Temporal trends in arthropod abundances after the transition to organic farming in paddy fields

<div><p>Organic farming aims to reduce the effect on the ecosystem and enhance biodiversity in agricultural areas, but the long-term effectiveness of its application is unclear. Assessments have rarely included various taxonomic groups with different ecological and economic roles. In paddy fields with different numbers of years elapsed since the transition from conventional to organic farming, we investigated changes in the abundance of insect pests, generalist predators, and species of conservation concern. The abundance of various arthropods exhibited diverse trends with respect to years elapsed since the transition to organic farming. Larval lepidopterans, <i>Tetragnatha</i> spiders, and some planthoppers and stink bugs showed non-linear increases over time, eventually reaching saturation, such as the abundance increasing for several years and then becoming stable after 10 years. This pattern can be explained by the effects of residual pesticides, the lag time of soil mineralization, and dispersal limitation. A damselfly (<i>Ischnura asiatica</i>) did not show a particular trend over time, probably due to its rapid immigration from source habitats. Unexpectedly, both planthoppers and some leafhoppers exhibited gradual decreases over time. As their abundances were negatively related to the abundance of <i>Tetragnatha</i> spiders, increased predation by natural enemies might gradually decrease these insect populations. These results suggest that the consideration of time-dependent responses of organisms is essential for the evaluation of the costs and benefits of organic farming, and such evaluations could provide a basis for guidelines regarding the length of time for organic farming to restore biodiversity or the economic subsidy needed to compensate for pest damage.</p></div

Regression models relating years since transition to organic farming (X) to abundance of arthropods (Y).

<p>Three different models (Michaelis-Menten, power law, linear) were applied, and the best model was chosen based on AIC. Here, ΔAIC is the difference of AICs between null and the best models. If ΔAIC was larger than 2, the best model was regarded as meaningful, and the fitted curve was drawn in Figs <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0190946#pone.0190946.g001" target="_blank">1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0190946#pone.0190946.g002" target="_blank">2</a>.</p

Relationships between the time since the transition to organic farming and the abundance of organisms (stink bug, planthopper, and leafhopper species).

<p>Regression curves were drawn when the best model had ΔAIC > 2 in comparison to the null model. Horizontal dashed lines indicate the mean values in control fields subjected to conventional farming.</p

Relationship between the spider abundance in late July and insect abundance in early August.

<p>Relationship between the spider abundance in late July and insect abundance in early August.</p

Results of two-way ANOVAs showing the effects of farming type and season on the abundance of various arthropods (*: p<0.05; **: p<0.01; ****:p<0.001).

<p>Results of two-way ANOVAs showing the effects of farming type and season on the abundance of various arthropods (*: p<0.05; **: p<0.01; ****:p<0.001).</p

Relationships between the time since the transition to organic farming and the abundance of organisms (Lepidoptera, <i>Tetragnatha</i> spider, and damselfly species).

<p>Regression curves were drawn when the best model had ΔAIC > 2 in comparison to the null model. Horizontal dashed lines indicate the mean values in control fields subjected to conventional farming.</p

Three study regions: Kazuno and Nagano in Japan (a) and Elgin in Western Cape, South Africa (b) shown with dots.

<p>Three study regions: Kazuno and Nagano in Japan (a) and Elgin in Western Cape, South Africa (b) shown with dots.</p

The basic information of the study sites.

<p>Annual mean temperature is average across the last 30 years.</p><p>^a A farmer is referred to as a ‘Co-op farmer’ when more than a half of their products are sold through the farmers’ cooperative or an equivalent institution, otherwise the farmer is a ‘Non-co-op farmer’.</p><p>The basic information of the study sites.</p

Incorporation of fallow weed increases phosphorus availability in a farmer’s organic rice fields on allophanic Andosol in eastern Japan

<p>We investigated the amount of soil phosphorus (P) in a farmer’s paddy fields under organic farming (OF) for various periods from 0 to 22 years as well as other farmers’ fields under conventional farming. All the fields are located in allophanic Andosol with long history of P fertilizer application, and some of them have been converted to OF across years. After conversion to OF, P was supplied only with winter fallow weeds mainly Foxtail (<i>Alopecurus aequalis</i>), rice residues (rice bran and straw), and guano. We determined total-P (Tot-P) and plant-available P (Av-P), which consists of Truog-P (Tru-P) and Bray-2-P under reducing condition with ascorbic acid (Asc-P), in soils of each field. For both Av-Ps, the ratio to Tot-P increased across years under OF following quadratic functions with both linear and quadratic terms being statistically significant. The ratios showed little changes for the initial 15 (Tru-P) or 10 (Asc-P) years and increased rapidly thereafter. These temporal changes in Av-P were consistent with the rapid increase of the amount of P accumulated in the winter fallow weeds and incorporated in the fields after beginning of OF. These results led us to the hypothesis that the incorporation of winter weeds has contributed to the increase of Av-P in the organic fields across years. We tested this hypothesis by investigating temporal changes of Av-P after suspending the weed incorporation for 2 consecutive years in plots of the organic fields. Both Av-Ps were significantly greater in plots with continued weed incorporation (CWI) than those in plots with its suspension. We further found that the increase of Asc-P in plots with CWI was 4.9-fold the input of total P in the incorporated weeds. This suggests that the incorporation of winter fallow weeds enhanced soil-P availability beyond the supply of P accumulated in the weeds.</p

Identification of a novel strain of green algae, <i>Parachlorella</i> sp binos (Binos). a

<p>, Microscopic image of <i>P.</i> sp <i>binos</i> (Binos) (x2000). <b>b</b>, India ink staining of <i>P.</i> sp <i>binos</i> (Binos) (x400). India ink was diluted with water to make a 10% solution. <b>c, d</b>, Phylogenetic analysis of members of the phylum Chlorophyta based on (<b>c</b>) 18S rRNA gene and (<b>d</b>) actin amino acid sequences. <b>e, f</b>, Transmission electron micrographs of (<b>e</b>) <i>P.</i> sp <i>binos</i> (Binos) and (<b>f</b>) <i>Chlorella</i> sp. <b>g</b>, Infrared spectra of the extracellular matrix of <i>P.</i> sp <i>binos</i> (red line) and purified sodium alginate (standard; black line) analyzed by Fourier transform infrared spectroscopy from 400 to 4000 cm⁻¹.</p