Optimisation of Nitrogen Supply from Sugarcane Residues in the Wet Tropics

Retention of sugarcane leaves and tops on the soil surface after harvesting has almost completely replaced burning of crop residues in the Australian sugar industry. Long term retention of residue is believed to improve soil fertility to the extent that nitrogen (N) fertilizer applications might be reduced by up to 40 kg N/ha/y. However, the fate of N in the extreme environment of the wet tropics is not known with certainty. Indices of potential N mineralisation and nitrification were developed and indicate that potential N fertility is greater in the wet tropics compared to more southern cane growing areas, and is enhanced under residue retention. Field results from the wet tropics support this prediction, but indicate high soil ammonium-N concentrations relative to nitrate-N.

Saprotrophic survival of Magnaporthe oryzae in infested wheat residues.

2016

Hydrothermal technologies for the production of fuels and chemicals from biomass

Biomass and non-food crop residues are seen as relatively low cost and abundant renewable sources capable of making a large contribution to the world’s future energy and chemicals supply. Signifi cant quantities of ethanol are currently produced from biomass via biochemical processes, but thermochemical conversion processes offer greater potential to utilize the entire biomass source to produce a range of products. This chapter will review thermochemical gasifi cation and pyrolysis methods with a focus on hydrothermal liquefaction processes. Hydrothermal liquefaction is the most energetically advantageous thermochemical biomass conversion process. If the target is to produce sustainable liquid fuels and chemicals and reduce the impact of global warming as a result of carbon dioxide, nitrous oxide, and methane emissions (i.e., protect the natural environment), the use of “green” solvents, biocatalysts and heterogeneous catalysts must be the main R&D initiatives. As the biocrude produced from hydrothermal liquefaction is a complex mixture which is relatively viscous, corrosive, and unstable to oxidation (due to the presence of water and oxygenated compounds), additional upgrading processes are required to produce suitable biofuels and chemicals.

Impact of nitrification inhibitor (DMPP) on soil nitrous oxide emissions from an intensive broccoli production system in sub-tropical Australia

Vegetable cropping systems are often characterised by high inputs of nitrogen fertiliser. Elevated emissions of nitrous oxide (N2O) can be expected as a consequence. In order to mitigate N2O emissions from fertilised agricultural fields, the use of nitrification inhibitors, in combination with ammonium based fertilisers, has been promoted. However, no data is currently available on the use of nitrification inhibitors in sub-tropical vegetable systems. A field experiment was conducted to investigate the effect of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on N2O emissions and yield from broccoli production in sub-tropical Australia. Soil N2O fluxes were monitored continuously (3 h sampling frequency) with fully automated, pneumatically operated measuring chambers linked to a sampling control system and a gas chromatograph. Cumulative N2O emissions over the 5 month observation period amounted to 298 g-N/ha, 324 g-N/ha, 411 g-N/ha and 463 g-N/ha in the conventional fertiliser (CONV), the DMPP treatment (DMPP), the DMMP treatment with a 10% reduced fertiliser rate (DMPP-red) and the zero fertiliser (0N), respectively. The temporal variation of N2O fluxes showed only low emissions over the broccoli cropping phase, but significantly elevated emissions were observed in all treatments following broccoli residues being incorporated into the soil. Overall 70–90% of the total emissions occurred in this 5 weeks fallow phase. There was a significant inhibition effect of DMPP on N2O emissions and soil mineral N content over the broccoli cropping phase where the application of DMPP reduced N2O emissions by 75% compared to the standard practice. However, there was no statistical difference between the treatments during the fallow phase or when the whole season was considered. This study shows that DMPP has the potential to reduce N2O emissions from intensive vegetable systems, but also highlights the importance of post-harvest emissions from incorporated vegetable residues. N2O mitigation strategies in vegetable systems need to target these post-harvest emissions and a better evaluation of the effect of nitrification inhibitors over the fallow phase is needed.

Effect of nitrification inhibitors (DMPP and 3MP+ TZ) on soil nitrous oxide emissions from a sub-tropical vegetable production system in Queensland, Australia

The use of nitrification inhibitors, in combination with ammonium based fertilisers, has been promoted recently as an effective method to reduce nitrous oxide (N2O) emissions from fertilised agricultural fields, whilst increasing yield and nitrogen use efficiency. Vegetable cropping systems are often characterised by high inputs of nitrogen fertiliser and consequently elevated emissions of nitrous oxide (N2O) can be expected. However, to date only limited data is available on the use of nitrification inhibitors in sub-tropical vegetable systems. A field experiment investigated the effect of the nitrification inhibitors (DMPP & 3MP+TZ) on N2O emissions and yield from a typical vegetable production system in sub-tropical Australia. Soil N2O fluxes were monitored continuously over an entire year with a fully automated system. Measurements were taken from three subplots for each treatment within a randomized complete blocks design. There was a significant inhibition effect of DMPP and 3MP+TZ on N2O emissions and soil mineral N content directly following the application of the fertiliser over the vegetable cropping phase. However this mitigation was offset by elevated N2O emissions from the inhibitor treatments over the post-harvest fallow period. Cumulative annual N2O emissions amounted to 1.22 kg-N/ha, 1.16 kg-N/ha, 1.50 kg-N/ha and 0.86 kg-N/ha in the conventional fertiliser (CONV), the DMPP treatment, the 3MP+TZ treatment and the zero fertiliser (0N) respectively. Corresponding fertiliser induced emission factors (EFs) were low with only 0.09 - 0.20% of the total applied fertiliser lost as N2O. There was no significant effect of the nitrification inhibitors on yield compared to the CONV treatment for the three vegetable crops (green beans, broccoli, lettuce) grown over the experimental period. This study highlights that N2O emissions from such vegetable cropping system are primarily controlled by post-harvest emissions following the incorporation of vegetable crop residues into the soil. It also shows that the use of nitrification inhibitors can lead to elevated N2O emissions by storing N in the soil profile that is available to soil microbes during the decomposition of the vegetable residues over the post-harvest phase. Hence the use of nitrification inhibitors in vegetable systems has to be treated carefully and fertiliser rates need to be adjusted to avoid excess soil nitrogen during the postharvest phase.

Rejection by pigs of mouldy grain containing deoxynivalenol

Weaner pigs on a farm near Beaudesert in south eastern Queensland refused to eat feed comprised largely of wheat and barley. Older pigs consumed small amounts and some prepubertal gilts subsequently displayed enlarged and reddened vulvas. Wheat, barley and triticale were grown on the farm during 1983, which was unusually and persistently wet. The wheat and triticale were harvested and stored for about 3 weeks with moisture contents above 14% before being fed. Samples of the wheat and triticale contained pale pink grains, which can indicate infection by the fungus Fusariurn grarninearurn Schw. On analysis 2 mycotoxins known to be produced by F. graminearurn were detected, deoxynivalenol (vomitoxin) which causes feed refusal and vomiting, and zearalenone which causes oestrogenic effects. Concentrations of deoxynivalenol in the wheat, triticale and barley were 34, 10, and <0.1 mg/kg respectively. Concentrations of zearalenone were 6.2, 2.8 and 0.1 mg/kg respectively. Subsequently, F. grarninearurn was isolated from grains and crop residues. Although the wet weather contributed to F. grarninearurn infection of the crops before harvest, most of the toxins probably developed during storage.

Tillage and the environment in sub-tropical Australia-Tradeoffs and challenges

Tillage is defined here in a broad sense, including disturbance of the soil and crop residues, wheel traffic and sowing opportunities. In sub-tropical, semi-arid cropping areas in Australia, tillage systems have evolved from intensively tilled bare fallow systems, with high soil losses, to reduced and no tillage systems. In recent years, the use of controlled traffic has also increased. These conservation tillage systems are successful in reducing water erosion of soil and sediment-bound chemicals. Control of runoff of dissolved nutrients and weakly sorbed chemicals is less certain. Adoption of new practices appears to have been related to practical and economic considerations, and proved to be more profitable after a considerable period of research and development. However there are still challenges. One challenge is to ensure that systems that reduce soil erosion, which may involve greater use of chemicals, do not degrade water quality in streams. Another challenge is to ensure that systems that improve water entry do not increase drainage below the crop root zone, which would increase the risk of salinity. Better understanding of how tillage practices influence soil hydrology, runoff and erosion processes should lead to better tillage systems and enable better management of risks to water quality and soil health. Finally, the need to determine the effectiveness of in-field management practices in achieving stream water quality targets in large, multi-land use catchments will challenge our current knowledge base and the tools available.

Abundance and mortality of overwintering pupae of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on the Darling Downs, Queensland, Australia

Surveys were conducted between 1997 and 2001 to investigate the incidence of overwintering Helicoverpa spp. pupae under summer crop residues on the Darling Downs, Queensland. Only Helicoverpa armigera was represented in collections of overwintering pupae. The results indicated that late-season crops of cotton, sorghum, maize, soybean, mungbean and sunflower were equally likely to have overwintering pupae under them. In the absence of tillage practices, these crops had the potential to produce similar numbers of moths/ha in the spring. There were expected differences between years in the densities of overwintering pupae and the number of emerged moths/ha. Irrigated crops produced 2.5 times more moths/ha than dryland crops. Overall survival from autumn-formed pupae to emerged moths averaged 44%, with a higher proportion of pupae under maize surviving to produce moths than each of the other crops. Parasitoids killed 44.1% of pupae, with Heteropelma scaposum representing 83.3% of all parasitoids reared from pupae. Percentage parasitism levels were lower in irrigated crops (27.6%) compared with dryland crops (40.5%). Recent changes to Helicoverpa spp. management in cotton/grain-farming systems in south-eastern Queensland, including widespread adoption of Bt cotton, and use of more effective and more selective insecticides, could lead to lower densities of overwintering pupae under late summer crops.

Influence of organic amendments on greenhouse gas emissions and N use efficiency in sub-tropical cropping systems

This thesis investigated the impact of organic sources of nutrients on greenhouse gas emissions (carbon dioxide, nitrous oxide and methane), nitrogen use efficiency and biomass production in subtropical cropping soils. The study was conducted in two main soil types in subtropical ecosystems, sandy loam soil and clay soil, with a variety of organic materials from agro-industrial residues and crop residues. It is important for recycling of agro-industrial residues and agricultural residues and the mitigation of greenhouse gas emissions and nitrogen use efficiency.

Undersowing in a northern climate: effects on spring cereal yield and risk of nitrate leaching

Disadvantages of invariable cereal cropping, concern of nutrient leaching and prices of nitrogen (N) fertilizer have all increased during last decades. An undersown crop, which grows together with a main crop and after harvest, could mitigate all those questions. The aim of this study was to develop undersowing in Finnish conditions, so that it suits for spring cereal farming as well as possible and enhances taking care of soil and environment, especially when control of N is concerned. In total, 17 plant species were undersown in spring cereals during the field experiments between 1991-1999 at four sites in South and Central Finland, but after selection, eight of them were studied more thoroughly. Two legumes, one grass species and one mixture of them were included in long-term trials in order to study annually repeated undersowing. Further, simultaneous broadcasting of seeds instead of separate undersowing was studied. Grain yield response and the capacity of the undersown crop to absorb soil N or fix N from atmosphere, and the release of N were of greatest interest. Seeding rates of undersown crops and N fertilization rates during annually repeated undersowing were also studied. Italian ryegrass (Lolium multiflorum Lam., IR) absorbed soil nitrate N (NO3-N) most efficiently in autumn and timothy (Phleum pratense L.) in spring. The capacity of other grass species to absorb N was low, or it was insufficient considering the negative effect on grain yield. Red clover (Trifolium pratense L.) and white clover (Trifolium repens L.) suited well in annually repeated undersowing, supplying fixed N for cereals without markedly increased risk of N leaching. Autumn oriented growth rhythm of the studied legumes was optimal for undersowing, whereas the growth rhythm of grasses was less suited but varied between species. A model of adaptive undersowing system was outlined in order to emphasize allocation of measures according needs. After defining the goal of undersowing, many decisions are to be done. When diminishing N leaching is primarily sought, a mixture of IR and timothy is advantageous. Clovers suit for replacing N fertilization, as the positive residual effect is greater than the negative effect caused by competition. A mixture of legume and non legume is a good choice when increased diversity is the main target. Seeding rate is an efficient means for adjusting competition and N effects. Broadcasting with soil covering equipment can be used to establish an undersown crop. In addition, timing and method of cover crop termination have an important role in the outcome. Continuous observing of the system is needed as for instance conditions significantly affect growth of undersown crop and on the other hand N release from crop residues may increase in long run.

Effect of added nitrogen on plant litter decomposition depends on initial soil carbon and nitrogen stoichiometry

Increasing organic carbon inputs to agricultural soils through the use of pastures or crop residues has been suggested as a means of restoring soil organic carbon lost via anthropogenic activities, such as land use change. However, the decomposition and retention of different plant residues in soil, and how these processes are affected by soil properties and nitrogen fertiliser application, is not fully understood. We evaluated the rate and extent of decomposition of 13C-pulse labelled plant material in response to nitrogen addition in four pasture soils of varying physico-chemical characteristics. Microbial respiration of buffel grass (Cenchrus ciliaris L.), wheat (Triticum aestivum L.) and lucerne (Medicago sativa L.) residues was monitored over 365-days. A double exponential model fitted to the data suggested that microbial respiration occurred as an early rapid and a late slow stage. A weighted three-compartment mixing model estimated the decomposition of both soluble and insoluble plant 13C (mg C kg−1 soil). Total plant material decomposition followed the alkyl C: O-alkyl C ratio of plant material, as determined by solid-state 13C nuclear magnetic resonance spectroscopy. Urea-N addition increased the decomposition of insoluble plant 13C in some soils (≤0.1% total nitrogen) but not others (0.3% total nitrogen). Principal components regression analysis indicated that 26% of the variability of plant material decomposition was explained by soil physico-chemical characteristics (P = 0.001), which was primarily described by the C:N ratio. We conclude that plant species with increasing alkyl C: O-alkyl C ratio are better retained as soil organic matter, and that the C:N stoichiometry of soils determines whether N addition leads to increases in soil organic carbon stocks.

Green and brown bridges between weeds and crops reveal novel Diaporthe species in Australia

Diaporthe (syn. Phomopsis) species are well-known saprobes, endophytes or pathogens on a range of plants. Several species have wide host ranges and multiple species may sometimes colonise the same host species. This study describes eight novel Diaporthe species isolated from live and/or dead tissue from the broad acre crops lupin, maize, mungbean, soybean and sunflower, and associated weed species in Queensland and New South Wales, as well as the environmental weed bitou bush (Chrysanthemoides monilifera subsp. rotundata) in eastern Australia. The new taxa are differentiated on the basis of morphology and DNA sequence analyses based on the nuclear ribosomal internal transcribed spacer region, and part of the translation elongation factor-1α and ß-tubulin genes. The possible agricultural significance of live weeds and crop residues ('green bridges') as well as dead weeds and crop residues ('brown bridges') in aiding survival of the newly described Diaporthe species is discussed.

The role of sustainable agriculture and renewable-resource management in reducing greenhouse-gas emissions and increasing sinks in China and India

This paper contains an analysis of the technical options in agriculture for reducing greenhouse-gas emissions and increasing sinks, arising from three distinct mechanisms: (i) increasing carbon sinks in soil organic matter and above-ground biomass; (ii) avoiding carbon emissions from farms by reducing direct and indirect energy use; and (iii) increasing renewable-energy production from biomass that either substitutes for consumption of fossil fuels or replaces inefficient burning of fuelwood or crop residues, and so avoids carbon emissions, together with use of biogas digesters and improved cookstoves. We then review best-practice sustainable agriculture and renewable-resource-management projects and initiatives in China and India, and analyse the annual net sinks being created by these projects, and the potential market value of the carbon sequestered. We conclude with a summary of the policy and institutional conditions and reforms required for adoption of best sustainability practice in the agricultural sector to achieve the desired reductions in emissions and increases in sinks. A review of 40 sustainable agriculture and renewable-resource-management projects in China and India under the three mechanisms estimated a carbon mitigation potential of 64.8 MtC yr(-1) from 5.5 Mha. The potential income for carbon mitigation is $324 million at $5 per tonne of carbon. The potential exists to increase this by orders of magnitude, and so contribute significantly to greenhouse-gas abatement. Most agricultural mitigation options also provide several ancillary benefits. However, there are many technical, financial, policy, legal and institutional barriers to overcome.

GHG footprint of major cities in India

Concentration of greenhouse gases (GHG) in the atmosphere has been increasing rapidly during the last century due to ever increasing anthropogenic activities resulting in significant increases in the temperature of the Earth causing global warming. Major sources of GHG are forests (due to human induced land cover changes leading to deforestation), power generation (burning of fossil fuels), transportation (burning fossil fuel), agriculture (livestock, farming, rice cultivation and burning of crop residues), water bodies (wetlands), industry and urban activities (building, construction, transport, solid and liquid waste). Aggregation of GHG (CO2 and non-CO2 gases), in terms of Carbon dioxide equivalent (CO(2)e), indicate the GHG footprint. GHG footprint is thus a measure of the impact of human activities on the environment in terms of the amount of greenhouse gases produced. This study focuses on accounting of the amount of three important greenhouses gases namely carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) and thereby developing GHG footprint of the major cities in India. National GHG inventories have been used for quantification of sector-wise greenhouse gas emissions. Country specific emission factors are used where all the emission factors are available. Default emission factors from IPCC guidelines are used when there are no country specific emission factors. Emission of each greenhouse gas is estimated by multiplying fuel consumption by the corresponding emission factor. The current study estimates GHG footprint or GHG emissions (in terms of CO2 equivalent) for Indian major cities and explores the linkages with the population and GDP. GHG footprint (Aggregation of Carbon dioxide equivalent emissions of GHG's) of Delhi, Greater Mumbai, Kolkata, Chennai, Greater Bangalore, Hyderabad and Ahmedabad are found to be 38,633.2 Gg, 22,783.08 Gg, 14,812.10 Gg, 22,090.55 Gg, 19,796.5 Gg, 13,734.59 Gg and 91,24.45 Gg CO2 eq., respectively. The major contributors sectors are transportation sector (contributing 32%, 17.4%, 13.3%, 19.5%, 43.5%, 56.86% and 25%), domestic sector (contributing 30.26%, 37.2%, 42.78%, 39%, 21.6%, 17.05% and 27.9%) and industrial sector (contributing 7.9%, 7.9%, 17.66%, 20.25%, 1231%, 11.38% and 22.41%) of the total emissions in Delhi, Greater Mumbai, Kolkata, Chennai, Greater Bangalore, Hyderabad and Ahmedabad, respectively. Chennai emits 4.79 t of CO2 equivalent emissions per capita, the highest among all the cities followed by Kolkata which emits 3.29 t of CO2 equivalent emissions per capita. Also Chennai emits the highest CO2 equivalent emissions per GDP (2.55 t CO2 eq./Lakh Rs.) followed by Greater Bangalore which emits 2.18 t CO2 eq./Lakh Rs. (C) 2015 Elsevier Ltd. All rights reserved.

土壤有机碳和氮的分布及其对气候变化的响应

碳、氮不仅是生物体必需的营养元素，也是重要的生态元素。大气中温室气体C02、N2O等浓度的增加使得碳、氮的生物地球化学循环及其温室气体的减缓排放措施研究成为全球变化研究中的热点问题。 土壤是陆地生态系统的核心，是连接大气圈、水圈、生物圈、岩石圈的纽带;它是陆生生物赖以生存的物质基础，是陆地生态系统中物质与能量交换的重要场所，其在全球碳、氮循环中起着十分重要的作用。一方面，土壤有机碳和氮的含量与分布直接关系到生态系统的生产力和生态系统的规模，同时土壤有机碳和氮的转化与迁移又直接影响到温室气体的组成与含量。而土壤本身又是生态系统中生物与环境相互作用的产物。因此，研究土壤有机碳和氮的分布、转化及其对全球变化的响应对于正确理解碳、氮的生物地球化学循环及其对全球变化的响应制定应对策略具有重要意义。 全球变化的陆地样带是从机理上理解陆地生态系统对全球变化的响应，预测全球变化对陆地生态系统的可能影响，实现预警、调节和减少全球变化不良影响，科学地规划和管理陆地生态系统的有效平台。目前，国际地圈一生物圈计划(IGBP)基于不同地区全球变化驱动因素的不同以及全球变化的潜在反馈作用强度的不同，在全球4个关键地区共启动了15条IG8P陆地样带。以水分为主要驱动力的中国东北样带(NECT：Northeast China Transect)即为IGBP的陆地样带之一。 本文以中国东北样带为平台，基于2001年对中国东北样带科学考察所采土壤样品的实测结果和气候资料分析了土壤有机碳和氮的梯度分布及其与土壤、气候等因子之间的关系;借助C02浓度升高和不同土壤湿度的模拟试验探讨了土壤有机碳和氮对气候变化的响应;根据作物残体还田的长期定位试验和盆栽试验研究了作物残体还田对土壤有机碳和氮转化的影响，讨论了农田生态系统通过作物残体还田对减缓温室气体排放的效应。主要结果和结论如下： (1).样带表层土壤有机碳平均为22.3土4.93 g．kg-1，下层土壤有机碳平均为8.9±1.20 g．kg-1。样带表层土壤活性有机碳平均为3.52±0.881 g.kg-1，占表层土壤有机碳的13.1±0.78%;下层土壤活性有机碳平均为1.14±0.250g.kg-l，占下层土壤有机碳的10.9±0.79%。样带土壤活性有机碳与土壤有机碳之间呈极显著正相关关系(相关系数r=0.993，P<0.001)。 (2).不同生态类型土壤有机碳和活性有机碳含量不同。中国东北样带东部(经度126°～131°)为温带针阔混交林山地，植被种类极其丰富，地带性土壤为暗棕壤，并且多为自然土壤，土壤有机碳和活性有机碳含量较高。但由于采样区局部地理环境、植被结构及人类干扰程度的不同，土壤有机碳和活性有机碳含量变异较大，平均为61.9±13.84 g．kg-1和10. 88±2.236g. kg-1。样带中部（经度119°～126°）为松辽平原栎林草原、农田区和大兴安岭山地草甸草原区，属半湿润向半干旱过渡的气候。该区域主要土壤类型为黑土、黑钙土、盐化或碱化草甸土及风沙土，土壤沙化、碱化严重，土壤有机碳和活性有机碳含量明显降低，平均为10.5±1.97 g．kg-l和1. 35±0.327 g．kg-1。样带中西部（经度113°～119°）为内蒙古高原草甸草原和典型草原区域，具有典型的半干旱气候特征。该区地带性土壤为栗钙土，局部丘陵区分布黑钙土，土壤有机碳和活性有机碳含量为14.6±1.65 g．kg-1和2.07±0.342g.kg-1。样带西部（经度111°～113°）为内蒙古高原荒漠草原区域，地带性土壤为棕钙土，土壤较为贫瘠，其有机碳和活性有机碳含量最低，平均为7.99±1.51 g．kg-1和0.51±0.216 g．kg-1。从总的趋势看，样带表层土壤有机碳和活性有机碳的梯度分布趋势一致，都呈现出随经度降低而下降的趋势，局部因土壤退化而出现波动。 (3).样带土壤有机碳和活性有机碳与土壤全量氮、磷、硫、锌及有效氮、磷、钾、锰、锌等均呈显著或极显著相关关系，与土壤PH、容重、持水量及孔隙度也呈显著或极显著相关关系。土壤表层有机碳和活性有机碳与降水量之间具有正的相关关系，其相关系数为r=0.677（P<0.001）和r=0.712（P<0.001）。但下层土壤有机碳和活性有机碳与降水量之间没有显著的相关关系。 (4).样带下层土壤有机碳和活性有机碳与经度之间仍具有显著的相关关系（r=0.454，P=0.026; r=0.473，P=0.020）。样带下层土壤有机碳和活性有机碳的变异小于表层。不同的生态系统，下层土壤有机碳和活性有机碳与表层土壤有机碳和活性有机碳的比率不同。总的来看，土壤活性有机碳含量随深度的增加而下降的幅度大于土壤有机碳。 (5).短期培养条件下，CO2浓度升高及干旱胁迫下，土壤有机碳的变化不大，其变异系数为1.28%;相比较之下，土壤活性有机碳对气候变化比较敏感，其变异系数为29.67%。不同土壤湿度，土壤活性有机碳含量发生变异的幅度因CO2浓度升高而降低。 (6).样带土壤全氮和有效氮与经度呈极显著正相关，其相关系数分别是r=0.695 (P<0.001)和0.636(P<0.001)。土壤表层全氮和有效氮的梯度分布与土壤有机碳的分布基本一致：沿经度呈现东高西低的趋势，局部由于土壤退化而出现低谷。样带除东部山区外，其它各部分土壤有效氮都很低，成为其植被生长的限制因子之一。样带下层土壤全氮和有效氮的含量低于表层，但样带不同部位下层土壤全氮和有效氮下降的幅度不同。总的来看，土壤全氮的剖面分布和土壤有机碳相似，而土壤有效氮则有所不同。 (7).土壤全氮和有效氮是土壤生化环境中两个重要的因子。样带土壤全氮和有效氮和土壤有机碳、全磷、全硫、全锌、土壤活性碳、有效磷、有效钾、有效锰、有效锌、土壤容重、田间持水量土壤总孔度等因子均呈显著或极显著的相关关系。 (8).样带表层土壤全氮和有效氮与降雨量之间呈极显著的正相关关系，相关系数分别是0.682(P<0.001)和0.688(P<0.001)。而下层土壤全氮和有效氮与降雨量之间的没有显著的相关关系（r=0.241，P=0.256; r=0.366，P=0.079）。土壤有效氮占全氮的比例与年均温呈显著正相关关系（相关系数r=0.390，p=0.044）。 (9).短期培养试验中，CO2浓度加倍和不同土壤湿度对土壤全氮和有效氮的影响没有达到显著水平。整个试验中土壤全氮和有效氮的变异较小（变异系数分别是5.55%和3.84%），但仍能反映一定的变化趋势。 (10)．玉米残体还田能够增加土壤氮素含量，减轻因其作为燃烧材料而造成的氮素损失和对大气的污染;玉米残体施入土壤，增加了土壤微生物氮含量，提高土壤氮活性，有利于土壤氮素养分的协调供应;玉米残体还田能够促进氮素从营养器官向籽粒中转移，提高氮素养分的利用效率。同时，玉米残体还田可以降低土壤NO3--N的累 积，减少肥料氮的损失4.7～5.6%。 (ll)．根据国内外文献和我们连续10年作物残体还田的肥料长期定位试验及盆栽试验结果，从减缓CO2排放、增加土壤碳固存、提高土壤生产力入手，分析了农业生态系统作物残体还田的必要性与可行性，讨论了农田作物残体还田，增加土壤碳固存对于减缓CO2排放、提高土壤生产力的作用与意义。提倡作物残体因地制宜地归还土壤，但作物残体还田后土壤固存与减缓温室气体排放的潜力还需要进一步进行研究。