Soil nitrogen mineralization as affected by water and temperature interactions

Summary The hypothesis that water and temperature interact to influence the rate of soil N mineralization was studied in laboratory incubation experiments with two contrasting soils. Small sample rings (10 mm tall, 50 mm diameter) were packed to uniform bulk density with 1–2 mm aggregates of Plano silt loam and Wacousta silty clay loam. Samples were brought to five different water potentials (–0.1, –0.33, –0.5, –1.0, –3.0 bars) using pressure-plate techniques, and the undisturbed sample rings were then incubated at 10–35°C for 3, 10 or 14 days. The concentration of soil exchangeable NH₄ ⁺-N and NO₃ ^–-N was measured at the end of each incubation period on replicate samples. The Q₁₀ of N mineralization was approximately 2 for all tested water potentials. Soil N mineralization was linearly related to water content or log water potential, but no water-temperature interaction was evident. The Q₁₀ was constant with water content, and the scaled water content-N mineralization relationship was constant with temperature. We recommend the use of scaling approaches for assessing interactive effects between water and other environmental factors on N turnover in soils.     

Soil carbon and nitrogen dynamics as affected by long-term tillage and nitrogen fertilization

Quantifying seasonal dynamics of active soil C and N pools is important for understanding how production systems can be better managed to sustain long-term soil productivity especially in warm subhumid climates. Our objectives were to determine seasonal dynamics of inorganic soil N, potential C and N mineralization, soil microbial biomass C (SMBC), and the metabolic quotient of microbial biomass in continuous corn (Zea mays L.) under conventional (CT), moldboard (MB), chisel (CH), minimum tillage (MT), and no-tillage (NT) with low (45kgNha^–1) and high (90kgNha^–1) N fertilization. An Orelia sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) in south Texas, United States, was sampled before corn planting in February, during pollination in May, and following harvest in July. Soil inorganic N, SMBC, and potential C and N mineralization were usually highest in soils under NT, whereas these characteristics were consistently lower throughout the growing season in soils receiving MB tillage. Nitrogen fertilization had little effect on soil inorganic N, SMBC, and potential C and N mineralization. The metabolic quotient of microbial biomass exhibited seasonal patterns inverse to that of SMBC. Seasonal changes in SMBC, inorganic N, and mineralizable C and N indicated the dependence of seasonal C and N dynamics on long-term substrate availability from crop residues. Long-term reduced tillage increased soil organic matter (SOM), SMBC, inorganic N, and labile C and N pools as compared with plowed systems and may be more sustainable over the long term. Seasonal changes in active soil C and N pools were affected more by tillage than by N fertilization in this subhumid climate. Received: 20 September 1996     

Soil microbial biomass carbon and nitrogen as affected by cropping systems

 The impacts of crop rotations and N fertilization on microbial biomass C (C_mic) and N (N_mic) were studied in soils of two long-term field experiments initiated in 1978 at the Northeast Research Center (NERC) and in 1954 at the Clarion-Webster Research Center (CWRC), both in Iowa. Surface soil samples were taken in 1996 and 1997 from plots of corn (Zea mays L.), soybeans (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha^–1 before corn and an annual application of 20 kg P and 56 kg K ha^–1. The C_mic and N_mic values were determined by the chloroform-fumigation-extraction method and the chloroform-fumigation-incubation method, respectively. The C_mic and N_mic values were significantly affected (P<0.05) by crop rotation and plant cover at time of sampling, but not by N fertilization. In general, the highest C_mic and N_mic contents were found in the multicropping systems (4-year rotations) taken in oats or meadow plots, and the lowest values were found in continuous corn and soybean systems. On average, C_mic made up about 1.0% of the organic C (C_org), and N_mic contributed about 2.4% of the total N (N_tot) in soils at both sites and years of sampling. The C_mic values were significantly correlated with C_org contents (r≥0.41**), whereas the relationship between C_mic and N_tot was significant (r≤0.53***) only for the samples taken in 1996 at the NERC site. The C_mic : N_mic ratios were, on average, 4.3 and 6.4 in 1996, and 7.6 and 11.4 in 1997 at the NERC and CWRC sites, respectively. Crop rotation significantly (P<0.05) affected this ratio only at the NERC site, and N fertilization showed no effect at either site. In general, multicropping systems resulted in greater C_mic : C_org (1.1%) and N_mic : N_tot (2.6%) ratios than monocropping systems (0.8% and 2.1%, respectively). Received: 9 February 1999     

Soil nitrogen mineralization not affected by grass species traits

Species N use traits was evaluated as a mechanism whereby Bromus inermis (Bromus), an established invasive, might alter soil N supply in a Northern mixed-grass prairie. We compared soils under stands of Bromus with those from three representative native grasses of different litter C/N: Andropogon gerardii (Andropogon), Nassella viridula (Nassella) and Pascopyrum smithii (Pascopyrum); in ascending order of litter quality. Net mineralization (per g soil N) measured in year-long laboratory incubations showed no differences in comparisons of Bromus with two of the three native grasses: Andropogon and Nassella. Higher mineralization in Pascopyrum stands relative to Bromus was consistent with its higher litter quality. However, an unusually high occurrence of an N-fixing legume in Pascopyrum stands, potentially favoring high mineralization rates, confounded any conclusions regarding the effects of plant N use on N mineralization. Instead of an initial flush of net mineralization, as would be expected in laboratory incubation, we observed an initial lag phase. This lag in net N mineralization coincided with high microbial activity (respiration) that suggests strong N limitation of the microbial biomass. Further support for the importance of immobilization initially came from modeling mineralization dynamics, which was explained better when we accounted for microbial growth in our model. The absence of strong differences in net mineralization beneath these grasses suggests that differences in plant N use alone were unlikely to influence soil N mineralization through substrate quality, particularly under strong N control of the microbial biomass.     

Microbial biomass and C mineralization in agricultural soils as affected by atrazine addition

This study examines the effects of atrazine on both microbial biomass C and C mineralization dynamics in two contrasting agricultural soils (organic C, texture, and atrazine application history) located at Galicia (NW Spain). Atrazine was added to soils, a Humic Cambisol (H) and a Gleyic Cambisol (G), at a recommended agronomic dose and C mineralization (CO₂ evolved), and microbial biomass measurements were made in non-treated and atrazine-treated samples at different time intervals during a 12-week aerobic incubation. The cumulative curves of CO₂–C evolved over time fit the simple first-order kinetic model [Ct = Co (1 − e ^−kt )], whose kinetic parameters were quantified. Differences in these parameters were observed between the two soils studied; the G soil, with a higher content in organic matter and microbial biomass C and lower atrazine application history, exhibited higher values of the total C mineralization and the potentially mineralizable labile C pool than those for the H soil. The addition of atrazine modified the kinetic parameters and increased notably the C mineralized; by the end of the incubation the cumulative CO₂–C values were 33–41% higher than those in the corresponding non-added soils. In contrast, a variable effect or even no effect was observed on the soil microbial biomass following atrazine addition. The data clearly showed that atrazine application at normal agricultural rates may have important implications in the C cycling of these two contrasting acid soils.     

Carbon and nitrogen mineralization as affected by drying and wetting cycles

Drying and rewetting of soil is an important process in soil aggregation, soil organic matter (SOM) decomposition, and nutrient cycling. We investigated the source of the C and N flush that occurs upon rewetting of dry soil, and whether it is from microbial death and/or aggregate destruction. A moderately well drained Kennebec silt loam (Fine-silty, mixed, superactive, mesic Cumulic Hapludoll) was sampled to a 10 cm depth. Soil under constant water content (CWC) was compared with soil subjected to a series of four dry-wet (DW) cycles during the experimental period (96 d) and incubated at 25 °C. Mineralized C and N were measured during the drying and rewetting periods. Aggregate size distributions were studied by separating the soil into four aggregate size classes (>2000, 250-2000, 53-250, and 20-53 μm) by wet sieving. Repeated DW cycles significantly reduced cumulative N mineralization compared with CWC. The reduction in cumulative mineralized C resulting from DW compared with CWC increased as the DW treatments were subjected to additional cycles. The flush of mineralized C significantly decreased with repeated DW cycles. There was no significant effect on aggregate size distributions resulting from to the DW cycles compared with CWC treatment. Therefore, the flush of mineralized C and N seemed to be mostly microbial in origin in as much as aggregate distribution was unaffected by DW cycles.     

长期施肥对黑土有机碳矿化和团聚体碳分布的影响

采用我国东北地区的连续施肥28 a的典型黑土,通过颗粒分组的方法,研究了长期不同施肥处理对土壤团聚体形成和有机碳在各级团聚体中的分布规律,以及各粒级团聚体对养分的贡献能力的影响。结果表明,施用有机肥促进了土壤中大颗粒团聚体(>0.25 mm)的形成,尤其以2~1 mm粒级增加的比例最大;而当化肥和有机肥配合施用后,主要促进土壤中<1 mm团聚体形成,尤其对0.5~0.25 mm粒级团聚体形成的促进作用最大。施肥(无论是有机肥还是化肥)能增加土壤有机碳含量,而且,化肥有机肥配合施用较单施化肥和单施有机肥的效果更好。随着团聚体粒级的降低,团聚体中有机碳的分配出现两个峰值,分别在2~1 mm和0.25~0.053 mm两个粒级中出现。单施化肥处理及化肥有机肥配施处理与无肥处理相比均增加了0.5~0.25 mm粒级团聚体对有机碳的贡献率;单施有机肥的处理与无肥处理相比增加了1~0.5 mm粒级团聚体对有机碳的贡献率。然而,对于东北的典型黑土,2~1 mm和0.25~0.053 mm两个粒级对土壤有机碳的保护作用最大,表明团聚体对有机碳的保护作用是碳分配和矿化分解作用的综合结果,通过调控施肥种类可以达到有效保护土壤质量和肥力的效果。     

土壤氮素矿化对烤烟产量和尼古丁含量的影响

Nitrogen （N） supply is the most important factor affecting yield and quality of flue-cured tobacco （FCT）. A field experiment and an in situ incubation method were used to study the effects of soil N mineralization in the later stages of growth on yield and nicotine content of FCT in Fenggang and Jinsha, Guizhou Province. The yield and market value of FCT at Fenggang were much lower than those at Jinsha. However, the nicotine content of middle and upper leaves was much higher at Fenggang than at Jinsha when the same rate of fertilizer N was applied, which might be due to a higher N supply capacity at the Fenggang site. At later stages of growth （7-16 weeks after transplanting）, the soil net N mineralization at Fenggang （56 kg N ha^-1） was almost double that at Jinsha （30 kg N ha^-1）. While soil NH4-N and NO3-N were almost exhausted by the plants or leached 5 weeks after transplanting, the N taken up at the later growth stages at Fenggang were mainly derived from soil N mineralization, which contributed to a high nicotine content in the upper leaves. The order of soil N contribution to N buildup in different leaves was： upper leaves 〉 middle leaves 〉 lower leaves. Thus, soil N mineralization at late growth stages was an important factor affecting N accumulation and therefore the nicotine content in the upper leaves.     

中国西南喀斯特地区周期性温度波动对土壤有机碳矿化的影响

The diurnal fluctuation in soil temperature may influence soil organic carbon (SOC) mineralization, but there is no consensus on SOC mineralization response to the cyclical fluctuation in soil temperature. A 56-d incubation experiment was conducted to investigate the effects of constant and variable temperatures on SOC mineralization. Three soils were collected from the karst region in western Guizhou Province, southwestern China, including a limestone soil under forest, a limestone soil under crops and a yellow soil under crops. According to the World Reference Base (WRB) classification, the two limestone soils were classified as Haplic Luvisols and the yellow soil as a Dystric Luvisol. These soils were incubated at three constant temperatures (15, 20 and 25 ^oC) and cyclically fluctuating temperatures (diurnal cycle between 15 and 25 ^oC). The results showed that the 56-d cumulative SOC mineralized (C₅₆) at the fluctuating temperatures was between those at constant 15 and 25 ^oC, suggesting that the cumulative SOC mineralization was restricted by temperature range. The SOC mineralization responses to the fluctuating temperatures were different among the three soils, especially in contrast to those at constant 20 ^oC. Compared with constant 20 ^oC, significant (P < 0.05) decreases and increases in C₅₆ value were found in the limestone soil under forest and yellow soil under crops at the fluctuating temperatures, respectively. At the fluctuating temperatures, the forest soil with lower temperature coefficient Q₁₀ (the relative change in SOC mineralization rate as a result of increasing the temperature by 10 ^oC) had a significantly (P < 0.05) lower SOC mineralization intensity than the two cropland soils. These indicated that differences in temperature pattern (constant or fluctuating) could significantly influence SOC mineralization, and SOC mineralization responses to the fluctuating temperatures might be affected by soil characteristics. Moreover, the warmer temperatures might improve the ability of soil microbes to decompose the recalcitrant SOC fraction, and cyclical fluctuations in temperature could influence SOC mineralization through changing the labile SOC pool size and the mineralization rate of the recalcitrant SOC in soils.     

Soil carbon and nitrogen mineralization: Theory and models across scales

In the last 80 years, a number of mathematical models of different level of complexity have been developed to describe biogeochemical processes in soils, spanning spatial scales from few μm to thousands of km and temporal scales from hours to centuries. Most of these models are based on kinetic and stoichiometric laws that constrain elemental cycling within the soil and the nutrient and carbon exchange with vegetation and the atmosphere. While biogeochemical model performance has been previously assessed in other reviews, less attention has been devoted to the mathematical features of the models, and how these are related to spatial and temporal scales. In this review, we consider ∼250 biogeochemical models, highlighting similarities in their theoretical frameworks and illustrating how their mathematical structure and formulation are related to the spatial and temporal scales of the model applications. Our analysis shows that similar kinetic and stoichiometric laws, formulated to mechanistically represent the complex underlying biochemical constraints, are common to most models, providing a basis for their classification. Moreover, a historic analysis reveals that the complexity and degree and number of nonlinearities generally increased with date, while they decreased with increasing spatial and temporal scale of interest. We also found that mathematical formulations specifically developed for certain scales (e.g., first order decay rates assumed in yearly time scale decomposition models) often tend to be used also at other spatial and temporal scales different from the original ones, possibly resulting in inconsistencies between theoretical formulations and model application. It is thus critical that future modeling efforts carefully account for the scale-dependence of their mathematical formulations, especially when applied to a wide range of scales.     

Soil organic carbon and nitrogen in a Mollisol in central Ohio as affected by tillage and land use

Minimum tillage practices are known for increasing soil organic carbon (SOC). However, not all environmental situations may manifest this potential change. The SOC and N stocks were assessed on a Mollisol in central Ohio in an 8-year-old tillage experiment as well as under two relatively undisturbed land uses; a secondary forest and a pasture on the same soil type. Cropped systems had 51±4 (equiv. mass) Mg ha⁻¹ lower SOC and lower 3.5±0.3 (equiv. mass) Mg ha⁻¹ N in the top 30 cm soil layer than under forest. Being a secondary forest, the loss in SOC and N stocks by cultivation may have been even more than these reported herein. No differences among systems were detected below this depth. The SOC stock in the pasture treatment was 29±3 Mg ha⁻¹ greater in the top 10 cm layer than in cultivated soils, but was similar to those under forest and no-till (NT). Among tillage practices (plow, chisel and NT) only the 0–5 cm soil layer under NT exhibited higher SOC and N concentrations. An analysis of the literature of NT effect on SOC stocks, using meta-analysis, suggested that NT would have an overall positive effect on SOC sequestration rate but with a greater variability of what was previously reported. The average sequestration rate of NT was 330 kg SOC ha⁻¹ year⁻¹ with a 95% confidence interval ranging from 47 to 620 kg SOC ha⁻¹ year⁻¹. There was no effect of soil texture or crop rotation on the SOC sequestration rate that could explain this variability. The conversion factor for SOC stock changes from plow to NT was equal to 1.04. This suggests that the complex mechanisms and pathways of SOC accrual warrant a cautious approach when generalizing the beneficial changes of NT on SOC stocks.     

pH change, carbon and nitrogen mineralization in paddy soils as affected by Chinese milk vetch addition and soil water regime

Purpose

The aim of the research was to explore the effect of Chinese milk vetch (CM vetch) addition and different water management practices on soil pH change, C and N mineralization in acid paddy soils.

Materials and methods

Psammaquent and Plinthudult paddy soils amended with Chinese milk vetch at a rate of 12 g?kg^?1 soil were incubated at 25 °C under three different water treatments (45 % field capacity, CW; alternating 1-week wetting and 2-week drying cycles, drying rewetting (DRW) and waterlogging (WL). Soil pH, dissolved organic carbon, dissolved organic nitrogen (DON), CO₂ escaped, microbial biomass carbon, ammonium (NH₄ ⁺) and nitrate (NO₃ ^?) during the incubation period were dynamically determined.

Results and discussion

The addition of CM vetch increased soil microbial biomass concentrations in all treatments. The CM vetch addition also enhanced dissolved organic N concentrations in all treatments. The NO₃–N concentrations were lower than NH₄–N concentrations in DRW and WL. The pH increase after CM vetch addition was 0.2 units greater during WL than DRW, and greater in the low pH Plinthudult (4.59) than higher pH Paleudalfs (6.11) soil. Nitrogen mineralization was higher in the DRW than WL treatment, and frequent DRW cycles favored N mineralization in the Plinthudult soil.

Conclusions

The addition of CM vetch increased soil pH, both under waterlogging and alternating wet–dry conditions. Waterlogging decreased C mineralization in both soils amended with CM vetch. Nitrogen mineralization increased in the soils subjected to DRW, which was associated with the higher DON concentrations in DRW than in WL in the acid soil. Frequent drying–wetting cycles increase N mineralization in acid paddy soils.     

Nitrogen mineralization as affected by cropping system

Abstract

The quantity of crop residues returned to the soil is controlled by the cropping system and in turn these residues affect the amount of mineral nitrogen (N) released by decomposition of organic matter. This study was conducted to evaluate the effect of four 20‐year‐old cropping systems which returned varying amounts of crop residue on N mineralization potential (N₀) of a Misteguay silty clay soil. N₀ was estimated from a long‐term incubation study using exponential and hyperbolic models. The N₀ ranges from 70 to 109 mg.kg^‐1 for the exponential model and 86 to 144 mg.kg^‐1 for the hyperbolic model. Values of N₀ are closely related to the estimated amount of crop residue returned. It requires 0.33 Mg.ha^‐1 of crop residue returned to increase N₀ one mg.kg^‐1. Estimated N₀ values are consistently greater for the hyperbolic than for the exponential model. However, there is a close relationship between the instantaneous rates of reaction of the two models suggesting both models can be used in laboratory incubation studies to estimate N₀ using a nonlinear least‐squares fitting technique. This study shows that the exponential and hyperbolic models are equally effective in distinguishing both the qualitative and quantitative changes in soil organic N due to the cropping system in place.     

Soil carbon and nitrogen changes as affected by tillage system and crop biomass in a corn–soybean rotation

A wide range of tillage systems have been used by producers in the Corn-Belt in the United States during the past decade due to their economic and environmental benefits. However, changes in soil organic carbon (SOC) and nitrogen (SON) and crop responses to these tillage systems are not well documented in a corn–soybean rotation. Two experiments were conducted to evaluate the effects of different tillage systems on SOC and SON, residue C and N inputs, and corn and soybean yields across Iowa. The first experiment consisted of no-tillage (NT) and chisel plow (CP) treatments, established in 1994 in Clarion–Nicollet–Webster (CNW), Galva–Primghar–Sac (GPS), Kenyon–Floyd–Clyde (KFC), Marshall (M), and Otley–Mahaska–Taintor (OMT) soil associations. The second experiment consisted of NT, strip-tillage (ST), CP, deep rip (DR), and moldboard plow (MP) treatments, established in 1998 in the CNW soil association. Both corn and soybean yields of NT were statistically comparable to those of CP treatment for each soil association in a corn–soybean rotation during the 7 years of tillage practices. The NT, ST, CP, and DR treatments produced similar corn and soybean yields as MP treatment in a corn–soybean rotation during the 3 years of tillage implementation of the second experiment. Significant increases in SOC of 17.3, 19.5, 6.1, and 19.3% with NT over CP treatment were observed at the top 15-cm soil depth in CNW, KFC, M, and OMT soil associations, respectively, except for the GPS soil association in a corn–soybean rotation at the end of 7 years. The NT and ST resulted in significant increases in SOC of 14.7 and 11.4%, respectively, compared with MP treatment after 3 years. Changes in SON due to tillage were similar to those observed with SOC in both experiments. The increases in SOC and SON in NT treatment were not attributed to the vertical stratification of organic C and N in the soil profile or annual C and N inputs from crop residue, but most likely due to the decrease in soil organic matter mineralization in wet and cold soil conditions. It was concluded that NT and ST are superior to CP and MP in increasing SOC and SON in the top 15 cm in the short-term. The adoption of NT or CP can be an effective strategy in increasing SOC and SON in the Corn-Belt soils without significant adverse impact on corn and soybean yields in a corn–soybean rotation.     

Significance of organic nitrogen uptake from plant residues by soil microorganisms as affected by carbon and nitrogen availability

Soil microorganisms can use a wide range of nitrogen (N) compounds. When organic N sources are degraded, microorganisms can either take up simple organic molecules directly (direct route), or organic N may be mineralized first and taken up in the form of mineral N (mineralization-immobilization-turnover [MIT] route). To determine the importance of the direct route, a microcosm experiment was carried out. Two types of wheat residue were added to soil samples, including younger residue with a carbon (C) to N ratio of 12 and older residue with a C to N ratio of 29. Between days 1 and 4, the gross N mineralization rate reached 8.4 and 4.0 mg N kg⁻¹ dry soil day⁻¹ in the treatment with younger and older residue, respectively. During the same period, there was no difference in protease activity between the two residue amended treatments. The fact that protease activity was not related to gross N mineralization, even though the products of protease activity are the substrates for N mineralization, suggests that not all organic molecules released from residue or soil N passed through the soil mineral N pool. In fact, when leucine and glycine were added, only 10 and 53% of the amino acid-N, respectively, was mineralized. The fraction of N taken up via the direct route was estimated to be 55 and 62% for the young and older residue, respectively. After 28 days of incubation, the proportion of amino acid-N mineralized had increased especially in the soil amended with older residue, suggesting that the MIT route became increasingly important. This result is supported by an increase in the activities of enzymes responsible for the intracellular assimilation of ammonium (NH₄⁺). Our results suggest that in contrast to what is proposed by many models of soil N cycling, both the direct and MIT routes were operative, with the direct route being the preferred route of residue N uptake. The direct route became less important over time and was more important in soil amended with older residue, suggesting that the direct route is favored by lower mineral N availabilities. An important implication of these findings is that when the direct route is dominant, gross N mineralization underestimates the amount of N made available from the residue.     

Spatial variability of glyphosate mineralization and soil microbial characteristics in two Norwegian sandy loam soils as affected by surface topographical features

The aim of this study was to investigate the possible influence of surface topographical features on the spatial variability of glyphosate degradation and some microbial characteristics in sandy loam soil. Soil samples were taken from the ploughed layer across an agricultural field after seedbed preparation for grain (Grue site), and down to 1 m depth under a ridge tilled field (Målselv site), both sites having similar soil textural characteristics (sandy loam soil). Laboratory experiments were performed looking at glyphosate mineralization and soil microbial activity at the Grue site, as well as microbial biomass, activity and substrate utilization patterns at the Målselv site. Microbial biomass and activity decreased, and substrate utilization patterns changed with increasing soil depth, reflecting naturally occurring changes in quantity and quality of soil organic carbon. Further, our results show that considerable spatial heterogeneity in the degradation rate of glyphosate and general carbon utilization exists even across small areas within a single agricultural field. This horizontal variability was observed over several spatial scales, and could not be clearly explained. It evidently arose from differences in environmental factors affecting microbial activity and growth, and topographical features controlling redistribution of water and matter flow patterns were correlated to the investigated soil microbial variables.     

不同施氮水平下土壤的生化性质对干湿交替的响应

以中国科学院封丘农业生态试验站水氮耦合长期试验地为研究平台,采集五个施氮水平(施氮0、150、190、230、270 kg hm-2)下表层0 ～ 20 cm土壤并测定其土壤肥力参数(土壤pH、全氮、全磷、全钾、碱解氮、速效磷、速效钾、有机碳).结果表明:施氮降低了土壤pH、速效磷、全钾,增加了全氮、碱解氮、有机碳,除有机碳随施氮水平的增加而增加外,其他肥力参数并未随施氮水平的增加而发生规律性变化.土壤经过0、3、6、10次干湿交替,培养60 d后测定其生物和化学性质(土壤铵态氮、硝态氮、溶解性有机碳、脲酶活性、脱氢酶活性、微生物生物量碳、土壤基本呼吸).双因素方差分析显示干湿交替次数对铵态氮、硝态氮、无机氮、溶解性有机碳、脱氢酶活性、微生物生物量碳和土壤基本呼吸均有极显著作用,而干湿交替次数和施氮水平对除脱氢酶活性以外的其他土壤性质均无交互作用.五个施氮水平下土壤硝态氮、无机氮、溶解性有机碳、脲酶活性、脱氢酶活性和微生物生物量碳均随干湿交替次数增加而增加,土壤基本呼吸随干湿交替次数增加而降低.高施氮水平(施氮超过190 kg hm-2)下土壤性质的变异系数更小并能更好地发生聚类.研究表明当土壤遭遇干湿交替时,高施氮水平下土壤更能维持其生化性质的稳定.     

Soil nitrogen availability as affected by fallow-maize systems on two soils in Kenya

Simple methods for the measurement of nitrogen (N) availability are needed to assess the effect of low-input, organically based land management systems on the N supply of tropical soils. Our objectives were to determine the effect of contrasting land-use systems (LUS) on soil N availability and to identify measures of N availability that correlated with maize (Zea mays L.) grain yield. The LUS at the two sites in Kenya involved growth of a maize crop following 17 months of either: (1) Sesbania sesban (L.) Merr. tree growth (sesbania fallow), (2) natural regrowth of vegetation without cultivation (natural fallow), (3) three crops of unfertilized maize (maize monoculture), or (4) bare uncultivated soil (bare fallow). Soil was collected before the post-fallow maize crop was sown. The LUS had no effect on total soil N or amount of N in the heavy fraction soil organic matter (SOM) (>150 μm, >1.37 Mg m^–3). Sesbania and natural fallows, as compared to maize monoculture, increased the N in light fraction SOM (>150 μm, <1.13Mgm^–3), N in intermediate fraction SOM (>150 μm, 1.13 to 1.37 Mg m^–3), ammonium-N and aerobic N mineralization at a depth of 0–15 cm. Maize yields were highest following the sesbania fallow. Nitrate-N, inorganic-N (ammonium plus nitrate) and anaerobic N mineralization correlated with maize grain yield at both sites. The relationship between maize yield and pre-season nitrate-N improved when the depth of soil sampling was increased to 1 m at one site (an Alfisol), but did not improve at the site with anion adsorption in the subsoil (an Oxisol). The sesbania fallow was more effective than the natural fallow in increasing available soil N. Maize yield was better related to pre-season nitrate than N in size-density fractions of SOM. Received: 5 May 1997     

Soil aggregation and the stabilization of organic carbon as affected by erosion and deposition

The importance of soil aggregation in determining the dynamics of soil organic carbon (SOC) during erosion, transportation and deposition is poorly understood. Particularly, we do not know how aggregation contributes to the often-observed accumulation of SOC at depositional sites. Our objective was to assess how aggregation affects SOC stabilization in comparison to interactions of SOC with minerals. We determined and compared aggregate size distributions, SOC distribution in density fractions, and lignin-derived phenols from aggregated soil samples at both eroding and depositional sites. The stabilization effect of aggregation was quantified by comparing mineralization from intact and crushed macro-aggregates. Deposition of eroded soil material resulted in carbon (C) enrichment throughout the soil profile. Both macro-aggregate associated SOC and C associated with minerals (heavy fraction) increased in their importance from the eroding to the depositional site. In the uppermost topsoil (0–5 cm), SOC mineralization from intact aggregates was larger at the depositional site than at the eroding site, reflecting the large input of labile organic matter (plant residues) promoting aggregation. Contrastingly, in the subsoil, mineralization rates were lower at the depositional site because of effective stabilization by interactions with soil minerals. Aggregate crushing increased SOC mineralization by 10–80% at the eroding site, but not at the depositional site. The content of lignin-derived phenols did not differ between eroding and depositional sites in the topsoil (24.6–30.9 mg per g C) but was larger in the subsoil of the eroding site, which was accompanied by higher lignin oxidation. Lignin data indicated minor effects of soil erosion and deposition on the composition of SOC. We conclude that SOC is better protected in aggregates at the eroding than at the depositional site. During transport disaggregation and consequently SOC mineralization took place, while at the depositional site re-aggregation occurred mainly in the form of macro-aggregates. However, this macro-aggregation did not result in a direct stabilization of SOC. We propose that the occlusion of C inside aggregates serves as a pathway for the eroded C to be later stabilized by organo-mineral interaction.     

绿肥根茬还田和化肥用量对土壤团聚性及碳氮分布的影响

通过3年定位试验探讨绿肥根茬还田和化肥用量对土壤团聚性及碳氮分布特征的影响.试验依据绿肥根茬还田和化肥用量共设置5个处理:100％化肥即常规施肥(100％CF);绿肥根茬还田(GM);100％化肥+绿肥根茬还田(100％CF+GM);90％化肥+绿肥根茬还田(90％CF+GM);80％化肥+绿肥根茬还田(80％CF+G...