BackgroundGrazed grassland management regimes can have various effects on soil fauna. For example, effects on earthworms can be negative through compaction induced by grazing animals, or positive mediated by increases in sward productivity and cattle dung pats providing a food source. Knowledge gaps exist in relation to the behaviour of different earthworm species i.e. their movement towards and aggregation under dung pats, the legacy effects of pats and the spatial area of recruitment. The present study addressed these knowledge gaps in field experiments, over 2 years, using natural and simulated dung pats on two permanent, intensively grazed pastures in Ireland.ResultsDung pats strongly affected spatial earthworm distribution, with up to four times more earthworms aggregating beneath pats, than in the control locations away from pats. In these earthworm communities comprising 11 species, temporally different aggregation and dispersal patterns were observed, including absence of individual species from control locations, but no clear successional responses. Epigeic species in general, but also certain species of the anecic and endogeic groups were aggregating under dung. Sampling after complete dung pat disappearance (27 weeks after application) suggested an absence of a dung pat legacy effect on earthworm communities. Based on species distributions, the maximum size of the recruitment area from which earthworms moved to pats was estimated to be 3.8 m2 per dung pat. Since actual grazing over 6 weeks would result in the deposition of about 300 dung pats per ha, it is estimated that a surface area of 1140 m2 or about 11% of the total grazing area can be influenced by dung pats in a given grazing period.ConclusionsThis study showed that the presence of dung pats in pastures creates temporary hot spots in spatial earthworm species distribution, which changes over time. The findings highlight the importance of considering dung pats, temporally and spatially, when sampling earthworms in grazed pastures. Published comparisons of grazed and cut grasslands probably reached incorrect conclusions by ignoring or deliberately avoiding dung pats. Furthermore, the observed intense aggregation of earthworms beneath dung pats suggests that earthworm functions need to be assessed separately at these hot spots.Electronic supplementary materialThe online version of this article (10.1186/s12898-018-0216-6) contains supplementary material, which is available to authorized users.

Macropore systems predominantly determine rapid water flow and solute transport in undisturbed soils. Repeated experiments are needed to investigate the relationship between the nature of the macropore network and the resulting water and solute transport under different hydraulic initial and boundary conditions. However, the large heterogeneity in soil macropore network structures renders each soil sample unique and multiple identical samples impossible. In addition, the fragile nature of soil strongly limits the possible number of repeated experiments on one individual sample. Micromodels that mimic the precise shape and location of the macropores in undisturbed soil are therefore necessary to allow repeated experiments. In this study we investigated whether such micromodels can be obtained using contemporary three‐dimensional (3‐D) printing techniques and materials. We used X‐ray computed tomography to digitize the 3‐D macropore structure of an undisturbed soil sample. We printed a subsection of this macropore system in five different materials. Four out of the five investigated materials had essential parts of their macropore system clogged with residual printing or printing‐aid material. Only one reprint, namely the prime‐gray sample that was printed using stereo lithography, exhibited no pore clogging and had the largest hydraulic conductivity of all investigated reprints. Prime gray showed subcritical water repellency with a medium contact angle of approximately 65°, which is similar to contact angles found in natural soil. We conclude that the 3‐D printing of undisturbed soil macropore systems is in principle possible with contemporary 3‐D printing systems.

Core Ideas Soil quality indicators such as S and PAWC are considered as static indicators with low sensitivity. The soil quality indicator AWr is considered a dynamic indicator with high sensitivity. Indirect pedotransfer function based SWRCs are appropriate for static baseline indicators. Soil monitoring programs should use AWr as a sensitive and reliable soil quality indicator. The quality of a soil is its ability to deliver functions providing ecosystem services, human health and well‐being. Soil physical quality (SPQ) values use different parts of the soil water retention curve (SWRC) to calculate SPQ. For example, the plant available water capacity (PAWC) method is the difference in water content between permanent wilting point and field capacity. The S‐index uses the slope of the SWRC at its inflection point and the relative air‐water energy (AWr) is the integral of “dry” divided by the “wet” area of the SWRC. Increasing demand for soil monitoring policies calls for reliable and sensitive soil quality indicators (SQIs). The objectives of the study were to assess the sensitivity and applicability of SPQ indicators using direct and indirect data inputs. The indirect approach provided sufficient data complexity for the PAWC and S‐index values, but the more complex AWr required the direct approach. PAWC and S‐index values were identified as static SPQ indicators. The values obtained from these approaches should be used to form baseline static datasets and therefore have an indicative role only. The AWr value was identified as a dynamic SPQ indicator and provided required sensitivity to pick up temporal changes in SPQ. This indicator could be used at multiple scales and could even guide grassland management in terms of SPQ. Higher SWRC data resolution will require more complex hydraulic models to fit and will ultimately improve the accuracy of soil hydraulic data and improve the sensitivity of AWr as a SPQ indicator.

Soil quality determines the ability of soil to deliver ecosystem services and can be inferred from physical, biological and chemical indicators either in isolation or in combination. Earthworms are good soil-quality indicators that contribute to both chemical and physical quality by maintaining soil structure and cycling nutrients. The presence of dung pats can increase earthworm abundance locally and consequently the network of pores that they create through their burrowing activity. Inevitably this affects soil structure and consequently will have a spatially distributed effect on soil physical quality (SPQ). The aim of this field study was to examine the relationship between SPQ and earthworm abundance under dung and non-dung pat areas from deposition to decay and beyond. The present spatial and temporal study compared SPQ indicator (integral air-water energy, AWr) results with earthworm abundance across control and simulated dung pat treatments. Results showed that existing earthworm populations in this grassland were already very large (>500 individuals m–2) and SPQ (AWr) remained in the ‘very good’ category throughout the experiment. Earthworm abundance under dung pats and SPQ exhibited a significant (P = 0.05) temporal trend. In general, the time of decay of the dung pats played a role in increasing earthworm abundance and SPQ. Earthworm abundance and macropore density data formed a similar, ‘hump’-shaped dynamic over time. However, when an earthworm abundance threshold was exceeded (equivalent to about >3000 individuals m–2), the increase of SPQ under dung was attenuated and did increase further only under the control sward with high earthworm abundance. After 11 weeks, for both treatments, AWr under dung pats was capped at 0.83% and AWr under control sward peaked at 1.34%. Future work should focus on (a) further exploration of the threshold where earthworm abundance becomes detrimental for SPQ and (b) using the AWr SPQ indicator within an actual grazed trial which incorporates a gradient of soil degradation.