Crop Rotation Trial Research Articles

Cover crop-driven shifts in soil microbial communities could modulate early tomato biomass via plant-soil feedbacks

Sustainable agricultural practices such as cover crops (CCs) and residue retention are increasingly applied to counteract detrimental consequences on natural resources. Since agriculture affects soil properties partly via microbial communities, it is critical to understand how these respond to different management practices. Our study analyzed five CC treatments (oat, rye, radish, rye-radish mixture and no-CC) and two crop residue managements (retention/R+ or removal/R−) in an 8-year diverse horticultural crop rotation trial from ON, Canada. CC effects were small but stronger than those of residue management. Radish-based CCs tended to be the most beneficial for both microbial abundance and richness, yet detrimental for fungal evenness. CC species, in particular radish, also shaped fungal and, to a lesser extent, prokaryotic community composition. Crop residues modulated CC effects on bacterial abundance and fungal evenness (i.e., more sensitive in R− than R+), as well as microbial taxa. Several microbial structure features (e.g., composition, taxa within Actinobacteria, Firmicutes and Ascomycota), some affected by CCs, were correlated with early biomass production of the following tomato crop. Our study suggests that, whereas mid-term CC effects were small, they need to be better understood as they could be influencing cash crop productivity via plant-soil feedbacks.

Exchangeable and Plant-Available Macronutrients in a Long-Term Tillage and Crop Rotation Experiment after 15 Years.

The status of macronutrients phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg) was assessed 15 years after the establishment of a long-term crop rotation and soil tillage trial with mouldboard ploughing (MP), no-till (NT), deep conservation tillage (CTd) and shallow conservation tillage (CTs). The mobile proportions of macronutrients in an Austrian Chernozem soil were determined to a depth of 50 cm with the single reagent extractant acetic acid (AA) and Mehlich 3 (M3), which uses several reagents as extractants. AA revealed less P and K, but more Ca and Mg compared to M3. Both extractants could capture the distribution pattern of the nutrients in the soil profile, but M3 showed higher differences among the soil layers. In the first 5 cm in NT, the P concentration was higher than in MP, CTd and CTs. The concentration of K was higher in NT, CTd and CTs than in MP in the first 10 cm of the soil. Phosphorus and K concentrations did not differ between tillage treatments below these soil layers, and S, Ca and Mg were similar in all soil layers. As none of the analysed elements except for Ca were fertilized and no accumulation of S, Ca and Mg was observed in the upper soil layer, the higher concentrations are attributed to accumulation through crop residues and then less leaching of P and K. Crop rotation did not affect the distribution of the analysed macronutrients in the soil but affected the nutrient uptake by winter wheat mostly due to the yield differences of winter wheat in the two crop rotations.

Effects of farming systems, tillage, and traffic practices on deep drainage and soil salt loads in the Queensland Murray–Darling and Fitzroy Basins using soil chloride

Context Cropping in the Queensland Murray–Darling and Fitzroy Basins has precursors for secondary salinity – high soil salt loads and increased drainage after clearing. Aims To measure changes in deep drainage, for key tillage, traffic, and cropping systems. Methods Steady-state and transient chloride (Cl) mass-balance were applied to Cl profiles from four tillage and crop rotation trials and one controlled traffic trial in southern and central Queensland, to determine deep drainage below the root zone. Key results Large downward movement of Cl occurred after clearing. Deep drainage from transient Cl balance for cropping was a small proportion of rainfall but considerably higher than under native vegetation. Deep drainage was consistently greater under zero than conventional tillage, for both winter and summer cropping. For example, deep drainage was greatest for zero tillage (∼45 mm/year) and least for conventional, stubble mulch, and reduced tillage (2–6.3 mm/year) at the highest-rainfall site (677 mm/year). Deep drainage was 12.7 and 7.9 mm/year for zero and conventional tillage, respectively, at the lowest-rainfall site (497 mm/year). Drainage under continuous wheat conventional tillage was more than twice that where some summer crops were included. At Billa Billa, continuous wheat had greater deep drainage by three to five times than continuous sorghum for three of four tillage systems. No drainage was detected during 6 years of opportunity cropping. A pasture legume ley had only 1.7 mm/year of deep drainage. Deep drainage was less for compacted than non-compacted treatments (23.3 vs 38.2 mm/year). Conclusion and implications Increased deep drainage with zero tillage and controlled traffic can be reduced using summer crops, particularly opportunity cropping where crops are planted when soil water is sufficient, and ley pastures.

Crop rotations differ in soil carbon stabilization efficiency, but the response to quality of structural plant inputs is ambiguous

Evaluate crop rotation diversity, perenniality, carbon (C) inputs, and C input quality as predictors of soil organic carbon (SOC) concentration and aggregate mean weight diameter (MWD). At a crop rotation trial in its 37th year in Ontario, Canada, species in rotations included corn (C, Zea mays L.), alfalfa (A, Medicago sativa L.), soybean (S, Glycine max (L.) Merr.), winter wheat (W, Triticum aestivum L.), and red clover (rc, Trifolium pratense L.). Rotations were: CC, CCAA, CCSS, CCSW, CCSWrc, AA. Soils (0–20 cm) were analyzed for aggregate MWD, aggregate C, and SOC concentrations. We estimated C inputs from historical yields and C input quality (C:N, lignin: N, or NMR-derived index) from structural plant tissues. C stabilization efficiency was estimated as the ratio of SOC stock per unit total or root C input. Crop rotation diversity failed to increase SOC concentrations or aggregate MWD. Perennialized rotations (CCSWrc, CCAA, AA) maintained the numerically highest SOC concentrations, and root C input increased SOC concentration. Out of 12 statistical tests relating C input quality to C stabilization efficiency, only 3 indicated a positive effect and 6 tests indicated a negative effect. Including a perennial forage such as alfalfa and limiting soybean frequency promotes aggregate MWD and SOC concentration more so than optimizing crop rotation diversity. Quality of structural plant inputs does not explain differences in C stabilization efficiency, possibly due to overriding influence of living root inputs.

Quality differences between organic and conventional field grown vegetables – experiences from a Swedish long-term field trial

There is a debate regarding differences in quality between conventionally and organically grown vegetables. Sometimes the arguments are ideology driven rather than fact based. It is complicated to compare trials performed with different cultivars grown under different climate and management conditions. Usually the results are hard to generalize. To get rid of some of the confounding factors, a long-term crop rotation trial has been performed at Onnestad in southern Sweden. A number of conventional and organic rotations have been grown at the same site for more than 25 years (started in 1987). Some of the rotations contain field grown vegetable crops. In order to evaluate the impact on produce quality samples of carrots (Daucus carota) (1 conventional, 2 organic rotations), beet root (Beta vulgaris) (1 conventional, 1 organic rotation), potato (Solanum tuberosum) (2 conventional, 1 organic rotation) and onion (Allium cepa) (1 conventional, 2 organic rotations) were collected in 2014 and 2015. Yield was documented at harvest. Samples were frozen for soluble solids content analysis and freeze-dried for dry matter content determination. In general quality differences were small and inconsistent when the vegetables from the different crop rotations were compared. The largest differences were found between years, but the different crops did not respond in the same way.

Crop Rotational Effects on Yield Formation in Current Sugar Beet Production - Results From a Farm Survey and Field Trials.

In Europe, the framework for sugar beet (Beta vulgaris L.) production was subject to considerable changes and for the future it is expected that sugar beet cultivation might concentrate around the sugar factories for economic reasons. Based on data from a national sugar beet farmers’ survey and multi-year crop rotation trials, the effects of cropping interval (number of years in between two subsequent sugar beet crops) and of preceding crops on sugar yield were elucidated under current Central European management conditions. The dominating sugar beet cropping interval was ≥4 years in the farm survey with pronounced differences between regions. However, the cropping intervals 2, 3, and ≥4 years did not affect the sugar yield. Therefore, significant differences in sugar yield between regions were assumed to be caused by multiple interactions between year, site, and farmers’ skills. Throughout Germany, the dominating preceding crops in sugar beet cultivation were winter wheat (Triticum aestivum L.) and winter barley (Hordeum vulgare L.). In the field trials, the sugar yield was 5% higher after pea (Pisum sativum L.) compared to maize (Zea mays L.) as preceding crop, while differences between the preceding crops pea and winter wheat, and wheat and maize were not significant. Repeated measurements of canopy development and leaf color during the growing season revealed a higher N-availability after pea as preceding crop. However, decreased growth after maize was not completely compensated for by high N-fertilizer doses. Overall, the causes for the differences in sugar yield between the preceding crops remained open. The results do not support concerns about substantial yield losses in sugar beet production due to a reduction in the cropping interval from 3 to 2 years. Nevertheless, short rotations with maize and sugar beet might increase the risk of Rhizoctonia solani crown and root rot infestation. Leguminous crops such as pea offer the potential for higher sugar beet yield with lower N-fertilizer doses.

Do Soil Tests Help Forecast Nitrogen Response in First‐Year Corn Following Alfalfa on Fine‐Textured Soils?

Core Ideas ISNT and PSNT concentrations did not differ among alfalfa stand ages in five long‐term trials. Only PSNT concentration increased during first‐year corn in three of five long‐term trials. The PSNT was less related to soil N mineralization and more related to available N than the ISNT. The PSNT more accurately predicted corn response or nonresponse to fertilizer N than the ISNT. The product of alfalfa stand age and PSNT separated N response in 87% of 15 independent site‐years. Improved methods are needed to predict grain yield response to fertilizer N for first‐year corn (Zea mays L.) following alfalfa (Medicago sativa L.) on fine‐textured soils. Data from 21 site‐years in the North Central Region of the United States were used to determine how Illinois soil nitrogen test (ISNT) and pre‐sidedress soil nitrate test (PSNT) concentrations change during alfalfa–corn rotations and how they relate to mineralized N, available N, and grain yield response to fertilizer N in first‐year corn. In five long‐term crop rotation trials, ISNT and PSNT taken in the alfalfa phase did not change with alfalfa stand age, but PSNT concentration in first‐year corn increased with alfalfa stand age in three of five trials. Across 21 site‐years, the PSNT more accurately predicted corn response or nonresponse to fertilizer N than the ISNT (67 vs. 52% of site‐years). The interaction of PSNT with alfalfa stand age resulted in 87% accuracy when validated with 15 independent site‐years; no N response was likely in first‐year corn when the product of PSNT × stand age exceeded 42. Incubation of soil from first‐year corn showed that the ISNT explained over twice as much variation in soil nitrate‐N accumulation after 17 wk than the PSNT (30 vs. 13%). Overall, the PSNT was less related to soil N mineralization and more related to aboveground corn N uptake, total available N, and grain yield response to fertilizer N in first‐year corn than the ISNT. This research provided a simple method for predicting when N response is likely in first‐year corn after alfalfa, but further research is needed to predict the size and mineralization rate of soil N pools after alfalfa termination to estimate the economically optimum nitrogen rate when fertilizer N response is likely.

Comparison of wheat-based rotation systems and monocropping systems under dryland Mediterranean conditions

Mono-cropping systems consisting of general low-yielding cereals are crucial productivity constraints in dry areas of the Mediterranean region. A crop rotation consisting of quinoa (Chenopodium quinoa Willd.)–wheat (Triticum aestivum L)-chickpea (Cicer arietinum) was compared with monocropping in the Mediterranean region of Turkey. A four year crop rotation trial was set up in Adana, Turkey. A total of nine crop rotation systems of Quinoa (Q), chickpea (L) and wheat (W) were considered in this study. The four year results revealed that there was significant difference in grain and biomass yields, but no significant difference in plant height, harvest index, and 1000 grain weight of wheat among the treatments in crop rotation except number of grains per spike and soil organic matter. Continuous wheat and chickpea-wheat-chickpea-wheat (LWLW) produced significantly greater grain yield in 2010/2011 and 2011/2012. QWLW and QWQW rotation produced significantly lower biomass yield in 2008/2009. The maximum biomass yield of LWLW rotation resulted in 2011/2012 growing season. Therefore, the effect of previous crop on wheat grain and biomass yield in the four years of the rotation study was significantly different. In the long run, the favorable effects of legume-based rotations on crop yields and water productivity are apparent. Keywords: crop rotation system, monocropping system, quinoa, Chenopodium quinoa, chickpea, dryland, yield DOI: 10.25165/j.ijabe.20171005.3443 Citation: Tekin S, Yazar A, Barut H. Comparison of wheat-based rotation systems and monocropping systems under dryland Mediterranean conditions. Int J Agric & Biol Eng, 2017; 10(5): 203–213.

Greater soil carbon stocks and faster turnover rates with increasing agricultural productivity

Abstract. Devising agricultural management schemes that enhance food security and soil carbon levels is a high priority for many nations. However, the coupling between agricultural productivity, soil carbon stocks and organic matter turnover rates is still unclear. Archived soil samples from four decades of a long-term crop rotation trial were analyzed for soil organic matter (SOM) cycling-relevant properties: C and N content, bulk composition by nuclear magnetic resonance (NMR) spectroscopy, amino sugar content, short-term C bioavailability assays, and long-term C turnover rates by modeling the incorporation of the bomb spike in atmospheric 14C into the soil. After > 40 years under consistent management, topsoil carbon stocks ranged from 14 to 33 Mg C ha−1 and were linearly related to the mean productivity of each treatment. Measurements of SOM composition demonstrated increasing amounts of plant- and microbially derived SOM along the productivity gradient. Under two modeling scenarios, radiocarbon data indicated overall SOM turnover time decreased from 40 to 13 years with increasing productivity – twice the rate of decline predicted from simple steady-state models or static three-pool decay rates of measured C pool distributions. Similarly, the half-life of synthetic root exudates decreased from 30.4 to 21.5 h with increasing productivity, indicating accelerated microbial activity. These findings suggest that there is a direct feedback between accelerated biological activity, carbon cycling rates and rates of carbon stabilization with important implications for how SOM dynamics are represented in models.

Comparison of wheat-based rotation systems and monocropping systems under dryland Mediterranean conditions

Mono-cropping systems consisting of general low-yielding cereals are crucial productivity constraints in dry areas of the Mediterranean region. A crop rotation consisting of quinoa (Chenopodium quinoa Willd.)–wheat (Triticum aestivum L)-chickpea (Cicer arietinum) was compared with monocropping in the Mediterranean region of Turkey. A four year crop rotation trial was set up in Adana, Turkey. A total of nine crop rotation systems of Quinoa (Q), chickpea (L) and wheat (W) were considered in this study. The four year results revealed that there was significant difference in grain and biomass yields, but no significant difference in plant height, harvest index, and 1000 grain weight of wheat among the treatments in crop rotation except number of grains per spike and soil organic matter. Continuous wheat and chickpea-wheat-chickpea-wheat (LWLW) produced significantly greater grain yield in 2010/2011 and 2011/2012. QWLW and QWQW rotation produced significantly lower biomass yield in 2008/2009. The maximum biomass yield of LWLW rotation resulted in 2011/2012 growing season. Therefore, the effect of previous crop on wheat grain and biomass yield in the four years of the rotation study was significantly different. In the long run, the favorable effects of legume-based rotations on crop yields and water productivity are apparent. Keywords: crop rotation system, monocropping system, quinoa, Chenopodium quinoa, chickpea, dryland, yield DOI: 10.25165/j.ijabe.20171005.3443 Citation: Tekin S, Yazar A, Barut H. Comparison of wheat-based rotation systems and monocropping systems under dryland Mediterranean conditions. Int J Agric & Biol Eng, 2017; 10(5): 203–213.

State Priekuļi Plant Breeding Institute – A Century of Agricultural Research and Plant Breeding

The State Priekuïi Plant Breeding Institute (previously Wenden, Cçsis or Priekuïi Experimental and Breeding Station) started its operation in 1913. The main aims of research have remained the same for the last century: to provide knowledge on crop management and to create crop varieties suitable to local growing conditions and farming systems, acceptable to consumer requirements. Supply to farmers of high quality seed material of cereals, potato, pea, clover and grasses is an essential part of the scope. Overall, 31 crop species have been involved in a wide range of studies. More than 100 different crop varieties have been bred since the beginning of the 20th century. Potato varieties ‘Brasla’, ‘Agrie Dzeltenie’, winter rye variety ‘Kaupo’, pea varieties ‘Vitra’, ‘Retrija’, barley variety ‘Idumeja’ and several clover and grass varieties are widely grown in farmers’ fields. The first hulless barley variety in the Baltic States, ‘Irbe’, and winter triticale variety ‘Inarta’ have been bred in the Institute recently. Long-term crop rotation trials have been run for more than 50 years. A number of outstanding scientists and agronomists have worked in the Institute: potato breeders E. Knappe and V. Gaujers, cereal breeders J. Lindermanis, M. Gaiíe, and M. Sovere, grass breeders P. Pommers, A. Apinis, and I. Holms, pea breeder M. Vitjaþkova, researchers on crop management R. Sniedze and V. Miíelsons, research manager and director U. Miglavs and others

Effect of permanent bed planting combined with controlled traffic on soil chemical and biochemical properties in irrigated semi-arid Mediterranean conditions

Improving agricultural soil quality in semi-arid regions is necessary for reducing soil erosion and improving water use. Conservation agriculture (CA) can increase soil quality and biodiversity, and reduce operational costs without losing crop productivity both under irrigated or rainfed conditions. However, few studies on soil chemical and biochemical status in irrigated farms under CA in the Mediterranean region are available. Permanent beds with crop residue retention (PB) have been proposed as an alternative to conventionally tilled beds with residue incorporation into the soil (CB). These two soil management systems combined with controlled traffic were compared during two different seasons (2009 and 2010) in a loamy alluvial Typic Xerofluvent soil under a maize (Zea mais L.)–cotton (Gossypium hirsutum L.) crop rotation trial established in 2007 in Southern Spain. Total organic carbon (TOC), water soluble carbon (WSC), Kjeldahl nitrogen (Kjel-N), dehydrogenase (DHA) and β-glucosidase (β-Glu) activities and microbial biomass carbon (MBC) and nitrogen (MBN) were analysed in soil from beds and furrows after crop harvest. Results indicated that Kjel-N, TOC and enzymatic activities were significantly higher in soil from furrows in PB than in CB, but practically no differences were found in soil from the bed zone. Moreover, traffic did not affect chemical and biochemical parameters in spite of its compacting effect. Major differences were found between samplings due to different quantity and nature of the residues (maize vs. cotton). Principal component analysis confirmed that TOC, Kjel-N and β-Glu (and DHA to a less extend) are useful indicators of soil management impact on soil quality in this irrigated Mediterranean conditions; however, this is not the case of WSC, a common indicator in rainfed conditions. Results confirmed that conservation agriculture is the better option to increase soil biological and biochemical quality in irrigated farms under Mediterranean semi-arid conditions.

Energy balance of five fodder cropping systems in the irrigated lowlands of Northern Italy

Extensification has recently become an important option in Western European agriculture, driven both by economic considerations (product surpluses together with the fact that developed countries cropping systems have been heavily relying on fossil energy) and growing public concern on the possible adverse effects of intensive farming on the environment and human health. The adoption of rational fodder crop rotations, with the rediscovery of the beneficial effect of the meadow, is viewed as a possible mean to reduce the impact of farming systems in the lowlands of Northern Italy, characterised by highly intensive cropping and animal husbandry. For this reason our study examines the effects of crop rotation on the energy balance during 1985-2007 period in a long-term crop rotation trial in Northern Italy comparing five fodder crop systems, different in the degree of crop intensification and for the presence or absence of the meadow: a 1-year continuous cereal double cropping (R1); a 3-year rotation (R3); a 6-year rotation (R6); a permanent meadow (PM); and a continuous grain maize cropping (CM). Each rotation was subjected to two input treatments, defined as high (mostly used in lowlands of northern Italy) and low (input reduction of ca. 30%) respectively, in terms of nutrient levels, herbicide doses, and soil tillage methods. The crop rotations exerted a marked influence on the energy balance. The most efficient rotations in terms of net energy production energy efficiency have been characterized by reduced length and presence of maize and catch-crops.

Forage and Food Legumes in a Multi-Year, Wheat-Based Rotation Under Drought-Stressed Conditions in Northern Syria’s Medium Rainfall Zone

Long-term crop rotation trials were designed to assess sustainability of alternatives to traditional fallow and monocropping. The trial described here (6 years) involved wheat (Triticum aestivum L.) in rotation with lentil (Lens culinaris L.), forage vetch (Vicia sativa), pasture medic (Medicago spp.), fallow and watermelon (Citrullus vulgaris). Barley (Hordeum vulgare L.) was compared with wheat for an additional 2 years. Cereal grain and straw yields were highest with fallow and watermelon followed by vetch, lentil and medic; the latter showed no differential effect of variable grazing intensity. Fertilizer N increased yields except in the low-rainfall years (less than 250 mm). Barley out-yielded wheat in terms of grain, but not straw. Medic yielded highest in dry matter, whereas lentil produced highest seed yield. Despite the difficulty of assessing crop and animal-oriented rotations because of non-commonality of outputs, economic considerations are foremost, but other benefits of rotations (soil quality, water-use relations) are also relevant to the overall assessment of cereal-based Mediterranean rotations. The study suggested barley rather than wheat as the desired cereal in rotation with legumes in this marginal-rainfall (350 mm) environment and provided support for the viability of vetch and lentil in the cropping system. Given the importance of sheep in the region's farming system, vetch is likely to have a major role in crop rotations.

Significance of the 50-year-old long-term experiments in Martonvásár in improving crop production

Long-term experiments are indispensable for investigations on the long-term effects of various crop production methods and technologies. The long-term experiments set up in Martonvásár by Béla Győrffy are now 50 years old and can be considered as part of the national heritage. The most important of these experiments involve crop rotation vs. monoculture trials, the comparison of fertilisation systems, studies on the interactions and carry-over effects of organic and mineral fertilisers, fertiliser rate experiments and polyfactorial experiments. The long-term experiments in Martonvásár form an integral part of maize and wheat research and provide a place for testing the agronomic responses of maize hybrids and wheat varieties. Valuable scientific results are obtained from these experiments regarding the reasons for yield depression in monocultures, the yieldincreasing effect of crop rotations, the comparative benefits of organic and mineral fertilisation, the agronomic responses of genotypes, the sustainability and yield stability of crop production techniques, and the interaction between various crop production factors. These results promote the improvement of maize and wheat production and are regularly incorporated into recommendation systems. The present generation of scientists has a responsibility to maintain these experiments, so that they can continue to serve their purpose in the coming decades.

Effect of mineral and organic fertilization on soil fertility as well as on the biomass production and N utilization of winter wheat (Triticum aestivum L.) in a long-term cereal crop rotation experiment (IOSDV)

In this trial the effect of mineral and organic fertilization was investigated on the biomass production (grain and straw), N balance, N uptake of winter wheat and some indices of N utilization (N harvest index %, N use efficiency, agronomic efficiency, apparent recovery efficiency) as well as soil fertility in a long-term field experiment. The bifactorial cereal crop rotation (w. wheat, w. barley, maize) trial was set up in 1984 in Keszthely, Hungary, as a part of the international mineral and organic nitrogen fertilization trial network (IOSDV). The statistical significance of the experimental factors (increasing rates of N fertilizer and organic fertilization) was tested by analysis of regression and variance. As a result of the experimental treatments it can be concluded that the increasing rate of N fertilization as well as the additional organic fertilizers influenced the biomass production and N uptake of winter wheat or soil fertility. The Corg content of soil increased significantly with the incorporation of both farmyard manure and straw. Thus, straw as a byproduct should be recycled into the soil mainly if there is no animal husbandry in the farm to preserve the C balance.

Seasonal Changes in Soil Organic Matter and Biomass and Labile Forms of Carbon as Influenced by Crop Rotations

Recent concerns about the role of carbon (C) in climate change and the implications about soil organic matter (SOM) for sustainable use of soils have underlined the need to examine the role of SOM in cropping systems, particularly in fragile ecosystems. Accordingly, we examined the changes that occur in total SOM and in its more reactive fractions, labile and biomass C, within a long‐term, cereal‐based crop rotation trial. The rotations were wheat (Triticum turgidum var durum) grown after vetch (Vicia sativa), medic (Medicago sativa), chickpea (Cicer arietinum), lentil (Lens culinaris), fallow, a summer crop, melon (Citrullus vulgaris), and after wheat (i.e., continuous wheat). Secondary treatments involved nitrogen (N) fertilizer application and variable grazing intensity. Uncropped microplots were established in the main rotation plots, and in the fallow and medic ones with variable grazing. Total SOM and labile and microbial biomass C were periodically measured in the rotations throughout the cropping season. Medic and vetch were highest in the three C forms, with fallow always lowest. All forms changed with sampling time throughout the season. Organic matter decreased from 1.48% in February to 1.15% in August after cropping. Although labile C followed a similar pattern, with a large falloff between the May and August sampling, biomass C increased initially, remained stable for a few months, and decreased at the last two samplings. Although all three C forms were highest in the zero‐grazing in the fallow and medic rotations, the effect of grazing was not significant. Thus, although organic C can be built up in the soil to varying extents depending on the crop rotation, it is a dynamic entity, especially the labile and biomass fractions, having implications for crop growth and soil quality.

Characterization of Rhizoctonia spp. Recovered from Crop Plants Used in Rotational Cropping Systems in the Western Cape Province of South Africa.

Isolates of Rhizoctonia spp. associated with barley, canola, clover, lucerne, lupin, annual Medicago spp. (medic), and wheat were recovered during the conduct of a 4-year (2000 to 2003) crop rotation trial in the Western Cape province of South Africa. These isolates were characterized by determining their anastomosis group (AG), in vitro optimum growth temperature, and pathogenicity toward emerging and 14-day-old seedlings of all the aforementioned crops. During the 4-year rotational trial, 428 Rhizoctonia isolates, in all, were obtained. The most abundant multinucleate AG was AG-4 HG-II (69%), followed by AG-2-1 (19%), AG-3 (8%), AG-2-2 (2%), and AG-11 (2%). The population of binucleate Rhizoctonia spp. comprised AG-K (53%), AG-A (10%), AG-I (5%), and unidentified AGs (32%). The optimal time for isolating Rhizoctonia spp. was found to be at the flowering or seedpod stage (20 to 22 weeks after planting). Temperature studies showed that isolates belonging to AG-2-2, AG-4 HG-II, and AG-K had significantly higher optimum growth temperatures than those from other AGs. In pathogenicity assays conducted on emerging as well as 14-day-old seedlings, isolates of AG-2-2 and AG-4 HG-II were the most virulent on all crops. Rhizoctonia solani AG-2-1 was highly virulent on canola, moderately virulent on medic and lupin, weakly virulent on lucerne and barley, and nonpathogenic on wheat. AG-11 isolates were moderate to weakly virulent on all crops, with the exception of barley and wheat. AG-3 was weakly virulent on canola, lupin, and medic. AG-K was the only binucleate Rhizoctonia sp. capable of inciting disease in our assays. This is the first comprehensive study to elucidate the identity and potential importance of Rhizoctonia spp. as a yield limiting factor in crop production systems in the Western Cape province of South Africa.

Crop Rotation with Forage Pearl Millet for Control of Root-Lesion Nematodes in On-Farm Trials with Potato

ABSTRACT In Canada, root-lesion nematode (Pratylenchus penetrans Cobb) (RLN) is a major soil pest in many crops. Crop damage is reduced by controlling the RLN populations below economic threshold level by soil fumigation, rotation with non-susceptible crops and growing RLN-re-sistant varieties. Agriculture Environmental Renewal Canada has developed a forage pearl millet (Pennisetum glaucum L.) variety “CFPM 101” (Canadian Forage Pearl Millet 101) adapted to Eastern Canadian growing conditions. On-farm crop rotation trials with CFPM 101 and corn (Zea mays) were conducted in commercial potato (Solanum tuberosum L.) fields in Ontario. In the following year, potato was grown. RLN populations were monitored three times (Spring, Summer, and Fall 2000) during the rotation crop and two times (Spring and Summer 2001) during RLN susceptible potato crop. Rotation crop, season, and crop Xyear interaction significantly affected the RLN levels. Soil RLN population did not change in rotation year 2000 and susceptible (p...

Decomposition of 13C and 15N labelled plant residue materials in two different soil types and its impact on soil carbon, nitrogen, aggregate stability, and aggregate formation

Increasing soil organic matter (SOM) is a major factor in overcoming soil degradation. An incubation experiment using 2 soil types (Red Clay and Black Earth) and 2 different rotations, a clover (Trifolium subterraneum)/cereal rotation and a long fallow/cereal rotation, from a long-term crop rotation trial located at Tamworth, NSW, Australia was conducted to investigate the decomposition of 3 different plant materials, medic (Medicago truncatula) (C : N = 13), rice straw (Oryza sativa) (C : N = 25) and flemingia leaf (Flemingia macrophylla) (C : N = 13), labelled with 13C and 15N. A control treatment with no added residue was also included. The impact of the residue decomposition on total organic carbon, labile carbon, total nitrogen, aggregate stability and the formation of large macro-aggregates from smaller macro-aggregates were studied. Total C (CT), stable carbon isotope composition (δ13C), total N (NT), and %15N excess were measured by catalytic combustion and an isotope ratio mass spectrophotometer, while labile C (CL) was determined by oxidation with KMnO4. Aggregate stability [mean weight diameter (MWD)] was determined by immersion wet sieving. Correlations of C fractions with MWD were also investigated. The location of the newly added plant residue materials within soil aggregates was studied using a soil aggregate eroding machine. Loss of C from the added plant residues was highest for the medic and lowest for the flemingia, while the rice straw initially lost C at a slower rate but by 200 days was equal to the medic. The medic treatment was the only residue to lose N by gaseous loss during the experiment and it was all lost during the first 10 days. In both soils, the addition of residues increased CT and CL compared with the control treatment, with flemingia showing the greatest increase. Factors other than their C : N ratio were clearly determining C turnover. Addition of medic residues resulted in a rapid increase in MWD in both soils in the first 10 days compared with that at the commencement of the experiment. However, this was not maintained for the 200 days by which time MWD had decreased, but it was still greater than the starting point. By contrast, the addition of flemingia leaf exhibited a slower but more sustained increase to have the highest MWD at 200 days, equal to that of the medic treatment at 10 days. There was a positive correlation of CL with MWD at 200 days for both soils. Results from the soil aggregate eroding machine showed that a higher percentage of CT was derived from added plant residues in the outer one-third of the soil aggregates than in the inner two-thirds, with the greatest difference being for the flemingia treatment. There was no difference between different residue materials in the amount of CT derived from the added residues in the inner parts of soil aggregates. These results showed that soil macro-aggregates were forming around a central old aggregate by binding of smaller aggregates to it, with products formed as a result of the breakdown of plant residues binding them together. From the results obtained, and those of other researchers, a concept of macro-aggregate formation under different agricultural systems is proposed.