Filter-feeders have differential bottom-up impacts on green and brown food webs

Rice-crayfish farming system promote subsoil microbial residual carbon accumulation and stabilization by mediating microbial metabolism process

Rice-crayfish farming systems (RCs) can help mitigate climate change by enhancing soil organic carbon (SOC) sequestration. However, the mechanisms that govern the responses of microbial residues carbon (MRC), a key component of SOC, in RCs are not fully understood. We conducted a 6-year field experiment comparing RCs and rice monoculture systems (RMs). Specifically, we explored how MRC formation and stabilization differ between the two systems and how those differences are linked to changes in the metabolic processes of microbes. Results showed that MRC levels in RCs were 5.2 % and 40.0 % higher in the topsoil and subsoil, respectively, compared to RMs, indicating depth-dependent effects. Notably, MRC accumulation and stabilization in RCs were promoted through a cascade of processes of dissolved organic carbon (DOC) accessibility-microbial metabolism-mineral protection. In addition, the mechanism of MRC accumulation in subsoil differed between the two systems. Specifically, RMs improved accessibility of DOC by reducing humification and aromaticity of subsoil DOC, which helped microbes access to resources at lower cost. This decreased the respiration rate of microbes, thereby increasing microbial carbon pump (MCP) efficiency and thus promoting MRC accumulation. By contrast, the crayfish in RCs facilitated carbon exchange between topsoil and subsoil through their burrowing behaviors. This increased carbon allocation for microbial metabolism in the subsoil, supporting a larger microbial population and thus enhancing the MCP capacity, while reducing MRC re-decomposition via enhanced mineral protection, further increasing subsoil MRC accumulation. That is, MRC accumulation in the subsoil of RCs was predominantly driven by microbial population numbers (MCP capacity) whereas that of RMs was mostly driven by microbial anabolic efficacy (MCP efficiency). Our findings reveal a key mechanism by which RCs promoted soil MRC accumulation and stabilization, highlighting the potential role of DOC accessibility-microbial metabolism-mineral protection pathway in regulating MRC accumulation and stabilization.

Top-down effects of filter-feeding fish and bivalves moderate bottom-up effects of nutrients on phytoplankton in subtropical shallow lakes: An outdoor mesocosm study

Biomanipulation has been widely used in the ecological restoration of eutrophic lakes for decades. However, biomanipulation is prone to failure if external nutrient loads are not reduced. In order to explore the importance of filter-feeding fish and bivalves on algal control, an outdoor mesocosm experiment was conducted using different nutrient concentrations. Four treatments simulating daily loads of nutrients in Lake Taihu were studied: current, two times, and three times average daily loads of nutrients with both fish (Aristichthys nobilis) and Asian clam (Corbicula fluminea) and as a control current daily loads without fish or bivalves. Results showed that stocking of filter-feeding fish and bivalves (80 g m-3 bighead carp; 200 g cm-2 clams) at two times daily nutrient loads could effectively control water column Chl a concentrations and phytoplankton biomass. At higher nutrient concentrations (TN ≥ 260 μg L-1 d-1; TP ≥ 10 μg L-1 d-1), top-down control of filter-feeding fish and bivalves was less effective and bottom-up effects resulted in significant increases of Chl a concentration. Thus, as phytoplankton biomass in freshwater ecosystems is determined by both the top-down effects of predators and the bottom-up effects of nutrients, external loadings should be controlled when filter-feeding fish and bivalves are used for algal control to ensure the efficacy of biomanipulation.

Identifying potential drivers of distribution patterns of invasive Corbicula fluminea relative to native freshwater mussels (Unionidae) across spatial scales

This study aimed to identify the importance of ecological factors to distribution patterns of the invasive Clam (Corbicula fluminea) relative to native mussels (family: Unionidae) across seven rivers within the Mobile and Tennessee basins, Southeast United States. We quantitatively surveyed dense, diverse native mussel aggregations across 20 river reaches and estimated mussel density, biomass, and species richness along with density of invasive C. fluminea (hereafter Corbicula). We measured substrate particle size, velocity, and depth in quadrats where animals were collected. Additionally, we characterized reach scale environmental parameters including seston quantity and quality (% Carbon, % Nitrogen, % Phosphorous), water chemistry (ammonium [ NH 4 + ], soluble reactive phosphorous [SRP]), and watershed area and land cover. Using model selection, logistic regression, and multivariate analysis, we characterized habitat features and their association to invasive Corbicula within mussel beds. We found that Corbicula were more likely to occur and more abundant in quadrats with greater mussel biomass, larger substrate size, faster water velocity, and shallower water depth. At the reach scale, Corbicula densities increased where particle sizes were larger. Mussel richness, density, and biomass increased with watershed area. Water column NH 4 + increased at reaches with more urban land cover. No land cover variables influenced Corbicula populations or mussel communities. The strong overlapping distribution of Corbicula and mussels support the hypothesis that Corbicula are not necessarily limited by habitat factors and may be passengers of change in rivers where mussels have declined due to habitat degradation. Whether Corbicula is facilitated by mussels or negatively interacts with mussels in these systems remains to be seen. Focused experiments that manipulate patch scale variables would improve our understanding of the role of species interactions (e.g., competition, predation, facilitation) or physical habitat factors in influencing spatial overlap between Corbicula and native mussels.

Using aquatic animals as partners to increase yield and maintain soil nitrogen in the paddy ecosystems

Whether species coculture can overcome the shortcomings of crop monoculture requires additional study. Here, we show how aquatic animals (i.e. carp, crabs, and softshell turtles) benefit paddy ecosystems when cocultured with rice. Three separate field experiments and three separate mesocosm experiments were conducted. Each experiment included a rice monoculture (RM) treatment and a rice-aquatic animal (RA) coculture treatment; RA included feed addition for aquatic animals. In the field experiments, rice yield was higher with RA than with RM, and RA also produced aquatic animal yields that averaged 0.52–2.57 t ha^-1. Compared to their corresponding RMs, the three RAs had significantly higher apparent nitrogen (N)-use efficiency and lower weed infestation, while soil N contents were stable over time. Dietary reconstruction analysis based on ¹³C and ¹⁵N showed that 16.0–50.2% of aquatic animal foods were from naturally occurring organisms in the rice fields. Stable-isotope-labeling (¹³C) in the field experiments indicated that the organic matter decomposition rate was greater with RA than with RM. Isotope ¹⁵N labeling in the mesocosm experiments indicated that rice used 13.0–35.1% of the aquatic animal feed-N. All these results suggest that rice-aquatic animal coculture increases food production, increases N-use efficiency, and maintains soil N content by reducing weeds and promoting decomposition and complementary N use. Our study supports the view that adding species to monocultures may enhance agroecosystem functions.

Monoculture, where only one type of crop is grown to the exclusion of any other organism, is a pillar of modern agriculture. Yet this narrow focus disregards how complex inter-species interactions can increase crop yield and biodiversity while decreasing the need for fertilizers or pesticides. For example, many farmers across Asia introduce carps, crabs, turtles or other freshwater grazers into their rice paddies. This coculture approach yields promising results but remains poorly understood. In particular, it is unclear how these animals’ behaviours and biological processes benefit the ecosystem.

To examine these questions, Guo, Zhao et al. conducted three separate four-year field experiments; they compared rice plots inhabited by either carp, mitten crabs or Chinese softshell turtles with fields where these organisms were not present.

With animals, the rice paddies had less weeds, better crop yields and steady levels of nitrogen (a natural fertiliser) in their soil. These ecosystems could breakdown organic matter faster, use it better and had a reduced need for added fertilizer. While animal feed was provided in the areas that were studied, carp, crabs and turtles obtained up to half their food from the field itself, eating weeds, algae and pests and therefore reducing competition for the crops.

This work helps to understand the importance of species interactions, showing that diversifying monocultures may boost yields and make agriculture more sustainable.

The role of cladocerans in green and brown food web coupling

ABSTRACT Cladocerans, an important zooplankton community, are consumers from the base of the food web of aquatic environments. We investigated the contribution of producers (phytoplankton and periphytic biofilm) and particulate organic carbon (POC). Collections were carried out in lakes of the Upper Paraná River Floodplain, the last stretch free of dams in the second-largest South American basin. Isotope ratios (δ13C and δ15N) were measured, besides the contributions of probable food sources to the cladocerans biomass. The phytoplankton constituted the source of carbon for cladocerans, followed by POC. Thus this work, in addition to emphasizing the importance of cladocerans in nutrient cycling, highlighted the need for the conservation of environments surrounding the lakes as they are the sources of organic matter for aquatic communities. Besides, the analyzed zooplanktonic organisms demonstrated their role in the interconnection between the green and brown food webs, which have been studied separately for a long time.