the period studied totaled 128 g m^?2. By comparison, only 40 g m^?2 was produced by the I₃₀ events greater than 10 mm h^?1. Even though the amount of soil lost is relatively insignificant from a quantitative standpoint, the organic matter content lost in the sediment (six times more than in the soil) is a permanent loss that threatens the development of the surface of the soil in this area when the vegetative cover is less than 40%. The soil here experiences a chronic loss of 0·02 mm annually as a consequence of frequent, moderate events, in addition to any loss produced by extraordinary events, which, though less frequent, are much more erosive. If moderate events are ignored, an important part of soil loss will be lost in the long run. Copyright © 2007 John Wiley & Sons, Ltd.     

退水期鄱阳湖薹草(Carex cinerascens)和藜蒿(Artemisia selengensis)洲滩湿地CO2通量变化及其影响因子

于2014年10月到2015年5月鄱阳湖退水期,利用密闭箱—气相色谱法对鄱阳湖北部星子县洲滩两种代表性的植被群落——薹草(Carex cinerascens)和藜蒿(Artemisia selengensis)进行CO_2通量的对比观测,结果表明:薹草和藜蒿湿地的生态系统呼吸具有明显季节变化模式,其最小值均出现在冬季,最大值均出现在春季,平均值分别为3291.80和2581.89mg CO_2/(m~2·h),退水期薹草和藜蒿湿地累积的CO_2通量分别为213.71±2.27和176.39±11.48 t CO_2/hm~2.较高的生物量是薹草湿地CO_2通量高于藜蒿湿地的原因.5 cm土温是影响薹草和藜蒿湿地CO_2通量季节变化最重要的影响因子,藜蒿湿地生态系统呼吸的温度敏感性指数(Q10)高于薹草湿地.水分、植物生物量和湿地CO_2通量之间无显著相关性.