Effects of agricultural intensification on soil-associated arthropod population dynamics, community structure, diversity and temporal variability over a seven-year period

While most studies focusing on the effects of agricultural intensification on soil biota are inherently short-term in nature, long-term (multiyear) studies are essential in assessing long-term temporal responses of soil biota to agronomic practices. We investigated the effects of three components of agricultural intensification, i.e. cultivation (disturbance), herbicide addition (modification of floristic composition) and mulching (resource addition) on soil-associated arthropods in an annual (maize) and a perennial (asparagus) cropping system over a 7 yr period. An additional treatment (hand-hoeing of weeds during the crop growing season) was used to represent minimal intensification. Many taxa of arthropods responded positively to mulching and to treatments which allowed high weed biomass in the non crop-production period, e.g. the hand-hoeing and cultivation treatments in the perennial crop. Herbicide treatments also facilitated high numbers of many taxa in the annual crop when this coincided with plot invasion by herbicide-tolerant weeds. Generally, arthropod taxa were positively correlated with weed biomass and negatively with crop plant biomass, probably because of the superior resource (litter) quality produced by the former. Ordination analyses indicated that arthropod community structure was often correlated with weed community structure. Mulching and allowing high weed biomass also promoted a high species richness of soil-associated Coleoptera, but coleopteran diversity was not related to weed species diversity. Analyses of temporal variability (inversely related to stability) of arthropod taxa across years revealed few treatment effects in the annual crop, but showed destabilising effects of weed reduction in the perennial crop. In the perennial crop, temporal variability was also positively correlated with crop biomass and negatively with weed biomass across plots. Our study shows that agricultural intensification is not consistently harmful to the soil fauna, that soil-associated arthropods are most responsive to management practices which affect the nature and quality of resource input, and that long-term experiments are essential for answering questions about how agricultural practices affect soil organisms against the natural backdrop of temporal variation.