Alice S.A. Johnston

Effects of agricultural management practices on earthworm populations and crop yield: validation and application of a mechanistic modelling approach

1. There is little consensus on how agriculture will meet future food demands sustainably. Soils and their biota play a crucial role by mediating ecosystem services that support agricultural productivity. However, a multitude of site-specific environmental factors and management practices interact to affect the ability of soil biota to perform vital functions, confounding the interpretation of results from experimental approaches. Insights can be gained through models which integrate the physiological, biological and ecological mechanisms underpinning soil functions. 2. We present a powerful modelling approach for predicting how agricultural management practices (pesticide applications and tillage practices) affect soil functioning through earthworm populations. By combining energy budgets and individual-based simulation models, and integrating key behavioural and ecological drivers, we accurately predict population responses to pesticide applications in different climatic conditions. 3. We use the model to analyse the ecological consequences of different weed management practices. Our results demonstrate that an important link between agricultural management (herbicide applications and zero, reduced and conventional tillage) and earthworms is the maintenance of soil organic matter (SOM). 4. We show how zero and reduced tillage practices can increase crop yields while preserving natural ecosystem functions. This demonstrates how management practices which aim to sustain agricultural productivity should account for their effects on earthworm populations, as their proliferation stimulates agricultural productivity. 5. Synthesis and Applications. Our results indicate that weed management that relies on tillage has longer term effects on soil biota than pesticide control, if the herbicides have short dissipation times. If pesticides known to be toxic to earthworms are applied, the risk of exposure will be reduced if irrigation is stopped around application time or if application is carried out during dry periods. Similarly, if the organic content of the soil is increased the recovery rate of earthworm populations can be increased. However, effects are not necessarily additive and the impact of different growing practices on earthworms will depend on their timing and the environmental conditions. Our model can be used to estimate the overall impact of different combinations of crop management activities in different regions to explore how earthworm populations respond. If our models are linked to crop yield models the costs and benefits of different crop solutions for both yields and biota could be estimated and aid optimisation of the trade-off between different ecosystem services

The influence of soil communities on the temperature sensitivity of soil respiration

Soil respiration represents a major carbon flux between terrestrial ecosystems and the atmosphere, and is expected to accelerate under climate warming. Despite its importance in climate change forecasts, however, our understanding of the effects of temperature on soil respiration (RS) is incomplete. Using a metabolic ecology approach we link soil biota metabolism, community composition and heterotrophic activity to predict RS rates across five biomes. We find that accounting for the ecological mechanisms underpinning decomposition processes predicts climatological RS variations observed in an independent dataset (n = 312). The importance of community composition is evident because without it RS is substantially underestimated. With increasing temperature, we predict a latitudinal increase in RS temperature sensitivity, with Q10 values ranging between 2.33 ± 0.01 in tropical forests to 2.72 ± 0.03 in tundra. This global trend has been widely observed, but has not previously been linked to soil communities.Using a model derived from metabolic theory, the authors identify the contributions of soil biota metabolism, community composition and heterotrophic activity to soil respiration. The approach accurately predicts variation in respiration with mean annual temperature (MAT) across five biomes.

Forecasting tillage and soil warming effects on earthworm populations

1. Healthy soils are crucial for sustainable food production, but tillage limits the biological regulation of essential ecosystem services. Better understanding of the mechanisms driving management effects on soil ecosystem engineers is needed to support sustainable management under environmental change. 2. This paper presents the EEEworm (Energy–Environment–Earthworm) model, a mechanistic individual-based model (IBM) of Lumbricus terrestris populations. L. terrestris is a dominant earthworm species in undisturbed habitats and is closely associated with numerous ecosystem services such as water flow regulation, soil structure and crop production. In reduced tillage agriculture a decline in mechanical disturbance allows for L. terrestris proliferation, whilst the activities of L. terrestris can replace many of the soil functions provided by tillage. 3. Extensive EEEworm validation with eight published studies (average R2 = 0.84) demonstrates a mechanistic approach which can extrapolate between diverse soil, management and weather conditions. EEEworm simulation experiments elucidate that a combination of direct and indirect tillage effects lead to population declines in tilled fields, with litter removal from the soil surface being the main driver. 4. We investigate the effects of different tillage intensities under historical and projected soil warming conditions, and find that future warmer and drier soils in our simulation exacerbate the effects of deep ploughing on L. terrestris population declines. These effects result from warmer and drier soil conditions increasing individual metabolic rates and tillage reducing food availability to meet energy demands. 5. Synthesis and applications. Pre-emptive strategies to mitigate climate change impacts on soil health in agroecosystems should focus on decreasing tillage intensity and retention of crop residues following tillage. EEEworm has the potential to benefit land managers, policy makers, risk assessors and regulators by providing a tool to forecast how soil systems respond to combinations of land management and environmental change. To allow better cost-benefit analysis of contrasting land management systems a future aim of mechanistic models like EEEworm is to incorporate the links between earthworm populations, soil functions and ecosystem services.