沼液沼渣对温室迷你黄瓜品质的影响

试验以化肥、农家肥为对照,研究了沼液、沼渣对迷你黄瓜品质的影响。结果表明,以沼渣为基肥,辅以植株叶面喷施沼液,对提高迷你黄瓜品质有明显的作用。其中施沼渣A2的处理对降低硝酸盐含量作用最明显,比CK1、CK2分别降低了14%和41%的硝酸盐。100%沼液(B2)叶面喷施可显著降低硝酸盐的含量,一次喷施可降低45%左右,其中A1B1、A1B2处理喷沼液后叶绿素含量平均提高了89%,A1B2处理的Vc上升了77%,综合评价后尤以A2B2处理效果最好,在保证高产的同时,Vc含量达48.63 mg/100 g鲜重,可溶性糖含量达22.16%,硝酸盐含量为90.74 mg/kg。     

添加调节N和P浓度后的植物残体对土壤呼吸作用、微生物量和养分有效性的影响

Microbial activity and nutrient release are known to be influenced by organic matter properties,but it is difficult to separate the effect of C/N ratio from that of C/P ratio because in most plant residues both ratios are either high or low.An incubation experimeut was conducted to investigate the effects of reducing the C/N and C/P ratios of slowly decomposable plant residues (young eucalyptus leaves,mature wheat straw,and sawdust) to those of rapidly decomposable residues (young kikuyu shoots) on soil respiration,microbial biomass,and N and P availability.The C/N and C/P ratios of the former were adjusted to 15 and 89,respectively,by adding N as (NH4)2SO4,P as KH2PO4 or both and residues were added at 10 g C kg-1 to a silt loam.Soil respiration was measured over 21 d;microbial biomass C (MBC) and available N and P were measured on days 0,7,and 21.Compared to the unamended soil,addition of kikuyu increased cumulative respiration 20-fold,MBC concentration 4 to 8-fold,and available P concentration up to 4-fold,whereas the increase in available N concentration was small and transient.Cumulative respiration and MBC concentration were low in the sawdust-amended soil and were not influenced by reducing the C/N and C/P ratios.Cumulative respiration with original wheat and eucalyptus was 30％-40％ of that with kikuyu.Reducing the C/N ratio alone or both C/N and C/P ratios increased cumulative respiration and MBC concentration 2-fold compared to the original wheat and eucalyptus,whereas reducing the C/P ratio had little effect.Throughout the experiment,the available N concentration after addition of residues with reduced C/N ratio increased in the following order of eucalyptus ＜ wheat ＜ sawdust.By independently lowering the C/N and C/P ratios,microbial activity was more limited by C and N than P.However,lowering the C/N ratio of very slowly decomposable sawdust had no effect on soil respiration and MBC concentration,suggesting that other properties such as concentration of poorly decomposable C compounds limited decomposition.     

Foliar Application of Urea and Triazone Nitrogen to Cotton

Upland cotton (Gossypium hirsutum) requires adequate nitrogen (N) for optimum yields. Foliar applications of urea to supplement soil applied N have been tried for many years across the Cotton Belt, but responses have been highly variable. No published information is available regarding response by irrigated cotton to foliar applied N in subtropical South Texas. This study investigated the response of cotton to foliar applied urea and triazone N over a three-year period near Weslaco, Texas. In all years, foliar applied urea tended to increase seed cotton yield when soil applied N was limiting. In the absence of soil applied N, the increase due to foliar urea was significant in two of the three years (28.7 and 15.7% increases). Foliar applied triazone N was ineffective at increasing seed cotton yields.     

Nitrogen availability mediates the effects of roots and mycorrhizal fungi on soil organic carbon decomposition: A meta-analysis

Plant roots and their associated mycorrhizal fungi critically mediate the decomposition of soil organic carbon (C), but the general patterns of their impacts over a broad geographical range and the primary mediating factors remain unclear. Based on a synthesis of 596 paired observations from both field and greenhouse experiments, we found that living roots and/or mycorrhizal fungi increased organic C decomposition by 30.9%, but low soil nitrogen (N) availability (i.e., high soil C:N ratio) critically mitigated this promotion effect. In addition, the positive effects of living roots and/or mycorrhizal fungi on organic C decomposition were higher under herbaceous and leguminous plants than under woody and non-leguminous plants, respectively. Surprisingly, there was no significant difference between arbuscular mycorrhizal fungi and ectomycorrhizal fungi in their effects on organic C decomposition. Furthermore, roots and/or mycorrhizal fungi significantly enhanced the decomposition of leaf litter but not root litter. These findings advance our understanding of how roots and their symbiotic fungi modulate soil C dynamics in the rhizosphere or mycorrhizosphere and may help improve predictions of soil global C balance under a changing climate.     

Nitrogen release from plant‐derived and industrially processed organic fertilizers used in organic horticulture

As a consequence of the BSE crisis, alternatives for fertilizers derived from animal residues are being sought for use in organic horticulture. Grain legumes (milled seeds of pea, yellow lupine, and faba bean) and organic fertilizers of industrially processed plant and microbial residues (Maltaflor®‐spezial, Phytoperls®, Agrobiosol®, Rizi‐Korn) were investigated as to their suitability as a replacement fertilizer. With four soils, incubation studies were conducted to determine net N mineralization of the organic fertilizers, and pot experiments were used to measure the apparent N utilization by perennial ryegrass. The objectives of this study were (1) to determine simple fertilizer characteristics that describe their N release and (2) to compare the suitability of both experimental setups to predict fertilizer N release. At the end of all experiments, net N mineralization and apparent N utilization from Rizi‐Korn was highest compared to all the other organic fertilizers, while pea performed relatively poor. This differentiation between the fertilizers developed during the first 2 weeks. Nitrogen release from the organic fertilizers as described by net N mineralization or apparent N utilization was significantly related to the N content of the fertilizers. Different soils modified this relationship. Two industrially processed fertilizers (Phytoperls®, Agrobiosol®) could not be included into a generalized relationship because N release from these fertilizers was low compared to their N content. It is discussed that the quality of fertilizer C and N affected the N release from the fertilizers. Both experimental setups, incubation and pot experiments, were suitable to describe the release of plant‐available N from the organic fertilizers. However, N release of fertilizers with a low net N mineralization in the incubation experiments was underestimated compared to plant N uptake of ryegrass in the pot experiments. It is concluded that the N content of organic fertilizers indicates, but not predicts their N release.     

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.     

立式连续干发酵装置的设计与产气特性

该研究设计的立式连续干发酵装置的几点特色包括搅拌升温调节罐、多点分布式进料、立式连续干发酵反应器和熟料回流混合过程,形成一种高固体浓度有机废弃物制取生物燃气的工艺方法及其发酵系统,可实现沼气工程的快速启动、厌氧发酵罐内无需搅拌、且节能,节水,减少沼液沼渣的排放量,保护环境,降低成本。采用上述设计的反应器进行发酵试验,研究熟料回流对产气特性的影响得出,应用回流的各组挥发性脂肪酸(volatile fatty acids,VFA)都有所降低,且回流比为1:7的VFA含量最低,氨氮没有累积,C/N接近25,产气效果最好。回流比1:9组,VFA含量累积最高,同时出现氨氮抑制产气的现象,所以得出熟料回流量过多,厌氧发酵系统就会受到抑制,且熟料回流对沼气中甲烷含量没有显著影响。     

沼渣营养价值及沼渣源饲料和其生产的猪肉重金属残留分析

对牛粪、玉米秸秆作为发酵原料生产沼气的发酵残留物--沼渣的营养价值及沼渣源配合饲料和沼渣源配合饲料生产的猪肉的重金属残留进行了分析.结果表明,风干沼渣中干物质、粗蛋白、粗脂肪、粗灰分、粗纤维含量分别为857.7mg·g-1、99.0mg·g-1、34.9mg·g-1、10.7mg·g-1、252.7mg·g-1;重金属元素镉、铅、铜含量分别为18.48P,g·kg-1、42.50μg·kg-1、20.80μg·kg-1,汞未检出.对照组、沼渣Ⅰ组、沼渣Ⅱ组饲料中镉、铅、铜、汞的含量均低于国家饲料卫生标准;猪肉中重金属镉、铅、铜含量对照组、沼渣Ⅰ组、沼渣Ⅱ组分别为0.033mg·kg-1、0.044mg·kg-1、0.058mg·kg-1,0.268mg·kg-1、0.364mg·kg-1、0.393mg·kg-1,5.555mg·kgM-1、7.180mg·kg-1、8.404mg·kg-1,汞均未检出.沼渣作为猪饲料源利用有一定的潜力,以2%～10%在饲料中添加不会引起猪肉重金属镉、铅、铜、汞含量超标.     

Manipulating the N release from N-rich crop residues by using organic wastes on soils with different textures

The potential to manipulate the N release from vegetable crop residues (cauliflower, leek) by using organic wastes was tested under field conditions on three soil textures during 2 years (silt loam, sandy loam and loamy sand). During the first year, incorporation of green waste compost and sawdust did not significantly increase microbial biomass N and did not lead to a significant N immobilization of crop residue‐N. During the second year, straw did increase microbial biomass N and showed a good N immobilization potential in all textures. The largest increase in microbial biomass N and the greatest N immobilization occurred in the loamy sand soil. The texture effect was probably because of better incorporation of the crop residues and immobilizer wastes in the loamy sand soil compared with the other textures. During spring, there was no consistent remineralization of immobilized N after the addition of malting sludge or vinasses in either year. This could be a result of the limited amount of N immobilized and available for remineralization in the first year or an unsuitable composition of the remineralizer wastes.     

可利用的碳和氮对丛枝菌根真菌萌发孢子氨基酸代谢的影响

The effects of carbon (C) and nitrogen (N) sources on N utilization and biosynthesis of amino acids were examined in the germinating spores of the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith after exposure to various N substrates,CO2,glucose,and/or root exudates.The N uptake and de novo biosynthesis of amino acids were analyzed using stable isotopic labeling with mass spectrometric detection.High-performance liquid chromatography-based analysis was used to measure amino acid levels.In the absence of exogenous N sources and in the presence of 25 mL L-1 CO2,the germinating AM fungal spores utilized internal N storage as well as C skeletons derived from the degradation of storage lipids to biosynthesize the free amino acids,in which serine and glycine were produced predominantly.The concentrations of internal amino acids increased gradually as the germination time increased from 0 to 1 or 2 weeks.However,asparagine and glutamine declined to the low levels;both degraded to provide the biosynthesis of other amino acids with C and N donors.The availability of exogenous inorganic N (ammonium and nitrate) and organic N (urea,arginine,and glutamine) to the AM fungal spores using only CO2 for germination generated more than 5 times more internal free amino acids than those in the absence of exogenous N.A supply of exogenous nitrate to the AM fungal spores with only CO2 gave rise to more than 10 times more asparagine than that without exogenous N.In contrast,the extra supply of exogenous glucose to the AM fungal spores generated a significant enhancement in the uptake of exogenous N sources,with more than 3 times more free amino acids being produced than those supplied with only exogenous CO2.Meanwhile,arginine was the most abundant free amino acid produced and it was incorporated into the proteins of AM fungal spores to serve as an N storage compound.     

以沼渣为原料固态发酵生产Bt生物农药

沼渣是沼气工程中产生的副产物,由于其富含营养物质及微量元素,而广泛应用于农业,畜牧业,养殖业等方面。Bt生物农药是一类具有选择性和高效性的杀虫剂,但是由于其原料的成本较高从而限制了生物农药的发展,而沼渣中富含微生物所需要的各种养分并且价格低廉,因此该文以生产沼气后的沼渣为资源,运用固态发酵,探索了利用沼渣制备苏云金杆菌生物农药的可行性。首先对沼渣营养成分进行了分析,分析表明其营养物质丰富,适合苏云金芽孢杆菌的生长与增殖。其次对菌种的沼渣培养基进行了优化,优化后的培养基质量分数为：50%的沼渣添加35%啤酒糟,10%玉米粉,5%豆饼粉,并与常规培养基和单纯沼渣的发酵进程进行了对比。在优化培养基条件下,发酵48 h后,芽孢数达到5.23×1010 CFU/g,毒力效价为16 100 IU/mg。在传统培养基中芽孢数2.55×1010 CFU/g,毒力效价12 500 IU/mg,而在单纯沼渣中Bt产量及毒性为1.74×108 CFU/g,6000 IU/mg。采用沼渣发酵制备Bt生物农药与传统培养基比较,降低了36.3%的生产成本,且发酵性能优良,也为沼渣的利用寻找到崭新的途径。     

Nitrogen turnover of legume seed meals as affected by seed meal texture and quality at different temperatures

The aim was to investigate how legume seed meal texture and corresponding quality affects N turnover at different temperatures. Therefore, the effect of size fractionation ‘fine’ and ‘coarse’ of seed meals of yellow lupin (Lupinus luteus L.), blue lupin (Lupinus angustifolius L.) and faba bean (Vicia faba L.) on net N mineralization and turnover was investigated in an incubation experiment at 5°C, 12°C and 20°C. The differences in N release from the two particle size fractions could not be detected at 12 and 20°C incubation temperature. Moreover, net N mineralization at 5°C was higher during incubation of the coarse particle size fractions than during incubation of the fine fraction. In contrast to the common understanding of temperature dependence of microbial processes, the overall influence of incubation temperature on net N mineralization was less expressed. The formation of microbial biomass was highest at 5°C. The subsequent decrease of soil microbial biomass was only partly reflected by net N mineralization suggesting the formation of microbial residues as a preliminary N sink. The control of the N release from legume seed meals seems to be dominated by the N-immobilizing effects of polyphenols at lower temperatures and of C-rich polymers (hemicelluloses) at higher temperatures.     

保护地蔬菜轮作体系中两种不同复合肥对产量的影响及其氮素动态特征

Yield and N uptake of tomato (Lycopersicum esculentum Mill.) and pepper (Capsicum annuum L.) crops in five successive rotations receiving two compound fertilizers (12-12-17 and 21-8-11 N-P₂O₅-K₂O) were studied to determine 1) crop responses, 2) dynamics of NO₃-N and NH₄-N in different soil layers, 3) N balance and 4) system-level N efficiencies. Five treatments (2 fertilizers, 2 fertilizer rates and a control), each with three replicates, were arranged in the study. The higher N fertilizer rate, 300 kg N ha^-1 (versus 150 kg N ha^-1), returned higher vegetable fruit yields and total aboveground N uptake with the largest crop responses occurring for the low-N fertilizer (12-12-17) applied at 300 kg N ha^-1 rather than with the high-N fertilizer (21-8-11). Ammonium-N in the top 90 cm of the soil profile declined during the experiment, while nitrate-N remained at a similar level throughout the experiment with the lower rate of fertilizer N. At the higher rate of N fertilizer there was a continuous NO₃-N accumulation of over 800 kg N ha^-1. About 200 kg N ha^-1 was applied with irrigation to each crop using NO₃-contaminated groundwater. In general, about 50% of the total N input was recovered from all treatments. Pepper, relative to tomato, used N more efficiently with smaller N losses, but the crops utilized less than 29% of the fertilizer N over the two and a half-year period. Local agricultural practices maintained high residual soil nutrient status. Thus, optimization of irrigation is required to minimize nitrate leaching and maximize crop N recovery.     

粪草比对干式厌氧发酵产沼气效果的影响

弄清粪草比对干式厌氧发酵产气效果的影响,对改进干式厌氧工程具有重要意义。该文按A、B、C、D设计了4组试验：A组为单独猪粪,B组为单独稻草,C组为猪粪与稻草质量比2︰1,D组为猪粪与稻草质量比1︰2。试验进行了62 d,反应温度设定为（35±1）℃,各组反应的TS（总固体）浓度均为30%。结果显示,产气曲线出现拐点的时间A组在第48 d,B组在第40 d,C组在第26d,D组在第25 d。C、D两组总产气量分别为15715 mL和13186 mL。而根据A、B两组单独原料产气量推算,C、D两组的产气潜力分别为15168 mL和13838 mL。实际测量值与计算值没有显著差别。调节粪草比可以从原料转化速率方面促进发酵效率,并不能提高原料的产气潜力。因此,粪草比改善的优势可能是改良原料结构和调节原料营养,而不是改善发酵原料的转化潜力。     

Apparent availability of nitrogen in composted municipal refuse

The use of composted municipal refuse on agricultural land requires prior knowledge of the interactions among compost, soil, and plants. Research into the availability of N in highly matured municipal refuse compost is particularly important considering the current concern about groundwater contamination by NO _inf3 ^sup- -N. A greenhouse pot bioassay was conducted to determine the percentage of short-term apparent bioavailable N of a highly matured refuse compost and its relative efficiency in supplying inorganic N to the soil-plant system in comparison with NH₄NO₃. Municipal refuse (after 165 days of composting) was applied at rates equivalent to 10, 20, 30, 40, and 50 t ha^-1 to a ferrallitic soil from Tenerife Island (Andeptic Paludult). NH₄NO₃ was applied at rates equivalent to the total N content of the compost treatments. Perennial ryegrass (Lolium perenne L.) was grown in 3-kg pots and the tops were harvested at regular intervals after seedling emergence. The compost increased dry matter yield, soil mineral N, and plant N uptake proportional to the applied rate. These increases were significantly higher than the control at an application rate of 20 t ha^-1. After 6 months the apparent bioavailable N ranged from 16 to 21%. The relative efficiency was 43% after 30 days. This suggests that large inputs of inorganic N into soil can be obtained with high rates of this kind of compost, with a potential for NO _inf3 ^sup- -N contamination. However, applied at moderate rates in our bioassay (<50 t ha^-1), compost showed a low N-supplying capacity to ryegrass, i.e. a small fraction of the mineralized compost N was used by plants in the course of time. This was ascribed to a partial biological immobilization. This pattern of N availability in highly matured municipal refuse compost, positive net mineralization but partial immobilization, is similar to the pattern of N availability in biologically active soils and is therefore extremely interesting for the conservation of N in agro-ecosystems.     

Nitrogen uptake by ryegrass from organic wastes applied to a sandy loam soil

Sandy‐textured Mediterranean soils are invariably depleted in organic matter and supply only small amounts of N to crops. To compensate for these deficiencies, we tested the N supply from six organic wastes applied to a Cambic Arenosol in pots growing ryegrass. The results showed that the behaviour of the wastes in supplying N to a ryegrass crop grown in this soil can be predicted by observing their performance in laboratory aerobic incubations. The N made available during these incubations fitted well to a one‐pool kinetic model.     

Soil organic matter and the extracellular microbial matrix show contrasting responses to C and N availability

An emerging paradigm in soil science suggests microbes can perform ‘N mining’ from recalcitrant soil organic matter (SOM) in conditions of low N availability. However, this requires the production of extracellular structures rich in N (including enzymes and structural components) and thus defies stoichiometric expectation. We set out to extract newly synthesised peptides from the extracellular matrix in soil and compare the amino acid (AA) profiles, N incorporation and AA dynamics in response to labile inputs of contrasting C/N ratio. Glycerol was added both with and without an inorganic source of N (10% ¹⁵N labelled NH₄NO₃) to a soil already containing a large pool of refractory SOM and incubated for 10 days. The resulting total soil peptide (TSP) and extracellular pools were compared using colorimetric methods, gas chromatography, and isotope ratio mass spectrometry. N isotope compositions showed that the extracellular polymeric substance (EPS) contained a greater proportion of products formed de novo than did TSP, with hydrophobic EPS-AAs (leucine, isoleucine, phenylalanine, hydroxyproline and tyrosine) deriving substantially more N from the inorganic source provided. Quantitative comparison between extracts showed that the EPS contained greater relative proportions of alanine, glycine, proline, phenylalanine and tyrosine. The greatest increases in EPS-peptide and EPS-polysaccharide concentrations occurred at the highest C/N ratios. All EPS-AAs responded similarly to treatment whereas the responses of TSP were more complex. The results suggest that extracellular investment of N (as EPS peptides) is a microbial survival mechanism in conditions of low N/high C which, from an evolutionary perspective, must ultimately lead to the tendency for increased N returns to the microbial biomass. A conceptual model is proposed that describes the dynamics of the extracellular matrix in response to the C/N ratio of labile inputs.     

Nitrogen turnover and N2O production in the forest floor of beech stands as influenced by forest management

Effects of forest management (thinning) on gross ammonification, net ammonification, net nitrification, microbial biomass, and N₂O production were studied in the forest floor of adjacent untreated control (“C”) and thinned (“T”) plots in three beech (Fagus sylvatica L.) stands in the Swabian Jura (Southern Germany) during three intensive field campaigns in the year 2004. The investigated sites are located less than 1 km apart on the slopes of a narrow valley. Due to different exposure (southwest, northeast, northwest), the three sites are characterized by warm‐dry microclimate (southwest site, SW) and cool‐moist microclimate (northeast site, NE; and northwest site, NW). Measurements at the NW site covered the second year (13 to 20 months) after thinning, and measurements at the SW and NE sites covered the sixth year (61 to 68 months) after thinning. Mean gross ammonification varied insignificantly across the six plots (range: 37.5 ? 31.2 to 51.0 ? 10.5 mg N (kg dry soil)^–1 d^–1). The SW site was characterized by very low net nitrification and nitrate (NO ) concentrations that were not significantly different between control and thinned plot. In contrast, for the thinned plot at the NE site (NET), significantly increased mean net nitrification (2.3 ? 1.2 mg N (kg dry soil)^–1 d^–1 at the NET plot vs. 0.4 ? 0.2 mg N (kg dry soil)^–1 d^–1 at the NEC plot) and mean extractable NO concentrations (43.9 ? 22.8 mg N (kg dry soil)^–1 at the NET plot vs. 4.1 ? 0.8 mg N (kg dry soil)^–1 at the NEC plot) were observed. The differences in net nitrification and NO concentrations across the research plots were related to differences in the forest‐floor C : N ratios: net nitrification increased exponentially below a threshold C : N value of about 25. The results of this study indicate that the forest floor of the warm‐dry SW site is very resistant to N loss triggered by thinning due to high C : N ratios around 30. Under the cool‐moist microclimate of the NE site, a significantly lower C : N ratio of 22.1 at the thinned plot (control plot: 26.7) coincided with significantly increased net nitrification. Thus, different responses of net nitrification to thinning under different microclimate appear to be triggered by different C : N ratios. Nitrous oxide production was mainly governed by forest‐floor water content, and since differences in water content at adjacent control and thinned plots were low, N₂O production was not significantly different between adjacent control and thinned plots.     

高寒草甸土CO2流通和氮矿化之间的关系以及对外加碳或氮的响应

In nutrient-limited alpine meadows,nitrogen(N) mineralization is prior to soil microbial immobilization;therefore,increased mineral N supply would be most likely immobilized by soil microbes due to nutrient shortage in alpine soils.In addition,low temperature in alpine meadows might be one of the primary factors limiting soil organic matter decomposition and thus N mineralization.A laboratory incubation experiment was performed using an alpine meadow soil from the Tibetan Plateau.Two levels of NH4NO3(N) or glucose(C) were added,with a blank without addition of C or N as the control,before incubation at 5,15,or 25 ℃ for 28 d.CO2 efflux was measured during the 28-d incubation,and the mineral N was measured at the beginning and end of the incubation,in order to test two hypotheses:1) net N mineralization is negatively correlated with CO2 efflux for the control and 2) the external labile N or C supply will shift the negative correlation to positive.The results showed a negative correlation between CO2 efflux and net N immobilization in the control.External inorganic N supply did not change the negative correlation.The external labile C supply shifted the linear correlation from negative to positive under the low C addition level.However,under the high C level,no correlation was found.These suggested that the correlation of CO2 efflux to net N mineralization strongly depend on soil labile C and C:N ratio regardless of temperatures.Further research should focus on the effects of the types and the amount of litter components on interactions of C and N during soil organic matter decomposition.     

Plant availability of catch crop sulfur following spring incorporation

Catch crops might reduce sulfate leaching and thereby increase the overall sulfur (S)‐use efficiency in crop rotations. At two experimental sites in Denmark (a sandy loam and a coarse sand), S uptake of catch‐crop species was measured. Furthermore, net release of S following incorporation of this material (S contents 0.13%–1.03%, C:S ratios of 40–329, and lignin contents of 1%–10.8%) was investigated in a pot experiment with spring barley in sandy soil. The catch crops showed huge differences in their ability to sequester S. The best catch crops (legumes on sandy loam), sequestered 10–12 kg S ha^–1, and the poorest catch crops (ryegrass and sorrel on coarse sand) sequestered less than 3 kg S ha^–1. The S‐mineralization rates were highest for crucifers (57%–85% of total S added) and lowest for legumes (up to 46% of total S added). Differences can partly be explained by the C:S ratio, whereas no significant relationship was found with the lignin content of the incorporated catch crops. Catch crops may help to avoid S deficiency and increase synchrony between plant demand and available soil S in a crop rotation. However, the release of S will not fulfil the need of S‐demanding crops and even for cereals, the mineralization will most often only make a contribution. In the case of legume catch crops, it is advisable to use a supplemental S source.