Amendment Placement Directs Soil Carbon and Nitrogen Cycling in Severely Disturbed Soils

The recovery of ecosystem processes in severely disturbed systems is often limited by biological resources in the soil. The objective of this study was to direct soil microbial biomass (SMB) size and activity with organic amendments. These amendments were applied to the soil at different amendment locations (incorporated versus surface‐applied) and amounts (none, light, and heavy) in a 2 × 3 factorial design. The size and activity of SMB, soil nutrients, and aboveground biomass were monitored over 3 years to determine the rate and direction of change. Contrary to expectations that SMB and carbon mineralization potential (C‐MIN) would be larger with amendment incorporation, SMB‐carbon was greatest in the surface‐heavy treatment and lowest in the incorporated‐control treatment. SMB‐nitrogen, C‐MIN, and organic carbon were greater in the surface than in the incorporated treatments and in amended plots compared to controls. This departure from expectations suggests that other factors, such as microclimate or vegetation, are interacting with the amendment to affect SMB. The degree of contribution, however, is unclear. The treatments only affected planted aboveground biomass early in the experiment, with greater total biomass in the surface‐light treatment in fall 2003. There was also a significant positive relationship between aboveground biomass and SMB in fall 2004. Inorganic nitrogen, total nitrogen, and the soil quality indicators qCO₂ and C_mic/C_org did not vary systematically with amendment treatment. In general, amendment addition did enhance soil biotic properties and supported increased vegetation, but the complication of incorporating the amendment was not necessary for promoting biological development in disturbed soils.     

Soil Carbon, Nitrogen and Phosphorus Dynamics as Affected by Solarization Alone or Combined with Organic Amendment

Soil solarization, alone or combined with organic amendment, is an increasingly attractive approach for managing soil-borne plant pathogens in agricultural soils. Even though it consists in a relatively mild heating treatment, the increased soil temperature may strongly affect soil microbial processes and nutrients dynamics. This study aimed to investigate the impact of solarization, either with or without addition of farmyard manure, in soil dynamics of various C, N and P pools. Changes in total C, N and P contents and in some functionally-related labile pools (soil microbial biomass C and N, K₂SO₄-extractable C and N, basal respiration, KCl-exchangeable ammonium and nitrate, and water-soluble P) were followed across a 72-day field soil solarization experiment carried out during a summer period on a clay loam soil in Southern Italy. Soil physico-chemical properties (temperature, moisture content and pH) were also monitored. The average soil temperature at 8-cm depth in solarized soils approached 55 °C as compared to 35 °C found in nonsolarized soil. Two-way ANOVA (solarization×organic amendment) showed that both factors significantly affected most of the above variables, being the highest influence exerted by the organic amendment. With no manure addition, solarization did not significantly affect soil total C, N and P pools. Whereas soil pH, microbial biomass and, at a greater extent, K₂SO₄-extractable N and KCl-exchangeable ammonium were greatly affected. An increased release of water-soluble P was also found in solarized soils. Yet, solarization altered the quality of soluble organic residues released in soil as it lowered the C-to-N ratio of both soil microbial biomass and K₂SO₄-extractable organic substrates. Additionally, in solarized soils the metabolic quotient (qCO₂) significantly increased while the microbial biomass C-to-total organic C ratio (microbial quotient) decreased over the whole time course. We argued that soil solarization promoted the mineralization of readily decomposable pools of the native soil organic matter (e.g. the microbial biomass) thus rendering larger, at least over a short-term, the available fraction of some soil mineral nutrients, namely N and P forms. However, over a longer prospective solarization may lead to an over-exploitation of labile organic resources in agricultural soils. Manure addition greatly increased the levels of both total and labile C, N and P pools. Thus, addition of organic amendments could represent an important strategy to protect agricultural lands from excessive soil resources exploitation and to maintain soil fertility while enhancing pest control.     

Microbial Activity in Organic Soils as Affected by Soil Depth and Crop

The microbial activity of Pahokee muck, a lithic medisaprist, and the effect of various environmental factors, such as position in the profile and type of plant cover, were examined. Catabolic activity for [7-¹⁴C]salicylic acid, [1,4-¹⁴C]succinate, and [1,2-¹⁴C]acetate remained reasonably constant in surface (0 to 10 cm) soil samples from a fallow (bare) field from late in the wet season (May to September) through January. Late in January, the microbial activity toward all three compounds decreased approximately 50%. The microbial activity of the soil decreased with increasing depth of soil. Salicylate catabolism was the most sensitive to increasing moisture deep in the soil profile. At the end of the wet season, a 90% decrease in activity between the surface and the 60- to 70-cm depth occurred. Catabolism of acetate and succinate decreased approximately 75% in the same samples. Little effect of crop was observed. Variation in the microbial activity, as measured by the catabolism of labeled acetate, salicylate, or succinate, was not significant between a sugarcane (Saccharum officinarum L.) field and a fallow field. The activity with acetate was insignificantly different in a St. Augustine grass [Stenotaphrum secundatum (Walt) Kuntz] field, whereas the catabolism of the remaining substrates was elevated in the grass field. These results indicate that the total carbon evolved from the different levels of the soil profile by the microbial community oxidizing the soil organic matter decreased as the depth of the soil column increased. However, correction of the amount of carbon yielded at each level for the bulk density of that level reveals that the microbial contribution to the soil subsidence is approximately equivalent throughout the soil profile above the water table.     

Carbon Partitioning and Assimilation as Affected by Nitrogen Deficiency in Cassava

Plants of cassava (Manihot esculenta Crantz) were raised in a sand root medium watered with nutrient solutions, under greenhouse conditions. As the N-supply increased, shoot dry mass was enhanced to a greater extent than root dry mass, thus leading to an increased shoot to root ratio. In leaves, contents of total soluble saccharides, non-reducing saccharides, and inorganic phosphate increased linearly with increasing N-supply. An opposite response was found for reducing saccharides and starch. In general, content of non-reducing saccharides was considerably greater than starch content. Activity of sucrose synthase was not detected, regardless of the N-treatments; by contrast, activity of neutral and acid invertases increased with increasing N-availability. Roots accumulated more total soluble saccharides, but less reducing saccharides and starch, as the N-supply increased. Photosynthetic rates decreased with increasing N-deficiency. Such a decrease was circumstantially associated to reducing saccharide, but not starch, accumulation. Results suggest a limited capacity for carbon export from source leaves under N-limitation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stratification and Storage of Soil Organic Carbon and Nitrogen as Affected by Tillage Practices in the North China Plain

Tillage practices can redistribute the soil profiles, and thus affects soil organic carbon (SOC), and its storage. The stratification ratio (SR) can be an indicator of soil quality. This study was conducted to determine tillage effects on the profile distribution of certain soil properties in winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) systems in the North China Plain (NCP). Three tillage treatments, including no till (NT), rotary tillage (RT), and plow tillage (PT), were established in 2001 in Luancheng County, Hebei Province. The concentration, storage, and SR of SOC and soil total nitrogen (TN) were assessed in both the wheat and maize seasons. Compared with RT and PT, the mean SRs for all depth ratios of SOC under NT increased by 7.85% and 30.61% during the maize season, and by 14.67% and 30.91% during the wheat season, respectively. The SR of TN for 0–5:30–50 cm increased by 140%, 161%, and 161% in the maize season, and 266%, 154%, and 122% in the wheat season compared to the SR for 0–5:5–10 cm under NT, RT and PT, respectively. The data indicated that SOC and TN were both concentrated in the surface-soil layers (0–10 cm) under NT but were distributed relatively evenly through the soil profile under PT. Meanwhile, the storage of SOC and TN was higher under NT for the surface soil (0–10 cm) but was higher under PT for the deeper soil (30–50 cm). Furthermore, the storage of SOC and TN was significantly related to SR of SOC and TN along the whole soil profile (P<0.0001). Therefore, SR could be used to explain and indicate the changes in the storage of SOC and TN. Further, NT stratifies SOC and TN, enhances the topsoil SOC storage, and helps to improve SOC sequestration and soil quality.     

Crop Management for Soil Carbon Sequestration

Reducing emissions of greenhouse gases (GHG) from agriculture is related to increasing and protecting soil organic matter (SOM) concentration. Agricultural soils can be a significant sink for atmospheric carbon (C) through increase of the SOM concentration. The natural ecosystems such as forests or prairies, where C gains are in equilibrium with losses, lose a large fraction of the antecedent C pool upon conversion to agricultural ecosystems. Adoption of recommended management practices (RMPs) can enhance the soil organic carbon (SOC) pool to fill the large C sink capacity on the world's agricultural soils. This article collates, reviews, and synthesizes the available information on SOC sequestration by RMPs, with specific references to crop rotations and tillage practices, cover crops, ley farming and agroforestry, use of manure and biosolids, N fertilization, and precision farming and irrigation. There is a strong interaction among RMPs with regards to their effect on SOC concentration and soil quality. The new equilibrium SOC level may be achieved over 25 to 50 years. While RMPs are being adapted in developed economies, there is an urgent need to encourage their adoption in developing countries. In addition to enhancing SOC concentration, adoption of RMPs also increases agronomic yield. Thus, key to enhancing soil quality and achieving food security lies in managing agricultural ecosystems using ecological principles which lead to enhancement of SOC pool and sustainable management of soil and water resources.     

Carbon Source Quality and Placement Effects on Soil Organic Carbon Status

Improved management of agricultural soils has potential for sequestering carbon (C) and reducing the accumulation of atmospheric carbon dioxide. Development of management practices to increase C sequestration is dependent on improved understanding of soil processes influencing long-term storage of C. A field study was conducted to compare surface C source quality and above- vs. belowground addition of annual or perennial plant biomass effects on particulate organic matter (POM), total labile C (TLC), and total organic C (TOC). Since microaggregate stabilization within macroaggregates is the main mechanism for sequestering C, aggregate size distribution, expressed as mean weight diameter (MWD), and wet aggregate stability (WAS) was also measured. After 5 years, POM decreased in plots receiving surface application of readily available substrate (sucrose and alfalfa pellets) and the bare surface control. Plots receiving plant additions [wood chips, growing wheat (Triticum aestivum L.) crop, growing switchgrass (Panicum virgatum L.) crop, and fallow receiving either wheat or switchgrass residue] exhibited higher TLC and TOC content. Plots receiving plant residue maintained MWD, and those supporting live plants exhibited increasing WAS. Surface plant residue protected the soil against raindrop impact and reduced the intensity of wetting and drying cycles allowing the development of larger more stable aggregates resulting in C accrual.     

Effects of Nitrogen and Phosphorus Fertilization on Soil Carbon Fractions in Alpine Meadows on the Qinghai-Tibetan Plateau

In grassland ecosystems, N and P fertilization often increase plant productivity, but there is no concensus if fertilization affects soil C fractions. We tested effects of N, P and N+P fertilization at 5, 10, 15 g m⁻² yr⁻¹ (N₅, N₁₀, N₁₅, P₅, P₁₀, P₁₅, N₅P₅, N₁₀P₁₀, and N₁₅P₁₅) compared to unfertilized control on soil C, soil microbial biomass and functional diversity at the 0–20 cm and 20–40 cm depth in an alpine meadow after 5 years of continuous fertilization. Fertilization increased total aboveground biomass of community and grass but decreased legume and forb biomass compared to no fertilization. All fertilization treatments decreased the C:N ratios of legumes and roots compared to control, however fertilization at rates of 5 and 15 g m⁻² yr⁻¹ decreased the C:N ratios of the grasses. Compared to the control, soil microbial biomass C increased in N₅, N₁₀, P₅, and P₁₀ in 0–20 cm, and increased in N₁₀ and P₅ while decreased in other treatments in 20–40 cm. Most of the fertilization treatments decreased the respiratory quotient (qCO₂) in 0–20 cm but increased qCO₂ in 20–40 cm. Fertilization increased soil microbial functional diversity (except N₁₅) but decreased cumulative C mineralization (except in N₁₅ in 0–20 cm and N₅ in 20–40 cm). Soil organic C (SOC) decreased in P₅ and P₁₅ in 0–20 cm and for most of the fertilization treatments (except N₁₅P₁₅) in 20–40 cm. Overall, these results suggested that soils will not be a C sink (except N₁₅P₁₅). Nitrogen and phosphorus fertilization may lower the SOC pool by altering the plant biomass composition, especially the C:N ratios of different plant functional groups, and modifying C substrate utilization patterns of soil microbial communities. The N+P fertilization at 15 g m⁻² yr⁻¹ may be used in increasing plant aboveground biomass and soil C accumulation under these meadows.     

The Effect of Soil Erosion on Europe’s Crop Yields

Abstract Soil erosion negatively affects crop yields and may have contributed to the collapse of ancient civilizations. Whether erosion may have such an impact on modern societies as well, is subject to debate. In this paper we quantify the relationship between crop yields and soil water available to plants, the most important yield-determining factor affected by erosion, at the European scale. Using information on the spatial distribution of erosion rates we calculate the potential threat of erosion-induced productivity losses. We show that future reductions in productivity in Europe as a whole are relatively small and do not pose a substantial threat to crop production within the coming century. However, within Europe there is considerable variability, and although productivity in northern Europe is not likely to be significantly reduced by soil erosion, for the southern countries the threat of erosion-induced productivity declines is stronger.     

秸秆还田与施肥对土壤酶活性和作物产量的影响

通过大田定位试验,在小麦-玉米轮作条件下,以小麦品种‘西农889’和玉米品种‘郑单958’为供试作物,采取不施肥秸秆不还田(CK)、秸秆还田(S)、秸秆还田+腐熟有机肥(SM)、秸秆还田+氮肥(SN)、秸秆还田+氮肥+磷肥(SNP)共5种处理,对不同处理下土壤电导率、蔗糖酶活性和脲酶活性的动态变化及作物产量进行了研究。结果显示:(1)秸秆还田后土壤的电导率变化呈先上升后下降趋势,不同处理间周年电导率平均值表现为SNP>SN>SM>S>CK,且差异显著。(2)秸秆还田配合施用氮肥处理的土壤蔗糖酶活性和脲酶活性最高,蔗糖酶活性最大值(70.62mg.g-1.d-1)为对照的1.36倍,脲酶活性最大值(3.58mg.g-1.d-1)比对照提高了9.15%。(3)土壤有机碳含量在S、SM处理之间差异不显著,而S、SM处理与CK、SN、SNP处理之间差异显著,SM处理比对照处理提高了8.91%。(4)土壤全氮含量在不同处理之间差异显著,并以SNP处理最高,其次是SM处理,S、SN处理再次之,且SNP、SM、S、SN处理土壤全氮含量分别比对照提高了19.8%、11.1%、9.88%和7.41%。(5)秸秆还田处理的作物产量显著高于CK,并以秸秆配施氮磷肥处理的小麦产量最高,比CK提高了50.6%;秸秆配施氮肥处理的玉米产量最高,比CK提高了34.3%。研究表明,秸秆还田配施有机肥、无机肥可以有效促进有机物矿质化,显著增加土壤养分含量,增强土壤酶活性,提高土壤有机碳含量,从而促进作物增产。     

Soil Phosphorus Fractions and Symbiotic Nitrogen Fixation across a Substrate-Age Gradient in Hawaii

We evaluated soil phosphorus (P) fractions, other soil characteristics, and rates of symbiotic N₂ fixation across a substrate-age gradient in Hawaii that was dominated by the leguminous tree Acacia koa (koa). Patterns of soil P observed on this gradient were compared to those on a slightly wetter gradient dominated by the nonfixer Metrosideros polymorpha (ohia). Along both gradients, concentrations of primary-mineral P fell sharply between the young and intermediate-aged sites, while labile inorganic P declined most steeply between the intermediate-aged and old sites. The most marked difference between the two gradients was that total soil carbon (C), nitrogen (N), and P, as well as nonoccluded organic P, were more variable across the ohia gradient, increasing to the intermediate-aged sites, then declining sharply at the old site. On the koa gradient, specific nitrogenase activity, measured by the acetylene-reduction (AR) assay, decreased three- to eightfold between the young site and the intermediate-aged and old sites, respectively. Nodule biomass showed no clear pattern. N₂ fixation rates, estimated by combining AR activity and nodule biomass measurements, were up to 8 kg N · ha⁻¹ · y⁻¹ at the young site and no more than 2 kg N · ha⁻¹ · y⁻¹ at the older sites, suggesting that koa may be a modest source of N in these Hawaiian forests. Received 26 September 2000; accepted 15 February 2002     

LLWR Techniques for Quantifying Potential Soil Compaction Consequences of Crop Residue Removal

Harvesting crop residues for bioenergy or bio-product production may decrease soil organic matter (SOM) content, resulting in the degradation of soil physical properties and ultimately soil productivity. Using the least limiting water range (LLWR) to evaluate improvement or degradation of soil physical properties in response to SOM changes has generally been hampered by the extensive amount of data needed to parameterize limiting factor models for crop production. Our objective was to evaluate five pedotransfer functions to determine their effectiveness in predicting soil water holding capacity in response to different SOM levels. Similarly, two other pedotransfer functions were evaluated to determine the effects of SOM on cone index values. Predictions of field capacity and wilting point water content as well as the cone index–water content–bulk density relationship of soil strength using the pedotransfer functions were compared with field data from two tillage experiments near Akron, CO that had a range of SOM concentrations. Equations previously developed by da Silva and Kay gave the best estimates of LLWR for the pedotransfer functions we evaluated. These equations were then used to illustrate LLWR changes in response to different soil and crop management practices on a Duroc loam near Sidney, NE. The results showed that tillage and, possibly, soil erosion decreased the LLWR as tillage intensity increased. Therefore, we recommend that crop residue removal rates be limited to rates that maintain or increase SOM content to ensure soil physical conditions are not degraded.     

代谢基因工程提高作物产量的分子靶标

应用基因工程技术对植物细胞内的代谢途径进行遗传修饰,已成功地使细胞代谢发生改变或合成新的化合物。光合作用,淀粉合成,氮素同化和水分利用等是形成作物产量的基础代谢。对这些代谢途径中的关键步骤和靶分子进行基因修饰以提高作物产量的研究已取得长足的进展,并正在发展成为提高作物产量的新途径。本文着重论述应用代谢基因工程提高作物产量的技术策略,研究现状,存在的问题,所面临的挑战和应用前景。     

Control of Nitrogen Loss from Forested Watersheds by Soil Carbon:Nitrogen Ratio andTree Species Composition

Leaching losses of nitrate from forests can have potentially serious consequences for soils and receiving waters. In this study, based on extensive sampling of forested watersheds in the Catskill Mountains of New York State, we examine the relationships among stream chemistry, the properties of the forest floor, and the tree species composition of watersheds. We report the first evidence from North America that nitrate export from forested watersheds is strongly influenced by the carbon:nitrogen (C:N) ratio of the watershed soils. We also show that variation in soil C:N ratio is associated with variation in tree species composition. This implies that N retention and release in forested watersheds is regulated at least in part by tree species composition and that changes in species composition caused by introduced pests, climate change, or forest management could affect the capacity of a forest ecosystem to retain atmospherically deposited N. Received 4 March 2002; Accepted 4 June 2002.     

不同杉木林分类型土壤团聚体生态化学计量特征

在野外调查的基础上,选择成土母质相同、坡向坡度相似、海拔基本一致的杉木-米老排、杉木-火力楠和杉木纯林3种杉木人工林采集土壤样品,通过干筛法分离＞2 mm、2～0.25 mm和＜0.25 mm 3个团聚体组分,研究其土壤团聚体有机碳、全氮、全磷的含量及其生态化学计量特征,以阐明不同杉木林分类型土壤团聚体碳氮磷生态化学计...     

Laccase Gene Composition and Relative Abundance in Oak Forest Soil is not Affected by Short-Term Nitrogen Fertilization

Anthropogenic nitrogen (N) deposition affects a wide range of soil processes including phenol oxidase (PO) activity and soil organic matter dynamics. Depression of phenol oxidase activity in response to N saturation is believed to be mediated by the activity of white-rot basidiomycetes, whose production of extracellular oxidative enzymes can be limited by high N availability. We examined the effect of short-term N deposition on basidiomycete laccase gene diversity and relative abundance in temperate oak forest soil in which significant decreases in phenol oxidase and increased SOM have been recorded in response to experimental N deposition. UniFrac was used to compare the composition of laccase genes between three control- and three nitrogen-fertilized (80 kg⁻¹ ha⁻¹ per year) oak forest soils. The relative abundance of laccase genes was determined from qPCR analysis of laccase and basidiomycete ITS gene abundances. Our results indicate that there was no significant shift in the composition of laccase genes between control- and N-fertilized soils, nor was there a significant change in the relative abundance of laccase genes. These data suggest that N deposition effects on mineral soil PO activity do not result from changes in laccase gene diversity of white-rot basidiomycetes but are likely the result of altered microbial abundance or expression in this ecosystem type. Furthermore, laccase gene composition may be tied to factors that structure microbial communities in general, as soil laccase gene communities are more similar to other forest soils than with the corresponding litter.     

土壤镉污染与作物

通过土壤镉污染对作物影响的盆栽模拟试验,以揭示重金属镉在土壤—植物系统中的转移、分布规律及其对植物生长、发育的影响。试验证明不同化合形态的镉施入土壤(砂壤质褐土,pH值8.2),水稻对镉吸收的多寡依次为CdCl2>CdSO4>CdO>CdS>CdCO3。白菜的镉吸收表现为CdSO4>CdCl2>CdO>CdCO3。土壤的不同镉浓度(施加CdCl2,以纯镉计)对作物影响的试验结果表明,可食部位达到食品污染标准(谷物含镉量0.4ppm、蔬菜0.2ppm为暂定标准)时的土壤镉污染临界值分别为：小麦、莴苣、白菜<1ppm,茄子、萝卜<2.5ppm,番茄、菜豆<20ppm。土壤因子的处理影响镉的活动性;降低土壤pH值,水稻的镉吸收增加。增施有机肥、ZnSO4、S、CaO、CaSO4可降低糙米含镉量13.4%一30%。白菜的镉吸收,由于增施有机肥、FeSO4、Fe2O3、CaO或S而降低菜叶含镉量28%一61%。以Cd,Zn比1:100或1:200施入土壤,叶内含镉量分别下降61%和76.4%,但白菜产量减少61%和76%。     

Soil Carbon and Nitrogen Storage in Upper Montane Riparian Meadows

Though typically limited in aerial extent, soils of high-elevation riparian wetlands have among the highest density of soil carbon (C) and nitrogen (N) of terrestrial ecosystems and therefore contribute disproportionally to ecosystem services such as water retention, forage production, wildlife habitat, and reactive N removal. Because much soil C and N is stored in labile forms in anaerobic conditions, management activities or environmental changes that lead to drying cause mineralization of labile soil organic matter, and loss of C and N. Meadows are focal points of human activities in mountain regions, often with incised stream channels from historically heavy grazing exacerbated by extreme runoff events. To quantify soil C and N stores in montane riparian meadows across hydrologic conditions, 17 meadows between 1950- and 2675-m elevation were selected in the central Sierra Nevada Range, California, that were classified using the proper functioning condition (PFC) system. Results indicate that C and N density in whole-solum soil cores were equivalent at forest edge positions of properly functioning, functioning at-risk, and nonfunctioning condition. Soils under more moist meadow positions in properly functioning meadows have at least twice the C, N, dissolved organic C, and dissolved organic N (DON) than those under nonfunctioning meadows. Densities of total N and DON, but not C, of functioning at-risk meadows are significantly lower (P < 0.05) than those of properly functioning meadows at mid-slope and stream-bank positions, suggesting accelerated loss of N early in degradation processes. Though variable, the soil attributes measured correspond well to the PFC riparian wetland classification system.     

Three-Year Soil Carbon and Nitrogen Responses to Sugarcane Straw Management

Green harvest sugarcane management has increased soil organic C and N stocks over time. However, emerging sugarcane straw removal to meet increasing bioenergy demands has raised concerns about soil C and N depletions. Thus, we conducted a field study in southeast Brazil over nearly three years (1100 days) for assessing soil C and N responses to increasing sugarcane straw removal rates. In order to detect the C input as a function of the different amounts of straw over three years, a field simulation was performed, where the original soil layer (0–0.30 m) was replaced by another from an adjacent area with low total C and δ¹³C. The treatments tested were as follows: (i) 0 Mg ha^?1 year^?1 (i.e., 100% removal), (ii) 3.5 Mg ha^?1 year^?1 (i.e., 75% removal), (iii) 7.0 Mg ha^?1 year^?1 (i.e., 50% removal), (iv) 14.0 Mg ha^?1 year^?1 (i.e., no removal), and (v) 21.0 Mg ha^?1 year^?1 (i.e., no removal + extra 50% of the straw left on the field). The results showed that sugarcane straw removal affected the soil C and total N pools. In the first 45 days of straw decomposition, a small but important straw-derived C portion enters into the soil as dissolved organic carbon (DOC). The lower the straw removal rate, the higher was straw-derived DOC content found into the soil, down to 0.50 m depth. After 3 years of management, keeping sugarcane straw on soil surface significantly increased C and N stocks within surface soil layer (0–0.025 m). Our findings suggest that under no straw removal management (i.e., 14 Mg ha^?1), approximately 364 kg ha^?1 of C and 23 kg ha^?1 of N are annually stored into this low-C soil. The contribution of the straw-derived C (C-C4) to the total soil C increases over time, which accounted for about 60% under no straw removal rate. The greatest contribution of the C storage preferentially occurs into the fraction of organic matter (<?0.53 μm) associated with soil clay minerals. We concluded that indiscriminate sugarcane straw removal to produce cellulosic ethanol or bioelectricity depletes soil C stocks and reduces N cycling in sugarcane fields, impairing environmental gains associated with bioenergy production. Therefore, this information, linked with other agronomic and environmental issues, should be taken into account towards a more sustainable straw removal management for bioenergy production in Brazil.     

Carbon and Nitrogen Mineralization in Relation to Soil Particle-Size Fractions after 32 Years of Chemical and Manure Application in a Continuous Maize Cropping System

Long-term manure application is recognized as an efficient management practice to enhance soil organic carbon (SOC) accumulation and nitrogen (N) mineralization capacity. A field study was established in 1979 to understand the impact of long-term manure and/or chemical fertilizer application on soil fertility in a continuous maize cropping system. Soil samples were collected from field plots in 2012 from 9 fertilization treatments (M₀CK, M₀N, M₀NPK, M₃₀CK, M₃₀N, M₃₀NPK, M₆₀CK, M₆₀N, and M₆₀NPK) where M₀, M₃₀, and M₆₀ refer to manure applied at rates of 0, 30, and 60 t ha⁻¹ yr⁻¹, respectively; CK indicates no fertilizer; N and NPK refer to chemical fertilizer in the forms of either N or N plus phosphorus (P) and potassium (K). Soils were separated into three particle-size fractions (2000–250, 250–53, and <53 μm) by dry- and wet-sieving. A laboratory incubation study of these separated particle-size fractions was used to evaluate the effect of long-term manure, in combination with/without chemical fertilization application, on the accumulation and mineralization of SOC and total N in each fraction. Results showed that long-term manure application significantly increased SOC and total N content and enhanced C and N mineralization in the three particle-size fractions. The content of SOC and total N followed the order 2000–250 μm > 250–53μm > 53 μm fraction, whereas the amount of C and N mineralization followed the reverse order. In the <53 μm fraction, the M₆₀NPK treatment significantly increased the amount of C and N mineralized (7.0 and 10.1 times, respectively) compared to the M₀CK treatment. Long-term manure application, especially when combined with chemical fertilizers, resulted in increased soil microbial biomass C and N, and a decreased microbial metabolic quotient. Consequently, long-term manure fertilization was beneficial to both soil C and N turnover and microbial activity, and had significant effect on the microbial metabolic quotient.