No abstract

Maintaining long‐term soil productivity requires development of cropping systems that provide maintenance or improvement in soil structure and an understanding of associated rhizosphere microbial populations. The objectives of this study were to determine the effects of several crops on soil biomass C and biomass N contents, their within‐season variability, and the relationships between changes in soil biomass C, biomass N, and soil structure on a Brookston clay loam soil (fine‐loamy, mixed, mesic Typic Argiaquoll). Soil microbial biomass C, biomass N, and soil‐structure parameters (wet aggregate stability [WAS], organic carbon [OC], dry aggregate mean weight diameter [MWD], bulk density, and total and air‐filled porosity) were measured during the third year of corn (Zea mays L.), soybean (Glycine max [L.], Merr.) alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), reed canarygrass (Phalaris arundinacea L.), orchardgrass (Dactylis glomerata L.), and no‐crop (bare, covered, and shaded) plots at monthly intervals (June, July, August, and September). Reed canarygrass resulted in greater biomass‐C contents than both the corn and soybean at all four sampling dates. Soil biomass C under alfalfa was significantly greater than under corn and soybean for both the first and last sampling dates. Forage species did not affect the soil biomass‐N content. No consistent effects of the no‐crop treatment on biomass C or biomass N were observed between sampling dates. Biomass C was significantly correlated with WAS, OC, and MWD for the cropping treatments. Wet aggregate stability was negatively correlated with soil water content for both the no‐crop and cropping treatments, indicating that improvements in structure were not solely the result of the cropping treatments and associated rhizosphere populations. Reed canarygrass resulted in greater soil biomass C/N than the alfalfa, corn, and orchard grass plots, suggesting that fungal activity, and therefore soil structure, may be preferentially enhanced in the presence of reed canarygrass. This study demonstrated the influence of forage species and seasonal variability on concurrent changes in microbial biomass and soil structural properties.

No abstract

Soil temperatures vary with depth and time within and among years. An experiment was conducted to quantitatively evaluate the effects of temperature on the development of the taproot and lateral roots of four soybean (Glycine max (L.) Merr.) cultivars at four temperatures. ‘Beeson’, ‘Hawkeye 63’, ‘Wayne’, and ‘Harosoy 63’ soybeans were grown at 17, 21, 25, and 29°C, using a specially designed, constant‐temperature, water bath system. The water bath was located in a temperature‐modified greenhouse where air temperature ranged from 20 to 35°C during the 12‐month experimental program. Serial measurements of taproot and lateral root tip location and number of primary nodes were recorded on alternate days until 19 days after emergence. A primary node was defined as the position at which one or more 1st order laterals emerge from the taproot. The rate of taproot extension increased with temperature, decreased with time at temperatures greater than 17°C, and was greater for the Beeson and Hawkeye 63 cultivars than for the Wayne and Harosoy 63 cultivars when averaged over time. The rate of lateral root extension increased with temperature, increased with time at temperatures less than 29°C, and was greater for the Beeson and Hawkeye 63 cultivars than for the Wayne and Harosoy 63 cultivars at temperature treatments greater than 17°C when averaged over time. The number of primary nodes varied with depth, time, and cultivar. The greatest number of nodes occurred 2.5 to 7.5 cm below the vermiculite surface. The Beeson and Hawkeye 63 cultivars had fewer primary nodes than the Wayne and Harosoy 63 cultivars. The results indicate that the depth of soybean root penetration can be altered to increase available water during periods of stress by cultivar selection and changes in soil temperature.

Rebound (decrease in bulk density after removal of mechanical stress) of five agricultural soils from four soil orders was measured at stresses of 1,000, 5,000 and 10,000 g cm−2 and pore water pressures from 0 to −1 bar.Rebound was usually < 0.05 g cm−3. It was larger in soils containing expanding clays than in soils containing nonexpanding clays. It increased with stress in soils containing expanding clays but not in soils containing nonexpanding clays. Rebound decreased with repeated loading. It could not be delineated in terms of soil type, pore water pressure, or water content. In the prediction of bulk density of unsaturated agricultural soils based on their compression characteristics, rebound is usually small enough that it can be neglected.