浑善达克沙地东南缘坡地风蚀速率变化的137Cs与210Pb复合示踪研究

【目的】坡地是人为活动频繁而生态较脆弱的地区,土壤侵蚀是造成丘陵区坡地土地流失的主要方式,因此探究丘陵区坡地不同位置和不同时间尺度风蚀速率的变化具有重要意义。【方法】以位于浑善达克沙地东南缘农牧交错带、生态环境脆弱、易受到土壤风蚀影响的内蒙古多伦县为研究区,选择一个植被类型为沙化草地的典型坡地,通过测量其坡顶、坡肩、背坡、坡麓等不同位置剖面的¹³⁷Cs和²¹⁰Pb比活度,探讨坡地不同位置的剖面¹³⁷Cs、²¹⁰Pb含量及分布特征,计算坡地不同位置的土壤侵蚀速率。【结果】坡地¹³⁷Cs分布较深,²¹⁰Pb含量随深度增加呈负指数递减,¹³⁷Cs、²¹⁰Pb平均活度值较低,受长期风蚀的影响,样品中砂含量高。【结论】除坡麓处于沉积状态外,坡地不同位置均受到轻度或微度风蚀作用的影响,风蚀速率相对大小为：坡肩>坡顶>背坡>坡麓。20世纪60年代以来土壤侵蚀速率显著降低,生态环境质量总体向好。     

等高绿篱-坡地农业复合系统土壤CO_2排放特征

基于2008年12月初至翌年6月初绿篱-冬小麦复合系统土壤CO2全生长季的观测,研究了绿篱复合种植及其枝叶不同还田(移出/AR、翻施/AI和表施/AC)方式下土壤CO2的排放特征。结果表明,整个冬小麦生长期间,各处理土壤CO2排放具有相似的季节变化趋势,小麦生长旺盛期明显高于越冬期和成熟期。绿篱复合种植及其枝叶还田显著影响土壤CO2排放,与作物单作(CK)处理相比,AR处理土壤CO2平均排放通量降低了9.07%,而AI和AC处理分别增加了35.70%和8.42%。土壤温度、作物生长及土壤微生物生物量碳是影响土壤CO2排放的主要因素。土壤CO2排放通量与土壤温度呈显著或极显著指数正相关,其中与10 cm处土壤温度的相关性最高;同时,土壤CO2排放与0~5 cm土层微生物生物量碳含量及小麦地下生物量均呈显著线性相关;土壤水分对土壤CO2排放的影响居于次要地位。     

黄土区退耕草地合理放牧可减少土壤CO2排放和土壤侵蚀

【目的】在退耕草地实施合理放牧,有助于减少土壤CO2排放、 减缓土壤侵蚀。为验证此假设,本研究选择黄土高原渭北旱原坡地,建立退耕草地放牧、 退耕草地不放牧和传统农业耕作三种处理的对比试验小区,定量研究了退耕草地合理放牧相对于退耕草地在减少土壤CO2排放和土壤侵蚀的作用及其影响因素,为探寻在我国西部退耕还草区实施畜牧业生产与环境保护的协调发展模式提供科学依据。【方法】在建立的退耕草地放牧、 退耕草地不放牧和传统农业耕作3种处理的试验小区,利用LI-8100 碳通量自动测量仪原位监测植物生长期(4~8月)和放牧前后土壤CO2排放速率的变化,同时利用时域反射仪(TDR)测定表层0—10 cm土壤含水量,用地温表测定土壤表层2 cm和5 cm的温度。利用环境放射性核素 7Be示踪技术监测较大降雨事件引起的土壤侵蚀速率,同时取样测定侵蚀区土壤有机碳含量,比较不同处理小区侵蚀导致的土壤有机碳流失量。【结果】观测期间,3种处理CO2平均排放速率大小顺序为退耕草地[3.69±0.39 μmol/(m2·s)]退耕草地放牧[3.00±0.44 μmol/(m2·s)]传统农耕地[1.99±0.22 μmol/(m2·s)],坡耕地退耕还草后土壤CO2排放增加了85%,而合理放牧使退耕草地土壤CO2排放量减少了19%。放牧后退耕草地土壤CO2排放速率平均减少了11%,减少值在2% ~ 41%之间。观测期内,退耕草地放牧后土壤侵蚀速率比农耕地和退耕草地分别减少了93% 和77%。坡耕地退耕还草后土壤CO2排放增加主要由于草被植物引起土壤有机碳储量增加和土壤侵蚀强度减小,放牧后退耕草地土壤CO2排放减少主要与动物踩踏引起土壤容重明显增加及草类植被地上部分向土壤中输入的有机碳的减少有关。水分、 温度影响因子无法解释3种处理间土壤CO2排放差异。【结论】合理放牧不仅能显著减少退耕草地土壤CO2排放,而且可以有效控制退耕草地土壤和有机碳侵蚀流失。放牧期间动物的踩踏作用引起草地土壤容重显著增加是退耕草地土壤CO2排放量和土壤侵蚀速率减少的主要原因。本研究结果揭示,在我国黄土高原和类似的退耕还草地区实施合理放牧既可以促进当地畜牧业生产,又能控制土壤侵蚀和减少CO2的排放,是一种值得探究的草地可持续发展管理模式。     

人工林植被细根防蚀拦沙作用的有效性

选择我国西南山地典型人工林为对象,调查全山坡人工林的植被盖度及根系密度,利用137Cs示踪技术确定人工林坡地土壤侵蚀速率,探讨人工林坡地植被盖度及其细根与土壤侵蚀的关系.结果表明:整个人工林坡地表现为严重土壤侵蚀,侵蚀速率达25.25 t/(hm2·a).不同类型植被覆盖下土壤侵蚀再分布速率为裸地(-36.32 t/(hm2·a))＜乔木(-0.57 t/(hm2·a))＜灌木(0.53 t/(hm2·a))＜草(0.60t/(hm2·a)).回归分析表明,人工林坡地土壤侵蚀速率与植被盖度无显著相关性,土壤侵蚀速率随＜1mm的植被细根密度的增加而降低,二者之间呈显著负相关关系(P＜0.01),但与＞10 mm径级的植被粗根密度没有显著相关性.人工林地植被细根控制土壤侵蚀的有效性可以用植被细根密度与土壤再分布速率的关系方程来定量评价.当植被细根密度为60～100 g/m2时,可以看作人工林植被防蚀拦沙的有效根密度.结果表明,植被细根在控制坡地土壤侵蚀比植被盖度更为重要.因此,禁牧保护植被细根是控制人工林地土壤侵蚀与土地退化的有效措施.     

水稻秸秆施用后土壤溶解性有机碳的变化与土壤CO_2排放

为了进一步认识水稻秸秆还田后土壤有机碳固定机制,以浙江两种典型水稻土淡涂泥、青紫泥为研究对象,并以不加秸秆为对照,研究秸秆施用下土壤溶解性有机碳(Dissolved Organic Carbon,DOC)含量以及CO_2产生速率的动态变化,以及秸秆分解产生DOC在土壤中的吸附作用。结果表明,添加秸杆显著增加了土壤CO_2排放,总矿化碳量表现为10%>5%>2%>CK;土壤CO_2产生速率的变化与DOC含量变化具有高度一致性,都表现为先快速上升-急剧下降-缓慢下降-趋于稳定的变化模式,两者呈显著正相关(P<0.01),但在石英砂处理中两者则无明显相关性。DOC吸附实验表明随着秸秆分解时间延长,所产生的DOC在土壤中的吸附作用增强。pH和本底DOC较低的青紫泥对DOC具有更强的吸附作用,其CO_2排放速率和总矿化碳量均显著低于淡涂泥,青紫泥更有利于秸秆还田有机碳的保留和稳定。该结果表明秸秆分解前期产生DOC主要以CO_2形式释放,分解后期产生DOC更倾向于土壤有机碳固定,增加土壤对DOC的吸附作用有利于土壤固碳。     

等高绿篱--坡地农业复合系统土壤N2O排放特征

N2O是一种重要的温室气体, 具有很强的温室效应。当前全球变化条件下, 人类活动和农业生产行为产生的N₂O排放增加是当前倍受关注的问题。本研究于2008年11月-2009年10月, 利用静态箱 气相色谱技术对亚热带地区紫穗槐(Amorpha fruticosa L.)绿篱枝叶还田条件下冬小麦 夏玉米轮作田土壤N₂O排放通量进行原位监测, 观测紫穗槐枝叶移出(AR)、翻施(AI)、表施(AC)及作物单作(CK)4种处理下整个生长季土壤N₂O的排放量, 对等高绿篱 坡地农业复合生态系统土壤N₂O排放通量变化及其影响机制进行研究。结果表明, 整个冬小麦 夏玉米轮作期, 4个处理土壤N₂O排放通量呈现出相似的季节变化特征, AR、AI、AC、CK处理全生长季的排放总量为127.62 mg·m^-2、209.66 mg·m^-2、208.73 mg·m^-2、77.52 mg·m^-2。作物不同生育阶段N₂O日均排放通量在冬小麦季表现为: 开花-成熟期>拔节-开花期>出苗-拔节期; 在夏玉米季表现为: 拔节-抽雄期>播种-拔节期>抽雄-成熟期。本试验综合评估了等高绿篱 坡地农业复合生态系统土壤N2O排放通量变化及其影响机制。研究显示, 土壤N₂O排放通量在冬小麦季与土壤温度相关性显著, 在夏玉米季与土壤水分相关性显著。在复合生态系统中紫穗槐复合种植及枝叶还田显著促进土壤N2O排放, 翻施处理产生的N2O量大于表施处理。     

喀斯特地区土壤-洞穴CO_2时空迁移变化特征

为研究土壤-洞穴CO_2含量动态变化特征,选择织金洞及其岩溶表层3个样地进行监测。研究表明:土壤CO_2与洞穴CO_2变化规律具有一致性,均表现为春升、夏高、秋降,冬稳定的特征。夏季至秋季土壤-洞穴CO_2均呈上升趋势,土壤CO_2为0.13 mmol mol~(-1),洞穴CO_2为0.44 mmol mol~(-1),存在累积效应。织金洞为多进口洞穴,随着距洞口不断的深入,在灵霄殿至飞鸟觅食CO_2达到高值区为1.51 mmol mol~(-1),两端分别为:1.34 mmol mol~(-1)、1.05 mmol mol~(-1),呈现两端低中间高的分布趋势。织金洞上覆土壤CO_2含量是大气CO_2含量的22~27倍,是洞内CO_2含量的7~9倍。随着大气CO_2进入到土壤,土壤CO_2含量呈直线上升,直到60 cm处开始下降,经过基岩溶管、溶隙等进入洞穴内部。另一部分洞穴空气CO_2由大气降水-土壤水-洞穴水PCO_2转化而来,研究区为重碳酸盐钙型水和硫酸盐钙型水。研究发现:洞穴CO_2高值区自1月随时间变化分布特征为:中部→中部、前半段过渡带→前半段→前半段、中部过渡带→中部的迁移过程。这种新发现对喀斯特地区洞穴气候环境研究与洞穴开发保护具有重要的理论意义和实践指导价值。     

菜地土壤CO2与N2O排放特征及其规律

为了解不同集约化类型菜地土壤CO2和N2O排放特征及影响因子,选取京郊20年露地老菜地(OV20)、3年菜地种植历史的露地新菜地(OV3)、3年大棚菜地(GV3),以及相邻的当地典型粮田玉米地(Maize)4个类型地块,研究了春黄瓜生育期间土壤CO2和N2O排放特征及影响因子。结果表明:1)春黄瓜生育期间的土壤CO2排放通量主要受土壤5 cm处温度(指数关系)和土壤水分(对数关系或二次抛物线关系)影响;期间玉米地土壤CO2平均排放通量为(346.8±56.5)mg.m-2.h-1,20年露地菜地、3年露地菜地有机肥处理、3年露地菜地配施处理、3年大棚菜地的土壤CO2平均排放通量分别是玉米地的1.38、1.21、1.39和1.56倍。2)土壤N2O排放通量与施肥活动密切相关,排放高峰都出现在氮肥施用后,并受土壤温度和水分的影响。基肥后土壤温度低(15～20℃),排放峰出现在第5 d,排放峰持续时间(长达20 d)与施肥量相关;追肥后土壤温度高(>20℃),排放高峰发生早(追肥后第3 d),但因追肥用量低,因此持续时间短(仅一周)。3)黄瓜生长期内玉米地N2O累积排放量为N(1.95±0.10)kg.hm-2,20年老菜地、3年大棚菜地和3年新菜地N2O累积排放量分别是同期大田玉米地的1.67、1.95和1.99倍。4)本实验中春黄瓜生长季菜地土壤化肥氮N2O排放系数在1.86%～4.71%之间,显著高于IPCC旱地排放缺省值1%。其中,新菜地排放系数高于老菜地,设施菜地排放系数高于露地菜地;但有机肥氮的N2O排放系数则远远低于化肥氮的排放系数,仅为0.11%。     

红外加热对大豆生态系统CO_2排放的影响

通过田间试验,采用静态箱-气相色谱法测定CO2排放通量,研究红外加热增加叶面温度对土壤、大豆-土壤系统CO2排放的影响。结果表明,红外加热叶面增温2℃促进了土壤CO2的排放,在鼓粒-成熟期对照与增温的排放通量分别为202.09±28.75、378.34±156.17mg·m-2·h-1,增温处理使CO2排放通量增加了87.21%,但未达到显著水平;增温使土壤CO2累积排放量显著增加了39.96%。对照和增温的大豆-土壤系统呼吸的气温敏感性系数Q10值分别为0.68和2.54,土壤呼吸的土壤温度Q10值分别为4.22和1.68。研究表明,增温能促进土壤CO2排放,增加大豆-土壤系统呼吸的Q10值,降低土壤呼吸的Q10值。研究结果可为气候变化条件下估算区域农田温室气体排放提供一定的科学依据。     

保护性耕作对山地结球甘蓝生长和土壤CO_2含量的影响

为了解耕作措施对山地蔬菜种植的影响,改善山地土壤的生态环境,笔者通过田间小区试验研究了免耕及秸秆还田方式对山地土壤CO2含量及结球甘蓝的生长、发育和产量的影响。结果表明,秸秆覆盖和免耕相结合即有利于山地土壤呼吸产生CO2,又有利于山地蔬菜生长和产量、产值提高;免耕对土壤呼吸产生CO2影响不明显,但有利于蔬菜生长;深耕条件下增施秸秆,有利于土壤呼吸产生CO2,但对当季蔬菜的生长有一定的负面影响。     

黄土区土壤CO2释放及其影响因素研究

研究表明黄土区土壤CO2释放具有一定特殊性。从当日清晨至次日晨土壤CO2释放量呈由高至低再变高的规律,其变化趋势大体与温度变化一致,但时间上有一定滞后性。土壤CO2释放量有明显季节变化,夏季日释放量最高,秋季次之,冬季最低。不同覆被土壤CO2释放量存在差异,裸地释放量较高。CO2释放量对土质变化敏感,致密土壤则释放量小。     

Regional surface flux of CO2 inferred from changes in the advected CO2 column density

A Lagrangian experiment was conducted over Iowa during the daytime (9:00–17:30 LT) on June 19, 2007 as part of the North American Carbon Program's Mid-Continent Intensive using a light-weight and operationally flexible aircraft to measure a net drawdown of CO₂ concentration within the boundary layer. The drawdown can be related to net ecosystem exchange when anthropogenic emissions are estimated using a combination of the Vulcan fossil fuel emissions inventory coupled with a source contribution analysis using HYSPLIT. Results show a temporally and spatially averaged net CO₂ flux of −9.0 ± 2.4 μmol m⁻² s⁻¹ measured from the aircraft data. The average flux from anthropogenic emissions over the measurement area was 0.3 ± 0.1 μmol CO₂ m⁻² s⁻¹. Large-scale subsidence occurred during the experiment, entraining 1.0 ± 0.2 μmol CO₂ m⁻² s⁻¹ into the boundary layer. Thus, the CO₂ flux attributable to the vegetation and soils is −10.3 ± 2.4 μmol m⁻² s⁻¹. The magnitude of the calculated daytime biospheric flux is consistent with tower-based eddy covariance fluxes over corn and soybeans given existing land-use estimates for this agricultural region. Flux values are relatively insensitive to the choice of integration height above the boundary layer and emission footprint area. Flux uncertainties are relatively small compared to the biospheric fluxes, though the measurements were conducted at the height of the growing season.     

外加碳酸盐增加石灰性土壤密闭培养过程中CO2的释放量

The closed-jar incubation method is widely used to estimate the mineralization of soil organic C. There are two C pools (i.e., organic and inorganic C) in calcareous soil. To evaluate the effect of additional carbonates on CO2 emission from calcareous soil during closed-jar incubation, three incubation experiments were conducted by adding different types (CaCO3 and MgCO3 ) and amounts of carbonate to the soil. The addition of carbonates significantly increased CO2 emission from the soil; the increase ranged from 12.0% in the CaCO3 amended soil to 460% in the MgCO3 amended soil during a 100-d incubation. Cumulative CO2 production at the end of the incubation was three times greater in the MgCO3 amended soil compared to the CaCO3 amended one. The CO2 emission increased with the amount of CaCO3 added to the soil. In contrast, CO2 emission decreased as the amount of MgCO3 added to the soil increased. Our results confirmed that the closed-jar incubation method could lead to an overestimate of organic C mineralization in calcareous soils. Because of its effect on soil pH and the dissolution of carbonates, HgCl2 should not be used to sterilize calcareous soil if the experiment includes the measurement of soil CO2 production.     

Spatial and temporal variability of soil CO2 emission in a sugarcane area under green and slash-and-burn managements

Soil management causes changes in physical, chemical, and biological properties that consequently affect soil CO₂ emission (FCO2). Here, we studied the soil carbon dynamics in areas with sugarcane production in southern Brazil under two different sugarcane management systems: green (G), consisting of mechanized harvesting that produces a large amount of crop residues left on the soil surface, and slash-and-burn (SB), in which the residues are burned before manual harvest, leaving no residues on the soil surface. The study was conducted during the period after harvest in two side-by-side grids installed in adjacent areas, having 60 points each. The aim was to characterize the temporal and spatial variability of FCO2, and its relation to soil temperature and soil moisture, in a red latosol (Oxisol) where G and SB management systems have been recently used. Mean FCO2 emission was 39% higher in the SB plot (2.87 μmol m⁻² s⁻¹) when compared to the G plot (2.06 μmol m⁻² s⁻¹) throughout the 70-day period after harvest. A quadratic equation of emissions versus soil moisture was able to explain 73% and 50% of temporal variability of FCO2 in SB and G, respectively. This seems to relate to the sensitivity of FCO2 to precipitation events, which caused a significant increase in SB emissions but not in G-managed area emissions. FCO2 semivariogram models were mostly exponential in both areas, ranging from 72.6 to 73.8 m and 63.0 to 64.7 m for G and SB, respectively. These results indicate that the G management system results in more homogeneous FCO2 when spatial and temporal variability are considered. The spatial variability analysis of soil temperature and soil moisture indicates that those parameters do not adequately explain the changes in spatial variability of FCO2, but emission maps are clearly more homogeneous after a drought period when no rain has occurred, in both sites.     

Relationship between soil CO2 concentrations and forest-floor CO2 effluxes

To better understand the biotic and abiotic factors that control soil CO₂ efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO₂ effluxes and soil CO₂ concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island. We used small solid-state infrared CO₂ sensors for long-term continuous real-time measurement of CO₂ concentrations at different depths, and measured half-hourly soil CO₂ effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO₂ diffusivity in undisturbed soil cores. The method accounts for the CO₂ production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity by a power law function, which was independent of soil depth. CO₂ concentration at all depths increased with increase in soil temperature, likely due to a rise in CO₂ production, and with increase in soil water content due to decreased diffusivity or increased CO₂ production or both. It also increased with soil depth reaching almost 10 mmol mol⁻¹ at the 50-cm depth. Annually, soil CO₂ efflux was best described by an exponential function of soil temperature at the 5-cm depth, with the reference efflux at 10 °C (F₁₀) of 2.6 μmol m⁻² s⁻¹ and the Q₁₀ of 3.7. No evidence of displacement of CO₂-rich soil air with rain was observed.Effluxes calculated from soil CO₂ concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO₂ efflux originated in the top 20 cm soil. Calculated CO₂ production varied with soil temperature, soil water content and season, and when scaled to 10 °C also showed some diurnal variation. Soil CO₂ efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO₂ storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO₂ efflux was proportional to CO₂ concentration at the 50-cm depth with the slope determined by soil water content, which was consistent with a simple steady-state analytical model of diffusive transport of CO₂ in the soil. The latter proved successful in calculating effluxes during 2004.     

南亚热带果园土壤二氧化碳释放变异性研究

Temporal variability in soil CO2 emission from an orchard was measured using a dynamic open-chamber system for measuring soil CO2 effiux in Heshan Guangdong Province, in the lower subtropical area of China. Intensive measurements were conducted for a period of 12 months. Soil CO2 emissions were also modeled by multiple regression analysis from daily air temperature, dry-bulb saturated vapor pressure, relative humidity, atmospheric pressure, soil moisture, and soil temperature. Data was analyzed based on soil moisture levels and air temperature with annual data being grouped into either hot-humid season or relatively cool season based on the precipitation patterns. This was essential in order to acquire simplified exponential models for parameter estimation. Minimum and maximum daily mean soil CO2 effiux rates were observed in November and July, with respective rates of 1.98 ± 0.66 and 11.04 ± 0.96 μmol m^-2 s^-1 being recorded. Annual average soil CO2 emission （FCO2） was 5.92 μmol m^-2 s^-1. Including all the weather variables into the model helped to explain 73.9% of temporal variability in soil CO2 emission during the measurement period. Soil CO2 effiux increased with increasing soil temperature and soil moisture. Preliminary results showed that Q10, which is defined as the difference in respiration rates over a 10 ℃ interval, was partly explained by fine root biomass. Soil temperature and soil moisture were the dominant factors controlling soil CO2 effiux and were regarded as the driving variables for CO2 production in the soil. Including these two variables in regression models could provide a useful tool for predicting the variation of CO2 emission in the commercial forest Soils of South China .     

Linking N2O emission to soil mineral N as estimated by CO2 emission and soil C/N ratio

Based on the N₂O and CO₂ emission data concomitantly measured from agricultural upland fields around the world, we developed an empirical model as follows: cumulative N₂O emission = aexp[b*(E_CO2/S_cn + F_n)] (R²_adj = 0.85∼0.87), where E_CO2 is the rate of heterotrophic respiration from soils, S_cn is the soil C/N ratio, and F_n is the chemical fertilizer N rate. The model parameters derived from the data from the soils without receiving chemical fertilizers were significantly different from the ones from the fertilized soils. This model indicates that CO₂ emission and soil C/N ratio can be used as scaling parameters to produce regional or global inventories of N₂O emission from agricultural soils.     

长期施肥下红壤旱地土壤CO2排放及碳平衡特征

在国家肥力网红壤旱地长期定位试验地上,采用静态箱/气相色谱法测定土壤CO2排放速率,同时利用根去除法区分根系对土壤呼吸的贡献,通过计算净生态系统生产力（NEP）,判断长期不同施肥下红壤旱地农田碳汇强度。结果表明,小麦、玉米生长季各处理的土壤和土体呼吸速率随着作物生长、温度升高均呈现明显的季节变化规律;玉米生长季土壤和土体累积呼吸量大于小麦生长季,小麦、玉米生长季均以NPKM处理土壤和土体呼吸累积呼吸量最大,且显著高于其它处理（P0.05）,NP和NPK处理次之,CK和NK处理最小（P0.05）;小麦、玉米生长季各处理根际呼吸占土壤呼吸的比例分别为7.6 %～17.4 %、4.7%～16.6 %,均以NPKM处理根际呼吸贡献率最大;小麦季NPKM处理、玉米季CK和NPKM处理的NEP值为负,是大气CO2的汇,且NPKM处理的净初级生产力与土壤呼吸的比值（NPP/Rs）最大,其它处理NEP值均为正,是大气CO2的源。有机无机肥配施（NPKM）相比其它处理具有较强的碳汇功能,是红壤旱地比较合理的施肥措施。     

FACE facility CO2 concentration control and CO2 use in 1990 and 1991

CO₂ treatment level control and CO₂ use are reported for free-air carbon dioxide enrichment (FACE) facility operations at the University of Arizona's Maricopa Agricultural Center in 1990 and 1991. These are required for evaluation of the validity of biological experiments conducted in four replicates of paired experimental and control plots in a large cotton field and the cost-effectiveness of the plant fumigation facility. Gas concentration was controlled to 550 γmol mol^-1 at the center of each experimental plot, just above the canopy. In both years, season-long (April–September) average CO₂ levels during treatment hours (05:00–19:00 h Mountain Standard Time) were 550 γmol mol⁻¹ measured at treatment plot centers when the facility was operating. Including downtime, the season average was 548 γmol mol⁻¹ in 1991. In 1990, the season averages for the four elevated CO₂ treatments varied from 522 to 544 γmol mol⁻¹, owing to extended periods of downtime after lightning damage. Ambient CO₂ concentration during treatment was 370 γmol mol⁻¹. Instantaneous measurements of CO₂ concentration were within 10% of the target concentration of 550 γmol mol⁻¹ more than 65% of the time when the facility was operating, and 1 min averages were within 10% of the target concentration for 90% of the time. The long-term average of CO₂ concentration measured over the 20 m diameter experimental area of one array at the height of the canopy was in the range 550–580 γmol mol⁻¹ during July 1991, with the higher values near the edges. In 1991, CO₂ demand averaged 1250 kg per array per 14 h treatment day, or 4 kg m⁻² of fumigated plant canopy. The FACE facility provided good temporal and spatial control of CO₂ concentration and was a cost-effective method for large-scale field evaluations of the biological effects of CO₂.     

A new technique to measure soil CO2 efflux at constant CO2 concentration

A new principle for measuring soil CO₂ efflux at constant ambient concentration is introduced. The measuring principle relies on the continuous absorption of CO₂ within the system to achieve a constant CO₂ concentration inside the soil chamber at ambient level, thus balancing the amount of CO₂ entering the soil chamber by diffusion from the soil. We report results that show reliable soil CO₂ efflux measurements with the new system. The novel measuring principle does not disturb the natural gradient of CO₂ within the soil, while allowing for continuous capture of the CO₂ released from the soil. It therefore holds great potential for application in simultaneous measurements of soil CO₂ efflux and its δ¹³C, since both variables show sensitivity to a distortion of the soil CO₂ profile commonly found in conventional chamber techniques.