Effect of P fertilizer application on N balance of soybean crop in the guinea savanna of Nigeria

Soybean (Glycine max (L.) Merr.) is becoming increasingly important in the cereal-based cropping system of the Nigerian Guinea savanna zone and this justifies research on its effects on soil N. Although soybean can obtain 50% or more of its N requirement from the atmosphere, the N contribution of the crop to the system depends on the amount of N contained in roots, haulms, and fallen leaves after grain harvest. At four sites in the northern Guinea savanna, the effects on N balance of P fertilizer and soybean varieties of different duration were tested. The varieties received P fertilizer at the rates of 0, 30, and 60 kg P ha⁻¹. The total N accumulated aboveground at harvest averaged 104 kg N ha⁻¹ in the early and medium varieties, and 135 kg N ha⁻¹ in the late varieties. Across all varieties and sites, total N content was increased by 40–47% when P was applied. Apparent N harvest index averaged 85% but was not significantly affected by variety or P rate. When only grain was exported, the calculated N balance of the early and the medium varieties was −2.6 to −12.2 kg N ha⁻¹ while the longer duration varieties had positive N balances ranging from 2 to 10.9 kg N ha⁻¹. The N accrual was negative when P was not applied and ranged from 2.4 to 5.2 kg N ha⁻¹ with P application. The interaction of variety and site on the N balance was significant at P<0.05. N balance at the southernmost site was −14.2 kg N ha⁻¹ compared with 2.6–10 kg N ha⁻¹ at the northern sites where N₂ fixation was higher. The estimate of N balance is reduced when soybean haulms are exported. A positive N contribution by soybean is, therefore, possible in a soybean–cereal rotation when: (i) P is applied, (ii) the soybean variety is late maturing, and (iii) only grain is exported.     

Residual benefits of promiscuous soybean to maize (Zea mays L.) grown on farmers’ fields around Minna in the southern Guinea savanna zone of Nigeria

Biological nitrogen fixation (BNF) by promiscuous cultivars of soybeans (Glycine max (L.) Merr.) in cereal-based cropping systems of Nigeria’s moist savanna zone offers a potential for minimizing the investment made by resource-poor farmers on nitrogen fertilizers. A 3-year trial was conducted on five farmers’ fields in the southern Guinea savanna zone of Nigeria to assess the residual effects of two successive crops of promiscuous soybean cultivars on the yield of a following maize (Zea mays L.) crop. The soybean cultivars, TGX1456-2E (medium maturity) and TGX1660-19F (late maturity), were grown in 1996 and 1997. Treatments, imposed only in the first year of the trial, were: (i) uninoculated, (ii) inoculated with a mixture of two Bradyrhizobium strains, and (iii) fertilized with 60 kg N ha⁻¹. A fourth treatment was a plot left to fallow. In 1998, all the previous soybean and fallow plots were sown to maize without any fertilizer application. Results in 1996 and 1997 showed a soybean response to inoculation in the first year, but differences due to the residual effect of inoculation in the second year were not significant. Both cultivars showed a similar response to inoculation but responses at the five sites were varied. Soybean cultivar 1456-2E fixed 43–52% of its N amounting to 56–70 kg N ha⁻¹ and cultivar 1660-19F derived 39–54% of its N from N₂-fixation which amounted to 51–78 kg N ha⁻¹. Both cultivars had a high N harvest index resulting in a net removal of 52–95 kg N ha⁻¹ when both grain and stover were exported. Even when the stover was returned, there was a depletion of 23–65 kg N ha⁻¹, with 1456-2E removing more N than 1660-19F. Arbuscular-mycorrhizal infection on maize roots was 11–27% and dependent on previous soybean treatments and farmers’ fields. Plant height, shoot biomass, grain yield, and N uptake of maize were significantly greater in plots previously sown to soybean than in the fallow plots. In general, plots sown to the late maturing cultivar 1660-19F exhibited better residual effect, producing larger yield parameters than the plots planted with medium maturing 1456-2E.     

Potential nitrogen enrichment of soil and surface water by atmospheric ammonia sorption in intensive livestock production areas

Elevated atmospheric NH₃ levels near intensive livestock operations can add significant N to local agroecosystems. In this study, the potential atmospheric NH₃ sorbed by soil and water was assessed over a 2-year period starting October 2000 in an intensive livestock production area in southern Alberta, Canada. Fifty-two uneven grid sampling sites were selected in the 53,905 ha study area. The sorption rate of atmospheric NH₃ was estimated weekly by exposing distilled water and air-dried soil samples to the atmosphere at the sampling sites. The increases in NH₄–N content in the samples after 1-week exposure was regarded as an index of the atmospheric NH₃ sorbed for that week. The NH₃ sorption rates were highly variable across the 52 sites, with water ranging from 4 to 125 kg ha⁻¹ year⁻¹ with a mean of 22 kg N ha⁻¹ year⁻¹ and soil from 5 to 84 kg N ha⁻¹ year⁻¹ with a mean of 20 kg N ha⁻¹ year⁻¹. Considerable variation in NH₃–N sorption across the study area reflects the effects of size, direction (upwind or downwind) and proximity of nearby livestock operations or other NH₃ sources and operators’ activities around the sampling sites. The NH₃ sorption rate at each site also varied considerably in response to weather conditions. The high rate of NH₃ input poses a direct risk of surface water eutrophication in intensive livestock operation areas. If fertilizer recommendations are not reduced to account for NH₃ sorption by soil, excess N may also contribute to eutrophication through runoff and leaching.     

The influence of source and timing of nitrogen fertilizers on yield and nitrogen use efficiency of four sweet potato cultivars

Nitrogen (N) availability is a very important determinant of crop yield. It is also one of the most expensive inputs in crop production. Thus, the efficient management of N by farmers with limited resource is a very important part of successful soil and crop management system. A field study was conducted on a Norfolk sandy loam soil at the George Washington Carver Agricultural Experiment Station, Tuskegee, AL, to evaluate the influence of fertilizer source and timing on the yield of four sweet potato [Ipomoea batatas (L.)] cultivars. Two N sources (urea, CO(NH₂)₂ and ammonium nitrate, NH₄NO₃), four sweet potato cultivars (‘Georgia Jet’, GA-Jet; ‘TU-82-155’, TU-155; ‘TU-1892’; and ‘Rojo Blanco’, Rojo-Bl) and three times (zero, single, and split applications) were used in a factorial designed experiment. Plants were harvested after 80 and 120 days and total and marketable yields determined. Nitrogen recovery efficiency, yield efficiency and physiological efficiency were also determined. Total and marketable yields of early maturing cultivars were significantly higher than of late maturing cultivars (P<0.05). The single application of N resulted in significantly higher yield in storage roots than split application (P<0.05). Physiological efficiency values were highly correlated with total marketable yields. Recovery rates increased with time. Late maturing cultivars tended to have higher N recovery and physiological efficiency than early maturing cultivars. These results suggest that cultivar maturity group should play an important part in N fertilization recommendations for sweet potato.     

Urease inhibitor reduces N losses and improves plant-bioavailability of urea applied in fine particle and granular forms under field conditions

A field lysimeter/mini plot experiment was established in a silt loam soil near Lincoln, New Zealand, to investigate the effectiveness of urea fertilizer in fine particle application (FPA), with or without the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT - “Agrotain”), in decreasing nitrogen (N) losses and improving N uptake efficiency. The five treatments were: control (no N) and ¹⁵N-labelled urea, with or without NBPT, applied to lysimeters or mini plots (unlabelled urea), either in granular form to the soil surface or in FPA form (through a spray) at a rate equivalent to 100 kg N ha⁻¹. Gaseous emissions of ammonia (NH₃) and nitrous oxide (N₂O), nitrate (NO₃⁻) leaching, herbage dry-matter (DM) production, N-response efficiency, total N uptake and total recovery of applied ¹⁵N in the plant and soil varied with urea application method and with addition of NBPT. Urea with NBPT, applied in granular or FPA form, was more effective than in application without NBPT: N₂O emissions were reduced by 7-12%, NH₃ emissions by 65-69% and NO₃⁻ leaching losses by 36-55% compared with granular urea. Urea alone and with NBPT, applied in FPA form increased herbage DM production by 27% and 38%, respectively. The N response efficiency increased from 10 kg DM kg⁻¹ of applied N with granular urea to 19 kg DM kg⁻¹ with FPA urea and to 23 kg DM kg⁻¹ with FPA urea plus NBPT. Urea applied in FPA form resulted in significantly (P < 0.05) higher ¹⁵N recovery in the shoots compared with granular treatments and this was improved further when urea in FPA form was applied with NBPT. These results suggest that applying urea with NBPT in FPA form has potential as a management tool in mitigating N losses, improving N-response efficiency and increasing herbage DM production in intensive grassland systems.     

Vegetation dynamics after compost amendment in a Mediterranean post-fire ecosystem

On Mediterranean calcareous soils, high fire frequency induces soil impoverishment and the development of stable Quercus coccifera garrigues. Organic amendment could increase soil fertility levels, and could alter the vegetation dynamics and the established dominance relationships. In this study, the plant cover evolution is monitored in an amended burnt shrub during two years. Three treatments are studied: control (D0), 50 t ha⁻¹ (D50) and 100 t ha⁻¹ (D100) of fresh co-composted sewage sludge and greenwastes.First, the spreading process leads to the squashing of the vegetation, whose intensity reaches a threshold on D100 and limits the woody species’ recovery after amendment. Consequently, the dominant herb Brachypodium retusum is favoured. On D50, woody species are favoured compared to herbs, probably due to the space colonization strategy of Q. coccifera after squashing. Thus, compost at both rates favours the two dominant resprouter species until they have colonized all the free space created by squashing.Secondly, compost has some fertilizing effects on Rosmarinus officinalis and Ulex parviflorus, that improves their cover on D50. However, compost also increases U. parviflorus’ sensitivity to drought.At the end of the experiment, a third compost effect appears, as seeder cover becomes greater than D0 on D50. After amendment, compost covers rocky stones, which creates a new territory where species with superficial rooting can establish (mulch effect).     

Differential N uptake by maize cultivars and soil nitrate dynamics under N fertilization in West Africa

Nitrate is prone to leaching in the sandy soils of the West African moist savannas. Better management of nitrogen (N) resources and maize cultivars with enhanced genetic capacity to capture and utilize soil and fertilizer N are strategies that could improve N-use efficiency. In two field experiments conducted at Zaria, northern Nigeria, five maize (Zea mays L.) cultivars planted early in the season were assessed under various N levels for differences in N uptake, soil N dynamics, and related N losses. Cultivar TZB-SR accumulated more N in the aboveground plant parts in both years than the other cultivars. All, except the semi-prolific late (SPL) variety, met about 50–60% of their N demand by the time of silking (64–69 DAP). In both years, SPL had the greatest capacity to take up N during the grain filling period, and it had the highest grain-N concentration and the least apparent N loss through leaching in the second year. There were no significant differences in soil N dynamics among cultivars in both years. At harvest, the residual N in the upper 90 cm of the profile under all the cultivars ranged from 56 to 72 kg ha⁻¹ in the first year and from 73 to 83 kg ha⁻¹ in the second year. Apparent N loss from 0 to 90 cm soil profile through leaching ranged from 35 to 122 kg ha⁻¹ in both years. N application significantly increased N uptake by more than 30% at all sampling dates in the second year of the experiment, but had no effect on apparent N loss. Results indicate that the use of maize cultivars with high N uptake capacity during the grain filling period when maximum leaching losses occur could enhance N recovery and may be effective in reducing leaching losses of mineral N in the moist savanna soils.     

水氮组合模式对双季稻甲烷和氧化亚氮排放的影响

为给双季稻水肥高效利用调控技术提供理论基础,设置间歇灌溉和淹水灌溉两种灌溉方式,高氮、中氮、低氮和不施氮这4种施肥方式,开展大田小区试验,探讨了水氮组合模式对双季稻CH4和N2O排放的影响.结果表明,间歇灌溉显著降低了CH4积累排放量,与淹水灌溉相比,早晚稻分别降低13.18~87.90 kg·hm-2和74.48~131.07 kg·hm-2,分别减排了24.4%~67.4%和42.5%~66.5%;但促进了N2O排放,早晚稻的增排量分别为0.03~0.24 kg·hm-2和0.35~1.53 kg·hm-2,分别比淹水灌溉增加6.2%~18.3%和40.2%~80.9%.总体上,间歇灌溉降低了稻田温室气体的增温潜势,其中早稻降低了18.8%~58.6%,晚稻降低34.4%~60.1%,两季综合降低2 388~4 151 kg·hm-2(以CO2eq计),下降41%~54%.通过相关分析发现,土壤CH4排放和土壤溶液Eh显著负相关,和溶液CH4浓度显著正相关.与淹水灌溉相比间歇灌溉模式有利于减排CH4,虽增排了N2O,但增温潜势显著减少.综合来看,间歇灌溉配施中氮更有利于双季稻种植.     

Rice mill waste as organic manure on a degraded Alfisol

In much of sub-Saharan Africa (SSA), the lack of organic soil amendments constitutes one of the principal causes for declining soil fertility in intensifying farming systems. The challenge, therefore, remains to increase the availability of organic inputs and to develop recommendations for their combination with inorganic fertilizers. An on-farm experiment was conducted in the northern Guinea savanna of Nigeria to evaluate the fertilizer effect of rice (Oryza sativa L.) mill waste (RMW) on a degraded Alfisol. The decomposition and nutrient (N and P) release patterns of RMW were studied using the litterbag technique, and the effect on maize yield and soil properties was determined. The RMW was applied at rates of 0, 5, 10, and 15 Mg DM ha⁻¹ and was applied either unburnt or burnt (farmers’ practice). In both years, compound fertilizer was broadcast during land preparation on all plots at rates of 40 kg N ha⁻¹, 17 kg P ha⁻¹, and 33 kg K ha⁻¹.Results obtained in the litterbag study showed that, at maize harvest, more than 90% of the P had been released from the decomposing RMW. However, around 60% of the organic C and 45% of the N still remained. Compared to the control treatment (0 Mg ha⁻¹ RMW), which yielded 0.55 Mg ha⁻¹, maize (Zea mays L.) grain yields were increased by 95% when 10 Mg ha⁻¹ of unburnt RMW was applied, and by 147% with 15 Mg ha⁻¹ (mean of 2 years). In contrast, burnt RMW did not result in significant yield increases. The cumulative application of 30 Mg ha⁻¹ of unburnt RMW significantly increased the soil organic carbon in the surface soil from 0.7% (0 Mg ha⁻¹ RMW) to 1.3%.The results of this investigation indicate that RMW constitutes a valuable organic input in the Guinea savanna if applied unburnt at rates of 10–15 Mg ha⁻¹ in combination with inorganic fertilizer. The repeated application of unburnt RMW may contribute to the rehabilitation of degraded soils through the buildup of soil organic matter.     

N and P dynamics in the litter layer and soil of Mexican semi-arid forests, state of Morelos

In central Mexico, it is common for farmers to retain useful trees in abandoned lands after maize cultivation, creating a park-like landscape of scattered trees for extensive livestock grazing, among other land uses (mature forests, secondary forests, and livestock grazing in secondary forests). Among these trees Acacia cochliancantha and Ipomoea arborescens are the most common species associated with this land use in the region. The objective of this study was to assess the effects of both tree species on soil N and P recycling. To this end, we measured N and P concentrations in leaves of both species; and the seasonal N and P (total and dissolved) content in the litter, and total N and P, inorganic N, and bicarbonate-extractable-P concentrations, and the N transformations in the soil, in samples collected under crown of Acacia and Ipomoea and in open areas. Trees of different species varied in their capacity to cycle N. The leaves of Acacia were richer in N than those of Ipomoea (29.7 and 25.0 mg N g⁻¹, respectively), and nutrient resorption was higher in leguminous trees than in Ipomoea (by 20% in the case of N, and 35% in the case of P). Acacia trees had higher effects on soils than Ipomoea trees, like consistent increases of N concentrations (by 30% in the case of total N, and by 50% in the case of inorganic N) and transformations (N mineralization and nitrification in rainy season increases by a factor of 20 and 36, respectively). On the other hand, Ipomoea produced senescent leaves and accumulated forest litter with less P concentration (0.8 and 0.7 mg P g⁻¹, respectively) in relation to Acacia (senescent leaves: 1.3 mg P g⁻¹; litter: 1.1 mg P g⁻¹), reflecting the lower availability of the nutrient in the soil. The total litter N and P pools decreased in the rainy season under crown of both species, as the dissolved P pool did. The total soil N and P concentrations did not change with sampling season. However, potential N transformations and bicarbonate P under both species were higher in dry season than in rainy season samples. Comparison with other land uses in the region suggests that the Acacia–Ipomoea system cycles low quantity of nutrients, but there are no notable differences in the availability of P in the soils. These results demonstrate that scattered trees improve the N and P cycling following the discontinuation of agricultural practices, and the effects will vary depending on the tree species.     

黄土丘陵区3种典型天然草地群落土壤呼吸对模拟降雨的响应

土壤呼吸是影响陆地生态系统碳循环的关键生态学过程之一,降雨是土壤呼吸变化的重要驱动因子.为探明黄土丘陵区退耕草地群落土壤呼吸对降雨的短期响应规律,选择白羊草(Bothriochloa ischaemum)、达乌里胡枝子(Lespedeza davurica)、铁杆蒿(Artemisia gmelinii)为优势种的3种草地群落为研究对象,于2014年6月和7月,以未降雨群落为对照,对比测定了2种模拟降雨量(15 mm和50 mm)下连续8 d的土壤呼吸速率及土壤水分和土壤温度等.结果表明:2种模拟降雨量对3种草地群落土壤温度和土壤含水量均具有显著影响(p0.05).50 mm降雨下群落土壤呼吸速率增加量高于15 mm降雨.3种群落土壤呼吸对降雨的响应程度存在明显差异,其中,土壤呼吸速率峰值高低顺序为:白羊草(1.75 g·m~(-2)·h~(-1))铁杆蒿(1.69 g·m~(-2)·h~(-1))达乌里胡枝子(1.12 g·m~(-2)·h~(-1)).0~10 cm土壤含水量与土壤呼吸速率呈显著对数函数关系.3种群落土壤呼吸温度敏感性系数Q_(10)值变化范围是1.31~2.01,且月份和降雨量处理间均存在差异.研究表明,准确评估降雨格局改变背景下退耕草地生态系统碳收支需考虑群落类型的差异.     

Atmospheric deposition contributes little nutrient and sediment to stream flow from an agricultural watershed

Atmospheric deposition of nutrients within agricultural watersheds has received scant attention and is poorly understood compared to nutrient transport in surface and subsurface water flow pathways. Thus, we determined the deposition of phosphorus (P), nitrogen (N), and sediment in a mixed land use watershed in south-central Pennsylvania (39.5 ha; 50% corn–wheat–soybean rotation, 20% pasture, and 30% woodland), in comparison with stream loads at several locations along its reach between 2004 and 2006. There was a significant difference in deposition rates among land uses (P < 0.05) with more P and N deposited on cropland (1.93 kg P and 10.71 kg N ha⁻¹ yr⁻¹) than pasture (1.10 kg P and 8.06 kg N ha⁻¹ yr⁻¹) and woodland (0.36 and 2.33 kg N ha⁻¹ yr⁻¹). Although not significant, sediment showed the same trends among land uses. A significant relationship was found between P in deposition and P in soil <10-m away from the samplers suggesting much of the deposited sample was derived from local soil. Samplers adjacent to the stream channel showed deposition rates (1.64 kg P and 8.83 kg N ha⁻¹ yr⁻¹) similar to those on cropland. However, accounting for the surface area of the stream, direct deposition of P, N, and sediment probably accounted for <3% of P and <1% of N and sediment load in stream flow from the watershed (1.41 kg P, 27.09 kg N, and 1343 kg sediment ha⁻¹ yr⁻¹ at the outlet). This suggests that strategies to mitigate nutrient and sediment loss in this mixed-land use watershed should focus on runoff pathways.     

Managing crop residues, fertilizers and leaf litters to improve soil C, nutrient balances, and the grain yield of rice and wheat cropping systems in Thailand and Australia

This paper reports on the influence that residue and fertilizer management have on nutrient balances, soil organic matter (SOM) dynamics, and crop yields of a flooded rice system in northeast Thailand (1992–1997) and a wheat–forage legume rotation in eastern Australia (1992–1998). Both soils had been subject to at least 18 years of cultivation and had lost up to 90% of the original labile (C_L) and 85% of the total carbon (C_T).For the rainfed rice cropping systems of northeast Thailand, a system is described in which small applications of leaf litter from locally grown trees are applied annually to rice paddy soils prior to transplanting. Annual applications of 1500 kg ha⁻¹ of leaf litter from different locally grown shrubs for five seasons resulted in increases in rice grain yield in 1997 of between 20 and 26% above the no-leaf litter control. Nutrient balances, determined by the difference between the inputs (fertilizer and added leaf litters) and outputs (grain and straw), indicated net positive balances of up to 457 kg N ha⁻¹, and 60 kg P ha⁻¹, after five seasons of leaf litter applications. Sulfur and potassium balances resulted in net deficits of up to −13 kg S ha⁻¹ and −52 kg P ha⁻¹, where no leaf litter was applied and rice straw was removed following harvest. Soil carbon (C) concentrations increased significantly only where higher fertilizer rate and rice stubble retention were combined.The poor management of fertilizers and crop residues, and excessive cultivation has also resulted in large soil fertility losses in the grain growing areas of Eastern Australia. After five wheat and two legume/fallow crops, negative N balances of up to −303 kg ha⁻¹ were calculated for the treatments where wheat stubble was not retained and bare fallow leys were used. The balance of nutrients such as K, which are contained in larger proportions in stubble, were found to be up to −362 kg ha⁻¹ on the straw-removed treatments and up to +29 kg ha⁻¹ on the straw-retained treatments. Forage legume leys resulted in short term increases in C_L and the carbon management index (CMI).Sustainable farming systems require that crop yields are stable through the maintenance of soil fertility and the balance of nutrients in the system. Increases in soil C levels require sustained periods of balanced fertilization and residue retention.     

Effect of application of ammonium thiosulphate on production and emission of methane in a tropical rice soil

Agricultural sources of atmospheric methane include flooded rice (Oryza sativa L.) paddies. However, certain soil nutrient management and cultural practices offer opportunities to reduce methane emissions. The effect of application of ammonium thiosulphate, a potential source of nitrogen and sulphur and also an inhibitor of nitrification and urease on methane production and emission from flooded alluvial (Typic Haplaquept) rice soil in India, was examined. Methane production and emission from control and urea-amended soil samples were almost identical. Application of ammonium thiosulphate to laboratory-incubated flooded soil (30 and 60 μg N g⁻¹ soil) and flooded rice fields (45.6 and 60 kg N ha⁻¹) effected a distinct inhibition of methane production and emission. Ammonium thiosulphate stimulated the population of sulphate-reducing bacteria (SRB) to a greater extent at 60 μg N g⁻¹ soil than at 30 μg N g⁻¹ soil. In ammonium thiosulphate-applied rice field plots, mean methane efflux decreased by about 38 and 60% at 45.6 and 60 kg N ha⁻¹, respectively, over that of control. Inhibition of methane production by ammonium thiosulphate is, at least in part, due to the stimulation of SRB. Results suggest the mitigation potential of ammonium thiosulphate on methane emission from flooded rice paddies.     

不同氮、磷肥用量下双季稻田的CH4和N2O排放

以红壤双季稻田为研究对象,采用静态暗箱-气相色谱法对2009年水稻生长期内不施肥（CK）,平衡施肥（BF）、减氮磷一（DNP1）、减氮磷二（DNP2）和增氮磷（INP）等5个处理的CH4和N2O排放通量以及环境因素进行观测.结果表明,早稻生长期间BF、DNP1、DNP2和INP的CH4平均排放通量为4.57、5.42、...     

Effects of elevated CO2 concentration and nitrogen supply on biomass and active carbon of freshwater marsh after two growing seasons in Sanjiang Plain, Northeast China

An experiments were carried out with treatments differing in nitrogen supply (0, 5 and 15 g N/m²) and CO₂ levels (350 and 700 μmol/mol) using OTC (open top chamber) equipment to investigate the biomass of Calamagrostis angustifolia and soil active carbon contents after two years. The results showed that elevated CO₂ concentration increased the biomass of C. angustifolia and the magnitude of response varied with each growth period. Elevated CO₂ concentration has increased aboveground biomass by 16.7% and 17.6% during the jointing and heading periods and only 3.5% and 9.4% during dough and maturity periods. The increases in belowground biomass due to CO₂ elevation was 26.5%, 34.0% and 28.7% during the heading, dough and maturity periods, respectively. The responses of biomass to enhanced CO₂ concentrations are differed in N levels. Both the increase of aboveground biomass and belowground biomass were greater under high level of N supply (15 g N/m²). Elevated CO₂ concentration also increased the allocation of biomass and carbon in root. Under elevated CO₂ concentration, the average values of active carbon tended to increase. The increases of soil active soil contents followed the sequence of microbial biomass carbon (10.6%) > dissolved organic carbon (7.5%) > labile oxidable carbon (6.6%) > carbohydrate carbon (4.1%). Stepwise regressions indicated there were significant correlations between the soil active carbon contents and plant biomass. Particularly, microbial biomass carbon, labile oxidable carbon and carbohydrate carbon were found to be correlated with belowground biomass, while dissolved organic carbon has correlation with aboveground biomass. Therefore, increased biomass was regarded as the main driving force for the increase in soil active organic carbon under elevated CO₂ concentration.     

外源碳和氮输入对降水变化下土壤呼吸的短期影响

利用野外原位小区控制试验,模拟研究了降水变化下草地生态系统土壤呼吸对外源碳和氮输入的响应.在2014年,以内蒙古锡林河流域温带典型草原为研究对象,测定了增加降水处理（CK）、增加降水配施氮肥处理[CN,2.5 g·（m²·a）^-1]、增加降水配施碳源处理[CG,24 g·（m²·a）^-1]和增加降水配施氮肥和碳源处理[CNG,2.5 g·（m²·a）^-1+24 g·（m²·a）^-1]下土壤呼吸的变化,并分析了土壤呼吸与土壤温度、土壤水分、土壤可溶性有机碳（DOC）、土壤微生物量碳（MBC）之间的关系.结果表明,在自然降水较多的第一次增加降水（FWE）阶段,CG处理和CNG处理168 h土壤CO₂累积通量显著增加,而CN处理168 h土壤CO₂累积通量无显著变化,并且CG处理和CNG处理土壤MBC含量显著高于CK处理和CN处理,同时,该阶段平均CO₂释放速率与土壤MBC含量正相关（P<0.05）.与FWE阶段相比,无自然降水的第二次增加降水（SWE）阶段各处理168 h土壤CO₂累积释放量显著降低,并且各处理MBC含量也显著降低（P<0.05）,仅有土壤DOC含量显著增加（P<0.05）,CG处理和CN处理168 h土壤CO₂累积通量显著降低（P<0.05）.两个降水阶段土壤呼吸速率与土壤温度或土壤体积含水量均有显著的正相关性（P<0.05）.因此,自然降水的分布对土壤水分的影响调控着外源氮和碳对半干旱草地生态系统土壤呼吸的作用效应.     

羟基磷灰石对Cd污染土壤中马铃薯生长及品质的影响

通过温室盆栽实验研究了施用羟基磷灰石改良Cd污染土壤对马铃薯生长及品质的影响.实验设置了3个Cd污染水平(0、5、10 mg.kg-1)、6个羟基磷灰石施用量(0、4、8、10、16、30 g.kg-1)和2个马铃薯品种(中薯3号、大西洋).结果表明,土壤Cd污染导致马铃薯单株产量下降(5 mg.kg-1的Cd污染土壤中降低24%～31%,10 mg.kg-1的Cd污染土壤中降低41%～45%),但是施用羟基磷灰石可以提高单株产量.相对于不施用羟基磷灰石,5 mg.kg-1的Cd污染土壤中施用10 g.kg-1的羟基磷灰石可以增产17%～39%,10 mg.kg-1的Cd污染土壤中施用30 g.kg-1的羟基磷灰石可以增产45%～58%.由于羟基磷灰石改善了Cd污染土壤环境,因此马铃薯器官中叶绿素含量和SOD活性明显上升,而MDA含量明显下降.施用羟基磷灰石也提高了马铃薯品质,马铃薯块茎中维生素C含量、淀粉含量以及蛋白质含量也明显提高.随着羟基磷灰石施用量由0 g.kg-1增加到30 g.kg-1,在5 mg.kg-1的Cd污染土壤中,马铃薯块茎Cd含量由0.87～0.95 mg.kg-1下降到0.13～0.21 mg.kg-1,降幅为78%～85%;在10 mg.kg-1的Cd污染土壤中,块茎Cd含量由1.86～1.93 mg.kg-1下降到0.52～0.65 mg.kg-1,降幅为66%～72%.实验表明,羟基磷灰石缓解土壤Cd毒性的主要机制是提高土壤pH值,降低土壤中有效态Cd含量,羟基磷灰石中的Ca阻碍土壤Cd向马铃薯迁移.但是羟基磷灰石对土壤Cd毒性的缓解效应是有限性的,过量施用可能对马铃薯生长和品质产生胁迫作用.在Cd污染土壤中施用适量的羟基磷灰石后中薯3号生长状况和品质好于大西洋,说明不同马铃薯品种对种植环境的改善有不同的响应.     

小麦-玉米轮作体系农田氮素淋失特征及氮素表观平衡

连续6年采用渗漏计法研究了不同施氮处理下陕西关中小麦-玉米轮作区农田土壤90 cm深度处氮素(N)淋失特征和土壤-作物体系氮素表观平衡状况.结果表明:该地区农田氮素淋溶主要发生在降雨量较多的玉米季,且集中在8月和9月.监测期内,TN和NO-3-N年平均流失量分别为2.72~23.07 kg·hm-2和1.53~18.72 kg·hm-2,年流失率分别为0.65%~3.44%和0.82%~3.32%,且年总氮、硝态氮流失量均随年施氮量增加呈指数增加.氮素淋失形态中,NO-3-N比例较高,可占总氮淋失量的56.00%~81.00%,且随着氮肥用量的降低,其占总氮淋失量的比例也随之减小.可见,施氮量的大小在一定程度上会影响淋失液中各形态氮的比例.氮素表观平衡结果显示,随着施氮量提高,氮素在土壤中的残留和表观氮盈余均呈现指数增加趋势.长期施氮条件下,土壤-作物体系氮素表观损失率的幅度为32.60%~55.20%,土壤表观残留率为-0.17%~8.20%.多年监测结果表明,优化施氮模式下,作物不仅可以获得较高的产量和氮肥利用率,农田氮素淋失量也大幅降低,在节约肥料资源的同时减轻了潜在的环境风险.     

Effects of soil properties, mulch and NPK fertilizer on maize yields and nutrient budgets on ferralitic soils in southern Benin

Four on-farm experiments examined whether modest applications of fertilizers in combination with prunings from native agroforestry trees would be an alternative to maintain the fertility of ferralitic soils in Benin. An application of about 1.9 t ha⁻¹ dry matter of mulch of Senna siamea combined with 30 kg N ha⁻¹, 22 kg P ha⁻¹ and 25 kg K ha⁻¹ as compound fertilizer was compared with (1) 60 kg N ha⁻¹, 43 kg P ha⁻¹ and 50 kg K ha⁻¹ as compound fertilizer alone, (2) mulch of S. siamea alone (about 3.2 t ha⁻¹ dry matter), and (3) a control treatment. Criteria were soil properties, yields, nutrient uptakes, and nutrient budgets. Application of sole mulch had no significant effects (P>0.05) on maize yields, while combined application of prunings and NPK fertilizers or sole NPK increased yields significantly (P<0.05). The most limiting nutrient was P. The local maize cultivar was efficient in P uptake, but not in internal nutrient utilization efficiency; mulch increased significantly the internal P utilization efficiency (P<0.05). Soil properties were interpreted with the QUEFTS (quantitative evaluation of the fertility of tropical soils) computer program. The predicted and measured yields were almost the same for maize without NPK. The measured responses to NPK were much lower than the responses calculated by QUEFTS. The calculated nutrient budgets were split into balances for available nutrients and for those not immediately available (NIA). Nutrient budgets were negative for the control and sole mulch treatments, and positive for the NPK treatments. Mulch improved the balances of NIA nutrients. The present experiment could not prove that combining NPK with mulch is the best option for sustainable agriculture. It may be more economical to apply lower rates of fertilizer to local maize than those applied in the two NPK treatments in the present study.