Influence of soil tillage on net N-mineralization under sugar beet

The shift from conventional to reduced soil tillage may affect physical and chemical soil properties and thereby net N-mineralization. The aim of this study was to quantify the effect of soil tillage on net N-mineralization at different depths. For this purpose the mineralization rate in the 0–10, 10–20, and 20–30 cm soil layer of conventionally and reduced tilled, N unfertilized plots under sugar beet was estimated with a field incubation method in 1993 and 1994. This method was compared to a balance method which takes into account N uptake of the crop and mineral N in the soil (0–90 cm). The mineralization rate in 1993 and 1994 was determined by the increase of soil temperature in spring and the change of water content in summer. Although the amount of cumulated mineralized N did not differ. soil tillage affected the mineralization rate in the three soil layers depending on the distribution of organic matter. In the reduced tilled soil the mineralization rate in the upper soil layer (0–10 cm) was higher than in conventionally tilled soil, whereas in the layer 10–20 cm depth the mineralization rate of the conventional treatment exceeded that of the reduced tilled treatment. These differences in mineralization rate between conventional and reduced tillage were more distinct in 1994 than in 1993. Since the amount of N mineralized during the season and the period of highest N-mineralization rate in the arable soil layer did not differ due to soil tillage the necessity of an increased N-fertilization in reduced tillage systems cannot be concluded from these results. This is supported by results of the white sugar yield, which showed no interaction between tillage and N fertilization. Results of the field incubation method differed considerably from those of the balance method. This is primarily due to lacking parameters in the N balance.     

Effects of compost input and tillage intensity on soil microbial biomass and activity under Mediterranean conditions

Organic amendment and tillage reduction are two common practices to contrast soil organic matter decline, thus promoting sustainable cropping and carbon sequestration. In a horticultural land use system under Mediterranean climate, we evaluated the 9-year effects of two compost inputs (15 and 30 t ha⁻¹ y⁻¹, low and high input, respectively) and two tillage intensities (intensive and reduced) on soil macronutrients concentration, microbial biomass and activity. Total organic C, total N and P_Olsen were smaller in plots amended at low input, whilst intensive tillage decreased them at both compost inputs. These decreases in intensively tilled plots was ascribed to the disruption of soil aggregates, with consequent microbial degradation of the physically protected organic matter by oxidative processes. On the contrary, reduced tillage increased the extractable C, likely due to a higher protection of the most labile soil C fraction from the mineralization. Similarly, microbial biomass C and N increased following both doubling compost input and reducing tillage intensity, with a greater effect by the first factor. The higher values of cumulative 10-day basal and 20-h glucose-induced respiration, and metabolic quotient in intensively tilled plots suggested that high tillage favoured soil aggregate disruption and C accessibility. This was also confirmed by higher values of dehydrogenase activity/total organic C in those plots. Intensive tillage caused a higher soil aeration and organic substrates accessibility to microflora, thus undoing the fertility benefits provided by the high compost input. However, also a low compost input coupled to reduced tillage seemed to accomplish soil sustainability needs.     

Microbial biomass in soils of Russia under long-term management practices

 Non-tilled and tilled plots on a spodosol (C_org 0.65–1.70%; pH 4.1–4.5) and a mollisol (C_org 3.02–3.13%, pH 4.9–5.3), located in the European region of Russia, were investigated to determine variances in soil microbial biomass and microbial community composition. Continuous, long-term management practices, including tillage and treatment with inorganic fertilizers or manure, were used on the spodosol (39 years) and mollisol (22 years). Total microbial biomass (C_mic), estimated by the substrate-induced respiration (SIR) method, and total fungal hyphae length (membrane filter technique) were determined seasonally over a 3-year period. Long-term soil management practices (primarily tillage and fertilizer application) led to decreases in total microbial biomass (80–85% lower in spodosol and 20–55% lower in mollisol), decreases in the contribution of C_mic to C_org (2.3- to 3.5-fold lower in spodosol and 1.2- to 2.3-fold lower in mollisol), and 50–87% decreases in total fungal hyphae length compared to non-tilled control plots. The contribution of fungi to total SIR in virgin mollisol and fallow spodosol plots was approximately 30%. However, the contribution of fungi to SIR was approximately two times greater in tilled spodosol plots compared to a fallow plot. In contrast, the contribution of fungi to SIR in tilled plots of mollisol was less (1.4–4.7 times) than for a virgin plot. In summary, long-term soil management practices such as tillage and treatment with organic or inorganic fertilizers are important determinants of soil microbial biomass and the contribution of fungi to total SIR. Received: 28 April 1998     

The influence of tillage on the proportion of organic carbon and nitrogen in the microbial biomass of medium-textured soils in a humid climate

Summary Microbial biomass C and N respond rapidly to changes in tillage and soil management. The ratio of biomass C to total organic C and the ratio of mineral N flush to total N were determined in the surface layer (0–5 cm) of low-clay (8–10%), fine sandy loam, Podzolic soils subjected to a range of reduced tillage (direct drilling, chisel ploughing, shallow tillage) experiments of 3–5 years' duration. Organic matter dynamics in the tillage experiments were compared to long-term conditions in several grassland sites established on the same soil type for 10–40 years. Microbial biomass C levels in the grassland soils, reduced tillage, and mouldboard ploughing treatments were 561, 250, and 155 g g^-1 soil, respectively. In all the systems, microbial biomass C was related to organic C (r=0.86), while the mineral N flush was related to total N (r=0.84). The average proportion of organic C in the biomass of the reduced tillage soils (1.2) was higher than in the ploughed soils (0.8) but similar to that in the grassland soils (1.3). Reduced tillage increased the average ratio of mineral N flush to total soil N to 1.9, compared to 1.3 in the ploughed soils. The same ratio was 1.8 in the grassland soils. Regression analysis of microbial biomass C and percent organic C in the microbial biomass showed a steeper slope for the tillage soils than the grassland sites, indicating that reduced tillage increased the microbial biomass level per unit soil organic C. The proportion of organic matter in the microbial biomass suggests a shift in organic matter equilibrium in the reduced tillage soils towards a rapid, tillage-induced, accumulation of organic matter in the surface layer.     

Influence of reduced tillage on earthworm and microbial communities under organic arable farming

Although reduced tillage is an agricultural practice reported to decrease soil erosion and external inputs while enhancing soil fertility, it has still rarely been adopted by European organic farmers. The objective of this study was to assess the long-term interactive effects of tillage (conventional (CT) vs. reduced (RT)) and fertilization (slurry (S) vs. composted manure/slurry (MCS)) on earthworms and microbial communities in a clay soil under spelt in an organic 6-year crop rotation. Earthworm populations (species, density and biomass, cocoons) were investigated by handsorting the soil nine years after initial implementation of the treatments. Soil microbial carbon (C_mic) and nitrogen (N_mic) were measured by chloroform-fumigation extraction and a simplified phospholipid fatty acid (PLFA) analysis was used to separate for populations of bacteria, fungi and protozoa. Significantly increased total earthworm density in RT plots was mainly attributed to increased numbers of juveniles. Moreover, we found five times more cocoons with RT. Species richness was not affected by the treatments, but tillage treatments had differentially affected populations at the species-level. In addition, cluster analysis at the community level revealed two distinct groups of plots in relation to tillage treatments. In RT plots C_mic increased in the 0–10 cm and 10–20 cm soil layers, while PLFA concentrations indicative of Gram-negative bacteria, fungi and protozoa only increased in the topsoil. Lower bacteria-to-fungi ratios in the upper soil layer of RT plots indicated a shift to fungal-based decomposition of organic matter whereas a higher C_mic-to-C_org ratio pointed towards enhanced substrate availability. Slurry application decreased microbial biomass and enhanced density of juvenile anecic earthworms but overall fertilization effect was weak and no interactions with tillage were found. In conclusion, tillage is a major driver in altering communities of earthworms and microorganisms in arable soils. The use of reduced tillage provides an approach for eco-intensification by enhancing inherent soil biota functions under organic arable farming.     

Effect of tillage intensity on N mineralization of different crop residues in a temperate climate

To evaluate the effect of tillage intensity on the N mineralization pattern of winter wheat residues, sugar beet residues, Italian ryegrass and maize residues undisturbed soil samples were taken from six sites under different tillage management. Site NT_K had been managed for 10 years under reduced tillage (RT), whereby the last 4 years the crops were sown using direct seeding (NT). Site RT_CSE had been managed for 20 years under reduced tillage (RT) and site RT_H for 3 years. For each site under RT a nearby site under conventional tillage (CT) was selected (CT_K, CT_CSE and CT_H). On site NT_K and site RT_CSE a significantly higher amount of SOC in the 0–10 cm was accumulated compared to the respective CT sites. Between site RT_H and site CT_H no such significant difference was found. However, the content of microbial biomass C (MB-C) and the β-glucosidase and urease activities were higher on all RT sites compared to the respective CT sites. This indicates that these microbiological and biochemical parameters seem to be very sensitive for alterations in management intensity. After 98 days, more N was immobilized under NT_K than under CT_K by adding winter wheat residues (expressed as kg ha⁻¹ and as % of total added N). This higher immobilization potential can be explained by a higher microbial activity and a change in microbial population. Under RT_CSE and RT_H net N immobilization of the winter wheat residues was found, but the pattern was less pronounced than for NT_K. However, when expressed as % of total N added, N immobilization of winter wheat residues was higher under CT than under RT, which indicates that high C:N residues when incorporated, decompose more slowly under RT than under CT. Similar results were found comparing the N mineralization pattern of maize residues under RT_H and CT_H. The residues of sugar beet and Italian ryegrass at site CT_H released N more rapidly and to a higher extent, 74.1% and 66.2%, respectively (expressed as % of total N added) than under RT_H at the end of the incubation. The slower mineralization of N rich crop residues under RT compared to CT means that there is less potential risk for nitrate leaching to occur, which may result in a higher N efficiency in RT compared to CT.     

Nitrogen pools and turnover in arable soils under different durations of organic farming: II: Source‐and‐sink function of the soil microbial biomass or competition with growing plants?

The following parameters were measured on seven field plots at 3 sites which had been under organic farming for different periods of time: mineral nitrogen (N _min) contents, in situ net nitrogen mineralization (N _net), soil microbial biomass carbon (C _mic), and nitrogen (N _mic) contents, and extractable organic N contents. The measurements were conducted every three weeks from spring 1995/1996 to autumn 1997. The objective was to test whether, under organic farming: 1) temporal fluctuations of N_mic contents over the course of the year are indicative for a source‐and‐sink function for plant‐available N of the soil microbial biomass, and 2) temporal variations in N_mic content can be related with in situ N_net or plant N uptake. N_min contents gradually increased after ploughing in autumn until late winter. During intensive plant growth in spring, values rapidly declined. In situ N_net fluctuated only moderately and reached high values during intensive plant growth (May—July) as well as after soil cultivation in autumn. The C_mic and N_mic contents generally were low in winter, increased in spring and reached maxima in late spring or summer. In spring, the increase in C_mic contents preceded the increase in N_mic contents, resulting in elevated C_mic:N_mic ratios until shooting of winter wheat. This corresponds to an uptake of available soil nitrogen by the plants at the expense of soil micro‐organisms. The subsequent increase in N_mic contents, coinciding with high plant N uptake rates, indicates an enhanced, plant‐induced N mobilization at that time. Possible mobilization mechanisms are discussed. Soil microbial biomass exerted a source‐and‐sink function for extractable organic N on some of the field plots. Estimates of in situ N_net measurements were neither correlated significantly with soil microbial biomass N, N_mic flux, N_mic turnover, nor with plant N uptake. Lower N_mic turnover rates on 41 years versus 3 years organically managed fields indicate a stabilizing effect of organic farming on soil microflora.     

Effects of tillage on deposition and utilization of 15N residual fertilizer

Differences in soil tillage can influence the utilization of fertilizer N. This study determined the effect of zero tillage (ZT) and conventional shallow tillage (CT), of three years duration, on the deposition and dynamics of residual ¹⁵N urea. The study was conducted on a Chernozemic soil in a semiarid region of the Canadian prairie. In the absence of tillage, microbial biomass N levels increased by 48% and mineral N levels decreased by 62%, at the 0–5-cm soil depth, compared to the CT soil. The ZT soil had 96% more residual N distributed in the non-biomass organic N, but 52% less in the mineral and microbial biomass forms of soil N, compared to the CT soil. These tillage induced changes affected the short-term availability of residual ¹⁵N as determined by N mineralization and plant utilization studies. Potentially mineralizable N (N₀) was higher, but release of ¹⁵N residual fertilizer was slower in the ZT soil, compared to CT soil. Initial plant uptake of soil N was similar between tillage systems, although three times as much residual fertilizer was derived from the CT as compared to the ZT soil. Mineralization or plant uptake reduced the differences in ¹⁵N residual fertilizer deposition between tillage soil samples. Tillage differences may influence the short-term N balance of soils receiving N fertilizer. Such changes are related to the greater level of microbial biomass in the surface of ZT soils.     

Losses of nitrate-nitrogen in water draining from under autumn-sown crops established by direct drilling or mouldboard ploughing

The leaching of nitrate-N under autumn-sown arable crops was measured using hydro-logically isolated plots, about 0.24 ha in area, from 1984–1988. Fluxes of water and nitrate moving over the soil surface (surface runoff), at the interface between topsoil and subsoil (interflow), and in the subsoil (drainflow) were monitored in plots with mole-and-pipe drain systems (drained plots); surface runoff and interflow only were monitored in ‘undrained’ plots. Half the drained and undrained plots were direct-drilled, and on the other half seedbeds were prepared by tillage to 200 mm. Tillage increased the total leaching loss of nitrate by 21 % compared with direct drilling in drained plots. About 95% or the nitrate moving from the soil was present in the water intercepted by the subsoil drains in these plots. In undrained plots less water and nitrate were collected in total; more of the nitrate was present in interflow on ploughed plots and in surface runoff in direct-drilled land. Losses of nitrate for the whole experiment from 1978-1988 were analysed. This showed that, between the harvest of one crop and the spring application of fertilizer to the next, loss of nitrate-N from ploughed land (L_p) was approximated by L_p=22+Fkg N ha^?1, where F was the autumn fertilizer-N applied. After fertilizer was applied in spring, loss of nitrate-N depended on rainfall such that for 100 mm rainfall about 30% of the fertilizer-N was lost by leaching. About 18% more nitrate-N was lost from direct-drilled land than from ploughed land in spring, but the total loss was generally small compared to that over winter. The apparent net mineralization of organic-N was measured in 1988. In autumn and winter there was little effect of tillage treatment (26 and 31 kg N ha^?1 on direct drilled and tilled plots respectively). However, over the year 83 kg N ha^?1 were mineralized in tilled plots, and 67 kg N ha^?1 in direct-drilled plots. Five factors governing the leaching of nitrate are assessed and this identified that fertilizer nitrogen application to the seedbed of winter sown crops and the mineralization of nitrogen from the residues of the previous crop are the most significant factors for nitrogen leaching in the UK.     

Changes in the fungal-to-bacterial respiratory ratio and microbial biomass in agriculturally managed soils under free-air CO2 enrichment (FACE) - A six-year survey of a field study

In soil ecology, microbial parameters have been identified as sensitive indicators of changes in the soil environment. The Braunschweig FACE project provided the opportunity to study the effects of elevated CO₂ (550 μmol mol⁻¹) as compared to ambient CO₂ (370 μmol mol⁻¹) on total microbial biomass (C_mic), C_mic-to-C_org ratio and the fungal-to-bacterial respiratory ratio together with total C_org, N_t, C:N ratio and pH over a six-year period. Field management followed a typical crop rotation system of this region with either a crop-related full nitrogen supply (N100) or 50% reduced N supply (N50). The soil microbial parameters responded to the elevated CO₂ treatment in varying intensities and time spans. The fungal-to-bacterial respiratory ratio was the most sensitive parameter in responding to an elevated CO₂ treatment with highly significant differences to ambient CO₂-treated control plots in the third year of CO₂ fumigation. After six years bacterial respiratory activity had increased in ascending order to 34% in FACE-treated plots (N50 and N100) as compared to control plots. Soil microbial biomass (C_mic) responded more slowly to the FACE treatment with highly significant increases of >12% after the fourth year of CO₂ fumigation. The C_mic-to-C_org ratio responded very late in the last two years of the CO₂ treatment with a significant increase of >7.0% only in the N100 variant. Total C_org and N_t were slightly but significantly increased under FACE around 10.0% with ascending tendency over time starting with the second year of CO₂ treatment. No significant FACE effects could be recorded for the C:N ratio or pH.These results suggest that under FACE treatment changes in the soil microbial community will occur. In our study the fungal-to-bacterial respiratory ratio was superior to total C_mic as microbial bioindicators in reflecting changes in the soil organic matter composition.     

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.     

Soil microbial biomass C and N changes in relation to forest conversion in the Northwestern Turkey

Tree species differ in their effect on soil development and nutrient cycling. Conversion of beech coppice to pine plantations can alter soil physical and chemical properties, which in turn may have significant impacts on soil microbial biomass C and N (C_mic, N_mic). The major objective of this study was to evaluate soil quality changes associated with the forest conversion in humid NW Turkey. Results from this study showed that levels of soil organic carbon (C_org), total nitrogen (N_t), moisture, C_mic and N_mic under beech coppice were consistently higher but levels of pH, CaCO₃ and EC were lower compared to pine plantation. Differences between the forest stands in C_mic and N_mic were mainly related to the size of the C_org stores in soil and to tree species. In addition, high level of CaCO₃ is likely to reduce pools of soil organic C and possibly even microbial biomass C and N in pine forests. The average C_mic:N_mic ratios were higher in soils under beech coppice than pine plantation, while C_mic:C_org and N_mic:N_t percentages were similar in both forest types. These results revealed the differences in microbial community structure associated with different tree species and the complex interrelationships between microbial biomass, soil characteristics, litter quantity and quality. Copyright © 2008 John Wiley & Sons, Ltd.     

Microbial C,N, and P relationships in moisture‐stressed soils of Potohar,Pakistan

In 11 rain‐fed arable soils of the Potohar plateau, Pakistan, the amounts of microbial‐biomass C (C_mic), biomass N (N_mic), and biomass P (P_mic) were analyzed in relation to the element‐specific total storage compartment, i.e., soil C_org, N_t, and P_t. The effects of climatic conditions and soil physico‐chemical properties on these relationships were highlighted with special respect to crop yield levels. Average contents of soil C_org, N_t, and P_t were 3.9, 0.32, and 0.61 mg (g soil)^–1, respectively. Less than 1% of P_t was extractable with 0.5 M NaHCO₃. Mean contents of C_mic, N_mic, and P_mic were 118.4, 12.0, and 3.9 µg (g soil)^–1. Values of C_mic, N_mic, P_mic, soil C_org, and N_t were all highly significantly interrelated. The mean crop yield level was closely connected with all soil organic matter– and microbial biomass–related properties, but showed also some influence by the amount of precipitation from September to June. Also the fraction of NaHCO₃‐extractable P was closely related to soil organic matter, soil microbial biomass, and crop yield level. This reveals the overwhelming importance of biological processes for P turnover in alkaline soils.     

Initial decomposition of post-harvest crown and root residues of poplars as affected by N availability and particle size

An incubation experiment was carried out to investigate the impacts of residue particle size and N application on the decomposition of post-harvest residues of fast-growing poplar tree plantations as well as on the microbial biomass. Crown and root residues, differing in their C/N ratios (crown 285, root 94), were ground to two particle sizes and incubated with and without application of inorganic nitrogen (N) for 42 days in a tilled soil layer from a poplar plantation after 1 year of re-conversion to arable land. Carbon and N mineralization of the residues, microbial biomass C and N, ergosterol contents, and recovery of unused substrate as particulate organic matter (POM) were determined. Carbon mineralization of the residues accounted for 26 to 29 % of added C and caused a strong N immobilization, which further increased after N addition. N immobilization in the control soil showed that even 1 year after re-conversion, fine harvest residues still remaining in the soil were a sink for mineral N. Irrespective of the particle size, C mineralization increased only for crown residues after application of N. Nevertheless, the overall decrease in amounts of POM-C and a concurrent decrease of the C/N ratio in the POM demonstrate the mineralization of easily available components of woody residues. Microbial biomass significantly decreased during incubation, but higher cumulative CO₂ respiration after N application suggests an increased microbial turnover. Higher ergosterol to microbial biomass C ratios after residue incorporation points to a higher contribution of saprotrophic fungi in the microbial community, but fungal biomass was lower after N addition.     

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。     

Long-term monitoring of microbial biomass, N mineralisation and enzyme activities of a Chernozem under different tillage management

 We investigated the influence of tillage (conventional, minimum and reduced) on selected soil microbial properties of a fine-sandy loamy Haplic Chernozem over a period of 8 years. The microbial biomass and soil microbial processes were affected mostly by type of tillage and to a lesser extent by the date of soil sampling. Whereas xylanase activity was significantly higher in the 0 to 10-cm soil layer of the reduced and minimum tillage systems within the first year of the experiment (protease and phosphatase activities were significantly higher in the second year), significant treatment effects on microbial biomass, N mineralisation and potential nitrification were observed after a 4-year period. The slow response of substrate-induced respiration to the change in type of tillage may have been due to the differences in the biomass C turnover rates. After a 4-year period, the stratification of the soil microbial biomass within the profile of reduced and minimum tillage systems was probably responsible for the more intensive soil microbial processes near the soil surface compared with conventional tillage. In the 20 to 30-cm layer, N mineralisation, potential nitrification and xylanase activity in the conventional treatment were significantly higher than in the minimum and reduced tillage plots due to buried organic materials. Discriminant analysis underlined the similarity of the enzyme activity patterns in the top layer of the reduced and minimum tillage treatments, and in both layers of the conventional tillage system. The trend towards a significant increase in functional diversity caused by reduced tillage became obvious within the first year of the experiment, and this effect was still manifest after 8 years. All relationships suggested that there were differences in available resources (e.g. organic matter) along the sequence of different tillage systems; this was reflected in part by enhanced enzymatic and microbial activities in the soil layers. In conclusion, this study showed that soils affected by tillage may be classified on the basis of their functional diversity. Therefore, the soil microbial properties chosen for microbiological soil monitoring (microbial biomass, N mineralisation and enzyme activities involved in C, N and P cycling) provide a reliable tool with which to estimate early changes in the dynamics and distribution of soil microbial processes within soil profiles. Received: 3 February 1998     

Tillage intensity and fertility level effects on nitrogen and carbon cycling in a vertisol

Abstract

Because of erosion problems, an effort has been undertaken to evaluate the effect of tillage intensity on carbon (C) and nitrogen (N) cycling on a vertisol. Soil samples at 0–10, 10–20, and 20–30 cm depth were collected from a split plot experiment with five different levels of tillage intensity on Houston Black soil (fine, montmorillonitic, thermic Udic Pellusterts). The experiment was a split plot design with 5 replications. The main plots were chisel tillage, reduced tillage, row tillage, strip tillage, and no tillage. The subplots were soil fertility levels with either high or low fertilizer application rate. Total N, total phosphorus (P), organic C, inorganic N, and C:N ratio were measured on soil samples as well as the potential C mineralization, N mineralization, C turnover, and C:N mineralization ratio during a 30 d incubation. Total P and organic C in soil were increased, with 0.9 and 0.8 kg P ha^‐1 and 20.6 and 20.0 kg C ha^‐1, for high and low soil fertility, respectively. Fertilizer application had no effect on either total N at the 0–10 cm depth, or on soil nutrient status below 10 cm. Potential soil N mineralization was decreased at the 0–10 cm depth and increased at the 20–30 cm depth by the high fertilizer treatment. Chisel tillage decreased total N and P in the 0–10 cm depth, with 1.4 and 1.6 kg N ha^‐1 and 0.8 and 0.9 kg P ha^‐1. However, chisel tillage increased total N and P at the 10–20 cm depth, with 1.3 and 1.2 kg N ha^‐1, and 0.72 and 0.66 kg P ha^‐1 for chisel tillage and no tillage, respectively. Tillage intensity increased C mineralization and C turnover, but reduced N mineralization at the 0–10 cm depth. The results indicate that intensively tilled soil had a greater capacity for C mineralization and for reductions in soil organic C levels compared to less intensively tilled systems.     

SOIL MICROBIAL BIOMASS AND NITROGEN MINERALIZATION RATES ALONG AN ALTITUDINAL GRADIENT ON THE COFRE DE PEROTE VOLCANO (MEXICO): THE IMPORTANCE OF LANDSCAPE POSITION AND LAND USE

A study was conducted to examine the responses of microbial activity and nitrogen (N) transformations along an altitudinal gradient. The gradient was divided into three parts. Three areas were sampled: upper part (UP): coniferous forest, corn field, and abandoned corn field; middle part (MP): tropical cloud forest, grassland, and corn field (COL); and lower part (LP): tropical deciduous forest and sugarcane. The results showed that soil microbial biomass carbon (C) and basal respiration were significantly higher in MP and UP than in LP, whereas the microbial quotient (C_mic/C_org) was higher in LP and MP than in UP. The metabolic quotient (qCO₂) was similar among gradient parts evaluated. Net N mineralization, ammonification, and nitrification rates were higher in UP than MP and LP. We found that in UP, the forest conversion to cropland resulted in no significant differences in microbial activity and N transformation rates between land uses. In MP, microbial biomass C, ammonification, and net N mineralization rates decreased significantly with conversion to cropland, but C_mic/C_org and nitrification were higher in COL. Basal respiration and qCO₂ were significantly lower in COL when compared with other land uses. In LP, lower microbial biomass C, C_mic/C_org, and nitrification rates but higher ammonification and net N mineralization rates were observed in tropical deciduous forest than in sugarcane. No significant differences in basal respiration and qCO₂ were found between uses of LP. Clearly, then, soil organic C is not equally accessible to the microbial community along the gradient studied. Copyright © 2012 John Wiley & Sons, Ltd.     

Microbial properties and nitrogen contents of arable soils under different tillage regimes

Attention is being paid to the use of different tillage regimes as a means of retaining soil organic carbon (SOC) and sequestering more SOC. Alongside earlier measurements of total SOC stocks under different tillage regimes, we have examined the distribution of nitrogen (N), microbial activity and the structure of the soil bacterial community from differently tilled plots under continuous barley. The plots were established 5 yr before sampling and have been maintained annually under conventional tillage (CT; moldboard ploughing to 20 cm and disking), deep ploughing (DP; ploughing to 40 cm and disking), minimum tillage (MT; disking to 7 cm) or zero tillage (ZT). Our earlier work showed there was no difference in SOC contents down to 60‐cm depth between the treatments, but now we report that there were significant differences in the total N and active microbial biomass (substrate‐induced respiration) contents of the same soils. The N contents of the CT, DP and MT treatments were not significantly different, but the ZT contained significantly more N, indicating either greater N retention under the ZT treatment or preferential loss from the more intensively tilled treatments, or a combination of both. The microbial biomass content was greater for the CT and DP treatments than for the MT and ZT treatments, indicating greater sensitivity to treatment effects of the microbial biomass pool than the total C pool, consistent with its more dynamic nature. Terminal restriction fragment length polymorphism (T‐RFLP) analyses of the soil bacteria DNA (a method of assessing the bacterial community structure) enabled the samples to be distinguished both according to SOC content, which is to be expected, and to tillage regime with the greatest differences in community structure occurring in the ZT treatment and the least in DP and CT treatments, reflecting the degree of homogenization or disturbance resulting from tillage.     

Microbial biomass in soils under different tillage and crop rotation systems

A long-term study on the effect of different crop rotations [soybean/wheat, S/W; maize/wheat, M/W or cotton/wheat, C/W] and tillage regimes [no-tillage (NT) or conventional tillage (CT)] on microbial biomass and other soil properties is reported. The experiment was established in 1976 in southern Brazil as a split-plot experimental design in three replications. Soil samples were taken in 1997 and 1998 at 0- to 5-, 5- to 10- and 10- to 20-cm depths and evaluated for microbial biomass C, N, P and S by direct extraction methods. The NT system showed increases of 103%, 54%, 36%, and 44% for microbial biomass C, N, P, and C_mic:C_org percentage, respectively at the 0- to 5-cm depth. NT systems also increased the C to N:S:P ratios. These results provide evidence that tillage or crop rotation affect microbial immobilization of soil nutrients. The larger amount of C immobilized in microbial biomass suggests that soil organic matter under NT systems provides higher levels of more labile C than CT systems.