A decline in soil biodiversity is generally considered to be the reduction of forms of life living in soils, both in terms of quantity and variety. Where soil biodiversity decline occurs, it can significantly affect the soils’ ability to function, respond to perturbations and recover from a disturbance. Several soil threats have been identified as having negative effects on soil biodiversity, including human intensive exploitation, land-use change and soil organic matter decline. In this review we consider what we mean by soil biodiversity, and why it is important to monitor. After a thorough review of the literature identified on a Web of Science search concerning threats to soil biodiversity (topic search: threat* “soil biodiversity”), we compiled a table of biodiversity threats considered in each paper including climate change, land use change, intensive human exploitation, decline in soil health or plastic; followed by detailed listings of threats studied. This we compared to a previously published expert assessment of threats to soil biodiversity. In addition, we identified emerging threats, particularly microplastics, in the 10 years following these knowledge based rankings. We found that many soil biodiversity studies do not focus on biodiversity sensu stricto, rather these studies examined either changes in abundance and/or diversity of individual groups of soil biota, instead of soil biodiversity as a whole, encompassing all levels of the soil food web. This highlights the complexity of soil biodiversity which is often impractical to assess in all but the largest studies. Published global scientific activity was only partially related to the threats identified by the expert panel assessment. The number of threats and the priority given to the threats (by number of publications) were quite different, indicating a disparity between research actions versus perceived threats. The lack of research effort in key areas of high priority in the threats to soil biodiversity are a concerning finding and requires some consideration and debate in the research community.

Decomposition is a key flux of terrestrial carbon to the atmosphere. Therefore, gaining a better understanding of how plant litter decomposes in soil, and what governs this process, is vital for global climate models. The Tea Bag Index (TBI) was introduced by Keuskamp et al. (2013) as a novel method for measuring litter decomposition rate and stabilisation. The TBI uses two types of tea bags representing fast (green tea) and slow (rooibos tea) decomposition substrates as standardised litter bags. To date, the TBI method has been used in over 2000 locations across the globe. However, before now, there has been no information on how the composition of the tea leaves change during incubation. These data are crucial in determining the validity of the use of the TBI method globally, to ensure the tea leaves decompose in a way that is representative of so-called “native” litters. To provide chemical underpinning of the TBI method, a laboratory incubation of the tea bags was conducted with destructive sampling at 0, 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, and 91 d. The incubated tea was analysed for total C and N. In addition, C was characterised as alkyl, O-alkyl, aromatic, or carbonyl C using solid-state ¹³C nuclear magnetic resonance spectroscopy with cross-polarization and magic angle spinning (CPMAS NMR). The results suggest that changes in carbon in both tea types are comparable to other litter studies, with a net decrease in total C and relative proportion of O-alkyl C fraction, which contains carbohydrates and cellulose. We conclude that the decomposition of tea leaves in the bags used in the TBI is representative of other litters.

It has been claimed that Zero Budget Natural Farming (ZBNF), a burgeoning practice of farming in India based on low-inputs and influenced by agro-ecological principles, has the potential to improve farm viability and food security. However, there is concern that the success of the social movement fueling the adoption of ZBNF has become out of step with the science underpinning its performance relative to other farming systems. Based on twenty field plot experiments established across six districts in Andhra Pradesh (SE India), managed by locally based farmer researchers, we present the first ‘on the ground’ assessment of ZBNF performance. We show that there is no short-term yield penalty when adopting ZBNF in small scale farming systems compared to conventional and organic alternatives. In terms of treatment response, we observed differences between agro-climatic zones, but in this initial evaluation we cannot recommend specific options tuned to these different contexts.

Zero Budget Natural Farming (ZBNF) is a grassroot agrarian movement and a state backed extension in Andhra Pradesh, and has been claimed to potentially meet the twin goals of global food security and environmental conservation. However, there is a lack of statistically evaluated data to support assertions of yield benefits of ZBNF compared to organic or conventional alternatives, or to mechanistically account for them. In order to fill this gap, controlled field experiments were established in twenty-eight farms across six districts, spanning over 800 km, over three cropping seasons. In these experiments, we compared ZBNF (no synthetic pesticides or fertilisers, home-made inputs comprising desi cow dung and urine with mulch) to conventional (synthetic fertilisers and pesticides) and organic (no synthetic pesticides or fertilisers, no mulch, purchased organic inputs, e.g. farmyard manure and vermicompost) treatments, all with no tillage. Comparisons were made in terms of yield, soil pH, temperature, moisture content, nutrient content and earthworm abundance. Our data shows that yield was significantly higher in the ZBNF treatment (z score = 0.58 ± 0.08), than the organic (z= −0.34 ± 0.06) or conventional (−0.24 ± 0.07) treatment when all farm experiments were analysed together. However, the efficacy of the ZBNF treatment was context specific and varied according to district and the crop in question. The ZBNF yield benefit is likely attributed to mulching, generating a cooler soil, with a higher moisture content and a larger earthworm population. There were no significant differences between ZBNF and the conventional treatment in the majority of nutrients. This is a particularly important observation, as intensive use of synthetic pesticides and fertilisers comes with a number of associated risks to farmers’ finances, human health, greenhouse gas emissions, biodiversity loss and environmental pollution. However, long-term field and landscape scale trials are needed to corroborate these initial observations.

Purpose: To build a more holistic understanding of soil pH change we assessed the synchronised effects of a contrived soil pH change on soil chemistry, vegetation growth and nutrition, and soil faunal abundance and diversity. Methods: We established a fifteen year old field experiment with a contrived pH gradient (pH 4.3 to 6.3) and measured the effect on soil chemistry, plant biomass and elemental composition and the impact of these changes on soil fauna (earthworms, nematodes, rotifers and tardigrades) and biological indices (based on ecological group structures of earthworms and nematodes). A single 20 × 20 × 20 cm soil block was excavated from each sample site to directly attribute biotic parameters in the block to the abiotic (soil) conditions. Results: Acidification affected the extractable concentrations of Al, Ca, Mn and P and the C:N ratio of the soil and caused a reduction in plant Ca (r_s for pH vs Ca = 0.804 p < 0.01), an increase in plant Mn (r_s = −0.450 p = 0.019), along with significant decrease in root:shoot ratio (r_s = 0.638, p < 0.01). There was a significant positive correlation between pH and earthworm index (r_s = 0.606, p < 0.01), and a negative correlation between pH and nematode index (r_s = −0.515, p < 0.01). Conclusion: Soil pH influenced the mobility of Ca, Al, Mn and P, which in turn has impacted on plant tissue chemistry and plant biomass ratios. Linked changes in soil chemistry and vegetation had a corresponding effect on the abundance and diversity of nematodes and earthworms in the soil blocks.