Changes in the soil organic N fractions of a tropical Alfisol fertilized with 15N-urea and cropped to maize or pasture

Qualitative and quantitative changes in soil and fertilizer-derived organic N fractions were assessed during a cropping season in an intertropical Alfisol, under maize and pasture, fertilized with¹⁵ N-urea. Before the sowing, after fertilizing and after the harvest, the organic N of top soil samples was fractionated by a two-step acid hydrolysis under reflux (H1 = 1 M HCl for 3 h; H2 = 3 M HCl for 3 h). The total hydrolysable N (HN) from H1 decreased significantly during the cropping season in both maize and pasture soils. Contrastingly, the content of HN from H2 and that of non-hydrolysable N did not vary significantly during the cropping season. The easily hydrolysable fractions, especially amino acid N, amino sugar N and amide N, were the most active N pools and the major source of N potentially available for plants. The urea-derived N that remained in the soil was mainly in organic forms at both 7 and 108 d after fertilizing (70–82% and 93–98%, respectively), higher figures being found in pasture than in maize soil. The total amount of urea-derived HN decreased significantly during the crop period in both maize and pasture soils. This decrease was largely due to the decline in HN from H1. The amount of non-hydrolysable urea-derived N was significantly higher in pasture than in maize soil and it decreases in the former and increases in the latter, during the cropping season. During the crop period, the decrease of urea-derived organic N was 4.6 to 9.1 times higher than that of native organic N. Shortly after fertilizing, the proportion of urea-derived N in the easily hydrolysable (H1) organic fractions was higher than that of soil N, whereas the reverse was true for the slowly hydrolysable (H2) or insoluble fractions. These differences were less marked, but still significant, at the end of cropping. The easily hydrolysable organic N fractions were more sensitive than total N to the impact of land use intensification and are, therefore, a more useful index for early detection of soil biological degradation.     

长期不同施肥模式对双季稻田土壤酸解有机氮组分的影响

为明确南方双季稻区长期不同施肥模式对稻田不同耕层(0～10和10～20 cm)土壤酸解有机氮组分及其含量的影响,本研究以长期(36年)定位施肥试验田为平台,系统分析了单独施用化肥(CF)、秸秆还田+化肥(RF)、30%有机肥+70%化肥(OM)和不施肥对照(CK)下双季稻田0～10和10～20 cm土壤酸解有机氮及其组分氨基酸氮、氨基糖氮、铵态氮和酸解未知氮含量的变化特征及其与土壤全氮、有机碳含量的关系。结果表明: 与CK相比,OM和RF处理均显著增加了双季稻田0～10和10～20 cm土壤全氮、碱解氮和有机碳含量。OM、RF和CF处理双季稻田0～10和10～20 cm土壤酸解有机氮含量均显著高于CK,分别比CK增加10.7%～42.6%和12.2%～51.5%。与CF和CK相比,OM和RF处理显著提高了双季稻田0～10和10～20 cm土壤氨基酸氮、铵态氮、酸解未知氮和氨基糖氮含量。不同施肥处理稻田0～10和10～20 cm土壤酸解有机氮和非酸解性氮含量大小顺序均表现为OM>RF>CF>CK。各施肥处理稻田0～10 cm土壤氨基酸氮、铵态氮、氨基糖氮和非酸解性氮含量均高于10～20 cm土壤。土壤酸解有机氮含量与土壤全氮、有机碳含量均存在极显著的正相关关系。综上,RF和OM处理有利于增加双季稻田0～10和10～20 cm土壤有机氮含量,增强稻田土壤供氮能力,提高土壤肥力。     

土壤有机氮组分的年际变化及其对秸秆还田的响应

阐明土壤有机氮组分的年际变化特征及其对秸秆还田的响应对合理调控土壤有机氮库和土壤可持续利用具有重要意义。在沈阳农田生态系统国家野外科学观测研究站进行田间微区试验(土壤类型为潮棕壤),设置单施氮肥(200 kg N·hm^-2,下同)、50%秸秆还田配施氮肥和100%秸秆还田配施氮肥3个处理,采用Bremner酸水解法对试验第1、3、6、9年的土壤有机氮组分进行分级。结果表明: 氨基酸态氮含量随着耕作年限的增加逐渐提升,提升幅度为39.8%;酸解未知态氮含量提升幅度为10.8%,且在第3年时最高;土壤总氮和其他有机氮组分含量随耕作年限变化不大。相对容易矿化的酸解总氮占土壤总氮的比例随耕作年限的增加逐渐增加,比较稳定的未酸解态氮占土壤总氮的比例随耕作年限的增加逐渐下降,说明随着耕作年限的增加土壤氮素有效性提高,土壤供氮能力增强。与单施氮肥相比,加入秸秆提高了土壤总氮和各酸解态氮含量,秸秆还田量越多,提升效果越明显。秸秆还田对酸解态氮组分的影响主要发生在试验第6、9年,增加的土壤总氮主要为氨基酸态氮和酸解未知态氮,从而提高了土壤中酸解态氮占土壤总氮的比例。秸秆还田能够提升土壤氮库容,提高土壤保氮供氮能力。     

The effect of cropping and fertiliser nitrogen rates on nitrogen balance in soil

Summary Fertilizer/soil N balance of cropped and fallow soil has been studied in a pot experiment carried out with grey forest soil (southern part of Moscow region) at increasing rates of¹⁵N labelled ammonium sulfate (0; 8; 16; 32 mg N/100 g of soil). The fertilizer¹⁵N balance has been shown to depend upon its application rate and the presence of growing plants. Fertilizer N uptake efficiency was maximum (72.5%) and gaseous losses-minimum (12.5%) at the application rate of 16 mg N/100 g of soil. Fertilizer N losses from the fallow soil were 130–220% versus those from the cropped soil. At the application of fertilizer N the plant uptake of soil N was 170–240% and the amount of soil N as N–NH₄ exchangeable + N–NO₃ in fallow was 350–440% as compared to the control treatment without nitrogen (PK).After cropping without or with N fertilizer application at the rates of 8 and 32 mg N/100 g of soil, a positive nitrogen balance has been found which is likely due to nonsymbiotic (associative) N-fixation. It has been shown that biologically fixed nitrogen contributes to plant nutrition.     

A study of soil nitrogen organic fractions and correlation with yield response of Sudan-sorghum hybrid grass on Quebec soils

Summary Acid-hydrolysable organic nitrogen fractions were determined before and after two crops of Sudan-sorghum hybrid grass grown in a growth chamber on twenty Quebec soils. The relationship between the organic nitrogen fractions and yield response to N fertilizer and N uptake was examined by correlation methods and compared with corresponding relations obtained using incubation and boiling water extraction methods as indices of soil N availability. Mean contributions to total N lost during growth of the two crops were: hydrolysable NH ₄ ⁺ -N−19%, amino acid N−16% and hexosamine N−2%. The amino acid N, hydrolysable NH ₄ ⁺ -N and total hydrolysable N were significantly correlated with percent yield, log percent yield decrement and N uptake. Better correlations were obtained with the incubation and boiling water extraction methods than with the organic-N fractions. Contribution from Department of Soil Science, Macdonald College of McGill University.     

Dynamics of fertilizer-derived organic nitrogen fractions in an arable soil during a growing season

Background

Inorganic fertilizer is one of the most important anthropogenic inputs which influences soil nutrient turnover in agricultural ecosystems. However, as the key process involved in the maintenance, transformation and stability of soil nitrogen (N), the incorporation and allocation of fertilizer N between different soil organic N (SON) fractions in a growing season remains largely unknown.

Methods

In this study, a field experiment was conducted in triplicate of micro-plots and a total of 200 kg N ha^?1 (¹⁵?N-labeled (NH₄)₂SO₄, 98 atom %) was applied as a basal dressing and two top dressings, at jointing and filling stages, respectively, to a maize crop during one growing season. The distribution and seasonal dynamics of fertilizer N in different SON fractions (i.e., amino acids, amino sugars, hydrolyzable ammonium N and acid insoluble-N) were measured by liquid/gas chromatography–mass spectrometry (LC/GC-MS) and element analysis-combustion-isotope ratio mass spectrometry (EA-C-IRMS) techniques. Path analysis was used to evaluate the transformation processes between organic N fractions derived from fertilizer and N supply strategy in soil-plant system.

Results

The accumulation of fertilizer-derived N in different organic fractions was season-specific. At jointing stage, preferential enrichment of ¹⁵?N was found in soil amino acids plus amino sugars, indicating the active biological immobilization of basal dressing fertilizer N. Nevertheless, there is still a small proportion of fertilizer N stabilized in the acid insoluble fraction. The accumulation of the residual fertilizer N in hydrolyzable ammonium N reached a maximum at filling stage and then declined significantly, implying the rapid release of the fertilizer N remained in mineral forms. The contents of amino acids changed slightly, but they played a very important role in mediating SON transformation.

Conclusion

The hydrolyzable ammonium N was a temporary pool for rapid fertilizer N retention and simultaneously was apt to release N for crop uptake in the current season. In contrast, the amino acids could serve as a transitional pool of available N in the soil-crop system, while the acid insoluble fraction was as a stable pool of fertilizer N. Importantly, there is an interim shift among different pools to maintain soil N turnover; hence N in the amino acid fraction mediates N supply and the depolymerization of SON constituents controls the proceeding of fertilizer N cycling in the soil-plant system.     

长期施肥对红壤性水稻土有机氮组分的影响

通过16年的田间定位试验,研究了长期不同施肥模式对红壤性水稻土有机氮组分的影响.结果表明：长期化肥处理对土壤各氮素含量的作用不明显;有机物料循环特别是有机肥和化肥配施显著提高了土壤矿质氮和有机氮含量,提高幅度分别为55.2%和38.8%.有机物料循环处理显著提高了酸解性氮组分,其对土壤铵态氮、氨基糖氮和未知氮含量的提高幅度分别为36.5%、68.4%和73.9%;有机物料与化肥配施后,氨基酸氮含量也显著提高,提高幅度达71.1%,但是降低了未知氮含量,降低幅度为34.5%.此外,各施肥处理土壤累积矿化氮量均随培养时间的延长呈增加趋势,有机物循环或配施化肥处理土壤矿化氮量均高于单施化肥处理.     

Effect of legume-based cropping systems on nitrogen mineralization potential of Vertisol

The quantity and patterns of net mineralization of soil nitrogen (N) were studied in Vertisols under different cropping systems in the semi-arid tropical areas. Eight cropping systems were selected; three contained pigeonpea (PP), one contained PP and cowpea (COP), and two contained chickpea (CP) as legume component crops, one included sequence cropping with nonlegumes during the rainy and postrainy seasons, and one system was kept fallow (F) during the rainy season and sown to sorghum (S) during the postrainy season. Cropping systems with PP as a component crop increased mineralizable N(N _o ) content two-fold in the soil compared with fallow + sorghum (F+S)–F+S system. The N mineralization rate constant (k) was not significantly affected by previous cropping history of the soil; however, a numerically higher rate constant was observed in the COP/PP intercrop, followed by sequential S+safflower (SF) system as compared to the other soils. Mineral N accumulation curves for six soils were more accurately described by the exponential model than the linear model. The active N fraction (N _o /N_tot %) varied between 8 and 16% for different systems and a direct relationship was observed between N _o /N_tot and total N for the soils under diverse cropping systems.ICRISAT JA (1638)     

长期施肥对潮棕壤有机氮组分的影响

依托长期定位试验(从1990年开始)的4个施肥处理(CK,不施肥;M,循环猪圈肥;NPK,单施化肥;NPK+M,化肥配施循环猪圈肥),探讨了长期施肥对潮棕壤有机氮各组分的影响并揭示了其年际变化特征.结果表明: 从试验开始至2015年,不同施肥模式下各有机氮组分呈现不同的变化趋势,其中酸解有机氮基本上表现为逐年增加的趋势;M和NPK+M处理氨基酸态氮也逐年上升,而CK和NPK处理氨基酸态氮持续下降,这可能与微生物对土壤中氨基酸的利用有关.在整个试验过程中(1990—2015年的平均值),NPK处理酸解铵态氮大幅度增加,比CK增加31.1%;在施加有机肥的2个处理中(M和NPK+M)有机氮各组分均有所提高.与M相比,NPK+M处理各有机氮组分增加的幅度更加明显,这表明有机无机肥配施能有效增加土壤有机氮,进而增强土壤供氮能力,提高土壤肥力水平.     

Losses and biogeochemical cycling of soil organic nitrogen with prolonged arable cropping in the South African Highveld – evidence from - and -amino acids

We know little about the mechanisms that cause rapid losses in the soil organic N pool during cropping. As the analysis of amino acid enantiomers can provide insight into both the fate of microbial N and the ageing of cells in the environment, we used this technique as a tool to examine how the pool of protein-bound N in subtropical Plinthosols responds to increasing duration of arable cropping. The samples comprised bulk soils (0–20 cm) and clay fractions from each of three agro-ecosystems in semiarid South Africa; the sites have been cropped for periods varying from 0 to 98 years. The amino acid enantiomers contributed 34% to the total N content. With increasing number of years a piece of land has been cropped, the amino acid concentrations declined bi-exponentially to about 30% of their initial level in the native grasslands. Changes of the remaining soil protein-N pool were indicated by alterations in the d-content of individual amino acids. As the years of arable cropping increased, the proportions of d-alanine and d-glutamic acid increased relative to the respective l-enantiomers. This was attributed to an accumulation of N in residues of bacterial cell walls. In contrast, the d/l-ratios of leucine and aspartic acid declined in the long-term cultivated plots, probably reflecting losses of old amino acid-N reserves at the most degraded arable land.     

长期施肥对土壤养分库的影响

扼要地概述了国内外近年来关于长期施肥条件下土壤养分库的动态与平衡方面的研究成果。主要介绍并讨论了土壤氮、磷、钾养分全量及有效量的动态变化 ,土壤有机碳含量的动态变化 ;铵态氮、氨基酸态氮、氨基糖态氮、酸解未知态氮、非水解残渣氮及微生物体氮在土壤氮库中的动态变化 ;Ca2 P、Ca8 P、Al P、Fe P、闭蓄态磷 ,活性有机磷、中度活性有机磷、中稳性有机磷、高稳性有机磷及微生物体磷在土壤磷库中的动态变化 ;土壤碳库中松结态腐殖质、稳结态腐殖质、紧结态腐殖质以及富里酸和胡敏酸的动态变化 ;长期施肥对土壤有机无机复合状况的影响 ;农田土壤养分库的养分循环与养分平衡。还指出了当前土壤养分库研究面临的一些问题 ,并提出了今后研究的几个热点内容     

Relationships of soil fertility and stem borers damage to yield in maize-based cropping system in Cameroon

Field trials were designed to investigate the effect of direct nitrate fertilisation and mucuna fallow on maize yield and borer attacks in the humid forest zone of Cameroon. A traditional maizecassava- groundnut system (farmers’ practice) was compared with a maize-cassava + 120 Kg N ha-1, a rotation system in which maize-cassava followed a mucuna fallow as well as with a maize monocrop grown after mucuna fallow and with a maize monocrop grown with 120 Kg N ha-1. Average egg batch densities of Busseola fusca (Lepidoptera: Noctuidae) were lower by 35–55% in inter- than monocrops but the effect of nitrogen on B. fusca oviposition was not different from that of mucuna fallow. Highest larval infestations were found in sole maize with nitrogen and sole maize after mucuna. Yield losses were 4–10 times higher in the farmers’ practice compared to maize-cassava after mucuna and maizecassava with nitrogen. Mixed cropping systems including farmers’ practice yielded higher total gross and net benefits compared to sole maize crops. But, they were higher in maize-cassava intercrop + N than for farmer’s practice.     

Long-Term Effect of Crop Rotation and Fertilisation on Bioavailability and Fractionation of Copper in Soil on the Loess Plateau in Northwest China

The bioavailability and fractionation of Cu reflect its deliverability in soil. Little research has investigated Cu supply to crops in soil under long-term rotation and fertilisation on the Loess Plateau. A field experiment was conducted in randomized complete block design to determine the bioavailability and distribution of Cu fractions in a Heilu soil (Calcaric Regosol) after 18 years of rotation and fertilisation. The experiment started in 1984, including five cropping systems (fallow control, alfalfa cropping, maize cropping, winter wheat cropping, and grain-legume rotation of pea/winter wheat/winter wheat + millet) and five fertiliser treatments (unfertilised control, N, P, N + P, and N + P + manure). Soil samples were collected in 2002 for chemical analysis. Available Cu was assessed by diethylene triamine pentaacetic acid (DTPA) extraction, and Cu was fractionated by sequential extraction. Results showed that DTPA-Cu was lower in cropping systems compared with fallow control. Application of fertilisers resulted in no remarkable changes in DTPA-Cu compared with unfertilised control. Correlation and path analyses revealed that soil pH and CaCO₃ directly affected Cu bioavailability, whereas available P indirectly affected Cu bioavailability. The concentrations of Cu fractions (carbonate and Fe/Al oxides) in the plough layer were lower in cropping systems, while the values in the plough sole were higher under grain-legume rotation relative to fallow control. Manure with NP fertiliser increased Cu fractions bound to organic matter and minerals in the plough layer, and its effects in the plough sole varied with cropping systems. The direct sources (organic-matter-bound fraction and carbonate-bound fraction) of available Cu contributed much to Cu bioavailability. The mineral-bound fraction of Cu acted as an indicator of Cu supply potential in the soil.     

Transformation of15N-labelled urea in rice-wheat cropping system

Summary In a udic chromusterts the transformation of an initial application of¹⁵N-urea @ 80 kg N ha^–1 to rice (Oryza sativa L.) in rice-wheat (R-W) and to wheat (Triticum aestivum L.) in wheat-rice (W-R) rotations was followed in 6 successive crops in each rotation. All rice crops were grown in irrigated wetland and wheat in irrigated upland conditions.The first wheat crop in W-R rotation utilized 22 kg fertilizer N ha^–1 as compared to 19 kg by the corresponding rice crop in R-W rotation. But the latter absorbed more soil N than the former. About 69% of the total N uptake in rice was derived from mineralization of soil organic N as compared to 61% in wheat.The succeeding wheat crop in R-W rotation utilized 6.7% of the residual fertilizer N in the soil but the corresponding rice crop in W-R rotation only 2.2%. The higher utilization appeared to be related to a greater incorporation of labelled fertilizer N in mineral and hexosamine fractions of the soil N. After the second crop in each rotation, the average residual fertilizer N utilization in the next 4 crops ranged between 3 and 4%.The total recovery of¹⁵N-urea in all crops amounted to 21.7 and 24.3 kg N ha^–1 in R-W and W-R rotation, respectively. At the end of the experiment, about 9 to 10 kg ha^–1 of the applied labelled N was found in soil upto 60 cm depth. Most of the labelled soil N (69–76%) was located in the upper 0–20 cm soil layer indicating little movement to lower depths despite intensive cropping for 4 years.     

Nitrogen compounds in the apoplastic sap of sugarcane stem: some implications in the association with endophytes

Several nitrogen compounds were identified and quantified in the apoplastic and symplastic sap of sugarcane stems. The sap of stems was composed mainly of soluble sugars, which constituted 95% of the total organic compounds detected. Sap also contained nitrogen compounds, with amino acids (50-70% of N) and proteins (20-30% of N), being the main nitrogenous substances, as well as inorganic forms as ammonium, nitrite and nitrate, in low concentrations (<20% of N). Serine, proline, alanine and aspartic acid together represented around 60% of the amino acids of the sap of both field grown and high nitrogen fertilized plants, and non-nitrogen fertilized plants inoculated with Gluconacetobacter diazotrophicus. The total amino acid content of apoplastic sap was six to nine times lower in non-nitrogen fertilized plants than in fertilized ones. The possible roles of these substances to regulate endophytic associations with sugarcane are also discussed.     

种植模式和施肥对黄土旱塬农田土壤团聚体及其碳分布的影响

本研究以长武黄土高原农业生态试验站33年长期定位试验处理为研究对象,选取撂荒(R)、小麦连作(CK/W)、小麦玉米轮作(L),小麦连作选取单施氮肥(N)、单施磷肥(P)、施氮磷肥(NP)、单施有机肥(M)、氮肥配施有机肥(NM)、磷肥配施有机肥(PM)、氮磷肥配施有机肥(NPM)共10种不同种植模式和施肥田间处理,运用干湿筛法和TOC法,分析长期不同施肥和种植模式下黄土旱塬农田土壤力稳性和水稳性团聚体数量、稳定性、各粒级团聚体中全碳和有机碳的分布特征,以及各粒级团聚体对碳分布的贡献。结果表明: 力稳性团聚体以>0.25 mm大团聚体为主,含量>67%,施肥使其减少;连作减少了微团聚体,而轮作增加了微团聚体且其效果大于施肥处理。水稳性团聚体以<0.25 mm微团聚体为主,含量>61%;施肥和耕作均减少了水稳性微团聚体。不同施肥方式和种植模式均降低了团聚体破坏率(PAD),增加了大团聚体(R_0.25)含量。施用有机肥显著提高了力稳性团聚体各粒级中全碳和有机碳含量;连作和轮作增加了各粒级团聚体中全碳含量,但轮作显著降低了有机碳含量。单施氮、磷肥使土壤全碳含量降低,而氮磷复合肥和有机肥使土壤全碳含量显著增加;种植模式对全碳的影响较施肥处理小,连作和轮作使土壤全碳增加。除单施氮、磷肥外,复合施肥和施用有机肥使土壤有机碳显著增加;不同种植模式对土壤有机碳的影响小于施肥处理,轮作使有机碳含量降低。微团聚体对土壤全碳和有机碳的贡献率最大,可达21.2%~33.6%;不同施肥和种植模式使微团聚体全碳的贡献率增加,施肥处理中NP和NPM显著增加了微团聚体的全碳和有机碳贡献率,轮作对微团聚体全碳和有机碳的增加趋势贡献最显著。     

三种集约化种植体系氮素平衡及其对地下水硝酸盐含量的影响

选取中国北方3种重要的集约化种植体系小麦玉米轮作、大棚蔬菜和果园,研究了3种体系年度氮素输入输出关系、土壤硝酸盐的累积、不同体系地下水硝态氮含量的动态变化.结果表明,大棚蔬菜年度化肥氮、有机肥氮、灌水带入的氮和总氮输入量分别为135.8、1881、402和36.56kg·hm^-2,分别为小麦玉米田的25、37.5、83.8和5.8倍,为果园的2.1、10.4、6.82和4.2倍.不同系统降水输入的氮在142～189kg·hm^-2之间.3个体系氮输出量分别为280、329和121kg·hm^-2.氮素年度盈余分别为349、332.7和74.6kg·hm^-2.0～90cm土层硝态氮累积量分别为22.1～2.75、1173和613kg·hm^-2,90～180cm土层硝态氮累积量分别为2.13～2.42、10.32和976kg·hm^-2.在0～180cm剖面中,小麦玉米田各层土壤硝态氮处于相对均一分布,大棚蔬菜以表层最高,30cm以下各层也远高于大田,果园土壤硝态氮累积随土壤深度而增加.3种体系均表现出硝酸盐的明显淋洗.大棚蔬菜区浅井地下水硝态氮含量99%超过了10mg·L^-1.而大棚深井和果园浅井超标率均为5%,小麦玉米深井为1%.大棚蔬菜区地下水硝态氮含量与井深呈指数函数降低关系.     

Carbon and nitrogen stocks in physical fractions of a subtropical Acrisol as influenced by long-term no-till cropping systems and N fertilisation

The long-term soil management effects on C and N stocks of soil physical fractions are still poorly understood for South American subtropical soils. This study aimed (i) to evaluate the influence of cereal- and legume-based cropping systems and N fertilisation on C and N stocks of the sand-, silt- and clay-size fractions of a no-tilled subtropical Acrisol in southern Brazil, (ii) to compute the Carbon Management Index (CMI) for those cropping systems using physical fractionation data, and (iii) to investigate the possible existence of finite capacity of those soil physical fractions to store C and N. Soil samples of a long-term experiment were collected from the 0–2.5 and 2.5–7.5 cm layers of three no-till cropping systems [fallow bare soil, oat/maize (O/M) and pigeon pea+maize (P+M)] under two N fertilisation levels (0 and 180 kg N ha^–1). However, for fallow bare soil, only the non-fertilised sub-plot was sampled. An adjacent native grassland soil was sampled as a reference. The C and N stocks of the three soil physical fractions were higher in the legume-based cropping system (P+M) than in O/M and bare soil, because of the higher residue input in the former. The P+M cropping system restored the C and N stocks in sand- and silt-size fractions to the same levels found in grassland soil. Higher C and N stocks in all physical fractions were also obtained with N fertilisation. The C and N stocks and the C:N ratio were most affected by cropping systems in the sand- and least in the clay-size fraction. Particulate organic matter was found in the silt-size fraction, showing this fraction is not only constituted by mineral-associated organic mater, as commonly believed. Taking grassland soil as reference (CMI = 100), the CMI ranged from 46, in O/M no N, to 517, in P+M with N, pointing to a better soil management in the latter. The clay-size fraction tended to show a finite capacity to store C and N (48.8 g C kg^–1 and 4.9 g N kg^–1 of clay), which was not verified in sand- and silt-size fractions. The adoption of no-tillage and legume-based cropping systems with high residue input are adequate soil management strategies to improve soil quality and make the agricultural production systems more sustainable in subtropical regions.     

长期双季稻绿肥轮作对水稻产量及稻田土壤有机质的影响

以中国农业科学院红壤实验站1982年布置的长期定位试验为研究对象,分析了长期双季稻绿肥轮作体系下水稻产量变化趋势、稻田土壤有机质变化特征及土壤活性有机质组成。结果表明,绿肥作物与双季稻轮作种植后,水稻产量显著高于冬闲对照,绿肥作物紫云英、油菜和黑麦草处理年平均水稻产量（1982-2008）分别为10.8 t?hm-2?a-1,10.2 t?hm-2?a-1和10.0 t?hm-2?a-1,较冬闲对照分别提高27.2%,20.5%和18.1%。试验前期（1982-1993）种植绿肥作物各处理之间水稻产量无显著差异,试验开展11年后（1994-2008）种植紫云英处理水稻产量显著高于油菜和黑麦草处理。长期双季稻绿肥轮作土壤有机质随年份显著增加,双季稻紫云英轮作土壤有机质积累速度最快,年增加0.31 g?kg-1,双季稻黑麦草次之,土壤有机质年增加0.28g?kg-1,双季稻油菜轮作土壤有机质年增加0.26g?kg-1。种植绿肥作物紫云英稻田土壤活性有机质显著高于其它处理。种植绿肥作物各处理土壤有机碳、全氮、土壤微生物量碳和土壤微生物量氮含量均显著高于冬闲对照。其中黑麦草和紫云英处理土壤微生物量碳含量及微生物熵显著高于油菜和冬闲对照处理。在湘南红壤丘陵双季稻区,种植绿肥作物对提高水稻产量、增加土壤有机质、提高土壤有机质活性具有重要意义,绿肥选择上以紫云英对水稻产量和稻田土壤培肥综合效果最好。     

Contribution of amino compounds to dissolved organic nitrogen in forest soils

Dissolved organic nitrogen (DON) may play an important role in plantnutrition and nitrogen fluxes in forest ecosystems. In spite of the apparentimportance of DON, there is a paucity of information concerning its chemicalcomposition. However, it is exactly this chemical characterization that isrequired to understand the importance of DON in ecosystem processes. Theprimaryobjective of this study was to characterize the distribution of free aminoacidsand hydrolyzable peptides/proteins in the DON fraction of Oa horizon leachatesalong an extreme edaphic gradient in northern California. Insitu soil solutions were extracted by centrifugation from Oahorizonscollected beneath Pinus muricata (Bishop pine) andCupressus pygmaea (pygmy cypress) on slightlyacidic/fertile and highly acidic/infertile sites. DON accounted for 77 to99% of the total dissolved nitrogen in Oa horizon leachates. Nitrogen infree amino acids and alkyl amines ranged from 0.04–0.07 mgN/L on the low fertility site to 0.45–0.49 mg N/L onthe high fertility site, and accounted for 1.5 to 10.6% of the DON fraction.Serine, glutamic acid, leucine, ornithine, alanine, aspartic acid andmethylamine were generally the most abundant free amino compounds. Combinedamino acids released by acid hydrolysis accounted for 48 to 74% of theDON, suggesting that proteins and peptides were the main contributor to DON inOa horizon leachates. Together, nitrogen from free andcombined amino compounds accounted for 59 to 78% of the DON. Most of theDON was found in the hydrophobic fraction, which suggests the presence ofprotein/peptide-polyphenol complexes or amino compounds associated withhumic substances. Because free and combined amino acids can be an importantnitrogen source for some plants, soil DON may play an important role in plantnutrition and ecosystem function.