Denitrification, N2O and CO2 fluxes in rice-wheat cropping system as affected by crop residues, fertilizer N and legume green manure

Use of renewable N and C sources such as green manure (GM) and crop residues in rice-wheat cropping systems of South Asia may lead to higher crop productivity and C sequestration. However, information on measurements of gaseous N losses (N₂O+N₂) via denitrification and environmental problems such as N₂O and CO₂ production in rice-wheat cropping systems is not available. An acetylene inhibition-intact soil core technique was employed for direct measurement of denitrification losses, N₂O and CO₂ production, in an irrigated field planted to rice (Oryza sativa L.) and wheat (Triticum aestivum L.) in an annual rotation. The soil was a coarse-textured Tolewal sandy loam soil (Typic Ustochrept) and the site a semi-arid subtropical Punjab region of India. Wheat residue (WR, C:N=94) was incorporated at 6 t ha^-1 and sesbania (Sesbania aculeata L.) was grown as GM crop for 60 days during the pre-rice fallow period. Fresh biomass of GM (C:N.=18) at 20 or 40 t ha^-1 was incorporated into the soil 2 days before transplanting rice. Results of this study reveal that (1) denitrification is a significant N loss process under wetland rice amounting to 33% of the prescribed dose of 120 kg N ha^-1 applied as fertilizer urea-N (FN); (2) integrated management of 6 t WR ha^-1 and 20 t GM ha^-1 supplying 88 kg N ha^-1 and 32 kg FN ha^-1 significantly reduced cumulative gaseous N losses to 51.6 kg N ha^-1 as compared with 58.2 kg N ha^-1 for 120 kg FN ha^-1 alone; (3) application of excessive N and C through applying 40 t GM ha^-1 (176 kg N ha^-1) resulted in the highest gaseous losses of 70 kg N ha^-1; (4) the gaseous N losses under wheat were 0.6% to 2% of the applied 120 kg FN ha^-1 and were eight- to tenfold lower (5-8 kg N ha^-1) than those preceding rice; (5) an interplay between the availability of NO₃^- and organic C largely controlled denitrification and N₂O flux during summer-grown flooded rice whereas temperature and soil aeration status were the primary regulators of the nitrification-denitrification processes and gaseous N losses during winter-grown upland wheat; (6) the irrigated rice-wheat system is a significant source of N₂O as it emits around 15 kg N₂O-N ha^-1 year^-1; (7) incorporation of WR in rice and rice residue (C:N=63) in wheat increased soil respiration, and increased CO₂ production in WR- and GM-amended soils under anaerobic wetland rice coincided with enhanced rates of denitrification; and (8) with adequate soil moisture, most of the decomposable C fraction of added residues was mineralized within one crop-growing season and application of FN and GM further accelerated this process.     

Effects of temperature, soil water status, and soil type on swine slurry nitrogen transformations

Manure N dynamics are affected by manure characteristics, soil factors, and environmental conditions. An incubation experiment was conducted to assess the relationship of these factors. The effects of temperature (11, 18, and 25°C), soil texture (three soils, silt loam to sandy loam), and soil water status (constant at 60% water filled pore space, WFPS, and fluctuating between 30% and 60% WFPS) on net mineralization and nitrification of swine manure N were assessed. Swine manure was applied at an equivalent rate of 350 kg total N ha^-1 to 250 g air-dry soil in 2-l canning jars. Subsamples were taken from each jar for NO₃^- and NH₄⁺ determination when fluctuating moisture treatment dried to 30% WFPS, with sampling continuing through four wet-dry cycles at each temperature. Manure NH₄⁺ was rapidly nitrified to NO₃^-. The relationship between NO₃^- accumulation and degree days after application (DDAA, 0°C base) could be described across temperatures using a single pool exponential model for each soil. More NO₃^- accumulated in coarser-textured soils (150-200 mg N kg^-1 soil), compared to 130 mg N kg^-1 soil in the silt loam soil. Fluctuating soil water status did not alter estimates of rate and extent of NO₃^- accumulation, but slowed NH₄⁺ disappearance somewhat.     

半干旱区施氮和灌溉条件下覆膜对春玉米产量及氮素平衡的影响

以典型半干旱区干湿砂质新成土（Ust-Sandic Entisols）为供试土壤进行田间试验,研究地膜覆盖、施氮及补充灌水量对春玉米（Zea mays L.）产量、土壤矿质氮（NO3--N和NH4+-N）及氮素平衡的影响。结果表明,0—100 cm土体范围内,随着土层加深,播前和收获后土壤NO3--N含量呈降低趋势,NH4+-N有所增加,但变幅不大;总矿质氮量（NO3--N和NH4+-N）表现为下降。说明地膜覆盖和施氮并没有使NO3--N深层累积量增加,这可能与土壤本身供氮能力严重不足有关。与不施氮相比,施氮各处理氮肥表观损失量增加;与不覆膜相比,作物氮素累积量比不覆膜显著增加（P0.05）。在低灌（80 mm）覆膜和高灌（160 mm）覆膜条件下,玉米的氮肥利用率均比不覆膜均提高了18.8%,说明覆膜低灌在相同施氮条件下,可节约80 mm灌水。但低灌（80 mm）与高灌（160 mm）不覆膜间氮肥利用率差异不显著,表明在相同施氮条件下,覆膜可有效提高氮肥利用率,减少氮素损失。综合考虑籽粒产量和氮肥利用率,“覆膜+补灌80 mm+施氮90 kg/hm2”可能为本试验条件下较优的栽培模式。     

Effect of rice straw management on nitrogen balance and residual effect of urea-N in an annual lowland rice cropping sequence

Two field experiments were conducted in 1999 (wet season) and 2000 (dry season) on a Ustic Endoaquerts in central Thailand to examine the impact of rice straw management practices on rice yield, N uptake and fertilizer-N use efficiency. Treatments included a combination of urea broadcast at a rate of 70 kg N ha^у with either straw or compost which were incorporated at a rate of 5 Mg ha^у. At maturity of the wet season rice, ¹⁵N recovery by the grain was low (11-14%) as well as straw-N derived from labeled N (5-7%). After harvest, 25-29% of applied N still remained in the soil, mainly in the 0 to 5-cm layer. Large amounts of fertilizer-N (53-55%) were lost (unaccounted for) from the soil/plant system during the first crop. Residual fertilizer-N recovery in the second rice crop was less than 3% from the original application. During both fallow seasons NO₃^m-N remained the dominant form of mineral N (NO₃^m + NH₄⁺) in the soil but its concentration was low. In the wet season grain yield response to N application was significant (P =0.05). Organic material sources did not significantly change grain yield and N accumulation in rice. In terms of grain yield and N uptake at maturity, there was no significant residual effect of fertilizer-N on the subsequent rice crop. These results indicated that the combined use of organic residues with urea did not decrease total N losses or increase crop yield or uptake of N compared to urea alone.     

Nitrogen balance of nitrogen-15 applied as ammonium sulphate to irrigated potatoes in sandy textured soils

Farmers are applying very high amounts of N fertilizer (sometimes >900 kg N/ha), commonly (NH₄)₂SO₄, to irrigated potato (Solanum tuberosum, L.) grown on sandy textured soils in the Cappadocia region of Turkey. To obtain information on potato yield, N uptake, N fertilizer residue in the soil and the portion of N fertilizer leached below 200 cm soil depth, nine field experiments were conducted at three different locations in 1992, 1993 and 1994. The N rates used in these experiments were 0, 200, 400, 600, 800 and 1,000 kg N/ha within a completely randomized block design with three replicates. N fertilizer was applied in two equal portions; one at planting and one just before the first irrigation. Although all yield data were used to find out the marketable tuber yield, the N rate response curve and the fate of applied fertilizer N was determined only for the 400 and 1,000 kg N/ha rates. Isotope microplots were established where ¹⁵N-labelled (NH₄)₂SO₄ was applied at 5.0 atom % and 2.5 atom % excess enrichments for the 400 kg N/ha and 1,000 kg N/ha rates, respectively. At harvest, marketable and dry tuber yield was determined for all N rates. Dry tuber and leaf plus vine yields were determined for the isotope microplots and they were analysed for the % N and ¹⁵N atom % excess. The % N derived from fertilizer and N use efficiency (%NUE) were calculated for the plant samples. The ¹⁵N-labelled residue left in 0-200 cm soil was also determined. The amount of N fertilizer leached below 200 cm soil depth was also calculated. ¹⁵N-labelled NO₃^- and total NO₃^- of the groundwater from wells were determined at different dates. Our results show that the optimum marketable tuber yield was obtained with 600 kg N/ha. Tuber N uptake was increased slightly, while leaf plus vine N uptake increased considerably when the N rate was increased from 400 to 1,000 kg N/ha. The %NUE values decreased nearly by half and the amount of N fertilizer in the 0-200 cm soil layer increased more than 3 times when the N rate was increased from 400 to 1,000 kg N/ha. Nearly half of the applied fertilizer N (45.6%) at 400 kg N/ha and more than half of the applied fertilizer N (60.8%) at 1,000 kg N/ha was still in the 0-200 cm soil layer after harvest. Four times more N fertilizer was leached below 200 cm soil depth when 1,000 kg N/ha N was applied instead of 400 kg N/ha. Our results also indicate that there is a potential contamination of groundwater due to leaching of the applied N fertilizer.     

Overwinter soil denitrification activity and mineral nitrogen pools as affected by management practices

During freeze-thaw events, biophysical changes occurring in soils can affect processes such as mineralization, nitrification and denitrification which control inorganic N balances in agro-ecosystems. To evaluate the impact of these climatic events on soil biochemical properties, a study was conducted comparing soil denitrification enzyme activity (DEA), dissolved organic C (DOC) and inorganic N levels before and after the winter season in plots under: (1) continuous corn (Zea mays L.) (CC) with annual chisel plow and disking, (2) corn-soybean (Glycine max L.) (CS) rotation with chisel plow every other year prior to planting soybean, and (3) corn-soybean-wheat (Triticum aestivum L.)/hairy vetch (Vicia villosa Roth) (CSW-V) with ridge tillage during the corn and soybean crops, and dairy manure application during the corn year. Soil cores were collected in late autumn and immediately after spring thaw at 0-5, 5-10, 10-15, and 15-30 cm depths. Regardless of management practices, freeze-thaw events resulted in significant (2-10 times) increases in NH₄⁺-N, NO₃^--N (P<0.001) and DOC (P<0.01) levels at all soil depths. Following freeze-thaw, DEA remained unchanged in the 5-30 cm depth but dropped significantly (P<0.01) in the 0-5 cm soil layer. In that layer, soils which had been chisel plowed during the previous growing season lost 78-84% of the DEA recorded during the fall, whereas in the plots amended with manure during the previous season, the loss of activity was 40-45%. These data indicate that frequent tillage, compared with manure additions, is more conducive to overwinter loss of DEA in surface layers of soils subject to freeze-thaw cycles.     

Effects of dicyandiamide, farmyard manure and irrigation on crop yields and ammonia volatilization from an alluvial soil under a rice (Oryza sativa L.)-wheat (Triticum aestivum L.) cropping system

Volatilization of NH₃ from soil is a major N-loss mechanism that reduces the efficiency of applied N fertilizers, and causes environmental pollution. Strategies are needed to reduce the loss. The influences of dicyandiamide (DCD), farmyard manure (FYM) and irrigation on NH₃ volatilization from an alluvial soil in rice (Oryza sativa L.)-wheat (Triticum aestivum L.) cropping system was studied using the acid trap method. The loss of NH₃ in the rice-wheat system ranged from 38.6 kg N ha^-1 from the unfertilized soil to 69.0 kg N ha^-1 in the treatment with urea+DCD. Substitution of 50% N provided through urea by FYM reduced NH₃-N volatilization by 10% in rice and wheat as compared to the urea treatment. Application of DCD increased NH₃ volatilization in wheat by 7% but in rice it had no effect. The irrigation level had no effect on NH₃ volatilization in rice but fewer irrigations with fewer splits of N in wheat resulted in higher NH₃ volatilization. Application of DCD and FYM with urea had similar effects on grain yield and N uptake by rice and wheat as that of the urea treatment. The study showed that integrated use of organic manure and chemical fertilizer has the potential to reduce the loss of N due to volatilization and thereby minimize environmental pollution. Nitrification inhibitors, which are reported to be useful in increasing the N-use efficiency by reducing the leaching and denitrification losses of N, however, may increase N loss due to volatilization.     

Soil carbon and nitrogen dynamics in Lumbricus terrestris. L. middens in four arable, a pasture, and a forest ecosystems

Lumbricus terrestris' middens contain large concentrations of organic material and have been characterized as microenvironments distinct from the surrounding soil. The direct and indirect consequences of midden formation on nutrient cycling dynamics and organic matter pools in various ecosystem types have not received much consideration. Therefore, we focused on the differences in C and N dynamics between midden and bulk soil samples in four corn (Zea mays L.) agroecosystems, a rotational pasture and a deciduous forest, in June, July and August of 1996, in Ohio, USA. Paired earthworm midden and bulk soil samples were analyzed for mineral N (NH₄⁺-N and NO₃^--N), dissolved organic N, microbial biomass N (MBN) and carbohydrate C (CarbC). Additionally, coarse litter, fine litter, particulate organic matter, and soil organic matter fractions were separated and analyzed for total C, total N and C:N ratios. Mineral and dissolved N levels were higher in the midden soil relative to those in the bulk soil for all ecosystem types, except for only NO₃^--N levels in two highly fertilized agroecosystems and in the pasture. MBN, CarbC, and total C and N levels for all organic fractions were significantly greater in the earthworm midden samples relative to these in the bulk samples across all ecosystem types. The plan defined by principal component analysis clearly separated two main groups: (1) includes the forest, the pasture and the less fertilized cornfields and the midden effect is to increase slightly the organic matter content and strongly the inorganic N content, and (2) includes the heavily fertilized agroecosystems and the midden effect is also to increase the organic matter content but to decrease the inorganic N content. We concluded that L. terrestris' middens significantly raised overall soil C and N levels relative to the bulk soil, in a variety of ecosystem types, and, given the abundance of earthworm middens, these macrosites should receive important attention when evaluating nutrient cycling processes at the systems level.     

Use of principal component analysis to assess factors controlling net N mineralization in deciduous and coniferous forest soils

Net N mineralization was studied in three different forest sites (Belgium): a mixed deciduous forest with oak (Quercus robur L. and Quercus rubra L.) and birch (Betula pendula Roth) as dominant species, a deciduous stand of silver birch (Betula pendula) and a coniferous stand of Corsican pine (Pinus nigra ssp. Laricio). The organic (F + H) layer and mineral soil at different depths (0-10, 10-20 and 20-30 cm) were sampled at three locations in the mixed deciduous forest (GE, GF1, GF2), at one location in the silver birch stand (SB) and one in the Corsican pine stand (CP). All samples were incubated over 10 weeks under controlled temperature and moisture conditions. The net N mineralization rates in the organic and upper mineral layer (0-10 cm) were found to be significantly different from the other layers and accounted for 66-95% of the total mineralization over the first 30 cm. Net N mineralization rates in the organic layer ranged from 4.2 to 27.3 mg N m^-2 day^-1. Net N mineralization and nitrification rates were positively correlated. For the mineral soil, net N mineralization rates decreased with depth and the upper 10 cm showed significantly higher rates, ranging from 8.9 to 33.5 mg N m^-2 day^-1. The rates of the 10-20 cm and 20-30 cm sublayers were similar, ranging from 1.2 to 7.4 mg N m^-2 day^-1. The net N mineralization rates for the total mineral layer (0-30 cm) ranged from 17.4 mg N m^-2 day^-1 (SB) to 36.1 mg N m^-2 day^-1 (CP). Both from PCA and multiple regression analysis, we could conclude that net N mineralization rates were closely related to the initial mineral N content (N_initial). Furthermore, significant correlations were observed between the net N mineralization rate, the total carbon (TC) and NH₄⁺-N content for the mineral layers and between net N mineralization rate, total nitrogen (TN), hemicellulose content and C/N for the organic layers.     

外加碳、氮对土壤氮矿化、固定与激发效应的影响

本文利用14C和15N对中国生黄绵土(坡地黄绵土)、菜园黄绵土和瑞典耕作草甸土的土壤氮矿化、固定与激发效应进行了研究。结果表明,外加碳、氮能促进土壤氮的矿化、固定与激发效应;促进作用的大小次序为外加NH4-15N大于外加NO3-15N,外加葡萄糖+NH4-15N大于外加葡萄糖+NO3-15N,外加麦秸+NH4-15N大于外加麦秸+NO3-15N,外加葡萄糖+NH4-15大于外加麦秸+NH4-15,外加葡萄糖+NO3-15N大于外加麦秸+NO3-15N;低肥力土壤高于高肥力土壤。在本文中提出了土壤净矿化氮的激发效应、土壤生物固定氮激发效应和土壤总矿化氮的总激发效应的概念,认为土壤氮的总激发效应更能反映土壤氮激发效应的实质。     

Effects of elevated carbon dioxide concentration on biological nitrogen fixation, nitrogen mineralization and carbon decomposition in submerged rice soil

Controlled-environment chambers were used to study the effects of elevated CO₂ concentrations on biological N fixation, N mineralization and C decomposition in rice soil. In three chambers, CO₂ concentration was maintained at 353ᆣ/396ᆫ µmol mol^-1 (day/night; ambient CO₂), while in another three, CO₂ was maintained at 667ᆸ/700ᆽ µmol mol^-1 (day/night; elevated CO₂) throughout the growing season. Rice (var. Nipponbare) seedlings were grown under either ambient or elevated CO₂ concentrations, and then transplanted into the soils in the corresponding chambers. At different growth stages, soil samples were taken from surface (0-1cm) and sub-surface (1-10cm) layers at the centre of four hills, then sieved (<1 mm) to remove root residues. Fresh soil was used to measure N fixation activity (using the acetylene reduction assay), NH₄⁺ content and organic C. Separate sets of soil samples were transferred to serum bottles and anaerobically incubated at 30°C for 30 days to measure potential rates of N mineralization and C decomposition. Under an elevated atmospheric CO₂ concentration, acetylene reduction activity significantly increased in the surface soil layer during the early cultivation stages and in the sub-surface soil layer during the latter part of cultivation. There was no difference in the amount of NH₄⁺ in fresh soils between elevated and ambient CO₂ chambers, while the rate of N mineralization was increased by elevated CO₂ during the latter part of cultivation. Soils from the elevated CO₂ chambers had obviously higher rate of C decomposition than that from the ambient CO₂ chambers. CH₄ production gradually increased with the growth of rice plants. These results suggest that elevated CO₂ affected biological N fixation, N mineralization and C decomposition in submerged rice soil during the different growth stages of rice.     

水氮互作对小麦土壤硝态氮运移及水、氮利用效率的影响

为给强筋小麦(Triticum aeativum L.)高产优质栽培的水、氮合理运筹提供理论依据,在高产地力条件下,选用强筋小麦品种济麦20,设置不施氮（N0）、施氮180 kg/hm2 （N1）、240 kg/hm2 （N2）3个施氮水平,每个施氮水平下设置不灌水（W0）、底墒水+拔节水+开花水（W1）、底墒水+冬水+拔节水+开花水（W2）、底墒水+冬水+拔节水+开花水+灌浆水（W3）4个灌水处理,每次灌水量均为60 mm,研究了水氮互作对麦田耗水量、土壤硝态氮运移、氮素利用效率和水分利用效率的影响。结果表明,（1）增加施氮量,开花期和成熟期0—140 cm各土层的土壤硝态氮含量显著升高;增加灌水时期,土壤硝态氮向深层的运移加剧,成熟期0—80 cm各土层的土壤硝态氮含量降低,120—140 cm土层的土壤硝态氮含量升高。N1W1处理在开花期0—60 cm土层的土壤硝态氮含量较高,成熟期土壤硝态氮向100—140 cm土层运移少,有利于植株对氮素的吸收。（2）随施氮量的增加,子粒产量先升高后降低,以N1最高。N1水平下,W1处理获得了较高的子粒产量、子粒氮素积累量、氮素利用效率、氮肥农学利用率和氮肥偏生产力;在此基础上增加冬水(W2),上述指标无显著变化;再增加灌浆水(W3),上述指标显著降低。（3）施氮提高了小麦对土壤水的利用能力,随施氮量增加,土壤供水量及其占总耗水量的比例显著升高。N1水平下,W1处理获得了最高的水分利用效率;再增加灌水时期,水分利用效率显著降低,开花至成熟阶段的耗水模系数显著升高,灌水量占总耗水量的比例升高,降水量和土壤供水量占总耗水量的比例降低。本试验条件下,施氮为180 kg/hm2,灌底墒水＋拔节水＋开花水3水的N1W1处理,是兼顾高产、高效的水氮运筹模式。     

Organic matter and aggregation in a degraded potato soil as affected by raw and composted pulp residue

Soils under intensive potato (Solanum tuberosum L.) production are often low in organic matter content and microbial activity. Pulp fibre residue addition may restore the quality of these soils. An experiment was initiated in which raw or composted pulp fibre waste was added to a Fredericton sandy loam (Orthic Humo-Ferric Podzol) in New Brunswick (Canada). Each material was applied at 45 Mg (dry weight) ha^-1 and 90 Mg (dry weight) ha^-1 before planting. Soil was under continuous potato (cv. Russet Burbank) for 3 years. The raw and the composted pulp greatly increased soil organic matter (SOM) content, the C/N ratio, macroaggregation, C mineralization, microbial biomass C (MBC) and enzyme activities for the whole duration of the experiment but had no effect on its total N. Most of the SOM increase was found in the macroorganic matter fraction. The SOM, C/N ratio, macroaggregation, C mineralization and MBC decreased from the time of application. No major difference was found between materials except for CO₂-C release in the incubation which was higher for the composted than for the raw pulp. This study indicated that pulp fibre, either applied raw or composted, increased SOM and macroaggregation and promoted microbial growth and activity in this potato soil which was low in C content. The beneficial effects on soil physical and biochemical properties were still present after 3 years of continuous cropping.     

Nitrogen mineralization potential of dairy manures and its relationship to composition

The objectives of this study were to determine the variability in mineralization of dairy manure N, to determine if N mineralization can be predicted by compositional factors or by near- or mid-infrared reflectance spectroscopy. Dairy manures (n =107) were collected from farms in Maryland, Virginia, Pennsylvania, New York, and Connecticut. The composition of these manures ranged from 14 to 386 g dry matter kg^-1, 0.9 to 9.5 kg total N/m³, and 0.3 to 4.7 kg NH₄⁺-N/m³. Manure-amended soil was aerobically incubated at 25°C and concentrations of NH₄⁺-N and NO₃^--N were determined at day 2 and day 56. The manures were highly variable in their N mineralization characteristics, ranging from a net mineralization of 54.9% to a net immobilization of 29.2% of the organic N. When compositional parameters were individually regressed against percentage mineralized organic N, the highest correlation coefficient (r) was 0.164. A stepwise regression of all 11 variables yielded a maximal r of 0.486. These results suggest that the availability of dairy manure organic N is highly variable and that the availability cannot be predicted from simple compositional parameters. No relationship was found between near-infrared spectral characteristics and N mineralization suggesting that no simple relationship exists between N mineralization and compositional characteristics. There appears to be some potential for the use of mid-infrared for determining the mineralization potential of manures.     

Different behavioral patterns of the earthworms Octolasion tyrtaeum and Diplocardia spp. in tallgrass prairie soils: potential influences on plant growth

This study addressed differences between Diplocardia spp. (a native North American earthworm) and Octolasion tyrtaeum (an introduced European species), with respect to behavior, influence on soil microbial biomass, and plant uptake of N in tallgrass prairie soils. We manipulated earthworms in PVC-encased soil cores (20 cm diameter) over a 45-day period under field conditions. Treatments included: (1) control with no earthworms, (2) Diplocardia spp. only, and (3) O. tyrtaeum only. Prior to addition of earthworms, seedlings of Andropogon gerardii (a dominant tallgrass) were established in each core, and a dilute solution of ¹³C-labeled glucose and ¹⁵N-labeled (NH₄)₂SO₄ was added to the soil to facilitate examination of earthworm/microbe/plant interactions. We found that Diplocardia spp. were significantly more active than O. tyrtaeum, and quickly assimilated ¹³C and ¹⁵N from the tracer. Individuals of Diplocardia spp. were present at shallower soil depths than O. tyrtaeum throughout the study. Contrary to expectation, this greater activity of Diplocardia spp. did not result in increased plant productivity. Rather, the activity of Diplocardia spp. was associated with less plant growth and smaller amounts of N acquired by A. gerardii seedlings compared to controls or O. tyrtaeum treatments. We observed few significant influences of earthworm treatments on microbial biomass C or N pool sizes, but the microbial C/N ratio was consistently greater in the presence of Diplocardia spp. relative to O. tyrtaeum. Results of this study indicate that activity of earthworms may enhance competition for N between microbes and plants during the growing season in tallgrass prairie.     

Impact of a change in tillage and crop residue management practice on soil chemical and microbiological properties in a cereal-producing red duplex soil in NSW, Australia

The effect of a change of tillage and crop residue management practice on the chemical and microbiological properties of a cereal-producing red duplex soil was investigated by superimposing each of three management practices (CC: conventional cultivation, stubble burnt, crop conventionally sown; DD: direct-drilling, stubble retained, no cultivation, crop direct-drilled; SI: stubble incorporated with a single cultivation, crop conventionally sown), for a 3-year period on plots previously managed with each of the same three practices for 14 years. A change from DD to CC or SI practice resulted in a significant decline, in the top 0-5 cm of soil, in organic C, total N, electrical conductivity, NH₄-N, NO₃-N, soil moisture holding capacity, microbial biomass and CO₂ respiration as well as a decline in the microbial quotient (the ratio of microbial biomass C to organic C; P <0.05). In contrast, a change from SI to DD or CC practice or a change from CC to DD or SI practice had only negligible impact on soil chemical properties (P >0.05). However, there was a significant increase in microbial biomass and the microbial quotient in the top 0-5 cm of soil following the change from CC to DD or SI practice and with the change from SI to DD practice (P <0.05). Analysis of ester-linked fatty acid methyl esters (EL-FAMEs) extracted from the 0- to 5-cm and 5- to 10-cm layers of the soils of the various treatments detected changes in the FAME profiles following a change in tillage practice. A change from DD practice to SI or CC practice was associated with a significant decline in the ratio of fungal to bacterial fatty acids in the 0- to 5-cm soil (P <0.05). The results show that a change in tillage practice, particularly the cultivation of a previously minimum-tilled (direct-drilled) soil, will result in significant changes in soil chemical and microbiological properties within a 3-year period. They also show that soil microbiological properties are sensitive indicators of a change in tillage practice.     

Short-term patterns of carbon and nitrogen mineralisation in a fallow field amended with green manures from agroforestry trees

The mineralisation of green manure from agroforestry trees was monitored with the objective to compare the temporal dynamics of mineralisation of litter from different species. Green manures from five agroforestry tree species were used on a fallow field during the long rainy season of 1997 (March-August) and from two species in the following short rainy season (September-January) in western Kenya. Different methods, i.e. measurements of isotopic ratios of C in respired CO₂ and of soil organic matter (SOM) fractions, soil inorganic N and mass loss from litterbags, were used in the field to study decomposition and C and N mineralisation. Soil respiration, with the separation of added C from old soil C by using the isotopic ratio of ¹³C/¹²C in the respired CO₂, correlated well with extractable NH₄⁺ in the soil. Mineralisation was high and very rapid from residues of Sesbania sesban of high quality [e.g. low ratio of (polyphenol+lignin)/N] and low and slow from low quality residues of Grevillea robusta. Ten days after application, 37% and 8% of the added C had been respired from Sesbania and Grevillea, respectively. Apparently, as much as 70-90% of the added C was respired in 40 days from high quality green manure. Weight losses of around 80%, from high quality residues in litterbags, also indicate substantial C losses and that a build-up of SOM is unlikely. For immediate effects on soil fertility, application of high quality green manure may, however, be a viable management option. To achieve synchrony with crop demand, caution is needed in management as large amounts of N are mineralised within a few days after application.     

氮肥后移对土壤氮素供应和冬小麦氮素吸收利用的影响

采用田间试验研究了氮肥后移对土壤氮素供应和冬小麦氮素吸收利用的影响。结果表明,与农民习惯施氮（N 300 kg/hm2,基肥和拔节肥各占1/2）比较,氮肥后移处理（N210kg/hm2,基肥、拔节肥和孕穗肥各占1/3）在不降低小麦产量的同时,大大提高了氮肥利用率,且全生育期氮素表观损失极低。过量施用氮肥(N 300 kg/hm2)明显提高了60 cm以下土层硝态氮含量,增加了其向地下水淋溶迁移的风险。氮肥后移可提高小麦成熟期0-20cm土层硝态氮积累量,降低其在20-100cm土层的积累。基于冬小麦不同生育阶段的氮素吸收量而进行氮肥后移是可行的,氮肥后移可节省氮肥30%,是较为理想的施氮方式。     

Nitrogen mineralization and crop uptake from surface-applied leaves of green manure species on a tropical volcanic-ash soil

Leaves of nine green manure (GM) species were surface applied to a tropical volcanic-ash soil at a rate of 100 kg N ha^-1 in order to evaluate their N-fertilizer value in a glasshouse experiment. GM treatments were compared to urea at two rates, 50 kg N ha^-1 (FN50) and 100 kg N ha^-1 (FN100), and to a control with no fertilizer application (FN0). Two weeks after treatment application, upland rice seedlings were sown in order to conduct N uptake studies. Soil volumetric moisture content was maintained close to 50%. In general, soil showed an initial increase in inorganic N followed by a rapid decline with time. After 2 weeks of evaluation FN100, FN50 and leaves of Mucuna pruriens var. Tlaltizapan and Indigofera constricta presented higher values of inorganic N (157-109 mg N kg^-1 soil); while, FN0 and leaves of Mucuna deerengianum, Cratylia argentea and Calliandra calothyrsus presented lower values (75-89 mg N kg^-1 soil). N recovery by rice, at 20 weeks after planting, was highest for FN100 (59.9%) followed by Canavalia brasiliensis (54.6%), Calliandra calothyrsus (47.4%) and M. pruriens var. IITA-Benin (32.4%); while, M. pruriens var. Tlaltizapan, FN50, Tithonia diversifolia and I. constricta presented lower N uptake (13-20%). Significant relationships were found between some quality parameters of GM evaluated (i.e. total N, fibers, lignin and polyphenol content), soil N availability and rice N uptake. These results suggest that GM that decomposed and released N slowly resulted in high N uptake when they were used at pre-sowing in a tropical volcanic-ash soil.     

Mineralisation of <Superscript>15</Superscript>N-labelled sheep manure in soils of different texture and water contents

In order to investigate the effect of soil water and texture on C and N mineralisation of applied organic matter, sheep manure was sandwiched between two halves of intact soil cores and incubated at 20°C. The soils contained 10.8% (L1), 22.4% (L3) and 33.7% (L5) clay, respectively, and were drained to seven different matric potentials in the range -15 to -1,500 hPa. Evolution of CO₂-C was determined during 4 weeks of incubation. Contents of NO₃^--N, ¹⁵N and microbial biomass N were determined at the end of the incubation. The net release of CO₂-C from the manure (estimated as the difference between soils with and without manure) and the total CO₂-C evolution from soils with manure was not related to soil water content. Most CO₂-C evolved from manure-amended soils in the least clayey L1 soil. The manure caused immobilisation of soil NO₃^--N but the soil matric potential had no major effects on the net NO₃^--N production. Less than 1% of the manure ¹⁵N was found as NO₃^--N at the end of the incubation. When unamended, the sandy L1 soil held the least N in microbial biomass but the largest increases in biomass N caused by manure application were found in this soil. Despite the higher increases in microbial biomass N in the L1 soil, the total content of microbial biomass N in soils with manure application peaked in the most clayey soil (L5). The recovery of manure ¹⁵N at the end of the incubation ranged from 89% to 102%. The variation in ¹⁵N recovery was not related to soil clay content nor to soil matric potential. The experimental set-up was designed to mimic field conditions where manure is left as a discrete layer surrounded by structurally intact soil. In this situation the soil clay content and the soil water level appeared to have little influence on the C and N turnover in the manure layer.