Carbon distribution and losses: erosion and deposition effects

Because of concerns about the eventual impact of atmospheric CO₂ accumulations, there is growing interest in reducing net CO₂ emissions from soil and increasing C storage in soil. This review presents a framework to assess soil erosion and deposition processes on the distribution and loss of C in soils. The physical processes of erosion and deposition affect soil C distribution in two main ways and should be considered when evaluating the impact of agriculture on C storage. First, these processes redistribute considerable amounts of soil C, within a toposequence or a field, or to a distant site. Accurate estimates of soil redistribution in the landscape or field are needed to quantify the relative magnitude of soil lost by erosion and accumulated by deposition. Secondly, erosion and deposition drastically alter the biological process of C mineralization in soil landscapes. Whereas erosion and deposition only redistribute soil and organic C, mineralization results in a net loss of C from the soil system to the atmosphere. Little is known about the magnitude of organic C losses by mineralization and those due to erosion, but the limited data available suggest that mineralization predominates in the first years after the initial cultivation of the soil, and that erosion becomes a major factor in later years. Soils in depositional sites usually contain a larger proportion of the total organic C in labile fractions of soil C because this material can be easily transported. If the accumulation of soil in depositional areas is extensive, the net result of the burial (and subsequent reduction in decomposition) of this active soil organic matter would be increased C storage. Soil erosion is the most widespread form of soil degradation. At regional or global levels its greatest impact on C storage may be in affecting soil productivity. Erosion usually results in decreased primary productivity, which in turn adversely affects C storage in soil because of the reduced quantity of organic C returned to the soil as plant residues. Thus the use of management practices that prevent or reduce soil erosion may be the best strategy to maintain, or possibly increase, the worlds soil C storage.     

菜地土壤氮素矿化和硝化作用的特征

采用培养试验对南京郊区 6 对菜地土和水稻土的土壤 N 素矿化和硝化作用特征进行了研究。菜地土为相同类型水稻土改种蔬菜约 20 年的土壤。结果表明,培养 28 天期间,6 对供试土壤中有 4 对土壤都是菜地土壤矿化 N 量低于相同类型水稻土,其日矿化速率也低于相应的水稻土,而其他 2 对供试土壤之间无明显差异。大多数菜地土的土壤硝化率低于相应的水稻土。培养28 天时的矿化率和硝化率与土壤 pH、速效 P 呈显著相关。     

采用15N同位素稀释法研究不同层次土壤氮素总转化速率

采用15N同位素稀释方法,开展短期(7天)室内培养实验,估算了一水稻土0~20、20~60和60~90 cm土层土壤主要N素转化过程的总转化速率,结果表明,标记N溶液加入后2 h内各土层土壤的总矿化、硝化、固定速率显著高于其他时间段(p<0.01)。2 h后,矿化速率在小范围内起伏。0~20 cm土层土壤N素的硝化速率随培养时间延长而降低,另外两层土壤则基本保持稳定,硝化速率的变化与硝化作用底物NH4+-N浓度的变化呈显著正相关。值得注意的是,外源无机N溶液加入后2 h内,大量NH4+-N和NO3--N被固定,并认为N素的非生物固定起主导作用。2 h后,出现了N素在固定与再矿化间反复转换的现象。实验结果表明,与净转化速率相比总转化速率能更好地描述单个N素转化过程,但由于外源N加入对N素转化的影响、再矿化作用以及忽略了N素转化过程中的气体损失、DNRA(硝态氮异化还原为铵)过程等,本研究结果与真实值间存在一定差异。     

关于我国现代农业发展中的有机肥问题

在化肥已成为肥料主体的背景下,讨论了中国现代农业发展中的有机肥问题。(1)有机肥的数量和质量。近几年来有机肥数量不断增加,每年有机肥资源量约40亿吨,但总体质量较差;(2)有机肥的产业化和商品化;(3)有机肥的矿质化和腐质化;(4)科学、高效地利用有机肥。     

长期施肥对黑土呼吸过程的影响

土壤呼吸是土壤有机C矿化分解,释放无机养分的重要生物化学过程。对公主岭地区长期有机肥(不施有机肥、施中量和高量有机肥处理)与化肥(不施化肥、施用N、NP、NPK化肥)配合施用的12个处理的黑土进行室内好气培养(196天),采用一级动力学方程模拟土壤的呼吸过程,结果表明,有机肥和化肥的施用能显著增加土壤呼吸释放的CO2 -C的累积量,提高土壤中潜在矿化的有机碳含量及其占土壤有机质的比例,促进土壤有机质中无机养分的释放,有利于提高土壤养分的有效性,改善黑土的供肥状况。有机肥与NPK化肥配合施用效果更为明显。     

温度和土地利用变化对土壤有机碳矿化的影响

[目的]研究温度和土地利用变化对土壤有机碳矿化的影响。[方法]以水田和林地土壤为研究对象,采用室内培养试验,测定土壤有机碳矿化量。[结果]水田和林地土壤有机碳矿化速率变化趋势均为前期较快,后期稳定。土壤有机碳矿化所释放的CO2-C累积量随着培养温度的升高而显著增加（P〈0.05）,温度升高10℃,水田和林地土壤释放的CO2-C累积量分别增加157.8%和135.8%,但林地和水田土壤CO2-C累积量的差异并不明显。[结论]温度对土壤有机碳矿化有明显的影响,而土地利用变化对有机碳矿化没有影响。     

绿肥中养分释放规律及对烟叶品质的影响

在田间条件下利用埋设玻璃滤纸包法研究了绿肥翻压后的分解规律及其翻压绿肥对烟叶品质的影响。结果表明，经过13周的分解，苜蓿中有机C的矿化率高达81％，N素矿化率为54％；黑麦草有机C的矿化率为63％，N素矿化率为22％。这些矿化N中约92％的N（苜蓿）和74％的N（黑麦草）是在翻压后的前6周释放的。在烟株的整个生长期内，由苜蓿释放的N素约为N43奴hm^-2，为了防止后期N素供应过多而导致上部烟叶烟碱含量过高，苜蓿宜提前翻压并从总N肥施用量中扣除由苜蓿矿化出的N。黑麦草因N素含量低和N素矿化量少，整个生长季中约释放了N10．5kghm^-2的N素，所以在黑麦草生长量不大的情况下，可将黑麦草全部翻压并忽略其释放的N素。 在没有减少化肥施用量的情况下，翻压苜蓿提高了上部烟叶中烟碱含量，降低了总糖和还原糖含量。而翻压黑麦草对烟叶中烟碱含量有所降低，总糖和还原糖含量适中，因而烟叶的品质得到了改善。     

松材线虫危害后马尾松林土壤氮素的矿化特性

选择合肥大蜀山森林公园受松材线虫危害程度不同的马尾松林为研究对象,采用室内模拟试验对林氮素的转化性质,增加了土壤氮素淋失的危险性。地土壤氮素矿化特征进行了研究。结果表明:不同林分土壤氮素转化速率差异显著(P<0.05),受损严重的林分氮素矿化速率显著高于轻度受损林分,特别是硝化速率更为显著(P<0.01)。松材线虫病危害改变了马尾松林土壤     

Tillage-induced environmental conditions in soil and substrate limitation determine biogenic gas production

Tillage changes soil environmental conditions and controls the distribution of residues in the soil, both actions that affect the production and emission of soil biogenic gases (CO₂, N₂O, and CH₄). The objective of this study was to determine how tillage-induced environmental conditions and substrate quality affect the mineralization rate of easily metabolizable compounds and the subsequent production of these gases. Carbon compounds, with and without nitrogen, were applied to soil cropped to maize under tilled and no-till systems. Following substrate application in the spring and summer, biogenic gases were measured periodically at the soil surface (flux) and within the profile (concentration) at 10-, 20-, and 30-cm depths (i.e., within, at the bottom of, and below the plough layer). Strong CO₂ and N₂O responses to sucrose and glycine in both the field and the laboratory indicate that the soil was C- and N-limited. Surface fluxes of CO₂ and N₂O were greater in soils amended with glycine than with sucrose and were greater in tilled than no-till soils. Transient emission of CH₄ following the addition of glycine was observed and could be attributed to inhibition of N mineralization and nitrification processes on CH₄ oxidation. Laboratory and field measurements indicated that the larger substrate-induced CO₂ emission from the tilled soils could not be attributed to differences in the total biomass or the basal respiratory activity of the soils. Thus, there appears to be no underlying difference in the functional capacity of the microbial communities under different tillage regimes. Comparison of gas profiles indicates relative accumulation of CO₂ at depth in soils under no-till, as well as greater decline in profile CO₂ content with time in the tilled compared to the no-till soil. These results support the conclusion that greater CO₂ efflux from the tilled soils resulted from more rapid gas diffusion through the profile. Hence, the observed differences in gas fluxes between tilled and no-till soils can be attributed to differences in physical environment.     

Impacts of anthropogenic N additions on nitrogen mineralization from plant litter in exotic annual grasslands

Urban regions of southern California receive up to 45 kg N ha^-1 y^-1 from nitrogen (N) deposition. A field decomposition study was done using ¹⁵N-labelled litter of the widespread exotic annual grass Bromus diandrus to determine whether elevated soil N is strictly from N deposition or whether N mineralization rates from litter are also increased under N deposition. Tissue N and lignin concentrations, which are inversely related in field sites with high and low N deposition, determine the rate at which N moves from plant litter to soil and becomes available to plants. The effect of soil N on N movement from litter to soil was tested by placing litter on high and low N soil in a factorial experiment with two levels of litter N and two levels of soil N. The litter quality changes associated with N deposition resulted in faster rates of N cycling from litter to soil. Concentrations of litter-derived N in total N, NH₄⁺, NO₃⁻, microbial N and organic N were all higher from high N/low lignin litter than from low N/high lignin litter. Litter contributed more N to soil NH₄⁺ and microbial N in high N than low N soil. At the end of the study, N mineralized from high N litter on high N soil accounted for 46% of soil NH₄⁺ and 11% of soil NO₃⁻, compared to 35% of soil NH₄⁺ and 6% of soil NO₃⁻ from low N litter on low N soil. The study showed that in high N deposition areas, elevated inorganic soil N concentrations at the end of the summer N deposition season are a result of N mineralized from plant litter as well as from N deposition.