Effects of mechanical energy inputs on soil respiration at the aggregate and ®eld scales

Cultivation machinery applies large amounts of mechanical energy to the soil and often brings about a decrease in soil organic carbon (SOC). New experiments on the effects of mechanical energy inputs on soil respiration are reported and the results discussed. In the laboratory, a speci®c energy, K, of 150 J kgÿ1, similar to that experienced during typical cultivation operations, was applied to soil aggregates using a falling weight. Respiration (carbon dioxide, CO2 emission) of the samples was then measured by an electrical conductimetric method. Basal respiration (when Kˆ0) measured on Chromic Luvisol aggregates, was found to increase with increasing SOC, from 1.88 mg CO2 g ÿ1 hÿ1 for a permanent fallow soil (SOCˆ11 g kgÿ1) to 8.25 mg CO2 gÿ1 hÿ1 for a permanent grassland soil (SOCˆ32 g kgÿ1). Basal respiration of a Calcic Cambisol, more than doubled (2.0±5.2 mg CO2 g ÿ1 hÿ1) with increasing gravimetric soil water contents. Mechanical energy inputs caused an initial burst of increased respiration, which lasted up to 4 h. Over the following 4±24 h period, arable soils with lower SOC contents, (11±21 g kgÿ1), respiration rates dropped back to a level, approximately 1.14 times higher than the basal value. However, grassland soils with higher SOC contents (28±32 g kgÿ1), increases in this longer-term respiration rate following 150 J kgÿ1 of energy, were negligible. A ®eld experiment, in which CO2 was measured by infra-red absorption, also showed that tillage stimulated increased levels of soil respiration for periods ranging from 12 h to more than one week. The highest respiration rates, 80 mg CO2 m ÿ2 hÿ1 were associated with high energy, powered tillage on clay soils. On the same soil, low energy draught tillage resulted in a respiration rate of approximately half this value. The results of these experiments are discussed in relation to equilibrium levels of soil organic matter. The application of known quantities of mechanical energy to soil aggregates under laboratory conditions, in order to simulate the effect of different cultivation practices, when combined with the subsequent measurement of soil respiration, can provide useful indication of the likely consequences of soil management on SOC. # 2000 Elsevier Science B.V. All rights reserved.