7750: Crop rotation and animal manure effects on soil. I. Organic carbon and tilth formation

Schjønning, P.; Munkholm, L.J. and Elmholt, S. (2006) Crop rotation and animal manure effects on soil. I. Organic carbon and tilth formation. [preprint]

Summary

Management influences key soil properties decisive for soil tilth but it is unclear, which management options are most effective. We investigated the effect of crop rotation and addition of animal manure on soil organic carbon fractions, the dispersibility of soil clay, and the intensity of fungal hyphae. We hypothesized that these soil attributes are interacting in the creation of soil tilth, and that management will influence this interaction. Soil was sampled six and seven years after the start of a field experiment with different cropping systems at two loamy sand soils. A system dominated by small grain cereals not receiving animal manure served as a reference treatment (‘CEREAL’). This system was compared to the same crop sequence but with application of animal manure (‘CEREAL+MANURE’, only at one location), and to a diversified crop rotation including a grass/clover but without addition of animal manure (‘CEREAL+GRASS’). A part of each field plot was compacted by a tractor. We measured soil organic carbon fractions and soil microbial biomass in whole-soil samples. Carbon fractions were measured in 1-2 mm aggregates as well. Clay dispersibility was measured following an end-over-end shaking procedure, and the soil content of fungal hyphae was estimated in samples of 1-2 mm aggregates. The content of soil organic carbon was lowest for the CEREAL system at both locations. Hot-water extractable soil organic carbon displayed the same pattern but the trend was less significant. Total soil organic carbon was higher in 1-2 mm aggregates than in whole-soil, while hot-water extractable soil organic carbon was lower in the aggregates. The carbon fractions in aggregates responded similarly to cropping systems as those in whole-soil samples. Soil microbial biomass was highest in the CEREAL+GRASS system but significantly only for one year and at one location. Clay dispersibility was highest in the CEREAL system but significant only at one location. The growth of fungal hyphae was enhanced by addition of animal manure but in particular by the versatile crop rotation. Soil compaction tended to increase clay dispersibility at both locations but the effect was not significant. We conclude that only 6-7 years of differentiated soil management significantly affected soil organic carbon fractions, and that the dispersibility of clay and the growth of fungal hyphae were influenced as well. The results confirm our hypothesis that management affects the tilth-forming processes discussed. Our investigations do not lend support for the hypothesis that labile organic fractions are more sensitive to management changes than are total organic carbon. Neither did the measurements on aggregates give more clear answers than measurements on whole-soil samples.

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