Total porosity increase and random roughness due to plowing were each significantly affected by the moisture content at tillage time. Their magnitudes were greatest at low moisture contents, decreased approximately linearly as soil moisture increased to the lower plastic limit (LPL), and then increased at moisture contents greater than LPL. The porosity before plowing more significantly affected tillage‐induced random roughness than total porosity increase. There were significant deviations from these generalities within each of the three soil associations: Barnes‐Aastad, Kranzburg‐Poinsett, and Nicollet‐Webster.In all three associations, the porosity decrease by subsqeuent disking and harrowing was more pronounced where plowing gave the greatest porosity increase. Only in the Kranzburg‐Poinsett association (high in silt content) were porosity increases observed from disking and harrowing near and above the LPL. Disking and harrowing nearly always reduced random roughness, and the reduction increased linearly as the random roughness of the plow treatment increased.
Nitrogen and phosphorus losses were determined in plant leachates from alfalfa, Kentucky bluegrass, barley straw stubble, and oats straw stubble. Plant samples harvested from the field were either leached immediately or were desiccated by drying or freezing before leaching. The extraction of soluble nitrogen and soluble phosphorus in leachates from aloealoea and bluegrass was greatly increased by drying or freezing. These same treatments, however, released only small amounts of soluble phosphorus from barley and oats straw. Estimated soluble nutrient losses indicate that leaching of alfalfa and bluegrass by surface runoff water could contribute substantial amounts of nitrogen and phosphorus to lakes and streams.The concentrations and amounts of the various nutrients in runoff from agricultural areas result from the interaction of many factors. These factors include type of crop, cultural and conservation practices, length and steepness of slope, amount and distribution of precipitation, soil infiltration and percolation characteristics, and size of contributing watershed. One factor, the leaching of crops and crop residues by precipitation and/or runoff, has not been thoroughly assessed, but under certain conditions may be an important source of nutrients in surface runoff.
Nutrients transported in surface runoff and interflow from an undisturbed aspen-birch (Populus tremuloides Michx., and Retula papyrifera Marsh.) forest (6.48 ha) in northern Minnesota were measured for 3 years. Surface runoff from snowmelt accounted for 97% of the average annual surface runoff and for 57% of the average annual water loss. Slope aspect influenced the amount, rate, and time of snowmelt runoff. In surface runoff, organic nitrogen (N) comprised 80% of the total N load, and organic (+ hydrolyzable) phosphorus (PI comprised 45% of the total P load. The quantities of cations in surface runoff were in the order of calcium (Ca) > potassium (K) > magnesium (Mg) > sodium (Na). More than 96% of all the nutrients in surface runoff were transported by snowmelt. The annual volumes of interflow varied only slightly during the 3 years. Compared with surface runoff, the amounts of all the nutrients (except Na) and their weighted concentrations decreased in interflow. These nutrient losses from the ecosystem can accumulate in surface waters.
Nutrient losses in surface runoff from a native prairie were determined for 5 years. The prairie, dominated by little bluestem (Andropogon scoparius Michx.), was on a Barnes loam soil in westcentral Minnesota. Snowmelt runoff accounted for 80% of the average annual runoff. After spring thaw, rainfall caused appreciable runoff (37% of annual) during 1 year only. Depending on chemical parameter, 63 to 88% of the average annual nutrient loads were transported by snowmelt runoff. Average annual total nitrogen (N) and total phosphorus (P) losses were 0.8 and 0.1 kg/ha, respectively, and organic N and organic (+ hydrolyzable) P comprised 68 and 82% of their respective annual losses. Average annual cation losses ranged from about 0.1 to 1.8 kg/ha and, except for 1 year, were in the order K > Ca> Mg > Na. Average weighted concentrations for all parameters were higher for rainfall runoff than for snowmelt runoff. Nutrient losses from native prairie represent natural levels for comparing the effects of different land uses on nutrient loads in surface runoff.
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