Graduate Thesis Or Dissertation
 

Long-term Impacts of Nitrogen Fertilizer, Tillage and Crop Residue on Soil and Plant Nutrients in Winter Wheat Cropping Systems Under Dryland Conditions

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  • Agricultural practices influence the nutrient dynamics of soil and plants, which may take more than a decade to be expressed in the drylands. The nutrient supplying capacity of soil is a crucial component of sustainable agriculture, and hence, this study can play an important role in policy making for the future of dryland agriculture. The objectives of this thesis were to evaluate the long-term impact of different methods of crop residue management, varying inorganic N application rates, different tillage systems, and tillage timing on soil pH and on plant essential macro and micronutrient in soil and tissue (wheat grain and straw) under winter wheat dryland cropping system of the inland Pacific Northwest (PNW). The studies were carried out on the three existing long-term experiments of Oregon State University, and were represented as (i) Crop residue study (CR), (ii) Tillage-Fertility study (TF), and (iii) Wheat-Pea study (WP). Responses included total N, C, S, and Mehlich III extractable P, K, Ca, Mg, Mn, Fe, Zn, Cu, and B in soil (four depths: 0-10, 10-20, 20-30, and 30-60 cm) and the total concentration of the same elements in plant tissue. The CR study examined the 84 years effect of residue burning, variable rates of inorganic N application, and organic amendments (farmyard manure and pea vines) on soil pH and the nutrient concentration in the wheat residue managed plots under winter wheat- fallow (WW-F) cropping system. No differences were observed between burned and unburned residue plots (excluding plots with the organic amendments) for nutrients in soil and wheat after 84 years of WW-F cropping. Residue incorporation with farmyard manure (FYM) reduced the rate of nutrient decline over time in soil, whereas inorganic N application decreased grain P, K, and Ca with the higher N rate application rates compared with the zero N application or FYM. In the TF study, the 75 years effect of moldboard plow (MP), sub-surface sweep (SP), and disk plow (DP) and variable rates of inorganic N on soil pH, and on soil and plant nutrients were investigated under WW-F. The soil under MP lost greater amount of total N, C, and extractable K, and Mg in the top 10 cm than the soil under SP and DP. The severity of C, N, Mehlich III extractable P, Ca, Mg, Cu, Mn, and Zn decline were lower in soil under DP than in soil under MP, and comparable with soil under SP for most nutrients. Long-term application of higher rates of N fertilizer improved soil C: N ratio and P accumulation in wheat grain. Decreased soil pH was also observed with higher N rates application in the TF study. The WP study consisted of 52 years effect of timing and tillage intensity on the same parameters as of CR and TF study under winter wheat- dry pea rotation. The treatments were spring tillage (ST), fall tillage (FT), disk/chisel tillage (DT/CT), and no tillage (NT). No differences were observed between ST and FT for nutrients in soil and wheat over time, however, NT had started to show its effect in 2015; greater soil total C, S, and extractable P, K, and Mn were evident under NT compared with FT, ST and DT/CT in the upper 10 cm soil depth. The NT plots were under minimum tillage until 1995, and under NT after that, so its long-term effect is yet to be seen. The results suggest that NT can be effective agricultural practice for improving resiliency of soil under dryland wheat production.
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  • This research was supported by the Oregon State University startup funds awarded to Dr. Valtcho D. Jeliazkov (Zheljazkov).
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