Effects of long-term application of different green manures on ferric iron reduction in a red paddy soil in Southern China

Dissimilatory Fe（Ⅲ） reduction is an important process in the geochemical cycle of iron in anoxic environment. As the main products of dissimilatory iron reduction, the Fe（Ⅱ） species accumulation could indicate the reduction ability. The effects of different green manures on Fe（Ⅲ） reduction in paddy soil were explored based on a 31-year rice-rice-winter green manure cropping experiment. Four treatments were involved, i.e., rice-rice-milk vetch （RRV）, rice-rice-rape （RRP）, rice-rice-ryegrass （RRG） and rice-rice-winter fallow （RRF）. Soils were sampled at flowering stage of milk vetch and rape （S1）, before transplantation （S2）, at tillering （S3）, jointing （S4）, and mature （S5） stages of the early rice, and after the harvest of the late rice （S6）. The contents of TFeHa （HCI-extractable total Fe）, Fe（Ⅱ）HCI （HCI-extractable Fe（Ⅱ） species） and Fe（Ⅲ）HCI （HCI- extractable Fe（Ⅲ） species） were measured. The correlations among those Fe species with selected soil environmental factors and the dynamic characteristics of Fe（Ⅱ）HCI accumulation were investigated. The results showed that TFeHc~ in RRF was significantly higher than those in the green manure treatments at most of the sampling stages. Fe（II）Ha increased rapidly after the incorporation of green manures in all treatments and kept rising with the growth of early rice. Fe（Ⅱ）Ha in RRG was quite different from those in other treatments, i.e., it reached the highest at the S2 stage, then increased slowly and became the lowest one at the S4 and S5 stages. Fe（Ⅲ）Ha showed oppositely, and Fe（Ⅱ）HCI/Fe（Ⅲ）HCI performed similarly to Fe（Ⅱ）HCI The maximum accumulation potential of Fe（Ⅱ）HCI was significantly higher in RRF, while the highest maximum reaction rate of Fe（Ⅱ）Ha accumulation appeared in RRG. Significant correlations were found between the indexes of Fe（Ⅱ）HCI accumulation and soil pH, oxidation-reduction potential （Eh） and total organic acids, respectively. In together, we found that long-term application of green manures decreased the TFeHa in red paddy soils, but promoted the ability of Fe（lll） reduction, especially the ryegrass; Fe（Ⅱ）Ha increased along with the growth of rice and was affected by soil conditions and environmental factors, especially the water and redox ability.

Integrated application of February Orchid(Orychophragmus violaceus) as green manure with chemical fertilizer for improving grain yield and reducing nitrogen losses in spring maize system in northern China

The development of more efficient management systems is crucial to achieving high grain yields with high nitrogen use efficiency（NUE）. February Orchid-spring maize rotation system is a newly established planting system with the benefits of ground cover and potential wind erosion in northern China. A field experiment was conducted to evaluate the effects of integrated application of February Orchid as green manure with reduction of chemical fertilizers（INTEGRATED） on spring maize yield, N uptake, ammonium volatilization, and soil residual mineral N in northern China. Compared to farmers＇ traditional fertilization（CON）, integrated application of February Orchid as green manure with 30% reduction of nitrogen fertilizers（INTEGRATED） increased maize grain yield and biomass by 9.9 and 10.2%, respectively. The 0–100 cm soil residual Nmin at harvest was decreased by 58.5% and thus nitrogen use efficiency was increased significantly by 26.7%. The nitrogen balance calculation further demonstrated that the INTEGRATED approach performed better than CON with lower apparent nitrogen loss（decreased by 48.9%） which evidenced by the ammonium volatilization of top-dressing fertilizer was decreased by 31.1%, the N_（min） movement to the deeper soil layers was reduced, and the apparent nitrogen leaching loss nearly equal to 0 under the INTEGRATED treatment. Therefore, in northern China, integrated application of green manure and chemical fertilizers is an efficient management approach for improving maize yields and NUE simultaneously.