Organic Matter Dynamics

There is a strong variation in the organic matter content and distribution between the plots (Table 1) that can be related to the effects of pH changes on the microbial activity and earthworm population (Shiel & Rimmer, 1984). Much of the change has occurred in the coarser organic matter fraction (Shiel, 1986) which has accumulated near the surface of the more acid plots (Plot 7) while the carbon content at depth on plot 7 has decreased - the upper 6 cm of plot 7 in Table 1 is an organic matter mat and is above the original surface level, so that plot 8 at 12-15 cm is equivalent to plot 7 at 18-21 cm. This surface mat was visible 50 years ago (Pawson, 1960) and now is apparent both to human and bovine visitors: the cattle prefer to lie on the plots with the thick surface organic mats.

Table 1. Effect of treatment on the volumetric carbon content (mgC cm^-3) by depth and the total carbon content (mg^C cm^-2) to 27 cm.

Both organic nitrogen and carbon have been affected. The change in organic matter with treatment is associated with a considerable change in soil bulk density and in the breakdown and accumulation rate of carbon in the soil (Table 2); this variation in breakdown suggests major variations in microbial activity (Shiel & Rimmer, 1984).

Table 2. The breakdown rate of endogenous and exogenous C, the change in C relative to plot 6 and the change in bulk density (BD) with depth.