Effects of intensity of agronomic practices on a soil ecosystem

Monitoring soil biological activity may provide useful information in assessing the sustainability of agricultural systems. From 1985 to 1992, we compared five cropping systems in southern Maryland. Four of the systems involved the production of wheat, soybean, or corn, but differed in intensity of tillage, herbicide use, crop biomass removal, and N fertilizer application. The fifth system was continuous grass. The objective was to assess how these cropping systems affected selected indicators of soil biological activity and nutrient cycling. From Fall, 1990 through Fall 1991 the upper layers (0-5 or 0-15 cm) of soil were studied to assess C and N contents, selected physical properties, extractable organic C, microbial respiration, decomposition of plant tissue, N mineralization, and earthworm (Lumbricidae) populations. On a dry weight basis total and labile C were significantly higher under grass and lower under conventionally tilled (CT) continuous corn than in the other three systems. In fall, there were significantly fewer earthworms (10.4 m-2) under the reduced input rotation than in any other system, and fewer under CT continuous corn than in the organic (corn-wheat-green manure) rotation or grass. In general, the rates of CO2 evolution were 30 to 35% lower in fall (11.5 to 22.5 kg C ha-1 d-1) than in spring (17.3 to 33.7 kg C ha-1 d-1). During both seasons grass had approximately twice as much soil metabolic activity as CT continuous corn. The absence of tillage clearly was important in increasing N mineralization capacity, since the two untilled systems had comparable rates of N release that were significantly greater than those for the other systems. Among the tilled systems, mineral N after laboratory incubation decreased with increasing frequency and intensity of tillage in the order: reduced input > organic > CT continuous corn. It was evident from a combination of measures that soil biological activity was enhanced in the systems that minimized tillage. The results underscored the value of grass for soil improvement.