EFFECTS OF SOIL TEXTURE AND STRUCTURE ON CARBON AND NITROGEN MINERALIZATION IN GRASSLAND SOILS

The hypotheses that disruption of soil structure increases mineralization rates in loams and clays more than in sandy soils and that this increase can be used to estimate the fraction of physically protected organic matter were tested, C and N mineralization was measured in undisturbed, and in finely and coarsely sieved moist or dried/remoistened soil. Fine sieving caused a temporary increase in mineralization. The relative increase in mineralization was much larger in loams and clays than in sandy soils and much larger for N than for C. The combination of remoistening and sieving of the soil gave a further increase in the mineralization flush after the disturbance. Again, the extra flush was larger in loams and clays than in sandy soils, and larger for N than for C. In loams and clays, small pores constituted a higher percentage of the total pore space than in sandy soils. The fraction of small pores explained more than 50% of the variation in the N mineralization rate between soils. There was also a good correlation between the small-pore fraction and the relative increase in N mineralization with fine sieving. For C, these relations were not clear. It is suggested that a large part of the organic matter that was present in the small pores could not be reached by micro-organisms, and was therefore physically protected against decomposition. Fine sieving exposed part of this fraction to decomposition. This physically protected organic matter had a lower C:N ratio than the rest of the soil organic matter. The increase in N mineralization after fine sieving can be regarded as a measure of physically protected organic matter.