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Effects of Soil Moisture, Texture, and Vegetation Type on C and N Transformations in Agricultural Fields of Northwestern Nevada
AuthorJohnson, Zachary C.
AdvisorVerburg, Paul S.
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Irrigation is a common requirement for agriculture in semi-arid areas in the Western United States due to relative scarcity of water resources. In these areas, oftentimes competing demands for water are present including agriculture, municipal use and protection of natural ecosystems, resulting in conflict situations. One example of an area where these conflicting demands are intertwined is the Walker Basin, a semi-arid region in northwestern Nevada. Increased agricultural and municipal water use has reduced streamflows into Walker Lake, a terminal desert lake, causing lake levels to drop. As a result, lake salinity has increased to levels that are detrimental to native fish populations. The study presented in this thesis focused on the effects of alternative crops on soil quality in the Walker River Basin. Alternative crops are proposed to reduce water use while maintaining economic viability. This study was part of a large research project focusing on the potential for reducing water use in the Walker Basin and its effects on agriculture, natural ecosystems and the economy in this area.A laboratory and field study were conducted to explore the effects of soil moisture, texture, temperature, and vegetation type on C and N transformations in soils from the Walker River Basin where alternative crops were planted. The primary objective of the laboratory study was to measure the effects of soil moisture, texture, and vegetation type on the C and N transformations in soils taken from two agricultural sites and a wildlife refuge site. The two agricultural field soils were planted with tef, amaranth, alfalfa, and switchgrass while the wildlife refuge area remained fallow. Carbon and N transformations in these soils were measured using constant temperature incubations with three moisture treatments. Carbon transformations in incubations of pre- and post-planting soils and N transformations in pre-planting soils generally increased with increasing moisture while N transformations in post-planting soils were highest in the 0.15 (g g-1) moisture treatments. High clay contents most likely contributed to lower CO2-C production rates and net changes in total extractable N for the wildlife refuge area soils in the pre-planting incubation. Clay appeared to have a negative effect on the CO2-C production rates in the post-planting incubation for the two agricultural fields, which was somewhat surprising since textural differences between the two fields in this incubation were relatively small. Aboveground biomass generally did not significantly affect the C and N fluxes. However, tef soils showed the highest C mineralization rates and had the largest aboveground biomass production. Generally, the post-planting incubation had higher C fluxes and lower N fluxes than the pre-planting incubation indicating that the quality of available N or the availability of organic-N may have decreased over the growing season since total C and N generally did not differ between pre- and post-planting incubations.The primary objective of the field study was to measure the effects of soil moisture, temperature, and vegetation type on C and N transformations in two agricultural fields of the Walker River Basin planted with tef, amaranth, alfalfa, and switchgrass. One field received three irrigation regimes (50%, 75% and 100% of estimated plant water use) while the other field only received the 50% irrigation treatment. Moisture was the primary driver for CO2-C respiration with higher moisture levels resulting in higher CO2-C respiration rates. The number of days since last irrigation application appeared to negatively affect the CO2-C respiration rates in our soils. Total above-ground biomass appeared to affect the C respiration rates positively and the net changes in inorganic-N negatively. The effects of vegetation type on the C and N transformations were not consistent over the establishment growing season. Field was the primary driver for changes in inorganic N with one field producing net positive changes and the other field producing net negative changes over the establishment growing season possibly due to differences in biomass production in combination with differences in mineralization rates. Soil temperature was not widely significant for this study but when it was significant it affected the C and N transformations negatively most likely because high temperatures often coincided with low soil moisture levels. Texture did not play a significant role in the C and N transformations because of relatively small differences in texture between the two fields. In the short-term, none of the alternative crops grown appeared to have immediate detrimental effects on soil quality versus the traditionally grown alfalfa. In fact, the tef and amaranth crops grew quite well over the establishment season without detrimentally impacting soil C and N transformations as measured in this study. As a result, these two alternative crops have high potentials for the future in this area.