不同施肥处理黑土覆膜后秸秆碳氮在团聚体中的固存特征

地膜覆盖是提高作物产量的重要措施,理解覆膜条件下黑土团聚体中外源碳和氮的固存特征,为深刻认识地膜覆盖措施的可持续应用提供理论依据。选取长期定位试验站(29年)不施肥(CK)、单施化肥(NPK)和有机肥配施化肥(MNPK)3个典型施肥处理,表层土壤(0—20 cm)添加¹³C¹⁵N双标记玉米秸秆后设置裸地和覆膜的田间原位微区培养试验,探讨不同施肥处理结合覆膜黑土团聚体中有机碳和全氮对秸秆来源碳和氮的响应。结果表明,与裸地相比,所有处理覆膜后微团聚体(<0.25 mm)中秸秆来源碳和氮的含量平均降低了26.49%和32.05%。覆膜MNPK与裸地处理相比大团聚体(>0.25 mm)中秸秆来源碳和氮的含量显著降低了35.58%和15.97%,但大团聚体中原土壤有机碳的含量提高了9.16%。在CK和NPK处理微团聚体中,秸秆来源碳占该粒级团聚体有机碳的比例表现为覆膜>裸地,而在MNPK处理各粒级团聚体中则表现为裸地>覆膜。无论覆膜与否,秸秆来源碳对团聚体有机碳和秸秆来源氮对团聚体全氮的贡献率受施肥处理的影响表现为CK>N...     

Temporal and spatial distribution of roots and competition for nitrogen in pea-barley intercrops – a field study employing 32P technique

Root system dynamics, productivity and N use were studied in inter- and sole crops of field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) on a temperate sandy loam. A ³²P tracer placed at a depth of 12.5, 37.5, 62.5 or 87.5 cm was employed to determine root system dynamics by sampling crop leaves at 0, 15, 30 and 45 cm lateral distance. ¹⁵N addition was used to estimate N₂ fixation by pea, using sole cropped barley as reference crop. The Land Equivalent Ratio (LER), which is defined as the relative land area under sole crops that is required to produce the yields achieved in intercropping, were used to compare the crop growth in intercrops relative to the respective sole crops.The ³²P appearance in leaves revealed that the barley root system grows faster than that of pea. P uptake by the barley root system during early growth stages was approximately 10 days ahead of that of the pea root system in root depth and lateral root distribution. More than 90% of the P uptake by the pea root system was confined to the top 12.5 cm of soil, whereas barley had about 25–30% of tracer P uptake in the 12.5 – 62.5 cm soil layer. Judging from this P uptake, intercropping caused the barley root system to grow deeper and faster lateral root development of both species was observed. Barley accumulated similar amounts of aboveground N when grown as inter- and sole crop, whereas the total aboveground N acquired by pea in the intercrop was only 16% of that acquired in the pea sole crop. The percentage of total aboveground N derived from N₂ fixation in sole cropped pea increased from 40% to 80% during the growth period, whereas it was almost constant at 85% in intercropped pea. The total amounts of N₂ fixed were 95 and 15 kg N ha^–1 in sole cropped and intercropped pea, respectively. Barley was the dominant component of the pea-barley intercrop, obtaining 90% of its sole crop yield, while pea produced only 15% of the grains of a sole crop pea. Intercropping of pea and barley improved the utilization of plant growth resources (LER > 1) as compared to sole crops. Root system distribution in time and space can partly explain interspecific competition. The ³²P methodology proved to be a valuable tool for determining root dynamics in intercropping systems.     

陇东雨养苹果覆膜对土壤团聚体结构稳定性与细根分布的影响

不同的果园管理方式可影响果树根系生长、分布与土壤团聚体稳定性、有机碳固存,进而改变“根-土”复合体响应关系。对西北陇东旱塬不同覆膜年限(2 a、4 a和6 a)苹果园表层土壤(0—20 cm)细根生长进行调查,并采用干筛法和湿筛法相结合的方式对土壤团聚体进行分级(>2 mm, 0.5—2 mm, 0.25—0.5 mm和<0.25 mm)。计算团聚体稳定性参数[> 0.25 mm机械稳定性团聚体含量(DR_0.25)、> 0.25 mm水稳性团聚体含量(WR_0.25)、平均重量直径(MWD)、平均几何直径(GMD)、团聚体破坏率(PAD)、水稳系数(WSC)]和团聚体有机碳含量。分析细根生长与土壤物理结构对长期覆膜的响应,探明土壤团聚体稳定性与有机碳固持关系,揭示黄绵土物理结构稳定机制。结果表明：6 a处理通过增加表层土壤黏粒和物理性黏粒比例,改变孔隙结构,抑制细根生长,其根量、根长和根表面积仅为对照(CK)的20.97%、24.66%和41.25%;降低表层土壤团聚体力稳性,其DR_0.25、机械稳...     

Impact of elevated CO2 on soil organic matter dynamics as related to changes in aggregate turnover and residue quality

Increasing global atmospheric CO₂ concentration can potentially affect C cycling in terrestrial ecosystems. This study was conducted to assess the impact of elevated CO₂ concentration on soil organic matter and aggregate dynamics in Lolium perenne and Trifolium repens pastures. Soil samples from a 6 year old `free air CO₂ enrichment' (FACE) experiment were separated in four aggregate size classes (<53, 53–250, 250–2000, and > 2000 m). Free light fraction (i.e. particulate organic matter (POM) outside of aggregates; free LF) and intra-aggregate-POM (i.e. POM occluded within the aggregate structure; iPOM) were isolated. The distinct ¹³C-signature of the CO₂ used to raise the ambient CO₂ concentration in FACE allowed us to calculate proportions of recently incorporated C (< 6 yr) in the physically defined soil fractions. The proportion of new C increased with increasing aggregate size class, except the two largest aggregate size classes had a similar proportion of new C; this indicates a faster turnover of macroaggregates compared to microaggregates. In addition, higher proportions of new C in macroaggregates under T. repens compared to L. perenne indicate a faster macroaggregate turnover under T. repens. This faster macroaggregate turnover is hypothesized to be a result of the higher residue quality (C:N ratio) of T. repens compared to L. perenne and reduces the potential of sequestering C under elevated CO₂. In the L. perenne soil, elevated CO₂ did not significantly increase total C, but led to: (1) a 54% increase in aggregation and (2) a 40% increase in total iPOM-C. It is hypothesized that the sequestration of iPOM-C induced by elevated CO₂ in the low residue quality, L. perenne treatment, resulted from an increase in the proportion of large macroaggregates with a slow turnover.     

Relationship between intracellular pH and N metabolism in maize (Zea mays L.) roots

In vivo ³¹P nuclear magnetic resonance (NMR) was used to characterize the effect of the N form (NO₃ vs. NH₄) and the external pH (4, 6, and 8), on the intracellular pH of root tips (0–5 mm) and root segments (5–30 mm). Ammonium-grown root tips were the most sensitive to changes in the external pH. In vivo ¹⁵N NMR was used to characterize the pathway of primary ammonium assimilation in the ammonium-grown roots and to compare the activity of the apical and more-basal root parts. The kinetics of ¹⁵NH₄ ⁺ incorporation showed that primary assimilation in both root tips and root segments followed the glutamine synthetase (GS) pathway. In agreement with the reported gradient of GS along the seminal root of maize, incorporation of label into glutamine amide was more rapid in tips than in segments. It is suggested that this higher GS activity increases the endogenous proton production and thus contributes to the greater dependence of the cytoplasmic pH on the external pH in the ammonium-treated root tips.     

Inter-row crop competition for band-injected ammonium nitrate

Inter-species competition for applied nutrients may be managed by weed control in cultivated communities. However, intra-crop competition may still occur. Thus, regarding a fertilizer band as a finite nutrient source, individual rows of a cereal crop may compete for the applied nitrogen. The aim was to explore the partition of applied N between crop rows affected by the displacement of the fertilizer band relative to the rows. In a micro-plot experiment, a ¹⁵N-ammonium–¹⁵N-nitrate solution was injected in four positions parallel to two rows of spring wheat (Triticum aestivum ssp. vulgare). The course of ¹⁵N crop uptake in each row was estimated by a sigmoid growth function using eight samplings during the elongation phase. The estimated parameters were used for calculating the asymptotic value (a), the time until a/2 was reached (T_0.5a), and the uptake rate at this time (MaxRate), and related to the distance between the crop row and fertilizer band. The a-value decreased by 5.1%-point recovery cm^–1, T_0.5a increased by 0.5 day cm^–1 and the MaxRate decreased by 0.3%-point recovery day^–1 cm^–1. Placement of the fertilizer band centred between two crop rows caused an even access to the applied nitrogen, but a parallel displacement from the centre by a few centimetres significantly affected the parameters. The proliferation of the roots in the soil and thus the time until the roots reached the fertilizer zone is assumed to cause the inter-row competition for the band-injected N source. The access to the fertilizer band was clearly reflected in the dry matter production causing an uneven crop stand. The consequences of inter-row crop competition for a band-injected N source with respect to grain yield and grain quality were not explored, but attention on these parameters are required, particularly in the production of high quality wheat grains.     

Enhanced nitrogen mineralization in mowed or glyphosate treated cover crops compared to direct incorporation

Information on how management by mowing and herbicide alter residue quality and nitrogen (N) inputs would be valuable to improve prediction of N availability. Mowing and glyphosate application are widely used by growers to limit cover crop growth and facilitate incorporation. A mixture of cover crops, hairy vetch (Vicia villosa L.), oriental mustard (Brassica junceaL.) and cereal rye (Secale cerealeL.), was investigated as a means to improve soil quality and optimize N availability. There is limited information on how mowing or glyphosate application influence cover crop decomposition and N mineralization from these heterogeneous residues. A rye cover crop was grown in the field over the winter and transferred to containers as an intact soil profile to conduct a greenhouse study. Management treatments (mowing and glyphosate) were imposed eight days before incorporation. Plant and soil N dynamics were monitored. The experiment was repeated with the addition of a tri-mixture cover crop. Inorganic NO₃^– in bare soil ranged from 6 to 10 g N g soil^–1 over 39 days. Similar or lower levels of soil NO₃^– were observed after rye residue incorporation, from 2 to 6 g N g^–1; consistent with N-immobilization. Application of untreated, mixed cover crop residues generally was associated with higher levels of soil inorganic NO₃^–, from 3 to 11 g N g^–1. For both rye and mixed residues, management by mowing or glyphosate enhanced N mineralization by 10 to 100%, compared to untreated residues. At the same time, application of mowing or glyphosate 8 days before cover crop incorporation seemed to reduce the amount of residues by about half compared to untreated controls. Belowground biomass was reduced more than aboveground, although recovery of senescent roots may have been incomplete. Management by glyphosate or mowing enhanced soil inorganic N availability in the short-term while simultaneously reducing carbon and N inputs.     

干旱胁迫下胡杨实生幼苗氮素吸收分配与利用

胡杨(Populus euphratica)是塔里木河流域荒漠河岸林的建群种,水分和氮素是限制胡杨幼苗的存活及早期生长的主要因子。利用~(15)N同位素示踪技术分析水和氮素的交互作用对胡杨幼苗不同生长阶段氮素的吸收分配利用及幼苗生长的影响,进一步探究氮素对胡杨实生苗早期形态建成的作用及对干旱胁迫的缓解效应,以期提高幼苗的存活率。实验以一年生胡杨实生幼苗为研究对象,采用温室内盆栽实验,设置4个干旱处理(D_1、D_2、D_3、D_4,土壤相对含水量为:20%—25%、40%—45%、60%—65%、80%—85%)和3种氮素水平(N_0、N_1、N_2:0、3、6 g/盆)测定胡杨幼苗的生长指标和各部分的Ndff、分配率及利用率。结果表明:胡杨幼苗在土壤相对含水量60%—65%(D_3)、氮素添加量3 g/盆(N_1)时,其生长表现为最佳状态;干旱胁迫下,不同氮素添加量对胡杨幼苗各部分的Ndff值存在显著差异,N_2低于N_1;随干旱胁迫减弱(D_3、D_4),植株在生长早期(25 d)根部吸收的~(15)N优先向地上部分转运,生长后期(75 d)植株Ndff最高,其中以根系中Ndff最高;不同生长期幼苗各部分的~(15)N分配存在显著差异,根系~(15)N分配率较高,但不同氮量处理间差异不显著;随生长期的推移,植株对~(15)NH_4~(15)NO_3的利用率表现为粗根最大,各处理中D_3N_1处理均显著高于其他处理。结论:轻度干旱胁迫下添加适量的氮素能够增强植株对氮素的吸收征调能力,优化水资源获取以维持生存的重要机制。     

Management of biological N2 fixation in alley cropping systems: Estimation and contribution to N balance

Alley cropping is being widely tested in the tropics for its potential to sustain adequate food production with low agricultural inputs, while conserving the resource base. Fast growth and N yield of most trees used as hedgerows in alley cropping is due greatly to their ability to fix N₂ symbiotically with Rhizobium. Measurements of biological N₂ fixation (BNF) in alley cropping systems show that some tree species such as Leucaena leucocephala, Gliricidia sepium and Acacia mangium can derive between 100 and 300 kg N ha^-1 yr^–1 from atmospheric N₂, while species such as Faidherbia albida and Acacia senegal might fix less than 20 kg N ha^-1 yr^-1. Other tree species such as Senna siamea and S. spectabilis are also used in alley cropping, although they do not nodulate and therefore do not fix N₂. The long-term evaluation of the potential or actual amounts of N₂ fixed in trees however, poses problems that are associated with their perennial nature and massive size, the great difficulty in obtaining representative samples and applying reliable methodologies for measuring N₂ fixed. Strategies for obtaining representative samples (as against the whole tree or destructive plant sampling), the application of ¹⁵N procedures and the selection criteria for appropriate reference plants have been discussed.Little is known about the effect of environmental factors and management practices such as tree cutting or pruning and residue management on BNF and eventually their N contribution in alley cropping. Data using the ¹⁵N labelling techniques have indicated that up to 50% or more of the tree's N may be below ground after pruning. In this case, quantification of N₂ fixed that disregards roots, nodules and crowns would result in serious errors and the amount of N₂ fixed may be largely underestimated. Large quantities of N are harvested with hedgerow prunings (>300 kg N ha^-1 yr^-1) but N contribution to crops is commonly in the range of 40–70 kg N ha^-1 season. This represents about 30% of N applied as prunings; however, N recoveries as low as 5–10% have been reported. The low N recovery in maize (Zea mays) is partly caused by lack of synchronization between the hedgerow trees N release and the associated food crop N demand. The N not taken up by the associated crop can be immobilized in soil organic matter or assimilated by the hedgerow trees and thus remain in the system. This N can also be lost from the system through denitrification, volatilization or is leached beyond the rooting zone. Below ground contribution (from root turnover and nodule decay) to an associated food crop in alley cropping is estimated at about 25–102 kg N ha^-1 season^-1. Timing and severity of pruning may allow for some management of underground transfer of fixed N₂ to associated crops. However many aspects of root dynamics in alley cropping systems are poorly understood. Current research projects based on ¹⁵N labelling techniques or ¹⁵N natural abundance measurements are outlined. These would lead to estimates of N₂ fixation and N saving resulting from the management of N₂ fixation in alley cropping systems.     

Effects of planted tree fallows on soil nitrogen dynamics,above-ground and root biomass,N2-fixation and subsequent maize crop productivity in Kenya

The effects of planted fallows of Sesbania sesban (L.) Merr. and Calliandra calothyrsus (Meissner) on soil inorganic nitrogen dynamics and two subsequent maize crops were evaluated under field conditions in the highlands of eastern Kenya. Continuous unfertilised maize, maize/bean rotation and natural regrowth of vegetation (weed fallow) were used as control treatments. The proportion of symbiotic N₂-fixation was estimated by measuring both leaf ¹⁵N enrichment and whole-plant ¹⁵N enrichment by the ¹⁵N dilution technique for Sesbania and Calliandra, using Eucalyptus saligna (Sm.) and Grevillea robusta (A. Cunn) as reference species. Above- and below-ground biomass and N contents were examined in Sesbania, Calliandra, Eucalyptus and Grevillea 22 months after planting. Both the content of inorganic N in the topsoil and the quantity of N mineralised during rainy seasons were higher after the Sesbania fallows than after the other treatments. Compared to the continuous unfertilised maize treatment, both residual crop yields were significantly higher when mineral N (one application of 60 kg N ha^–1) was added. Furthermore, the second crop following the Sesbania fallow was significantly higher than the continuous maize crop. The above-ground biomass of the trees at final harvest were 31.5, 24.5, 32.5 and 43.5 Mg ha^–1 for the Sesbania, Calliandra, Grevillea and Eucalyptus, respectively. For the total below-ground biomass the values for these same tree species were 11.1, 15.5, 17.7, and 19.1 Mg ha^–1, respectively, of which coarse roots (>2 mm), including tap roots, amounted to 70–90%. About 70–90% of the N in Sesbania, and 50–70% in Calliandra, was derived from N₂-fixation. Estimates based on leaf ¹⁵N enrichment and whole-plant ¹⁵N enrichment were strongly correlated. The N added by N₂-fixation amounted to 280–360 kg N ha^–1 for Sesbania and 120–170 kg N ha^–1 for Calliandra, resulting in a positive N balance after two maize cropping seasons of 170–250 kg N ha^–1 and 90–140 kg N ha^–1, for Sesbania and Calliandra, respectively. All the other treatments gave negative N balances after two cropping seasons. We conclude that Sesbania sesban is a tree species well suited for short duration fallows due to its fast growth, high nutrient content, high litter quality and its ability to fix large amounts of N₂ from the atmosphere.     

林龄对刺槐人工林土壤团聚体、有机碳和细菌群落的影响

探究刺槐人工林土壤团聚体、有机碳及细菌群落的变化特征,可为提升土壤质量与功能提供理论依据。以黄土高原沟壑区不同林龄刺槐人工林(8年生、18年生和30年生)和刚撂荒的农耕地(CK)为研究对象,利用最适湿度筛分法获得不同粒径团聚体,测定其有机碳含量和细菌群落结构特征。结果表明:(1)各林龄土壤团聚体均以>0.25 mm粒径为主,含量为92.74%-95.78%。土壤团聚体稳定性随着林龄的增加显著提高,以30年生林地最高,团聚体平均重量直径(MWD)、几何平均直径(GMD)分别较CK显著增加48.19%和91.38%(P<0.05)。(2)各林龄均以<0.25 mm粒径有机碳含量最高。土壤有机碳含量和>2 mm粒径团聚体有机碳含量均随着林龄的增长而显著增加。(3)团聚体细菌群落主要由变形菌门(Proteobacteria)、放线菌门(Actinobacteria)和酸杆菌门(Acidobacteria)组成。随着林龄的增加,各粒径团聚体中放线菌门相对丰度先降低后增加,18年生林地最低。酸杆菌门变化趋势与放线菌门相反。变形菌门无明显变化。土壤有机碳、pH、全氮和全磷是影响团聚体细菌群落的主要因素。(4)>2 mm和2-0.25 mm粒径团聚体中,己科河菌门(Rokubacteria)对团聚体稳定性影响最大,既对团聚体稳定性产生直接正效应,又通过增加有机碳的含量间接提高团聚体稳定性。在<0.25 mm粒径团聚体中,有机碳含量对其稳定性的影响最大,有机碳含量越高,团聚体稳定性越强。综上,营造刺槐人工林有利于提高土壤团聚体稳定性,促进土壤有机碳的积累,可作为该区域生态恢复的有效措施。在生长发育过程中,人工林可通过改变土壤团聚体细菌群落和有机碳含量从而影响团聚体的稳定性。     

Nitrogen accumulation in sole and mixed stands of sweet-blue lupin (Lupinus angustifolius L.), ryegrass and oats

Nitrogen fixation was measured in monocropped sweet-blue lupin (Lupinus angustifolius), lupin intercropped with two ryegrass (Lolium multiflorum) cultivars or with oats (Avena sativa) on an Andosol soil, using the ¹⁵N isotope dilution method. At 117 days after planting and at a mean temperature below 10°C, monocropped lupin derived an average of 92% or 195 kg N ha⁻¹ of its N from N₂ fixation. Intercropping lupin with cereals increased (p<0.05) the percentage of N derived from atmospheric N₂ (% Ndfa) to a mean of 96%. Compared to the monocropped, total N fixed per hectare in intercropped lupin declined approximately 50%, in line with the decrease in seeding rate and dry matter yield. With these high values of N₂ fixation, selection of the reference crop was not a problem; all the cereals, intercropped or grown singly produced similar estimates of N₂ fixed in lupin. It was deduced from the ¹⁵N data that significant N transfer occurred from lupin to intercropped Italian ryegrass but not to intercropped Westerwoldian ryegrass or to oats. Doubling the ¹⁵N fertilizer rate from 30 to 60 kg N ha⁻¹ decreased % Ndfa to 86% (p<0.05), but total N fixed was unaltered. These results indicate that lupin has a high potential for N₂ fixation at low temperatures, and can maintain higher rates of N₂ fixation in soils of high N than many other forage and pasture legumes.     

Nitrogenase activity in the papyrus swamps of Lake Naivasha,Kenya

Nitrogenase activity (acetylene reduction activity) was found to occur universally in the Cyperus papyrus swamp in Lake Naivasha. Low rates of acetylene reduction activity (0.9–104.9 nmol C₂H₄ g d.wt. roots^-1 h^-1) were associated with excised roots of C. papyrus but higher rates of activity (89.0–280.4 nmol C₂H₄ g d.wt. roots^-1 h^-1) were associated with intact root systems of the plant. It was estimated that nitrogen fixation associated with young roots alone could supply about 26% of the nitrogen requirements of growing papyrus plants. Acetylene reduction activity in the lake bottom sediments was generally low and associated with adjacent papyrus stands. Plate counts of putative aerobic and facultatively anaerobic N₂-fixing bacteria associated with papyrus roots showed the presence of high numbers of diazotrophs (5.4 × 10⁶ CFU g d.wt. roots^-1). Fewer numbers of N₂-fixing bacteria were detected in the sediments (1.9 × 10³-3.2 × 10⁴ CFU g d.wt. sediment^-1).     

秦岭天然林凋落物去除对土壤团聚体稳定性及细根分布的影响

研究凋落物去除对秦岭天然林土壤团聚体稳定性及细根分布特征的影响机制,为森林生态治理提供理论依据。结果表明:(1)凋落物去除导致云杉林10-40 cm土层土壤中粒径大于0.25 mm水稳性团聚体含量(WR_0.25)较对照显著降低(P<0.05);混交林10-40 cm土层的大于0.25 mm机械稳定性团聚体含量(DR_0.25)显著降低(P<0.05);红桦林表层0-10 cm土壤团聚体的平均质量直径(MWD)显著降低(P<0.05);(2)凋落物去除导致三种天然林0-20 cm土层的细根生物量密度(FRBD)较对照显著降低44.18%-57.24%,细根体积(FRV)显著降低24.64%-60.41%;三种天然林中红桦林0-40 cm各土层的FRV最高;(3)凋落物去除导致云杉林0-40 cm土层的土壤容重较对照显著增加5.24%-13.04%,三种天然林0-40 cm土层的土壤有机碳含量显著降低7.92%-25.21%,全氮含量显著降低10.17%-18.10%;(4)相关分析表明,云杉林土壤的团聚体破坏率(PAD)和土壤不稳定团粒指数(E_LT)与FRBD、细根根长密度(FRLD)和FRV均呈极显著负相关,混交林土壤的PAD与FRBD呈极显著负相关关系(r=-0.814),红桦林土壤团聚体MWD与FRBD、FRLD和细根比根长(FSRL)均呈显著负相关关系(r=-0.777、-0.771和-0.786)。综上,凋落物去除在总体上降低了天然林土壤团聚体的稳定性及有机碳和全氮含量,并且不利于林木细根的生长。     

Estimation of residual N effect of faba bean and pea on two succeeding cereals using15N methodology

A field experiment was conducted using¹⁵N methodology to study the effect of cultivation of faba bean (Vicia faba L.), pea (Pisum sativum L.) and barley (Hordeum vulgare L.) on the N status of soil and their residual N effect on two succeeding cereals (sorghum (Sorghum vulgare) followed by barley). Faba bean, pea and barley took up 29.6, 34.5 and 53.0 kg N ha^–1 from the soil, but returned to soil through roots only 11.3, 10.8 and 5.7 kg N ha^–1, respectively. Hence, removal of faba bean, pea and barley straw resulted in a N-balance of about –18, –24, and –47 kg ha^–1 respectively. A soil nitrogen conserving effect was observed following the cultivation of faba bean and pea compared to barley which was of the order of 23 and 18 kg N ha^–1, respectively. Cultivation of legumes resulted in a significantly higher A_N value of the soil compared to barley. However, the A_N of the soil following fallow was significantly higher than following legumes, implying that the cultivation of the legumes had depleted the soil less than barley but had not added to the soil N compared to the fallow. The beneficial effect of legume cropping also was reflected in the N yield and dry matter production of the succeeding crops. Cultivation of legumes led to a greater exploitation of soil N by the succeeding crops. Hence, appreciable yield increases observed in the succeeding crops following legumes compared to cereal were due to a N-conserving effect, carry-over of N from the legume residue and to greater uptake of soil N by the succeeding crops when previously cropped to legumes.     

Root distributions partially explain 15N uptake patterns in Gliricidia and Peltophorum hedgerow intercropping systems

The relative distributions of tree and crop roots in agroforestry associations may affect the degree of complementarity which can be achieved in their capture of below ground resources. Trees which root more deeply than crops may intercept leaching nitrogen and thus improve nitrogen use efficiency. This hypothesis was tested by injection of small doses of (¹⁵NH₄)₂SO₄ at 21.8 atom% ¹⁵N at different soil depths within established hedgerow intercropping systems on an Ultisol in Lampung, Indonesia. In the top 10 cm of soil in intercrops of maize and trees, root length density (L_rv) of maize was greater than that of Gliricidia sepium trees, which had greater L_rv in this topsoil layer than Peltophorum dasyrrachis trees. Peltophorum trees had a greater proportion of their roots in deeper soil layers than Gliricidia or maize. These vertical root distributions were related to the pattern of recovery of ¹⁵N placed at different soil depths; more ¹⁵N was recovered by maize and Gliricidia from placements at 5 cm depth than from placements at 45 or 65 cm depth. Peltophorum recovered similar amounts of ¹⁵N from placements at each of these depths, and hence had a deeper N uptake distribution than Gliricidiaor maize. Differences in tree L_rv across the cropping alley were comparatively small, and there was no significant difference (P<0.05) in the uptake of ¹⁵N placed in topsoil at different distances from hedgerows. A greater proportion of the ¹⁵N recovered by maize was found in grain following ¹⁵N placement at 45 cm or 65 cm depth than following placement at 5 cm depth, reflecting the later arrival of maize roots in these deeper soil layers. Thus trees have an important role in preventing N leaching from subsoil during early crop establishment, although they themselves showed a lag phase in ¹⁵N uptake after pruning. Residual ¹⁵N enrichment in soil was strongly related to application depth even 406 days after ¹⁵N placement, demonstrating the validity of this approach to mapping root activity distributions.     

Symbiotic N2-fixation by the cover crop Pueraria phaseoloides as influenced by litter mineralization

The perennial legume Pueraria phaseoloides is widely used as a cover crop in rubber and oil palm plantations. However, very little knowledge exists on the effect of litter mineralization from P. phaseoloides on its symbiotic N₂-fixation. The contribution from symbiotic N₂-fixation (Ndfa) and litter N (Ndfl) to total plant N in P. phaseoloides was determined in a pot experiment using a ¹⁵N cross-labeling technique. For determination of N₂-fixation the non-fixing plant Axonopus compressus was used as a reference. The experiment was carried out in a growth chamber during 9 weeks with a sandy soil and 4 rates of ground litter (C/N=16,2.8% N). P. phaseoloides plants supplied with the highest amount of litter produced 26% more dry matter and fixed 23% more N than plants grown in soil with no litter application, but the percentage of Ndfa decreased slightly, but significantly, from 87 to 84%. The litter N uptake was directly proportional to the rate of application and constituted 10% of total plant N at the highest application rate. Additionally, a positive correlation was found between litter N uptake and the amount of fixed N₂. The total recovery of litter N in plants averaged 26% at harvest (shoot + root) and was not affected by the quantity added. A parallel incubation experiment also showed that, as an average of all litter levels, 26% of the litter N was present in the inorganic N pool. The amounts of fertilizer and soil N taken up by plants decreased with litter application, probably due to microbial immobilization and denitrification. It is concluded that, within the litter levels studied, litter mineralization will result in a higher amount of N₂-fixed by P. phaseoloides.     

Biological Nitrogen Fixation in Sugar Cane: A Key to Energetically Viable Biofuel Production

The advantages of producing biofuels to replace fossil energy sources are derived from the fact that the energy accumulated in the biomass is captured directly from photosynthesis and is thus renewable, and that the cycle of carbon dioxide fixation by the crop, followed by burning of the fuel makes no overall contribution to atmospheric CO₂ or, consequently, to global warming. However, these advantages are negated if large quantities of fossil fuels need to be used to grow or process the biofuel crop. In this regard, the Brazilian bioethanol program, based on the fermentation/distillation of sugar cane juice, is particularly favorable, not only because the crop is principally hand harvested, but also because of the low nitrogen fertilizer use on sugar cane in Brazil. Recent ¹⁵N and N balance studies have shown that in some Brazilian cane varieties, high yields are possible without N fertilization because the plants are able to obtain large contributions of nitrogen from plant-associated biological N₂ fixation (BNF). The N₂-fixing acid-tolerant bacterium Acetobacter diazotrophicus was first found to occur within roots, stems, and leaves of sugar cane. Subsequently, two species of Herbaspirillum also have been found to occur within the interior of all sugar cane tissues. The discovery of these, and other N₂-fixing bacteria that survive poorly in soil but thrive within plant tissue (endophytic bacteria), may account for the high BNF contributions observed in sugar cane. Further study of this system should allow the gradual elimination of N fertilizer use on sugar cane, at least in Brazil, and opens up the possibility of the extension of this efficient N₂-fixing system to cereal and other crops with consequent immense potential benefits to tropical agriculture.     

Effects of soil pH and nitrogen fertility on the population dynamics of Thielaviopsis basicola

Black root rot of tobacco, caused by Thielaviopsis basicola, is generally severe at soil pH values >5.6 and suppressed under more acidic conditions (pH < 5.2). Soil acidifying fertilizers containing NH₄–N are generally recommended for burley tobacco production in North Carolina, but the effects of N form and application rate on development of black root rot and on the population dynamics of T. basicola have not been determined. Greenhouse and laboratory studies were conducted to evaluate the effects of N form (NH₄ ⁺ or NO₃ ^–) and rate on pathogen and disease parameters at several initial soil pH levels. A moderately-conducive field soil, initial pH 4.7, was adjusted to a pH of 5.5 or 6.5 by the addition of CaOH₂, then amended with the desired nitrogen form and rate. Pathogen populations were determined over time. In addition, spore production in extracts of roots from plants grown in the various nitrogen and pH treatments was determined. Finally, because tobacco responds to acidic soil conditions and exposure to NH₄–N by accumulating high concentrations of the polyamine putrescine, the toxicity of putrescine on vegetative growth and reproduction of T. basicola was investigated. Low soil pH and high levels of NH₄–N suppressed reproduction of T. basicola in soil and in root extract, while use of NO₃–N and depletion of NH₄–N resulted in rapid increases in populations of T. basicola. At 20 mM, putrescine inhibited hyphal growth by 60% and aleuriospore production by 98%. Fertilizers that reduced soil pH also reduced reproduction by T. basicola, and thus have potential for management of black root rot by suppressing populations of T. basicola over multiple years of crop production. The suppression of T. basicola and black root rot observed with NH₄–N amendments may partially be due to development of an inhibitory environment in the root and not solely to changes in rhizosphere pH.     

中亚热带山区土壤不同形态铁铝氧化物对团聚体稳定性的影响

以福建省三明市本底条件一致的三片林分的土壤作为研究对象,采用湿筛法测定了水稳定性团聚体粒径分布,分析了不同形态铁铝氧化物含量与0.25 mm大团聚体数量及团聚体平均重量直径(MWD)的关系。结果表明:(1)不同形态铁铝氧化物含量呈现出米槠次生林米槠人工林杉木人工林,游离结晶态(Fed、Ald)无定形(Feo、Alo)络合态(Fes、Als)。(2)0.25 mm大团聚体含量呈现米槠次生林米槠人促林杉木人工林,林分之间差异显著,MWD值的趋势与之相似。(3)线性回归分析表明:不同形态铁铝氧化物均与0.25 mm水稳定性大团聚体数量及MWD值达到显著甚至极显著相关,但通过分析相关系数R和显著性P说明氧化铝可能比氧化铁更有助于大团聚体的形成与稳定,无定形及络合态铁铝氧化物比游离态铁铝氧化物更能促进大团聚体的形成与稳定。