Preemergence Herbicidal Activity Of Soybean Meal And Its Ecological Effects On Soil Microbes

Synthetic herbicides are often used for weed control in agriculture ecosystem and turfgrass management. Public awareness of the widespread use of herbicides and the possible negative effects of their residues on the environment have led to increased efforts in the search for natural herbicidal active products. Soybean meal (SBM) is a byproduct of soybean oil extrcation, it contains 40%-48% protein, and is commonly used as an animal feed.In this paper, SMB and soybean meal hydrolysate (SBH) were studied for inhibitory effect on the germination of plant seed (also called preemergence herbicidal activity) on various weeds and turf plants in the growth chamber and in the greenhouse, and the effects of SBM on mature turf plant, turf quality and soil microbial diversity were also investigated. Besides, the preemergence herbicidal machnism of corn gluten meal (CGM) with similar composition to SBM was explored in growth chamber experiment. The above-mentioned studies are trying to illustrate the preemergence herbicidal mechanism of SBM and other protein, and provide theory basises for developing SBM as a natural preemergence herbicide. The main research results were as follows.(1) The preemergence herbicidal activity of SBM was evaluated on 8 plants in growth chamber and in greenhouse. The results showed that the root growth of germinating seeds can be inhibited effectively by SBM, and the seeds germination rate had been reduced. There were differences among species in their response to SBM treatment. In general, monocotyledonous plants were not susceptible to SBM as dicotyledonous plants. Weeds, i.e. Chrysanthemum indicum, Ageratum conyzoides, Kyllinga brevifolia and Bidens pilosa produced no roots with SBM at 7 mg/mL in growth chamber experiment, whereas reductions in root length of turf plants, i.e. Lolium perenne, Lolium muliiflorum and Trifolium hybridum had 86%, 52% and 79%, respectively. The preemergence herbicidal activity of SBM significantly decreased under the sterile condition. For example, L. muliiflorum and Trifolium pretense exhibited only < 32% reduction in root length with SBM at 10 mg/mL under the sterile condition, but (> 90%) reduction at the same SBM level under nonsterile condition, suggesting that the microbes play a very important role in the herbicidal activity of SBM. The pH of SBM media with various application rates were between 6.63 to 7.10 and the ammonia-nitrogen contents were between 1.07 to 1.63 mg/mL before the experiment. After cultured for 7 days under nonsterile condition, the pH rose to 8.26-8.86, and the ammonia-nitrogen contents reached 25.98 to 232.47 mg/L, whereas the nitric nitrogen and the nitrite nitrogen contents were very low and remained similar level before and after the experiment, proposing that free ammonia produced by the raised pH and ammonia-nitrogen of SBM after the cultivation were the main active herbacidal material. Herbicidal efficiency of SBM was compared between two application methods in the greenhouse. In general, the preplant-incorporated (PPI) treatments were more effective than soil-surfaced applications (PRE).(2) SBH were prepared by Alcalase 2.4 L, and herbicadal activity on L. perenne was tested in the growth chamber and greenhouse condition. Results showed that SBH exhibited a stronger herbicidal activity than SBM. SBH, however, prolonged L. perenne root length and shoot length at 14 to 17% and 12 to 14%, respectively, under sterile condition, while the germination did not change significantly, showing that SBH had no inhibitory effect on seed germination under sterile condition. This also suggested that microbes played an improtant role in the preemergence herbicidal activity of SBH. The similar changing trends of pH and ammonia-nitrogen content between SBH and SBM demonstrated that they had a similar preemergence herbicidal mechanism: free ammonia releasing from decomposed SBH by microbes was the herbicidal activity substance.(3) A series of hydrolyzhed soybean meal peptides (HSP) with different molecular weight (MW) (> 10000 Dal, 5000-10000 Dal, 3000-5000 Dal, 1000-3000 Dal, < 1000 Dal) were prepared by ultrafiltration, and then were subjected to L perenne bioassay. The results indicated that there was no significant difference among these different HSP at the same N content. Subsequently, a hexapeptide (Tyr-Ser-Tyr-Pro-Pro-Arg) was separated and purified from the HSP fraction with WM < 1000 Dal, and all the HSP fractions with different WM had no significant difference on the perennial ryegrass root inhibition, further confirming that the herbicidal activity was resulted from the proteins or peptides in SBM and SBH, and the herbicidal activity of SBH was not related to peptide's MW. (4) Growth chamber germination experiment was conducted to study the preemergence herbicial activity of CGM and CGH. Results proposed that on the sterile condition, the herbicidal activity of CGM was dramatically decreased, and CGH simultaneously promoted the root and shoot growth of germinating L perenne. pH and ammomia-nitrogen content in CGM and CGH media did not change greatly under the sterile condition, but there was a dramatically increasing under the nonsterile condition. In addition, the preemergence herbicidal activity did not related to MW of hydrolyzed corn gluten peptides. These results concluded that there was a similar herbicidal mechanism between CGM and SBM.(5) SBM stress stimulated the activities of SOD, CAT, POD in the period of seed germinating and early seedling development of perennial ryegrass, and the peak of antioxidant enzymes activity occurred at 7-12 d. So did the content of soluble protein, soluble sugar and MDA.(6) SBM contained about 7% N by weight and provided an additional N source to turfgrass with well-developed root systems in autumn and winter. Nitrate reeducates activity, net photosynthetic rate, turf biomass, total nitrogen and chlorophyll content of Zoysia tenuifolia were enhanced by SBM treatments. The coloration and equitability of the turf were therefore improved, and the green stage also prolonged.(7) The impact of SBM on soil microbial diversity was studied by analyzing the soil enzyme activity and using the traditional incubation method and the molecular biological technology, such as PCR-DGGE and RAPD. Most soil enzyme activities did not change sigficantly, especially during the later experimental period, there was no obvious difference between SBM treatments and control, except that the high rate of SBM treatment (300 g/m~2) for a long time promoted the protein enzyme activity. The number of soil microorganism was increased by SBM treatments in a certain period. The impact of SBM on soil microorganism reduced gradually with decomposition, absorption and seepage nutrient of SBM. During the later period along with autoregulatory ability of soil microorganism, the number of soil microorganism was similar between the SBM treatment and the control. The analysis of PCR-DGGE fingerprinting revealed that the richness, diversity and uniformity index of lawn soil bacteria was enhanced by the high rate of SBM application within 14 days, but after 28 days there was no obvious difference of these targets between all treatments and control. SBM at low amount played little effect on the diversity of soil bacteria. The RAPD fingerprinting analysis demonstrated that the diversity of soil microorganism could be increased in a certain time after the treatments of SBM, but there was no obvious difference of soil microorganism diversity among all treatments and control after 28 days.