Plant species effects on soil macrofauna density in grassy arable fallows of different age

Soil macrofauna response to integrated soil fertility management under maize monocropping in Zimbabwe

The continued decline in both populations and diversity of soil macrofauna in agroecosystems remains a major concern for sustainable food production in Southern Africa. Macrofauna abundance and diversity were examined in a maize monocropping system following repeated incorporation of Calliandra calothyrsus, Crotalaria juncea, cattle manure, maize stover, and Pinus patula sawdust at 1.2 or 4.0 t C ha⁻¹ with or without nitrogen (120 kg N ha⁻¹) for 13 years (seasons). All treatments received phosphorus (P), potassium (K), and sulphur (S) at 16.0, 14.7 and 4.6 kg ha⁻¹, respectively, at each planting time in December. Using the ISO/TSBF (Tropical Soils Biology and Fertility Institute) monoliths procedure, macrofauna were collected to a depth of 0.30 m fortnightly from February to mid-May 2016. Lumbricus terrestris and Isoptera were significantly (P < 0.001) affected by organic treatment and seasonal time, while organic treatment alone significantly affected Elateridae. A fluctuating trend characterised Diplopoda, L. terrestris and Isoptera with two general peaks for L. terrestris and a significant Isoptera peak under maize stover. Rainfall significantly influenced L. terrestris and Isoptera abundance, while within-season sampling time explained the observed fluctuations in Diplopoda, Holotrichia serrata, Araneae, Acrididae, and Formicidae. While Araneae was prevalent under maize stover, Elateridae, H. serrata and Araneae showed no distinct pattern in abundance. At low application rate, higher Shannon-Wiener diversity indices were apparent except for maize stover. Increasing application rate of the different organics amplified stimulation, abundance and persistence of macrofauna, and significantly increased diversity relative to the control. Application of N-based mineral fertiliser appeared to be detrimental to selective macrofauna (e.g., L. terrestris, Isoptera and Elateridae). Monte Carlo permutation test identified ammonium nitrate as the most influential and detrimental variable followed by organic resource quality, soil organic carbon (SOC) and application rate. Long-term co-application of organic and inorganic resources thus stimulated macrofaunal populations and maintained diversity. To sustain the productivity of the predominantly maize-based smallholder cropping systems, it is proposed that extension packages promote the inclusion of a minimum of at least 1.2 t carbon ha⁻¹ annually towards improved soil health.

Trophic level and basal resource use of soil animals are hardly affected by local plant associations in abandoned arable land

Plants provide resources and shape the habitat of soil organisms thereby affecting the composition and functioning of soil communities. Effects of plants on soil communities are largely taxon-dependent, but how different functional groups of herbaceous plants affect trophic niches of individual animal species in soil needs further investigation. Here, we studied the use of basal resources and trophic levels of dominating soil meso- and macrofauna using stable isotope ratios of carbon and nitrogen in arable fallow systems 3 and 14-16 years after abandonment. Animals were sampled from the rhizosphere of three plant species of different functional groups: a legume (Medicaco sativa), a nonlegume herb (Taraxacum officinale), and a grass (Bromus sterilis). We found virtually no consistent effects of plant identity on stable isotope composition of soil animals and on thirteen isotopic metrics that reflect general food-web structure. However, in old fallows, the carbon isotope composition of some predatory macrofauna taxa had shifted closer to that of co-occurring plants, which was particularly evident for Lasius, an aphid-associated ant genus. Trophic levels and trophic-chain lengths in food webs were similar across plant species and fallow ages. Overall, the results suggest that variations in local plant diversity of grassland communities may little affect the basal resources and the trophic level of prey consumed by individual species of meso- and macrofauna belowground. By contrast, successional changes in grassland communities are associated with shifts in the trophic niches of certain species, reflecting establishment of trophic interactions with time, which shapes the functioning and stability of soil food webs.

Diversity and abundance of macro-invertebrates on abandoned cattle kraals in a semi-arid savanna

Abandoned cattle (Bos taurus) kraals are sources of habitat heterogeneity in dystrophic semi-arid African savannas with a strong positive effect on soil nutrients and plant productivity. However, little is known regarding how macro-invertebrate assemblages vary between abandoned kraals and the surrounding savanna matrix. We tested whether herbaceous biomass and basal and aerial covers and soil nutrients have an effect on aboveground and belowground macro-invertebrate assemblages. Twelve abandoned kraals were contrasted with their paired control plots for soil characteristics, herbaceous productivity, and macro-invertebrate assemblages in Save Valley Conservancy, Zimbabwe. Abandoned kraals had significantly higher concentrations of soil nitrogen (N), phosphorus (P), potassium (K), and calcium (Ca) as well as herbaceous biomass and basal and aerial covers than control plots. Both aboveground and belowground macro-invertebrate species richness were higher on abandoned kraals. However, only belowground macro-invertebrate diversity (Shannon H' and Hill number 1) was significantly higher on abandoned kraals. Soil nutrients and herbaceous productivity had positive and significant correlations with the dominant taxa (Coleoptera, Hymenoptera, Hemiptera, Isoptera, and Myriapoda) on abandoned kraals. These results add to the growing body of evidence that abandoned kraals exert significant effects on savanna spatial heterogeneity years later, with implications on ecosystem processes and functioning.

Oribatida (Acari) in grassy arable fallows are more affected by soil properties than habitat age and plant species

Oribatid mites are one of the numerically dominant arthropod groups in soils. They play an important role in soil food webs via regulating the decomposition of organic matter and propagating microorganisms within the soil. To our knowledge, the influence of different plant functional groups on oribatid mites has not been studied in abandoned farmland with undisturbed succession before. The density and assemblage structure of oribatid mites in nine grassy arable fallows relative to three habitat age classes (2-3, 6-8, 12-15 years) and three selected plant species (legume: Medicago sativa, forb: Taraxacum officinale, grass: Bromus sterilis) were investigated in soil associated with single plants. Mite density declined marginally not significant with habitat age because of high abundances of the ubiquitous species Tectocepheus velatus sarekensis and Punctoribates punctum in young and mid-aged fallows and their subsequent decline in old fallows. Oribatid mite density and species assemblage were not affected by plant species. Only P. punctum had significantly higher densities in B. sterilis samples than in T. officinale samples due to a higher amount of fine roots. Distance-based linear models revealed that 65% of the variation in mite assemblage was explained by soil properties, soil type, exposition and geographic position, while habitat age was of minor importance. Canonical correspondence analysis revealed that the mite assemblage was best explained by soil organic and microbial carbon, water content and pH.