Fixed position study on basic cropping system of paddy field in downland region of South Sichuan [China]. II. The effect on yield increase and influence to soil fertility by the different patterns of multiple cropping system in paddy fields

abonos verdes absorption de substances nutritives arroz arroz irrigado crop yield culture multiple engrais vert fertilidad del suelo green manures irrigated rice multiple cropping nutrient uptake paddy soils rendement des cultures rendimiento de cultivos rice riz soil fertility trigo wheats