Compost Application Rates Research Articles

Effect of composting of palm oil mill wastes and cow dung or poultry manure on <i>Amaranthus hybridus</i> growth and yield

Objective: To evaluate the effect of shelter and different type of manure on degradation of palm oil mills wastes during composting and on growth and yield of African spinach (Amaranthus hybridus) grown on acrisol .Methodology and results: Palm oil mills wastes were composted, with poultry manure or cow dung with and without shelter. The experimental trials used the split-split plot design with composting method as a principalfactor, the types of manure and rates of compost as secondary factors. The composting of these wastes with manure under shelter improved their decomposition significantly. The compost made with poultry manure undershelter gave the highest plant growth and yield. The compost application rate of 20t/ha was not enough to increase plant growth but increased its yield (20.1t/ha versus 17.9t/ha).Conclusion and application of findings: Palm oils mills waste can be composted and used to grow Amaranthus hybridus. However, it is better to compost these wastes with poultry manure than cow dung under shelter. Inaddition, farmers need to apply at least 20t/ha to have high yields.KEY WORDS: acrisol; African spinach; Guinean climate; manure; shelter,

Effects of composted oil palm bunch wastes and chemical fertilizer on growth of oil palm seedling under water stress condition

A greenhouse study was conducted at the University of Nigeria, Nsukka to compare the effects of sole and integrated use of composted oil palm bunch waste and chemical fertilizer for improving the growth of oil palm seedlings under water stress condition. The experiment was a 3x3x4 factorial in a completely randomized design with three replications, comprising 3 factors thus irrigation intervals, NPKMg compound fertilizer and composted oil palm bunch waste. Water stress was imposed by observing irrigation intervals of 7days, 14days and 21days after irrigating the soil to field capacity. NPKMg (12:12:17:2) fertilizer was applied at 0g, 28g and 42g per seedling/polybag (based on field application rates of 0 Kg/ha, 1400 Kg/ha and 2100 Kg/ha), while the composted bunch waste was applied at 0g, 100g, 200g and 300g per seedling/polybag. Result showed that 14 days irrigation intervals gave the highest (P<0.05) effect on oil palm seedlings growth parameters compared to the 7 and 21 days intervals of irrigation. Similarly, application of 42g NPKMg fertilizer showed the highest (P>0.05) effects on growth parameters of oil palm seedlings relative to other rates of mineral fertilizers. Results also indicated that application 300gcompost/seedlings gave highest (P<0.05) effect on all growth parameters compared to other compost application rates. Interaction between the composted bunch waste, chemical fertilizer and irrigation intervals had no significant effect on all the growth parameters studied. However, combined application of 42g NPKMg compound fertilizer and 300g compost/seedlings was most effective in improving the growth of oil palm seedlings under 14 day intervals of irrigation and is therefore recommended.

The Effect of Water Hyacinth (<i>Eichhornia crassipes</i>) Compost on Tomato (<i>Lycopersicon esculentum</i>) Growth Attributes, Yield Potential and Heavy Metal Levels

The potential of different water hyacinth compost application rates in influencing growth attributes, yield and heavy metal accumulation of lead (Pb), copper (Cu), nickel (Ni) and zinc (Zn) in tomato fruit was studied in Masvingo. Four treatments of water hyacinth compost rates of 0, 37, 55.6 and 74.1 t·ha-1 were each replicated three times and applied in a randomized complete block design set up. Results showed that water hyacinth compost application rates significantly affected plant height, days to maturity and yield but had no influence on the number of tomato fruits per plant. The plant height at application rate of 74.1 t·ha-1 was 25%, 56% and 63% higher than the control at week 6, 9 and 12, respectively. At application rates of 56.6 t·ha-1, plant heights were 11%, 13% and 12% higher than the control whilst marginal plant height differences of -4%, 6% and 4% were recorded between application rate of 34.7 t·ha-1 and the control at week 6, 9 and 12, respectively. Tomato plants under compost rates of 34.7, 56.6 and 74.1 t·ha-1 in comparison to the control delayed maturity by 10, 17 and 20 days, respectively. Yields of 52, 55, 60 and 68 t·ha-1 were realized from hyacinth compost rates of 0, 34.7, 56.6 and 74.1 t·ha-1, respectively. Heavy metal concentrations increased with increase in the water hyacinth compost rate but at all application rates, the average concentrations were 85%, 93% and 86% lower than the Codex Alimentarious Commission permissible levels for Pb, Cu and Zn. Water hyacinth compost at a rate of 74 t·ha-1 therefore can be used for increased tomato yield without exposing consumers to heavy metal toxicity.

밭 토양에서 바닥재와 축분퇴비의 혼합시용이 토양의 중금속 함량 및 효소활성에 미치는 영향

Abstract BACKGROUND: Coal combustion bottom ash(BA) has high carbon and calcium content, and alkaline pH, which might improve nutrient cycling in soil related to microbial enzyme activities as it is used as soil amendment. However, it contains heavy metals such as copper(Cu), manganese (Mn), and zinc(Zn), which could cause heavy metals accumulation in soil. Compost might play a role that stabilize BA. The objective of this study was to evaluate effect of combined application of BA and compost as soil amendment on heavy metals concentration, enzyme activities, chemical properties, and crop yield in upland soil.METHODS AND RESULTS: BA was applied at the rate of 0, 20, 40, and 80 Mg/ha under different rate of compost application (0 and 30 Mg/ha) in radish (Raphanus sativus var) field. Combined application of BA and compost more improved chemical properties such as pH, EC, OM, total nitrogen, available phosphate, and exchangeable cations of soil than single application of BA. Water soluble Mn and Zn concentration in soil significantly decreased with increasing application rate of BA. Decrease in those metals concentration was accelerated with combined application of BA and compost. Urease and dehydrogenase activities significantly increased with increasing application rate of BA. Phosphotase activities were not affected with single application of BA but increased with combined application of BA and compost. Radish yield was not affected by application rate of BA.CONCLUSION: From the above results, combined application of BA and compost could be used as soil amendment to improve chemical properties and enzyme activities of soil without increase in heavy metal concentration and decrease in crop yield in upland soil.Research Article Open Access

Effect of organic fertilization on nutrient concentration and accumulation in nectarine (Prunus persica var. nucipersica) trees: The effect of rate of application

The aim of the present investigation was to evaluate the effects of annual application of three rates of compost on nutrient concentration and accumulation in tree organs, after 11 years of continuous application of a municipal solid waste (MSW) compost in a commercial nectarine orchard. Trees were subjected, since their plantation (2001), to the following MSW compost rate: 0, 5 and 10tdwha−1yr−1, split in May (60%) and September (40%). In 2011/2012 biomass of pruned wood, thinned fruits, fruit pulp and stones at harvest, and abscised leaves were weighted and analyzed for macro (N, P, K, Ca, Mg) and micro (Fe, Mn, Cu, and Zn) nutrients concentration. In addition, leaves and fruit pulp at harvest were analyzed for heavy metal (Cd, Cr, Hg and Pb) concentration. Plant part biomass (with the exception of thinned fruits) was linearly related to compost application rate. The highest rate of compost increased N and decreased Ca and Mg concentration in leaves sampled in summer, whereas no differences among treatments were observed for the other nutrients. At the end of the season leaf N, K and Zn were remobilized to storage plant part, while P, Ca, Mg, Fe and Mn reached the highest values in abscised leaves. Nitrogen, K, Mg, Cu, Mn and Zn accumulation in the plant part analyzed increased as a consequence of the application of compost at 10tha−1 when compared with compost at 5tha−1 and control. Phosphorous accumulation was increased by compost at the highest rate compared with control; no differences were observed for Ca and Fe accumulation. Nitrogen and P were mainly accumulated in fruit, abscised leaves and pruned wood; K was allocated mainly into fruits, while Ca and Mg were found highest in abscised leaves. Concentration of heavy metals in leaves and fruits were not affected by treatments.

Enzymatic Activities in Soil Cultivated with Coffee (Coffea arabica L. cv. ‘Bourbon’) and Amended with Organic Material

The objective of this study was to investigate the growth of coffee (Coffea arabica L. cv. ‘Bourbon’) in soil amended with compost, vermicompost, and bokashi at 25%, 50%, and 66% while mycorrhizal colonization and activity of acid and alkaline phosphatases and urease in soil were monitored. Treatments with 25% compost as well as higher application rates of compost and bokashi (50% or 66%) increased coffee growth. Shoot fresh weight increased 5 times when 25% bokashi or compost was applied, 8 times when 50% was applied and 10 times when 66% was applied compared to the control plants without organic fertilizer. Acid and alkaline phosphatase and urease activity increased when organic fertilizer was applied to soil. Application of organic fertilizers increased the growth and fresh weight of root and shoot of coffee plants and improved the root mycorrizal colonization and enzymatic activity of the soil.

Impacts of compost application on the formation and functioning of arbuscular mycorrhizas

With rising fertilizer costs, and concerns about the environmental impacts of excessive fertilizer use, organic matter is increasingly seen as an important source of nutrients in agriculture. This study investigated the impact of different rates of compost addition on the formation and functioning of arbuscular mycorrhizas (AM). A glasshouse experiment was conducted in which a mycorrhiza defective tomato mutant, and its mycorrhizal wild-type progenitor were grown in soil amended with a municipal greenwaste derived compost, at four rates of application. Impacts on the formation and functioning of AM were quantified. While compost addition at low levels of application had little effect on the formation of AM, at higher rates a small decrease in colonization of roots by AMF was observed. Both AM and compost application had a significant impact on plant growth and/or nutrition. Although the formation of AM had relatively little impact on plant growth, plants grew progressively better with increasing compost supply. In contrast, the formation of AM had a strong positive effect on plant P and Zn acquisition, especially at lower rates of compost application. AM and compost can have an important role in agricultural systems, especially those that place a strong emphasis on biologically regulated nutrient supply systems.

Sewage sludge compost use in bioenergy production – a case study on the effects on Cynara cardunculus L energy crop

Abstract As energy obtained from biomass is expected to significantly contribute to the future renewable energy pool, knowledge regarding energy crops management is crucial in order to maximize productivity while establishing positive environmental synergies. In this sense, the application of waste materials as organic amendments represents an interesting option for the growth of energy crops in Mediterranean regions. This is the first three-year trial (2008–2011) that assesses the effect of different sewage sludge compost treatments, applied as basal dressing, on the productivity the energy crop Cynara cardunculus L. (cynara). The experiment was conducted in Alicante (Southeastern Spain), testing four compost application rates: 0, 30, 50 and 70 t ha−1. Cynara yields (dry basis) were higher in the 2010/11 comparatively with 2008/09 for aboveground biomass (2063 vs 1202 g m−2), seed yield (320 vs 153 g m−2), oil yield (81 vs 33 g m−2) and energy yield (37 vs 21 MJ m−2). Concerning fertilisation treatments, composted sludge exerted positive effects, mainly between 50 and 70 t ha−1 application rates, primarily improving the productivity of cynara: aboveground biomass (2195 g m−2) and energy (38 MJ m−2) yields of plants amended with 70 t ha−1 were 68% higher than control plants, while seeds (308 g m−2) and oil yields (103 g m−2) increased by 40%. Therefore, the use of SSC enhanced cynara's productivity (aboveground biomass and seeds yields primarily). The present research provides valuable data to growers and researchers interested in the maximization of environmental positive synergies (soil protection, waste reuse, bioenergy production), as a way to enhance the sustainability of Mediterranean agrosystems.

Optimization of field scale biopiles for bioremediation of petroleum hydrocarbon contaminated soil at low temperature conditions by response surface methodology (RSM)

Ex-Situ Bioremediation has been increasingly viewed as an appropriate remediation technology for hydrocarbon contaminated soils under cold climates conditions in countries like Canada. A response surface methodology (RSM) based on a factorial design was performed to investigate and optimize the effects of the microbial consortia application rate and amount of mature compost amendment on the TPH removal (964 μg g−1 initial concentration). 18 field-scale biopiles (16 m3 each) were constructed, maintained and subjected to different microbial consortium and mature compost application rates under cold climate conditions over a period of 94 days. TPHs removal rates in the range of 74–82% was observed in the treatments setups where mature compost and microbial consortia were used simultaneously, compared to an average 48% of TPHs removal in control setup.The interaction between these two factors were studied and modelled using a statistical regression model, which showed that the microbial consortia application rate, the mature compost amendment and their interactions had a significant effect on TPHs degradation with a coefficient of determination (R2) of 0.88. Furthermore, using a numerical optimization approach, the optimum rates predicted via RSM were estimated at 4.1 ml m−3 and 7% for microbial consortia and compost application rates to obtain a maximum TPH removal of 90.7%.

Effects of Sewage Sludge Compost on Cynara cardunculus L. Cultivation in a Mediterranean Soil

ABSTRACT The aim of this study was to identify an optimal sewage sludge compost (SSC) application rate to cultivate Cynara cardunculus L. (cynara) in a Mediterranean soil. Five SSC treatments corresponding to 0, 2, 4, 6, and 8 kg compost/m2 (T0, T1, T2, T3, and T4, respectively) were applied to an agricultural plot (South-Eastern Spain). Twelve cynara plants per treatment were grown (January–June). SSC treatments enhanced Cox, Nk, Pburriel, Zn, and Fe. The optimal rates were found between T2 and T3. Cynara leaves’ elemental composition was scarcely affected by SSC. Only Fe increased, differing T2, T3, T4 plants (over 1000 mg Fe/kg) from T0 plants (701 mg Fe/kg). Cynara morphological traits were not affected by SSC treatments. Cynara results did not indicate a clear optimal SSC dose. However, applications between 4 and 6 kg of SSC/m2 are recommended in order to enhance soil organic matter and fertility, factors of special importance in Mediterranean environments.

밭토양에서 퇴비시용과 비닐멀칭이 토양탄소 축적에 미치는 영향

Abstract BACKGROUND: In most studies, soil carbon sequestration has been evaluated simply with change of soil organic carbon content. So far, information regarding stability of soil organic carbon is limited. METHODS AND RESULTS: This study was conducted to determine changes in soil organic carbon (SOC) content and stability of carbon in response to compost application rates and plastic mulching treatment during the hot pepper growing season. Under the pot experiment condition, compost was mixed with an arable soil at rates corresponding to 0, 10, 20, and 40 Mg/ha. To determine effects of plastic mulching on soil carbon sequestration, plastic mulching and no mulching treatments were set up in soils amended with the application rate of 20 Mg/ha. The SOC content did not significantly increase with application of compost and plastic mulching at harvest time. No significant changes in bulk density with compost application and plastic mulching was found. These might result from short duration of experiment. While hot water extractable organic carbon content significantly decreased with compost application and plastic mulching, humic substances increased. Belowground biomass of hot pepper was biggest at the recommended application rate (20 Mg/ha) of compost.

논토양에서 퇴비시용 및 경운이 토양탄소 축적과 안정화에 미치는 영향

So far, most studies associated with soil carbon sequestration have been focused on long term aspect. However, information regarding soil carbon sequestration in short term aspect is limited. This study was conducted to determine changes of soil organic carbon content and stability of carbon in response to compost application rate and tillage management during rice growing season(150 days) in short term aspect. Under pot experiment condition, compost was mixed with an arable soil at rates corresponding to 0, 6, 12, and 24 Mg/ha. To determine effect of tillage on soil carbon sequestration, till and no-till treatments were set up in soils amended with application rate of 12 Mg/ha. Compost application and tillage management did not significantly affect soil organic carbon(SOC) content in soil at harvest time. Bulk density of soil was not changed significantly with compost application and tillage management. These might result from short duration of experiment. While hot water extractable organic carbon(HWEOC) content decreased with compost application, humic substances(HS) increased. Below ground biomass of rice increased with application of compost and till operation. From the above results, continuos application of compost and reduce tillage might improve increase in soil organic carbon content and stability of carbon in long term aspect.

Effects of two composts and two grasses on microbial biomass and biological activity in a salt-affected soil

The effectiveness of compost supply at several doses (0, 50, 100, and 150t/ha) to a saline soil was studied using municipal solid waste (MSW) and palm waste (PW) composts. The experiment was carried out in pots under cultivation of Polypogon monspeliensis (halophyte forage species) and Hordeum vulgare (common forage species) and lasted three months. The investigation focused on some selected soil physico-chemical properties, soil microbial biomass, and ten soil enzymatic activities; Arylsulfatase (ARY), dehydrogenase (DEH), β-glycosidase (β-GLU), protease (PRO), urease (URE), invertase (INV), Fluorescein diacetate hydrolase (FDAH), catalase (CAT), acid and alkaline phosphatases (PHO). Both amendments improve markedly the saline soil quality. They ameliorate the physico-chemical properties. The increase of soil pH is regarded as an interesting fact and is usually proportional to the compost application rate. Electrical conductivity increased proportionally to the applied rates. Soil carbon and nitrogen amounts were also improved and the highest raise (7.5-folds) was noted for carbon. According to the substantial increase of the organic matter, levels of measured microbial biomass and several enzyme activities in saline soil were improved. DEH activity which proposed as a measure of overall microbial activity exhibited a significant increase only at dose 2 (100t/ha). Consequently, One hundred tones of composts per hectare, under which some enzymes exhibited an optimal of activity and metal accumulation can be minimized, appeared an interesting rate for saline soil amendment.

Production and reuse of waste in rural area with high density of greenhouse

Agricultural activities cause the production of considerable amounts of waste sometimes dangerous that must be properly handled to avoid negative impacts on rural areas and on agroecosystems. The estimation of qualitative and quantitative characteristics of agricultural waste products and the capacity of rural land of transposing organic matter deriving from the processes of composting, is a key point for the planning and management of the waste integrated cycle. The aims of this study are the evaluation of the quantities of various types of agricultural waste on territorial scale, the amount of compost that can be used in rural areas affected by different cultures and the effectiveness of community composter in the treatment of vegetable agricultural waste for the production of green composted soil. These assessments were carried out in an area of study characterized by a high spatial density of greenhouses. The methodological procedure used is based on the use of agricultural waste production coefficients and maximum application rates of compost for cultivation. The results show the role and potential of the agricultural areas in the waste cycle from production to the potential reuse of recovered material.

Effects of soil surface management practices on soil and tree parameters in a ‘Cripps Pink’/M7 apple orchard 1. Mineral nutrition

Effects on soil, leaf and fruit element concentrations of organic (compost, straw mulch and hand weeding) and integrated (inorganic fertilisers and herbicide usage; IP) soil surface management practices in the tree rows, in combination with weed covers, cover crops and straw mulch in the work rows, were investigated in a seven-year trial. The trial took place on a gravelly soil in a ‘Cripps Pink’/M7 apple orchard in the Elgin area, South Africa. The organic treatments promoted higher phosphorus (P) and potassium (K) concentrations in the soil, leaves and fruit than the IP treatments. Leaf and fruit nitrogen (N) levels were normal in both the organic and IP treatments. Compost application rates that supply adequate N therefore deliver excessive amounts of P and K. Use of such composts should be based on P and K contents, supplementing where necessary with N. In both the IP and organic treatments, element concentrations were higher in the tree rows than the work rows. Treatments applied to the work rows did not affect soil and tissue element concentrations consistently. Standardisation of compost composition is required, as is balanced delivery of N, P and K.

Influence of compost on the mobility of arsenic in soil and its uptake by bean plants (Phaseolus vulgaris L.) irrigated with arsenite-contaminated water

The influence of compost on the growth of bean plants irrigated with As-contaminated waters and its influence on the mobility of As in the soils and the uptake of As (as NaAsIIIO2) by plant components was studied at various compost application rates (3·104 and 6·104 kg ha−1) and at three As concentrations (1, 2 and 3 mg kg−1). The biomass and As and P concentrations of the roots, shoots and beans were determined at harvest time, as well as the chlorophyll content of the leaves and nonspecific and specifically bound As in the soil.The bean plants exposed to As showed typical phytotoxicity symptoms; no plants however died over the study. The biomass of the bean plants increased with the increasing amounts of compost added to the soil, attributed to the phytonutritive capacity of compost. Biomass decreased with increasing As concentrations, however, the reduction in the biomass was significantly lower with the addition of compost, indicating that the As phytotoxicity was alleviated by the compost. For the same As concentration, the As content of the roots, shoots and beans decreased with increasing compost added compared to the Control. This is due to partial immobilization of the As by the organic functional groups on the compost, either directly or through cation bridging. Most of the As adsorbed by the bean plants accumulated in the roots, while a scant allocation of As occurred in the beans. Hence, the addition of compost to soils could be used as an effective means to limit As accumulation in crops from As-contaminated waters.

Effects of olive solid waste and olive solid waste compost application on soil properties and growth of Solanum lycopersicum

Mediterranean countries are the largest global producers of olive oil. One by-product of this industry, and a major environmental problem, is olive solid waste (OSW). This study aims to explore the potential of OSW and olive solid waste compost (OSWC) for reuse and recycling as an agricultural fertiliser. Typical Sandy and Loamy agricultural soils were augmented with increasing amounts of OSW and OSWC (0, 4, 8 and 10%) in order to investigate the effect on soil characteristics and growth of tomato (Solanum lycopersicum). Plant growth was dramatically reduced by the application of OSW Composting of OSW results in a break-down of toxic compounds. Consequently, augmentation of soils with OSWC enhanced seed-germination, plant and root biomass and total nitrogen content. Application of 4–8% OSWC produced the optimal effect on soil parameters across all treatments, while plant growth subsequently decreased when OSWC application was increased to 10%. These results suggest that composting of OSW has the greatest potential beneficial impacts for the use of this agri-industrial waste as an agricultural fertiliser. To ensure optimal impact on crop production application of OSWC should be within 100–210 tonnes per hectare, and not exceed 260 tonnes, significantly greater than current standard compost application rates.

Soil respiratory and enzyme activities in leachate-contaminated soils with different application rate of cow manure compost: a laboratory study

The experiment was conducted to evaluate the effects of cow manure compost (CMC) application on respiratory and enzyme activities of leachate-polluted soils. Soils treated with various levels of leachate (0, 12.5, 25, 37.5, and 50 ml leachate per 300 g soil) were amended with 0, 25 and 50 g CMC, respectively. Soil pH, respiration and enzyme activities were evaluated by regularly repeated sampling. Addition of CMC neutralized pH and maintained it within the range of 6.4–6.7 in a long run. Respiration was increased neither by leachate nor by CMC. This probably results from deterioration of soil compaction, biotoxicity of leachate, or increased fungi proportion. Enzyme activities increased with increasing CMC application but decoupled with leachate addition rate, reasonably attributed to pH neutralization and high content of humus in CMC. Thus, pH value and humus content could be considered as key items affecting soil enzyme activities.

Composted municipal waste effect on chosen properties of calcareous soil

Abstract A 3-year field study was conducted to assess effects of composted municipal waste on some properties, distribution of Zn, Cu in a calcareous soil and uptake of these metals by wheat. The treatments were 0, 25, 50 and 100 Mg ha-1 of municipal solidwastewhichwas applied in three consecutive years. The application of composted municipal waste increased the saturated hydraulic conductivity, the aggregate stability,the organic carbon content and electrical conductivity, whereas it slightly decreased the soil pH and bulk density. A significant increase in the concentration of Zn and Cu were observed with increasing number and rate of compost application. The distribution of Zn and Cu between the different fractions in untreated and treated soils showed that the majority of Zn and Cu were in the residual form. Finally, the levels of Zn and Cu were higher in grains of wheat grown in composttreated plots compared to that grown in the control plots.

Structure, functions and interguild relationships of the soil nematode assemblage in organic vegetable production

The abundance and metabolic footprints of soil nematodes were quantified during four of eight years of an intensive organic vegetable production system. Treatment variables included cover crop mixtures and compost application rates. The abundances of bacterivore and fungivore nematodes were enhanced by the annual use of winter cover crops but showed no relationship to the level of residual soil organic matter. Metabolic footprints, based on biomass and respiratory activity, were calculated for functional guilds and ecosystem services of the nematode assemblage. The enrichment footprint, representing the ecosystem service of nutrient mineralization, was related to the level of soil organic carbon. It was strongly related to the metabolic footprint of bacterivore nematodes and both were enhanced in treatments that were cover cropped annually. Cover-cropped treatments also had a slightly higher herbivore footprint, suggesting support of some taxa of plant-feeding nematodes. The structure footprint, reflecting the metabolic activity of higher trophic level nematodes, including the predators of opportunistic species, did not differ among cover crop and compost amendment treatments. However, enrichment footprints were correlated with bacterivore footprints, which represented the predominantly bacterivore resources available to predators. Abundance of predators increased as a function of the abundance of those nematode prey that can be amplified by organic inputs. The functional connectance, represented by spatial co-location, between predators and amplifiable prey was greater in treatments with a greater abundance of predators. The functional connectance between predators and herbivore prey, representing a management target, was strongly related to the functional connectance between predators and amplifiable prey. We conclude that cover crops not only affect organisms at the entry level of the web but that resources are also transferred to higher trophic links which increases top-down pressure on plant-parasitic nematodes.