Nitrogen transformations in a peaty soil improved for pastoral agriculture

Abstract

Abstract. Microbial transformations and chemical distribution of N were compared in two upland stagnohumic gley soils at the same site. In 1981 one soil was improved for more intensive agriculture by lime and fertilizer applications and reseeding with a grass/clover mixture. The other soil (referred to here as unimproved) was also reseeded in 1981, but no lime was added and the sward has subsequently reverted to rough grazing land. Improvement resulted in greater soil microbial activity. The improved soil showed greater rates of denitrification, net N immobilization and N fixation (acetylene reduction) and contained more microbial biomass N than the unimproved soil. However, no major differences in the distribution of organic N fractions were detected. This indicates that a large amount of the soil N present before improvement did not undergo mineralization and remained unavailable to plants.

Carbon flow in an upland grassland: effect of liming on the flux of recently photosynthesized carbon to rhizosphere soil

“…Lime application has been shown to increase plant production, microbial biomass and net N mineralization in areas with heathland herbs, grasses and Sitka‐spruce plantations (Neale et al , 1997). Liming of upland pasture areas in northern Britain increased soil microbial activity and rates of microbial C and N transformations (Isabella & Hopkins, 1994; Hopkins, 1997). The effect of lime application on the increment of basal soil respiration and biomass C was detected in an upland soil pasture even 15 years after improvement (Webster et al , 2000).…”

Decomposition in a peaty soil improved for pastoral agriculture

“…The effect of the lime addition on soil pH was restricted to the 0-5 cm layer (Dampney, 1985) and the samples were taken from this depth. The pH of the improved soil was 6.8 and that of the unimproved soil was 3.7 (Isabella & Hopkins, 1994).…”

“…The fact that the enhanced C02 production could be detected 13 years after lime application must be due to increased respiration by the soil microorganisms, since evolution of C02 from the CaCO3 added to the acidic soil would have ended long before. This is consistent with increases in the size ofthe soil microbial biomass and in the rates of nitrogen transformation in the improved soil (Hopkins et ul., 1990;Isabella & Hopkins, 1994). Improved pastures may, therefore, be acting as persistent hot-spots of C02 emission in the uplands.…”

“…The soils were taken from the trial site used by Isabella & Hopkins (1994) was needed for pasture improvement (Dampney, 1985) and is on a cambic stagnohumic gley (USDA: Humaquept) of the Wilcocks series (Soil Survey of England and Wales, 1979). The 0 horizon contains 0.55 g C/g soil (dry weight), is about 40cm deep, and contains about 900 t C/ha (Hopkins, 1988).…”

“…Pasture improvement removes some of the constraints on microbial activity, in particular acidity, in upland soils. For example, the numbers of microorganisms are increased (Maltby, 1984) and increases in the rates of the major microbial transformations of nitrogen can be detected more than 10 years after improvement (Hopkins etal., 1990;Isabella& Hopkins, 1994). In the present study, the effects of pasture improvement on the decomposition of plant material and COz release were investigated thirteen years after improvement.…”