Physical protection of mineralizable C in aggregates from long-term pasture and arable soil

Depending on agricultural management, soil aggregation can provide physical protection of organic matter against rapid decomposition. Within a given soil series, farm management affects the quality and quantity of organic inputs, soil disturbance and biological activity, and thereby the processes of aggregate formation (biogenic vs. physicogenic). We determined the physical protection of readily mineralizable organic matter against mineralization in undisturbed aggregates from a conventional arable field and a permanent pasture (>70 years). Soil samples from the two fields were incubated at constant temperature and moisture content, corresponding to field capacity. The increase in CO, evolution due to crushing (<250 mu) of the aggregates was used to estimate the macroaggregate-protected C fraction. The fraction of C protected at the microstructural level was estimated from the increase in CO, evolution after ball-mill grinding. In addition, aggregate size distribution and bulk density and porosity of undisturbed soil and macroaggregates were determined. Unprotected C fractions were not significantly different between the management systems and ranged from 1.9% to 2.4% of total organic C. In the arable soil, 1.4% of total soil organic C was physically protected in macroaggregates. Crushing of macroaggregates did not significantly increase C mineralization in the pasture soil. The results indicate that mineralization was considerably suppressed in the dominantly large and dense physicogenic macroaggregates from the arable field, but not in the dominantly porous, biogenic macroaggregates of the pasture soil. However, the protection in macroaggegates from the arable soil is not likely to be effective on the long-term because of the low water stability and the disrupting forces of cultivation under field conditions. A relatively high additional C mineralization from ground compared to crushed soil material, especially in the upper layer of the pasture soil, suggests a more important C protection at the microstructural level. Higher C protection in microaggregates from the pasture soil was supported by a previous micromorphological study of soil microstructures in thin sections of the considered management systems. (C) 2003 Elsevier B.V. All rights reserved.