Recycling of nitrogen in herbivore feces: plant recovery,herbivore assimilation,soil retention,and leaching losses

Herbivores directly and indirectly affect ecosystem functioning in forests. Feces deposition is a direct effect that supplies ephemeral N pulses to soils. Herbivore-mediated changes in plant N allocation and uptake are indirect effects that can also influence soil N availability. These effects may interact if defoliation influences the ability of plants to recover fecal N, and this may affect subsequent generations of herbivores. We added ¹⁵N-enriched insect feces (frass) to a series of replicated red oak, Quercus rubra, mesocosms that had been damaged experimentally and then followed the frass N over the course of 2 years. In the first season, some frass N was mineralized in the soil and leached in organic form from the mesocosms within 1 week of deposition. Within 1 month, frass N had been acquired by the oaks and enriched the foliage; late-season herbivores assimilated the frass N within the same growing season. In the second season, herbivore damage from the previous year lowered total leaf N contents and ¹⁵N recovered in the foliage. A subsequent cohort of early-season herbivores fed on this foliage consequently derived less of their N from the previous year’s frass, and feral leaf rollers colonized fewer of these saplings. The 0- to 5-cm soil fraction was the largest N sink measured, and 42% of the frass N was recovered in the soil. The results demonstrate that: (1) some frass N can be recycled rapidly into foliage and assimilated by successive cohorts of herbivore within the same season; (2) damage can affect N allocation in the following year’s foliage, influencing N availability to and host selection by herbivores; and (3) leaching losses occur soon after deposition but are buffered by soil pools, which are the largest sinks for frass N.     

Effects of alternative prey on predation by small mammals on gypsy moth pupae

Previous work shows that predation by small mammals is a dominant cause of mortality of low-density gypsy moths in North America and that declines in small mammal density result in increases in gypsy moth density. Here we examined whether predation by small mammals is density dependent by way of a type III functional response, and how predation is influenced by alternative prey. First we showed that the preference of predators for gypsy moth pupae was low compared to other experimental prey items, such as mealworm pupae and sunflower seeds. Predation on gypsy moth pupae was characterized by a type II functional response with percent predation highest at the lowest prey densities, whereas the functional response to sunflower seeds was characterized by a type III functional response in which predation increased with increasing prey density. These results suggest that predation by small mammals is unlikely to stabilize low-density gypsy moth populations.     

Inorganic Nitrogen Derived from Foraging Honey Bees Could Have Adaptive Benefits for the Plants They Visit

In most terrestrial ecosystems, nitrogen (N) is the most limiting nutrient for plant growth. Honey bees may help alleviate this limitation because their feces (frass) have high concentration of organic nitrogen that may decompose in soil and provide inorganic N to plants. However, information on soil N processes associated with bee frass is not available. The objectives of this work were to 1) estimate the amount of bee frass produced by a honey bee colony and 2) evaluate nitrogen mineralization and ammonia volatilization from bee frass when surface applied or incorporated into soil. Two cage studies were conducted to estimate the amount of frass produced by a 5000-bee colony, and three laboratory studies were carried out in which bee frass, surface-applied or incorporated into soil, was incubated at 25^oC for 15 to 45 days. The average rate of bee frass production by a 5,000-bee colony was estimated at 2.27 to 2.69 g N month⁻¹. Nitrogen mineralization from bee frass during 30 days released 20% of the organic N when bee frass was surface applied and 34% when frass was incorporated into the soil. Volatilized NH₃ corresponded to 1% or less of total N. The potential amount of inorganic N released to the soil by a typical colony of 20,000 bees foraging in an area similar to that of the experimental cages (3.24 m²) was estimated at 0.62 to 0.74 g N m⁻² month⁻¹ which may be significant at a community scale in terms of soil microbial activity and plant growth. Thus, the deposition of available N by foraging bees could have adaptive benefits for the plants they visit, a collateral benefit deriving from the primary activity of pollination.     

Soil C and N dynamics within a precipitation gradient in Mediterranean eucalypt plantations

Soil C and N dynamics were evaluated in five eucalypt plantations within a precipitation gradient (500–2,000 mm) in Portugal. Soil physical and chemical properties, total and labile (particulate organic matter, hydrolyzable, hot water soluble and microbial) soil C and N pools, and C and N mineralization were measured to characterize the C and N dynamics and their controlling factors within this gradient. Contents of total and labile soil organic C and N were positively correlated with the mean annual precipitation. A similar relationship was observed for net N mineralization (anaerobic and long-term aerobic incubation), gross N mineralization (¹⁵N isotope dilution technique) and C mineralization. In contrast, rates of C and N mineralization (per unit of C and N) were higher in the driest sites due to their higher proportion of particulate organic matter C. Net and gross N mineralization were strongly correlated and showed similar controlling factors (mean annual precipitation, total and labile C and N and extractable P contents), suggesting that net N mineralization during long-term aerobic incubation reflects gross N transformations. Although, gross NO₃–N production and gross NO₃–N immobilization were observed in all sites, net nitrification in the drier sites was not observed in the first weeks of the study. Our results suggest that, under Mediterranean conditions, mean annual precipitation is the major factor determining the C and N dynamics in soils with Eucalyptus plantations.     

Soil carbon and nitrogen mineralization following deposition of insect frass and greenfall from forests under elevated CO2 and O3

Elevated CO₂ and O₃ alter tree quality and the quality of herbivore inputs, such as frass, to forest soil. Altered quality or quantity of herbivore inputs to the forest floor can have large impacts on belowground processes. We collected green leaves and frass from whitemarked tussock moth caterpillars from aspen-birch stands at the Aspen Free Air CO₂ Enrichment (FACE) site near Rhinelander, WI, USA. Small or large quantities of frass, greenfall, or a 1:1 ratio of frass and greenfall were added to microcosms for each FACE treatment (control, +CO₂, +O₃, +CO₂+O₃). We measured initial frass and greenfall quality, and recorded microbial respiration, and nitrate leaching over 40 days. Elevated carbon dioxide (eCO₂) and tropospheric ozone (eO₃) significantly altered the carbon, nitrogen, and condensed tannin content of insect frass and green leaves. Although FACE treatments affected input quality, they had minimal effect on microbial respiration and no effect on nitrogen leaching. In contrast, input quantity substantially influenced microbial respiration and nitrate leaching. Respiratory carbon loss and nitrate immobilization were nearly double in microcosms receiving large amounts of herbivore inputs than those receiving no herbivore inputs. Small amounts of herbivore inputs, however, did not significantly alter microbial respiration or immobilization, suggesting that effects of herbivore inputs on soil processes will be detected only at moderate to high herbivory/input levels. These results suggest that subtle changes in frass and greenfall quality may not affect soil nutrient cycling. In contrast, environmental change induced increases in insect population size or frass and greenfall inputs to the soil may substantially impact nutrient cycling.     

Spatial analysis of gypsy moth populations in Sardinia using geostatistical and climate models

• 1 Spatial fluctuations of the Sardinian population of the gypsy moth Lymantria dispar (L.) (Lepidoptera: Lymantriidae) were characterized using geostatistical and climate models. Data on gypsy moth egg mass abundance recorded at 282 permanent monitoring sites from 1980 to 2004 were incorporated in a geographic information system with the vegetational, geomorphological and pedological features of the sites.
• 2 Statistical analyses revealed that the relative outbreak frequency was related to the predominant host tree, slope and elevation of the monitoring sites, whereas there was no correlation between outbreak frequency and exposure and soil type.
• 3 By using bioclimatic modelling, probability maps of gypsy moth outbreaks were generated. The model identified a probability surface with climatic conditions favourable to gypsy moth outbreaks and thus potentially subject to defoliation. The maps included 92 sites where outbreaks never occurred, suggesting that the Sardinian climate may not be a determinant factor for gypsy moth outbreaks.
• 4 The geostatistical method cokriging with outbreak frequency as a covariate was found to be the most suitable technique to estimate gypsy moth egg mass abundance. Semivariograms showed spatial correlation of egg mass abundance within the range 18.5–53 km. The results obtained were used to create regional gypsy moth distribution maps by cokriging, which demonstrated the outbreak foci and different infestation levels at each monitoring area. These results can help to delimit the treatment areas and develop rational gypsy moth management programmes.

     

European field collections and Canadian releases ofCeranthia samarensis (Dipt.: Tachinidae), a parasitoid of the gypsy moth

A programme to collect, import and release into Canada the gypsy moth parasitoid,Ceranthia samarensis (Diptera: Tachinidae) is described. The parasitoid's potential for biological control in Canada is also discussed. The parasitoid was collected in Europe by exposing experimental gypsy moth larvae in areas where local gypsy moth populations were at low densities. Following field exposure, the host larvae were returned to the laboratory and parasitoids reared from them. This technique has shown thatC. samarensis is the suffers 7–16% hyperparasitism. From 83–90% of theC. samarensis typically enter diapause as pharate adults within the puparia. Laboratory tests of post-exposure host rearing conditions indicate that constant temperatures disrupt the normal parasitoid diapause and that this effect can not be offset by use of either static long or short photoperiods or natural daylengths. Shipping and cold-storage procedures for puparia are described. Post-storage time to emergence of adultC. samarensis decreased with longer cold storage periods and with higher post-storage incubation temperatures. Emergence requires 112 degree-days above a threshold of 8°C after a period of at least 8 months cold storage. Releases of adultC. samarensis into field cages at four locations in southern Ontario are documented. While dissection of host larvae from the field cages has failed so far to demonstrate evidence of parasitism, we remain hopeful that some establishment of the parasitoid has occurred.      

Virulence and fitness of the fungal pathogen Entomophaga maimaiga in its host Lymantria dispar, for pathogen and host strains originating from Asia, Europe, and North America

In this study, we tested (1) whether non-North American gypsy moth strains are susceptible to North American isolates of Entomophaga maimaiga and (2) the potential for erosion in the efficacy of E. maimaiga in controlling gypsy moth. We used bioassays to assess the variability in virulence (measured as time to death) as well as fitness of the pathogen (measured as spore production) in four gypsy strains challenged with six E. maimaiga isolates, using host and pathogen strains originating from Asia, Europe, and North America. We found that all E. maimaiga isolates tested were pathogenic to all strains of Lymantria dispar, regardless of the geographical origin of the fungal isolate, with at least 86% mortality for all combinations of fungal isolate and gypsy moth strain. We therefore conclude that Asian gypsy moths are susceptible to North American strains of E. maimaiga. No significant interactions between fungal isolates and gypsy moth strains with regard to time to death were found, indicating that each fungal isolate had the same overall effect on all the gypsy moth strains tested. However, fungal isolates differed significantly with regard to virulence, with a Russian isolate being the slowest to kill gypsy moth (5.1+/-0.1 days) and a Japanese isolate being the overall fastest to kill its host (4.0+/-0.1 days). Fungal isolates also differed in fitness, with variability in types of spores produced. These differences in virulence and fitness were, however, not correlated with geographical origin of the fungal isolate. Gypsy moth strains had no or only little effect on fungal virulence and fitness. Based on our studies with laboratory-reared gypsy moth strains, erosion of successful control of gypsy moth by E. maimaiga seems unlikely.     

Intestinal modifications of oak leaf tannins by Lymantria dispar L. (Lep., Lymantriidae) and possible effect on larval development

The mode of action of polyphenols within an insect is not well understood and it would be useful to investigate the behaviour pattern of single phenolic compounds within the gut of an insect in this context. In the present study, the phenolic composition of the food of the gypsy moth, Lymantria dispar , was compared with that of its frass by using high-performance liquid chromatography. The main difference was a shift in the concentration of phenolics in the frass with short retention times to those with longer ones, which, additionally, resulted in the appearance of new phenolic compounds. These new phenolics were hydrolysable tannins because after a cleavage via hydrolysation the hydrolysate contained high amounts of gallic acid. The appearance of these new phenolics in the frass of the larvae influenced growth negatively, as there was a negative correlation (P < 0.05) between their concentrations and the growth percentages of the larvae.     

Microbial responses to P addition in six South African forest soils

Forests growing on highly weathered soils are often phosphorus (P) limited and competition between geochemical and biological sinks affects their soil P dynamics. In an attempt to elucidate the factors controlling the relative importance of these two sinks, we investigated the relationship of between soil microbial growth kinetics and soil chemical properties following amendments with C, N and P in six South African forest soils. Microbial growth kinetics were determined from respiration curves derived from measurements of CO₂ effluxes from soil samples in laboratory incubations. We found that microbial growth rates after C + N additions were positively related to NaOH-extractable P and decreased with soil depth, whereas the lag time (the time between substrate addition and exponential growth) was negatively related to extractable P. However, the growth rate and lag time were unrelated to the soil’s sorption properties or Al and Fe contents. Our results indicate that at least some of the NaOH-extractable inorganic P may be biologically available within a relatively short time (days to weeks) and might be more labile than previously thought. Our results also show that microbial utilization of C + N only seemed to be constrained by P in the deeper part of the soil profiles.     

Climate suitability and management of the gypsy moth invasion into Canada

The gypsy moth has become established throughout southern Canada east of Lake Superior where the climate is suitable for the completion of its univoltine life cycle. The spread of the gypsy moth to the north and west in Canada has so far been prevented by climatic barriers and host plant availability as well as by aggressive eradication of incipient populations. Climate change is expected to increase the area of climatic suitability and result in greater overlap with susceptible forest types throughout Canada, especially in the west. At the same time, the gypsy moth is spreading west in the USA into states bordering western Canadian provinces. These circumstances all lead to a greatly increased risk of further invasion into Canadian forests by the gypsy moth. Management actions need to be intensified in different ways in different parts of the country to reduce the impacts of spread in eastern Canada and to prevent the gypsy moth from invading western regions.     

Microbial dynamics and carbon and nitrogen cycling following re-wetting of soils beneath two semi-arid plant species

Sporadic summer rainfall in semi-arid ecosystems can provide enough soil moisture to drastically increase CO₂ efflux and rates of soil N cycling. The magnitudes of C and N pulses are highly variable, however, and the factors regulating these pulses are poorly understood. We examined changes in soil respiration, bacterial, fungal and microfaunal populations, and gross rates of N mineralization, nitrification, and NH₄⁺ and NO₃^– immobilization during the 10 days following wetting of dry soils collected from stands of big sagebrush (Artemisia tridentata) and cheatgrass (Bromus tectorum) in central Utah. Soil CO₂ production increased more than tenfold during the 17 h immediately following wetting. The labile organic C pool released by wetting was almost completely respired within 2–3 days, and was nearly three times as large in sagebrush soil as in cheatgrass. In spite of larger labile C pools beneath sagebrush, microbial and microfaunal populations were nearly equal in the two soils. Bacterial and fungal growth coincided with depletion of labile C, and populations peaked in both soils 2 days after wetting. Protozoan populations, whose biomass was nearly 3,000-fold lower than bacteria and fungi, peaked after 2–4 days. Gross N mineralization and nitrification rates were both faster in cheatgrass soil than in sagebrush, and caused greater nitrate accumulation in cheatgrass soil. Grazing of bacteria and fungi by protozoans and nematodes could explain neither temporal trends in N mineralization rates nor differences between soil types. However, a mass balance model indicated that the initial N pulse was associated with degradation of microbial substrates that were rich in N (C:N <8.3), and that microbes had shifted to substrates with lower N contents (C:N =15–25) by day 7 of the incubation. The model also suggested that the labile organic matter in cheatgrass soil had a lower C:N ratio than in sagebrush, and this promoted faster N cycling rates and greater N availability. This study provides evidence that the high N availability often associated with wetting of cheatgrass soils is a result of cheatgrass supplying substrates to microbes that are of high decomposability and N content.     

Induced plant defenses breached? Phytochemical induction protects an herbivore from disease

Although wound-induced responses in plants are widespread, neither the ecological nor the evolutionary significance of phytochemical induction is clear. Several studies have shown, for example, that induced responses can act against both plant pathogens and herbivores simultaneously. We present the first evidence that phytochemical induction can inhibit a pathogen of the herbivore responsible for the defoliation. In 1990, we generated leaf damage by enclosing gypsy moth larvae on branches of red oak trees. We then inoculated a second cohort of larvae with a nuclear polyhedrosis virus (LdNPV) on foliage from the damaged branches. Larvae were less susceptible to virus consumed on foliage from branches with increasing levels of defoliation, and with higher concentrations of gallotannin. Defoliation itself was not related to any of our chemistry measures. Field sampling supported the results of our experiments: death from virus among feral larvae collected from unmanipulated trees was also negatively correlated with defoliation. In 1991, defoliation and gallotannin were again found to inhibit the virus. In addition, gallotannin concentrations were found to be positively correlated with defoliation the previous year. Compared with previous results that demonstrated a delecterious effect of induction on gypsy moth pupal weight and fecundity, the inhibition of the virus should confer an advantage to the gypsy moth. Since leaf damage levels increase as gypsy moth density increases, and since leaf damage inhibits the gypsy moth virus, there is the potential for positive feedback in the system. If phytochemical induction in red oak can inhibit an animal pathogen such as LdNPV, it suggests to us that induction in red oak is a generalized response to tissue damage rather than an adaptive defense against herbivores.     

Storage of Resting Spores of the Gypsy Moth Fungal Pathogen, Entomophaga maimaiga

The fungal pathogen, Entomophaga maimaiga causes epizootics in populations of the important North American forest defoliator gypsy moth ( Lymantria dispar ). Increasing use of this fungus for biological control is dependent on our ability to produce and manipulate the long-lived overwintering resting spores (azygospores). E. maimaiga resting spores undergo obligate dormancy before germination so we investigated conditions required for survival during dormancy as well as the dynamics of subsequent germination. After formation in the field during summer, resting spores were stored under various moisture levels, temperatures, and with and without soil in the laboratory and field. The following spring, for samples maintained in the field, germination was greatest among resting spores stored in plastic bags containing either moistened paper towels or sterile soil. Resting spores did not require light during storage to subsequently germinate. In the laboratory, only resting spores maintained with either sterile or unsterilized soil at 4°C (but not at 20 or -20°C) germinated the following spring, but at a much lower percentage than most field treatments. To further investigate the effects of relative humidity (RH) during storage, field-collected resting spores were placed at a range of humidities at 4°C. After 9.5 months, resting spore germination was highest at 58% RH and no resting spores stored at 88 or 100% RH germinated. To evaluate the dynamics of infections initiated by resting spores after storage, gypsy moth larvae were exposed to soil containing resting spores that had been collected in the field and stored at 4°C for varying lengths of time. No differences in infection occurred among larvae exposed to fall-collected soil samples stored at 4oC over the winter, versus soil samples collected from the same location the following spring. Springcollected resting spores stored at 4°C did not go into secondary dormancy. At the time that cold storage of soil containing resting spores began in spring, infection among exposed larvae was initiated within a few days after bringing the soil to 15°C. This same pattern was also found for spring-collected resting spore-bearing soil that was assayed after cold storage for 2-7 months. However, after 31-32 months in cold storage, infections started 14-18 days after soil was brought to 15°C, indicating a delay in resting spore activity after prolonged cold storage.     

Compound effects of induced plant responses on insect herbivores and parasitoids: implications for tritrophic interactions

1. Induced plant responses can affect herbivores either directly, by reducing herbivore development, or indirectly, by affecting the performance of natural enemies. Both the direct and indirect impacts of induction on herbivore and parasitoid success were evaluated in a common experimental system, using clonal poplar trees Populus nigra (Salicales: Salicaceae), the gypsy moth Lymantria dispar (L.) (Lepidoptera: Lymantriidae), and the gregarious parasitoid Glyptapanteles flavicoxis (Marsh) (Hymenoptera: Braconidae). 2. Female parasitoids were attracted to leaf odours from both damaged and undamaged trees, however herbivore‐damaged leaves were three times more attractive to wasps than undamaged leaves. Parasitoids were also attracted to herbivore larvae reared on foliage and to larval frass, but they were not attracted to larvae reared on artificial diet. 3. Prior gypsy moth feeding elicited a systemic plant response that retarded the growth rate, feeding, and survival of gypsy moth larvae, however induction also reduced the developmental success of the parasitoid. 4. The mean number of parasitoid progeny emerging from hosts fed foliage from induced trees was 40% less than from uninduced trees. In addition, the proportion of parasitised larvae that survived long enough to issue any parasitoids was lower on foliage from induced trees. 5. A conceptual and analytical model is provided to describe the net impacts of induced plant responses on parasitoids, and implications for tritrophic interactions and biological control of insect pests are discussed.     

Discrimination and production of disparlure enantiomers by the gypsy moth and the nun moth

ABSTRACT. Two odour receptor cells were physiologically identified within male antennal hair sensillae of the gypsy moth, Lymantria dispar L, and the nun moth, L. monacha L. In the gypsy moth, one cell responded to (+)-disparlure, while a neighbouring cell responded to (-)-disparlure. In the nun moth both cells responded to (+)-disparlure. The lack of sensitivity to (-)-disparlure in the nun moth was corroborated by electroantennogram (EAG) recordings, which indicated no affinity to this enantiomer. Single cell responses of male gypsy moth to different concentrations of the synthetic enantiomers of disparlure were then compared to responses elicited by hexane extracts of female glands of both species. The gypsy moth's extracts stimulated almost exclusively the receptor cell specialized for (+)-disparlure, while both cells were simultaneously stimulated by the extracts of the nun moths. From the response characteristic of the cells it is estimated that pheromone production of the nun moth is about 10% (+) and 90% (-)-disparlure, and that of the gypsy moth is almost 100% (+)-disparlure. Stimulation of the antenna of each species by female gland extracts of both species did not indicate the presence of receptors for other hexane elutable pheromone components in either species.     

Persistence of tree related patterns in soil nutrients following slash-and-burn disturbance in the tropics

Individual trees are known to influence soil chemical properties, creating spatial patterns that vary with distance from the stem. The influence of trees on soil chemical properties is commonly viewed as the agronomic basis for low-input agroforestry and shifting cultivation practices, and as an important source of spatial heterogeneity in forest soils. Few studies, however, have examined the persistence of the effects of trees on soil after the pathways responsible for the effects are removed. Here, we present evidence from a Mexican dry forest indicating that stem-related patterns of soil nutrients do persist following slash-and-burn removal of trees and two years of cropping. Pre-disturbance concentrations of resin extractable phosphorus (P), bicarbonate extractable P, NaOH extractable P, total P, total nitrogen (N) and carbon (C), KCl extractable nitrate (NO₃ ^-), and net N mineralization and nitrification rates were higher in stem than dripline soils under two canopy dominant species of large-stemmed trees with contrasting morphologies and phenologies (Caesalpinia eriostachys Benth. and Forchhammeria pallida Liebm.). These stem effects persisted through slash burning and a first growing season for labile inorganic and organic P, NaOH inorganic P, and plant-available P, and through a second growing season for labile organic P, NaOH organic P, and plant-available P. While stem effects for extractable NO₃ ^-, net nitrification rates, total N and C disappeared after felling and slash burning, these stem effects returned after the first growing season. These results support the view that tree-influenced patterns of soil nutrients do persist after tree death, and that trees contribute to the long-term spatial heterogeneity of forest soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Gypsy moth cell lines divergent in viral susceptibility

Summary A series of cell lines unique in insect virus susceptibility pattern have been isolated from the ovaries of the gypsy moth (Lymantria dispar: Lepidoptera: Lymantriidae) on a synthetic medium with mammalian and avian serum supplementation. Growth curves showed the poorest growth occurring on peptone-based media with somewhat better growth on amino-acid-based media. The best growth was obtained with combined media. Serological study distinguished the present cell lines from one another and from cell lines derived from other insect species grown routinely in the same laboratory. Baculovirus susceptibility among the new lines varied from no response to a specific complete replication response upon challenge by the homologous (gypsy moth) nuclear polyhedrosis virus. This research was funded in part through a reimbursable agreement with the U.S. Forest Service.     

Spatial analysis of harmonic oscillation of gypsy moth outbreak intensity

Outbreaks of many forest-defoliating insects are synchronous over broad geographic areas and occur with a period of approximately 10 years. Within the range of the gypsy moth in North America, however, there is considerable geographic heterogeneity in strength of periodicity and the frequency of outbreaks. Furthermore, gypsy moth outbreaks exhibit two significant periodicities: a dominant period of 8–10 years and a subdominant period of 4–5 years. In this study, we used a simulation model and spatially referenced time series of outbreak intensity data from the Northeastern United States to show that the bimodal periodicity in the intensity of gypsy moth outbreaks is largely a result of harmonic oscillations in gypsy moth abundance at and above a 4 km² scale of resolution. We also used geographically weighted regression models to explore the effects of gypsy moth host-tree abundance on the periodicity of gypsy moths. We found that the strength of 5-year cycles increased relative to the strength of 10-year cycles with increasing host tree abundance. We suggest that this pattern emerges because high host-tree availability enhances the growth rates of gypsy moth populations.     

Alternate partial root-zone irrigation induced dry/wet cycles of soils stimulate N mineralization and improve N nutrition in tomatoes

Given the same amount of irrigation volume, applying alternate partial root-zone irrigation (PRI) has improved crop N nutrition as compared to deficit irrigation (DI), yet the mechanisms underlying this effect remain unknown. Therefore, the objective of this study was to investigate whether PRI induced soil dry/wet cycles facilitate soil organic N mineralization hereby contributing to the improvement of N nutrition in tomatoes. The plants were grown in split-root pots in a climate-controlled glasshouse and were subjected to PRI and DI treatments during early fruiting stage. ¹⁵N-labeled maize residues were incorporated into the soils. Results showed that PRI resulted in 25% higher net ¹⁵N mineralization than did DI, indicating that the enhanced mineralization of soil organic N alone could account for the 16% increase of N accumulation in the PRI than in the DI plants. The higher net N mineralization under PRI was coincided with an intensified soil microbial activity. In addition, even though soil chloroform fumigation labile carbon (CFL-C, as an index of microbial biomass) was similar for the two irrigation treatments, a significant increase of chloroform fumigation labile nitrogen (CFL-N) was found in the PRI wetting soil. Consequently, the C:N ratio of the chloroform fumigation labile pool was remarkably modified by the PRI treatment, which might indicate physiological changes of soil microbes or changes in labiality of soil organic C and N due to the dry/wet cycles of soils, altering conditions for net N mineralization. Moreover, in both soil compartments PRI caused significantly less extractable organic carbon (EOC) as compared with DI; whilst in the PRI wetting soil significantly higher extractable organic nitrogen (EON) was observed. A low EOC:EON ratio in the PRI wetting soil may indicate an increasing net mineralization of the organic N as a result of microbial metabolism. Conclusively, PRI induced greater microbial activity and higher microbial substrates availability are seemingly responsible for the enhanced net N mineralization and improved N nutrition in tomato plants.