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Understanding changes in soil organic C (SOC) and total soil N (TSN) is important for evaluating C fluxes and optimizing N management. We evaluated long-term SOC and TSN changes under dryland rotations for historical stubble-mulch (HSM) and graded terrace (GT) plots on a clay loam soil in Bushland, TX. Compared with adjacent grassland with no history of cultivation, stored SOC in the surface 0.30 m of HSM declined by 41% after 86 yr of cultivation, with half of the estimated changes occurring during the first 20 yr. In the HSM plots under a winter wheat (Triticum aestivum L.)-fallow rotation, SOC and TSN (0.0-0.152 m) were significantly greater with decreasing tillage intensity (P < 0.05) in 1977 for treatments imposed in 1941. On GT plots under a winter wheat-sorghum [Sorghum bicolor (L.) Moench]-fallow rotation, SOC and TSN storage under no-till were not significantly (P = 0.396 and P = 0.261, respectively) different from stubble- mulch tillage 30 yr after treatments were imposed in 1984. Calculated export of N in wheat and sorghum grain from 1927 to 2013 from GT (1.6 Mg ha-1) accounted for 80% of the difference between TSN in grassland and GT plots. From 1927 to 1960, TSN decline exceeded N in exported grain by 1.2 Mg ha-1 and may explain present-day accumulation of NO3-N at 1 to 6 m in the unsaturated zone. Since 1966, crops have probably been assimilating NO3-N located deeper in the soil profile to supplement N requirements. [ABSTRACT FROM AUTHOR]

Abstract: The main goal of this research was to measure cotton water use, and to determine irrigation water scheduling parameters associated with optimal seed-lint yield and irrigation water use efficiency, which are poorly understood in the Central Asian Republic of Uzbekistan. A cotton (Gossypium hirsutum L.) field experiment with drip irrigation in comparison to furrow (conventional) irrigation was conducted on a deep silt loam soil (Calcic Xerosol) at the Central Experiment Station of the Uzbekistan National Cotton Growing Research Institute at Tashkent in 2003, 2004 and 2005. To investigate irrigation scheduling, the field capacity (F C) index was adopted, which was 0.30m3 m−3 in this soil. Irrigations were scheduled when soil water in the root zone was depleted to specific fractions of F C, e.g., 70% of F C, for each of three main plant growth periods (germination–squaring; squaring–flowering; beginning of maturation–maturation). Crop water use, which we here define as the sum of transpiration and evaporation, was established using the soil water balance approach on a weekly basis. Soil profile water content was determined using a neutron moisture meter (NMM), which was calibrated in polyvinyl chloride (PVC) access tubes for each differing soil layer. Under drip irrigation and the optimal mode (70–70–60% of F C) of irrigation scheduling, 18–42% of the irrigation water was saved in comparison with furrow irrigated cotton grown under the same condition; and irrigation water use efficiency increased by 35–103% compared with that of furrow irrigation. Seed-lint cotton yield was increased 10–19% relative to that for furrow irrigated cotton. The irrigation scheduling rule developed here should be considered an improved practice for drip irrigated cotton that is applicable to irrigated Calcic Xerosols of Uzbekistan. [Copyright &y& Elsevier]

Previous research established growth-stage-specific irrigation scheduling for upland cotton (Gossypium hirsutum L.) in Uzbekistan. We report further investigation of irrigation scheduling for two cotton varieties and its effect on seed-lint yield and irrigation crop water productivity with and without use of a wetting front detector (WFD) to trigger irrigation cessation. A WFD is a device buried in the soil to sense when the wetting front from irrigation reaches a certain depth. The depth may be adjusted upon installation of the WFD. Field trials were conducted in silt loam soil near Tashkent, Uzbekistan, in 2016, 2017, and 2018. The growth stages of germination to flowering, flowering to boll formation, and maturation were considered for the development of irrigation scheduling regimes with respect to field capacity water content (Fc). The best growth, development and seed-lint yield for the Sultan cotton variety were achieved with irrigations scheduled at soil water content levels of 70%, 75%, and 65% of Fc during the aforementioned three growth stages, respectively, and were achieved for variety UzPITI-103 with irrigations scheduled at 70%, 70%, and 60% of Fc during the aforementioned plant growth stages, respectively. There were appreciable varietal differences in water requirement with the early maturing Sultan variety having a larger water requirement. In contrast, irrigation scheduling with larger Fc values did not result in increasing the yield for the UzPITI-103 cotton variety. In comparison with irrigation without WFD, the use of the WFD decreased irrigation duration by 2.0–3.5 h, net irrigation amount by 7–90 m3 ha−1, irrigation run off by 69–134 m3 ha−1, and deep percolation below the root zone by 10–17 m3 ha−1, while increasing irrigation crop water productivity by 0.08–0.10 kg m−3 and seed-lint yield by 0.04–0.10 Mg ha−1. Irrigation scheduling regimes based on percentages of Fc and irrigation with WFD should be considered applicable practices for upland cotton varieties on silt loam soils of the central zone of Uzbekistan and for similar soil-climatic conditions of Central Asian countries. ● Varieties differed in water requirement, optimum irrigation scheduling, and yield. ● Use of wetting front detectors (WFDs) decreased water losses and improved yield. ● Automated WFDs led to reduced irrigation and increased crop water productivity. [ABSTRACT FROM AUTHOR]

Improvements in water use efficiency of the irrigated agriculture of Uzbekistan begin with determination of crop water use under its different climates, soils, and management practices. The neutron moisture meter (NMM) is a key tool for determination of crop water use, which we define here as being equal to transpiration and evaporation from the soil surface, i.e., the evapotranspiration. We accurately field calibrated NMMs at six locations in Uzbekistan, in soils ranging from deep, uniform silt loams of loessial origin to highly stratified alluvial soils near the Amu Darya River. In all soils, separate calibrations were found for the 10-cm depth due to closeness to the soil-air interface. Near Tashkent and at the Syrdarya Branch Station, the soil below 10 cm was divided into two layers based on the increased CaCO3 content in the lower of the two layers. Distinctly different calibration equation slopes were found for these layers. At the Kashkadarya Branch Station, a single calibration was sufficient for the soil below 10 cm. At the Khorezm Branch Station, an abrupt change in soil texture at the 90-cm depth required separate calibration equations for the 30- to 70-cm depth range (silt loam) and the 110- to 170-cm depth range (fine sand). Overall, the root mean square errors (RMSEs) of calibration ranged from 0.009 to 0.025 m³ m-3 and r² values ranged from 0.91 to 0.99. Data gathered provide an excellent illustration of why calibration efforts should organize soil water content data by depth range. Two examples of profile water content measurement for crop water use studies are given. [ABSTRACT FROM AUTHOR]