Effects of long-term organic amendment on the fertility of soil, nodulation, yield, and seed quality of soybean in a soybean-wheat rotation system

Organic amendment has long been recognized as an effective approach to improve soil fertility and plant nutrition. Most studies on long-term organic amendment in fields have focused on cereals. The effects of long-term organic amendment on legumes remain largely unknown. Here we studied the impact of organic amendments, including straw and manure on the soil fertility, nodulation, seed yield, and seed quality in a 27-year experiment. The long-term fertilizer treatments were laid out using a randomized complete block design designed with three replications in the first year of experimentation (1990). Fertilization treatments were CK (no fertilization), NPK (inorganic nitrogen, phosphorus and potassium fertilizers), NPKS (Straw together with inorganic N, P, and K fertilizers), and NPKM (manure together with inorganic N, P, and K fertilizers). The layout was subsequently maintained in the following years to clearly understand the effect of long-term application of each treatment. Maize (Zea mays L.)-wheat (Triticum aestivum L.) rotation was the main long-term experiment cropping system from 1990 to 2011; from 2012 it becomes wheat-soybean (Glycine max L.) rotation. We found that NPKS, NPKM, and NPK increased organic matter by 34.7%, 36.24%, and 17.17%, respectively, and total nitrogen by 28.9%, 29.7%, and 25.2%, respectively, in comparison with CK. Organic amendment also increased enzymatic activity of phenol oxidases and β-1,4-glucosidases. The NPKS and NPKM increased soybean yields by 30.8% and 29.6%, respectively, in comparison with NPK alone and CK. The soybean yields significantly correlated with NO3-N, available phosphorus, available potassium and soil organic matter. Both the inorganic fertilizers and organic amendments did not significantly alter soybean nodule number and size. NPKS and NPKM applications significantly reduced fat content by 1.17% and 1.26%, respectively, as opposed to NPK fertilized treatments alone (0.34%), compared with those under CK, but did not affect soybean protein. Overall, the combination of straw returning with inorganic fertilizers and manure with inorganic fertilizers improves soil fertility and increases soybean productivity. However, organic amendments together with inorganic fertilizers might have an undesirable effect on soybean fat content. Organic amendments combined with NPK application have the highest value of protein content and increased nitrogen to the soil. High nitrogen reduced fat and increased protein content. Therefore, rate of N inputs should be adjusted under the application of organic amendments together with inorganic fertilizers in fluvo-aquic soil.