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In hilly areas, agroforestry can be a more sustainable way of producing food and other products and services than agriculture based on sole-cropping. However, research is needed to evaluate and quantify formation of natural terraces in agroforestry and their contribution to soil conservation. This study quantified natural terrace formation and examined its role in reducing soil and nutrient losses during early stages of agroforestry with fruit trees, contour grass strips and maize or coffee in agroforestry systems on sloping land in northwest Vietnam. Two agroforestry systems, comprising longan (Dimocarpus longan L.)-mango (Mangifera indica L.)-maize (Zea mays L.)-guinea grass (Panicum maximum Jacq.) (fruit-maize-AF) and son tra (Docynia indica (Wall.) Decne.)-coffee (Coffea arabica L.)-guinea grass (fruit-coffee-AF) were compared with sole-cropped maize (sole-maize) and sole-cropped coffee (sole-coffee), respectively. Terrace formation was evaluated over five years using erosion pins placed above grass strips and the volume of terrace formed was estimated. Soil and nutrient losses were quantified using soil traps. The results showed that terraces formed as the systems developed, through gradual deposition of soil sediment above the living grass strips and trees. Accumulated soil sedimentation above the grass strips during the five-year study period raised the soil surface by 4.0 cm in fruit-maize-AF and 4.2 cm in fruit-coffee-AF, and the volume of terraces generated by the grass strips was 0.26 and 0.43 m3/m respectively. The fruit-maize-AF and fruit-coffee-AF systems significantly reduced losses of soil, soil organic carbon (SOC) and associated nutrients (N, P, K) compared with sole-maize and sole-coffee already in the first two years, while the reductions were greater from year 3 onwards. On average across experiments and years, the agroforestry systems reduced soil, SOC, N, P and K losses by 27–76%, 21–78%, 20–82%, 24–82% and 22–84%, respectively. These findings show that agroforestry with fruit trees, grass strips and crops could be a useful management practice and viable option for sustainable agricultural systems on sloping land, by reducing soil (and carbon and nutrient) losses through terrace formation.

Leguminous service crops (SCs) can provide multiple services to cropping systems, reducing the reliance on external resources if sufficient biomass is produced. However, rapid light and temperature reductions limit post-harvest cultivation of SCs in Northern Europe. A novel practice of intercropping SCs in two consecutive crops (spring–winter cereal) to extend the period of SCs growth, and hence improve yield and reduce weeds, was tested. Three spatial and temporal arrangements of SCs and cash crops were investigated, as well as three SC mixtures, characterized by their longevity and frost sensitivity. Compared to no SC, the best performing mixture, frost-tolerant annuals, increased grain and N yield of winter wheat by 10% and 19%, respectively, and reduced weed biomass by 15% and 26% in oats and winter wheat, respectively. These effects were attributed to high biomass production and winter survival. However, this SC reduced oat yields by 15% compared to no SC. Furthermore, SC growth and service provision varied largely between experiments, driven by the weather conditions. Extending the SC’s growth period by intercropping in two consecutive cereal crops has potential, but locally adapted species choices and establishment strategies are needed to ensure SC vitality until termination.

Combined use of lime, animal manure and inorganic fertilisers is effective in replenishing the fertility of degraded acid soils. However, many smallholder farmers lack access to sufficient amounts of these inputs to improve the fertility and reduce the aluminium toxicity of Ferralsols. Organic manures are available but often have low nutrient content, which limits their ability to supply nutrients to soils. In a two-factor field experiment over four seasons on an Anthropic Ferralsol in Southern Province, Rwanda, we assessed (i) the effect of cattle manure on soil properties at a reduced rate affordable to smallholder farmers compared with that of NPK fertiliser applied, with and without lime also at a reduced rate, and (ii) the effect of supplementing grass in a basal cattle diet with legume leaves on manure quality and its effect on soil properties. Manure from cattle fed only the grass Chloris gayana (grass-only manure) and from cattle fed C. gayana supplemented with Acacia angustissima leaves (grass+legume manure) was applied at 5 t dry matter ha-1 (25% of the recommended rate) at the beginning of each growing season. NPK was applied as split doses supplying a total rate of 70 kg N ha-1. Lime was applied annually at a rate of 2.0 t CaO ha-1, which was 25% of the rate required to neutralise total acidity at the site. All amendments were applied only to the soil surrounding the maize plants (planting stations), which is estimated at 25% of the plot area. Maize stover was left on plots after harvest and planting stations were retained over all growing seasons. All treatments altered soil properties at the planting stations. Lime generally increased pH but there was no significant difference between lime plus manure treatments and non-limed manure treatments. Soil organic carbon concentration and cation exchange capacity were higher in manure and NPK treatments than in non-fertilised treatments. The manure treatment increased soil water-holding capacity compared with the NPK and non-fertilised treatments. There was no significant difference in total N, Ca2+, Mg2+ and K+ between the NPK and manure treatments. Micro-dosing animal manure can thus replace mineral fertiliser plus lime for soil fertility replenishment in smallholder farming. Grass+legume manure contained higher concentrations of total N, Ca, Mg, K and Na than grass-only manure, but its effect on soil properties did not differ significantly from that of grass-only manure.

The solution to reducing existing yield gaps on smallholder farms lies in understanding factors limiting yield in areas with agricultural intensification potential. This study applied an integrated analysis approach comprising Classification and Regression Tree (CART), generalized linear mixed model (GLMM), and factor analysis (FA), to explain soil and management‐related factors influencing maize yield gaps, in order to enhance yields. The study was conducted in Mukuyu and Shikomoli in western Kenya, sites with, respectively, high and low agroecological potential regarding soil fertility. Maize yield gaps were quantified by comparing yields on the 90th percentile of farms to yields determined in 189 fields on 70 randomly sampled smallholdings. Soil and management‐related factors were determined at early and late maize development stages. Maize yield on the 90th percentile of farms in Mukuyu and Shikomoli was 5.1 and 4.8 t/ha, respectively, and the average yield gap was 1.8 and 2.6 t/ha, representing 35% and 54% unachieved yield for Mukuyu and Shikomoli, respectively. In FA, soil was revealed to be the main factor influencing maize yield gaps at both sites, rather than management‐related variables. The CART method identified maize density, chlorophyll values, maize height, and depth to compact layer as consistent factors affecting yield at both sites, while GLMM identified soil texture (silt content) as important. According to CART, weed cover at early stages and maize density at late stages were the most limiting factor in maize production in Mukuyu and Shikomoli, respectively. Generalized linear mixed model analysis identified agroecology‐specific factors influencing maize yield gaps as soil‐available phosphorus and zinc, plus weed pressure at early maize stages in Mukuyu, and plus soil cation exchange capacity and exchangeable magnesium in Shikomoli. Through an integrated approach, it was possible to identify both consistent and agroecology‐specific factors limiting crop yields. This can increase the applicability of the findings to smallholder farms.

This study investigated the effect of cutting height (30 and 100 cm) on chemical composition and in vitro digestibility in field-grown Acacia angustissima (Mill.) Kuntze. (Prairie acacia), Leucaena pallida Britton & Rose (guaja) and Mimosa scabrella Bentham. (bracatinga) legumes and their value in supplementing a basal diet of Chloris gayana Kunth. (Rhodes grass). Cutting height did not affect major chemical composition. Crude protein (CP) was highest in A. angustissima and lowest in M. scabrella, while neutral detergent fibre (NDF) and acid detergent fibre (ADF) were highest in M. scabrella. Degradation parameters were greater at 100 cm cutting height. L. pallida showed high in vitro organic matter digestibility but A. angustissima had the highest metabolisable energy. Addition of A. angustissima, L. pallida and M. scabrella to the basal diet increased improved the nutritional value and increased the CP content from 8.4 to 19.8% and 18.1 and 16.1%, respectively. Cutting height of A. angustissima, L. pallida and M. scabrella had no effect on their nutritional value. Their other functions and benefits will determine choice of cutting height as management practice when used on farms. Key words: Legume trees, cutting height, chemical composition, in vitro degradability.

International audience; AbstractSustainable intensification of agricultural production is needed to ensure increased productivity relative to inputs. Short-term yield returns and labor input are major determinants of the fate of sustainable intensification options on smallholder farms in sub-Saharan Africa because labor shortage is often acute, and most farmers lack access to labor-saving technologies. We assessed the relationship between maize grain yield change and labor input from a total of 28 published papers (631 data pairs) including subsets of data pairs within specific sustainable intensification practices. Among the reviewed technologies, manually dug planting basins showed ratios between the change in yield and change in labor inputs (ΔY/ΔL) below 1, suggesting that labor demand increased more than yield. In contrast, ridging showed average ΔY/ΔL values ≥ 2. No-till showed high ΔY/ΔL (average ≥ 1.7) when combined with herbicides but average ΔY/ΔL ≤ 1 (total labor) when manually weeded. Manually weeded rotations showed average ΔY/ΔL ≥ 1 and manually weeded intercropping systems average ΔY/ΔL around 1. The relations revealed four scenarios: high yield returns but low labor demand, high yield returns and labor demand, low yield returns and labor demand, and low yield returns but high labor demand. High yield with high labor demand requires mostly investments in machinery and/or herbicides to reduce labor input. Low yield with low labor demand requires improved crop management, whereas low yield with high labor demand requires a combination of improved crop management and investments to reduce labor. This is the first comprehensive assessment showing that the sustainable intensification options being considered for smallholder farmers may increase crop yield but also labor demand. Options that include mechanization and herbicides at low cost are likely to be adopted due to their reduction effect on drudgery and total labor input.

Animal manure provides plant nutrients and also affects soil nutrient availability, pH buffering and soil physical properties through its contribution to soil organic matter pools. However, the quality and quantity of manure are often low on smallholder farms in sub-Saharan Africa and the initial effect of manuring on crop yield may be small or even negative. In a two-factorial experiment over four seasons in southern Rwanda, the fertiliser value to a maize crop of manures produced by cattle fed a basal diet of only Chloris gayana grass or a mixed C. gayana-Acacia angustissima diet was compared with that of NPK 17−17−17 and no fertiliser. The potential liming effect of the manures was also evaluated through inclusion (or not) of travertine as the second factor. All amendments were applied only to maize planting holes. The crop failed in season 1 due to drought, but manure application (5 t ha−1) approximately doubled maize yield compared with the unfertilised control during seasons 2–4, while NPK (70 kg N ha−1) increased yield by 3- to 4-fold, with corresponding improvements in crop performance indicators. The mixed diet increased manure quality and maize yield compared with the grass diet in season 4. Liming showed a consistent tendency to improve crop performance indicators and yield, but significant differences were only identified in some cases, possibly because the pH increase was small. The results suggest that in regions where manure availability is limiting, application of reduced rates only to planting holes may be an efficient technology. Enhanced animal feed can result in higher quality manure, and ultimately increase crop yield, if nutrient losses during manure handling and storage can be limited.

The recent interest in multi-functional agricultural landscapes has not been matched with formal assessment of the roles that trees play across the spectrum of ecosystem services (ESs) provided in Sub-Saharan Africa (SSA). A structured literature review (1995–2014) assessed 350 journal articles about provision of one or more ESs by trees on farms and in agricultural landscapes in SSA. This revealed information on 15 ESs from studies in 23 countries covering arid (1% of studies), semi-arid (49%), sub-humid (26%) and humid (24%) agro-ecological zones. The majority of the studies reported provisioning (39%) and supporting (35%) followed by regulating (26%) ESs while studies on cultural services were scarce. Beneficial impacts of trees were dominating (58%), in particular in semi-arid zones where they were associated with enhancing water and nutrient cycling. A decline in some ESs was reported in 15% of the studies, while 28% found no effect of trees. Although the effects of trees were mainly positive...

The fertiliser effect of adding wood ash or crushed rock to a low-fertility soil, compared with an unamended control, was assessed in a pot experiment with a perennial ryegrass-red clover mixture. Dinitrogen (N2) fixation by the clover and translocation of fixed N to the grass were determined using 15N natural abundance. The wood ash produced the highest accumulated clover biomass over two cuts, followed by the crushed rock. Chemical analyses suggested that the increase was due to K supply by the amendments. The wood ash also led to larger amounts of fixed N compared with the control. However, N2 fixation was not increased as much as biomass amount, leading to dilution of plant N. There were minor or no treatment effects on mineralisation from soil N pools. This indicates that good-quality wood ash can be successfully used as a multi-element soil amendment to enhance clover growth on low-fertility soils.

Forage crops provide micronutrients as well as energy, protein and fibre to ruminants. However, the micronutrient concentrations of forage plant species differ, legumes generally having higher concentrations than grasses. In addition to that there are also strong effects of soil type. Typically, the concentrations of one or several micronutrients in forage are too low to meet the nutritional requirement of dairy cows. We hypothesized that the overall micronutrient (Co, Cu, Fe, Mn, Mo, Zn) concentrations of forage mixtures are affected by the red clover dry matter (DM) proportion and site effects. This hypothesis was tested at three contrasting sites. The results showed that increased red clover proportion increased the overall concentrations of several micronutrients in the mixtures at all sites. At the site with the widest range of red clover proportion (0–70%) in the mixture, the Co, Cu and Fe concentrations more than doubled between the lowest and highest red clover DM proportion. At the other two sites a smaller increase in red clover proportion (from 10% to 25% or from 25% to 50%) also increased the overall concentrations of Co by up to 80% but less for other micronutrients. One of the sites generally had higher micronutrient concentrations in the crop and removed larger amounts of micronutrients with the harvested biomass compared to the other two sites. This could be explained by differences in pH and micronutrient concentrations of the soils at the sites. We conclude that increased red clover proportion in the sward has the potential to increase the overall micronutrient concentrations but that the effect of the soil is also a controlling factor.

A renewed interest in trace elements (TE), as micronutrients as well as potentially toxic elements, and new options for multi-element analysis has led to an increased number of scientists engaging in TE studies. Accreditation, certification and quality control of TE analyses often applies only to the last step in the sample chain when prepared samples are sent to the laboratory for digestion/extraction and subsequent analysis. However, all stages of the chain from initial sampling to final analysis require an understanding of the specific challenges involved in TE studies and an awareness of the contamination risks as well as approaches to limit these. Contamination can potentially be introduced during all stages of handling and preparation of plant samples, e.g. through dust and the materials that make up the different work surfaces, tools and containers used. Milling devices originally used during preparation of two sets of archived herbage samples were tested to indicate the degree of contamination that can arise from milling. For example, some of the milling devices tested showed effects on several TE concentrations while also increasing the variability between samples. A titanium knife mill which was included for comparison gave the best results, showing no measurable contamination by TE of primary interest, while it allowed a high throughput of samples. To enhance the quality of data on TE in bulky plant material such as herbage and to ensure future usability of newly archived samples, we suggest that field handbooks and sample preparation protocols (where needed) are revised to include precautions against TE contamination in all handling steps. This will ensure reliable data on concentrations of micronutrients and potential toxic TE in plant material.

Slash and burn agriculture is no longer feasible due to the rising population pressure and a dwindling resource base. Crop production intensification is required to produce more food per unit area of land, while rebuilding soil fertility. We explored the impact of reallocation of limited resources on crop productivity across farms of different resource endowment in central Mozambique. The results suggested that decreasing the cropped area and concentrating resources (fertiliser, manure and labour within the farmers’ means) to smaller fields can increase total farm maize production for poor as well as relatively richer farmers in central Mozambique, but that the fertility of the retained and abandoned land is important for the overall outcome. Concentrating resources to smaller areas would in most cases also lead to increased concentrations of soil organic carbon (SOC), indicating positive effects on soil fertility and crop productivity in the long term. However, this apparently does not apply when only fertiliser is used and if harvest residues and weeds are removed or burnt. Organic inputs are thus also required, such as retention of crop residues, and applications of manure if available can further increase SOC. The study has revealed that sustainable intensification or crop production requires that multiple constraints be addressed simultaneously. Success of crop intensification options will also depend on proper targeting to different farm types as well as variability in soil fertility. Differences in resource ownership and bio-physical circumstances lead to different opportunities for individual farmers even within the same area. While targeted interventions for individual farmers might not be feasible, targeting defined farmer/endowment groups may be a sustainable pathway to increase productivity. Although smallholder farmers in sub-Saharan Africa (SSA) have limited assets, the study revealed opportunities to increase crop productivity without substantial capital inputs, but more efficient use and targeting.

Rising costs and pressure on supplies of commercial mineral fertilizers and increasing markets for organically produced foods and feeds have led to a growing interest in soil amendments to supply plant nutrients. Rockdust is a by-product of quarrying and its effectiveness to supply plants with nutrients has been a contested issue and there have been no assessments of its effect on soil biota other than plants. The aim of this study was to assess the effect of a commercially-available volcanic rockdust application on crop growth and element concentrations for a wide range of macro and microelements and the response of soil microbial communities to rockdust due to the potential alteration in soil mineralogy. A three-year controlled outdoor-growing experiment was conducted on three different soil types with two wheat cultivars in the first year following rockdust application and with forage species in the third year. Our results show that the tested rockdust had no positive or negative effect on plant growth or nutrient composition. In addition, the microbial response to added substrates, a sensitive measure of changes in soil environment, were unaltered by the rockdust. As the rockdust had no nutrient or toxic effect it can probably be considered as an inert material which at least causes no harm but equally has no demonstrable ecological or agricultural benefit.

This study examined whether the N release pattern during decomposition of plant materials used as green manures can be modified in the short term through mixing plant materials with different level...

Nitrogen (N) recycling to the regrowth of mulched red clover (Trifolium pratense L.) and mulched mixed red clover/perennial ryegrass (Lolium perenne L.) leys was determined in field experiments during three consecutive years using 15N-labelled shoot material. Nitrogen recycling was greater in the pure clover stands than in the mixed stands in the beginning of the growing season, but increased successively in the mixed stands so that it was similar (14–15.5%) in both stands at the end of the season. This recycling of N from the mulch led to increased biomass accumulation but did not alter stand composition in the mixed stands. Mulch-derived N was incorporated into the soil organic N in both pure clover and mixed stands which thus contributed to building up soil fertility. An approximately similar proportion of N remained unaccounted for in mulched pure clover and mixed stand leys and presumably represented gaseous losses. To exploit the benefits of green manure leys in the humid temperate zone while minimising the negative environmental impact, these should be harvested rather than mulched.

Mixed ley crops including legumes are important in agriculture, giving high quality forage as well as adding N to the cropping system. These leys are also increasingly being used as green manure crops. Due to the large N 2 fixing capacity of the leguminous crops the N input is large, causing large internal N flows in the ley systems. Nitrogen budgets are thus intricate but important for evaluation of the N efficiency and environmental footprint of the production systems including mixed leys. Three field experiments were established on sandy soils during three consecutive years. Total quantities of N fixed symbiotically by red clover ( Trifolium pratense L.) in pure and mixed (with perennial ryegrass ( Lolium perenne L.)) stands, including N transferred to the grass, were determined under different cutting regimes. Estimates of N transfer from red clover to ryegrass were derived through (a) dilution of 15 N label in microplots of immobilised 15 N-labelled soil, and (b) recovery of 15 N label by direct transfer from leaf-labelled clover plants in mezotrons. The estimated all-season amount of N transferred from red clover to ryegrass determined from isotope dilution was 0.7–6.8 g m −2 , with no significant differences between intact and harvested stands, but mulched stands had more transferred N than harvested stands during the last growth period. All-season N transfer corresponded to an average of 30 and 57% of grass N (in harvested and intact stands, respectively) and 13 and 26% of clover N (in harvested and intact stands, respectively) with an increasing proportion being transferred successively during the growing season. The estimated amounts of N transferred from leaf-labelled clover to the companion ryegrass were 0.4–4.7 g m −2 , i.e. somewhat lower than the isotope dilution estimates. This N corresponded to approximately 16% of grass N and 15% of clover N, irrespective of cutting strategy. The discrepancy between results obtained by the two methods was attributed to leaf fall being prevented in the direct transfer plots, but not in the dilution-based estimates. In addition, the mulch in plots for direct transfer estimates was replaced with non-labelled mulch, whereas the ‘native’ mulch was left in the plots for estimation through dilution. The N transfer contributed strongly to the N budget of the companion ryegrass, especially in the stands where leaf fall contributed to the transfer. The uptake of clover-derived N by a companion grass may have implications for ley composition and feeding value as well as for the effects of mixed green manure and catch crops in agriculture.

Cutting strategy effect on N2 fixation and distribution of fixed N above and below ground in red clover (Trifolium pratense L.) and mixed red clover/perennial ryegrass (Lolium perenne L.) green manure leys was quantified in field experiments including in situ mezotrons and microplots. Symbiotically fixed N in clover, transfer of fixed N to grass in the mixed stands and the fate of 15N contained in mulch were estimated by isotope dilution. Below ground clover-derived N was estimated by leaf labelling. Total N2 fixation was estimated by correcting fixed N in plant shoots with plant-derived N below ground and recycled N from mulch. The total N2 fixation was larger in harvested and mulched stands (average 45 g m−2) than in the intact stands (32 g m−2). Of the fixed N, 53% (intact), 46% (harvested) and 60% (mulched) was found below ground. The average recycling of N in mulch was 21% and contributed 13.7% (pure clover) and 2.2% (mixed) of the clover N in the regrowth. Recycling of N did not decrease N2 fixation in the mulched compared with harvested stands. The results indicate that cutting regime should be considered when estimating total amounts of N fixed by green manure leys.

Leguminous leys are important sources of nitrogen (N), especially in forage-based animal production and organic cropping. Models for estimating total N2 fixation of leys—including below-ground plant-derived N (BGN)—are based on grazed or harvested leys. However, green manure leys can have different proportions of above-ground plant-derived N (AGN) and BGN when subjected to different cutting regimes. To investigate the effects of cutting on N distribution in white clover, a pot experiment was carried out using 15N techniques to determine N2 fixation, N rhizodeposition and root C and N content of cut and uncut white clover (Trifolium repens L. cv. Ramona) plants. Percentage N derived from air (%Ndfa) was lower in uncut (63%) than in cut (72%) plants, but total Ndfa was not significantly affected by cutting. The higher reliance on N2 fixation in cut plants was thus counterbalanced by lower biomass and total N content. With BGN taken into account, total plant-derived N increased by approximately 50% compared with AGN only. Cutting did not affect the proportion of BGN to standing shoot biomass N after regrowth, but decreased the proportion of BGN to total shoot biomass production during the entire growth period. Thus, estimates of N fixation in green manure leys should consider management practices such as cutting regime, as this can result in differences in above- and below-ground proportions of plant-derived N.

The C mineralisation pattern during the early stage of decomposition of plant materials is largely determined by their content of different carbohydrates. This study investigated whether detailed plant analysis could provide a better prediction of C mineralisation during decomposition than proximate analysis [neutral detergent solution (NDF)/acid detergent solution (ADF)]. The detailed analysis included sugars, fructans, starch, pectin, cellulose, lignin and organic N. To determine whether differences in decomposition rate were related to differences in hemicellulose composition, the analysis particularly emphasised the concentrations of arabinose and xylose in hemicelluloses. Carbon dioxide evolution was monitored hourly in soil amended with ten different plant materials. Principal component and regression analysis showed that C mineralisation during day 1 was closely related to free sugars, fructans and soluble organic N components (R2 = 0.83). The sum of non-cellulose structural carbohydrates (intermediate NDF/ADF fraction) was not related to C mineralisation between days 1 and 9. In contrast, a model including starch and protein in addition to the non-cellulose structural carbohydrates, with the hemicelluloses replaced by arabinose and xylose, showed a strong relationship with evolved CO2 (R2 = 0.87). Carbon mineralisation between days 9 and 34 was better explained by xylan, cellulose and lignin (R2 = 0.72) than by lignocellulose in the ADF fraction. Our results indicated that proximate analyses were not sufficient to explain differences in decomposition. To predict C mineralisation from the range of plant materials studied, we propose a minimum set of analyses comprising total N, free sugars, starch, arabinose, xylan, cellulose and lignin.

Many parts of the world are investigating the efficacy of recycling nutrient resources to agriculture from different industry and domestic sectors as part of a more circular economy. The complex nature of recycled products as soil amendments coupled to the large diversity of soil types and their inherent properties make it difficult to optimize the benefits and minimize the risks from potentially toxic elements often present in recycled materials. Here we investigated how wheat grain cadmium (Cd) concentration was affected by soil amendments, namely human urine and biogas digestate compared to traditional farm manures and mineral fertilizers. We show that Cl(-) inadvertently added to soils with e.g. urine or biogas digestate strongly increased crop Cd concentrations, largely by mobilizing inherent soil Cd. This resulted in wheat grain Cd levels that could result in exceeding recommended WHO limits for dietary intake. This was evident even in soils with low inherent Cd content and when Cd inputs were low. The future of a circular economy that helps to underpin global food security needs to ensure that the effects of applying complex materials to different types of agricultural land are fully understood and do not jeopardize food safety.

Applying by-products as soil amendments to agricultural systems is growing in popularity. We aimed to assess the efficacy of some contemporary by-products to provide nutrients to crops as well as the potential harm of adding toxic elements to the environment. Four different by-products widely available in Northern Europe were tested for their effects on two nutrient-poor agricultural soils in terms of increasing available macro- and micro-nutrients as well as toxic elements. Assessing soil microbial community as a sensitive tool for evaluating soil quality was conducted with the focus on microbial activity, carbon metabolism and on Rhizobium/Agrobacterium. Wood ash increased pH and CaEDTA, KEDTA and MgEDTA in the soils. The only increase in EDTA-extractable micronutrients in the soils was observed by applying pot ale, increasing Cu. None of the amendments increased the availability of Pb and Cd in the soils. Soils amended with the by-products thus remained similar to the unamended control but were quite different from fully mineral fertilised soils. There were no detectable adverse effects on the physiological and genetic profiles of microbial communities. The by-products were moderately beneficial and did not change the soil microbial community as much as the fully fertilised treatment with mineral fertilisers. Changes in the microbial community profiles were probably due to direct effects on microbes limited by K, Ca and N as opposed to indirect effects on plant growth. This is potentially significant in understanding how to improve impoverished and marginal soils as microbial activity affects many other ecosystem functions.

Two experiments were carried out on four soils of differing pH, organic C and heavy metal content; in the first the microbial substrate utilization efficiency after growth on glucose was determined, and in the second the microbial maintenance requirement during C limitation. In the first experiment growth characteristics and changes in chloroform-labile total C(Ctot) and ninhydrin-N were determined as well as the fate of the added glucose C(gluC) over 12 d. In the second experiment themicrobial biomass was labelled by additions of 14C-glucose in extremely small amounts so that thebiomass was still subject to C limitation. In the first experiment there was an exponential increase in the rate of CO2 production upon glucose addition, indicating exponential microbial growth. Microbial growth characteristics derived from the respiration curves showed differences between soils, but noconsistent response to low pH or heavy metal stress. The gluC data did not give a valid estimate of the substrate utilization efficiency, but indicated a lower extractability of the glucose-derived biomass C in the (NH4)2SO4- and the sewage sludge-amended soils than in the Ca(NO3)2-fertilized and FYM-amended soils. Changes in biomass C and N were contradictory so that we could not conclude whether or not microorganisms subjected to stress were characterized by a lower substrate utilization efficiency. The totC specific respiration rate during conditions of C-limitation in the second experiment was higher in the (NH4)2SO4-fertilized and in the sewage sludge-amended soil compared to that in the Ca(NO3)2-fertilized and FYM-amended soils, indicating a higher maintenance requirement in the former soils. Respiration of gluC reflected differences in the specific respiration rate. Loss of microbially-incorporated gluC was larger in the (NH4)2SO4- and the sewage sludge-amended soils. The results indicate that higher maintenance requirements may contribute to the lower biomass C-to-soil organic C ratio that characterized the acid- and the heavy metal-contaminated soil.

A number of microbial properties previously shown to be sensitive to heavy metal toxicity were determined in soils from field experiments at Brunnby and Robacksdalen in Sweden. The properties investigated were the acetylene reduction activity (ARA) potential of blue-green algae and heterotrophic soil bacteria, population size of Rhizobium leguminosarum bv. trifolii, size of the microbial biomass, basal respiration, specific respiration rate of the biomass, bacterial community metal tolerance, phospholipid fatty acid pattern of the soil microbial community and lag period and specific microbial growth rate determined after glucose addition to soil. In the Brunnby soils, sewage sludge applications between 1966 and 1989 had increased the soil C content from 2.3 to 2.6% and reduced soil pH from 6.1 to 5.8. Concentrations of Cd, Cr, Cu, Pb and Zn in the soil had increased by up to 76%, but had not reached the current lower EC limits for soils. Most of the measured microbial properties were affected by the sludge additions, although effects were generally moderate. We observed reductions of between 15 and 80% in autotrophic and heterotrophic ARA potential, in numbers of rhizobia and in the biomass C-to-organic C ratio, and increases between 25 and 76% in the specific microbial respiration rate, and in the lag time and the specific microbial growth rate upon glucose addition. There were significant differences in the community structure determined by phospholipid fatty acid (PLFA) patterns between the high sludge treatment and the control and low sludge treatment, and in the bacterial community Cu tolerance between the high sludge treatment and the control. Basal respiration was not significantly affected by past sludge additions. At Robacksdalen, additions of metal salt solutions between 1979 and 1991 had increased soil concentrations of Cd, Cu and Pb by up to 23%, but soil concentrations were nevertheless below the background concentrations at Brunnby. In spite of the low metal concentrations, small, but statistically significant effects of metal addition on the specific respiration rate, lag time before the onset of microbial growth upon glucose addition and on potential autotrophic and heterotrophic ARA were found. The findings are discussed in relation to current legislation for soil protection.