Effects Of New Agronomic Practices On Biodiversity Of Weed Community In Rice-Wheat Croping System

By using new agronomic methods, such as labour-saving cultivations (i.e. dry direct seeding, wet direct seeding, seedling throwing, mechanized-transplanting, wheat-rice interplanting et al.), no-tillage combining with mulch of straw, and rice-duck farming, the farmers can be liberated from the burdensome labour of the field, the agro-ecosystem can be improved and the weed infestation can be weakened or controlled. The application of new agronomic methods can affect the weed community through changing the environment of the field. Systemic study had not been done up to now about the manner and degree of the influence.Since the rural industrial structure adjustment, labour-saving cultivation have been applied in rice cultivation. Conversion of the rice cultivating mode from conventional manual transplanting to labour-saving cultivating have directly affected the structure of the potential weed community in paddy field. In the paper the quantitative characteristics of the potential weed communities (weed seedbank) in paddy fields was investigated under six rice cultivating modes:dry direct seeding, wet direct seeding, seedling throwing, mechanized-transplanting, wheat-rice interplanting and conventional manual transplanting. The result indicated that the overall density of weed seedbank with dry direct seeding was up to 228,416 seeds/m2, significantly higher than the other modes, and followed by the wheat-rice interplanting mode. The seedbank density under mechanized-transplanting, seedling throwing, wet direct seeding and manual transplanting were not reached 1/3 of the dry direct seeding. Regarding the seedbank density of different type of weed, seed density of paddy weed was higher than that of the weeds in wheat field and weeds in autumn crops field under whichever rice cultivating mode. Sedge weed seed density under dry direct seeding and broad leaf weeds under wheat-rice interplanting were significantly higher than all weed types under the other different rice cultivating modes. Except dry direct seeding, seekbank density of broad leaf weeds was higher than grass weeds, sedge weeds in corresponding rice cultivating modes. The test result of seedbank densities in different soil layers showed that the density in topsoil under mechanized-transplanting was significantly higher than under lower layer. At the same time, the seedbank densities in different layers under dry direct seeding were significantly higher than corresponding layers in the other rice cultivating modes. Comparison of Margalef index among different rice cultivation modes exhibited that the conventional manual transplanting was the highest, and the seedling throwing was the lowest. Species diversity indexes under wet direct seeding and wheat-rice interplanting, such as Shannon-Wiener index, Simpson index, Pielou evenness index, were higher than other modes. Through comparison of the relative importance of the weed seeds, the dominant weeds in the potential weed community were listed in those six rice cultivation modes. The result from analysis of ecological conditions among different rice cultivation modes indicated that the difference of water management was one of the main factors in affecting the potential weed community in paddy field. It is suggested that the different labour-saving cultivations could be alternatively used to effectively and persistently control the potential weed community in paddy fields.Labour-saving cultivation of rice farming compared to the traditional manual transplant is different significantly in the farming practices and water management, which will directly affect the structure of weed community in the paddy fields. In order to reveal the characteristics of the weed community and weed infestation in paddy field under long-term labour-saving cultivation,138 random sampling fields of 14 sites were surveyed to obtain the weed important value in the paddy fields of middle Jiangsu Province with seven scales by visualization. Thirty six weed species, belonging to 33 genus of 18 families, were found in this survey. The families including the most weed species were Gramineae, Cyperaceae, Compositae, Scrophulariaceae, Alismataceae and Lythraceae. Four weed species Echinochloa crusgalli (L.) Beauv., Ammannia baccifera L., Eclipta prostrata (L.) L. and Leptochloa chinensis (L.) Nees were showed high dominance and frequency in all the surveyed three regions, and their frequency is at or near 100 percent. However, there were regional differences in weed infestation level, Yangzhou in the weeds against the most, followed in Taizhou, Nantong relative minimum. Different cultivation technique corresponds to different dominant weeds, such as Monochoria vaginalis (Burm. F.) Presl ex Kunth and Ammannia baccifera L. in mechanized-transplanting mode; Ammannia baccifera L., Digitaria sanguinalis (L.) Scop., Monochoria vaginalis (Burm. F.) Presl ex Kunth and Lobelia chinensis Lour. in wheat-rice interplanting mode; Echinochloa crusgalli (L.) Beauv., Monochoria vaginalis (Burm. F.) Presl ex Kunth and Ammannia baccifera L. in seedling throwing mode; Echinochloa crusgalli (L.) Beauv., Ammannia baccifera L. and Monochoria vaginalis (Burm. F.) Presl ex Kunth in wet direct seeding mode; Echinochloa crusgalli (L.) Beauv., Ammannia baccifera L. and Leptochloa chinensis (L.) Nees in dry direct seeding mode. Comparing with the conventional manual transplanting mode, the weed community in paddy field with labour-saving cultivation techniques not only had higher species richness, but also had higher dominance. The order of the weed infestation, from low to high, was conventional manual transplanting< mechanized-transplanting< seedling throwing< wet direct seeding< dry direct seeding< wheat-rice interplanting.To solve the weed problem in paddy field under labour-saving cultivating mode, conversion of cultivating mode was tried in this study. After the conversion from dry direct seeding (DDS) to wet direct seeding (WDS), wheat-rice interplanting (WRI), manual transplanting (MAT), mechanized-transplanting (MET),14 weed species were found in the field of WRI, and 12 weed species were found respectively in the other cultivating modes. 10 species were in common under the five cultivating modes. Weed density under WDS was the most (994 individual/m2), followed with the WRI (959 individual/m2) and DDS (CK,792 individual/m2). There were not significant difference between WDS,DDS and WRI. Weed density under MET (374 individual/m2) and MAT (264 individual/m2) were very significantly lower than DDS, WDS and WRI. Moreover, dynamics of the weed occurrence and the dominant species had changed because of the cultivating mode conversion. All in all, the weed community in paddy field had not followed the characteristics of the previous year after the conversion of rice cultivating mode, and the troublesome weed problem was controlled in some degree.Effect of tillage and straw returning (mulch with straw) on the weed occurrence was studied at Yangtze river valley. The result showed that density order of the treatments from high to low was:No tillage-No tillage-No returning> No tillage-No tillage-Returning> Rotary tillage-No tillage-No returning> Rotary tillage-No tillage-Returning> No tillage-Rotary tillage-No returning> No tillage-Rotary tillage-Returning> Rotary tillage-Rotary tillage-No returning> Rotary tillage-Rotary tillage-Returning. Based on analysis of multi-factor variance and the Bray-Curtis similarity index, it was clear that the tillage types used in previous crop affected the weed density in the current crop significantly:weed density under rotary tillage was lower than that of no-tillage, with density of 932 individual/m and 1149 individual/m respectively; and weed density was affected very significantly by the tillage type used in the current crop:848 individual/m2 under rotary tillage and 1233 individual/m2 under no tillage; the treatment with straw mulching could reduce the weed occurrence to some extent, but the effect was not significant; there was no significant interaction among the three testing factor. The dominant weed species were confirmed by comparing the relative importance.The observation of the changes and dynamics of the weed seedbank and the weed control effect under long-term rice-duck farming system (RDFS) were studied from 1999 to 2008.The result showed that density of the whole seedbank decreased 90.73% accumulatively after nine successive years. Seed reduction of broad-leaf weed and sedge weed was around 95%, however, that of grass weed was only 76%. Seeds of paddy field weed reduced much more than those of wheat field weed. RDFS made the vertical distribution of weed seed uniform in the soil. Base on the ecological indices, it could be found that the seedbank had gradually evolved to the direction of fewer species, lower density and lower diversity after long-term RDFS. After nine years of continuous RDFS, the community structure of seedbank had evolved from Lindernia procumbens+Mazus japonicus+Cardamine hirsuta+Alopecurus aequalis to Alopecurus aequalis+Rotala indica+Mazus japonicus+Cyperus difformis. RDFS made the density of the paddy field weeds decreased annually. Reduction trend was best fitted by the damping model y= k+a·ebx, and the parameter b could reflect the rate of declination of the weed populations. Our results provided an important proof for the positive effect of RDFS on the control of the overground weed community and the underground potential weed community. All in all, RDFS is an effective weed management strategy based on ecological method.