华南受干扰林和成熟林氮素流失对模拟氮沉降的响应

Current nitrogen (N) leaching losses and their responses to monthly N additions were investigated under a disturbed pine (Pinus massoniana) forest and a mature monsoon broadleaf forest in southern China. N leaching losses from both disturbed and mature forests were quite high (14.6 and 29.2 kg N ha^-1 year^-1, respectively), accounting for 57% and 80% of their corresponding atmospheric N inputs. N leaching losses were substantially increased following the first 1.5 years of N applications in both forests. The average increases induced by the addition of 50 and 100 kg N ha^-1 year^-1 were 36.5 and 24.9 kg N ha^-1 year^-1, respectively, in the mature forest, accounting for 73.0% and 24.9% of the annual amount of N added, and 14.2 and 16.8 kg N ha^-1 year^-1 in the disturbed forest, accounting for 28.4% and 16.8% of the added N. Great N leaching and a fast N leaching response to N additions in the mature forest might result from long-term N accumulation and high ambient N deposition load (greater than 30 kg N ha^-1 year^-1 over the past 15 years), whereas in the disturbed forest, it might result from the human disturbance and high ambient N deposition load. These results suggest that both disturbed and mature forests in the study region may be sensitive to increasing N deposition.     

Changes in C storage by terrestrial ecosystems: How C-N interactions restrict responses to CO2 and temperature

A general model of ecosystem biogeochemistry was used to examine the responses of arctic tundra and temperate hardwood forests to a doubling of CO₂ concentration and to a 5°C increase in average growing season temperature. The amount of C stored in both ecosystems increased with both increased CO₂ and temperature. Under increased CO₂, the increase in C storage was due to increases in the C∶N ratio of both vegetation and soils. Under increased temperature, the increased C storage in the forest was due to a shift in N from soils (with low C∶N ratios) to vegetation (with high C∶N ratios). In the tundra, both a shift in N from soils to vegetation and an increase in C∶N ratios contributed to increased C storage under higher temperatures. Neither ecosystem sequestered N from external sources because the supply rate was low.     

Soil microbial properties under N and P additions in a semi-arid, sandy grassland

Surface (0–15 cm) soil samples were collected from a semi-arid, sandy grassland in Keerqin Sandy Lands, Northeast China to study changes in soil microbial and chemical properties after five consecutive years of nitrogen (N) and phosphorus (P) additions. Nitrogen and P additions and their interactions negligibly affected soil organic carbon and total N contents, while P addition significantly increased soil total P content. Soil pH was significantly decreased by N addition, which significantly increased net nitrification rate, whereas it did not affect net N mineralization rate. No significant effects of N and P additions and their interactions on basal respiration were detected. In addition, N addition significantly decreased microbial biomass C (MBC) and N, and thus microbial quotient, but increased dissolved organic C and microbial metabolic quotient due to the significant decrease of MBC. Our results suggest that in the mid-term the addition of N, but not P, can change soil microbial properties, with a possible decline in soil quality of semi-arid, sandy grasslands.     

Stoichiometric responses of soil microflora to nutrient additions for two temperate forest soils

The ratios of soil carbon (C) to nitrogen (N) and C to phosphorus (P) are much higher in Chinese temperate forest soils than in other forest soils, implying that N and P might limit microbial growth and activities. The objective of this study was to assess stoichiometric responses of microbial biomass, enzyme activities, and respiration to N and P additions. We conducted a nutrient (N, P, and N + P) addition experiment in two temperate soils under Korean pine (Pinus koraiensis) plantation and natural broadleaf forest in Northeast China and measured the microbial biomass C, N, P; the activities of β-glucosidase (BG), N-acetyl-β-glucosaminidase (NAG), and acid and alkaline phosphomonoesterase (AP); and the microbial respiration in the two soils. Nitrogen addition increased microbial biomass N and decreased microbial biomass C-to-N ratio and microbial respiration in the two soils. Nitrogen addition decreased NAG activity to microbial biomass N ratio, P addition decreased AP activity to microbial biomass P ratio, and N, P, and N + P additions all increased BG activity to microbial biomass C ratio. These results suggest that microbial stoichiometry is not strictly homeostatic in response to nutrient additions, especially for N addition. The responses of enzyme activities to nutrient additions support the resource allocation theory. The N addition induced a decline in microbial respiration, implying that atmospheric N deposition may reduce microbial respiration, and consequently increase soil C sequestration in the temperate region.     

Impact of inorganic nitrogen additions on microbes in biological soil crusts

Many studies have shown that changes in nitrogen (N) availability affect the diversity and composition of soil microbial community in a variety of terrestrial systems, but less is known about the responses of microbes specific to biological soil crusts (BSCs) to increasing N additions. After seven years of field experiment, the bacterial diversity in lichen-dominated crusts decreased linearly with increasing inorganic N additions (ambient N deposition; low N addition, 3.5 g N m⁻² y⁻¹; medium N addition, 7.0 g N m⁻² y⁻¹; high N addition, 14.0 g N m⁻² y⁻¹), whereas the fungal diversity exhibited a distinctive pattern, with the low N-added crust containing a higher diversity than the other crusts. Pyrosequencing data revealed that the bacterial community shifted to more Cyanobacteria with modest N additions (low N and medium N) and to more Actinobacteria and Proteobacteria and much less Cyanobacteria with excess N addition (high N). Our results suggest that soil pH, together with soil organic carbon (C), structures the bacterial communities with N additions. Among the fungal communities, the relative abundance of Ascomycota increased with modest N but decreased with excess N. However, increasing N additions favored Basidiomycota, which may be ascribed to increases in substrate availability with low lignin and high cellulose contents under elevated N conditions. Bacteria/fungi ratios were higher in the N-added samples than in the control, suggesting that the bacterial biomass tends to dominate over that of fungi in lichen-dominated crusts after N additions, which is especially evident in the excess N condition. Because bacteria and fungi are important components and important decomposers in BSCs, the alterations of the bacterial and fungal communities may have implications in the formation and persistence of BSCs and the cycling and storage of C in desert ecosystems.     

Inhibition of Extracellular Enzyme Activities in a Forest Soil by Additions of Inorganic Nitrogen

Abstract

Forest ecosystems are known to be nitrogen (N) limited, and productivity often increases with N additions. However, recent studies have suggested that additions of excessive N through atmospheric deposition may affect soil processes adversely. In this study, we conducted a laboratory‐based manipulation experiment to reveal the effects of N additions on extracellular enzyme activities in a temperate forest soil. Three different levels of N additions (water‐only control, low N addition, and high N addition) were applied to soil columns (2.5‐cm diameter×12 cm height) over 4 weeks. Overall, the additions of N decreased specific activities (enzyme activity per log bacterial cell number) of urease, glutaminase, phosphatase, and arylsulfatase significantly. However, dehydrogenase activity was increased with the high N addition. The results suggest that N deposition may impede nutrient cycling, particularly mineralization of organic nutrients. In addition, this enzyme inhibition may be one of the mechanisms for N retention observed in forest floors.     

Responses of soil microbes and their interactions with plant community after nitrogen and phosphorus addition in a Tibetan alpine steppe

Journal of Soils and Sediments - Nitrogen (N) and phosphorus (P) additions are the widely used restoration management for degraded grasslands. However, soil microbial community responses to N and P...     

Plant material addition affects soil nitrous oxide production differently between aerobic and oxygen-limited conditions

It is a common agricultural practice for crop residues to be plowed into the soil or left on the soil surface. Soil addition of crop residues can considerably modify soil microbial activity and net N mineralization, and in general such modifications are negatively related to the C:N ratios of crop residues. Yet, little is known on the impacts of crop residues of different C:N ratios on soil nitrous oxide (N₂O) production under different aeration conditions via nitrification and denitrification. In this study, an 84-day laboratory incubation was conducted under aerobic and O₂-limited conditions and soil N₂O production was measured every 3 days after the addition of plant materials with a wide range of C:N ratios from 14 to 297. Two aerobic conditions were created by adjusting the water content of soil at a bulk density of 1.1 g cm⁻³ to 30% water-filled pore space (WFPS) and 60% WFPS, and two O₂-limited conditions were made by 90% WFPS and fluctuation between 90% and 30% WFPS. Each fluctuation cycle lasted 9 days and soil water content was readjusted to 90% WFPS at the end of each cycle. We also measured microbial respiration activity and net N mineralization periodically (i.e., 3, 7, 14, 28, 42, 56, 70, and 84 days) during the incubation and microbial biomass C at the end of incubation. At aerobic conditions, soil amendments of plant materials, regardless of their C:N ratios, all enhanced soil N₂O production. However, net N mineralization was dependent on plant material C:N ratios, being significantly higher or lower than the control for C:N ratios ∼15 and C:N ratios ≥44, respectively. Such inconsistent responses indicated that nitrifiers mediating nitrification and therefore byproduct N₂O production could strongly compete with heterotrophic microbes for NH₄⁺ and therefore net N mineralization was not a good predictor for nitrification-associated N₂O production. Interestingly, plant material additions reduced soil N₂O production by up to ∼95% at O₂-limited conditions, perhaps due to NO₃⁻ limitation. Soil NO₃⁻ production via nitrification could be low at O₂-limited conditions, and soil NO₃⁻ availability could be further reduced due to increases in microbial biomass and thus microbial N assimilation after plant material additions. This NO₃⁻ limitation might enhance N₂O reduction to N₂, by which denitrifiers could harvest more energy from the consumption of limited NO₃⁻. Nonetheless, our results revealed contrasting differences in N₂O production between aerobic and O₂-limited conditions following soil amendments of plant materials.     

Influence of pool substitution on the interpretation of fertilizer experiments with 15N

The uptake of labelled and unlabelled N by wheat was measured in pot and field experiments with ¹⁵N-labelled fertilizer. Soils from two sites on the same series were used in the pot experiment; one had been bare-fallowed for 22 years and contained 1.6% organic C, the other had been under grass for many years and contained 3.8% organic C. Fertilizer N increased the uptake of unlabelled soil N in both soils, i.e. there was a positive ‘added nitrogen interaction’ (ANI). There was no ANI in the field experiment. A simulation model is used to show how positive ANIs can arise as a result of ‘pool substitution’—labelled inorganic fertilizer N standing proxy for unlabelled inorganic soil N that would otherwise have been immobilized. In the low-organic fallow soil, pool substitution accounted for the whole of the observed ANI and fertilizer N did not enhance either gross or net mineralization of soil N. Pool substitution also operated in the high organic grassland soil, but here net mineralization of soil N increased with increasing additions of fertilizer, giving rise to a ‘real’ ANI in addition to the larger ‘apparent’ ANI caused by pool substitution. This increase in net mineralization is probably caused by a decrease in immobilization of N as fertilizer N additions increase, not by an increase in gross mineralization of soil N. For pool substitution to operate, fertilizer N and soil inorganic N must occupy the same pool. This occurred in the pot experiment but not in the field experiment, where fertilizer and soil inorganic N remained separate and there was no ANI. When pool substitution occurs, fertilizer use efficiency is predictably lower as measured by the isotopic method than as measured by the conventional non-isotopic procedure.     

头季稻氮肥运筹对再生稻干物质积累、产量及氮素利用率的影响

为探讨头季稻不同肥料运筹方式对再生稻产量和氮素利用率的影响,以杂交稻组合"Ⅱ优航2号"为材料,在头季施氮量225.00kg·hm-2的基础上,研究了不同基蘖穗肥氮素配比[3种基蘖肥与穗肥配比分别为8:2(N1)、7:3(N2)、6:4(N3)]头季稻-再生季稻氮素累积量、干物质生产、产量及氮素利用率的特性。结果表明:与N1、N2相比,头季成熟期N3处理氮素累积量分别增加9.26%、3.54%,头季齐穗期～头季成熟期N3处理氮素转移量分别增加21.47%、6.76%,整个生育期N3处理干物质净积累总量分别增加5.10%、4.78%。N3处理头季产量最高,达12431kg·hm-2,极显著高于N1、N2处理;氮肥利用率达46.44%,比N1、N2处理提高14.81%、5.43%;氮肥农学利用率达20.66kg·kg-1,比N1、N2处理提高14.97%、12.34%。研究结果还表明,头季不同基蘖穗肥氮素配比对再生稻再生季的影响不显著。     

Effects of tree species and N additions on forest floor microbial communities and extracellular enzyme activities

Forest nitrogen (N) retention and soil carbon (C) storage are influenced by tree species and their associated soil microbial communities. As global change factors alter forest composition, predicting long-term C and N dynamics will require understanding microbial community structure and function at the tree species level. Because atmospheric N deposition is increasing N inputs to forested ecosystems across the globe, including the northeastern US, it is also important to understand how microbial communities respond to added N. While prior studies have examined these topics in mixed-species stands, we focused on the responses of different tree species and their associated microbial communities within a single forest type - a northern hardwood forest in the Catskills Mountains, NY. Based on prior studies, we hypothesized that N additions would stimulate extracellular enzyme activities in relatively labile litters, but suppress oxidative enzyme activities in recalcitrant litters, and tested for independent tree species effects within this context. During the 2007 growing season (May-June), we measured enzyme activities and microbial community composition (using phospholipid fatty acid analysis - PLFA) of the forest floor in single-species plots dominated by sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), red oak (Quercus rubra), American beech (Fagus grandifolia) and eastern hemlock (Tsuga canadensis), species whose litters range from relatively labile to recalcitrant. Half the plots were fertilized with N by adding NH₄NO₃ (50 kg ha⁻¹ y⁻¹) from 1997 to 2009. Non-metric multidimensional scaling (NMS) and multi-response permutation procedures (MRPP) were used to examine microbial community structure and relationship to enzyme activities.We found that in response to N additions, both microbial community composition and enzyme activities changed; however the strength of the changes were tree species-specific and the direction of these changes was and not readily predictable from prior studies conducted in mixed-species stands. For example, in contrast to other studies, we found that N additions caused a significant overall increase in fungal biomass that was strongest for yellow birch (24% increase) and weakest for sugar maple (1% increase). Contrary to our initial hypotheses and current conceptual models, N additions reduced hydrolytic enzyme activities in hemlock plots and reduced oxidative enzyme activity in birch plots, a species with relatively labile litter. These responses suggest that our understanding of the interactions between microbial community composition, enzyme activity, substrate chemistry, and nutrient availability as influenced by tree species composition is incomplete. NMS ordination showed that patterns in microbial community structure (PLFA) and function (enzyme activity) were more strongly influenced by tree species than by fertilization, and only partially agreed with the structure-function relationships found in other studies. This finding suggests that tree species-specific responses are likely to be important in determining the structure and function of northeastern hardwood forests in the future. Enhanced understanding of microbial responses to added N in single and mixed-species substrates with varying amounts of lignin and phenols may be needed for accurate predictions of future soil C and N dynamics.     

祁连山青海云杉林叶片—枯落物—土壤的碳氮磷生态化学计量特征

以祁连山排露沟流域青海云杉(Picea crassifolia)林为研究对象,研究其不同海拔梯度叶片—枯落物—土壤间的碳、氮、磷生态化学计量学特征,并对其相关性进行分析。结果表明,在不同海拔梯度上,叶片、枯落物和土壤C∶N比的变化范围分别为22.95~36.72、21.41~41.61、12.41~20.70,均值大小依次为枯落物叶片土壤,C∶P和N∶P比的变化范围分别为510.2~739.8、398.6~698.1、134.1~219.7和18.13~26.86、6.71~26.28、7.96~16.56,均值大小依次均为叶片枯落物土壤。随海拔梯度的增加,除土壤C∶N比差异性不显著外(p0.05),叶片和枯落物的碳、氮、磷化学计量比在不同海拔间的差异显著性各不相同。叶片、枯落物和土壤C∶N比两两均具有显著正相关(p0.05),叶片与枯落物及土壤与枯落物C∶P比均具有显著负相关(p0.05),叶片与土壤C∶P比及不同组分N∶P比之间相关性均不显著(p0.05)。该研究结果有助于进一步了解青海云杉林碳、氮、磷在不同组分间的相互作用规律与机制。     

氮肥运筹对滴灌甜菜产量、氮素吸收和氮素平衡的影响

以甜菜品种"Beta356"为材料,研究了氮素运筹[甜菜叶丛快速增长期、块根膨大期和糖分积累期的氮素追施比例分别为6∶3∶1、5∶3∶2、4∶4∶2(用N1、N2、N3表示),不施氮素的处理为对照(用CK表示)]对滴灌甜菜产量、氮素吸收和氮素平衡的影响。结果表明:各处理甜菜产糖量为N3N2N1CK,氮素运筹间差异不显著。与N1处理相比,适当降低叶丛快速生长期的氮素施用比例,有利于提高氮肥表观利用率和氮肥表观残留率,降低氮肥表观损失率。其中N2处理的氮肥表观利用率和氮肥表观残留率分别比N1和N3处理提高了-3.00%和22.00%,108.22%和-0.14%,N2处理的氮肥表观损失率分别比N1和N3处理降低了262.40%和65.25%。综合考虑产量、氮素利用和氮素平衡认为,N2处理具有较高经济效益和环境效益,是北疆滴灌甜菜合理氮素运筹模式。     

Responses of Soil Acid Phosphomonoesterase Activity to Simulated Nitrogen Deposition in Three Forests of Subtropical China

Soil acid phosphomonoesterase activity(APA)plays a vital role in controlling phosphorus(P)cycling and reflecting the current degree of P limitation.Responses of soil APA to elevating nitrogen(N)deposition are important because of their potential applications in addressing the relationship between N and P in forest ecosystems.A study of responses of soil APA to simulated N deposition was conducted in three succession forests of subtropical China.The three forests include a Masson pine(Pinus massoniana)forest (MPF)-pioneer community,a coniferous and broad-leaved mixed forest(MF)-transition community and a monsoon evergreen broad-leaved forest(MEBF)-climax community.Four N treatments were designed for MEBF:control(without N added),low-N(50 kg N ha^-1 year^-1),and medium-N(100 kg N ha^-1 year^-1)and high-N(150 kg N ha^-1 year^-1),and only three N treatments(i.e.,control, low-N,medium-N)were established for MPF and MF.Results showed that soil APA was highest in MEBF,followed by MPF and MF.Soil APAs in both MPF and MF were not influenced by low-N treatments but depressed in medium-N treatments.However,soil APA in MEBF exhibited negative responses to high N additions,indicating that the environment of enhanced N depositions would reduce P supply for the mature forest ecosystem.Soil APA and its responses to N additions in subtropical forests were closely related to the succession stages in the forests.     

黄土高原不同年限刺槐土壤化学计量特征分析

通过对陕北黄土高原4种不同年限(10,15,25,40年)刺槐林地0—200cm土壤的采集与分析,研究不同刺槐种植时间对土壤营养元素及化学计量比的影响。在每个林龄的林地内设3个小区,每个小区采用随机采样法选取3个采样点分层采集土壤样品,进行土壤碳、氮、磷、钾测定,计算化学计量比。结果表明:(1)有机碳和全氮、全钾含量随刺槐年限增加呈基本增大的趋势,全磷随刺槐年限的增长变化不大;有机碳、全氮和全磷呈极显著的正相关关系;(2)在0—200cm土层,有机碳、全氮空间分布基本一致,随土层深度增加均呈先减少后趋于稳定的变化趋势;全磷在整个空间中的分布较为均匀,其空间变异性低于有机碳和全氮;全钾含量高于碳氮磷含量,且随土壤深度变化不大;(3)土壤C∶N比,C∶P比,N∶P比和C∶N∶P比均随刺槐年限的增长呈先降低后升高的趋势,都在15年处有最低值;(4)土壤C∶N比随土层深度在一定范围波动,C∶P和N∶P比均随土层深度的增加呈先减少后趋于稳定的变化趋势。     

硝态氮和铵态氮供应比例对雷竹碳、氮、磷化学计量的影响

【目的】硝态氮（NO₃--N）和铵态氮（NO₄+-N）是土壤中容易被植物吸收利用的两种无机态氮,对植物养分吸收的影响不同。研究不同比例硝态氮（NO₃--N）和铵态氮（NO₄+-N）供应下植物器官碳（C）、氮（N）、磷（P）化学计量特征,有助于了解土壤养分对植物体内C、N、P营养元素分配规律的影响。【方法】采用盆栽方法,以一年生雷竹（Phyllostachys violascens）为试材,进行了NO₃--N和NO₄+-N配比试验。在供氮量均为12.5 g/pot的前提下,设5个硝、铵供应比例处理:1:0、2:1、1:1、1:2、0:1。试验处理20天后,取雷竹竹冠上、中、下部叶片和细根样品,测定其C、N、P含量,并对其异速生长关系进行分析。【结果】不同硝铵比例处理间雷竹叶片和细根C含量差异不显著,N、P含量差异显著。随着氮素供应中NO₄+-N比例的增加,叶片和细根的N、P含量均在硝铵比为1:1、1:2时显著高于其他处理,C:N、C:P、N:P总体上呈降低趋势,表明生长速率提高;叶片和细根N与C、N与P的Ⅱ类线性回归斜率在硝铵比为1:1、1:2时显著增大,表明相同N供应水平下,硝铵比为1:1、1:2时,C、P有更多的积累量。【结论】不同硝铵比显著影响着雷竹叶片和细根C、N、P的化学计量特征,合理的硝铵混合比例可促进雷竹对C的固定和N、P吸收,以硝铵比为1:1、1:2较适宜雷竹生长与养分积累。     

灌溉方式和氮肥运筹对免耕厢沟栽培杂交稻氮素利用及产量的影响

【目的】免耕厢沟是四川重点推广的水稻栽培模式。研究该模式下不同灌溉方式和氮肥运筹对水稻干物质累积、转运和氮素利用效率等的影响,可为免耕厢沟水稻栽培水肥管理提供依据。【方法】 以杂交中稻F优498为试验材料,采用两因素裂区设计,主区为传统水层灌溉(W1)和干湿交替灌溉(W2)两种灌溉方式,副区为氮肥运筹模式,在总施氮量为150 kg/hm2条件下,设置基肥 ∶蘖肥 ∶穗肥分别为6 ∶2 ∶2(N1)、 4 ∶2 ∶4(N2)、 2 ∶2 ∶6(N3)等3种氮肥运筹模式,以不施氮(N0)为对照,研究免耕厢沟模式下,杂交稻在齐穗期、成熟期各处理下干物质氮素积累、茎鞘的干物质转运、产量及其构成因子以及氮素利用效率。【结果】 灌溉方式和氮肥运筹对水稻主要生育期干物质量和氮吸收、转运及产量具显著影响及互作效应。干湿交替灌溉能扩“库”增“源”,保证足够的穗数,提高干物质积累量;淹水灌溉无效分蘖较多,群体质量变差,对干物质积累、氮素吸收、产量造成不利影响。适宜的前氮后移能为水稻整个生育期提供比较平衡的氮素供应,促进氮素的吸收、 提高氮素积累、协调氮素分配;N2模式下氮素表观利用率达69%以上,氮肥的农学利用效率、表观利用率比N1(6 ∶2 ∶2)和N3(2 ∶2 ∶6)分别高4.50%~36.85%、 8.09%~28.54%,增产7.47% ~15.76%。合理的水氮管理显著提高各生育期的氮素积累量,促进齐穗后叶和茎鞘氮素向穗的运转量。【结论】 干湿交替灌溉(W2)和氮肥运筹4 ∶2 ∶4(N2)为本试验条件下的最优水氮运筹模式,其充分发挥了水氮耦合优势,促进齐穗后“源”(茎鞘、叶)氮素向“库”(穗)的运转,有利于高产群体构建,有效提高氮素利用率,提高水稻每穗实粒数和结实率,增产效果显著。     

Climatic Gradient,Cropping System,and Crop Residue Impacts on Carbon and Nitrogen Mineralization in No‐Till Soils

Abstract

In the West Central Great Plains of the United States, no‐till management has allowed for increased cropping intensity under dryland conditions. This, in turn, has affected the carbon (C) and nitrogen (N) mineralization dynamics of these systems. In this region, moisture stress increases from north to south due to an increase in evapotranspiration (ET), resulting in a climatic gradient that affects cropping system management. The objectives of this study were to determine the interaction of cropping system intensification and climatic gradient (ET) on C and N mineralization and to determine if the presence or absence of crop residue on the soil surface affects C and net N mineralization. Two cropping systems, winter wheat‐fallow (WF) (Triticum aestivium L.) and winter wheat‐corn (sorghum)‐millet‐fallow (WCMF) [Zea mays (L.), Sorghum bicolor (L.) Moench, Panicum milaceum (L.)] were studied at three locations across this aforementioned ET gradient. The treatments had been in place for 8 yrs prior to sampling in the study. These results showed that the more intense cropping system (WCMF) had a higher laboratory C mineralization rate at two of the three locations, which the study concluded resulted from larger residue biomass additions and larger quantities of surface residue and soil residue at these locations (Soil residue is defined as recognizable crop residue in the soil that is retained on a 0.6 mm screen). However, no differences in N mineralization occurred. This is most likely due to more N immobilization under WCMF as compared to WF. Presence or absence of crop residue on the surface of undisturbed soil cores during incubation affected potential C and net N mineralization more than either cropping system or location. Soil cores with the surface residue intact mineralized as much as 270% more C than the same soils where the surface crop residue had been removed. In laboratory studies evaluating the relative differences in cropping systems effects on C and N mineralization, the retention of crop residue on the soil surface may more accurately access the cropping system effects.     

Predictors of gross N mineralization and immobilization during decomposition of stabilized organic matter in agricultural soil

Long-term additions of different types of organic amendments affect the amount of soil organic matter. Less is known about how this in turn affects carbon (C) and nitrogen (N) mineralization from the pool of stabilized soil organic matter, or the extent to which gross N immobilization influences the net amount of N mineralized. Soils, differing in the quantity and quality of organic matter inputs they had received since 1956, were sampled approximately 6 or 18 months after the most recent applications of organic amendments. Two laboratory experiments were carried out to: (i) evaluate if, and how, the organic amendments had affected C mineralization, gross and net N mineralization; (ii) examine the relation between gross N immobilized and free-light fraction of organic matter; and (iii) assess predictors for gross N mineralization and immobilization rates in soils. The amount of soil organic C and N were major determinants of C and gross N mineralization, but not of net N mineralization. Carbon mineralization was related to gross N mineralization, but the ratio between the two was not constant. Gross N immobilization was related to the amount of free-light fraction material in the soil with 90% variation explained. For most common organic amendments applied in autumn, our results support the use of total soil organic N and C mineralization as predictors of gross N mineralization from stabilized soil organic matter. In addition, we propose that the amount of free-light fraction material present in the soil in spring is adequate as a predictor of the immobilization potential of the soil, without a need to consider the C-to-N ratio of this material.     

秸秆还田下寒地水稻实现高产高氮肥利用率的氮肥运筹模式

  【目的】  秸秆还田是东北寒地水稻种植区培肥土壤的重要措施,研究调整水稻基蘖氮肥与穗氮肥比例,为促进寒地水稻氮肥的合理施用提供科学依据。  【方法】  田间试验于2016、2017年在吉林省进行,供试水稻品种为吉粳511。上一季水稻收获后,秸秆9000 kg/hm2粉碎至10 cm左右,翻压还田。在总施氮 (N) 量200 kg/hm2不变的前提下,设置5个基蘖肥与穗肥比例处理5∶5 (N 5∶5)、6∶4 (N 6∶4)、7∶3 (N 7∶3)、8∶2 (N 8∶2) 和9∶1 (N 9∶1),以不施氮肥 (N0) 为对照。在水稻6个生育期调查植株生物量和氮素含量,成熟期测定产量及产量构成因素。计算了氮素积累与转运特征,以及氮素利用效率。  【结果】  与N0处理相比,施氮提高了水稻穗数、穗粒数和结实率,进而显著提高了产量,以N 8∶2处理的水稻产量最高。水稻返青期至拔节期,氮积累量随基蘖氮肥占总施氮量比例的增加而增加,而齐穗期至成熟期阶段则表现为随基蘖氮肥占总施氮量比例的增加先增后减,氮素积累总量以N 8∶2处理最高。施氮显著提高了氮素转运量和齐穗后积累氮素对籽粒氮积累量贡献率,其中氮素转运量随基蘖氮肥占总施氮量比例的增加而增加,而齐穗后积累氮素对籽粒氮积累量贡献率随基蘖氮肥占总施氮量比例的增加先增后降,以N 8∶2处理最高。随基蘖氮肥占总施氮量比例的提高,氮素回收率、农学利用率、偏生产力和生理利用率均呈现先增后减趋势,均以N 8∶2处理最高。相关分析结果表明,水稻齐穗期前后氮素积累量与水稻产量均呈显著或极显著正相关 (r = 0.8943～0.9476),其中水稻齐穗后氮积累量与产量的相关性高于齐穗前。  【结论】  在秸秆还田条件下,基蘖氮肥与穗氮肥比例为8∶2最有利于提高水稻齐穗期至成熟期氮积累量,促进氮素向籽粒的转运,使水稻产量和氮素利用效率协同提高。因此,在本试验条件下,总施氮量200 kg/hm2,基蘖肥与穗肥比例为8∶2的施氮制度是优化水稻氮素积累特性及获得高产的理想运筹模式。