Effects of nitrogen substitution isonitrogen substitution on soil structural environment and root viability in the root zone of double-cropping rice

【Objective】The purpose of the study was to explore the effects of organic fertilizer isonitrogen substitution on the rice and root zone soil structural environment,soil microbial mass carbon,rice root growth,and to provide a theoretical basis for the optimization of reduced fertilizer enhanced efficiency planting system matched to intensive doublecropping rice production in the south.【Method】Based on the environment of a typical zone of double-cropping indica rice in Nanning,Guangxi,four types of localized treatments were set up:no fertilization control(CK),inorganic nitrogen (T1,100% chemical fertilizer),inorganic nitrogen with organic nitrogen(T2,green manure+chemical fertilizer),and organic nitrogen(T3,green manure+biochar+silkworm excrement+nitrogen-free chemical fertilizer). Rice plant samples and root zone soil samples were collected at 7,14,21,35 and 42 d after transplanting during the critical period from greening after transplanting to the formation of the root system in sunny fields. The growth of rice roots and changes in the structure of soil aggregates and microbial mass carbon content were systematically analyzed.【Result】Isonitrogen substitution promoted the formation of soil water-stable aggregates and increased the microbial mass carbon content. In the early rice season,the total content of small and medium-sized aggregates(classes II to V)in each treatment showed T2 (21.44%)>T3(18.85%)>T1(13.4%)>CK(7.65%). The content of microaggregates(classes VI to VII)was higher in the T2 and T3 treatments than that in the CK and T1 treatments,and the mean weight diameter(MWD)of soil waterstable aggregates in the T2 treatment was lower than that in the other treatments. In the late rice season,the total content of small and medium-sized aggregates(classes II to V)in each treatment was in the order of T2(21.44%)>T3(18.85%)> CK(12.11%)>T1(7.70%),with higher microaggregate content and lower MWD in the T2 and T3 treatments(classes VI to VII). Geometric mean diameter(GMD)of both T2 and T3 treatments of double-cropping rice was significantly lower than that of T1 and CK treatments(P<0.05,the same below),and microbial mass carbon content was in the order of T2> T3>T1>CK. Isonitrogen substitution changed rice root volume and root viability and enhanced plant nutrient uptake. In early rice season,42 d after transplanting,root volume performance and root active absorption area performance were T2> T1>T3>CK. In the late rice season,the root volume performance at 42 d after transplanting was consistent with that of the early rice season,and the root active absorbing area performance was T1>T2>CK>T3. Isonitrogen substitution affected the root shoot ratio and dry matter accumulation of rice. In the early rice season,with the prolongation of transplanting time,the root shoot ratio of each treatment showed an overall trend of increasing and then decreasing,all of which peaked at 21 d after rice transplanting. In the late rice season,the peak root shoot ratio was not significant in all treatments. At 42 d after transplanting,the dry matter weight of plants differed significantly among treatments,both showing T2>T1>T3>CK.【Conclusion】Green manure,biochar and silkworm excrement can promote the formation of soil waterstable aggregates,increase the microbial mass carbon content,and facilitate the growth and development of rice roots. Organic fertilizer isonitrogen substitution production model matching the planting system can be an important optimization measure for reducing fertilizer and increasing efficiency of intensive rice in southern double-cropping indica rice area.