R.N. Watson

PLANT REMOVALS IN PERENNIAL GRASSLAND: VEGETATION DYNAMICS, DECOMPOSERS, SOIL BIODIVERSITY, AND ECOSYSTEM PROPERTIES

The consequences of permanent loss of species or species groups from plant communities are poorly understood, although there is increasing evidence that individual species effects are important in modifying ecosystem properties. We conducted a field experiment in a New Zealand perennial grassland ecosystem, creating artificial vegetation gaps and imposing manipulation treatments on the reestablishing vegetation. Treatments consisted of continual removal of different subsets or “functional groups” of the flora. We monitored vegetation and soil biotic and chemical properties over a 3-yr period. Plant competitive effects were clear: removal of the C3 grass Lolium perenne L. enhanced vegetative cover, biomass, and species richness of both the C4 grass and dicotyledonous weed functional groups and had either positive or negative effects on the legume Trifolium repens L., depending on season. Treatments significantly affected total plant cover and biomass; in particular, C4 grass removal reduced total plant biomass in summer, because no other species had appropriate phenology. Removal of C3 grasses reduced total root biomass and drastically enhanced overall shoot-to-root biomass ratios. Aboveground net primary productivity (NPP) was not strongly affected by any treatment, indicating strong compensatory effects between different functional components of the flora. Removing all plants often negatively affected three further trophic levels of the decomposer functional food web: microflora, microbe-feeding nematodes, and predaceous nematodes. However, as long as plants were present, we did not find strong effects of removal treatments, NPP, or plant biomass on these trophic groupings, which instead were most closely related to spatial variation in soil chemical properties across all trophic levels, soil N in particular. Larger decomposer organisms, i.e., Collembola and earthworms, were unresponsive to any factor other than removal of all plants, which reduced their populations. We also considered five functional components of the soil biota at finer taxonomic levels: three decomposer components (microflora, microbe-feeding nematodes, predaceous nematodes) and two herbivore groups (nematodes and arthropods). Taxa within these five groups responded to removal treatments, indicating that plant community composition has multitrophic effects at higher levels of taxonomic resolution. The principal ordination axes summarizing community-level data for different trophic groups in the soil food web were related to each other in several instances, but the plant ordination axes were only significantly related to those of the soil microfloral community. There were time lag effects, with ordination axes of soil-associated herbivorous arthropods and microbial-feeding nematodes being related to ordination axes representing plant community structure at earlier measurement dates. Taxonomic diversity of some soil organism groups was linked to plant removals or to plant diversity. For herbivorous arthropods, removal of C4 grasses enhanced diversity; there were negative correlations between plant and arthropod diversity, presumably because of negative influences of C4 species in the most diverse treatments. There was evidence of lag relationships between diversity of plants and that of the three decomposer groups, indicating multitrophic effects of altering plant diversity. Relatively small effects of plant removal on the decomposer food web were also apparent in soil processes regulated by this food web. Decomposition rates of substrates added to soils showed no relationship with treatment, and rates of CO2 evolution from the soil were only adversely affected when all plants were removed. Few plant functional-group effects on soil nutrient dynamics were identified. Although some treatments affected temporal variability (and thus stability) of soil biotic properties (particularly CO2 release) throughout the experiment, there was no evidence of destabilizing effects of plant removals. Our data provide evidence that permanent exclusion of plant species from the species pool can have important consequences for overall vegetation composition in addition to the direct effects of vegetation removal, and various potential effects on both the above- and belowground subsystems. The nature of many of these effects is driven by which plant species are lost from the system, which depends on the various attributes or traits of these species.

Response of soil microbial biomass dynamics, activity and plant litter decomposition to agricultural intensification over a seven-year period

Soil microorganisms and the processes that they govern are essential for long-term sustainability of agricultural systems, but most studies on agricultural effects on the soil microflora are inherently short-term. We investigated the effects of three aspects of agricultural intensification, i.e. cultivation (disturbance), herbicide addition (modification of plant composition) and mulching (resource addition) on soil biological properties such as microbial biomass and activity over 7 yr in annual (maize) and perennial (asparagus) cropping systems. The mulching treatment had strong, usually positive effects on both substrate-induced respiration (SIR) and CO2–C release from chloroform-fumigated soil throughout the study. In the perennial crop, treatments allowing high weed biomass caused large increases in microbial biomass and respiration after yr 3, and in both sites microbial biomass was positively correlated with weed biomass and negatively with crop plant biomass. This latter effect appears due to the high decomposability of weed residues relative to those from crop plants. Microbial biomass was also enhanced in atrazine-treated plots in the annual crop but only during the final year, presumably due to beneficial effects of plot invasion by herbicide-tolerant weeds. Mulching often also enhanced the microbial metabolic quotient (qCO2), the bacteria-to-fungal biomass ratio and within-year temporal variability of the microbial biomass, all of which are indicative of greater turnover and instability of the microbial biomass. Other treatments generally had smaller effects on these properties, although in the perennial crop an intense summer drought in yr 4 caused a large elevation in the metabolic quotient in the herbicide-treated (low weed) plots relative to the other plots, suggesting that high quality weed residues have stabilising effects. Temporal variability across years of both SIR and CO2–C release from fumigated soil was greatest in the herbicide-treated plots in the perennial crop, suggesting that high weed biomass (producing easily degradable organic matter) has stabilising effects. Decomposition rates of added litter were partially consistent with the microbial biomass data, with the highest litter breakdown rates usually occurring in the mulched plots. Our study shows that soil biological properties such as microbial biomass and activity are not necessarily adversely affected by agricultural intensification and that consequences of intensification mainly depend upon practices which alter the quality and quantity of residue inputs. Further, our results underline the need for long,-term field experiments, and several of the effects we identified could only have been detected through an experiment of several years duration.

Responses of soil nematode populations, community structure, diversity and temporal variability to agricultural intensification over a seven-year period

Because soil nematode populations play a key role in regulating the turnover of microbial communities and respond to changes in environmental conditions, shifts in their abundance and composition can be useful indicators of soil conditions. In this study nematode communities and other ecosystem variables were investigated over 7 yr under an annual (Zea mays) and a perennial (Asparagus officinale) crop using three weed management practices (cultivation, herbicide application, mulching) which can be related to agricultural intensification. Crop productivity and soil conditions did not change significantly during the trial. All management practices influenced the nematode fauna but the greatest long-term effects were from sawdust mulching. In the mulched plots there was an initial flush of both total and bacterial-feeding nematodes but both subsequently declined, which was coincident with enhanced populations of top predatory nematodes. The apparent negative interaction between bacterial-feeding and predatory nematodes was also demonstrated through the former being significantly (P<0.001) negatively correlated with soil carbon, bacterial mass and weed biomass and the latter being positively correlated with the same variables. Herbicide use did not exert any consistent detrimental effects on nematode communities and the nematode fauna in the herbicide treated plots tended to have greater diversity (as indicated by the Shannon–Weiner index) than that in many of the other plots. The effects of cultivation varied, but under the perennial crop the greatest number of total and bacterial-feeding nematodes were commonly at 5–10 cm depth in cultivated plots. While most treatments had relatively little general effect on the composition of the nematode fauna over the study period, several important specific effects were only apparent after at least 3 yr. Thus to effectively evaluate the relative effects of different agricultural practices in the long-term it is necessary to sample until the ecosystem has achieved some degree of equilibrium rather than monitoring only initial cropping cycles.

Relationships between nematodes, soil microbial biomass and weed-management strategies in maize and asparagus cropping systems

Abstract Five weed-management strategies (sawdust mulching, repeated spring-summer cultivation, hand-hoeing, two herbicide treatments) were applied to asparagus and maize cropping systems near Hamilton, New Zealand. Assessments of 27 nematode populations on four sampling occasions over an entire cropping cycle are related to published microbial, arthropod and environmental data. Under asparagus cropping abundance of 11 nematode populations (at genus or family level) in 0–5 cm soil showed significant treatment effects on at least two sampling occasions; under maize 6 populations showed treatment effects. Overall, the most obvious trends were for some taxa of bacterial feeding nematodes to have their greatest abundances under different treatments. The ratio of bacterial feeding to fungal feeding nematodes varied significantly with time and treatment, and indicates shifts in trophic structure of the nematode fauna. Canonical correspondence analysis demonstrated that nematode populations were more strongly related to environmental factors at the prior sampling than those at the contemporary sampling time. Under asparagus cropping the sawdust mulch was the predominant factor affecting ordinations; bacterial and fungal feeding nematodes were most abundant or showed greatest treatment responses, but the increase in populations of predacious nematodes (Nygolaimus, Mononchidae, Aporcelaimidae) may be responsible for absence of marked increases in these other groups. Under maize, effects were similar but less significant. Helicotylenchus and Pratylenchus were present under the maize crop but not under the asparagus crop. The responses of nematode taxa to weed management practices were very variable but, given the range of life history strategies within trophic groups, responses follow a predictable pattern. Detailed correlation of management-induced changes in nematode populations and biological environmental factors is confounded by the effect of nematode feeding activity on the microbial populations. Overall, the results confirm the important influence of microfaunal grazing on microfloral populations and the cycling of plant nutrients in the soil.

Response of soil microbial biomass and plant litter decomposition to weed management strategies in maize and asparagus cropping systems

Abstract Five weed management strategies (sawdust mulching, repeated spring-summer cultivation, hand-hoeing and two herbicide treatments) were applied to each of two cropping systems (maize and asparagus) near Hamilton, New Zealand. Assessments of the response of microbial activity and biomass were made over an entire annual cropping cycle (from August 1990 to October 1991). Soil respiration and substrate-induced respiration (SIR) were strongly stimulated by sawdust mulch over the experimental period, probably as a result of the enhanced soil moisture status, but the other treatments did not exert any strong consistent effects. Use of the selective inhibitor technique demonstrated temporary stimulatory effects of mulching, cultivation and (occasionally) herbicide application on both the bacterial and fungal components of the soil system. The fumigation-incubation technique also suggested that mulching had stimulatory effects on microbial activity and biomass but only when control values were not subtracted. Most of the effects detected occurred in the top 5 cm of the mineral soil. Placement of litter-bags on the surface and at 10cm depth indicated that litter decomposition was often most rapid in the sawdustmulched plots, probably as a result of enhanced abiotic decomposition. Soil respiration and SIR were also greatest for the litter placed on the mulched plots, over most of the annual cropping cycle. We concluded that weed management strategies which influence soil moisture contents are likely to induce the most significant responses by the soil microflora.

Effects of chitin amendment of soil on microorganisms, nematodes, and growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.)

Effects of soil amendment with crabshell chitin on the growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.), and on populations of soil bacteria, fungi, and plant-parasitic and free-living nematodes were investigated in a pot trial. Five soil samples were collected from Te Puke (Paengaroa Shallow Sand, a Typic Hapludand) and five from Hamilton (Bruntwood silt loam, an Aquic Hapludand), New Zealand. Subsamples of each soil were either amended with chitin or unamended and planted with white clover and ryegrass. The ryegrass shoot weight in amended soil was greater (P<0.01), most probably due to N mineralised from chitin. A significantly lower (P<0.01) root: shoot ratio of ryegrass in the amended soil also suggested improved N availability, and therefore less root mass was needed to support a given shoot mass. A reduction in nodulation was observed in 12-day-old white clover seedlings (P<0.05) and also in 6-week-old seedlings (P<0.01). The shoot weight of white clover was significantly lower (P<0.05) in amended soil, possibly due to phytotoxic effects of chitin. Chitin increased (P<0.01) the populations of bacteria and fungi by 13-fold and 2.5-fold, respectively. The cyst nematode of white clover, Heterodera trifolii, was significantly reduced in chitin-amended soil, possibly due to increased levels of chitinase produced by rhizosphere microorganisms. Two other plant-parasitic nematodes, Pratylenchus spp. and Tylenchus spp., were also reduced in ryegrass roots and in soil as a result of the chitin amendment. However, the total number of free-living nematodes increased 5.4-fold in amended soil.

Optimising the Whitehead and Hemming tray method to extract plant parasitic and other nematodes from two soils under pasture

Three variations of the Whitehead and Hemming tray method for extracting vermiform nematodes from soil samples, and the decant and sieve method, were compared using a silt loam and a clay loam soil under long-term pasture. Comparisons showed that greatest nematode recovery was achieved when 50 g of soil was placed in a tray lined with two-ply paper tissue and extracted for 48 h with 500 ml water. Recovery of the total nematode fauna and of plant parasitic nematodes from the tray extract was significantly better (P ≤ 0.001) after allowing nematodes to sediment in a 1 l straight-sided beaker than in a 15 cm diam. filter funnel. After 48 h extraction on trays, this method recovered 77% of the total numbers of all nematodes (72.5% of the plant parasites) extracted over 144 h with daily collection from the trays. The optimum extraction duration was different for Paratylenchus nanus compared to Pratylenchus sp., apparently related to differences in their mode of parasitism, with root-dwelling stages of Pratylenchus being recovered at longer times. There was a significant treatment × soil interaction for Meloidogyne sp., recovery of which was improved by beaker sedimentation in silt loam but not clay loam soil, compared to funnel sedimentation. A significant treatment × soil interaction was also observed for H. trifolii, the beaker method being better at recovering this nematode in clay loam than silt loam soil, compared to the decant and sieve method.

Effects of plant-parasitic nematodes and rhizosphere microorganisms on the growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.)

A pot trial was carried out to study the effects of plant-parasitic nematodes and rhizosphere microorganisms on the growth of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). Plants were grown together in Horotiu sandy loam (Vitric Hapludand). The treatments consisted of: untreated field soil (UT); soil frozen to −20°C to reduce nematodes (FR); soil fumigated with chloroform to kill most soil organisms (FU); and fumigated soil inoculated with a layer of frozen soil to reintroduce organisms present in frozen soil (FUI). The mean wet weights of white clover and ryegrass grown in UT soil for 6–9 weeks were only 10 and 60%, respectively, of those grown in FR soil and 5.8 and 56%, respectively, of those grown in FU soil. The severe growth reduction of white clover in UT was attributed to early invasion of roots by two nematode species, Heterodera trifolii and Meloidogyne hapla, which were detected in stunted 7-day-old seedlings. In the FR and FUI treatments, the freeze-thaw process appeared to have killed the second stage juveniles but not the eggs, which hatched to release more juveniles to invade white clover roots. Plant growth in FR, and to a lesser extent in FUI treatments, was reduced in comparison with FU, where there was no nematode invasion. The bacterial numbers in surface-sterilized roots were unaffected by any of the treatments. This suggests that the entry of bacteria into root tissues is independent of the wounding caused by nematodes. The mean bacterial numbers of the medians (means of log cfus across treatments and plant species of the median numbers across replicates) for “total” bacteria, fluorescent pseudomonads, Gram-negative bacteria and Gram-positive bacteria from surface-sterilized roots were 4.26, < 2, 2.94 and 3.08 for the four treatments, respectively. The most common bacterial genera identified were Pseudomonas and Bacillus. Fungi isolated from surface-sterilized roots included Fusarium oxysporum, Codinaea fertilis and many sterile fungi, with F. oxysporum being the most common identified fungus in FU and FUI treatments. C. fertilis was killed by the soil freezing process. F. oxysporum did not appear to be associated with a decrease in dry matter production of either white clover or ryegrass, but C. fertilis may have adversely affected the ryegrass dry matter production in the treatment UT.

Identification and host range assessment of Paratylenchus nanus (Tylenchida: Tylenchulidae) and Paratrichodorus minor (Triplonchida: Trichodoridae)

Adult and juvenile characters were used to identify Paratylenchus nanus Cobb, 1923 and Paratrichodorus minor (Colbran, 1956) Siddiqi, 1974 from soils under pasture in New Zealand. In glasshouse tests of 15 pasture plants, common in New Zealand, all good hosts (ratio of final to initial population >1) of P. nanus were grasses, namely Dactylis glomerata , Lolium multiflorum and L. perenne ( Neotyphodium endophyte-infected and-free), all new host records for this species. Good hosts of P.minor included those listed for P.nanus and Festuca arundinacea (endophyte-free) , Poa annua , Trifolium pratense , T. repens and T. subterraneum . Except for T. repens , these are all new host records for P.minor in New Zealand.

Microbial pathogens and plant parasitic nematodes in pastures with declining vigour.

Abstract Two years after establishment, areas of low vigour (LV) were noticed within a white clover (Trifolium repens)/ryegrass (Lolium perenne) pasture. These areas became progressively larger, and, two years later, the pasture had low vigour, comprising only 3% white clover. Clover tissue nitrogen and phosphorus levels were greater in high vigour (HV) than in LV herbage. There were no differences (P > 0.05) in “total” bacteria, fluorescent pseudomonads, or proportions of deleterious bacteria in LV and HV white clover roots (mean logio CFUs for total count and fluorescent pseudomonads were 9.4 and 8.8, respectively, and the proportions of deleterious bacteria encountered in these groups were 6.9% and 17.5%). There was a 9‐fold difference (P < 0.01) in the white clover dry matter between LV areas and HV areas in February, increasing to 25‐fold (P< 0.01) in March. The total dry matter yield differences, which included grasses and clover, were 2.7‐ and 2‐fold (P < 0.01) for the two months. More fungi were isolated (P < 0.05) from LV white clover roots than from HV roots (164 and 104 isolates, respectively). This was mostly due to greater numbers (P < 0.01) of Codinaea fertilis in LV roots (111 cf. 41 in HV). In ryegrass roots, there were also greater numbers (P < 0.01) of total fungi in LV roots (97 cf. 50 total isolates in HV), mostly due to higher populations of Fusarium oxysporum in LV roots (29 isolates cf. 5 in HV). There was no difference (P > 0.05) in the Soil Pathogenicity Index between LV and HV soil. The plant parasitic nematode populations in 250 ml of soil in LV and HV areas were 42.7 and 9.7 for Heterodera and 28.1 and 3.9 for Meloidogyne, respectively.