N.L. Schon

Influence of summer irrigation on soil invertebrate populations in a long-term sheep irrigation trial at Winchmore (Canterbury)

Abstract Soil invertebrates have limited defences against moisture stress but little is understood of how irrigation influences their community. This paper investigates the influence of five summer irrigation schedules on earthworms over a 15-month period in a long-term pasture irrigation study. The influence of the two extreme rates of irrigation on other soil invertebrates was also examined. The effects of frequent drought periods under dryland pasture favoured small and short-lived nematodes and oribatids. During wetter months, earthworm abundances were similar between dryland and irrigated treatments, while earthworms tended to migrate down the soil profile and aestivate in response to soil water deficits during summer. The higher abundance of earthworms in the summer months under irrigation may partly explain the lower soil carbon found under irrigation, due to enhanced rates of soil organic matter turnover, despite higher primary production and inputs of carbon. It is thus important to consider irrigation schedules not only to optimise plant growth, but also to optimise the invertebrate community and its activity.

Influence of phosphorus inputs and sheep treading on soil macrofauna and mesofauna in hill pastures

Abstract Earthworm abundance increased and Oribatida abundance decreased on a Brown soil with increasing inputs of phosphorus (P) fertiliser and sheep stocking rates. To determine if the same response to increasing management intensity is found in Allophanic soil, samples were collected from a long-term sheep-grazed trial with two contrasting management regimes, low and high intensity. As pastures on the Allophanic soil became more productive and were grazed with more sheep, there was a tendency for earthworm abundance to increase and oribatid abundance to decrease. Both these trends were consistent with those on the Brown soil, supporting the potential of earthworms and Oribatida as biological indicators. Oribatid mites, which produce nutrient rich faecal pellets, were positively correlated with porosity. The higher earthworm abundances in more intensive sheep grazed pastures may have a positive influence on ecosystem services such as air and water movement, and organic matter incorporation.

Effects of dairy cow treading pressures and food resources on invertebrates in two contrasting and co-occurring soils

It is often difficult to compare the effects of land use on soil invertebrates across different soil types, as management practices are often adjusted to compensate for soil differences. A mosaic of two contrasting and co-occurring soils offered a unique opportunity to examine the influence of common management practices on soil invertebrates (macrofauna, mesofauna, and nematodes). Treatments established on a well-structured Andosol soil and co-occurring poorly structured Gleysol included a legume-based pasture grazed at 2.3 cows/ha and nitrogen (N) fertilised pastures grazed at 3 and 3.8 cows/ha, with the 3.8 cows/ha treatment also receiving maize supplementation. Low abundance (<13 000 individuals/m2) and diversity (four species) of Oribatida was a feature of both soils, reflecting the low porosity of the two contrasting, co-occurring pastoral soils, despite the Gleysol soil being more susceptible to treading than the Andosol. The lack of difference might reflect the ongoing disturbance from livestock treading on both soils. Nematode trophic groups behaved most predictably across both soils, with plant-feeding and bacterial-feeding nematodes increasing with the use of N fertiliser. Despite potentially more organic material available for incorporation into the soil profile with increasing inputs of N fertiliser and use of feed supplement, lower abundances of anecic earthworms, Collembola, and Oribatida are reported. Both direct and indirect effects of livestock treading on the decomposer community in intensive systems might be factors limiting the incorporation of organic matter from the soil surface into the profile to sustain soil carbon.

Effect of nitrogen fertilizer on nitrogen pools and soil communities under grazed pastures

Abstract Nitrogen (N) fertilizer increases pasture production in New Zealand in a near linear manner and affects pasture composition, soil below-ground communities and N losses. We monitored N fertilized plots established in long-term low-fertility pasture over different time periods to compare changes in N availability on below-ground soil communities (particularly ammonia-oxidizing archaea [AOA] and ammonia-oxidizing bacteria [AOB] as they appear to be sensitive to change). Although the most significant effects were seen in the 30 gN (300 kg/ha) treatment, there were indications that even 5 gN had effects on archaea. AOA gene copies dominated in control and 5 gN treatments but decreased in the 10 gN treatment. The ratio of AOA to AOB in 10 gN was lower and more similar to 30 gN suggesting that the AOA/AOB ratio may be a sensitive indicator of change in N status. In the 30 gN treatment, both the soil C/N ratio and the fungal phospholipid fatty acids were reduced (consistent with changes in DNA profiles) and microbes were suppressed. The number of AOB gene copies significantly increased in this treatment and corresponded to a switch from ammonium-N to nitrate-N as the dominant inorganic form of N in the 56-day incubation. This was consistent with increased soil ammonium-N and nitrate-N concentrations, leading to increased nitrate-N leaching that occurred at a threshold of between 10 gN and 30 gN, and suggesting that, with 30 gN, nitrification and nitrate leaching are influenced more by AOB than archaea. Generally there was no significant change in mesofauna, microfauna or bacterial DNA profiles with the treatments. In a high-fertility pasture, DNA profiles for bacterial, fungal and archaeal groups clustered away from low-fertility pasture suggesting that changes in soil communities, with increased soil fertility, take more time to be fully expressed.

Influence of soil faunal communities on nitrogen dynamics in legume-based mesocosms

Soil invertebrates play an important part in nutrient supply. It has been suggested that invertebrates have the greatest influence on nitrogen (N) availability in soils of low N fertility, and therefore invertebrates may be less important in pastoral systems with high N inputs. The influence of invertebrates on N cycling, and the fate of 15N-labelled plant litter, in a low- or high-N environment and in a soil with low or high bulk density were explored by the introduction of different elements of the invertebrate community in constructed soil ryegrass–white clover mesocosms. At high bulk density and low N, the N made available by invertebrates resulted in higher plant growth, without any increases in N losses to the environment. At high N, where pasture growth was not limited by N, the N made available by invertebrates increased both herbage N% and the amount of N in leachate. More of the 15N-labelled plant litter decomposed in high-N than low-N mesocosms. Invertebrates increased the plant uptake of surface-applied 15N plant litter. The influence of invertebrates was dependent on bulk density, suggesting that invertebrates in compacted soils improved soil structure and N availability. This mesocosm study highlights the important role of invertebrates in N supply across a range of soil conditions.

Quantification of soil macro pore size and shape using resin impregnation

Abstract Subtle changes in soil pore size and shape as a consequence of changes in land use and management practices can have significant impacts on the movement of air, water, plant roots and soil biota. This paper describes a resin impregnation technique adapted for use in New Zealand pasture soils to quantify these soil pore attributes. Image analysis of the resin impregnated soil enables detailed assessment of soil pore shape and macro pore size distribution (>50 µm).