The Pattern And Mechanism Of Fertilization Effect On Soil Nematode Community In A Paddy Rice-upland Wheat Agroecosystem

Soil nematodes are the most abundant microfauna in the soil food web occurring at multiple trophic levels. They contribute a lot to the nutrient cycling and respond sensitive to the environmental variation, thus are expected to be greatly influenced by fertilization and soil condition. However, the mechanism of soil nematodes respond to fertilization and environmental change remains unclear. We conducted a 5 years fertilized experimental study in a double-cropping system with paddy rice and upland wheat in eastern China. Six fertilizer treatments were compared, including control (CK), 100% chemical fertilizer (CF),pig manure + 50% chemical fertilizer (MCF), straw + 100% chemical fertilizer (SCF), pig manure + straw + 50% chemical fertilizer (MSCF) and compound pig manure-chemical fertilizer (CMCF). Our study aim to investigate the pattern and mechanism of soil nematode abundance and community structure responded to fertilization in the paddy rice-upland wheat agroecosystem.A meta-analysis and field fertilization experiment were included in this study. The meta-analysis was used to evaluate the fertilizer effects on soil nematode communities by collecting data points from published articles. Fertilizer effects were considered from two aspects: the application amount of chemical or organic fertilizer and the types of organic fertilizer. The field fertilization experiment was studied from four directions: 1)investigated the seasonal dynamic variation of soil nematode communities respond to fertilization in 5 years; 2) compared the soil nematode communities in the rhizosphere and bulk soil of paddy rice and upland wheat respond to organic and chemical fertilizers; 3)evaluated nematode body size responds to fertilization and its application as ecological indices; 4) evaluated how fertilization impacted nematode trophic groups via changes in trophic resources (soil microbes and plant roots) and the soil environment (moisture condition and pH).Our main results were as follows:1. Meta-analysis showed that nematode species richness, structural index (SI) and omnivore-predator abundance were increased with the organically-fertilized regime and declined in inorganically fertilized regimes. species richness and total nematode abundance increased with increasing carbon (C) inputs from organic fertilizers, whereas greater nitrogen (N) application rates from fertilizers significantly reduced the species richness,Shannon's diversity (H')and omnivore-predator nematode abundance. Furthermore,organic fertilizers differed in their impact on soil nematodes, as those with C-rich crop residues rich supported larger free-living nematode populations and greatly promoted H', SI,maturity index (MI) and enrichment index (EI), whereas N-rich animal manure was more effective in controlling plant-feeding nematodes. Results suggests that the application of C-rich crop residues is the most effective practice to enhance soil biodiversity and preserve the ecological resilience.2. Dominant nematode genera were different between rice phase and wheat phase, for the rice phase were the plant-feeding nematode Hirschmanniella and algae-feeding nematode Rhabdolaimus, whereas dominant genera in the wheat phase were the fungal-feeding nematode Filenchus and bacterial-feeding nematodes Cephalobus,Eucephalobus and Acrobeloides. Total nematode abundance but not dominant genera responded to fertilization consistently in both phases. The CMCF treatment raised 1.3-6.2 times higher Hirschmanniella abundance but sharply reduced the Rhabdolaimus population by 1.2-7.8 times in the rice phase. In addition, SCF and MSCF treatments increased 60%higher Filenchus numbers but decreased 90 % Acrobeloides population in the wheat phase.These results indicated that in this double-cropped system, soil moisture condition is the determinant of the nematode community structure, whereas fertilization controls the total nematode abundance and population size.3. The rhizosphere effect on the nematode community was greater in upland wheat(aerobic soil) than in paddy rice (flooded soil). There was a greater nematode abundance and enrichment index (EI), but less nematode diversity and lower structure index (SI) in the rhizosphere than bulk soil of upland wheat. In contrast,the paddy rice system had similar abundance, diversity and ecological indices in the rhizosphere and bulk soil. Fertilization had significant effect on soil nematodes in both phases. There was almost no interaction between sampling location (rhizosphere and bulk soil) and fertilization on any measure of soil nematode communities, indicating that fertilization did not enhance the rhizosphere effect on soil nematode communities.4. A trend of increasing body size in fertilized plots was observed for most genera,with a relatively small increase in the size of small-bodied bacterivores and fungivores and a relatively large increase in the size of large-bodied omnivores. Fertilized plots had significantly greater CWM of body size than the CK treatment, although total nematode abundance increased significantly in the MSCF treatment only. Discriminant and multiple regression analyses showed that CWM of body size was positively correlated with the soil organic C, total N, available P and available K concentrations, which responded to fertilizer inputs. In contrast, soil fertility was weakly related to following abundance-based indicators:Margalefs richness index, Shannon's diversity index, summed maturity index (EMI) and enrichment index (EI) in both phases, indicating that CWM of body size has obvious advantages in indicating the variation of soil fertility and soil environment.5. The structural equation modeling (SEM) showed that the fertilization-induced changes in food resources (soil microbes, plant roots) and soil environment (soil moisture,pH) had effect on nematode trophic groups differed between rice phase and wheat phase.Plant roots significantly affected the abundance of fungal-feeding, plant-feeding and omnivore nematodes in the wheat phase, but not in the rice phase. Soil environment had strong relationship with the bacterial- and fungal-feeding nematodes in the wheat phase, but low relationship with all nematode trophic groups in the rice phase. Specifically, the effect of fertilization on the linkages between bacteria and bacterial-feeding nematode communities differed in rice and wheat phases. The Eucephalobus genera were correlated with many bacterial groups in both rice and wheat phases, including phyla Proteobacteria(?,? and ?) and Acidobacteria (subg,roups 3, 4, 6 and 7). Other nematodes such as Panagrolaimus were correlated with many bacterial phylum in the wheat phase but only one phyla of bacteria in the rice phase, while Tylopharynx shown the opposite result. These results indicated that environmental variation of paddy rice and upland wheat can be responsible for the different fertilization-induced effect on nematode trophic groups as well as linkages between dominant nematode genera and dominant bacterial phylum differed between rice phase and wheat phase.