Humic substances in soil and crop sciences: P. MacCarthy, C.E. Clapp, R.L. Malcolm and P.R. Bloom (Editors), American Society of Agronomy and Soil Science Society of America, Madison, WI, 1990, 281 pp., price US$ 30.00, ISBN: 0-89118-104-0

Studies were conducted to determine changes in organic matter and microbial biomass carbon in comparison with structural stability at the surface soil (0–5 cm) of a Charlottetown fine sandy loam, an Orthic Podzol, at three tillage and grassland sites situated in Prince Edward Island. The tillage experiments, established for 3–5 years, included comparisons of mouldboard ploughing, direct drilling, shallow tillage, and chisel ploughing. Two indices of soil structural stability were used: mean weight diameter (MWD) and aggregation index (AI). The latter index assigns a weight factor to aggregate size ranges based on their value for plant germination and root growth.Direct drilling and reduced tillage increased the level of soil organic carbon by 10–17%, relative to mouldboard ploughing. Organic carbon was more enriched in 1–2 mm and 4.75–9.00 mm macroaggregates, especially the former, compared with whole soil. The MWD of aggregates after wet sieving was 33% and 55%, relative to the grassland sites, for mouldboard ploughing and direct drilling, respectively. On these soils of similar mineralogy and particle size, a close linear relationship (r=0.942) was observed between organic carbon (r=0.947) and nitrogen (r=0.923). The AI was significantly correlated to both organic carbon and microbial biomass carbon using power regression. In contrast to MWD, the AI under direct drilling and reduced tillage, associated with an organic carbon level of 2.5%, approximated the AI under grass. Overall, the study showed that minimum tillage systems in humid climates can improve structural stability at the soil surface of fine sandy loams over a relatively short time frame.     

Specific response of fungal and bacterial residues to one-season tillage and repeated slurry application in a permanent grassland soil

The dynamics of fungal and bacterial residues to a one-season tillage event in combination with manure application in a grassland soil are unknown. The objectives of this study were (1) to assess the effects of one-season tillage event in two field trials on the stocks of microbial biomass, fungal biomass, microbial residues, soil organic C (SOC) and total N in comparison with permanent grassland; (2) to determine the effects of repeated manure application to restore negative tillage effects on soil microbial biomass and residues. One trial was started 2 years before sampling and the other 5 years before sampling. Mouldboard ploughing decreased the stocks of SOC, total N, microbial biomass C, and microbial residues (muramic acid and glucosamine), but increased those of the fungal biomarker ergosterol in both trials. Slurry application increased stocks of SOC and total N only in the short-term, whereas the stocks of microbial biomass C, ergosterol and microbial residues were generally increased in both trials, especially in combination with tillage. The ergosterol to microbial biomass C ratio was increased by tillage, and decreased by slurry application in both trials. The fungal C to bacterial C ratio was generally decreased by these two treatments. The metabolic quotient qCO₂ showed a significant negative linear relationship with the microbial biomass C to SOC ratio and a significant positive relationship with the soil C/N ratio. The ergosterol to microbial biomass C ratio revealed a significant positive linear relationship with the fungal C to bacterial C ratio, but a negative one with the SOC content. Our results suggest that slurry application in grassland soil may promote SOC storage without increasing the role of saprotrophic fungi in soil organic matter dynamics relative to that of bacteria.     

Influence of tillage systems on biological properties of a Typic Argiudoll soil under continuous maize in central Argentina

Farmers are increasingly using zero tillage in Central Argentina to replace other tillage systems. Intensive tillage decreases soil organic matter content and causes physical degradation. The objective of this work was to evaluate changes in some soil biological properties induced by different tillage systems. A 6 year experiment in which continuous maize (Zea mays L.) was grown using three tillage systems (conventional tillage, reduced tillage and zero tillage) was carried out at Córdoba Province, Argentina, on a Typic Argiudoll. Variations in total organic C content, microbial biomass C, metabolic quotient (qCO₂) and the proportion of the organic C present in the microbial biomass were evaluated at two sampling depths (0–5 and 5–15 cm). Additional samples from a nearby site (undisturbed grassland) were also taken and considered as a control. Concentrations of soil organic C and microbial biomass C were higher under zero tillage as compared with conventional tillage, at the 0–5 cm soil depth. Differences were not evident among tillage systems at the 5–15 cm soil depth. An analysis of the microbial biomass C content, in relation to the organic C, revealed higher values at the 0–5 cm soil depth only for those systems which provoke less disturbance of the soil (i.e. reduced tillage and zero tillage). Significantly greater amounts of CO₂---C were released from zero tillage and reduced tillage soils than from conventionally tilled soils. This release was positively correlated with microbial biomass C. qCO₂ values were not significantly different between tillage systems. Zero tillage proved to be more efficient in the conservation of organic C and microbial biomass C. The tillage system's impact on respiration was due to its effect on the microbial biomass.     

Microbial biomass and N mineralization in relation to N supply of sugar beet under reduced tillage

Reduced tillage may affect N supply of plants by influencing soil microbial biomass and thereby N release. The aim of this study was to evaluate changes in microbial biomass due to tillage in relation to N mineralization and to assess the contribution to the N supply of sugar beet. For this purpose, in a field trial near Göttingen in 1995 microbial biomass and net N mineralization were determined in an in situ incubation of ploughed and reduced tilled soil in plots which were not given application of mineral N fertilizer. In reduced tilled soil the increase in mineral N concentration in the upper 10 cm of soil was mainly attributed to an increase in microbial biomass. The organic matter was more easily decomposable, indicated by the increase in C_mic/C_org and N_mic/N_t ratios; this was further supported by the enhanced turnover of microbial biomass in reduced tillage plots. A regression function was used to relate seasonal fluctuations of microbial biomass, soil moisture and soil temperature to N mineralization rate. There was a good agreement between measured and calculated N mineralization rate. Reduced tillage affected N mineralization by affecting the quantity and quality of microbial biomass. In 0–30 cm soil depth 169 kg N/ha were mineralized, 30 kg more N than in ploughed soil. However, despite improved N availability, the N uptake of sugar beet was decreased in reduced tilled soil. Because the N concentration in plants did not differ, it was concluded that sugar beet growth in reduced tilled soil was impaired due to other factors than N supply.     

Modification of porespace by tillage in two stagnogley soils with contrasting management histories

The soil porespace was studied in two long-term tillage experiments on two clayey stagnogleys in Southern England. The soils differed in respect of mineral and organic composition and previous management history. In both soils the total volume of pores and the volume fraction of macropores in the topsoil horizon declined with direct drilling compared with annual ploughing. This difference between tillage treatments appeared to develop more slowly in the soil that was formerly under continuous arable cultivation than in the soil that was previously in long-term grassland. Fluid transport coefficients were greater in ploughed topsoil in both soils; however, at the boundaries between topsoil and subsoil, and in the upper subsoil, permeability and gaseous diffusivity were greater after direct drilling. At a long-term arable site, soil was more consolidated below the depth of ploughing or shallow tillage, whereas in a former grassland soil ploughing disrupted the continuity of channel-type macropores.     

不同耕作方式下玉米农田土壤养分及土壤微生物活性变化

通过连续3年的野外调查与室内分析试验,研究了不同耕作方式（翻耕、旋耕、免耕）对玉米农田土壤养分及土壤微生物活性的影响。结果表明,不同耕作方式下土壤pH值略显酸性,土壤容重与土壤总孔隙度变化趋势相反;土壤容重基本表现为:免耕 > 翻耕 > 旋耕;土壤总孔隙度TSP基本表现为:翻耕 > 旋耕 > 免耕,不同耕作方式差异均显著（p<0.05）。不同耕作方式下土壤养分（有机碳、全氮含量）和有效养分（有效磷、铵态氮和硝态氮）均呈现出一致性规律,大致表现为翻耕 > 旋耕 > 免耕,不同耕作方式下土壤全磷含量差异均不显著（p > 0.05）;与免耕相比,土壤微生物量碳和氮、土壤微生物数量（细菌、真菌、放线菌、固氮菌和纤维素菌）均有明显的增加,大致表现为:翻耕 > 旋耕 > 免耕。土壤微生物活度的变化范围为0.38~0.69,依次表现为翻耕 > 旋耕 > 免耕,不同耕作方式下土壤微生物活度差异均显著（p<0.05）。不同耕作方式下土壤微生物量碳周转率高于氮周转率,说明微生物量碳更新比微生物量氮快,其中翻耕处理下土壤微生物量碳和氮更新较旋耕和免耕快。通径分析发现,不同土壤环境因子对土壤微生物活度产生直接和间接负作用,其中有机碳、全氮、硝态氮、铵态氮、细菌数量对土壤微生物活度产生直接效应;土壤微生物量碳、微生物量氮、真菌数量和固氮菌数量对土壤微生物活度产生间接效应。     

Microbial biomass nitrogen pool in soils from a warm temperate grassland,and from deciduous and evergreen forests in Chiba,central Japan

We evaluated the status of the microbial biomass N pool in grassland, and in deciduous and evergreen forest soils in Chiba, central Japan. Microbial biomass N, a labile fraction of total N in the soil, ranged from 6.96 g N m^-2 (15 cm depth) in the grassland to 24.8 g in the deciduous and 20.7 g in the evergreen soils, on a landscape basis. Thus the pattern in the grassland and in the forest soils differed. The N flush measured by a fumigation-incubation method indicated that in the grassland soil microbial biomass N was underestimated by a factor of 2.6 compared with the results from a fumigation-extraction method, because of heavy N immobilization in the microbial biomass. This was in contrast to results from the forest soils, which did not immobilize N. Thus, the forest soils were in a steady-state condition compared with the grassland which formed a seral phase in the ecological succession. Simple correlation coefficients indicated a significant positive relationship between biomass N and organic C in the soil and the N concentration in the litter, the main component of organic matter in the soils of the three ecosystems.     

Spatial patterns of soil biological and physical properties in a ridge tilled and a ploughed Luvisol

The present study was conducted to determine the spatial heterogeneity of bulk density, soil moisture, inorganic N, microbial biomass C, and microbial biomass N in the ridge tillage system of Turiel compared to conventional mouldboard ploughing on three sampling dates in May, July, and August. The soil sampling was carried out under vegetation representing the ridge in a high spatial resolution down the soil profile. Bulk density increased with depth and ranged from 1.3 g cm⁻³ at 10 cm depth to 1.6 g cm⁻³ at 35 cm in ploughed plots and from 1.0 g m⁻³ at 5 cm to 1.4 g m⁻³ at 35 cm in the ridges. In the ploughed plots, the contents of microbial biomass C and microbial biomass N remained roughly constant at 215 and 33 μg g⁻¹ soil, respectively, throughout the experimental period. The microbial biomass C/N ratio varied in a small range around 6.4. In the ridged plots, the contents of microbial biomass C and microbial biomass N were 5% and 6% higher compared to the ploughed plots. Highest microbial biomass C contents of roughly 300 μg g⁻¹ soil were always measured in the crowns in July. The lowest contents of microbial biomass C of 85–137 μg g⁻¹ soil were measured in the furrows. The ridges showed strong spatial heterogeneity in bulk density, soil water content, inorganic nitrogen and microbial biomass.     

Tillage effects on microbial biomass and nutrient distribution in soils under rain-fed corn production in central-western Mexico

Quantifying how tillage systems affect soil microbial biomass and nutrient cycling by manipulating crop residue placement is important for understanding how production systems can be managed to sustain long-term soil productivity. Our objective was to characterize soil microbial biomass, potential N mineralization and nutrient distribution in soils (Vertisols, Andisols, and Alfisols) under rain-fed corn (Zea mays L.) production from four mid-term (6 years) tillage experiments located in central-western, Mexico. Treatments were three tillage systems: conventional tillage (CT), minimum tillage (MT) and no tillage (NT). Soil was collected at four locations (Casas Blancas, Morelia, Apatzingán and Tepatitlán) before corn planting, at depths of 0–50, 50–100 and 100–150 mm. Conservation tillage treatments (MT and NT) significantly increased crop residue accumulation on the soil surface. Soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were highest in the surface layer of NT and decreased with depth. Soil organic C, microbial biomass C and N, total N and extractable P of plowed soil were generally more evenly distributed throughout the 0–150 mm depth. Potential N mineralization was closely associated with organic C and microbial biomass. Higher levels of soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were directly related to surface accumulation of crop residues promoted by conservation tillage management. Quality and productivity of soils could be maintained or improved with the use of conservation tillage.     

THE EFFECTS OF GRINDING ON MICROBIAL AND NON-MICROBIAL ORGANIC MATTER IN SOIL

Grinding more than doubled the respiration rate of two silt loam soils, one arable and one grassland. The increases were smaller when the grinding treatment was given to portions of soils that had previously been fumigated with CHCI₃and incubated, a treatment that greatly decreased microbial biomass. The results indicate that the flush of decomposition caused by grinding was in part derived from killed organisms and in part from enhanced decomposition of non-biomass sections of the soil organic matter. Grinding killed about a quarter of the biomass in both soils. Carbon from killed organisms accounted for a quarter of the extra CO₂–C evolved after grinding in the arable soil and almost half in the grassland soil. The extra non-biomass organic matter decomposing after grinding amounted to about 0.5% of the soil organic carbon in both soils. This non-biomass material rendered decomposable by grinding had a higher C/N ratio than the organic matter decomposing in unground soil.     

Characterization of soil physical properties and organic matter under long-term primary tillage in a humid climate

Chisel ploughing is considered to be a potential conservation tillage method to replace mouldboard ploughing for annual crops in the cool-humid climate of eastern Canada. To assess possible changes in some soil physical and biological properties due to differences in annual primary tillage, a study was conducted for 9 years in Prince Edward Island on a Tignish loam, a well-drained Podzoluvisol, to characterize several mouldboard and chisel ploughing systems (at 25 cm), under conditions of similar crop productivity. The influence of primary tillage on the degree of soil loosening, soil permeability, and both organic matter distribution throughout the soil profile and organic matter content in soil particle size fractions was determined. At the time of tillage, chisel ploughing provided a coarser soil macrostructure than mouldboard ploughing. Mouldboard ploughing increased soil loosening at the lower depth of the tillage zone compared to chisel ploughing. These transient differences between primary tillage treatments had little effect on overall soil profile permeability and hydraulic properties of the tilled/non-tilled interface at the 15–30 cm soil depth. Although soil microbial biomass, on a volume basis, was increased by 30% at the 0–10 cm soil depth under chisel ploughing, no differences were evident between tillage systems over the total tillage depth. Mouldboard ploughing increased total orgainc carbon by 43% at the 20–30 cm soil depth, and the carbon and nitrogen in the organic matter fraction ≤ 53 μm by 18–44% at the 10–30 cm soil depth, compared to chisel ploughing.     

Effect of freeze-thaw events on mineralization of soil nitrogen

Summary In humid regions of the United States there is considerable interest in the use of late spring (April–June) soil NO ₃ ^– concentrations to estimate fertilizer N requirements. However, little information is available on the environmental factors that influence soil NO ₃ ^– concentrations in late winter/early spring. The influence of freeze-thaw treatments on N mineralization was studied on several central Iowa soils. The soils were subjected to temperatures of-20°C or 5°C for 1 week followed by 0–20 days of incubation at various temperatures. The release of soluble ninhydrin-reactive N, the N mineralization rate, and net N mineralization (mineral N flush) were observed. The freeze-thaw treatment resulted in a significant increase in the N mineralization rate and mineral N flush. The N mineralization rate in the freeze-thaw treated soils remained higher than in non-frozen soils for 3–6 days when thawed soils were incubated at 25°C and for up to 20 days in thawed soils incubated at 5°C. The freeze-thaw treatments resulted in a significant release of ninhydrin-reactive N. These values were closely correlated with the mineral N flush (r ²=0.84). The release of ninhydrin-reactive N was more closely correlated with biomass N (r ²=0.80) than total N (r ²=0.65). Our results suggest that freeze-thaw events in soil disrupt microbial tissues in a similar way to drying and re-wetting or chloroform fumigation. Thus the level of mineral N released was directly related to the soil microbial biomass. We conclude that net N mineralization following a spring thaw may provide a significant portion of the total NO ₃ ^– present in the soil profile.     

Microbial biomass and mineralization-immobilization of nitrogen in some agricultural soils

Summary The chloroform fumigation-incubation method (CFIM) was used to measure the microbial biomass of 17 agricultural soils from Punjab Pakistan which represented different agricultural soil series. The biomass C was used to calculate biomass N and the changes occurring in NH₄ ⁺-N and NO₃ ^–-N content of soils were studied during the turnover of microbial biomass or added C source. Mineral N released in fumigated-incubated soils and biomass N calculated from biomass C was correlated with some N availability indexes.The soils contained 427–1240 kg C as biomass which represented 1.2%–6.9% of the total organic C in the soils studied. Calculations based on biomass C showed that the soils contained 64–186 kg N ha^–1 as microbial biomass. Immobilization of NCO₃ ^–-N was observed in different soils during the turnover of microbial biomass and any net increase in mineral N content of fumigated incubated soils was attributed entirely to NH₄ ⁺-N.Biomass N calculated from biomass C showed non-significant correlation with different N availability indexes whereas mineral N accumulated in fumigated-incubated soils showed highly significant correlations with other indexes including N uptake by plants.     

Carbohydrate composition in relation to structural stability, compactibility and plasticity of two soils in a long-term experiment

The effects of tillage on soil organic carbon content, carbohydrate content, monosaccharide composition, aggregate stability, compactibility and plasticity were investigated in a field experiment on a gleysol and on a cambisol under winter barley in South-East Scotland. Two long-term treatments (direct drilling and conventional mouldboard ploughing for 22 years) were compared with short-term direct drilling and broadcast sowing plus rotavation for 5 years. Carbohydrate released sequentially to cold water, hot water, 1.0 M HCl and 0.5 M NaOH was determined after hydrolysis as reducing sugar equivalent to glucose in both fresh and air-dried samples. All other measurements were made on dry soils only. About 3% of the soluble carbohydrate was extracted by cold water, 10% by hot water, 12% by HCl and 75% by NaOH from both the dry and fresh soils. The total reducing sugars of the fractions were proportional to the total organic carbon determined by dichromate oxidation or C analysis. Organic carbon and carbohydrates were concentrated near the surface of the direct drilled soil, but were more uniformly distributed with depth in the ploughed soil. The surface soil under direct drilling was more stable, less compactible and had greater plasticity limits than under ploughing. However, particle size distributions were unaffected by tillage so that differences in soil properties were attributed to differences in the quantity and quality of organic matter. Differences in compactibility, structural stability and plasticity limits between depths and tillage treatments correlated with total carbon and with total carbohydrates. The hot water extractable carbohydrate fraction correlated best with aggregate stability and the NaOH fraction correlated best with compactibility and plastic limit. Both fractions were greatest in the long-term direct drilled soil. The hot water fraction had a galactose plus mannose over arabinose plus xylose ratio of 1.0–1.6 in comparison to 0.4–0.7 in the NaOH fraction indicating that the microbial contribution within the hot water-soluble fraction was the greater. The hot-water fraction was likely to contain more exocellular microbial polysaccharides involved in the stabilizing of soil aggregates. The hot-water and NaOH carbohydrate fractions may be good indicators of soil organic matter quality relevant to the preservation of good soil physical conditions.     

稻田系统管理措施对微生物生物量C、N、P的长期影响

A 12-year field experiment was conducted to investigate the effect of different tillage methods and fertil-ization systems on microbial biomass C,N and P of a gray fluvo-aguic soil in rice-based cropping system .Five fertilization treatments were designed under conventional tillae(CT) or on tillage(NT) system:no fertilizer(CK) ; chemical fertilizer only(CF) ; combining chemical fertilizer with pig manure(PM); combining chemical fertilizer with crop straw (CS) and fallow (F). The results showed that biomass C,N and P were enriched in the surface layer of no-tilled soil,whereas they distributed relatively evenly in the tilled soil,which might result from enrichment of crop resdue,organic manure and mineral fertilzer,and surficial developent of root systems under NT.Under the cultivation system NT had slightly greater biomass C,N and P at 0-5 cm depth ,significantly less biomass C,N and P at 5-15 cm depth ,less microbial biomass C,N and equivalent biomass P at 15-30 cm depth as compared to CT,indicating hat tillage was beneficial for the multiplication of organims in the plowed layer of soil.Under the fallow system,biomass C,N and P in the surface layer were significantly greater for NT than CT while their differences between the two tillage methods were neligible in the deeper layers.In the surface layer,biomass C,N and P in the soils amended with oranic manure combined with mineral fertilizers were significantly greater than those of the treatments only with mineral fertilizers and the control.Soils without fertilzer had the least biomass nutrient contents among the five fertilization treatments.Obviously,the long-term application of organic manure could maintain the higher activity of microorganisms in soils.The amounts of biomass C,N and P in the fallowed soils varied with the tillage methods;they were much greater under NT than under CT,especially in the surface layer,suggesting that the frequent plowing could decrease the content of organic matter in the surface layer of the fallowed soil.     

Drivers of microbial respiration and net N mineralization at the continental scale

The dominant pools of C and N in the terrestrial biosphere are in soils, and understanding what factors control the rates at which these pools cycle is essential in understanding soil CO₂ production and N availability. Many previous studies have examined large scale patterns in decomposition of C and N in plant litter and organic soils, but few have done so in mineral soils, and fewer have looked beyond ecosystem specific, regional, or gradient-specific drivers. In this study, we examined the rates of microbial respiration and net N mineralization in 84 distinct mineral soils in static laboratory incubations. We examined patterns in C and N pool sizes, microbial biomass, and process rates by vegetation type (grassland, shrubland, coniferous forest, and deciduous/broadleaf forest). We also modeled microbial respiration and net N mineralization in relation to soil and site characteristics using structural equation modeling to identify potential process drivers across soils. While we did not explicitly investigate the influence of soil organic matter quality, microbial community composition, or clay mineralogy on microbial process rates in this study, our models allow us to put boundaries on the unique explanatory power these characteristics could potentially provide in predicting respiration and net N mineralization. Mean annual temperature and precipitation, soil C concentration, microbial biomass, and clay content predicted 78% of the variance in microbial respiration, with 61% explained by microbial biomass alone. For net N mineralization, only 33% of the variance was explained, with mean annual precipitation, soil C and N concentration, and clay content as the potential drivers. We suggest that the high R² for respiration suggests that soil organic matter quality, microbial community composition, and clay mineralogy explain at most 22% of the variance in respiration, while they could explain up to 67% of the variance in net N mineralization.     

Effect of Long-Term Soil Management on the Mutual Interaction Among Soil Organic Matter, Microbial Activity and Aggregate Stability in a Vineyard

Vineyard management practices to enhance soil conservation principally focus on increasing carbon (C) input, whereas mitigating impacts of disturbance through reduced tillage has been rarely considered. Furthermore, information is lacking on the effects of soil management practices adopted in the under-vine zone on soil conservation. In this work, we evaluated the long-term effects (22 years) of alley with a sown cover crop and no-tillage (S + NT), alley with a sown cover crop and tillage (S + T), and under-vine zone with no vegetation and tillage (UV) on soil organic matter (SOM), microbial activity, aggregate stability, and their mutual interactions in a California vineyard in USA. Vegetation biomass, microbial biomass and activity, organic C and nitrogen (N) pools, and SOM size fractionation and aggregate stability were analysed. Soil characteristics only partially reflected the differences in vegetation biomass input. Organic C and N pools and microbial biomass/activity in S + NT were higher than those in S + T, while the values in UV were intermediate between the other two treatments. Furthermore, S + NT also exhibited higher particulate organic matter C in soil. No differences were found in POM C between S + T and UV, but the POM fraction in S + T was characterized by fresher material. Aggregate stability was decreased in the order: S + NT > UV > S + T. Tillage, even if shallow and performed infrequently, had a negative effect on organic C and N pools and aggregate stability. Consequently, the combination of a sown cover crop and reduced tillage still limited SOM accumulation and reduced aggregate stability in the surface soil layer of vineyards, suggesting relatively lower resistance of soils to erosion compared to no-till systems.     

Nitrogen cycling in organic farming systems with rotational grass–clover and arable crops

Organic farming is considered an effective means of reducing nitrogen losses compared with more intensive conventional farming systems. However, under certain conditions, organic farming may also be susceptible to large nitrogen (N) losses. This is especially the case for organic dairy farms on sandy soils that use grazed grass–clover in rotation with cereals. A study was conducted on two commercial organic farms on sand and loamy sand soils in Denmark. On each farm, a 3‐year‐old grass–clover field was selected. Half of the field was ploughed the first year and the other half was ploughed the following year. Spring barley (Hordeum vulgare L.) was sown after ploughing in spring. Measurements showed moderate N leaching during the pasture period (9–64 kg N ha^?1 year^?1) but large amounts of leaching in the first (63–216 kg N ha^?1) and second (61–235 kg N ha^?1) year after ploughing. There was a small yield response to manure application on the sandy soil in both the first and second year after ploughing. To investigate the underlying processes affecting the residual effects of pasture and N leaching, the dynamic whole farm model farm assessment tool (FASSET) was used to simulate the treatments on both farms. The simulations agreed with the observed barley N‐uptake. However, for the sandy soil, the simulation of nitrate leaching and mineral nitrogen in the soil deviated considerably from the measurements. Three scenarios with changes in model parameters were constructed to investigate this discrepancy. These scenarios suggested that the organic matter turnover model should include an intermediate pool with a half‐life of about 2–3 years. There might also be a need to include effects of soil disturbance (tillage) on the soil organic matter turnover.     

Effect of different soil cultivation systems,including no-tillage,on electro-ultrafiltration extractable organic nitrogen

The effect of different long-term soil-cultivation systems (ploughing, two types of cultivator, notillage) on organic N, extracted with the electroultrafiltration (EUF) technique, was studied in two arable soils, a Luvic Phaeozem derived from loess and a Eutric Cambisol. A modified EUF extraction procedure (the 80°C fraction extended from 5 to 90 min) was used to investigate the release of organic N from the ploughing and no-tillage treatment, and the content of hydrolysable N was measured in the combined filtrates. No-tillage and the two non-turning cultivation systems led to an accumulation of EUF organic N in the 0- to 10-cm depth compared to the ploughing treatment. In the lower horizon (15–25 cm) the reverse pattern was found in the loamy soil, with higher concentrations after ploughing than after reduced tillage. However, in the sandy soil all four cultivation treatments showed similar values in the 15- to 25-cm depth. During the time of investigation (May 1987 to February 1989) an EUF organic N accumulation occurred, which was about twice as high in the loamy as in the sandy soil. Therefore we conclude that in the sandy soil the mineralization of organic N was faster, and that reduced tillage retarded its degradation. In the total 0- to 25-cm depth, this delay was not observed in the loamy soil. The N release rates were much lower in the sandy than in the loamy soil and they were higher for the notillage than for the ploughing treatment. Only 30–40% of the total organic N desorbed was hydrolysable and the amino acid composition indicates that part of it originated from microbial Cells. The overall evaluation showed clearly that EUF-extractable organic N is a sitespecific factor.