旱地长期定位施肥对土壤剖面硝态氮分布与累积的影响

在 １５ ａ长期定位试验基础上研究了黄土高原旱地长期施用不同用量和配比的氮、磷肥对土壤剖面中硝态氮分布和累积的影响。结果表明：长期大量施用氮肥，在土壤剖面 １００～１８０ ｃｍ之间形成硝态氮累积层，峰值出现在 １４０ ｃｍ处，最大值为 ６７．９２ｍｇ／ｋｇ（单施 Ｎ １８０ ｋｇ／ｈｍ~２）； 配合施用磷肥可以降低土壤剖面硝态氮质量分数，根据试验，提出了旱地合理施肥的氮磷肥用量。     

灌淤土施氮后土壤硝态氮的动态变化

通过室内模拟试验,探讨不同水分条件下灌淤土施用不同氮肥后硝态氮随时间的变化规律,并在田间条件下,测定不同氮肥形态和数量对土壤硝态氮含量的影响。灌淤土施氮后土壤硝态氮含量变化与土壤含水量及氮肥种类有关。施肥9d后,土壤中的硝态氮迅速增加;土壤水分低于田间持水量的50%或水分过饱和都明显影响灌淤土的硝化作用;施用大颗粒尿素产生的硝态氮最少,淋失或流失的几率小。灌淤土土体中硝态氮的残留与施氮种类及数量有直接关系。施肥使土壤表层硝态氮显著增加,施用大颗粒尿素尤为突出,但施大颗粒尿素后,60cm土体中残留的硝态氮总量最少。随着施氮量增加,表层土体中硝态氮含量增加。合理的施肥水平一般不会造成硝态氮的大量累积,而过量施氮则导致硝态氮明显积累,对通气透水性好的灌淤土,容易造成硝态氮淋失。     

灌淤土施氮后土壤硝态氮的动态变化

通过室内模拟试验，探讨不同水分条件下灌淤土施用不同氮肥后硝态氮随时间的变化规律，并在田间条件下，测定不同氮肥形态和数量对土壤硝态氮含量的影响。灌淤土施氮后土壤硝态氮含量变化与土壤含水量及氮肥种类有关。施肥9d后，土壤中的硝态氮迅速增加；土壤水分低于田间持水量的50％或水分过饱和都明显影响灌淤土的硝化作用；施用大颗粒尿素产生的硝态氮最少，淋失或流失的几率小。灌淤土土体中硝态氮的残留与施氮种类及数量有直接关系。施肥使土壤表层硝态氮显著增加，施用大颗粒尿素尤为突出，但施大颗粒尿素后，60cm土体中残留的硝态氮总量最少。随着施氮量增加，表层土体中硝态氮含量增加。合理的施肥水平一般不会造成硝态氮的大量累积，而过量施氮则导致硝态氮明显积累，对通气透水性好的灌淤土，容易造成硝态氮淋失。     

施氮对植物生长、硝态氮累积及土壤硝态氮残留的影响

施用N肥可提高叶类蔬菜的产量,但施用过量也会造成土壤N污染。为了研究当年施肥量对作物硝态氮累积及土壤无机氮残留量的影响,采用土壤盆栽试验,研究了5个施氮水平下3种叶类蔬菜(油菜、小白菜、菠菜)的生长、硝态氮累积和土壤硝态氮残留的变化。结果表明,施氮过高抑制了植物的生长,其中对菠菜的抑制作用最强。3种蔬菜硝态氮累积对施N的反应不同,油菜在施N0.40g时硝态氮含量达到最高,为N1742.2μg·g-1FW;小白菜在0.60g最高,为N1635.6μg·g-1FW;而菠菜则在0.80g最高,为N865.2μg·g-1FW。施N量与土壤硝态氮残留量之间呈显著正相关关系,说明施氮量越高土壤中硝态氮的残余就越大,对土壤的污染就越严重。     

施肥对华北高产区土壤NO-3-N淋失与作物NO-3-N含量及产量的影响

利用4a的平衡施肥定位试验，研究太行山山前平原高产区冬小麦、夏玉米轮作制度下施肥对潮褐土中硝态氮的分布、移动、积累、植株吸收以及作物产量的影响。结果表明，土壤剖面中硝态氮含量与施肥量直接相关，过量施用氮肥使硝态氮在土壤中大量积累并向下层快速移动；氮磷对作物的养分供应存在着既相互促进又相互竞争的关系，施用适量磷肥可以促进小麦、玉米对氮素的吸收，提高作物产量，减少氮素在土壤中积累和淋失，但施磷量太高，由于氮磷之间的竞争作用，作物吸氮量反而下降，从而导致土壤中硝态氮的积累和淋失加剧，施用钾肥抑制了土壤硝态氮积累，促进了两季作物植株对氮素的吸收，从减少土壤硝态氮积累和淋失的角度，提出该区合理的施肥配比为组合N2P2K2，即ρA(N)=200kg hm^-2，ρA(P)=32．5kg hm^-2，ρA(K)=150kg hm^-2。图6参13。     

不同有机肥对矿区复垦土壤氮素矿化的影响

为揭示同一施氮水平下不同有机肥对矿区复垦土壤的氮素矿化过程的影响,以山西省孝义市水峪煤矿采煤塌陷复垦土壤为研究对象,在200 mg/kg施氮水平下(以全氮计),施用鸡粪、猪粪和牛粪,并以不施肥处理为空白对照,在培养温度为20℃和30℃下,进行为期161 d的室内恒温培养试验,分析不同有机肥对矿区复垦土壤氮素矿化的影响,从而为矿区复垦土壤的快速培肥熟化提供理论科学依据.结果表明:(1)在整个培养时期,各施肥处理的硝态氮含量、硝态氮累积量以及累积矿化量总体呈上升趋势,尤其在培养56-84 d迅速增加.在培养结束时(161 d),不同施肥处理的硝态氮含量、硝态氮累积量以及累积矿化量总体表现为鸡粪处理猪粪处理牛粪处理空白处理(P 0.05).(2)培养初始(0 d),不同施肥处理的铵态氮含量均最高,其中鸡粪、猪粪和牛粪处理之间差异不显著,但三者均显著高于空白处理(P 0.05).各施肥处理的铵态氮含量在培养0-14 d随着培养时间延长迅速降低,在14-161 d各施肥处理的铵态氮含量基本保持不变,维持在较低水平,培养结束时各处理铵态氮含量均低于1.71 mg/kg.(3)随温度升高,各施肥处理的土壤硝态氮含量、硝态氮累积量和累积矿化量增加,不同温度对各施肥处理的土壤硝态氮含量影响显著(P 0.05),但不同温度对各施肥处理的土壤硝态氮累积量和累积矿化量影响不显著.总的来说,有机肥的施入促进了矿区复垦土壤氮素矿化,提高了土壤氮素有效性,施用鸡粪对提高土壤有效氮效果最好,不同施肥处理对矿区土壤矿化效果总体表现为鸡粪处理猪粪处理牛粪处理空白处理,并且30℃下各有机肥的矿化效果优于20℃.(图2表5参50)     

不同施氮水平对玉米产量、氮素利用效率及土壤无机氮含量的影响

通过研究不同施氮水平对玉米产量、氮素利用率及土壤硝态氮(NO3--N)和铵态氮(NH4+-N)残留量的影响,为氮肥的合理利用提供依据.在黑龙江省农业科学院科技园区布置田间小区试验,结果显示:玉米产量随施氮量增加而增加,施氮量为165 kg·hm-2时,氮肥利用率最高,当施氮量高于165 kg·hm-2,产量反而有降低的趋势,过量施氮也并不能增加玉米对氮素的吸收,因而氮素利用率也随施氮量的增加而降低.玉米收获后土壤剖面无机态氮质量分数的变化因施氮量的不同而表现出差异,0~80 cm土层硝态氮积累量随氮肥输入量的增加而显著增加,以表层(0~40 cm)硝态氮质量分数最高,中间层(60~80 cm)质量分数最低,100 cm以下土层以施氮量为165 kg·hm-2的处理土壤硝态氮积累量最低,降低了硝态氮淋溶风险;铵态氮的质量分数相对较低,不同的施氮量对土壤铵态氮质量分数的影响主要在0~20 cm土层,铵态氮质量分数与施氮量并无显著的相关关系.综合考虑玉米产量、氮素利用率与生态环境效益,以165 kg·hm-2(优化施氮量)为最佳氮肥施用量.     

长期施肥对河西灌漠土有机氮组分的影响

在长期小麦和玉米轮作的基础上,通过单施化肥以及化肥与其它有机肥料的配合使用,研究不同施肥对河西灌漠土有机氮组分的影响.采用1965年Bremner提出的土壤有机氮分级方法,对22年有机肥与化肥定位试验中的耕层土壤有机氮组成进行分级.结果表明,长期施用化肥和有机肥对河西灌漠土全氮和土壤有机氮组成有显著影响,施肥对不同形态有机氮的影响取决于肥料种类.与单施化肥相比,有机肥与化肥配合施用后显著提高了土壤全氮、酸解性氮与非酸解性氮的含量.在酸解性氮中,氨基酸态氮和酸解未知氮显著增加,氨基糖态氮含量也有所提高.与不施肥相比,施用氮肥显著提高了土壤全氮、酸解性氮与非酸解性氮的含量.在酸解性氮中,化学氮肥显著提高了铵态氮的含量,对氨基酸氮、氨基糖氮和酸解未知态氮影响较小.施用磷、钾肥对河西灌漠土全氮和有机氮各组分含量影响较小.     

上海市蔬菜地土壤硝态氮状况研究

以上海市郊不同管理方式下菜地表层土壤采样测定土壤硝态氮含量为基础,并以水稻土等土壤作为对照,以期了解上海蔬菜地土壤硝态氮的现状,为菜地的合理施肥提出科学依据。结果表明,由于管理方式不同,土壤的硝态氮的NO3--N质量分数差异明显。大棚蔬菜地土壤中NO3--N明显高于其它其他用地管理方式下的土壤,依次为:w(大棚蔬菜地)>w(露天蔬菜地)>w(传统自留地),而且土壤硝态氮的积累是全剖面性的,而非仅在表层,如在80~100cm土层,大棚土壤硝态氮也为农田的好几倍。而且,大棚蔬菜地土壤盐渍化明显,主要特点之一是硝态氮积累,盐分高的土壤一般硝态氮也高。此外,长期大量的N肥投入引起了土壤酸化。土壤pH与土壤NO3--N质量分数呈线性负相关,经统计检验相关性达极显著水平。     

小麦/玉米和蚕豆/玉米间作对土壤硝态氮累积和氮素利用效率的影响

提高氮肥利用效率,减少氮肥过量施用对环境造成的污染,是保障国家粮食生产和生态环境安全的关键.文章以甘肃河西灌区为试验地点,在0、300、450 kg·hm-2氮水平下,探讨了小麦(Triticum aestivum L.)/玉米(Zea mays L.)和蚕豆(Vicia faba L.)/玉米间作对土壤硝态氮累积和分布的影响.结果表明:随氮肥用量增加,0～60 cm土层土壤硝态氮相对累积量增加,100～200 cm土层降低.在3个氮水平下,蚕豆间作土壤硝态氮含量和累积量都低于单作.小麦间作在300 kg·hm-2氮水平下土壤硝态氮累积量都低于单作,但在0和450 kg·hm-2氮水平下,小麦收获后间作与单作近似,玉米收获后小麦间作土壤硝态氮累积量都低于单作.与小麦和蚕豆间作的玉米在300 kg·hm-2氮水平下土壤硝态氮累积量都低于单作玉米,在0和450 kg·hm-2氮水平下则表现不同.在300 kg·hm-2氮水平下,两种间作氮肥当季利用率都明显高于相应的单作,施氮量增加到450 kg·hm-2时,则表现不一样.     

不同类型土壤镉含量对香芋镉吸收的影响

为探明香芋(Colocasia esculenta)镉(Cd)超标的关键原因,在湖南临武香芋主产区分别选择紫色页岩和第四纪红壤发育的Cd污染水稻土,研究土壤Cd含量、Cd活性及土壤基本理化性质对香芋Cd吸收的影响,以期为香芋的安全种植提供技术支撑。结果表明,两个试验点所有香芋Cd质量分数皆超过食品安全国家标准(0.1 mg·kg^-1),且各试验点内田块间土壤Cd含量、pH差异显著,而土壤有机质和阳离子交换量变异较小;紫色页岩发育的中碱性水稻土壤Cd活性、Cd在土壤-香芋系统中的转移系数(TFstem/taro、TFtaro/soil)皆低于第四纪红壤发育的酸性水稻土;紫色页岩发育的中碱性水稻土中,香芋Cd含量主要受土壤全Cd、TFtaro/soil的正调控;而第四纪红壤发育的酸性水稻土壤中,香芋Cd含量既受土壤全Cd、DTPA-Cd、TFtaro/soil的正调控,还受土壤pH的负调控。为实现香芋的安全生产,紫色页岩发育的中碱性水稻土中,应优先选择Cd吸收能力较低的香芋品种,或选择土壤全Cd低于0.65 mg·kg^-1的土壤进行种植;而在第四纪红壤发育的酸性水稻土壤中,则可优先选择Cd吸收能力较低的香芋品种,或选择土壤全Cd含量低于0.41 mg·kg^-1的土壤,或调理土壤pH至6.26以上,以及钝化土壤DTPA-Cd至0.19 mg·kg^-1以下的技术措施,抑制土壤Cd向香芋的迁移转运。     

Effect of nitrogen fertilization on caffeine production in coffee (Coffea arabica)

Nitrogen (N) based secondary metabolite production is thought to be costly to plants because N is required for growth, as well as, the synthesis of these compounds. Therefore, variation in N availability may result in variation in N-based secondary metabolite production. Here, we determine the effect of N fertilization on caffeine (N-based alkaloid) production in coffee (Coffea arabica) seedlings. A growth chamber experiment was performed with three N treatments applied to seedlings. N fertilization increased plant growth, leaf biomass, and plant N. Caffeine concentration in phloem exudates was greater in high-N fertilized plants relative to intermediate- and low-N plants. However, leaf, stem, root, and total overall caffeine concentration and content did not differ across N treatments. These results suggest caffeine in coffee is strongly regulated by genetic factors, and environment is likely less important to caffeine phenotype. This is among the first studies to investigate the effect of N fertilization on caffeine within the phloem, which has important implications for herbivores that are sensitive to caffeine and plant N and feed from the phloem of coffee.     

Nitrogen effects on decomposition: a five-year experiment in eight temperate sites

The influence of inorganic nitrogen (N) inputs on decomposition is poorly understood. Some prior studies suggest that N may reduce the decomposition of substrates with high concentrations of lignin via inhibitory effects on the activity of lignin-degrading enzymes, although such inhibition has not always been demonstrated. I studied the effects of N addition on decomposition of seven substrates ranging in initial lignin concentrations (from 7.4% to 25.6%) over five years in eight different grassland and forest sites in central Minnesota, USA. I predicted that N would stimulate the decomposition of lignin-poor substrates but retard the decomposition of lignin-rich substrates. Across these sites, N had neutral or negative effects on decomposition rates. However, in contrast to my hypothesis, effects of N on decomposition were independent of substrate initial lignin concentrations, and decomposition of the lignin fraction was unaffected by N fertilization. Rather, substrate-site combinations that exhibited more rapid decomposition rates in the control treatment were affected more negatively by addition of N fertilization. Taken together, these results suggest that decreased decomposition with added N did not result from inhibition of lignin-degrading enzyme activity, but may have resulted from abiotic interactions between N fertilizer and products of microbial degradation or synthesis or from N effects on the decomposer community. Low initial substrate N concentrations and N fertilization both stimulated N immobilization, but the differences among substrates were generally much larger than the effects of fertilization. This study suggests that atmospheric N addition could stimulate ecosystem carbon sequestration in some ecosystems as a result of reduced rates of forest floor decomposition.     

贵州麻江马尾松天然中龄林施肥研究

采用正交设计法对贵州麻江马尾松天然中龄林进行施肥试验，结果表明，施肥对林木生长具有良好的效果．至第７年，单株材积平均为对照的１３２．３％；所施元素中Ｎ、Ｐ、Ｋ对单株材积均有很大影响，且都达到０．１以上显著水平；而施Ｐ的自第４年至第７年均有良好的反应，其中以Ｐ２的效果最好；施肥的最优化组合为Ｎ２Ｐ２Ｋ２．     

施氮水平对小麦-玉米轮作体系氨挥发与氧化亚氮排放的影响

试验采用密闭室间歇通气法研究华北平原冬小麦（Triticum aestivum Linneaus）-夏玉米（Zea mays Linneaus）轮作体系在不同施氮水平下农田氨挥发和N2O排放。结果表明：冬小麦季和夏玉米季的氨挥发速率与N2O排放通量呈现出季节性动态变化,且随着施氮量的增加而增加,均在施肥后第2~3天出现峰值。玉米季氨挥发量和N2O排放量均高于小麦季,分别占整个轮作周期气态损失的53.5%~68.9%和54.7%~82.3%。轮作体系中氨挥发净损失量（以N计）为4.28~31.31 kg.hm-2,占施氮量的3.17%~5.80%;N2O净排放损失量（以N计）为50.95~1051.03 g.hm-2,占施氮量的比例为0.04%~0.23%。因此,施氮量是影响冬小麦-夏玉米轮作体系气态损失重要因素,且夏玉米季是控制控制气态损失的关键时期。     

不同施肥处理对红壤晚稻田CH_4排放的影响

选取不同施肥处理的双季稻田为研究对象,采用静态箱-气相色谱法对晚稻田CH4排放通量进行观测。结果表明,与不施肥对照(T1)相比,各施肥处理CH4排放通量均有不同程度增加。其中秸秆还田+化肥处理(T5)CH4平均排放通量为9.96mg.m-2.h-1,比增氮磷施肥处理(T4)和对照分别增加26.1%和120.0%;平衡施肥处理(T2)和减氮磷施肥处理(T3)CH4平均排放通量比对照增加20%左右。说明施化肥可能提高水稻植株运输能力,进而增加CH4排放,但并未发现施化肥处理(T1、T2、T3和T4)之间CH4排放存在显著差异。同时对相关环境因素的分析表明,各处理CH4排放通量与土壤5cm深处温度间存在指数函数关系,并与田间水层厚度呈正相关关系(P<0.05)。综合考虑温室效应和稻谷产量,认为T2为推荐施肥方式,即N、P2O5和K2O施用量分别为180、90和135kg.hm-2,在插秧前1d施入占总N量70%的碳铵和全部磷肥、钾肥(过磷酸钙和氯化钾)作为基肥,并在分蘖期(2008年7月19日)追施占总N量30%的尿素。     

15N-Nitrate signature in low-order streams: effects of land cover and agricultural practices.

Many studies have shown that intensive agricultural practices significantly increase the nitrogen concentration of stream surface waters, but it remains difficult to identify, quantify, and differentiate between terrestrial and in-stream sources or sinks of nitrogen, and rates of transformation. In this study we used the delta15N-NO3 signature in a watershed dominated by agriculture as an integrating marker to trace (1) the effects of the land cover and agricultural practices on stream-water N concentration in the upstream area of the hydrographic network, (2) influence of the in-stream processes on the NO3-N loads at the reach scale (100 m and 1000 m long), and (3) changes in delta15N-NO3 signature with increasing stream order (from first to third order). This study suggests that land cover and fertilization practices were the major determinants of delta15N-NO3 signature in first-order streams. NO3-N loads and delta15N-NO3 signature increased with fertilization intensity. Small changes in delta15N-NO3 signature and minor inputs of groundwater were observed along both types of reaches, suggesting the NO3-N load was slightly influenced by in-stream processes. The variability of NO3-N concentrations and delta15N signature decreased with increasing stream order, and the delta15N signature was positively correlated with watershed areas devoted to crops, supporting a dominant effect of agriculture compared to the effect of in-stream N processing. Consequently, land cover and fertilization practices are integrated in the natural isotopic signal at the third-order stream scale. The GIS analysis of the land cover coupled with natural-abundance isotope signature (delta15N) represents a potential tool to evaluate the effects of agricultural practices in rural catchments and the consequences of future changes in management policies at the regional scale.     

N fertilization effects on denitrification and N cycling in an aggrading forest.

We investigated N cycling and denitrification rates following five years of N and dolomite amendments to whole-tree harvested forest plots at the long-term soil productivity experiment in the Fernow Experimental Forest in West Virginia, USA. We hypothesized that changes in soil chemistry and nutrient cycling induced by N fertilization would increase denitrification rates and the N2O:N2 ratio. Soils from the fertilized plots had a lower pH (2.96) than control plots (3.22) and plots that received fertilizer and dolomite (3.41). There were no significant differences in soil %C or %N between treatments. Chloroform-labile microbial biomass carbon was lower in fertilized plots compared to control plots, though this trend was not significant. Extractable soil NO3- was elevated in fertilized plots on each sample date. Soil-extractable NH4+, NO3-, pH, microbial biomass carbon, and %C varied significantly by sample date suggesting important seasonal patterns in soil chemistry and N cycling. In particular, the steep decline in extractable NH4+ during the growing season is consistent with the high N demands of a regenerating forest. Net N mineralization and nitrification also varied by date but were not affected by the fertilization and dolomite treatments. In a laboratory experiment, denitrification was stimulated by NO3- additions in soils collected from all field plots, but this effect was stronger in soils from the unfertilized control plots, suggesting that chronic N fertilization has partially alleviated a NO3- limitation on denitrification rates. Dextrose stimulated denitrification only in the whole-tree-harvest soils. Denitrification enzyme activity varied by sample date and was elevated in fertilized plots for soil collected in July 2000 and June 2001. There were no detectable treatment effects on N2O or N2 flux from soils under anaerobic conditions, though there was strong temporal variation. These results suggest that whole-tree harvesting has altered the N status of these soils so they are less prone to N saturation than more mature forests. It is likely that N losses associated with the initial harvest and high N demand by aggrading vegetation is minimizing, at least temporarily, the amount of inorganic N available for nitrification and denitrification, even in the fertilized plots in this experiment.     

Sinks for nitrogen inputs in terrestrial ecosystems: a meta-analysis of 15N tracer field studies

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (< 1 week after 15N tracer application), total ecosystem 15N recovery was negatively correlated with fine-root and soil 15N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15N tracer application), total ecosystem 15N retention was negatively correlated with foliar natural-abundance 15N but was positively correlated with mineral soil C and N concentration and C:N, showing that plant and soil natural-abundance 15N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15N tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg N x ha(-1) x yr(-1) above which most ecosystems showed net losses of applied 15N tracer in response to N fertilizer addition.     

长期施肥对水稻土酶活性及理化特性的影响

为了明确不同施肥种类对水稻土土壤肥力与酶活性的影响,以30年（1981年至今）长期定位试验地为基础,研究不同施肥处理（CK,N,P,K,NPK,2倍NPK,NPK＋猪粪）水稻田耕层（0-20cm）土壤酶活性与养分的变化趋势及其相关性。结果表明：1）施P（配施或单施）增加土壤全P、速效P含量,平衡施肥配施有机肥（NPK＋猪粪处理）土壤的有机质、全P、速效P、全N、速效N均显著增加,所有施肥处理间pH值差异均不显著;2）除了NPK处理土壤脲酶活性最高外,平衡施肥配施有机肥土壤转化酶、酸性磷酸酶、过氧化氢酶活性均较其他处理显著增加;3）酸性磷酸酶活性与土壤全P量呈负相关,过氧化氢酶、脲酶、转化酶与全N、有机质、速效N、速效P呈显著或极显著正相关;因此,水田土壤长期平衡施肥配施有机肥,能显著提高土壤肥力,增加土壤酶活性,有益于土壤生产力的持续提高。