Food and habitat preferences of the earthworm Lumbricus terrestris L. for cover crops

Some of the widely used cover crops in temperate agroecosystems (including species of the Brassicaceae and Poaceae) have been shown to exhibit allelopathic effects. In particular, various Brassicaceae have been reported to act as biofumigants against a variety of soil-borne pests through the release of i.a. isothiocyanates (ITC) from glucosinolate precursors. Although these allelochemicals may potentially reduce the need for application of synthetic pesticides, their effects on beneficial soil ecosystem engineers such as earthworms (Lumbricidae) have not been assessed yet.Food choice chambers were used to assess short-term food preferences of Lumbricus terrestris L. for lacy phacelia (Phacelia tanacetifolia BENTH.), the Poaceae Italian ryegrass (Lolium multiflorum LAM.) and oats (Avena sativa L.), and the Brassicaceae yellow mustard (Sinapis alba L.) and rapeseed (Brassica napus L.) while litter bags were used to study long-term litter preference under field conditions. Habitat preference of L. terrestris was determined through simple habitat preference units.Ryegrass residues were a preferred food resource, both in fresh and partly decomposed state, over mustard, phacelia or rapeseed residues, and these were in turn more fed on than oats. Litter disappeared at a fast and variable rate from litter bags under field conditions. No clear relationships with residue C:N ratio were observed. Habitats in which living oat plants were present were avoided in comparison to habitats with bare soil or yellow mustard plants.In conclusion, brassicaceous cover crops are not preferred nor avoided by L. terrestris compared to the other cover crops in this study. Decreased ITC release and earthworm exposure under field conditions may further lower the risk of harmful effects, but further research on the long-term impact is needed.     

Influence of cover crops on potential nitrogen availability to succeeding crops in a Southern Piedmont soil

Winter cover crops are essential in conservation tillage systems to protect soils from erosion and for improving soil productivity. Black oat (Avena strigosa Schreb) and oilseed radish (Raphanus sativus L.) could be useful cover crops in the southeastern USA, but successful adoption requires understanding their influence on N availability in conservation tillage systems. Black oat and oilseed radish were compared to crimson clover (Trifolium incarnatum L.) and rye (Secale cereale L.) for biomass production and effects on N mineralization during the summer crop growing season from fall 1998 through summer 2002 near Watkinsville, GA. Rye produced 40 to 60% more biomass, although N contents were less than the other cover crops. Oilseed radish and black oat N contents were similar to crimson clover. Black oat, oilseed radish, and crimson clover C/N ratios were less than 30, whereas rye averaged 39. Amount of N mineralized in 90 days (N _min90) measured with in situ soil cores was 1.3 to 2.2 times greater following black oat, crimson clover, and oilseed radish than following rye. No differences in N _min90 were found between black oats, crimson clover, and oilseed radish in 1999 and 2000. The amount of potentially mineralizable N (N ₀) was not different due to cover crop, but was 1.5 times greater in 2000 and 2002 than in 1999. The rate of N mineralization (k) was 20 to 50% slower following rye than the other three cover crops. Black oat and oilseed radish biomass production and soil N mineralization dynamics were more similar to crimson clover than to rye, which indicates that they could be used as cover crops in the southeast without significant changes in N recommendations for most crops.

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Release of carbon and nitrogen from fodder radish (Raphanus sativus) shoots and roots incubated in soils with different management history

Plant roots are generally considered to decompose slower than shoots and contribute more to accumulation of soil organic matter, and management history is expected to shape the structure and function of decomposer communities in soil. Here we study the effect of chemical characteristics of shoots and roots from fodder radish (Raphanus sativus oleiformis L.), a widely used cover crop, on the release of their C and N after addition to soil. Shoots and roots were incubated for 180?d at 20°C using four soils with different management histories (organic versus mineral fertiliser, with and without use of cover crops), and the release of CO₂ and extractable mineral N was determined. More shoot C than root C was mineralised during the first 10?d of incubation. After 180?d, 58% of the C input was mineralised with no difference between shoots and roots. At the end of incubation, shoots had released more N (42% of shoot N) than roots (28% of root N). Moreover, management history did not affect net mineralisation of added plant C. Residues incubated in soil with a management history involving cover crops showed an enhanced net N mineralisation. Therefore, long-term decomposition of C added in radish shoots and roots is unaffected by differences in chemical characteristics or soil management history. However, the net mineralisation of N in shoots is faster than for N in roots, and net N mineralisation of added materials is higher in soil with than without a history of cover crops.

Abbreviations: CC: cover crop; IF: inorganic fertilizer; M: manure     

Effect of Catch Cropping on Phosphorus Bioavailability in Comparison to Organic and Inorganic Fertilization

ABSTRACT

Phosphorus (P) is a limited resource and its efficient use is a main task in sustainable agriculture. In a 3-year field experiment the effects of catch cropping [oil radish (Raphanus sativus), buckwheat (Fagopyrum esculentum), serradella (Ornithopus sativus), ryegrass (Lolium westerwoldicum), and phacelia (Phacelia tanacetifolia)] of organic fertilization (cattle manure and biowaste compost) and of inorganic fertilization (Triple-Superphosphate) on plant and soil parameters were investigated on a P-poor loamy sand in Northeast Germany. The catch crops were sown in September and remained on the plots until next spring. Then the main crops oilseed rape (Brassica napus), spring barley (Hordeum vulgare), or spring wheat (Triticum aestivum) were cultivated. The yield and P uptake of the main crops were determined. Furthermore, in the soil the organic matter content, pH, phosphorus (P) in soil solution (Psol), double-lactate and oxalate P content, P sorption capacity, and degree of P saturation were measured. All applied forms of fertilizer affected the P contents in soil and the yields and P uptakes of main crops. For green fertilization especially phacelia was found to contribute to the P supply of the main crops, since it increased the P uptake as well as the P contents in soil significantly. The cultivation of ryegrass led to a reduction of the P availability in soil. For example, in average of the three years the Psol content was 0.35 mg L^{? 1}when phacelia was cultivated and 0.22 mg L^{? 1} when ryegrass was cultivated. The cultivation of phacelia had a comparable effect on soil and plant parameters as the organic and mineral fertilization. An improved P availability and P utilization by catch cropping can reduce the need for external P input which may help to save the limited P resources worldwide.     

Cover crop effects on soil structure and early sugar beet growth

Cover crops can improve soil properties, especially soil structure, through organic matter input and rooting activity. However, large variations exist among cover crops, which may lead to differences in the extent of these effects. In this study, cover crops with differing properties were compared regarding soil structure and subsequent sugar beet growth. Field experiments were conducted at two Luvisol sites in Central Germany. Four cover crops (oil radish, saia oat, spring vetch and winter rye) were compared with fallow. Cover crop effects on soil water, N_min content, soil structure and subsequent early sugar beet growth were studied. Additionally, sugar beet received either no or optimal N fertilizer application. Rye and radish had the highest and vetch the lowest above- and belowground biomass. Soil water content was hardly affected by cover cropping, while topsoil N_min contents in April were increased. Penetration resistance was lowered, and aggregate stability was increased by the cover crops, especially oil radish, while values after spring vetch were similar to those of fallow. Differences among the cover crops might be because of a differing root biomass. Independent of N fertilizer application, sugar beet biomass in May tended to be higher after all cover crops, in particular under oil radish. The higher aggregate stability and lower penetration resistance were found to be beneficial for early sugar beet growth. Thus, sugar beet can benefit from a 1-year cultivation of preceding cover crops. Modifications of this effect through cover crop root biomass and architecture as well as repeated cover cropping need to be investigated in further studies.     

Cover crop growth and impact on N leaching as affected by pre‐ and postharvest sowing and time of incorporation

In Northern Europe, cover crops are traditionally established before spring crops by undersowing, but some cover crops might also have an effect if preharvest sown before spring crops and even winter crops. The effects of cover crop sowing date, sowing technique and succeeding main crop on biomass production, N uptake, nitrate leaching and soil inorganic N were tested in lysimeters and in the field. Cruciferous cover crops (oil radish, white mustard) were sown preharvest by broadcasting into winter wheat in July and were allowed to grow until a following winter wheat was established in September. Other preharvest cover crops were left in place until late autumn. For comparison, the same cruciferous cover crops were established postharvest after light harrowing. Perennial ryegrass undersown in spring barley was also included. Aboveground N uptake in preharvest cover crops amounted to a maximum of 24 kg N/ha in September before sowing winter wheat. When left until late autumn, preharvest oil radish took up a maximum of 66 kg N/ha, and ryegrass and postharvest cover crops 35 kg N/ha. Preharvest establishment of cruciferous cover crops before a spring‐sown crop thus seems promising. The soil was depleted of inorganic N to the same extent in late autumn irrespective of cover crop type, sowing time and technique within winter wheat or spring barley. However, the reduction in nitrate leaching of preharvest cover crops incorporated after 2 months and followed by winter wheat was only half of that achieved by cover crops left until late autumn or spring.     

Growth of legume and nonlegume catch crops and residual‐N effects in spring barley on coarse sand

The aim of this experiment was to investigate the growth and residual‐nitrogen (‐N) effects of different catch‐crop species on a low–N fertility coarse sandy soil. Six legumes (white clover [Trifolium repens L.], red clover [Trifolium pratense L.], Persian clover [Trifolium resupinatum L.], black medic [Medicago lupulina L.], kidney vetch [Anthyllis vulneraria L.], and lupin [Lupinus angustifolius L.]), four nonlegumes (ryegrass [Lolium perenne L.], chicory [Cichorium intybus L.], fodder radish [Raphanus sativus L.], and sorrel [Rumex Acetósa L.]), and one mixture (rye/hairy vetch [Secale cereale L./Vicia villosa L.]) were tested in a field experiment with three replicates in a randomized block design. Four reference treatments without catch crops and with N application (0, 40, 80, and 120 kg N ha^–1) to a succeeding spring barley were included in the design. Due to their ability to fix N₂, the legume catch crops had a significantly larger aboveground dry‐matter production and N content in the autumn than the nonlegumes. The autumn N uptake of the nonlegumes was 10–13 kg N ha^–1 in shoots and approx. 9 kg ha^–1 in the roots. The shoot N content of white clover, black medic, red clover, Persian clover, and kidney vetch was 55–67 kg ha^–1, and the root N content in white clover and kidney vetch was approx. 25 kg ha^–1. The legume catch crops, especially white and red clover, seemed to be valuable N sources for grain production on this soil type and their N fertilizer–replacement values in a following unfertilized spring barley corresponded to 120 and 103 kg N ha^–1, respectively. The N fertilizer–replacement values exceeded the N content of shoots and roots.     

Nitrogen mineralization and availability of mixed leguminous and non-leguminous cover crop residues in soil

Whereas non-leguminous cover crops such as cereal rye (Secale cereale) or annual ryegrass (Lolium multiflorium) are capable of reducing nitrogen (N) leaching during wet seasons, leguminous cover crops such as hairy vetch (Vicia villosa) improve soil N fertility for succeeding crops. With mixtures of grasses and legumes as cover crop, the goal of reducing N leaching while increasing soil N availability for crop production could be attainable. This study examined net N mineralization of soil treated with hairy vetch residues mixed with either cereal rye or annual ryegrass and the effect of these mixtures on growth and N uptake by cereal rye. Both cereal rye and annual ryegrass contained low total N, but high water-soluble carbon and carbohydrate, compared with hairy vetch. Decreasing the proportion of hairy vetch in the mixed residues decreased net N mineralization, rye plant growth and N uptake, but increased the crossover time (the time when the amount of net N mineralized in the residue-amended soil equalled that of the non-amended control) required for net N mineralization to occur. When the hairy vetch content was decreased to 40% or lower, net N immobilization in the first week of incubation increased markedly. Residue N was significantly correlated with rye biomass (r=0.81, P<0.01) and N uptake (r=0.83, P<0.001), although the correlation was much higher between residue N and the potential initial N mineralization rate for rye biomass (r=0.93, P<0.001) and N uptake (r=0.99, P<0.001). Judging from the effects of the mixed residues on rye N Concentration and N uptake, the proportion of rye or annual ryegrass when mixed with residues of hairy vetch should not exceed 60% if the residues are to increase N availability. Further study is needed to examine the influence of various mixtures of hairy vetch and rye or annual ryegrass on N leaching in soil. Received: 10 March 1997     

Effects of cereal roots on detachment rates of single- and double-drilled topsoils during concentrated flow

Farmers in Europe want to control soil erosion in ways that are easily incorporated in their normal practices. We have investigated the possibility of reducing soil erosion by concentrated flow (i.e. rill and gully erosion) through increasing the root density of cereal crops. In situ root density measurements on cereal fields were combined with laboratory flume experiments on samples, taken in single‐ and double‐drilled fields, of which the above‐ground biomass was clipped. During the laboratory experiments no significant effect of root densities on critical shear stress or channel erodibility was observed because of interactions with other changing parameters (e.g. ageing effects). Therefore, the expected relative detachment rates as a function of plant root density were calculated using an empirical equation. During the first 75 days of the crop growth season relative soil detachment rates for single‐drilled field parcels can be reduced up to 50% compared with a rootless field, whereas relative soil detachment rates in double‐drilled field parcels can be reduced up to 60% in this period. Thereafter, plant roots in double‐drilled field parcels reduce relative soil detachment rates on average by 9% compared with single‐drilled field parcels (up to an absolute maximum of 90% compared with rootless soils). During the growing season, not only root density increases but also the vegetation cover changes, which enhances soil protection from erosion. Therefore, cereal roots will help to conserve the soil when seed is drilled at double rates, especially during the early growth stages and in fields with medium risk of concentrated flow.     

Nitrogen uptake,nitrate leaching and root development in winter‐grown wheat and fodder radish

Early seeding of winter wheat (Triticum aestivum L.) has been proposed as a means to reduce N leaching as an alternative to growing cover crops like fodder radish (Raphanus sativus L.). The objective of this study was to quantify the effect of winter wheat, seeded early and normally, and of fodder radish on N dynamics and root growth. Field experiments were carried out on a humid temperate sandy loam soil. Aboveground biomass and soil inorganic N were determined in late autumn; N uptake and grain yield of winter wheat were measured at harvest. Nitrate leaching was estimated from soil water samples taken at 1 m depth. Root growth was measured late autumn using the core break and root washing methods. Winter wheat root growth dynamics were followed during the growing season using the minirhizotron method. The 2013–2014 results showed that early seeding of wheat improved autumn growth and N uptake and reduced N leaching during the winter compared with the normal seeding time. Early‐seeded wheat (WW_early) was, however, not as efficient as fodder radish at reducing N leaching. Proper establishment of WW_early was a prerequisite for benefiting from early seeding, as indicated by the 2012–2013 results. Early seeding improved root growth throughout the 2013–2014 growing season compared with normal seeding time, but had no significant effect on crop grain yield. Our results indicate the potential of using early seeding as a tool to limit drought susceptibility and increase nutrient uptake from the subsoil.     

Application of white mustard and oats in the phytostabilisation of soil contaminated with cadmium with the addition of cellulose and urea

Purpose

Determination of the effectiveness of white mustard and oats in immobilising cadmium as a soil contaminant and determining the role of cellulose and urea in restoring homeostasis in soil under pressure from Cd²⁺.

Materials and methods

Soil samples were contaminated with cadmium (CdCl₂·2¹/₂H₂O) at 0, 4, 8 and 16 mg Cd²⁺ kg^?1. In order to reduce the negative impact of Cd²⁺, cellulose was introduced to the soil at the following rates: 0 and 15 g kg^?1 and urea at 80 and 160 mg N kg^?1. The yield of the above-ground parts and roots was determined on days 40 and 80 of the experiment, along with the cadmium content in the plant material. The enzyme activity was also determined, and the physical and chemical properties of the soil were determined on the day of the oats’ (aftercrop) harvest.

Results and discussion

Contamination of soil with Cd²⁺ at 4 to 16 mg kg^?1 d.m. of soil reduced the yield of white mustard and oats. The tolerance index (TI) values indicate that oats (aftercrop) is more tolerant than white mustard of soil contamination with Cd²⁺. Cadmium accumulated more intensely in roots compared with the above-ground parts of the plants. The translocation index (TF) indicates smaller Cd²⁺ translocation from roots to above-ground parts, as it was below 1 in both plants. An addition of cellulose and nitrogen offsets the adverse impact of cadmium on plants. Arylsulphatase was the most sensitive to soil contamination with Cd²⁺, followed by dehydrogenases, catalase, β-glucosidase and urease, and alkaline phosphatase and acid phosphatase were the least sensitive. Contamination of soil with Cd²⁺ changed its physical and chemical properties only slightly.

Conclusions

White mustard and oats have phytostabilisation potential with respect to soil contaminated with cadmium. Cellulose introduced to the soil and fertilisation with urea alleviated the negative impact of cadmium on the growth and development of plants.

     

Root growth conditions in the topsoil as affected by tillage intensity

Many studies have reported impeded root growth in topsoil under reduced tillage or direct drilling, but few have quantified the effects on the least limiting water range for root growth. This study explored the effects of tillage intensity on critical soil physical conditions for root growth in the topsoil. Samples were taken from a 7-year tillage experiment on a Danish sandy loam at Foulum, Denmark (56°30′ N, 9°35′ E) in 2008. The main crop was spring barley followed by either dyer's woad (Isatis tinctoria L.) or fodder radish (Raphanus sativus L.) cover crops as subtreatment. The tillage treatments were direct drilling (D), harrowing 8-10 cm (H), and ploughing (P) to 20 cm depth. A chisel coulter drill was used in the H and D treatments and a traditional seed drill in the P treatment. Undisturbed soil cores were collected in November 2008 at soil field moisture capacity from the 4-8 and 12-16 cm depths.We estimated the critical aeration limit from either 10% air-filled porosity (ε_a) or relative gas diffusivity (D/D₀) of 0.005 or 0.02 and found a difference between the two methods. The critical limit of soil aeration was best assessed by measuring gas diffusivity directly. Root growth was limited by a high penetration resistance in the D and H soils (below tillage depth). Poor soil aeration did not appear to be a significant limiting factor for root growth for this sandy loam soil, irrespective of tillage treatment. The soil had a high macroporosity and D/D₀ exceeded 0.02 at field capacity. Fodder radish resulted in more macropores, higher gas diffusivity and lower pore tortuosity compared to dyer's woad. This was especially important for the H treatment where compaction was a significant problem at the lower depths of the arable layer (10-20 cm depth). Our results suggest that fodder radish could be a promising tool in the amelioration of soil compaction.     

Cover cropping influences physico-chemical soil properties under direct drilling soybean

In seeking effective methods to prevent soil degradation, conservation tillage plays an important protective role. Apart from significantly reducing production costs, cover crops contributes to beneficial changes in the soil environment. A three-year field experiment included three cover crops (winter rye, winter oilseed rape, and white mustard) subjected to mulching or desiccation and to the action of a herbicide at three rates (100%, 75%, and 50%). The study evaluated soil moisture and the content of organic matter, phosphorus, potassium, and magnesium in two soil layers (0–15?cm and 15–30?cm). Cover cropping had a positive effect on soil organic matter content. More organic matter (by 4.7%) was recorded in the topsoil layer (0–15?cm). Among the cover crops most favorable effect on the content of organic matter in the soil had white mustard (an increase of 14.2%) compared to the control. Moreover, rye and white mustard mulch increased the soil content of phosphorus and magnesium, while oilseed rape mulch increased the potassium content. At the critical growth stages (the flowering/pod set) of soybean (Glycine max (L.) Merril), soil moisture was dependent on mulching treatment and soil layers.     

The release and fate of nitrogen from catch-crop materials decomposing under field conditions

The release and fate of nitrogen from ¹⁵N-labelled perennial ryegrass (Lolium perenne L.) and white mustard (Sinapis alba L.) catch crops were studied in field microplots. The initial decline in ¹⁵N-labelled organic N, after incorporation of the material in early December, was more rapid from mustard containing 2.6% N than from ryegrass containing 1.4% N. After 9 months of decomposition, the residual organic ¹⁵N from the two materials declined at the same rate; the average decay constant for the following 2 years of decomposition was 0.30 a^?1. After 33 months of decomposition, 23% and 34% of the mustard and ryegrass ¹⁵N, respectively, was recovered in organic residues in the topsoil. Seven per cent of the ryegrass N was leached below 45 cm in micro-lysimeters during the winter following incorporation. Three spring barley (Hordeum vulgare L.) crops, which succeeded ryegrass incorporation, accumulated 19%, 4% and 2%, respectively, of the ryegrass N in the above-ground plant parts. Perennial ryegrass swards recovered a total of 26% of the ryegrass and 22% of the mustard catch-crop N within 2 years. After 2 years of decomposition in unplanted soil, 82% of the ryegrass N was accounted for. The ¹⁵N that was not accounted for may be present in the 10–45 cm depth, or it may have been lost by denitrification.     

Long-term winter cover cropping effects on corn (Zea mays L.) production and soil nitrogen availability

 This study was conducted to determine effects of long-term winter cover cropping with hairy vetch, cereal rye and annual ryegrass on soil N availability and corn productivity. From 1987 to 1995, with the exception of the first year of the study, the cover crops were seeded each year in late September or early October after the corn harvest and incorporated into the soil in late April or early May. Corn was seeded 10 days to 2 weeks after the cover crop residues had been incorporated, and N fertilizer was applied as a side-dressing at rates of 0, 67, 134, or 201 kg N ha^–1 each year. While the average annual total N input from the above-ground biomass of the cover crops was highest for hairy vetch (72.4 kg N ha^–1), the average annual total C input was highest for cereal rye (1043 kg C ha^–1) compared with the other cover crops. Hairy vetch was the only cover crop that significantly increased pre-side-dressed NO₃ ^–-N (N_i) corn biomass and N uptake at 0 N. At an N fertilizer rate of 134 kg N ha^–1 or higher, the cover crops had a minimal effect on corn biomass. This indicated that even after 9 years of winter cover cropping, the effect of the cover crops on corn growth resulted primarily from their influence on soil N availability. The amount of available N estimated from the cover crops (N_ac) was significantly correlated with relative corn biomass production (r ²=0.707, P<0.001). The total amount of available N, comprising N_ac and N added from fertilizer (N_f), was strongly correlated (r ²=0.820, P<0.001)) with relative corn biomass production. The correlation was also high for the available N comprising N_i and N_f (r ²=0.775, P<0.001). Although cereal rye and annual ryegrass did not improve corn biomass production in the short term, they benefited soil organic N accumulation and gradually improved corn biomass production compared with the control over the long term. Received: 10 August 1999     

Role of Cover Crops and Planting Dates for Improved Weed Suppression and Nitrogen Recovery in No till Systems

ABSTRACT

Cover crops improve the recovery and recycling of nitrogen and impart weed suppression in crop production. A two-year study with six weekly plantings of cover crops including non-winterkilled species (hairy vetch, Vicia villosa L.; winter rye Secale cereale L.) and winterkilled species (oat, Avena sativa L.; forage radish, Raphanus sativus L.) were assessed for effects on growth of forage rape (Brassica napus L.) and weed suppression. Early planting of cover crops gave the highest biomass and highest nitrogen accumulation. Delaying planting from early-September to mid-October suppressed cover-crop biomass by about 40%. Forage radish produced more biomass in the fall than other cover crops but was winter killed. Spring biomass was highest with rye or vetch. All cover crops suppressed weeds, but suppression was greatest under rye or hairy vetch. Hairy vetch accumulated the largest nitrogen content. Forage rape plants yielded more biomass after a cover crop than after no-cover crop.     

Populations of Pseudomonas solanacearum biovar 3 in naturally infested soil

Populations of Pseudomonas solanacearum biovar 3 were monitored in a clay loam soil sampled from the root zone of infected tomato plants during 1978, 1979 and 1980. Soil numbers increased during symptom development and declined with the death of infected plants. The decline in population size in the soil was continuous where no cover crop was planted between the autumn and spring crops. This decline in population size was interrupted, however, following the planting of an oats cover crop numbers decreased with the ploughing under of the oats. Rainfall was associated with high soil numbers but soil temperature did not appear to directly affect population size. Soil populations in the root zone of susceptible tomato plants cultivar Floradel reached a maximum 1000-fold greater than in soil from the root zone of a resistant line. P. solanacearum survived in bare fallow soil for 21 months. Tomatoes planted 2 months later wilted rapidly.     

Effects of cover crops sown in autumn on N and P leaching

A field experiment with separately tile-drained plots was used to study the ability of oilseed radish (Rhaphanus sativus L.), as a cover crop sown after harvest of a main crop of cereals or peas, to reduce nitrogen (N) and phosphorus (P) leaching losses from a clay loam in southern Sweden over 6 years. In addition to oilseed radish in pure stand, two cover crop mixtures (hairy vetch (Vicia villosa) and rye (Secale cereale) for 3 years and oilseed radish in mixture with buckwheat (Fagopyrum esculentum) for 2 years) were tested. The cover crop plots (three replicates per treatment) were compared with unplanted plots as a control. Plots cropped with oilseed radish during autumn (August–November) had significantly smaller yearly mean N concentration in drainage water over 5 of 6 years compared with unplanted controls. Mineral N content in the soil profile in autumn was significantly less in oilseed radish plots than for control plots in all years. The cover crop mixtures of hairy vetch and rye or buckwheat and oilseed radish also showed the potential to reduce soil mineral N in autumn and N concentration in drainage water, compared with unplanted controls. The cover crops had no impact on P leaching. In conclusion, oilseed radish has the ability to reduce leaching losses of N, without increasing the risk of P leaching.     

Arbuscular mycorrhizal fungi-mediated nitrogen transfer from vineyard cover crops to grapevines

Cover crops are often planted in between vineyard rows to reduce soil erosion, increase soil fertility, and improve soil structure. Roots of both grapevines and cover crops form mutualistic symbioses with arbuscular mycorrhizal (AM) fungi, and may be interconnected by AM hyphae. To study nutrient transfer from cover crops to grapevines through AM fungal links, we grew grapevines and cover crops in specially designed containers in the greenhouse that restricted their root systems to separate compartments, but allowed AM fungi to colonize both root systems. Leaves of two cover crops, a grass (Bromus hordeaceus) and a legume (Medicago polymorpha), were labeled with 99 atom% ¹⁵N solution for 24 h. Grapevine leaves were analyzed for ¹⁵N content 2, 5, and 10 days after labeling. Our results showed evidence of AM fungi-mediated ¹⁵N transfer from cover crops to grapevines 5 and 10 days after labeling. N transfer was significantly greater from the grass to the grapevine than from the legume to the grapevine. Possible reasons for the differences between the two cover crops include lower ¹⁵N enrichment in legume roots, higher biomass of grass roots, and/or differences in AM fungal community composition. Further studies are needed to investigate N transfer from grapevines to cover crops and to determine net N transfer between the two crops throughout their growing seasons, in order to understand the significance of AM fungi-mediated interplant nutrient transfers in the field.     

Cover crop identity determines root fungal community and arbuscular mycorrhiza colonization in following main crops

Cover crops (CC) can promote nutrient retention and recycling for main crops yet may also promote soilborne pathogens or suppress beneficial root symbionts such as arbuscular mycorrhizal fungi (AMF). We investigated how root fungal communities of main crop are affected by preceding CC monocultures and mixtures and by main crop identity. We expected that AMF abundance and diversity in main crops are promoted by AM-host CC, and suppressed by non-AM-host CC, and that mixtures of CC species can promote beneficial and suppress pathogenic root fungi. Our full-factorial field experiment comprised crop rotation in sand soil with different CC treatments (monocultures of radish [AM non-host], ryegrass, clover, vetch [AM hosts], mixtures of radish + vetch, ryegrass + clover and fallow) and two main crops (oat and endive). At peak crop growth, we investigated the root fungal communities in the main crops using microscopy and high throughput sequencing (Illumina MiSeq). Cover crop identity was of prime importance and CC legacy overruled main crop identity in determining root fungal communities in main crops. Compared with fallow, CC with ryegrass increased AMF colonization and richness in both main crops and of non-AMF in oat. Legacies of ryegrass, ryegrass + clover and vetch resulted in distinct root fungal communities in the main crops, while the legacy of CC with radish were similar to the legacy of fallow. Root fungal community in crops after clover had highest abundance of representative fungal pathogens in contrast with the other CC treatments that resulted in fungal communities where pathogens were scarce. Oppositely to expected, CC mixtures did not enhance fungal symbionts or suppressed pathogens. Overall, fungal communities in roots of the main crops in our field experiment were determined by the preceding CC species in monoculture, rather than by the CC AMF preference or functional group. This research highlights that the choice of CC determines the root fungal community in main crop which may influence crop quality.