Nitrogen mineralization of leguminous crops in soils

Organic‐N production by legumes is a key benefit of growing cover crops and green manures. A soil sample was mixed with legume residue commonly used as green manure in Kenya at a rate of 500 mg N (kg soil)^–1. Silica sand equal to the weight of the soils was added and mixed thoroughly. The mixture was packed in a leaching tube and leached with 100 mL of 5 mM of CaCl₂ ^. 2H₂O and incubated at 30°C. The leaching was repeated every 2 weeks for a total of 16 weeks and analyzed for N as NH , NO , and NO . Five legume residues and five different soils were used in this study. Nitrogen mineralization of the legume residues conformed to an exponential model. Application of a two‐components exponential model showed two phases of N mineralization. The relationship between the organic N remaining after each incubation period and time of incubation was controlled by two first‐order reactions. The initial fast rate (k₁) changed to a slow rate (k₂) at incubation times ranging from 2 to 8 weeks, depending on the legume residue and the soil used. The percentage of N in each phase varied among the legume residue and soils. Linear regression analyses showed that net cumulative amounts of N mineralized from individual legume residues was significantly correlated with the total polyphenols and polyphenol‐to‐N ratios for two soils. Nitrogen mineralization of dolichos and field bean was significantly and negatively correlated with clay and sand, respectively; of field bean and alfalfa was significantly correlated with C_mic; and of dolichos significantly but negatively correlated with the total N and organic N in soils. Linear regression analysis of the pooled data showed that net cumulative amounts of N mineralized and percentage N mineralized were significantly correlated with C : N ratios of the residues (r = 0.44 and 0.48 at p < 0.05, respectively), and that net cumulative N mineralized was significantly correlated with (lignin + polyphenols)‐to‐N ratios (r = 0.41 at p < 0.05) and with lignin contents (r = 0.61 at p < 0.001).     

Importance of organic nitrogen fractions in sandy soils,obtained by electro-ultrafiltration or CaCl2 extraction,for nitrogen mineralization and nitrogen uptake of rape

Summary Sandy soils have low reserves of mineral N in spring. Therefore organic-bound N is the most important pool available for crops. The objective of the present investigation was to study the importance of the organic-bound N extracted by electro-ultrafiltration and by a CaCl₂ solution for the supply of N to rape and for N mineralization. Mitscherlich-pot experiments carried out with 12 different sandy soils (Germany) showed a highly significant correlation between the organic N extracted (two fractions) and the N uptake by the rape (electroultrafiltration extract: r=0.76^***; CaCl₂ extract: r=0.76^***). Organic N extracted by both methods before the application of N fertilizer was also significantly correlated with N mineralization (electro-ultrafiltration extract: r=0.75^***; CaCl₂ extract: r=0.79^***). N uptake by the rape and the mineralization of organic N increased with soil pH and decreased with an increasing C:N ratio and an increasing proportion of sand in the soils. Ninety-eight percent of the variation in N uptake by the rape was determined by the differences in net mineralization of organic N. This show that in sandy soils with low mineral N reserves (NO _inf3 ^sup- -N, NH ₄ ⁺ -N) the organic soil N extracted by electro-ultrafiltration or CaCl₂ solutions indicates the variance in plant-available N. Total soil N was not related to the N uptake by plants nor to N mineralization.     

畜禽废弃物、施用畜禽废弃物牧场以及草芽中氨基化合物的研究

Amino compounds(ACs),i.e.,amino acids and amino sugars,are the major forms of organic nitrogen(N)in animal manure and soil.To increase our understanding on the effect of long-term poultry litter(PL)application on soil AC pools and turnover,in this study,we determined the contents of 21 ACs in 23 PL samples,15 soil samples collected from 0–20,20–40,and 40–60 cm layers of five pasture plots with 0,5,10,15 and 20 years of PL applications,and 5 grass shoot samples grown on these pasture fields.The contents of 21 ACs were simultaneously determined by methanesulfonic acid hydrolysis/extraction and anion chromatography-pulsed amperometry.PL application increased soil total and individual AC contents with a distribution pattern similar to that of AC in PL.The highest AC-N concentrations were observed in the soils with 10-or 15-year PL applications,inconsistent with the order of annual application rates or cumulative applied PL amounts.Application of PL increased the AC contents in grass shoots whereby the highest increase of most ACs was with the shoots from the fields that received PL for 5 years.These observations suggested that both freshly applied and residual PL had contributions to the soil AC-N,and that PL application also accelerated AC-N transformation in soil.     

Differentiating the mineralization dynamics of the originally present and newly synthesized amino acids in soil amended with available carbon and nitrogen substrates

Newly synthesized amino acids are the principle compounds created after inorganic nitrogen (N) is rapidly immobilized into microbial tissues. However, little is known about the mineralization kinetics of these newly synthesized amino acids compared to the amino acids originally present in the soil, and how substrate availability controls their mineralization. With ¹⁵N isotope tracing, the newly synthesized (¹⁵N-labeled) amino acids can be differentiated from the amino acids originally present (unlabeled) in soil, making it possible to evaluate the mineralization of the newly synthesized amino acids in tandem with the original amino acids. As amino acids can serve as both N and carbon (C) sources for microorganisms, the mineralization dynamics of amino acids may be manipulated by the availability of extraneous C and N. In this study, an aerobic 30-week intermittent leaching experiment was conducted, using glucose as C source and (¹⁴NH₄)₂SO₄ as N source, following separate additions to soil. The newly synthesized amino acids were determined by an isotope-based high performance liquid chromatography/mass spectrometry (HPLC/MS). The newly synthesized soil amino acids mineralized faster than the original ones, which indicated more rapid cycling of N in the newly synthesized soil amino acids pool. Glucose addition significantly decreased the mineralization of both the newly synthesized and the original amino acids. However, when inorganic N was abundant, the newly synthesized amino acids decomposed rapidly, and preferentially as a C source and energy, while N addition inhibited the mineralization of the original amino acids in the soil. We conclude that the presence of readily degradable C (e.g. glucose) and inorganic N controls the mineralization of newly synthesized and original amino acid pools in soil differently, which is a crucial mechanism in adjusting the N supply and sequestration processes in soil ecosystems.     

Nitrogen mineralization in relation to site history and soil properties for a range of Australian forest soils

Rates of N mineralization were measured in 27 forest soils encompassing a wide range of forest types and management treatments in south-east Australia. Undisturbed soil columns were incubated at 20°C for 68 days at near field-capacity water content, and N mineralization was measured in 5-cm depth increments to 30 cm. The soils represented three primary profile forms: gradational, uniform and duplex. They were sampled beneath mature native Eucalyptus sp. forest and from plantations of Pinus radiata of varying age (<1 to 37 years). Several sites had been fertilized, irrigated, or intercropped with lupins. The soils ranged greatly in total soil N concentrations, C:N ratios, total P, and sand, silt, and clay contents. Net N mineralization for individual soil profiles (0–30 cm depth) varied from 2.0 to 66.6 kg ha^-1 over 68 days, with soils from individual depths mineralizing from <0 (immobilization) to 19.3 kg ha^-1 per 5 cm soil depth. Only 0.1–3.1% of the total N present at 0–30 cm in depth was mineralized during the incubation, and both the amount and the percentage of total N mineralized decreased with increasing soil depth. N fertilization, addition of slash residues, or intercropping with lupins in the years prior to sampling increased N mineralization. Several years of irrigation of a sandy soil reduced levels of total N and C, and lowered rates of N mineralization. Considuring all soil depths, the simple linear correlations between soil parameters (C, N, P, C:N, C:P, N:P, coarse sand, fine sand, silt, clay) and N mineralization rates were generally low (r<0.53), but these improved for total N (r=0.82) and organic C (r=0.79) when the soils were grouped into primary profile forms. Prediction of field N-mineralization rates was complicated by the poor correlations between soil properties and N mineralization, and temporal changes in the pools of labile organic-N substrates in the field.     

温度和湿度对高寒湿地土壤碳矿化和氮矿化的影响

Relationships between carbon (C) production and nitrogen (N) mineralization were investigated in two alpine wetland soils of the Tibetan Plateau using laboratory incubation under different temperatures (5, 15, 25, and 35 ℃) and water saturation (noninundation and inundation). A significant positive relationship was found between CO2 production and N mineralization under increasing temperatures from 5 to 35 ℃ with the same water saturation condition in the marsh soil (r2 ＞ 0.49, P ＜ 0.0001) and the peat soil (r2 ＞ 0.38, P ＜ 0.002), and a negative relationship with water saturation increasing at the same temperature, especially 25 and 35 ℃, in the marsh soil (r2 ＞ 0.70, P ＜ 0.009) and the peat soil (r2 ＞ 0.61, P ＜ 0.013). In conclusion, temperatures and water saturation could regulate the relationship between CO2 production and net N mineralization in the Tibetan alpine marsh and peat soils.     

Nitrogen mineralization from an organically managed soil and nitrogen accumulation in lettuce

The potential of an organically managed Cambic Arenosol to supply nitrogen (N) from either an applied commercial organic fertilizer (granulated hen manure), a compost produced on‐farm, or four different mixtures of both fertilizers was studied in a laboratory incubation and a pot experiment with lettuce. In the incubation experiment, a significant higher apparent N mineralization occurred after hen‐manure application (53.4% of the organic N applied) compared to compost (4.5%) or mixed‐fertilizer application (8.7% to 16.7%). The apparent N mineralization in a mixed treatment consisting of compost and half rate of hen manure (15.4% of the organic N applied) was significantly higher than that estimated based on the N mineralization for compost and hen‐manure treatments (7.6%), proving that a combined application of both fertilizers enhanced organic‐N mineralization when compared to separate fertilizer supply. In the pot experiment, a higher lettuce fresh‐matter yield was obtained with hen manure (1.9 kg m^–2) than with compost (1.7 kg m^–2) or unfertilized control treatment (1.3 kg m^–2). Combined application of compost with only a half rate of hen manure led to yields (2.0 kg m^–2) equal to those obtained with only hen manure. A good correlation was observed between the N‐mineralization incubation data and the N accumulated by lettuce plants in the pot experiment (r = 0.983). Hence, in the organic production of baby‐leaf lettuce, a mixture of compost and hen manure appears to be a good fertilization alternative, since it allows a reduction by half of the typical amount of commercial fertilizer usually applied (granulated hen manure), cutting fertilization costs, and providing an amount of available N that allows maintaining lettuce yields.     

不同氮素形态配施氨基酸对翠冠梨生长及品质的影响研究

为探讨不同氮素配施氨基酸对梨树生长、产量及品质的影响,连续两年在8年生翠冠梨上开展了等氮条件下单施硝铵和尿素及配施氨基酸的研究。结果表明,单施硝铵处理较单施尿素处理显著增加了SPAD值、叶面积和百叶重,分别比尿素处理增加了2.8%、5.9%和7.2%。硝铵配施氨基酸处理显著增加了单果重。单施硝铵处理较单施尿素处理显著增加了单株产量、硬度和可溶性糖,有增加可溶性固形物和可滴定酸含量的趋势。花后110天硝铵配施氨基酸处理比单施硝铵处理显著增加了果糖、葡萄糖和蔗糖含量。各种糖在不同生育期所占比例不同,花后46天果实中山梨醇所占比例最高,约占总糖含量的69%~73%,葡萄糖和果糖比例相当;随着果实的发育,果糖和蔗糖比例逐渐增加,山梨醇比例降低。总体看来,硝铵配施氨基酸处理更有利于梨树生长和果实品质的提高。     

Differential Utilization of Dissolved Organic and Inorganic Nitrogen by Wheat (Triticum Aestivum L.)

A study was conducted under greenhouse conditions on wheat to investigate the utilization of dissolved organic nitrogen (N) in comparison with conventionally applied inorganic N sources (INS). Nitrogen was applied at a rate of 90 kg N ha^?1 in an inorganic form, an organic high molecular weight (MW) form (casein, haemoglobin, albumin), and an organic low MW amino acid form (glycine, alanine, valine). Inorganic N sources recorded the maximum response (126% to 150%) in total dry matter (DM) production while dissolved organic nitrogen (DON) sources showed 61% to 116% increase in comparison to the control treatment. Glycine gave the maximum DM production, which was comparable with both INS treatments. In hydroponics, greater utilization occurred and the shoots had a higher N content in comparison to those grown in soil. The concentration of DON and NO₃^? in soil after wheat harvest was similar in all the treatments.     

Nitrogen turnover in bare soil planted subsequently with grass as investigated by electro‐ultrafiltration (EUF)

Changes of EUF‐extractable nitrogen (N) (nitrate, ammonium, organic N) in 20 arable bare soils, subsequently planted with ryegrass (Lolium multiflorum L.) and cutting three times were investigated in pot experiments. All 20 soils responded qualitatively in the same way. During the period of bare soil, there was a significant increase of EUF‐extractable nitrate (EUF NO ), while extractable ammonium (EUF NH ) remained on the same level and organic N (EUF N_org) decreased. This decrease, however, was not significant. From sowing until the first cutting of the grass, EUF‐NO concentration decreased to almost zero. This low EUF‐NO level was maintained throughout the subsequent experimental period (three cuttings of grass). During the growth of the first cutting, EUF N_org decreased while EUF NH remained constant, however, on a low level. EUF NH fell during the growth of the second and third cutting. In this period, however, the N supply of the grass was insufficient. EUF N_org decreased during the growth of the second cutting, but increased during the growth of the third cutting. This shows that the EUF‐N_org fraction represents a transient pool, which gains and loses N. EUF NO , EUF NH , and EUF N_org correlated with the N uptake of the grass. Strongest correlation for EUF NO was found for the first cutting (p < 0.001), and for EUF NH and EUF N_org for the second and third cutting (p < 0.001). Total soil N was not correlated with the N uptake of the grass. EUF N_org was only about 2% of the total N. This relatively small EUF‐N_org fraction, however, is relevant for the mineralization of organic soil N, and the N quantity indicated by EUF N_org is in the range of the N amount mineralized in arable soils within a growing season.     

Effects of biochar,urea and their co‐application on nitrogen mineralization in soil and growth of Chinese cabbage

Experiments were conducted to study the effect of soil applications of kunai grass (Imperata cylindrica) biochar (0 and 10 t/ha) and laboratory grade urea (0, 200 and 500 kg N/ha) and their co‐application on nitrogen (N) mineralization in an acid soil. The results of an incubation study showed that the biochar only treatment and co‐application with urea at 200 kg N/ha could impede transformation of urea to ammonium‐N (NH₄⁺‐N). Soil application of biochar together with urea at 500 kg N/ha produced the highest nitrate‐N (NO₃^?‐N) and mineral N concentrations in the soil over 90 days. Co‐application of urea N with biochar improved soil N mineralization parameters such as mineralization potential (N_A) and coefficient of mineralization rate (k) compared to biochar alone. In a parallel study performed under greenhouse conditions, Chinese cabbage (Brassica rapa L. ssp. chinensis L.) showed significantly greater (P < 0.05) marketable fresh weight, dry matter production and N uptake in soil receiving urea N at 500 kg/ha or co‐application of biochar with urea N compared to the control. Application of biochar only or urea only at 200 kg N/ha did not offer any short‐term agronomic advantages. The N use efficiency of the crop remained unaffected by the fertilizer regimes. Applications of biochar only at 10 t/ha did not offer benefits in this tropical acid soil unless co‐applied with sufficient urea N.     

Experimental evaluation of methods to quantify dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in soil

A significant proportion of the total nutrient in soil solution can be bound to organic molecules and these often constitute a major loss from soil to freshwater. Our purpose was to determine whether chemical extractants used for measuring inorganic N could also be used to quantify dissolved organic nitrogen (DON) and carbon (DOC) in soil. In a range of soils, DOC and DON were extracted with either distilled water or 2 M KCl and the amount recovered compared with that present in soil solution recovered by centrifugal-drainage. The recovery of DON and DOC from soil was highly dependent upon the method of extraction. Factors such as soil sampling strategy (number of samples over space and time), sample preparation (sieving and drying), soil storage, extraction temperature, shaking time, and soil-to-extractant volume ratio all significantly affected the amount of DOC and DON extracted from soil. To allow direct comparison between independent studies we therefore propose the introduction of a standardized extraction procedure: Replicate samples of unsieved, field-moist soil extracted as soon as possible after collection with distilled water, 0.5 M K₂SO₄ or 2 M KCl at a 1:5 w/v ratio for 1 h at 20 °C.     

Contribution of α-amino N to extractable organic nitrogen (DON) in three soil types from the Scottish uplands

Organic nitrogen (DON) was extracted from two improved pasture soils, one of which had been re-colonized by acid heath vegetation, and a blanket peat. Although the quantities extracted in H₂O, 10 mM CaCl_2, 500 mM K₂SO₄ and 50 mM Na₂HPO₄ were not consistent, mean extractable DON as a proportion of total N was greater in the two grazed pastures (0.4%) than in the peat (0.2%). Averaged over the four extractants, free α-amino N was greater in the peat and least in the improved pasture soil and accounted for 26% of DON in the peat and less than 5% in the mineral soil. Amino N increased after 6 M HCl hydrolysis, and this combined N contributed 56% to DON in extracts of the mineral soil compared with only 36% in the peat This variation in the relative contributions of free and combined amino N to DON indicated qualitative differences in the composition of DON between the three soils.     

A field study of gross rates of N mineralization and nitrification and their relationships to microbial biomass and enzyme activities in soils treated with dairy effluent and ammonium fertilizer

Abstract. Gross N mineralization and nitrification rates were measured in soils treated with dairy shed effluent (DSE) (i.e. effluent from the dairy milking shed, comprising dung, urine and water) or ammonium fertilizer (NH₄Cl) under field conditions, by injecting ¹⁵N-solution into intact soil cores. The relationships between gross mineralization rate, microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) as affected by the application of DSE and NH₄Cl were also determined. During the first 16 days, gross mineralization rate in the DSE treated soil (4.3–6.1 μg N g^?1 soil day^?1) were significantly (P 14;< 14;0.05) higher than those in the NH₄Cl treated soil (2.6–3.4 μg N g^?1 soil day^?1). The higher mineralization rate was probably due to the presence of readily mineralizable organic substrates in the DSE, accompanied by stimulated microbial and extracellular enzyme activities. The stable organic N compounds in the DSE were slow to mineralize and contributed little to the mineral N pool during the period of the experiment. Nitrification rates during the first 16 days were higher in the NH₄Cl treated soil (1.7–1.2 μg N g^?1 soil day^?1) compared to the DSE treated soil (0.97–1.5 μg N g^?1 soil day^?1). Soil microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) increased after the application of the DSE due to the organic substrates and nutrients applied, but declined with time, probably because of the exhaustion of the readily available substrates. The NH₄Cl application did not result in any significant increases in microbial biomass C, protease or urease activities due to the lack of carbonaceous materials in the ammonium fertilizer. However, it did increase microbial biomass N and deaminase activity. Significant positive correlations were found between gross N mineralization rate and soil microbial biomass, protease, deaminase and urease activities. Nitrification rate was significantly correlated to biomass N but not to the microbial biomass C or the enzyme activities. Stepwise regression analysis showed that the variations of gross N mineralization rate was best described by the microbial biomass C and N.     

An evaluation of plant-available soil nitrogen in selected sandy soils by electro-ultrafiltration, KCl, and CaCl2 extraction methods

 Improving the precision in estimating the nitrogen (N) requirement for citrus trees on sandy soils is important for increasing N efficiency by the trees and minimizing potential losses of N in commercial citrus production areas. In this study, representative Florida soils were sampled from major citrus production areas and the electro-ultrafiltration (EUF) technique was used to measure the concentrations of total EUF-extractable nitrogen (EUF-N_t), ammonium-N (EUF-NH₄ ⁺–N) and nitrate-N (EUF-NO₃ ^––N). Available organic N (N_org) was calculated as: EUF-N_t–(NH₄ ⁺–N+NO₃ ^––N). The N concentrations in the EUF extraction were greater than those by the KCl or CaCl₂ method. The N_org fraction, estimated by the EUF method, varied from 4.4 to 40.8 mg kg^–1 soil, equivalent to 10 to 91 kg N ha^–1 (for the top 15 cm depth soil) and was positively correlated with the total soil N determined by the Kjeldahl method. The presence of appreciable amounts of N_org in these soils indicates that these soils contain high proportions of the total soil N in easily mineralizable N_org forms. This study demonstrates that the EUF-extractable organic bound N must be considered in developing N fertilizer recommendations for citrus. Received: 13 January 1999     

Gross nitrogen mineralization and nitrification rates and their relationships to enzyme activities and the soil microbial biomass in soils treated with dairy shed effluent and ammonium fertilizer at different water potentials

 Gross N mineralization and nitrification rates and their relationships to microbial biomass C and N and enzyme (protease, deaminase and urease) activities were determined in soils treated with dairy shed effluent (DSE) or NH₄ ⁺ fertilizer (NH₄Cl) at a rate equivalent to 200 kg N ha^–1 at three water potentials (0, –10 and –80 kPa) at 20  °C using a closed incubation technique. After 8, 16, 30, 45, 60 and 90 days of incubation, sub-samples of soil were removed to determine gross N mineralization and nitrification rates, enzyme activities, microbial biomass C and N, and NH₄ ⁺ and NO₃ ^– concentrations. The addition of DSE to the soil resulted in significantly higher gross N mineralization rates (7.0–1.7 μg N g^–1 soil day^–1) than in the control (3.8–1.2 μg N g^–1 soil day^–1), particularly during the first 16 days of incubation. This increase in gross mineralization rate occurred because of the presence of readily mineralizable organic substrates with low C : N ratios, and stimulated soil microbial and enzymatic activities by the organic C and nutrients in the DSE. The addition of NH₄Cl did not increase the gross N mineralization rate, probably because of the lack of readily available organic C and/or a possible adverse effect of the high NH₄ ⁺ concentration on microbial activity. However, nitrification rates were highest in the NH₄Cl-treated soil, followed by DSE-treated soil and then the control. Soil microbial biomass, protease, deaminase and urease activities were significantly increased immediately after the addition of DSE and then declined gradually with time. The increased soil microbial biomass was probably due to the increased available C substrate and nutrients stimulating soil microbial growth, and this in turn resulted in higher enzyme activities. NH₄Cl had a minimal impact on the soil microbial biomass and enzyme activities, possibly because of the lack of readily available C substrates. The optimum soil water potential for gross N mineralization and nitrification rates, microbial and enzyme activities was –10 kPa compared with –80 kPa and 0 kPa. Gross N mineralization rates were positively correlated with soil microbial biomass N and protease and urease activities in the DSE-treated soil, but no such correlations were found in the NH₄Cl-treated soil. The enzyme activities were also positively correlated with each other and with soil microbial biomass C and N. The forms of N and the different water potentials had a significant effect on the correlation coefficients. Stepwise regression analysis showed that protease was the variable that most frequently accounted for the variations of gross N mineralization rate when included in the equation, and has the potential to be used as one of the predictors for N mineralization. Received: 10 March 1998     

Cobalt and molybdenum stimulate compounds of primary metabolism,nitrogen forms,and photosynthetic pigments in peanut plants (Arachis hypogaea L.)

Abstract

The objective of this study was to evaluate the effects of cobalt (Co) and molybdenum (Mo) doses in the treatment of seeds on the biosynthesis of nitrogen compounds, photosynthetic pigments, sugars, and production of peanut plants. The doses of Co and Mo used were 0, 2, 3, and 4?mL kg^?1 seed, which were applied immediately before sowing. Seeds treated with Co and Mo at a dose of 4?mL kg^?1 yielded peanut plants with higher concentrations of photosynthetic pigments, carotenoids, and sucrose in leaves. Application of Co and Mo doses also increased biological nitrogen fixation by increasing the concentration of allantoic acid, nitrate, ammonia, and amino acids in leaves. The concentration of total amino acids corresponded to most of the nitrogen compounds (on average 50%), followed by the concentrations of nitrate (35%), ammonia (11%), allantoic acid (7%), and allantoin (0.2%). Application of 4?mL kg^?1 increased the production of total amino acids compared with the control treatment. Pod yield was not affected by the Co and Mo doses; however, treatment of peanut seeds with 4?mL kg^?1 was the most viable alternative for increased production of primary metabolism compounds, nitrogen forms, and photosynthetic pigments in peanut plants. This study provides important information regarding the role of Co and Mo in the biological nitrogen fixation of peanut plants. Future experiments should be conducted using a dose of 4?mL kg^?1 with different genotypes to verify the potential for increasing peanut yield.     

Nitrogen accumulation in soybean [Glycine max (L.) Merr.] is increased by manure compost application in drained paddy fields as a result of increased soil nitrogen mineralization

The application of manure compost is an effective way to increase soybean [Glycine max (L.) Merr.] yield and nitrogen (N) fertility in drained paddy fields. We investigated changes in soil N mineralization during soybean cultivation using reaction kinetics analysis to determine the contribution of increased N mineralization after manure compost application (at a rate of 0 to 6?kg?m^?2) on N accumulation and seed yield of soybean under drained paddy field conditions. The seed yield and N accumulation decreased markedly in the second and third year of the experiment, but soil N mineralization increased in both years. No decrease in soil N mineralization occurred even after two soybean crops. Soil N availability was not the main cause of decreased soybean yield in the second and third years. The differences in plant aboveground N content between plots with and without manure compost was similar to the increase in N mineralization caused by manure compost application in the second and third years. The application of 6?kg?m^?2 of manure compost increased the amount of ureide-N and nitrate-N in soybean in the third year. Our results suggest that manure compost application increases soil N mineralization and soybean N₂ fixation, resulting in increased N accumulation and seed yield. However, the soybean yield remained less than 300?g?m^?2 in the second and third years (i.e., below the yield in the first year) at all levels of manure compost application due to the remarkable decrease of N accumulation in the second and the third crops.     

Leaf-cutting ant (Atta Sexdens) and nutrient cycling: deep soil inorganic nitrogen stocks, mineralization, and nitrification in Eastern Amazonia

Nest excavation and agricultural activities of the leaf-cutting ant Atta sexdens create complex belowground heterogeneity in secondary forests of Eastern Amazonia. We examined the effects of this heterogeneity on inorganic-N stocks, net mineralization, and net nitrification to test the hypothesis that the bulk soil of the nests has higher net rates of mineralization and nitrification than soil that was not affected by the influences of ant nests, throughout the profile. This study was conducted in a secondary forest at Fazenda Vitoria, near Paragominas in the Eastern Brazilian Amazon, where a previous study showed that the bulk soil of ant nests had elevated NO₃⁻. The results of the inorganic-N measurements were consistent with the previous study, showing elevated NO₃⁻ deep in the soil profile of the nests. However, neither net mineralization nor net nitrification were significantly greater at depth in the mineral soil of the nests compared to soil that was not influenced by nests (P=0.05), although variability was higher in the nest soil. These results suggest that the NO₃⁻ may have diffused into the surrounding mineral from the N-rich organic matter buried by the ants in chambers within the deep soil.     

不同土地利用方式土壤温室气体排放对碳氮添加的响应

揭示不同土地利用方式下土壤N2O产生机制及其CO2和CH4的排放,有助于土壤温室气体减排措施的制定。本研究以长沙金井河流域酸性红壤上菜地、稻田、茶园和林地土壤为研究对象,控制温度和土壤含水量,采用静态培养-气相色谱法,研究4种利用方式土壤N2O、CO2和CH4的排放对不同碳氮和硝化抑制剂添加的响应。结果表明,由于土壤pH较低,酸性红壤外加氮源后仅有较小的N2O排放。葡萄糖能够促进尿素添加后N2O的排放及土壤反硝化作用N2O的排放。异养硝化作用可能是酸性红壤N2O产生的主要途径。硝化抑制剂双氰胺（DCD）对酸性红壤N2O减排无明显效果。碳氮添加后土壤N2O的总排放量表现为茶园 > 菜地 > 稻田 > 林地。外源有机碳能够显著促进4种利用方式土壤CO2的排放,表现为茶园、稻田 > 菜地、林地。但除稻田土壤CH4排放增加外,菜地、茶园和林地土壤CH4排放对外源有机碳无明显响应。