Study On The Formation Of Weed Community And Its Nutrient Driving Mechanism In Rice-wheat Rotation System

Weeds are common components of the farmland ecosystem, the soil weed seed-bank is a demonstration of the existence of a weed community and a link of the growth phases of an ecosystem. Together with the above-ground weed community, the seed bank forms the weed community complex. The maintenance of an appropriate amount of weeds plays a role in protecting the biological diversity of farmlands, whereas excessive amounts of weeds often compete with crops for water, fertilizer, and light. Fertilization can alter the natural succession process of the weed community in farmland by directly improving the yield and competitive advantage of crops and changing the nutrient content and structure of the soil. The different responses of various weeds to nutrients can directly alter the structure of the weed community. Proper nutrient management can improve the competitive relationship between crops and weeds and alter the species composition of the weed community, effectively controlling the occurrence of some malignant weeds, meanwhile, proper nutrient management can retain a certain level of biological diversity among controllable weeds.We studied the heterogeneity of a soil weed seed-bank and the heterogeneity of weed community diversity in a rice-wheat rotation system after the long-term application of different organic or non-organic fertilizers, analyzed the effects of major nutrients on the characteristics of the weed during 2010-2012. We sampled the soil in the Taihu area after a 31-year long-term fertilization experiment, identified and counted the species of weed seeds in the surface soil of 12 differentially treated areas using microscopic examination, provided information on nutrient management for the coordination of weed control in wheat field, bio-diversity protection and noxious weeds decrease. Chemical fertilizer treatments were selected as follows:CO (no fertilizer), CN (N fertilizer), CNP (N plus P fertilizer), CNK (N plus P fertilizer), CPK (P plus K fertilizer) and CNPK (N plus P and K fertilizers); manure treatments were selected as follows: MO (manure), MN (manure plus N fertilizer), MNP (manure plus N and P fertilizer), MNK (manure plus N and K fertilizer), MPK(manure plus P and K fertilizer), MNPK(manure plus N, P and K fertilizer). We analyzed the Simpson, Shannon, Margalef, and Pielou indexes of these samples, at the same time, principal component analysis (PCA) and redundancy analysis (RDA) were executed to mining the relationship between fertilizer management and weed community. We also studied explored the impact of farmland nutrient habitats on the density and diversity of the weed community. The present study explored the structural characteristics of the weed community, the diversity index, and the primary environment-impacting factors under different conditions of fertilization and provided a basis for the ecological management of farmland weeds. The main results were as follows:1 The nutrient indexes of the differently treated soils varied significantly. Compared with the initial soil nutrients, for the no-nitrogen treatments, the soil total nitrogen contents increased by 4.1% and 23.1% in the CO-and CPK-treated soil, respectively, and increased by 46.8% in the MPK treatment, the maximum difference of the soil total phosphorus between the treatments CO and MPK reached as high as 2572.2 mg kg-1. For the no-phosphorus fertilization treatments, the soil total phosphorus content was the same in the N-and NK-treated soil as in the initial soil and increased by 600% in the MPK treatment. The available potassium contents in all of the treatments decreased to variable degrees compared with the initial values, with the most significant decrease in the no-potassium treatment. The application of organic fertilizers had significant effects on the soil nutrient indexes. Compared with the non-organic fertilizers, the organic fertilizer treatment increased the soil levels of total nitrogen, total phosphorus, alkaline hydrolyzed nitrogen, available phosphorus, and organic materials by 18.16%,112.74%,11.82%, 159.42%, and 20.22%, respectively. The available potassium content decreased by 15.47%, and the total phosphorus content of the MNP treatment was higher than that of the CNP treatment by 111.17%. These results indicate that the application of organic fertilizers can increase the index values of soil nutrients, with the exception of the available potassium in soil. The yields of rice and wheat in different treatment plot fluctuated and varied greatly among different years affected by climate and other factors. The wheat yield was the highest for the balanced NPK fertilization, which was increased compared with the CPK, CNK, and CNP treatments by 142.09%(P< 0.05), 58.23%(P< 0.05), and 14.10%(P< 0.05), respectively. From statistic results of long-term experiment, increased trends of the yields of wheat and rice were found among different years. The increasing trend of rice yield was more significant than wheat, and yield of wheat fluctuated comparatively larger than rice. A deficiency of either N or P elements in the soil significantly decreased the rice and wheat yield. Although the content of available K decreased by varying degrees compared with the initial soil values, depending on the treatment, the differences in the available K content did not significantly affect the crop yields.2 The density of the soil weed seed-bank differed significantly among the tested soils, with the lowest density in the MNPK-treated soil, which had only 20.25% of the maximum density in the CPK-treated soil. The average seed density of the different organic fertilizer treatments was 48.71% lower than that in the soil treated with the chemical fertilizer (F=29.822, p<0.001). When comparing the soil weed seed-bank density of the organic fertilizer treatments with the corresponding chemical fertilizer treatments, the weed-seed density of the MPK treatment was 74.80% lower than that of the CPK treatment (t=25.218, p<0.001), with the most obvious decline. The MNP treatment only had a 0.039% decrease relative to the CNP treatment (t=0.607, p=0.576), with an insignificant difference. Therefore, the application of the organic fertilizers significantly reduced the density of the soil weed seed-bank, and at the same time, organic fertilizers could minimize the effects that the chemical fertilizers exerted on the soil weed seed-bank density. Thirty species of weed seeds were detected in the soil samples, belonging to 15 families. The seed density of Monochoriav aginalis of the Pontederiaceae family, Lindernia procumbens of the Scrophulariaceae family, and Ammannia baccifera of the Lythraceae family accounted for 20.78%,15.83%, and 10.37% of the total density of the seed-bank, respectively (Table 2), and were therefore considered the dominant species. Polygonum lapathifolium of the Polygonaceae family, Gnaphalium affine of the Asteraceae family, Ammannia arenari H. of the Lythraceae family, Eleocharis acicularis of the sedge family, and Cyperus difformis accounted for 9.04%,6.02%,5.54%,5.82%, and 5.77% of the total density of the seed bank, respectively, and were considered the subdominant species. These weeds had high seed densities and were widely distributed in the various treatments. These weeds are also the main weed populations that harm the farmland of the Taihu area. Compared to the chemical fertilizer treatments, the average species number in the organic fertilizer treatments was further reduced by 2.61 with long-term application, the average evenness index increased by 0.15, and the average Shannon index and Simpson index changed insignificantly. The community diversity index of the weed seed-bank differed between the organic and corresponding non-organic fertilizer treatments: multiple community diversity indexes demonstrated no significant difference between the CO and MO treatments, but the values in these treatments were significantly higher than in the other treatments. Among all of the treatments, the balanced fertilization treatment (MNPK treatment) using both organic and non-organic fertilizers not only maintained higher diversity and evenness indexes but also had a significantly higher species number and Margalef index than the other treatments, suggesting that balanced fertilization with organic and non-organic fertilizers can maintain the stability of a farmland ecosystem. The variance analyses showed that the treatments with chemical fertilizers and chemical fertilizers plus organic fertilizers had significant effects on the number and diversity index of species (Table 4). The organic fertilizers had significant effects on the species number, Margalef index, and equitability but had no significant effect on the Simpson index or Shannon index. The long-term application of organic fertilizers could significantly reduce the density of a soil weed seed-bank; non-organic fertilizers and a combination of non-organic and organic fertilizers had a significant influence on the number of species and diversity of weeds. The application of organic fertilizers could improve the Simpson, Shannon, and Pielou indexes of a soil weed seed-bank community and stabilize the community structure. In terms of the soil nutrient system itself, the soil organic materials and total nitrogen content are the main environmental factors affecting the distribution of a soil weed seed bank. Balanced fertilization can affect the dominant position of dominant weed populations, increase the degree of serenity, and improve the productivity and stability of farmland ecosystems.3 Among the weeds, Conyza Canadensis, Vicia saliva, Alopecurus aequalis Sobol, Mazus japonicus and Lapsana apogonoides showed high density and were widely distributed in every treatment area. These plants are the main dominant weed populations that affect wheat in the Taihu area. The long-term application of nitrogen and organic fertilizers could significantly reduce the density of weeds; non-organic fertilizers and a combination of non-organic and organic fertilizers had a significant influence on the number of species and diversity of weeds, the application of organic fertilizers could reduce the community diversity indices of weed. In terms of the soil nutrient system itself, the soil organic materials and nitrogen content are the main environmental factors affecting the distribution of weed image. The results also indicate that the application of a balanced fertilizer allow for all weed species present to shrive, and then affect the dominant position of dominant weed populations, increase the degree of serenity, and improve the productivity and stability of farmland ecosystems.4 Among the weeds, Eleocharis yokoscensis, Monochoriav aginalis, Ammannia arenara, and HerbaCeratopteridis showed high density and were widely distributed in this area. These weeds are the main dominant weed populations that affect rice in this area. Rice-wheat crop rotation is an important mode of production of Chinese farmland ecosystems, and the areas employing this method of production are also the high-yield regions in China. The long-term application of nitrogen and phosphorus fertilizers could significantly reduce the density of weeds, this study also showed that phosphorus fertilizer applications significantly reduced the density of sedge and effectively increase the density of M. aginalis, A. arenara and HerbaCeratopteridis grew best in no fertilizer treatment. The application of nitrogen and phosphorus fertilizers could reduce the richness index and increase the evenness index of weed community. In the absence of application of exogenous substances (CO) the biodiversity of the farmland ecosystem in this experimental study was maintained at a high level, the no-fertilizer treatment was adequate to meet the growing needs of most of the weeds. At heading stage of rice, the eigenvalues of the first and second ordination axes were 0.901 and 0.08, respectively. The correlation coefficients between the total P, Olsen P, total N, and available N, in the soil and the first ordination axis were-0.887、-0.816、-0.810、-0.794, respectively. In terms of the soil nutrient system itself, the soil organic materials, nitrogen and phosphorus content are the main environmental factors affecting the distribution of weed image. The results also indicated that the application of different fertilizers affected the dominant position of dominant weed populations and changed the weed community in local area.