Search

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]

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]