Interactions between tillage energy, soil structural stability and organic matter
In agricultural production, disturbance of the soil by cultivation occurs regularly. Mechanical energy applied in this way can have a adverse effect on soil stability, a lowering of soil organic matter (S_OM), and a increase in CO2 emissions. These changes result in unwanted environmental consequences and compromise the ability of soil to maintain a competitive and sustainable agricultural industry. As agricultural systems evolve, it becomes important to develop a indication of their sustainability with regard to soil structure, well before any serious consequences become apparent. The am of this work was to quantify the effect of mechanical energy (in particular tillage) on soil structural stability and the loss of SOM. New laboratory techniques were devised in which mechanical energy was applied to a range of soils at different water contents with measurements made of stability and the mineralization of SOM. Techniques used for characterising stability involved measuring mechanically-dispersed clay, cm using a turbid metric technique and the proportion of water stable aggregates (>250 m). A re-examination of the statistical theory of brittle fracture showed that soil friability, F1, could be quantised using the coincident of Variation of tensile-strength of a population of similar sized aggregates. Specific energies associated with different cultivation practices, were simulated using a falling weight and results indicated that the sensitivity of soil to mechanical damage was essentially zero at soil water contents below the plastic limit (wpL). With increasing soil water content, sensitivity to destabilization increased sharply. The empirical model to characterise these phenomena was evaluated under field conditions where the energy consumption of different tillage implements, operating at different soil water contents, was measured directly. Good agreement between the level of destabilization measured in the field and those in the laboratory was obtained at similar specific energy values. C.W. Watts, 2003. Cranfield University, Silsoe. The field experiments also showed that increased levels of cm following tillage were responsible for stronger and less friable day aggregates. More experiments on a soil with SOC values ranging from all to 32 g/kg enabled the original model to be refined, linking cm, SOC and soil water content to disruptive energy. This led to the development of a index, S, which quantise the sensitivity of soil to destabilization by mechanical energy inputs and provides a method for identifying soils at risk. The effect of mechanical energy on the mineralization of SOC was measured using the falling-weight. Mineralization was characterised by measuring soil respiration using data-logging, conductinetric respirometers, built to monitor CO2 emissions following applications of mechanical energy. Changes in respiration were characterised by the respiration ratio, rr (defied a respiration following the application of energy divided by basal respiration). Higher values of, rr, were associated with greater energy levels, particularly on soils with lower SOC. Increased respiration was also measured following tillage in the field, particularly from soils following tillage at high energy levels where the effect lasted for several weeks. In this work three physically based measures of soil quality (S, F1 and rr) have been used to quantify the effects of tillage of different intensities on soil structural stability and the loss of organic matter. Parameters common to these measures of soil quality are SOC content, soil-water content and tillage intensity. Results of this work indicate that organic matter, physically protected by stable soil structures, can b exposed to mineralization when the structure in destabilized during tillage, particularly a the soil becomes wetter (w>w1ºL). The practical consequences of this work concluded that increasing the levels of SOC, cultivating the soil at water contents below the plastic limit and a reduced energy input, provide the best practical approach to maintaining soil physical quality. The new methodologies developed here have helped improve understanding of the effects of mechanical energy on soil structural condition. They provide a sound basis to answer a range of questions relating to soil physical quality and the consequences of different soil management practices for soil behaviour in the environment, thus enabling the boundaries between good and bad practices to b better defied.